WO1990003666A1 - State of charge of redox cell - Google Patents

State of charge of redox cell Download PDF

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Publication number
WO1990003666A1
WO1990003666A1 PCT/AU1989/000252 AU8900252W WO9003666A1 WO 1990003666 A1 WO1990003666 A1 WO 1990003666A1 AU 8900252 W AU8900252 W AU 8900252W WO 9003666 A1 WO9003666 A1 WO 9003666A1
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Prior art keywords
positive
negative
electrolyte
redox flow
flow cell
Prior art date
Application number
PCT/AU1989/000252
Other languages
French (fr)
Inventor
Maria Skyllas-Kazacos
Barry George Maddern
Michael Kazacos
Jaqui Joy
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Unisearch Limited
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Publication date
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Publication of WO1990003666A1 publication Critical patent/WO1990003666A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04746—Pressure; Flow
    • H01M8/04753—Pressure; Flow of fuel cell reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/661—Metal or alloys, e.g. alloy coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/64—Carriers or collectors
    • H01M4/66—Selection of materials
    • H01M4/663—Selection of materials containing carbon or carbonaceous materials as conductive part, e.g. graphite, carbon fibres
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04186—Arrangements for control of reactant parameters, e.g. pressure or concentration of liquid-charged or electrolyte-charged reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
    • H01M8/0432—Temperature; Ambient temperature
    • H01M8/04365—Temperature; Ambient temperature of other components of a fuel cell or fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04701—Temperature
    • H01M8/04708—Temperature of fuel cell reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04858—Electric variables
    • H01M8/04865—Voltage
    • H01M8/0488—Voltage of fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04858—Electric variables
    • H01M8/04895—Current
    • H01M8/0491—Current of fuel cell stacks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04858—Electric variables
    • H01M8/04949—Electric variables other electric variables, e.g. resistance or impedance
    • H01M8/04953—Electric variables other electric variables, e.g. resistance or impedance of auxiliary devices, e.g. batteries, capacitors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
    • H01M8/184—Regeneration by electrochemical means
    • H01M8/188—Regeneration by electrochemical means by recharging of redox couples containing fluids; Redox flow type batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0002—Aqueous electrolytes
    • H01M2300/0005—Acid electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04186—Arrangements for control of reactant parameters, e.g. pressure or concentration of liquid-charged or electrolyte-charged reactants
    • H01M8/04194—Concentration measuring cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • This invention relates to a method of determining the state of charge of a redox flow cell through which positive and negative electrolytes flow, a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow, a redox flow cell system in which the state of charge can be determined, a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow and a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current.
  • redox flow cells have many advantages over conventional batteries, they do have a particular disadvantage in that energy is lost as a result of pumping the positive and negative electrolytes through the respective half-cells.
  • the minimum flow rate per cell required is referred to as the stoichiometric flow rate.
  • Another object is to provide a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
  • Another object is to provide a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
  • Yet another object is to provide a redox flow cell system in which the state of charge can be determined.
  • a further object is to provide a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
  • Yet a further object is to provide a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current.
  • a method of determining the state of charge of a redox flow cell through which positive and negative electrolytes flow the redox flow cell having:
  • a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow the redox flow cell having:
  • a third embodiment of this invention there is provided a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
  • the methods of the first, second and third embodiments can include:
  • the methods of the first and second embodiments further include the step of determining the state of charge of the cell or a parameter related thereto from the characteristic(s).
  • the change in the flow rates of the positive and negative electrolytes is determined from the state of charge of the cell.
  • the methods of the invention can include measuring characteristics of the positive and/or negative electrolytes related to the state of charge of the cell by:
  • A. (1) measuring inlet and outlet open circuit redox flow cell voltages between the positive and negative electrolytes of the redox flow cell and determining the difference therebetween;
  • X and Y is an ion selected from ions such as Cr, Fe, Mn, Ni, Al, Mo, Ru, La, Ti, Pb, etc. measuring the absorption of a charged or discharged X m+ /X n+
  • negative electrolyte is a function of concentration of X m+ /X n+ posit1ve electrolyte and/or Y a+ /Y b+ ;
  • a redox flow cell having:
  • a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, which-system comprises:
  • a positive electrolyte pump means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
  • adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change(s) in the flow rates of the positive and negative electrolytes, whereby the cell provides the selected discharge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
  • a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current, which system comprises:
  • a redox flow cell having:
  • adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell requires the selected charge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
  • the systems of the fourth, fifth and sixth embodiments can include: (i) Means to measure a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell; or (ii) Means to measure a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell; or
  • the positive electrolyte is recirculated to the positive compartment via the means for transporting oositive electrolyte by the positive electrolyte pump.
  • the negative electrolyte is recirculated to the negative compartment via the means for transporting negative electrolyte by the negative electrolyte pump.
  • the means to measure in the apparatus of the invention can be means to measure the following characteristics of the positive and/or negative electrolytes as follows:
  • A. means to measure inlet and outlet open circuit redox flow cell voltages between the positive and negative electrolytes of the redox flow cell and determining the difference therebetween;
  • electrolyte means to measure the absorption of a charged or discharged negative V 2+ /V 3+ electrolyte at a selected
  • the positive compartments of the first to fourth embodiments each include a positive electrode and the negative compartments of the first to fourth embodiments each include a negative electrode.
  • the positive and negative electrodes can be any shape desired. It is preferred that the positive and negative electrodes are rectangular-plate shaped.
  • the positive and negative electrodes can be carbon or graphite felt, mat, plate, rod, knit, fibre, and cloth; carbon impregnated teflon; carbon impregnated polyethylene; carbon impregnated polypropylene; carbon
  • the positive electrode can also be carbon or graphite felt, mat, plate, rod, knit, fibre, and cloth, carbon impregnated teflon, carbon impregnated polyethylene, carbon impregnated polypropylene, carbon impregnated polystyrene, carbon impregnated polyvinylchloride and carbon impregnated polyvinylidenechloride, impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag.
  • the negative electrode can also be carbon and graphite felt, mat, plate, rod, knit, fibre, and cloth, carbon impregnated teflon, carbon impregnated
  • Other types of caroon electrodes can also be used.
  • the positive and negative electrodes can also De non carbon electrodes such as platinised Ti; platinised Ru; platinised
  • a dimensionally stabilized anode (DSA - Ti or Ti alloy core, coated at least partially with titanium dioxide which coating is coated in turn with a noble metal coating selected from the group consisting of Pt, Pd, Os, Rh,
  • Ru, Ir and alloys thereof is a suitable positive electrode or V 2 O 5 coated Pb or Ti.
  • the Dositive anolytes and the negative catholytes in the case of an all-vanadium cell for example, comprise an electrolyte which is typically an aqueous solution which includes at least one of the following H 2 SO 4 , trifluoromethanesulphonic acid, Na 2 SO 4 , K 2 SO 4 , H 3 PO 4 ,
  • arylsulphonic acid such as p-toluenesulphonic acid, benzenesulphonic acid, naphthalenesulphonic acid, C 1 -C 8 alkylsulphonic acid such as
  • methylsulphonic acid and ethylsulphonic acid, acetic acid or mixtures thereof in a concentration of from 0.01M to 6.0M. It is especially preferred to use H 2 SO 4 in a concentration of from 0.25M to 4.5M, more preferably 0.5M to 4M.
  • the electrolyte typically has vanadium ions in sufficient concentration for high discharge capacity in the discharge cell, for example, 0.25M to 3.5M, preferably 1M to 3M, and more preferably 1.5M to 2.5M are typical in the charge and discharge cells of the invention.
  • the vanadium ions in the electrolyte can oe prepared by dissolving an oxide, sulphate, phosphate, nitrate, halide or other salt or complex of vanadium which is soluble in the electrolyte. It is especially preferable to dissolve vanadyl sulphate in 0.5M to 3.5M H 2 SO 4 or
  • the electrolyte is typically stirred or agitated preferably with a mechanical stirrer.
  • a cell used in a system of the invention typically cells of the "memorane-type", that is each type of cell employs a membrane rather than a diaphragm to separate a positive compartment from a negative compartment.
  • the membrane employed is typically sheet-like and can transport electrolyte ions whilst at the same time being hydraulically-impermeable in contrast to a diaphragm (typically asbestos) which allows restricted electrolyte transfer between compartments.
  • the ionically conducting separator can be a microporous separator or a membrane fabricated from a polymer based on perfluorocarboxylic acids or a proton exchange polymer such as sulphonated polystyrene, sulphonated polyethylene or a substantially fluorinated sulpnonic acid polymer such as Nafion (Trade Mark) or membranes of Flemion (Trade Mark) or Selemion (Trade Mark) material as manufactured by Asahi Glass Company.
  • Discharging and charging a cell are typically conducted in sealed air tight cells and can be conducted under an inert atmosphere such as nitrogen, argon, helium or neon or mixtures thereof although an inert atmosphere can be avoided in a sealed system.
  • an inert atmosphere such as nitrogen, argon, helium or neon or mixtures thereof although an inert atmosphere can be avoided in a sealed system.
  • All-vanadium redox cells can be operated over a broad temperature range, e.g. -5°C to 99°C but are typically operated in the temperature range 0°C to 40°C.
  • the redox flow cell includes monopolar and bipolar type cells.
  • a bipolar cell typically includes a plurality of positive compartments eacn having a positive electrode therein and a plurality of negative
  • a bipolar cell is typically of tne flat plate- or filter press-type.
  • the electrolyte storage vessels may also oe equipped with liquid level detectors which will detect any changes to the electrolyte levels resulting from evaporation or water decomposition during charging.
  • valves open and allow water to be delivered by gravity from a separate holding tank into the electrolyte storage tanks. When the desired level is once more established the valves are automatically closed.
  • the electrolyte storage vessels may also be equipped with temperature sensors to ensure that the solution temperature does not exceed 40°C at which point the V(V) solution would be in danger of decomposing and precipitating. If the temperature reaches 40°C, the solution charging is ceased and the system is allowed to discharge to approximately 80%
  • a heat-exchanger may be incorporated in the catholyte flow loop to prevent excessive heating of solutions.
  • the present invention describes methods and apparatuses for
  • state-of-charge of the flow cell can be monitored by utilizing two electrodes, one in each type of half-cell, and measuring the open circuit voltage therebetween.
  • the monitoring device is connected nydraulically but not electrically in the cell stack.
  • a open-circuit voltmeter is placed both before and after the cell stack to monitor the state-of-charge of the electrolytes as they enter and leave the positive and negative compartments. The difference between the open-circuit voltage before and after the positive and negative compartments is a measure of the 7.
  • ⁇ E cell should be 0.12 volts when the solutions are fully charged
  • the all-vanadium redox flow cell can include an open-circuit cell which is hydraulically connected but not electrically connected to the cell stack.
  • the anolyte and catholyte flow through each half-cell and the open-circuit voltage of the system can be continuously monitored and used to include state-of-charge of system as well as to regulate charging and discharging between tne desired limits e.g. 10% to 90% state-of-charge, by control system.
  • the open-circuit cell voltage can be used as an indication of the system state-of-charge it must be assumed that the two half-cells are at the same state-of-charge i.e. the system is balanced. If the electrolytes were to become unbalanced, however, it would not be possible to determine the imbalance from the open-circuit voltage, nor would the state-of-charge be accurately indicated by E oc . Ideally, therefore, each of the 1/2-cell electrolyte potentials should be monitored so that the system balance can be measured together with the state-of-charge.
  • an inert metal indicator electrode could be utilized.
  • Electrolyte conductivity can also be used to continuously monitor state of charge and regulate charging and discharging between desired limits. Since electrolyte conductivity varies linearly with the
  • Conductivity varies linearly with SOC however, so for a particular conversion per pass, a constant value of ⁇ (conductivity) between inlet and outlet positive and/or negative electrolyte would be set to control the pump flow rates.
  • the positive electrolyte conductivity increases by approximately 11 ms/cm for each 107. increase in SOC for the range 0 to 907. SOC, as shown in Figure 13. Thus, by measuring the conductivity of the positive
  • This ⁇ (conductivity) value is independent of the solution state-of-cnarge and would be a much simpler pump control method than the ⁇ E oc approach described previously.
  • Temperature compensation can readily be performed by using temperature probes and conductivity meters or circuitry capable of correcting for temperature variations.
  • Fig. 1 depicts schematically a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
  • Fig. 2 depicts schematically an alternative redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
  • Fig. 3 depicts schematically a non-linear circuit for recirculation control of the positive and negative electrolytes
  • Fig. 4 depicts schematically an alternative non-linear circuit for recirculation control of the positive and negative electrolytes
  • Fig. 5 depicts schematically a cross section of an electrolyte absorption probe in a pipe
  • Fig. 6 depicts schematically a front view'of the electrolyte absorption probe of Fig. 5;
  • Fig. 7 depicts schematically another alternative redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
  • Fig. 8 is a plot of ⁇ E cell calculated for different value of SOC, x and fraction conversion y;
  • Fig. 9 is a plot of solution ootential of V 2+ /V 3+ and
  • Fig. 10 depicts cell voltage as a function of % state-of-cnarge for an al l-vanadi um cel l employi ng a Sel emion membrane and 1 .5M VOSO 4 i n 2M
  • Fig. 11 depicts a calibration curve for 1.0M V(IV)/V(V) half cell
  • Fig. 12 depicts a calibration curve for 1.0M V(II)/V(III) half cell
  • Fig. 13a depicts a plot of conductivity of 2M V + 2M H 2 SO 4
  • Fig. 13b depicts a plot of conductivity of negative vanadium cell electrolyte as a function of state-of-charge (2M V in 3M H 2 SO 4 ).
  • Fig. 13c depicts a plot of conductivity of a positive electrolyte ( 2M V in 3M H 2 SO 4 ) as a function of state-of-charge.
  • Fig. 14 depicts a plot of UV-visible spectra for 2 M positive electrolytes at different states-of-charge. Curves 1-10 correspond to state-of-charge values of 1.0, 0.95, 0.90, 0.80, 0.60, 0.40, 0.20, 0.10, 0.05 and 0 respectively.
  • Fig. 15a depicts a plot of UV-visible spectra for 2 molar negative electrolysis at different states-of-charge. Curves 1-4 correspond to state-of-charge values 1,0. 0.95,09 and 0.8 respectively.
  • Fig. 15b depicts a plot of UV-visible spectra for 2 molar negative electrolytes at different states-of-charge. Curves 5-7 correspond to state-of-charge values of 0.6, 0.4, and 0.2 respectively.
  • Fig. 15c depicts a plot of UV-visible spectra for 2 molar negative electrolytes at different states-of-charge. Curves 8-10 correspond to state-of-charge values 0.1, 0.5 and 0 respectively.
  • Fig. 16 depicts a plot of aosorbance of 2 molar negative electrolyte at 750 nm as a function of state-of-charge.
  • Fig. 17 depicts a plot of absorbance of 2 molar negative electrolyte as a function of state-of-cnarge.
  • Curve 1 corresponds to absorbance at the minimum in spectrum at 450-500 nm
  • curve 2 is apsorpance at 700-850 nm mi ni mum.
  • a redox flow cell system 100 for providing a selected discharge voltage/current from redox flow cell 101 through which positive and negative electrolytes flow.
  • the positive electrolyte consists of 0.25M to 2.5M pentavalent/tetravalent vanadium ions in 0.25M - 5M H 2 SO 4 .
  • the negative electrolyte consists of 0.25M to 2.5M
  • Cell 101 has a negative compartment 102 having negative electrode 114 disposed therein, positive compartment 103 having positive electrode 115 disposed therein and ionically conducting separator 104 generally a Selemion CMV membrane. Negative electrode 114 and positive electrode 115 are electrically coupled via means to charge/discharge 116. Separator 104 is operatively disposed between compartments 102 and 103 to provide ionic communication between positive electrolyte in the positive compartment 103 and negative electrolyte in compartment 102.
  • System 100 includes positive electrolyte storage/flowthrough reservoir 105 and negative electrolyte
  • Positive electrolyte pump 1 is connected to pipe 107 to recirculate positive electrolyte between positive compartment 103 and storage reservoir 105 via pipes 107 and 107A.
  • Negative electrolyte pump 2 is connected to pipe 108 to recirculate negative electrolyte between negative compartment 102 and storage reservoir 106 via pipes 108 and 108A.
  • Electrolyte voltage probes 110 and 111 (which can be Hg/Hg 2 SO 4 electrode for example) are placed sealably through apertures (not snown) in pipes 108A and 107A respectively and are connected electrically to voltmeter 109 via wires 118 and 119.
  • Voltmeter 109 measures the open circuit voltage between inlet positive electrolyte flowing via pipe 107A into comoartment 103 and inlet negative electrolyte fowing via pipe 108A into negative comoartment 102.
  • an inlet open-circuit cell containing a membrane and electrodes which can be graphite plates, glassy carbon, platinum or other noble metals for example can be placed in line in pipes 108A and 107A.
  • the membrane acts as an ionic conductor between the electrolytes in pipes 108A and 107A and the electrodes are connected to voltmeter 109 to measure the potential difference between the electrolytes in pipes 108A and 107A which
  • Electrolyte voltage probes 112 and 113 are placed in apertures (not shown) in pipes 107 and 108 respectively and are connected electrically to voltmeter 117 via wires 120 and 121 respectively.
  • Voltmeter 117 measures the open circuit voltage between outlet positive electrolyte flowing via pipe 107 into reservoir 105 and outlet negative electrolyte via pipe 108 into reservoir 106.
  • an outlet open-circuit cell containing a membrane and electrodes which can be graphite plates, glassy carbon, platinum or other noble metals, for example, can be placed in line in pipes 107 and 108.
  • the membrane acts as an ionic conductor between the electrolytes in pipes 107 and 108 and the electrodes are connected to voltmeter 117 which measures the potential difference between the electrolytes in pipes 107 and 108 which corresponds to the open circuit voltage between the outgoing electrolytes.
  • Adjusting means 122 is connected electrically to voltmeters 109 and 117 via wires 123 and 124 to receive output signals corresponding to the open circuit voltage measured by voltmeters 109 and 117 via wires 123 and 124. Adjusting means 122 is connected electrically to pumps 1 and 2 via wires 125 and 126 respectively.
  • pump 1 recirculates pentavalent/tetravalent vanadium ions through positive compartment 103 and through reservoir 105 via pipes 107 and 107A.
  • Pump 2 recirculates di vaient/trivalent vanadium ions tnrougn negative comoartment 102 and through reservoir 106 via pipes 108 and 108A.
  • Electrical energy is withdrawn from cell 101 by loading an external circuit in the means to charge/discharge 116.
  • the incoming open circuit voltage between the incoming negative electrolyte flowing through pipe 108A and the incoming positive electrolyte flowing through pipe 107A is measured by voltmeter 109 which determines the voltage difference measured by electrolyte voltage probes 110 and 111.
  • the outgoing open circuit voltage between the outgoing negative electrolyte flowing through pipe 108 and the outgoing positive electrolyte flowing through pipe 107 is measured by voltmeter 117 which determines the voltage difference between voltage probes 112 and 113.
  • Output signals corresponding to the incoming open circuit voltage and the outgoing open circuit voltage are sent to adjusting means 122 via wires 123 and 124 respectively.
  • recirculation control of the positive and negative electrolytes is shown in block diagrammatic form in Fig. 3.
  • Separate electrically controlled pumps 1 and 2 for conveyi ng positi ve and negati ve el ectrolytes through compartments 103 and 102 respectively, are energised by common line 12 from a power control unit 13 responding to digital pump commands on line 14 generated from computing block 15 and converted to analogue command signals on line 40 by D/A converter 16.
  • the pumping flow rate is automatically adjusted to produce a selected differential open circuit voltage
  • ⁇ E oc cell value dependent upon three factors, (1) the state of cnarge of the electrolyte, (2) the state of the cell charge, and (3) the current drain thereon.
  • Factor (1) is represented in the drawing by signal " ⁇ E oc measured” 34 applied on input line 17 to one side of a second computing block 18.
  • Output line 27 from computing block 18 is applied as input to computing block 15.
  • the otner input 19 to computing block 18 is derived from a ROM 20 whicn by a look-up table for a selected value of by" 35, ano corrected for temperature, determines on the output line 19 what the state-of-charge of the positive and negative electrolytes should be.
  • the internal table of the ROM 20 is addressed on input line 21 by the state of charge of the cells derived through a look-up table in a second ROM 22 which in turn is addressed by the open circuit cell voltage "Eoc i measured" 28 via input line 29 and the actual temperature "Temp. T” 30 via line 31.
  • the signal line 21 is applied as a first input to a computing block 23 which also receives on input line 24 an indication of the current drain on the batteries as an input "IC/ d measured" 32 and on a third input 25 a signal "manual constant C" 33 which provides manual control to modify the pump response to current drain.
  • An output F s is applied on output line 26 to a second input to the computing block 15 thereby to derive a pump command on line 14 which correlates the three factors (1), (2) and (3) referred to above.
  • FIG. 4 An alternative non-linear circuit for adjusting means 122 is shown in Fig. 4 where like designating numerals are applied to like componentry of Fig. 3.
  • a linear controller 27 generates pump commands on output line 14 based upon the error differential between the measured ⁇ E oc 34 and a value of ⁇ E oc calculated 19 from the state of charge of the cell.
  • the control parameters 36, 37 and a manually set constant 38, 39 are also inputs to the control strategy incorporated in the linear controller 27.
  • the states of charge of the positive and negative electrolytes and what those charges should be, are derived through ROM's 20 and 22 in a similar manner to that previously described in connection with Fig. 3.
  • a redox flow cell system 200 for providing a selected discnarge voltage/current from redox flow cell 201 through which positive and negative electrolytes flow.
  • Cell 201 has a negative compartment 202 having negative electrode 214 di sposed therein, positive compartment 203 having positive electrode 215 disposed therein and ionically conducting separator 204 generally a Selemion CMV membrane. Negative electrode 214 and positive electrode 215 are electrically coupled via means to charge/discharge 216. Separator 204 is operatively disposed between compartments 202 and 203 to provide ionic communication between positive electrolyte in the positive compartment 203 and negative
  • System 200 includes positive electrolyte storage/flowthrough reservoir 205 and negative electrolyte
  • Positive electrolyte pump 1 is connected to pipe 207 to recirculate positive electrolyte between positive compartment 203 and storage reservoir 205 via pipes 207 and 207A.
  • Negative electrolyte pump 2 is connected to pipe 208 to recirculate negative electrolyte between negative compartment 202 and storage
  • V 2+ /V 3+ negative electrolyte absorption probe 210 is placed sealably through an aperture (not shown) in pipe 208A and is connected electrically to voltmeter 209 via wire 218.
  • Meter 209 measures a voltage or current from probe 210 related to the absorption of incoming V 2+ /V 3+ negative electrolyte at about 750nm.
  • V 2+ /V 3+ negative absorption probe 212 is placed through an aperture
  • Adjusting means 222 is connected electrically to
  • Adjusting means 222 is connected electrically to pumps 1 and 2 via wires 225 and 226 respectively.
  • pump 1 recirculates pentavalent/tetravalent vanadium ions through positive compartment 203 and through reservoir 205 via pipes 207 and 207A.
  • Pump 2 recirculates divalent/trivalent vanadium ions through negative compartment 202 and through reservoir 206 via pipes 208 and 208A.
  • Electrical energy is withdrawn from cell 201 by loading an external circuit in the means to charge/discharge 216.
  • the incoming absorption of V 2+ /V 3+ incoming negative electrolyte flowing tnrough pipe 208A is measured by probe 210 and an output signal related thereto is determined as an incoming voltage by voltmeter 209.
  • Fig. 5 depicts a cross sectional section of a pipe 600 having a V 2+ /V 3+ negative absorption probe 210 inserted through an aperture 602 in pipe 600.
  • Probe 210 has an outer casing 603 which has a transverse aperture 604 extending from side 605 through to side 606 of probe 210.
  • An apsorption system 607 is located within casing 603 about aperture 604.
  • System 607 has an array of infrared light emitting diodes 608 which emit infrared light of about 750nm and an array of silicon diode detectors 609 which are located opposite diodes 608 to detect light emitted therefrom.
  • Diodes 608 are housed in compartment 610 which has a window 611 opposite detectors 609 and detectors 509 are housed in compartment 612 which has a window 613 opposite diodes 608. Hence light emitted by diodes 608 can pass through windows 611 and 613 and be detected by detectors 609. Diodes 608 are connected electrically to power supply 614 via wires 615.
  • Detectors 609 can be connected electrically to voltmeter 209 depicted in Fig. 2 via wires 218.
  • Fig. 5 depicts a front view of probe 210 which clearly shows aperture 604 extending from side 605.
  • diodes 608 are powered by power supply 614 to emit light of about 750nm which passes through windows 611 and 613 and is detected by detectors 609. A portion of V 2+ /V 3+ negative electrolyte 601 which is flowing through pipe 600 passes through aperture 604. Output signals related to the absorption of V 2+ /V 3+ negative electrolyte
  • a redox flow cell system 500 for providing a selected discharge voltage/current from redox flow cell 501 through which positive anc negative electrolytes flow.
  • the positive electrolyte consists of 0.25M to 2.5M pentavalent/tetravalent vanadium ions in 0.25M - 5M H 2 SO 4 .
  • the negative electrolyte consists of 0.25M to 2.5M
  • Cell 501 has a negative compartment 502 having negative electrode 514 disposed therein.
  • positive compartment 503 having positive electrode 515 disposed therein and ionically conducting separator 504 generally a Selemion CMV memorane.
  • Negative electrode 514 and positive electrode 515 are electrically coupled via means to charge/discharge 516.
  • Separator 504 is operatively disposed between compartments 502 and 503 to provide ionic communication between positive electrolyte in the positive compartment 503 and negative
  • System 500 includes positive electrolyte storage/flowthrough reservoir 505 and negative electrolyte
  • Positive electrolyte pump 1 is connected to pipe 507 to recirculate positive electrolyte between positive compartment 503 and storage reservoir 505 via pipes 507 and 507A.
  • Negative electrolyte pump 2 is connected to pipe 508 to recirculate negative electrolyte between negative compartment 502 and storage
  • Meter 509 measures a voltage or current from probe 510 related to the conductivity of incoming V 2+ /V 3+ negative electrolyte.
  • electrolyte conductivity probe 511 is placed sealably through an aperture (not shown) in pipe 508A and is connected electrically to conductivity meter 509A via wire 519.
  • Meter 509A measures a voltage or current from probe 519 related to the conductivity of incoming V 4+ /V 5+ positive electrolyte.
  • V 2+ /V 3+ negative conductivity meter probe 512 is placed through an aperture (not shown) in pipe 508 and is connected electrically to conductivity meter 517 via wire 520.
  • Meter 517 measures a voltage or current from probe 512 related to the conductivity of outgoing V 2 +/V 3+ negative electrolyte.
  • V 4+ /V 5+ positive conductivity meter probe 512A is oiaced through an aperture (not shown) in pipe 508 and is connected electrically to conductivity meter 517A via wire 521.
  • Meter 517A measares a voltage or current from probe 512A related to the conductivity of outgoing V 4+ /V 5+ positive electrolyte. Adjusting means 522 is
  • conductivity meters 509, 509A, 517 and 517A vi a wires 523, 523A , 524 and 524A respectively to recei ve output si gnals corresponding to the voltage or current measured by conductivity meters
  • Adjusting means 522 is connected electrically to pumps 1 and 2 via wires 525 and 526 respectively.
  • electrolyte rebalance one can employ oxalic acid additions from reservoir 527 via line 528 to positive electrolyte storage/flowthrough reservoir 505 periodically, e.g. if system capacity drops by 10% or if +ve & -ve side out of balance by e.g. 10% add stoichiometric amount of oxalic acid to +ve electrolyte+
  • the positive electrolyte in reservoir 505 can be agitated by bubbling N 2 through to assist reaction and allow escape of CO 2 through vents. After several hours of reaction battery 501 can be reused and system capacity will gradually be restored - see Fig. 18. In the case of experiments which led to the results shown in Fig. 18 when excess oxalic acid was added only a slight increase in capacity is observed. If required amount is added capacity is restored.
  • the chemical reductant can also be KHC 2 O 4 .H 2 O, K 2 C 2 O 4 ,
  • Other chemical reductants can be used.
  • a reducing organic water-soluble compound such as a reducing organic water-soluble mercapto group-containing compound including SH-containing water-soluble lower alcohols (including SH-containing C 1 -C 1 2 primary, secondary and tertiary alkyl alcohols), SH-containing C 1 -C 12 primary, secondary and tertiary alkyl carboxylic acids, SH-containing C 1 -C 12 primary, secondary and tertiary alkyl amines and salts thereof,
  • SH-containing water-soluble lower alcohols including SH-containing C 1 -C 1 2 primary, secondary and tertiary alkyl alcohols
  • SH-containing C 1 -C 12 primary, secondary and tertiary alkyl carboxylic acids SH-containing C 1 -C 12 primary, secondary and tertiary alkyl amines and salts thereof
  • SH-containing C 1 -C 1 2 primary, secondary and tertiary alkyl amine acids and di- or tripeptides such as 2-mercaptoethylamine hydrochloride,
  • Reductants such as (NH 4 ) 2 C 2 O 4 NH 4 HC 2 O 4 .H 2 O, SO 2 ,
  • (NH 4 ) 2 SO 6 and H 2 are particularly advantageous as reductants since at least some of the reaction product is gaseous permitting higher concentrations of vanadium ions to be prepared and reducing further treatment of electrolyte to remove unwanted products.
  • pump 1 recirculates pentavalent/tetravalent vanadium ions througn positive compartment 503 and througn reservoir 505 via pipes 507 and 507A.
  • Pumo 2 recirculates divalent/trivalent vanadium ions through negative compartment 502 and through reservoir 506 via pipes 508 and 508A.
  • Electrical energy is withdrawn from cell 501 by loading an external circuit in the means to charge/discharge 516.
  • the incoming conductivity of V 2+ /V 3+ incoming negative electrolyte flowing through pipe 508A is measured by probe 510 and an output signal related thereto is determined by conductivity meter .
  • the incoming conductivity of V 4+ /V 5+ incoming positive electrolyte flowing through pipe 507A is measured by probe 511 and an output signal related thereto is determined by conductivity meter 509A.
  • the outgoing conductivity of V 2+ /V 3+ outgoing negative electrolyte flowing through pipe 508 is measured by probe 512 and an output signal related thereto is determined by
  • conductivity meter 517 The outgoing conductivity of V 4+ /V 5+ outgoing positive electrolyte flowing through pipe 507 is measured by probe 512A and an output signal related thereto is determined by conductivity meter
  • conductivities and the determined outgoing conductivities are transmitted to adjusting means 522 via wires 523, 523A, 524 and 524A respectively.
  • Adjusting means 522 determines the difference between the conductivities of the incoming and outgoing negative electrolytes and the difference between the conductivities of the incoming and outgoing positive electrolytes and adjusts the pump speeds of pumps 2 and 1, so that cell 501 outputs a selected discharge voltage.
  • Two analogous circuits to that shown in Fig. 3 or to that shown in Fig. 4 can be utilized, one for controlling pump 1 and one for controlling pump 2; except that ⁇ E oc is replaced by the difference between the
  • Fig. 9 demonstrates that the solution potentials of V 2+ /V 3+ in
  • H 2 SO 4 and V 4+ /V 5+ in H 2 SO 4 changes only slightly over a wide
  • E E° - cell cell
  • Fig. 10 shows the results of experiments in which the open-circuit voltage of an all-vanadium redox cell employing Selemion CMV membrane, as a function of the system's state-of-charge.
  • the results in this diagram demonstrates the feasibility of utilizing open-circuit voltage to monitor state-of-charge of the cell and thus control the charging and discharging processes between the required limits e.g. 10% to 90% state-of-charge.
  • Figs. 11 and 12 show the potentials of the positive and negative
  • Figs. 13a - 13c show the linear variation in the conductivities of both the positive (V 4+ /V 5+ ) and negative (V 2+ /V 3+ ) electrolytes of the vanadium redox cell as a function of state-of-charge.
  • the results in this diagram show that by simply measuring the conductivity of each solution with a standard probe, a simple meter can be calibrated to indicate solution state-of-charge directly for each half-cell electrolyte.
  • V 4+ (blue) - V 5+ (yellow) a spectrophotometric method could also be
  • Figure 14 shows a series of
  • curve 2 is absorbance at 700-850 nm minimum.
  • a simple detector can thus be employed to monitor the absorption by the solution of
  • the present invention discloses a method and apparatus which provide the necessary solution flowrate for a selected discharge voltage/current from a redox flow cell particularly an all-vanadium redox flow cell.
  • the method and apparatus are particularly useful in practical applications since a redox flow cell can be operated with minimum pumping energy so as to provide the required constant current and/or voltage output over a given period of time.
  • a redox flow cell can be operated so as to provide variable current and/or voltage output so as to meet demand requirements.
  • the latter method and apparatus are particularly useful in practical applications since a redox flow cell can be operated so that it requires a constant current and/or voltage input over a given period of time.
  • a redox flow cell can be operated with a variable current and/or voltage input under which conditions considerable pumping energy can be saved by adjusting the pump flow rates to the minimum required for the current involved and the SOC of the system.

Abstract

A redox flow cell system (100) in which the state of charge can be determined is disclosed. The system (100) includes a redox flow cell (101) having a negative compartment (102), a positive compartment (103) and an ionically conducting separator (104) operatively disposed between the positive and negative compartments and in contact with positive and negative electrolytes in compartments (103) and (102) to provide ionic communication therebetween; a positive electrolyte pump (1); means (107), (105) and (107A) for transporting positive electrolyte between the positive compartment (103) and the positive electrolyte pump (1), operatively associated with the positive compartment (103) and the positive electrolyte pump (1); a negative electrolyte pump (2); means (108), (106) and (108A) for transporting negative electrolyte between the negative compartment (102) and the negative electrolyte pump (2), operatively associated with the negative compartment (102) and the negative electrolyte pump (2); means to measure (110), (111) and (109) a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes; and means (122) to determine the state of charge of the cell from the characteristic(s), operatively associated with the means to measure (110), (111) and (109). Also disclosed are a method of determining state of charge of a redox flow cell, a method of providing a selected discharge voltage/current from a redox flow cell, a method of providing a selected charge voltage/current from a redox flow cell, a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell, and a redox flow cell system having a redox flow cell which is adaptable to require a selected charge voltage/current.

Description

STATE OF CHARGE OF REDOX CELL
TECHNICAL FIELD
This invention relates to a method of determining the state of charge of a redox flow cell through which positive and negative electrolytes flow, a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow, a redox flow cell system in which the state of charge can be determined, a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow and a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current.
BACKGROUND ART
Although redox flow cells have many advantages over conventional batteries, they do have a particular disadvantage in that energy is lost as a result of pumping the positive and negative electrolytes through the respective half-cells. Depending on the state of charge of the flow cell and the current used to charge or discharge the flow cell, the minimum flow rate per cell required is referred to as the stoichiometric flow rate. For an all-vanadium redox flow cell, employing recirculating 2M vanadium ions in the positive and negative electrolytes of each half-cell, the
stoichiometric flow rate Fs is given by:
Fs = 1/(3.2 X SOC) ml/min
where I = charge or discharge current in Amps
SOC = state-of-charge of system hence if a current of 10 amps is being drawn from the vanadium redox flow cell, the stoichiometric flow rate required for the positive and negative electrolytes through each half-cell, would be 3.1 ml/min if the battery was fully charged (SOC = 1), and 31 ml/min when the battery was only 10% charged (SOC = 0.1).
To minimise pumping energy requirements in flow cells, it has been proposed to vary the flow rate of the positive and negative electrolytes through the respective half-cells as a function of state-of-charge of the flow cell.
OBJECTS OF INVENTION
It is an object of this invention to provide a method of determining the state of charge of a redox flow cell through which positive and
negative electrolytes flow.
Another object is to provide a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
Another object is to provide a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
Yet another object is to provide a redox flow cell system in which the state of charge can be determined.
A further object is to provide a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow.
Yet a further object is to provide a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current.
DISCLOSURE OF INVENTION
According to a first embodiment of this invention there is provided a method of determining the state of charge of a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative comoartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell; and
determining the state of charge of the redox cell from the
characteristic(s).
According to a second embodiment of this invention there is provided a method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell;
determining change in flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, from the characteristic(s), required to provide the selected discharge voltage/current; and
adjusting flow rates of the positive and negative electrolytes through the positive and negative comoartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell provides the selected discharge
voltage/current.
According to a third embodiment of this invention there is provided a method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disϋosed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characteristic(s) related to state of charge of the cell; determining the change in the flow rates of the positive and negative electrolytes from the characterisec(s), whereby the cell requires the selected charge voltage/current; and
adjusting the flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell requires the selected charge voltage/current
The methods of the first, second and third embodiments can include:
(i) Measuring a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell; or
(ii) Measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell; or
(iii) Measuri ng a characteristic(s) of the positive ana negative electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
Typically the methods of the first and second embodiments further include the step of determining the state of charge of the cell or a parameter related thereto from the characteristic(s). In this case, the change in the flow rates of the positive and negative electrolytes is determined from the state of charge of the cell.
Generally the methods of the invention can include measuring characteristics of the positive and/or negative electrolytes related to the state of charge of the cell by:
A. (1) measuring inlet and outlet open circuit redox flow cell voltages between the positive and negative electrolytes of the redox flow cell and determining the difference therebetween;
(2) measuring conductivity of the positive and/or negative
electrolyte entering and/or leaving the redox flow cell;
(3) in the case of a redox flow cell having a negative V2+/V3+
electrolyte, measuring the absorption of a charged or discharged negative V2+/V3+ electrolyte at a selected wavelength or within a selected wavelength range in which the absorption of the negative V2+/V3+ negative electrolyte is a function of concentration of V2+/V3+ typically entering and leaving the redox flow cell; or
(4) in the case of a redox flow cell having a Xm+/Xn+
positive electrolyte and Ya+/YD+
negative electrolyte, where X and Y is an ion selected from ions such as Cr, Fe, Mn, Ni, Al, Mo, Ru, La, Ti, Pb, etc. measuring the absorption of a charged or discharged Xm+/Xn+
positive electrolyte and/or Ya+/Yb+
negative electrolyte at a selected waveiength(s) or within a selected wavelength range in which the absorption of the Xm+/Xn+ positive electrolyte and/or Ya+/Yb+
negative electrolyte is a function of concentration of Xm+/Xn+ posit1ve electrolyte and/or Ya+/Yb+ ;
3. determining the state of charge of the redox flow cell or a parameter related thereto from:
(1) the inlet or outlet open circuit redox flow cell voltages;
(2) the conductivity of the positive and/or negative electrolyte
entering and/or leaving the cell;
(3) the absorption of a charged or discharged negative V2+/V3+
electrolyte at the selected wavelength or within the selected wavelength range; or
(4) the absorption of Xm+/Xn+ positive electrolyte and/or
Ya+/Yb+ negative electrolyte at the selected wavelength or within the selected wavelength range
C. determining the magnitudes of the changes in the flow rates of the positive and negative electrolytes from the state-of-charge of the cell or a parameter related thereto required to provide the selected discharge voltage or the selected charge voltage; and
D. adjusting the flow rates of the positive and negative electrolytes through the cell in accordance with the magnitudes of the changes in the flow rates of the positive and negative electrolytes whereby the redox cell provides the selected discharge voltage or requires a selected charge voltage.
According to a fourth embodiment of this invention there is provided a redox flow cell system in which the state of charge can be determined which system comprises:
a redox flow cell having:
(a) a negative comoartment;
(b) a positive comoartment; and (c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump;
means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
a negative electrolyte pump;
means for transporting negative electrolyte between the negative compartment and tne negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a characterisec(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes; and
means to determine the state of charge of the cell from the
characteristic(s), operatively associated with the means to measure.
Typically the state of charge of a redox flow cell is monitored continuously during operation.
According to a fifth embodiment of this invention there is provided a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, which-system comprises:
a redox flow celljiaving:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting seoarator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump; means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
a negative electrolyte pump;
means for transporting negative electrolyte between the negative compartment and the negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a cnaracteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes;
means to determine change(s) in flow rates of the positive and negative electrolytes from the characteristic(s), whereby the cell provides the selected discharge voltage/current, which means to determine is operatively associated with the means to measure; and
adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change(s) in the flow rates of the positive and negative electrolytes, whereby the cell provides the selected discharge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
According to a sixth embodiment of this invention there is provided a redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current, which system comprises:
a redox flow cell having:
(a) a negative comoartment;
(b) a positive comoartment; and (c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump;
means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
a negative electrolyte pump;
means for transporting negative electrolyte between the negative compartment and the negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes;
means to determine change in flow rates of the positive and negative electrolytes from the characteristic(s), whereby the cell requires a selected charge voltage/current, which means to determine is operatively associated with the means to measure; and
adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell requires the selected charge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
The systems of the fourth, fifth and sixth embodiments can include: (i) Means to measure a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell; or (ii) Means to measure a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell; or
(iii) Means to measure a characterisec(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a characterιstic(s) of the negative and positive electrolytes leaving the redox flow cell.
Generally the positive electrolyte is recirculated to the positive compartment via the means for transporting oositive electrolyte by the positive electrolyte pump.
Also generally the negative electrolyte is recirculated to the negative compartment via the means for transporting negative electrolyte by the negative electrolyte pump.
The means to measure in the apparatus of the invention can be means to measure the following characteristics of the positive and/or negative electrolytes as follows:
A. (1) means to measure inlet and outlet open circuit redox flow cell voltages between the positive and negative electrolytes of the redox flow cell and determining the difference therebetween; or
(2) means to measure conductivity of the positive and/or negative electrolyte entering and/or leaving the redox flow cell;
(3) in the case of a redox flow cell having a negative V2+/V3+
electrolyte, means to measure the absorption of a charged or discharged negative V2+/V3+ electrolyte at a selected
wavelength or within a selected wavelength range in which the absorption of the negative V2+/V3+ negative electrolyte is a function of concentration of V2+/V3+ tyoically entering and leaving the redox flow cell; or
(4) in the case of a redox flow cell having a negative V2+/V3+ electrolyte, means to measure tne absorption of a charged or discharged Xm+/Xn+ positive electrolyte and/or Ya+/Yb+
negative electrolyte at a selected wavelength or
within a selected wavelength range in which the absorption of the Xm+/Xn+ positive electrolyte and/or Ya+/Yb+ negative electrolyte is a function of concentration of Xm+/Xn+ and/or Ya+/Yb+.
In operation, the positive compartments of the first to fourth embodiments each include a positive electrode and the negative compartments of the first to fourth embodiments each include a negative electrode. The positive and negative electrodes can be any shape desired. It is preferred that the positive and negative electrodes are rectangular-plate shaped.
The positive and negative electrodes can be carbon or graphite felt, mat, plate, rod, knit, fibre, and cloth; carbon impregnated teflon; carbon impregnated polyethylene; carbon impregnated polypropylene; carbon
impregnated polystyrene; carbon impregnated polyvinylchloride; carbon impregnated polyvinylidenechloride; glassy carbon; non-woven carbon fibre material; and cellulose. The positive electrode can also be carbon or graphite felt, mat, plate, rod, knit, fibre, and cloth, carbon impregnated teflon, carbon impregnated polyethylene, carbon impregnated polypropylene, carbon impregnated polystyrene, carbon impregnated polyvinylchloride and carbon impregnated polyvinylidenechloride, impregnated with and/or coated with Au, Mn, Pt, Ir, Ru, Os, Re, Rh, Sb, Te, Pb and/or Ag. The negative electrode can also be carbon and graphite felt, mat, plate, rod, knit, fibre, and cloth, carbon impregnated teflon, carbon impregnated
polyethylene, carbon impregnated polypropylene, carbon impregnated
polystyrene, carbon impregnated polyvinylchloride and carbon impregnated polyvinylidenechloride, imoregnated with anα/or coated with Pb, Si, Tl, Hg, Cd, In, Ag, Be, Ga, Sb, As, Zn, Ca and/or Mg. Other types of caroon electrodes can also be used. The positive and negative electrodes can also De non carbon electrodes such as platinised Ti; platinised Ru; platinised
Ir; platinised Pd; Pt; Pt black; Au; Pd; Ir; Ru; Os; Re; Rh; Hg or Ag. A dimensionally stabilized anode (DSA - Ti or Ti alloy core, coated at least partially with titanium dioxide which coating is coated in turn with a noble metal coating selected from the group consisting of Pt, Pd, Os, Rh,
Ru, Ir and alloys thereof) is a suitable positive electrode or V2O5 coated Pb or Ti.
The Dositive anolytes and the negative catholytes in the case of an all-vanadium cell for example, comprise an electrolyte which is typically an aqueous solution which includes at least one of the following H2SO4, trifluoromethanesulphonic acid, Na2SO4, K2SO4, H3PO4,
Na3PO4, K3PO4, HNO3, KNO3, NaNO3, sulphonic acid, C8-C14
arylsulphonic acid such as p-toluenesulphonic acid, benzenesulphonic acid, naphthalenesulphonic acid, C1-C8 alkylsulphonic acid such as
methylsulphonic acid and ethylsulphonic acid, acetic acid or mixtures thereof in a concentration of from 0.01M to 6.0M. It is especially preferred to use H2SO4 in a concentration of from 0.25M to 4.5M, more preferably 0.5M to 4M.
In the case of an all-vanadium cell the electrolyte typically has vanadium ions in sufficient concentration for high discharge capacity in the discharge cell, for example, 0.25M to 3.5M, preferably 1M to 3M, and more preferably 1.5M to 2.5M are typical in the charge and discharge cells of the invention. The vanadium ions in the electrolyte can oe prepared by dissolving an oxide, sulphate, phosphate, nitrate, halide or other salt or complex of vanadium which is soluble in the electrolyte. It is especially preferable to dissolve vanadyl sulphate in 0.5M to 3.5M H2SO4 or
V2O5 or NH4VO3 or NaVO3 in 0.5M to 6M H2SO4 by electrolytic
dissolution.
During discharging and charging of the cell the electrolyte is typically stirred or agitated preferably with a mechanical stirrer.
A cell used in a system of the invention typically cells of the "memorane-type", that is each type of cell employs a membrane rather than a diaphragm to separate a positive compartment from a negative compartment. The membrane employed is typically sheet-like and can transport electrolyte ions whilst at the same time being hydraulically-impermeable in contrast to a diaphragm (typically asbestos) which allows restricted electrolyte transfer between compartments. Thus the ionically conducting separator can be a microporous separator or a membrane fabricated from a polymer based on perfluorocarboxylic acids or a proton exchange polymer such as sulphonated polystyrene, sulphonated polyethylene or a substantially fluorinated sulpnonic acid polymer such as Nafion (Trade Mark) or membranes of Flemion (Trade Mark) or Selemion (Trade Mark) material as manufactured by Asahi Glass Company.
Discharging and charging a cell are typically conducted in sealed air tight cells and can be conducted under an inert atmosphere such as nitrogen, argon, helium or neon or mixtures thereof although an inert atmosphere can be avoided in a sealed system.
All-vanadium redox cells can be operated over a broad temperature range, e.g. -5°C to 99°C but are typically operated in the temperature range 0°C to 40°C.
The redox flow cell includes monopolar and bipolar type cells. A bipolar cell typically includes a plurality of positive compartments eacn having a positive electrode therein and a plurality of negative
comoartments each having a negative electrode therein and wherein each of the compartments are separated by a membrane. A bipolar cell is typically of tne flat plate- or filter press-type.
The electrolyte storage vessels may also oe equipped with liquid level detectors which will detect any changes to the electrolyte levels resulting from evaporation or water decomposition during charging.
To prevent excessive water loss which will lead to solution
supersaturation and possible crystallization of vanadium compounds, when the liquid levels drop below the critical point, valves open and allow water to be delivered by gravity from a separate holding tank into the electrolyte storage tanks. When the desired level is once more established the valves are automatically closed.
The electrolyte storage vessels may also be equipped with temperature sensors to ensure that the solution temperature does not exceed 40°C at which point the V(V) solution would be in danger of decomposing and precipitating. If the temperature reaches 40°C, the solution charging is ceased and the system is allowed to discharge to approximately 80%
state-of-charge. This maximum state-of-charge is maintained until the temperature drops below 40°C.
Alternatively a heat-exchanger may be incorporated in the catholyte flow loop to prevent excessive heating of solutions.
The present invention describes methods and apparatuses for
controlling the pump flowrate depending on both state-of-charge and the magnitude of current employed. In one desirable form, the inventor has found that state-of-charge of the flow cell can be monitored by utilizing two electrodes, one in each type of half-cell, and measuring the open circuit voltage therebetween. In this method of monitoring the flow cell the monitoring device is connected nydraulically but not electrically in the cell stack. A open-circuit voltmeter is placed both before and after the cell stack to monitor the state-of-charge of the electrolytes as they enter and leave the positive and negative compartments. The difference between the open-circuit voltage before and after the positive and negative compartments is a measure of the 7. conversion of the fluids on a single pass through the positive and negative comoartments As the monitored open circuit voltage decreases with decreasing state-of-charge of the flow cell, the flow rates of the positive and negative electrolytes through the redox cell are increased to maintain cell efficiency.
A difficulty with this type of monitoring technique is that when the current of the flow cell is increased or reduced, the stoichiometric flow rate can also change drastically (e.g. if I = 1 Amp, F = 0.31 ml/min at SOC = 1 and if I = 60 Amps, Fs = 18.60ml/min at SOC = 1).
This difficulty is overcome by the present invention which discloses a method of adjusting the flow rates of the positive and negative
electrolytes through the battery in accordance with the change in the flow rates of the negative and positive electrolytes through the redox flow cell
The difference in the open-circuit cell voltages ΔEoc is a
measure of the % conversion of the electrolyte. A simple equation can be derived which relates ΔEoc to a % conversion per pass, as follows.
For the positive 1/2 cell:
V5+ + e = V4+
E+ = E°+ -
Figure imgf000018_0001
For the negative 1/2 cell:
v3+ + e = V2+
E- = Eº- -
Figure imgf000018_0002
For inlet cell:
Figure imgf000018_0012
- E+ - E- = Eº
cell
Figure imgf000018_0003
f
Figure imgf000018_0004
For outlet cell :
Figure imgf000018_0005
Let x = state-of-charge, SOC
y = fraction conversion per pass
C° = initial vanadium concentration (V2+ or V5+)
Then * = xC° and = (1-x)C°
Figure imgf000018_0007
Figure imgf000018_0006
= (x-xy)C° and
Figure imgf000018_0008
= [(1-x)+xy]Cº
Also * = xC° and - (1-x)C°
Figure imgf000018_0010
= (x-xy)Cº and = C(1-x) + xy]Cº
Figure imgf000018_0009
Figure imgf000018_0011
2
Figure imgf000019_0001
The difference between the inlet and outlet open circuit cell voltages is thus given by ΔEoC or:
OR
Figure imgf000019_0002
For different values of SOC, x and fraction conversion, y at 25°C, therefore values of ΔEcell can be calculated and these are plotted in Fig. 8.
Thus, if a fraction conversion of 0.4 per pass is required,
ΔEcell should be 0.12 volts when the solutions are fully charged,
dropping down to approximately 0.028 volts when the solutions are only 107. charged. The pumping flow rate is then adjusted to give the desired
ΔEcell value depending on the state-of-charge of the cell stack, as indicated by the inlet open-circuit cell voltage (i.e. l) for
Figure imgf000019_0003
example. Other state-of-charge indicators could also be used, however, and two alternative approaches are described below.
The all-vanadium redox flow cell can include an open-circuit cell which is hydraulically connected but not electrically connected to the cell stack. The anolyte and catholyte flow through each half-cell and the open-circuit voltage of the system can be continuously monitored and used to include state-of-charge of system as well as to regulate charging and discharging between tne desired limits e.g. 10% to 90% state-of-charge, by control system.
Although the open-circuit cell voltage can be used as an indication of the system state-of-charge it must be assumed that the two half-cells are at the same state-of-charge i.e. the system is balanced. If the electrolytes were to become unbalanced, however, it would not be possible to determine the imbalance from the open-circuit voltage, nor would the state-of-charge be accurately indicated by Eoc. Ideally, therefore, each of the 1/2-cell electrolyte potentials should be monitored so that the system balance can be measured together with the state-of-charge.
Theoretically an inert metal indicator electrode could be utilized.
However, in a large scale redox flow cell system use of such an electrode would be impractical since solution potential measurement also requires a stable reference electrode potential. Reference electrodes, however are inherently unreliable due to a large number of interferences that can lead to drifts in the measured potential. Furthermore, the inventors have found that the solution potential changes only slightly over a wide range of states-of-charge so that sensitivity is poor.
Two alternative forms of the invention have also been disclosed, namely, one which utilizes conductivity measurements and one which uses absorption measurements based on colorometric or spectrophotometric principles, using the different colours of the charged and discharged anolyte and catholyte to determine and display the system state-of-charge allow accurate monitoring of system balance and state-of charge of a vanadium redox flow cell, but can also be applied to any cell system in which the electrolytes undergo changes in conductivity (e.g. Zn/Br2 cell) or colour. Thus for example: at negative V(II) V(III)
violet green
at positive V(IV) V(V)
blue yellow
Figure imgf000021_0001
Electrolyte conductivity can also be used to continuously monitor state of charge and regulate charging and discharging between desired limits. Since electrolyte conductivity varies linearly with the
state-of-charge of both the negative and positive 1/2-cell electrolytes (illustrated in Fig. 13), changes in conductivity between the inlet and outlet solutions could also be used to set the required conversion per pass and thus the flow rate through the cell. ΔEoc values do not change linearly with SOC and therefore require a more complex algorithm or look-up tables to determine appropriate value for pump control.
Conductivity varies linearly with SOC however, so for a particular conversion per pass, a constant value of Δ (conductivity) between inlet and outlet positive and/or negative electrolyte would be set to control the pump flow rates.
The positive electrolyte conductivity increases by approximately 11 ms/cm for each 107. increase in SOC for the range 0 to 907. SOC, as shown in Figure 13. Thus, by measuring the conductivity of the positive
electrolyte at the inlet and outlet of the cell stack, a conversion per pass of, for example, 30% could be maintained by setting a value of Δ (conductivity) equal to 11 X 3 = 33 ms/cm. This Δ (conductivity) value is independent of the solution state-of-cnarge and would be a much simpler pump control method than the ΔEoc approach described previously.
The variation in conαuctivity with SOC illustrated in Figure 13 applies to a temperature of 25°C and for a solution compositions of 2M vanadium sulphate in 2 M H2SO4. The conductivity varies considerably with solution composition and temperature. These plots cannot, therefore, be employed if the solution composition differs from the above.
Temperature compensation can readily be performed by using temperature probes and conductivity meters or circuitry capable of correcting for temperature variations.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 depicts schematically a redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
Fig. 2 depicts schematically an alternative redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
Fig. 3 depicts schematically a non-linear circuit for recirculation control of the positive and negative electrolytes;
Fig. 4 depicts schematically an alternative non-linear circuit for recirculation control of the positive and negative electrolytes;
Fig. 5 depicts schematically a cross section of an electrolyte absorption probe in a pipe;
Fig. 6 depicts schematically a front view'of the electrolyte absorption probe of Fig. 5;
Fig. 7 depicts schematically another alternative redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow;
Fig. 8 is a plot of ΔEcellcalculated for different value of SOC, x and fraction conversion y;
Fig. 9 is a plot of solution ootential of V2+/V3+ and
V4+/V5+ n H2SO4 as a function of state of charge;
Fig. 10 depicts cell voltage as a function of % state-of-cnarge for an al l-vanadi um cel l employi ng a Sel emion membrane and 1 .5M VOSO4 i n 2M
H7SO4 electrolyte (charging current = 20ma.cm-2, electrode area =
200cm~ and the cell was discharged across a 0.5 ohm resistor);
Fig. 11 depicts a calibration curve for 1.0M V(IV)/V(V) half cell;
Fig. 12 depicts a calibration curve for 1.0M V(II)/V(III) half cell;
Fig. 13a depicts a plot of conductivity of 2M V + 2M H2SO4
positive and negative electrolytes as a function of state-of-charge.
Fig. 13b depicts a plot of conductivity of negative vanadium cell electrolyte as a function of state-of-charge (2M V in 3M H2SO4).
Fig. 13c depicts a plot of conductivity of a positive electrolyte ( 2M V in 3M H2SO4) as a function of state-of-charge.
Fig. 14 depicts a plot of UV-visible spectra for 2 M positive electrolytes at different states-of-charge. Curves 1-10 correspond to state-of-charge values of 1.0, 0.95, 0.90, 0.80, 0.60, 0.40, 0.20, 0.10, 0.05 and 0 respectively.
Fig. 15a depicts a plot of UV-visible spectra for 2 molar negative electrolysis at different states-of-charge. Curves 1-4 correspond to state-of-charge values 1,0. 0.95,09 and 0.8 respectively.
Fig. 15b depicts a plot of UV-visible spectra for 2 molar negative electrolytes at different states-of-charge. Curves 5-7 correspond to state-of-charge values of 0.6, 0.4, and 0.2 respectively.
Fig. 15c depicts a plot of UV-visible spectra for 2 molar negative electrolytes at different states-of-charge. Curves 8-10 correspond to state-of-charge values 0.1, 0.5 and 0 respectively.
Fig. 16 depicts a plot of aosorbance of 2 molar negative electrolyte at 750 nm as a function of state-of-charge.
Fig. 17 depicts a plot of absorbance of 2 molar negative electrolyte as a function of state-of-cnarge. Curve 1 corresponds to absorbance at the minimum in spectrum at 450-500 nm, curve 2 is apsorpance at 700-850 nm mi ni mum.
BEST MODE AND OTHER MODES FOR CARRYING OUT THE INVENTION
Referring to Fig. 1 a redox flow cell system 100 for providing a selected discharge voltage/current from redox flow cell 101 through which positive and negative electrolytes flow. The positive electrolyte consists of 0.25M to 2.5M pentavalent/tetravalent vanadium ions in 0.25M - 5M H2SO4. The negative electrolyte consists of 0.25M to 2.5M
divalent/trivalent vanadium ions in 0.25M - 5M H2SO4. Cell 101 has a negative compartment 102 having negative electrode 114 disposed therein, positive compartment 103 having positive electrode 115 disposed therein and ionically conducting separator 104 generally a Selemion CMV membrane. Negative electrode 114 and positive electrode 115 are electrically coupled via means to charge/discharge 116. Separator 104 is operatively disposed between compartments 102 and 103 to provide ionic communication between positive electrolyte in the positive compartment 103 and negative electrolyte in compartment 102. System 100 includes positive electrolyte storage/flowthrough reservoir 105 and negative electrolyte
storage/flowthrough reservoir 106. Positive electrolyte pump 1 is connected to pipe 107 to recirculate positive electrolyte between positive compartment 103 and storage reservoir 105 via pipes 107 and 107A.
Negative electrolyte pump 2 is connected to pipe 108 to recirculate negative electrolyte between negative compartment 102 and storage reservoir 106 via pipes 108 and 108A. Electrolyte voltage probes 110 and 111 (which can be Hg/Hg2SO4 electrode for example) are placed sealably through apertures (not snown) in pipes 108A and 107A respectively and are connected electrically to voltmeter 109 via wires 118 and 119. Voltmeter 109 measures the open circuit voltage between inlet positive electrolyte flowing via pipe 107A into comoartment 103 and inlet negative electrolyte fowing via pipe 108A into negative comoartment 102. Alternatively an inlet open-circuit cell containing a membrane and electrodes which can be graphite plates, glassy carbon, platinum or other noble metals for example can be placed in line in pipes 108A and 107A. In this case the membrane acts as an ionic conductor between the electrolytes in pipes 108A and 107A and the electrodes are connected to voltmeter 109 to measure the potential difference between the electrolytes in pipes 108A and 107A which
corresponds to the open circuit voltage between the incoming electrolytes.
Electrolyte voltage probes 112 and 113 are placed in apertures (not shown) in pipes 107 and 108 respectively and are connected electrically to voltmeter 117 via wires 120 and 121 respectively. Voltmeter 117 measures the open circuit voltage between outlet positive electrolyte flowing via pipe 107 into reservoir 105 and outlet negative electrolyte via pipe 108 into reservoir 106. Alternatively, an outlet open-circuit cell containing a membrane and electrodes which can be graphite plates, glassy carbon, platinum or other noble metals, for example, can be placed in line in pipes 107 and 108. In this case the membrane acts as an ionic conductor between the electrolytes in pipes 107 and 108 and the electrodes are connected to voltmeter 117 which measures the potential difference between the electrolytes in pipes 107 and 108 which corresponds to the open circuit voltage between the outgoing electrolytes.
Adjusting means 122 is connected electrically to voltmeters 109 and 117 via wires 123 and 124 to receive output signals corresponding to the open circuit voltage measured by voltmeters 109 and 117 via wires 123 and 124. Adjusting means 122 is connected electrically to pumps 1 and 2 via wires 125 and 126 respectively.
In operation pump 1 recirculates pentavalent/tetravalent vanadium ions through positive compartment 103 and through reservoir 105 via pipes 107 and 107A. Pump 2 recirculates di vaient/trivalent vanadium ions tnrougn negative comoartment 102 and through reservoir 106 via pipes 108 and 108A. Electrical energy is withdrawn from cell 101 by loading an external circuit in the means to charge/discharge 116. The incoming open circuit voltage between the incoming negative electrolyte flowing through pipe 108A and the incoming positive electrolyte flowing through pipe 107A is measured by voltmeter 109 which determines the voltage difference measured by electrolyte voltage probes 110 and 111. The outgoing open circuit voltage between the outgoing negative electrolyte flowing through pipe 108 and the outgoing positive electrolyte flowing through pipe 107 is measured by voltmeter 117 which determines the voltage difference between voltage probes 112 and 113. Output signals corresponding to the incoming open circuit voltage and the outgoing open circuit voltage are sent to adjusting means 122 via wires 123 and 124 respectively.
One suitable non-linear circuit for adjusting means 122 for
recirculation control of the positive and negative electrolytes is shown in block diagrammatic form in Fig. 3. Separate electrically controlled pumps 1 and 2 , for conveyi ng positi ve and negati ve el ectrolytes through compartments 103 and 102 respectively, are energised by common line 12 from a power control unit 13 responding to digital pump commands on line 14 generated from computing block 15 and converted to analogue command signals on line 40 by D/A converter 16.
As previously described the pumping flow rate is automatically adjusted to produce a selected differential open circuit voltage
("ΔEoc cell value") dependent upon three factors, (1) the state of cnarge of the electrolyte, (2) the state of the cell charge, and (3) the current drain thereon. Factor (1) is represented in the drawing by signal "ΔEoc measured" 34 applied on input line 17 to one side of a second computing block 18. Output line 27 from computing block 18 is applied as input to computing block 15. The otner input 19 to computing block 18 is derived from a ROM 20 whicn by a look-up table for a selected value of by" 35, ano corrected for temperature, determines on the output line 19 what the state-of-charge of the positive and negative electrolytes should be. The internal table of the ROM 20 is addressed on input line 21 by the state of charge of the cells derived through a look-up table in a second ROM 22 which in turn is addressed by the open circuit cell voltage "Eoci measured" 28 via input line 29 and the actual temperature "Temp. T" 30 via line 31.
The signal line 21 is applied as a first input to a computing block 23 which also receives on input line 24 an indication of the current drain on the batteries as an input "IC/d measured" 32 and on a third input 25 a signal "manual constant C" 33 which provides manual control to modify the pump response to current drain. An output Fs is applied on output line 26 to a second input to the computing block 15 thereby to derive a pump command on line 14 which correlates the three factors (1), (2) and (3) referred to above.
An alternative non-linear circuit for adjusting means 122 is shown in Fig. 4 where like designating numerals are applied to like componentry of Fig. 3. In this instance a linear controller 27 generates pump commands on output line 14 based upon the error differential between the measured ΔEoc 34 and a value of ΔEoc calculated 19 from the state of charge of the cell. The control parameters 36, 37 and a manually set constant 38, 39 are also inputs to the control strategy incorporated in the linear controller 27. The states of charge of the positive and negative electrolytes and what those charges should be, are derived through ROM's 20 and 22 in a similar manner to that previously described in connection with Fig. 3.
Referring to Fig. 2 a redox flow cell system 200 for providing a selected discnarge voltage/current from redox flow cell 201 through which positive and negative electrolytes flow. The positive electrolyte consi sts of 0.25M to 2.5M pentavalent/tetraval ent vanadi um ions i n 0.25M - 5M H2 SO4. The negati ve el ectrolyte consi sts of 0.25M to 2.5M
divalent/trivalent vanadium ions in 0.25M - 5M H2SO4. Cell 201 has a negative compartment 202 having negative electrode 214 di sposed therein, positive compartment 203 having positive electrode 215 disposed therein and ionically conducting separator 204 generally a Selemion CMV membrane. Negative electrode 214 and positive electrode 215 are electrically coupled via means to charge/discharge 216. Separator 204 is operatively disposed between compartments 202 and 203 to provide ionic communication between positive electrolyte in the positive compartment 203 and negative
electrolyte in compartment 202. System 200 includes positive electrolyte storage/flowthrough reservoir 205 and negative electrolyte
storage/flowthrough reservoir 206. Positive electrolyte pump 1 is connected to pipe 207 to recirculate positive electrolyte between positive compartment 203 and storage reservoir 205 via pipes 207 and 207A.
Negative electrolyte pump 2 is connected to pipe 208 to recirculate negative electrolyte between negative compartment 202 and storage
reservoir 206 via pipes 208 and 208A. V2+/V3+ negative electrolyte absorption probe 210 is placed sealably through an aperture (not shown) in pipe 208A and is connected electrically to voltmeter 209 via wire 218. Meter 209 measures a voltage or current from probe 210 related to the absorption of incoming V2+/V3+ negative electrolyte at about 750nm.
V2+/V3+ negative absorption probe 212 is placed through an aperture
(not shown) in pipe 208 and is connected electrically to voltmeter 217 via wire 220. Voltmeter 217 measures measures a voltage or current from probe 212 related to the absorption of outgoing V2+/V3+ negative electrolyte at about 750nm. Adjusting means 222 is connected electrically to
voltmeters 209 and 217 via wires 223 and 224 to receive output signals corresponding to the voltage or current measured by voltmeters 209 and 217 via wires 223 and 224. Adjusting means 222 is connected electrically to pumps 1 and 2 via wires 225 and 226 respectively.
In operation pump 1 recirculates pentavalent/tetravalent vanadium ions through positive compartment 203 and through reservoir 205 via pipes 207 and 207A. Pump 2 recirculates divalent/trivalent vanadium ions through negative compartment 202 and through reservoir 206 via pipes 208 and 208A. Electrical energy is withdrawn from cell 201 by loading an external circuit in the means to charge/discharge 216. The incoming absorption of V2+/V3+ incoming negative electrolyte flowing tnrough pipe 208A is measured by probe 210 and an output signal related thereto is determined as an incoming voltage by voltmeter 209. The outgoing
absorption of V2+/V3+ outgoing negative electrolyte flowing through pipe 208 is measured by probe 212 and an output signal related thereto is determined by voltmeter 217 as an outgoing voltage. Output signals corresponding to the incoming voltage and the outgoing voltage are sent to adjusting means 222 via wires 223 and 224 respectively. Adjusting means 222 then determines the difference between the absorptions of the incoming and outgoing negative electrolytes and adjusts the pump speeds of pumps 2 and 1, so that cell 201 outputs a selected discharge voltage. An
analogous circuit to that show in shown in Figs. 3 or 4 can be utilized, for controlling pumps 1 and pump 2, except that ΔEoc is replaced by the difference between the absorptions of the incoming and outgoing negative electrolytes.
Fig. 5 depicts a cross sectional section of a pipe 600 having a V2+/V3+ negative absorption probe 210 inserted through an aperture 602 in pipe 600. Probe 210 has an outer casing 603 which has a transverse aperture 604 extending from side 605 through to side 606 of probe 210. An apsorption system 607 is located within casing 603 about aperture 604. System 607 has an array of infrared light emitting diodes 608 which emit infrared light of about 750nm and an array of silicon diode detectors 609 which are located opposite diodes 608 to detect light emitted therefrom. Diodes 608 are housed in compartment 610 which has a window 611 opposite detectors 609 and detectors 509 are housed in compartment 612 which has a window 613 opposite diodes 608. Hence light emitted by diodes 608 can pass through windows 611 and 613 and be detected by detectors 609. Diodes 608 are connected electrically to power supply 614 via wires 615.
Detectors 609 can be connected electrically to voltmeter 209 depicted in Fig. 2 via wires 218.
Fig. 5 depicts a front view of probe 210 which clearly shows aperture 604 extending from side 605.
In operation, diodes 608 are powered by power supply 614 to emit light of about 750nm which passes through windows 611 and 613 and is detected by detectors 609. A portion of V2+/V3+ negative electrolyte 601 which is flowing through pipe 600 passes through aperture 604. Output signals related to the absorption of V2+/V3+ negative electrolyte
flowing through aperture 604 are detected by detectors 609 which pass via wires 218 to voltmeter 209 depicted in Fig. 2. Any change in the concentration ratio of V2+/V3+ in the negative electrolyte results in a corresponding change in the degree of absorption of the V2+/V3+ negative electrolyte and this is reflected in the flux of light detected by detectors 609.
Referring to Fig. 7 a redox flow cell system 500 for providing a selected discharge voltage/current from redox flow cell 501 through which positive anc negative electrolytes flow. The positive electrolyte consists of 0.25M to 2.5M pentavalent/tetravalent vanadium ions in 0.25M - 5M H2SO4. The negative electrolyte consists of 0.25M to 2.5M
divalent/trivalent vanadium ions in 0.25M - 5M H2SO4. Cell 501 has a negative compartment 502 having negative electrode 514 disposed therein. positive compartment 503 having positive electrode 515 disposed therein and ionically conducting separator 504 generally a Selemion CMV memorane. Negative electrode 514 and positive electrode 515 are electrically coupled via means to charge/discharge 516. Separator 504 is operatively disposed between compartments 502 and 503 to provide ionic communication between positive electrolyte in the positive compartment 503 and negative
electrolyte in compartment 502. System 500 includes positive electrolyte storage/flowthrough reservoir 505 and negative electrolyte
storage/flowthrough reservoir 506. Positive electrolyte pump 1 is connected to pipe 507 to recirculate positive electrolyte between positive compartment 503 and storage reservoir 505 via pipes 507 and 507A.
Negative electrolyte pump 2 is connected to pipe 508 to recirculate negative electrolyte between negative compartment 502 and storage
reservoir 506 via pi pes 508 and 508A . Negati ve el ectrolyte conducti vi ty probe 510 i s pl aced sealably through an aperture (not shown) in pipe 508A and is connected electrically to conductivity meter 509 via wire 518.
Meter 509 measures a voltage or current from probe 510 related to the conductivity of incoming V2+/V3+ negative electrolyte. Positive
electrolyte conductivity probe 511 is placed sealably through an aperture (not shown) in pipe 508A and is connected electrically to conductivity meter 509A via wire 519. Meter 509A measures a voltage or current from probe 519 related to the conductivity of incoming V4+/V5+ positive electrolyte. V2+/V3+ negative conductivity meter probe 512 is placed through an aperture (not shown) in pipe 508 and is connected electrically to conductivity meter 517 via wire 520. Meter 517 measures a voltage or current from probe 512 related to the conductivity of outgoing V2+/V3+ negative electrolyte. V4+/V5+ positive conductivity meter probe 512A is oiaced through an aperture (not shown) in pipe 508 and is connected electrically to conductivity meter 517A via wire 521. Meter 517A measares a voltage or current from probe 512A related to the conductivity of outgoing V4+/V5+ positive electrolyte. Adjusting means 522 is
connected electrically to conductivity meters 509, 509A, 517 and 517A vi a wires 523, 523A , 524 and 524A respectively to recei ve output si gnals corresponding to the voltage or current measured by conductivity meters
509, 509A, 517 and 517A. Adjusting means 522 is connected electrically to pumps 1 and 2 via wires 525 and 526 respectively.
Imbalance occurs due to H2 evolution at negative electrode and
O2 oxidation of V2+ → V3+ if system not perfectly sealed. For
electrolyte rebalance, one can employ oxalic acid additions from reservoir 527 via line 528 to positive electrolyte storage/flowthrough reservoir 505 periodically, e.g. if system capacity drops by 10% or if +ve & -ve side out of balance by e.g. 10% add stoichiometric amount of oxalic acid to +ve electrolyte+
2V5+ + H2C2O4
Figure imgf000032_0001
2V4+ + H2O + 2CO2
The positive electrolyte in reservoir 505 can be agitated by bubbling N2 through to assist reaction and allow escape of CO2 through vents. After several hours of reaction battery 501 can be reused and system capacity will gradually be restored - see Fig. 18. In the case of experiments which led to the results shown in Fig. 18 when excess oxalic acid was added only a slight increase in capacity is observed. If required amount is added capacity is restored.
Chemical reductants other than oxalic acid could oe used.
The chemical reductant can also be KHC2O4.H2O, K2C2O4,
Na2C2O4, (NH4)2C2O4NH4HC2O4.H2O, LiHC2O4.H2O,
NaHC2O4.H2O, Li2C2O4, SO2, H2SO3, NaHSO3
Na2SO3, Na2S2O3, Na2S2O4, Na2S2O5, Na2S2O6,
Li2SO3, Li2SO6, KHSO3, K2SO3, K2S2O3, K2S2O4,
k2S2O5, K2S2O6, NH4HSC3, (NH4)2SO3, (NH4)2SO4, (NH4)2SO5, N2H4, H2N2H2.H2O, H2N2H2.H2SO4,
(NH4)2SO6, NaBH4, LiBH4, KBH4, Be(BH4)2, D2, T2,
CaH2, MgH2, H2 or calcium and magnesium salts of sulphurous acid, alkali-hydrogen-phosphites (Li, K, Na), alkali hypophosphites (Li, K, Na), hydroxyl amines, pyrosulphurous acid and dithioneous acid. Other chemical reductants can be used. For example, in principle it should possible to use a reducing organic water-soluble compound such as a reducing organic water-soluble mercapto group-containing compound including SH-containing water-soluble lower alcohols (including SH-containing C1 -C1 2 primary, secondary and tertiary alkyl alcohols), SH-containing C1-C12 primary, secondary and tertiary alkyl carboxylic acids, SH-containing C1-C12 primary, secondary and tertiary alkyl amines and salts thereof,
SH-containing C1 -C1 2 primary, secondary and tertiary alkyl amine acids and di- or tripeptides such as 2-mercaptoethylamine hydrochloride,
2-mercaptoethanol, 2-mercaptopropionylglycine, 2-mercaptopropionic acid, cystenylglycine, cysteine, carbamoyl cysteine, homocysteine, glutathione, cysteine hydrochloride ethyl ester and acetylcysteine. In principle it should also be possible to employ photocatalytic reduction and
photoreduction at a semiconductor photocathode.
Reductants such as (NH4)2C2O4NH4HC2O4.H2O, SO2,
H2C2O4, NH4HSO3, (NH4)2SO3, (NH4)2SO4,
(NH4)2SO5, N2H4, H2N2H2.H2O, H2N2H2.H2SO4,
(NH4)2SO6 and H2 are particularly advantageous as reductants since at least some of the reaction product is gaseous permitting higher concentrations of vanadium ions to be prepared and reducing further treatment of electrolyte to remove unwanted products.
In operation pump 1 recirculates pentavalent/tetravalent vanadium ions througn positive compartment 503 and througn reservoir 505 via pipes 507 and 507A. Pumo 2 recirculates divalent/trivalent vanadium ions through negative compartment 502 and through reservoir 506 via pipes 508 and 508A. Electrical energy is withdrawn from cell 501 by loading an external circuit in the means to charge/discharge 516. The incoming conductivity of V2+ /V3+ incoming negative electrolyte flowing through pipe 508A is measured by probe 510 and an output signal related thereto is determined by conductivity meter . The incoming conductivity of V4+/V5+ incoming positive electrolyte flowing through pipe 507A is measured by probe 511 and an output signal related thereto is determined by conductivity meter 509A. The outgoing conductivity of V2+/V3+ outgoing negative electrolyte flowing through pipe 508 is measured by probe 512 and an output signal related thereto is determined by
conductivity meter 517. The outgoing conductivity of V4+/V5+ outgoing positive electrolyte flowing through pipe 507 is measured by probe 512A and an output signal related thereto is determined by conductivity meter
517A. Output signals corresponding to the determined incoming
conductivities and the determined outgoing conductivities are transmitted to adjusting means 522 via wires 523, 523A, 524 and 524A respectively.
Adjusting means 522 then determines the difference between the conductivities of the incoming and outgoing negative electrolytes and the difference between the conductivities of the incoming and outgoing positive electrolytes and adjusts the pump speeds of pumps 2 and 1, so that cell 501 outputs a selected discharge voltage. Two analogous circuits to that shown in Fig. 3 or to that shown in Fig. 4 can be utilized, one for controlling pump 1 and one for controlling pump 2; except that ΔEoc is replaced by the difference between the
conductivities of the incoming and outgoing negative electrolytes in one of the circuits and by the difference between the conductivities of the incoming and outgoing positive electrolytes in the other circuit. EXAMPLE 1
Fig. 9 demonstrates that the solution potentials of V2+/V3+ in
H2SO4 and V4+ /V5+ in H2SO4 changes only slightly over a wide
range of states-of-charge. From equation (1) page 15,
E = E° - cell cell
Figure imgf000035_0001
where E°cell = 1.35 volts, therefore,
Eoc 1.35 -
Figure imgf000035_0002
Fig. 10 shows the results of experiments in which the open-circuit voltage of an all-vanadium redox cell employing Selemion CMV membrane, as a function of the system's state-of-charge. The results in this diagram demonstrates the feasibility of utilizing open-circuit voltage to monitor state-of-charge of the cell and thus control the charging and discharging processes between the required limits e.g. 10% to 90% state-of-charge. Figs. 11 and 12 show the potentials of the positive and negative
electrolytes (vs SCE reference electrode) as a function of state-of-charge of each half-cell. The results in these diagrams show these relationships can be employed to detect the state-of-charge of each half-cell and determine when the system is becoming unbalanced.
EXAMPLE 2
Figs. 13a - 13c show the linear variation in the conductivities of both the positive (V4+/V5+) and negative (V2+/V3+) electrolytes of the vanadium redox cell as a function of state-of-charge. The results in this diagram show that by simply measuring the conductivity of each solution with a standard probe, a simple meter can be calibrated to indicate solution state-of-charge directly for each half-cell electrolyte. EXAMPLE 3
Since the colour of the electrolytes changes during charge and discharge of the vanadium cell e.g. V2+(violet) - V3=(green) and
V4+(blue) - V5+ (yellow) a spectrophotometric method could also be
employed to monitor state-of-charge. Figure 14 shows a series of
spectrograms for 2 M V4+/V5+ solutions corresponding to different
states-of-charge of the positive electrolyte (e.g. curve 1 = 100% SOC and curve 10 = 0% SOC). A 1cm3 cuvette was employed and as can be seen, over most of the range (957. to 10% SOC) the absorption is too great to al low measurement.
In the case of the negative (V2+/V3+) electrolyte, however, 2 minima in the absorption curves are observed, as illustrated in Figure 15a
- 15c. The position of the two minima, varies as a function of
composition, however, plotting the absorption of the solution at 750nm as a function of state-of-charge, a linear relationship is obtained over the range 5% to 100% SOC as seen in Fig. 16. In Fig. 17 a plot of absorbance of 2 molar negative electrolyte as a function of state-of-charge is shown. Curve 1 corresponds to absorbance at the minimum in spectrum at
450-500 nm, curve 2 is absorbance at 700-850 nm minimum. A simple detector can thus be employed to monitor the absorption by the solution of
750 nm radiation passing through a flat-sided tube through which the negative electrolyte flows on its way into and out of the cell stack. The value of absorption measured can be readily translated into a
state-of-charge value of the fluids.
INDUSTRIAL APPLICABILITY
The present invention discloses a method and apparatus which provide the necessary solution flowrate for a selected discharge voltage/current from a redox flow cell particularly an all-vanadium redox flow cell. The method and apparatus are particularly useful in practical applications since a redox flow cell can be operated with minimum pumping energy so as to provide the required constant current and/or voltage output over a given period of time. Alternatively, a redox flow cell can be operated so as to provide variable current and/or voltage output so as to meet demand requirements. Also disclosed is a method of determining change in flow rates of positive and negative electrolyte through a redox cell whereby the cell requires a selected charge voltage/current and a redox flow cell system in which flow rates of positive and negative electrolytes through a redox flow cell can be changed, whereby the cell requires a selected charge voltage/current. The latter method and apparatus are particularly useful in practical applications since a redox flow cell can be operated so that it requires a constant current and/or voltage input over a given period of time. Alternatively, a redox flow cell can be operated with a variable current and/or voltage input under which conditions considerable pumping energy can be saved by adjusting the pump flow rates to the minimum required for the current involved and the SOC of the system.

Claims

1. A method of determining the state of charge of a redox flow cell througn which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characterisec(s) of the positive and/or negative electrolytes related to state of charge of the cell; and
determining the state of charge of the redox cell from the
characteristic(s).
2. The method as defined in claim 1 wherein the redox flow cell is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
3. The method as defined in claim 1 wherein the method comprises: measuring a characterisec(s) of the positive electrolyte entering the redox flow ce l l and l eavi ng the redox flow cel l .
4. The method as defined in claim 2 wherein the method comprises: measuring a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
5. The method as defined in claim 1 wherein the method comorises: measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
6. The method as defined in claim 2 wnerein the method comprises: measuring a cnaracteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
7. The method as defined in claim 1 wherein the method comprises: measuring a characteristic(s) of the positive and negative
electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
8. The method as defined in claim 2 wherein the method comprises: measuring a characteristic(s) of the positive and negative
electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
9. The method as defined in any one of claims 1 to 8 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
10. The method as defined in any one of claims 1 to 8 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
11. The method as defined in any one of claims 2, 4, 6 or 8 wherein the characteristic is the UV-Visible absorption of the V2+/V3+
electrolyte.
12. A method of providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characteristic(s) of the positive and/or negative electrolytes related to state of cnarge of the cell;
determining change i n flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, from the characteristic(s), required to provide the selected discharge voltage/current; and
adjusting flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell provides the selected discharge
voltage/current.
13. The method as defined in claim 12 wherein the redox flow cell is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
14. The method as defined in claim 12 wherein the method comprises: measuring a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
15. The method as defined in claim 13 wherein the method comprises: measuring a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
16. The method as defined in claim 12 wherein the method comprises: measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
17. The method as defined in claim 13 wherein the method comprises: measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
18. The method as defined in claim 12 wherein the method comprises: measuring a characteristic(s) of the positive and negative
electrolytes entering the redox flow cell and measuring a characterisec(s) of the negative and positive electrolytes leaving the redox flow cell.
19. The method as defined in claim 13 wherein the method comprises: measuring a characteristic(s) of the positive and negative electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
20. The method as defined in any one of claims 12 to 19 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
21. The method as defined in any one of claims 12 to 19 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
22. The method as defined in any one of claims 13, 15, 17 or 19 wherein the characteristic is the UV-Visible absorption of the V2+/V3+ electrolyte.
23. A method of providing a selected charge voltage/current from a redox flow cell through which positive and negative electrolytes flow, the redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
which method comprises:
measuring a characteristic(s) related to state of charge of the cell; determining the change in the flow rates of the positive and negative electrolytes from the characteristic(s), whereby the cell requires the selected charge voltage/current; and
adjusting the flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell requires the selected charge voltage/current
24. The method a s def i ned i n c l a i m 23 wher e i n tne redox f l ow ce l l is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
25. The method as defined in claim 23 wherein the method comprises: measuring a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
26. The method as defined in claim 24 wherein the method comprises: measuring a characterisec(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
27. The method as defined in claim 23 wherein the method comprises: measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
28. The method as defined in claim 24 wherein the method comprises: measuring a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
29. The method as defined in claim 23 wherein the method comprises: measuring a characteristic(s) of the positive and negative
electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
30. The method as defined in claim 24 wherein the method comprises: measuring a characterisec(s) of the positive and negative
electrolytes entering the redox flow cell and measuring a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
31. The method as defined in any one of claims 23 to 30 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
32. The method as defined in any one of claims 23 to 30 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
33. The method as defined in any one of claims 24, 26, 28 or 30 wherei n the characteri sti c i s the UV-Vi si bl e absorption of the V2+ /V3+ electrolyte.
34. A redox flow cell system in which the state of charge can be determined which system comprises:
a redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump;
means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
a negative electrolyte pump;
means for transporting negative electrolyte between the negative compartment and the negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes; and
means to determine the state of charge of the cell from the
characteristic(s), operatively associated with the means to measure.
35. The system as defined in claim 34 wherein the redox flow cell is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
36. The system as defined in claim 34 wherein the system comprises: means to measure a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving tne redox flow cell.
37. The system as defined in claim 35 wherein the system comprises: means to measure a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
38. The system as defined in claim 34 wherein the system comprises: means to measure a characterisec(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
39. The system as defined in claim 35 wherein the system comprises: means to measure a characterisec(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
40. The system as defined in claim 34 wherein the system comprises: means to measure a characteristic(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a
characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
41. The system as defined in claim 35 wherein the system comprises: means to measure a characterisec(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a
characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
42. The system as defined in any one of claims 34 to 41 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
43. The system as defined in any one of claims 34 to 41 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
44. The system as defined in any one of claims 35, 37, 39 or 41 wherein the characteristic is the UV-Visible absorption of the V2+/V3+ electrolyte.
45. The system of claim 34 wherein the means for transporting positive electrolyte is adapted to recirculate positive electrolyte to the positive compartment and the means for transporting negative electrolyte is adapted to recirculate negative electrolyte to the negative compartment.
46. A redox flow cell system for providing a selected discharge voltage/current from a redox flow cell through which positive and negative electrolytes flow, which system comprises:
a redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump;
means for transporting positive electrolyte between the positive compartment and the positive electrolyte pump, operatively associated with the positive compartment and the positive electrolyte pump;
a negative electrolyte pump;
means for transporting negative electrolyte between the negative compartment and the negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a characteristic(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes;
means to determine change(s) in flow rates of the positive and negative electrolytes from the characteristic(s), whereby the cell provides the selected discharge voltage/current, which means to determine is operatively associated with the means to measure; and
adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative comoartments respectively, in accordance with the determined change(s) in the flow rates of the positive and negative electrolytes, whereby the cell provides the selected discharge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
47. The system as defined in claim 46 wherein the redox flow cell is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
48. The system as defined in claim 46 wherein the system comprises: means to measure a characteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
49. The system as defined in claim 47 wherein the system comprises: means to measure a character! stic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
50. The system as defined in claim 46 wherein the system comprises: means to measure a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
51. The system as defined in claim 47 wherein the system comprises: means to measure a characterisec(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
52. The system as defined in claim 46 wherein the system comprises: means to measure a characteristic(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a
characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
53. The system as defined in claim 47 wherein the system comprises: means to measure a cnaracteristic(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
54. The system as defined in any one of claims 46 to 53 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
55. The system as defined in any one of claims 46 to 53 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
56. The system as defined in any one of claims 47, 49, 51 or 53 .whereJn the characteristic is the UV-Visible absorption of the V2+/V3+ electrolyte.
57. The system of claim 46 wherein the means for transporting positive electrolyte is adapted to recirculate positive electrolyte to the positive compartment and the means for transporting negative electrolyte is adapted to recirculate negative electrolyte to the negative compartment.
58. A redox flow cell system having a redox flow cell through which positive and negative electrolytes flow, which cell is adaptable to require a selected charge voltage/current, which system comprises:
a redox flow cell having:
(a) a negative compartment;
(b) a positive compartment; and
(c) an ionically conducting separator operatively disposed between the positive and negative compartments and in contact with the positive and negative electrolytes to provide ionic communication therebetween;
a positive electrolyte pump;
means for transporting positive electrolyte between the positive compartment and the positive electrolyte Dump, operatively associated with the positive comoartment and the positive electrolyte pump;
a negative electrolyte pump; meaπs for transporting negative electrolyte between the negative compartment and the negative electrolyte pump, operatively associated with the negative compartment and the negative electrolyte pump;
means to measure a characterisec(s) of the positive and/or negative electrolytes related to state of charge of the cell, operatively associated with the positive and/or negative electrolytes;
means to determine change in flow rates of the positive and negative electrolytes from the characteristic(s), whereby the cell requires a selected charge voltage/current, which means to determine is operatively associated with the means to measure; and
adjusting means for adjusting pumping speeds of the positive and negative electrolyte pumps and thereby change flow rates of the positive and negative electrolytes through the positive and negative compartments respectively, in accordance with the determined change in the flow rates of the positive and negative electrolytes, whereby the cell requires the selected charge voltage/current, which adjusting means is operatively associated with the means to determine and the positive and negative electrolyte pumps.
59. The system as defined in claim 58 wherein the redox flow cell is an all-vanadium redox flow cell, the positive electrolyte contains
V4+/V5+ and the negative electrolyte contains V2+/V3+.
60. The system as defined in claim 58 wherein the system comprises: means to measure a characterise c(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
61. The system as defined in claim 59 wherein the system comprises: means to measure a cnaracteristic(s) of the positive electrolyte entering the redox flow cell and leaving the redox flow cell.
62. The system as defined in claim 58 wherein the system comprises: means to measure a cnaracteri sticks; of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
63. The system as defined in claim 59 wherein the system comprises: means to measure a characteristic(s) of the negative electrolyte entering the redox flow cell and leaving the redox flow cell.
64. The system as defined in claim 58 wherein the system comprises: means to measure a characteristic(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a
characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
65. The system as defined in claim 59 wherein the system comprises: means to measure a characteristic(s) of the positive and negative electrolytes entering the redox flow cell and means to measure a
characteristic(s) of the negative and positive electrolytes leaving the redox flow cell.
66. The system as defined in any one of claims 58 to 65 wherein the characteristic is the open circuit voltage between the positive electrolyte and the negative electrolyte.
67. The system as defined in any one of claims 58 to 65 wherein the characteristic is the conductivity of the positive electrolyte and/or the negative electrolyte.
68. The system as defined in any one of claims 59, 61, 63 or 65 wherein the characteristic is the UV-Visible absorption of the V2+/V3+ electrolyte.
69. The system of claim 58 wherein the means for transporting positive electrolyte is adapted to recirculate positive electrolyte to the positive compartment and the means for transporting negative electrolyte is adapted to recirculate negative electrolyte to the negative comoartment.
70. The system of any one of claims 34, 46, 58 further including means to reoaiance the positive and/or negative electrolytes wnich means to rebalance is operatively associated with the positive and/or negative electrolytes.
71. The system of any one of claims 35, 47, 59 further including means to rebalance the positive and/or negative electrolytes which means to rebalance is operatively associated with the positive and/or negative electrolytes.
72. The system of claim 71 wherein the means to rebalance comprises a reservoir containing a chemical reductant which is operatively associated with positive electrolyte.
73. The system of claim 72 wherein the chemical reductant is selected from the group consisting of KHC2O4.H2O, K2C2O4,
Na2C2O4, (NH4)2C2O4NH4HC2O4.H2O, LiHC2O4.H2O,
NaHC2O4.H2O, Li2C2O4, SO2, H2SO3, NaHSO3
Na2SO3, Na2S2O3, Na2S2O4, Na2S2O5, Na2S2O6,
Li2SO3, Li2SO5, KHSO3, K2SO3, K2S2O3, K2S2O2.
K2S2O5, K2S2O6, NH4HSO3, (NH4)2SO3, (NH4)2SO4,
(NH4)2SO5, N2H4, H2N2H2.H2O, H2N2H2.H2SO4,
(NH4)2SO5, NaBH4, LiBH4, KBH4, Be(BH4)2, D2, T2,
CaH2, MgH2, H2 or calcium and magnesium salts of sulphurous acid,
alkali-hydrogen-phosphites (Li, K, Na), alkali hypophosphites (Li, K, Na), hydroxyl amines, pyrosulphurous acid and dithioneous acid. Other chemical reductants can be used. For example, in principle it should possible to use a reducing organic water-soluble compound such as a reducing organic water-soluble mercapto group-containing compound including SH-containing water-soluble lower alcohols (including SH-containing C1-C12 primary, secondary and tertiary alkyl alcohols), SH-containing C1-C12 primary, secondary and tertiary alkyl carooxylic acids, SH-containing C1-C12 primary, secondary and tertiary alkyl amines and salts tnereof,
SH-containing C1-C12 primary, secondary and tertiary alkyl amine acids and di- or tripeptides such as 2-mercaptoethyl amine hydrochloride,
2-mercaptoethanol, 2-mercaptopropionylglycine, 2-mercaptopropionic acid, cystenylglycine, cysteine, caroamoyl cysteine, homocysteine, glutathione, cysteine hydrochloride ethyl ester and acetyl cysteine and mixtures thereof.
74. The system of claim 72 wherein the chemical reductant is oxalic acid.
75. The method of any one of claims 1, 12 or 23 wherein the cell further includes means to rebalance the positive and/or negative
electrolytes which means to rebalance is operatively associated with the positive and/or negative electrolytes and wherein the method further comprises the step of rebalancing the state of charge of the positive and/or negative electrolytes when their state of charge becomes unbalanced.
76. The method of claim 75 wherein the means to rebalance comprises a reservoir containing a chemical reductant which is operatively associated with positive electrolyte.
77. The system of claim 76 wherein the chemical reductant is selected from the group consisting of KHC2O4.H2O, K2C2O4,
Na2C2O4, (NH4)2C2O4NH4HC2O4.H2O, LiHC2O4.H2O,
NaHC2O4.H2O, Li2C2O4, SO2, H2SO3, NaHSO3
Na2SO3, Na2S2O3, Na2S2O4, Na2S2O5, Na2S2O6,
Li2SO3, Li2SO6, KHSO3, K2SO3, K2S2O3, K2S2O4,
K2S2O5, K2S2O6, NH4HSO3, (NH4)2SO3, (NH4)2SO4,
(NH4)2SO5, N2H4, H2N2H2.H2O, H2N2H2.H2SO4,
(NH4)2SO6, NaBH4, LiBH4, KBH4, Be(BH4)2, D2, T2,
CaH2, MgH2, H2 or calcium and magnesium salts of sulphurous acid,
alkali-hydrogen-phosphites (Li, K, Na), alkali hypophosphites (Li, K, Na), hydroxyl amines, pyrosulDhurous acid and dithioneous acid. Other chemical reductants can be used. For example, in principle it snould possible to use a reducing organic water-soluble compound such as a reducing organic water-soluble mercapto group-containing compound including SH-containing water-soluble lower alcohols (including SH-containing C1 -C1 2 primary, secondary and tertiary alkyl alcohols), SH-containing C1-C12 primary, secondary and tertiary alkyl carboxylic acids, SH-containing C1-C12 primary, secondary and tertiary alkyl amines and salts thereof,
SH-containing C1-C12 primary, secondary and tertiary alkyl amine acids and di- or tripeptides such as 2-mercaptoethyl amine hydrochloride,
2-mercaptoethanol, 2-mercaptopropionylglycine, 2-mercaptopropionic acid, cystenylglycine, cysteine, carbamoyl cysteine, homocysteine, glutathione, cysteine hydrochloride ethyl ester and acetyl cysteine and mixtures thereof.
78. The system of claim 76 wherein the chemical reductant is oxalic acid.
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