US20170058420A1

US20170058420A1 - Process of increasing energy conversion and electrochemical efficiency of a scaffold material using a deposition material

Info

Publication number: US20170058420A1
Application number: US14/842,812
Authority: US
Inventors: Marc-Antoni Goulet; Erik Anton Kjeang
Original assignee: Simon Fraser University
Current assignee: Simon Fraser University
Priority date: 2015-09-01
Filing date: 2015-09-01
Publication date: 2017-03-02

Abstract

A process for increasing the energy conversion and electrochemical efficiency of a scaffold material using a deposition material comprises flowing by at least one surface of the scaffold material a solution which comprises the deposition material, forming agglomerations of the deposition material with at least one surface of the scaffold material, wherein the deposition material fills pores on the at least one surface of the scaffold material (“scaffold pores”) thereby increasing the surface area of the scaffold material, electrically connecting deposition material to the scaffold material via the formation of agglomerations, wherein said scaffold material is conductive and flow-through and wherein deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.

Description

FIELD OF THE INVENTION

The present invention relates to improving the efficiency of electrochemical cells.

BACKGROUND ON THE INVENTION

In a bid to develop efficient and compact electrochemical energy conversion devices much attention has been devoted towards increasing the electrochemical surface area (ESA) per unit volume of electrode. Whether for batteries, fuel cells or capacitors, electrodes with nanoscopic features and porosity are necessary to achieve the highest energy densities. Among the options available for conductive electrode materials, carbon continues to be principal due to its low cost, chemical resistance and potentially high surface area. As an inexpensive and abundant material, natural carbon has typically been processed into high surface area particles for electrochemical applications such as activated carbon for electrosorption or carbon black as a catalyst support in fuel cells. As a randomly aggregated electrode material, these powders require some form of structural support which may interfere with the mass transport of reactants. In addition, amorphous natural carbon with a low degree of graphitization suffers from poor electrical conductivity which is exacerbated by high contact resistance between particles when packed too loosely.
A more synthetic approach to manufacturing carbon materials with large active surface area per unit volume has involved pyrolysis of polymer precursors with a desired architecture and pore size distribution such as woven cloths, felts, papers or foams.^1,2Commercial vanadium redox flow batteries which do not require catalysts still rely upon graphite felt as electrodes today with a recent trend towards more compact carbon papers made from pyrolyzed polyacrilonitrile (PAN) flbers.³Typically composed of micrometer sized fibers, these materials fall short of the range needed to support the current densities required by modern energy applications. To attain the ideal nanometer scale pore size distribution while maintaining good electrical conductivity, many recent efforts have been made towards the development of carbon nanofoams.^4,5Unfortunately these materials typically suffer from poor mechanical properties such as brittleness and micrometer-sized cracks which skew their overall pore size distribution⁶. ¹S. K. Nataraj, K. S. Yang, T. M. Aminabhavi, Polyacrylonitrile-based nanofibers—A state-of-the-art review, Prog. Polym. Sci. 37 (2012) 487-513.²A. L. Dicks, The role of carbon in fuel cells, J. Power Sources. 156 (2006) 128-141³D. S. Aaron, Q. Liu, Z. Tang, G. M. Grim, A. B. Papandrew, A. Turhan, et al., Dramatic performance gains in vanadium redox flow batteries through modified cell architecture, J. Power Sources. 206 (2012) 450-453.⁴A. L. M. Reddy, S. R. Gowda, M. M. Shaijumon, P. M. Ajayan, Hybrid nanostructures for energy storage applications, Adv. Mater. 24 (2012) 5045-5064.⁵S. Chabi, C. Peng, D. Hu, Y. Zhu, Ideal three-dimensional electrode structures for electrochemical energy storage, Adv. Mater. 26 (2014) 2440-2445.⁶J. W. Lee, E. Kjeang, Nanofluidic fuel cell, J. Power Sources. 242 (2013) POWER-D-13-00735R1.
One method for increasing the active electrode area has been through the synthesis or deposition of high surface area nanomaterials onto a scaffold such as the powders, felts and papers mentioned above⁷. In the case of metallic catalysts, electrodeposition has been used to synthesize an array of nanoscale geometries with very high surface area such as nanoflowers and nano-onions. Most often these nanoparticles are deposited on supporting mesoscopic carbon powders and dispersed as inks for physical deposition. Depending on the design of the final cell and the reaction chemistry involved, these catalyst inks have been physically deposited with an array of methods including doctor-blading⁸, immersion⁹, spray coating¹⁰and inkjet printing¹¹. Other methods of catalyst deposition such as sputtering, chemical and physical vapor deposition are unlikely to be adopted commercially due to their higher cost. ⁷S. Litster, G. McLean, PEM fuel cell electrodes, J. Power Sources. 130 (2004) 61-76⁸G. Bender, T. a. Zawodzinski, A. P. Saab, Fabrication of high precision PEFC membrane electrode assemblies, J. Power Sources. 124 (2003) 114-117.⁹N. Arjona, M.-a. Goulet, M. Guerra-Balcazar, J. Ledesma-Garcia, E. Kjeang, L. G. Arriaga, Direct Formic Acid Microfluidic Fuel Cell with Pd Nanocubes Supported on Flow-Through Microporous Electrodes, ECS Electrochem. Lett. 4 (2015) F24-F28.¹⁰V. Meille, Review on methods to deposit catalysts on structured surfaces, Appl. Catal. A Gen. 315 (2006) 1-17.¹¹A. D. Taylor, E. Y. Kim, V. P. Humes, J. Kizuka, L. T. Thompson, Inkjet printing of carbon supported platinum 3-D catalyst layers for use in fuel cells, J. Power Sources. 171 (2007) 101-106.
More recently, synthesized carbon nanomaterials such as graphene and carbon nanotubes (CNTs) have received interest as electrode materials and catalyst supports due to their exceptionally high surface area. In addition, their ordered graphitic structure endows them with high electrical conductivity and mechanical strength¹². In the case of chemistries which do not rely on precious metal catalysts such as the vanadium redox battery, these nanomaterials have the potential to greatly increase the electrochemically active surface area (ESA) of a supporting carbon electrode such as graphite felt or carbon paper¹³. ¹²M. F. L. De Volder, S. H. Tawfick, R. H. Baughman, a J. Hart, Carbon nanotubes: present and future commercial applications—SOM, Science (80-.). 339 (2013) 535-539.¹³P. Han, Y. Yue, Z. Liu, W. Xu, L. Zhang, H. Xu, et al., Graphene oxide nanosheets/multi-walled carbon nanotubes hybrid as an excellent electrocatalytic material towards VO2+/VO2+ redox couples for vanadium redox flow batteries, Energy Environ. Sci. 4 (2011) 4710.
Despite the promise of these nanomaterials, there are challenges to be overcome. There currently remains a need for processes which are efficient and cost effective to improve the energy efficiency of electrochemical processes. These processes are largely influenced by the surface area of, for example, the electrodes, and for flow-through systems such as redox flow batteries and wastewater treatment the pore size distribution is also critical. Creating high surface area electrodes with good mass transport characteristics at a low cost is the current challenge in all electrochemical industries.
It is an object of the present invention to obviate or mitigate the above challenges and disadvantages.

SUMMARY OF THE INVENTION

The present invention provides a process of increasing the energy conversion and electrochemical efficiency of a flow-through, conductive scaffold material, by connecting therewith a deposition material which increases the surface area of the scaffold material, making scaffold material more available for reactions.
In another aspect, the invention further provides flow-through, conductive scaffold material connected with a deposition material which increases the surface area of the scaffold material, making the scaffold material more available for reactions with electrolytes.
The present invention provides, in particular, a process to enhance the performance of flow-through porous electrodes in electrochemical cells by flowing a deposition material, comprising nanomaterials in a preferably aqueous solution, through the electrode, wherein nanomaterials form self-assembling agglomerations on a surface of the electrode (for example surface of an electrode substrate) and wherein nanomaterials connect electrically with the electrode substrate.
The present invention provides a process for increasing the energy conversion and electrochemical efficiency of a scaffold material using a deposition material, which comprises flowing by at least one surface of the scaffold material a solution which comprises the deposition material, forming agglomerations of the deposition material with at least one surface of the scaffold material, wherein the deposition material fills pores on the at least one surface of the scaffold material (“scaffold pores”) thereby increasing the surface area of the scaffold material, electrically connecting deposition material to the scaffold material via the formation of agglomerations, wherein said scaffold material is conductive and flow-through and wherein deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.
The present invention provides a scaffold material with increased energy conversion and electrochemical efficiency properties which comprises a base scaffold material which is electrically conductive and flow-through, coupled with a deposition material deposited in pores on at least one surface of the scaffold material (“scaffold pores”) forming agglomerations of the deposition material with at least one surface of the scaffold material thereby increasing the surface area of the scaffold material and wherein the deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.
It is previously known to use carbon nanomaterials (such as graphene and carbon nanotubes) as electrode materials and catalyst supports due to their exceptionally high surface area. What has not been addressed is the engagement of nanoparticle deposition materials on scaffold materials, such as electrodes, in a manner which is simple, uses only water as a solvent to carry the deposition material, and which is non-damaging to the substrate or scaffold material. In the process of the invention, although a variety of solvent can be used, no special or harmful solvents need to be used beyond water. One key to the process is that the deposition materials (preferably nanomaterials), in this process form/self assemble as agglomerations with the surface of scaffold material/substrate. Without any interfering materials, the agglomerations electrically connect the deposition material to the scaffold material/substrate and thereafter form part of an enhanced efficiency, working electrode.
Without limiting the general range of applications, the process and structure of the present invention are especially suited to any electrochemical flow-through cell and any conductive, flow-through (porous) substrate/electrode material. For example, this process may be used for any electrochemical reactor, such as flow batteries, fuel cells, reactors for electrochemical processes, energy storage, energy conversion, electrodeposition, electrowinning, (electro)chemical synthesis' redox flow batteries' (liquid/solid based), and for capacitive deionization′ (CDI) for industrial processes (such as wastewater treatment). This is a non-limiting list.
These and other objects and advantages of the present invention will become more apparent to those skilled in the art upon reviewing the description of the preferred embodiments of the invention, in conjunction with the figures and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures set forth embodiments in which like reference numerals denote like parts. Embodiments are illustrated by way of example and not by way of limitation in all of the accompanying figures in which:

FIG. 1A is an annotated image of complete oxidation of 50% V(II) by 1 mm of carbon paper WE and subsequent complete reduction by 4 mm Toray CE;

FIG. 1B is an annotated image of discharge of vanadium reactants by co-laminar flow cell;

FIG. 2 shows SEM images of CNT deposition;

FIG. 3 are graphs depicting electrode enhancement in an analytical cell a) EIS, b) Tafel kinetics;

FIG. 4 are graphs depicting the effect of CNT deposition on Rct-1 and the correlated increase in both a) capacitance and b) exchange current of the carbon paper in V2+/V3+ electrolyte;

FIG. 5 is a graph depicting in situ EIS of Y-Junction cell;

FIG. 6 are graphs depicting in situ a) polarization and b) power density curves of Y-Junction cell;

FIG. 7 is a graph depicting pressure drop across V2 electrode as a function of flow rate for different CNT loading; and

FIGS. 8A, 8B, 8C, and 8D each illustrate schematics of the four principal reactions of the process of the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. As such this detailed description illustrates the invention by way of example and not by way of limitation. The description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations and alternatives and uses of the invention, including what we presently believe is the best mode for carrying out the invention. It is to be clearly understood that routine variations and adaptations can be made to the invention as described, and such variations and adaptations squarely fall within the spirit and scope of the invention.
In other words, the invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured. Similar reference characters denote similar elements throughout various views depicted in the figures.
This description of preferred embodiments is to be read in connection with the accompanying drawings, which are part of the entire written description of this invention. In the description, corresponding reference numbers are used throughout to identify the same or functionally similar elements. Relative terms such as “horizontal,” “vertical,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and are not intended to require a particular orientation unless specifically stated as such. Terms including “inwardly” versus “outwardly,” “longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” (if used) is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship.
In the present disclosure and claims, the word “comprising” and its derivatives including “comprises” and “comprise” include each of the stated integers but does not exclude the inclusion of one or more further integers. The term track and channel may be interchanged herein.
The term “variation” of an invention means an embodiment of the invention, unless expressly specified otherwise. A reference to “another embodiment” or “another aspect” in describing an embodiment does not imply that the referenced embodiment is mutually exclusive with another embodiment (e.g., an embodiment described before the referenced embodiment), unless expressly specified otherwise.
The term “including” and variations thereof mean “including but not limited to”, unless expressly specified otherwise.
The terms “a”, “an” and “the” mean “one or more”, unless expressly specified otherwise.
The term “plurality” means “two or more”, unless expressly specified otherwise.
The term “herein” means “in the present application, including anything which may be incorporated by reference”, unless expressly specified otherwise.
The term “whereby” is used herein only to precede a clause or other set of words that express only the intended result, objective or consequence of something that is previously and explicitly recited. Thus, when the term “whereby” is used in a claim, the clause or other words that the term “whereby” modifies do not establish specific further limitations of the claim or otherwise restricts the meaning or scope of the claim.
The term “e.g.” and like terms mean “for example”, and thus does not limit the term or phrase it explains. For example, in a sentence “the computer sends data (e.g., instructions, a data structure) over the Internet”, the term “e.g.” explains that “instructions” are an example of “data” that the computer may send over the Internet, and also explains that “a data structure” is an example of “data” that the computer may send over the Internet. However, both “instructions” and “a data structure” are merely examples of “data”, and other things besides “instructions” and “a data structure” can be “data”.
The term “respective” and like terms mean “taken individually”. Thus if two or more things have “respective” characteristics, then each such thing has its own characteristic, and these characteristics can be different from each other but need not be. For example, the phrase “each of two machines has a respective function” means that the first such machine has a function and the second such machine has a function as well. The function of the first machine may or may not be the same as the function of the second machine.
The term “i.e.” and like terms mean “that is”, and thus limits the term or phrase it explains. For example, in the sentence “the computer sends data (i.e., instructions) over the Internet”, the term “i.e.” explains that “instructions” are the “data” that the computer sends over the Internet.
The present invention provides a process for depositing materials upon a surface of a scaffold material, which deposition increases the energy conversion and electrochemical efficiency of the scaffold material. As used herein, scaffold material comprises any base material which is porous, electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase. Examples of scaffold material includes carbon paper, carbon/graphite felt, carbon cloth, carbon foam, metal foam (Ni, etc), carbon aerogel, carbon nanofoam, and packed bed electrodes.
As used herein, conductivity is the measure of a material's ability to carry or conduct an electric current. It is often given as percent of the copper standard, which is 100% IACS, (International Annealed Copper Standard). For example, silver has an IACS of 105 and has the highest conductivity. For electrochemical or electrocatalytic applications, surface areas of the conductive scaffold material are an important factor which affect the electrochemical reaction performance. Other factors include, fast transport of electrons and efficient mass transfer of reactants and products.
As used herein, an electrode is a conductor that passes an electrical current from one medium to another, usually from a power source to a device or material. It can take a number of different forms, including a wire, a plate, or a rod, and is most commonly made of metal, such as copper, silver, lead, or zinc, but can also be made of a non-metallic substance that conducts electricity, such as carbon or graphite. types of electrode materials. Some of the most prominent alloys and materials are copper, graphite, brass, silver, and platinum. Copper is second only to silver in terms of bulk electrical conductivity. Copper has better strength than silver, but offers inferior oxidation resistance. Copper is a common base metal for electrical contact and electrode applications. It is also used in alloys with graphite, tellurium, and tungsten, and is used to make brass and bronze. Copper has better EDM wear resistance than brass, but is more difficult to machine than either brass or graphite. Copper is also more expensive than graphite.
Graphite and carbon are used in a variety of electrode applications. Graphite, flake graphite, and graphitic carbon have a hexagonal, crystalline structure that cleaves or shears easily, making graphite a soft material and effective lubricant. Graphite is the most commonly used EDM electrode material because of its good machinability, wear resistance, and low cost. Like carbon, graphite is a non-metallic substance with an extremely-high sublimation temperature which provides resistance to high-temperature arcs. Fine, grain-sized graphite tends to have better erosion and wear performance, but costs more. Carbon is very inter, corrosion resistant, and electrochemically noble compared to many metals, which make carbon a useful material for electrochemical and electrowinning electrodes.
Titanium is a non-ferrous metal with excellent corrosion resistance, good fatigue properties, and a high strength-to-weight ratio. Titanium's excellent corrosion properties result in the use of titanium for electrochemical processes such as electroplating, electrophoresis, electrodeposition, electroforming, electro-hydrolysis, electrochlorination, electrofluorination, and electrolysis.
Brass is an alloy of copper and zinc. Brass materials are used to form EDM wire and small tubular electrodes. Brass does not resist wear as well as copper or tungsten, and has a lower conductivity than copper, but is much easier to machine and can be die-cast or extruded for specialized applications. EDM wire does not need to provide EDM wear or arc erosion resistance since new wire is fed continuously during the EDM wiring cutting process.
Silver has the highest conductivity of all metals. The high conductivity, softness (low hardness), and high resistance to oxidation make silver an excellent choice for contact materials. Silver is strengthened with copper and other alloy additions, but at the sacrifice of conductivity. Fine silver is silver with very high purity (99.99% Ag). Pure or fine silver is too soft for most commercial applications, but the material is used as a starting component to form other silver based alloys.
Platinum and palladium have very high erosion and corrosion resistance with low contact resistance. Platinum forms useful alloys with iridium, ruthenium, and tungsten. Palladium forms useful alloys with copper and ruthenium. Major drawbacks of these metals are high cost and the development of high contact resistance films in the presence of organic vapors
As used herein, deposition material is electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase. Examples include carbon nanotubes, graphene, carbon nanoparticles, graphite flakes, carbon black, fullerenes, carbon oxides, graphene oxide, functionalized carbon materials, doped carbon materials, catalysts, catalyst nanoparticles, metal nanoparticles, metal nanorods, metal supported catalyst materials, carbon supported catalyst materials, metal particles, etc. The preferred deposition material will have a tendency, through Van der Waals forces, to self-agglomerate or bind to itself to form “clusters”. This force or tendency can also be used, as in the present invention, to bind the deposition material to a surface of an electrode/scaffold instead.
For greater clarity, the process of the invention comprises i) breaking the existing bonded agglomerates of the deposition material by, for example, sonication, followed by ii) diluting the broken agglomerates of the deposition material (to prevent/reduce interaction and re-agglomeration of the deposition material during flow through the scaffold material) and iii) flowing a solvent comprising the broken agglomerates through at least one surface of a scaffold material such the broken agglomerates bond and re-agglomerate with the surface of the scaffold material.
As used herein, a carbon nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale. A nanometer is one-billionth of a meter, or about 10,000 times smaller than a human hair. CNTs are unique because the bonding between the atoms is very strong and the tubes can have extreme aspect ratios. A carbon nanotube can be as thin as a few nanometers yet be as long as hundreds of microns.
The process of the invention uses a deposition material (which is self-agglomerating but which bonds have been broken) and which comprises flowing by at least one surface of the scaffold material (or preferably through a 3D electrode scaffold) a solution which comprises the broken or non-agglomerated deposition material, re-forming agglomerations of the broken or non-agglomerated deposition material with at least one surface of the scaffold material, wherein the deposition material fills pores on the at least one surface of the scaffold material (“scaffold pores”) thereby increasing the surface area of the scaffold material, electrically connecting deposition material to the scaffold material via the formation of new agglomerations, wherein said scaffold material is conductive and flow-through and wherein deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.
It is a key aspect of the process of the invention, initially to break any bonded agglomerates of the deposition material (for example, by sonication), forming broken or non-agglomerated deposition material before treatment of the scaffold material. The broken or non-agglomerated deposition material is then diluted to prevent re-agglomeration of the deposition material during processing/the flow process. The diluted broken or non-agglomerated deposition material is then flowed through the scaffold material, forming new agglomerates of the deposition material and the scaffold material.
By using the flow distribution of the system itself as a depositing mechanism, the deposition material is deposited on the surface of the flow-through, porous scaffold material where the flow is greatest and therefore where the deposition material is most needed. Much like a natural dam in a river, the deposition materials will fill gaps in the scaffold (in the scaffold pores) and (by creating new agglomerations) will create more active surface area while simultaneously forcing the flow into smaller pores, thereby improving the mass transport of reactants (i.e. creating less distance to the reaction, for example, electrode, surface). Local convective mass transport of reactants in the vicinity of the active electrode surface is thereby achieved. The process is very simple, inexpensive and can be used incrementally during operation or maintenance of working cells as well as applied to cells during manufacture.
The process of the invention has several industrial advantages over other deposition methods described in the literature. Without the strict requirement for any additional chemicals, the only additional material cost is the deposition material being deposited. For an aqueous system, such as a vanadium redox flow battery, the suspending liquid (also referred to interchangeably herein as “solution”) can simply be the electrolyte itself, or a simpler derivative thereof such as sulfuric acid or water. To be clear, the solution for use in the process of the invention includes water, an electrolyte (in liquid form) which comprises at least one reactant for an electrochemical reaction or any suitable acid, for example sulfuric acid. The key aspect is that the deposition material is diluted in a solution and therein, placed in a state unfavourable for (self) re-agglomeration. In many cases, water can easily and preferentially be used.
Depending on the nature of the deposited material, this makes the process more environmentally benign by avoiding additional chemicals which may require special handling or disposal procedures.
This process can be applied preferentially with nanomaterials, as the deposition material (for example carbon nanotubes) and can be used with any nanomaterials that have the potential to adhere, either physically by Van der Waals forces or chemically by surface functionalization, to the surface of the scaffold material (for example electrode) of interest. In addition, when applied specifically to liquid based flow cells such as but not limited to: aqueous based vanadium redox flow batteries, solvent based lithium-ion flow batteries, methanol fuel cells or capacitive deionizers for wastewater treatment; the process of the invention can be used in situ to enhance the electrochemically active surface area of the scaffold without affecting the electrochemical reactions during operation. Even though the surface area enhancement may wash out over time the flowing deposition, treatment may be reapplied, as desired, in order to recover optimal performance without opening the electrochemical cell. In one aspect of the invention a minimal concentration of deposition materials (for example nanomaterials) may be further mixed into the electrochemical cell reactants themselves in order to continuously maintain the deposition on the surface of the scaffold material.
FIG. 8 (a-d) illustrate schematics of the four principal reactions of the process of the invention: FIG. 8a ) providing the schematic of the breakdown of the agglomerated deposition material 5, by any suitable mechanical agitation 7, into a temporary suspension. As noted herein, a variety of solutions are suitable for use in creating this suspension with the key being i) the agglomerations are broken down; and ii) that state is maintained (by dilution of the deposition material in a suspension). FIG. 8b ) illustrates a flow based electrochemical cell at 10 comprising electrodes 8 and scaffold material 12. Temporary suspension 14 (comprising deposition material 15) is pressure driven through the electrodes/scaffold material 12 from intake point 16 to outlet point 18. FIG. 8c ) illustrates the actual deposition of deposition material 15 on scaffold material 12. FIG. 8d ) illustrates the transition from deposition of deposition material 15 on scaffold material 12 to actual new agglomeration 20 formation, comprising deposition material 15 and scaffold material 12.
Deposition Process
It has been found that the desired deposition materials of the invention have tendency, through Van der Waals forces, to sediment or self-agglomerate into undesirable clusters in a liquid (whether electrolyte or derivative thereof); which hinders the deposition of the deposition material onto the actual electrode scaffold (the desired outcome). The process of the present invention delays this agglomeration or sedimentation process sufficiently to form a temporary suspension of the desired deposition material by using very small quantities (very low weight percent) of deposition material. In the example presented below, sonication of a 0.001% by weight mixture of carbon nanotubes (CNTs) in water leads to a temporary suspension in which CNT fiber bundles can be visually observed to agglomerate over the course of an hour. As such, the deposition material in the suspension can properly bind to the surface of the electrode when the suspension is forced through the electrode (at least) within this timeframe.
When the temporary suspension is injected into an electrochemical cell, the deposition material becomes lodged within the pores at the entrance (or deeper within the electrode), at which point the binding or agglomeration process begins. Binding or agglomeration of the deposition material on the electrode surface can occur through the same physical forces and interaction (e.g.: Van der Waals bonds) which lead to self-agglomeration of the deposition itself. The binding can also be achieved or enhanced through more specific chemical modification of the deposition material or electrode surface prior to deposition in order to form chemical bonds during deposition (hydrogen bonds, metallic bonds and covalent bonds).
In the example presented below, a 0.001% by weight mixture of CNTs in water was deemed sufficient to provide enough time to flow the temporary suspension through the electrode scaffold and produce a beneficial deposition. The exact concentrations to be used for other applications will depend on the physical nature of the material to be deposited and the liquid used to create the temporary suspension and determining this is well within the purview of a skilled technician in this field.
Forming the scaffold/deposition material agglomerations and thereby “coating” the scaffold material, in this way, increases the number of possible contact points open to electrochemical interaction (on the electrode) with an electrolyte. It is important to appreciate, as illustrated in FIGS. 8c ) and 8 d), that there is a transition from deposition of the deposition material 15 on scaffold material 12 (FIG. 8 c)) to actual new agglomeration 20 formation (FIG. 8d )), comprising deposition material 15 and scaffold material 12. The early or first deposition event(s) will perhaps not be considered full agglomeration, while subsequent events at the same site generally are. The process of the present invention is not, therefor, intended to be limited to full and complete agglomeration comprising deposition material 15 and scaffold material 12. In some instances and reactions, there may simply be deposition as per FIG. 8c ) and not full agglomeration, as per FIG. 8d ). When the term “agglomeration” is used in the claims, it is intended to include both levels of reaction.
Application of the present invention can yield high quality porous nano-structured coated electrodes wherein the substrate material of the electrode-metal current collector is with the coating material, i.e. the deposition material lodges inside the voids or pores formed during the fabrication of the electrode-metal current collector.
An apparatus may be used for high rate deposition of the deposition material on the scaffold material (for example, an electrode or metal current collector). The apparatus comprises a housing extending to a nozzle and having at least one ultrasonic chamber.
A fluid injection device fluidly communicates with the housing for injecting fluid, such as, for example, and aqueous solution of deposition material onto the surface of a scaffold material, to collide with such surface.
Pressure Driven Flow
An apparatus is required for formation of the temporary suspension. In the present example, a sonication apparatus was used for thirty minutes to break up the agglomerates of the deposition material within the deposition liquid and form this temporary suspension. This step can be achieved through other existing means such as mechanical agitation means including mechanical stirrers, centrifuges, etc. There are a wide variety of agitation means which will work very well.
An apparatus is required for containment and pressure driven flow of the temporary suspension through the scaffolding electrode. In the example below, the temporary suspension was housed within a 10 ml syringe, and flow was mechanically driven via a syringe pump at rates from 10-1000 μl/min. A sufficient pressure driven flow may easily be achieved by other existing means, such as peristaltic pumps, gravity induced flow, screw pumps, gear pumps, etc. . . . The process of the invention should not be limited to any one flow system. In one preferred aspect, the flow of temporary suspension/solution comprising the de-agglomerated deposition material is driven into the scaffold material under pressure.
Uses of the Deposition Process
The context of use of the process of the invention is primarily and preferably within any flow-based electrochemical cell or reactor. Oxidation-reduction or redox reactions take place in electrochemical cells. An electrochemical cell has three component parts: an electrolyte and two electrodes (a cathode and an anode). The electrolyte is usually a solution of water or other solvents in which ions are dissolved. Molten salts such as sodium chloride are also electrolytes. When driven by an external voltage applied to the electrodes, the ions in the electrolyte are attracted to an electrode with the opposite charge, charge-transferring (also called faradaic or redox) reactions can take place. Only with an external electrical potential (i.e. voltage) of correct polarity and sufficient magnitude can an electrolytic cell decompose a normally stable, or inert chemical compound in the solution. The electrical energy provided can produce a chemical reaction which would not occur spontaneously otherwise.
There are two types of electrochemical cells. Spontaneous reactions occur in galvanic (voltaic) cells; nonspontaneous reactions occur in electrolytic cells. Both types of cells contain electrodes where the oxidation and reduction reactions occur. Oxidation occurs at the electrode termed the anode and reduction occurs at the electrode called the cathode. The anode of an electrolytic cell is positive (cathode is negative), since the anode attracts anions from the solution. However, the anode of a galvanic cell is negatively charged, since the spontaneous oxidation at the anode is the source of the cell's electrons or negative charge. The cathode of a galvanic cell is its positive terminal. In both galvanic and electrolytic cells, oxidation takes place at the anode and electrons flow from the anode to the cathode.
The redox reaction in a galvanic cell is a spontaneous reaction. For this reason, galvanic cells are commonly used as batteries. Galvanic cell reactions supply energy which is used to perform work. The energy is harnessed by situating the oxidation and reduction reactions in separate containers, joined by an apparatus that allows electrons to flow. A common galvanic cell is a fuel cell. Both flow batteries and capacitors can be considered both galvanic and electrolytic cells for discharging and charging operations, respectively)
The redox reaction in an electrolytic cell is nonspontaneous. Electrical energy is required to induce the electrolysis reaction. An example of an electrolytic cell is shown below, in which molten NaCl is electrolyzed to form liquid sodium and chlorine gas. The sodium ions migrate toward the cathode, where they are reduced to sodium metal. Similarly, chloride ions migrate to the anode and are oxided to form chlorine gas. This type of cell is used to produce sodium and chlorine. The chlorine gas can be collected surrounding the cell. The sodium metal is less dense than the molten salt and is removed as it floats to the top of the reaction container.
Electrochemical cells within which the process of the invention may be applied includes use in capacitive deionization (CDI) systems. CDI is a technology to deionize water by applying an electrical potential difference over two porous carbon electrodes. Anions, ions with a negative charge, are removed from the water and are stored in the positively polarized electrode. Likewise, cations (positive charge) are stored in the cathode, which is the negatively polarized electrode. The operation of a conventional CDI system cycles through two phases: an adsorption phase where water is desalinated and a desorption phase where the electrodes are regenerated. During the adsorption phase, a potential difference over two electrodes is applied and ions are adsorbed from the water. The ions are transported through the interparticle pores of the porous carbon electrode to the intraparticle pores, where the ions are electrosorbed in the so-called electrical double layers (EDLs). After the electrodes are saturated with ions, the adsorbed ions are released for regeneration of the electrodes. The potential difference between electrodes is reversed or reduced to zero. In this way, ions leave the electrode pores and can be flushed out of the CDI cell resulting in an effluent stream with a high salt concentration, the so-called brine stream or concentrate. Part of the energy input required during the adsorption phase can be recovered during this desorption step.
For a high performance of the CDI cell, high quality electrode materials are of utmost importance. While carbon is the choice as porous electrode material, as high salt electrosorption capacity is important, the specific surface area and the pore size distribution of the carbon accessible for ions should be large on the electrode. The ions should be able to move fast through the pore network of the carbon and the conductivity of the carbon should be high. For this reason, the process of deposition in accordance with the invention, which enhances the specific surface area and optimizes pore size distribution, is particularly suitable and beneficial to the CDI cell. Activated carbon (AC) is the commonly used material, as it is the most cost efficient option and it has a high specific surface area. It can be made from natural or synthetic sources. Other carbon materials used in CDI research are, for example, ordered mesoporous carbon, carbon aerogels, carbide-derived carbons. Today, CDI is mainly used for the desalination of brackish water, which is water with a low or moderate salt concentration (below 10 g/L).[
The process of the invention may benefit any electrochemical reactor, such as flow batteries, fuel cells, reactors for electrochemical processes, energy storage, energy conversion, electrodeposition, electrowinning, wastewater treatment, and (electro)chemical synthesis,
This invention is further illustrated by the following examples which should not be construed as limiting. The contents of all references cited throughout this application, as well as the figures and tables are incorporated herein by reference.
While the forms of process and apparatus and system described herein constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise forms. As will be apparent to those skilled in the art, the various embodiments described above can be combined to provide further embodiments. Aspects of the present processes, devices, and systems (including specific components thereof) can be modified, if necessary, to best employ the processes, devices, and systems and components and concepts of the invention. These aspects are considered fully within the scope of the invention as claimed. For example, the various methods described above may omit some acts, include other acts, and/or execute acts in a different order than set out in the illustrated embodiments.
Further, in the methods taught herein, the various acts may be performed in a different order than that illustrated and described. Additionally, the processes can omit some acts, and/or employ additional acts.
These and other changes can be made to the present systems, methods and articles in light of the above description. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the invention is not limited by the disclosure, but instead its scope is to be determined entirely by the following claims.

Example 1

In both examples seen in FIG. 1, a customized microfluidic channel is used to force vanadium electrolyte through the electrodes. In the first case depicted in FIG. 1-a), the electrochemical cell in question is an analytical device used for characterization of materials. As such, it can be used to characterize, as it has in the present example, to characterize the deposition process. This method is used in addition to electrochemical impedance spectroscopy (EIS) to quantify the performance improvement of a novel CNT deposition process of the invention within flow-through porous electrodes. This deposition process is shown to increase the ESA of the carbon paper and the exchange current density of the V2+/V3+ reaction by an order of magnitude. It is also shown that this process of the invention can be used in situ to enhance the reaction rates of the previously published/made microfluidic co-laminar flow cell (CLFC) based on vanadium redox reactants and carbon paper flow-through porous electrodes as seen in FIG. 1-b). This class of electrochemical cell is based on membraneless co-laminar flow to achieve reactant separation and has been shown to benefit from flow-through porous electrodes¹⁴. Using the process of the invention, flowing deposition of CNTs is used to increase the surface area of the anode of the cell in FIG. 1-b) to offset the slower intrinsic kinetics of the V2+/V3+ reaction in particular. This process is shown to increase the overall power density of the co-laminar flow cell by 20-30% with a minimal effect on the required pumping pressure. ¹⁴M.-A. Goulet, E. Kjeang, Co-laminar flow cells for electrochemical energy conversion, J. Power Sources. 260 (2014) 186-196.
The reactants in this study are the V2+/V3+ and VO2+/VO2+ redox couples prepared by charging a commercial 1.7 M vanadium 4 M sulfuric acid stock solution with a conventional flow battery. For the purpose of studying the kinetics of the limiting reaction, the microfluidic analytical flow cell in FIG. 1-a) is operated with a 50% state of charge (SOC) V2+/V3+ solution in order to have equal concentrations of reductant and oxidant. The negative equilibrium potential of this redox couple (−0.255 V vs. SHE) permits the use of a silver foil as a reference electrode (RE), with 180 μm thick Toray carbon paper (TGP 060) serving as both the working electrode (WE) and counter electrode (CE) as described in Goulet¹⁵. For discharge performance tests, the co-laminar flow cell shown in FIG. 1-b) uses the same carbon paper electrodes with both V2+/V3+ and VO2+/VO2+ reactants charged to roughly 90% SOC representative of typical battery discharge operation. In all cases the carbon papers are heat treated to increase hydrophilicity. The carbon nanotubes (CNT) for electrode enhancement are 10-30 μm in length and <8 nm in outer diameter. A temporary suspension of 0.001% wt of these CNTs in deionized water is prepared by sonication for one hour and injected immediately afterwards through the carbon paper. The carbon paper electrodes are rinsed with DI water to remove loosely deposited CNTs before being used with vanadium reactants. ¹⁵M.-A. Goulet, M. Eikerling, E. Kjeang, Direct measurement of electrochemical reaction kinetics in flow-through porous electrodes, Electrochem. Commun. 57 (2015) 14-17.
Device Fabrication
The microfluidic channels for both flow cells pictured in FIG. 1 are fabricated by soft lithography of poly(dimethylsiloxane) (PDMS) from a photoresist template. The analytical flow cell consists of a single inlet and outlet with a uniform channel height of 150 μm to compress the electrodes and a channel width of 2 mm in the upstream WE section which gradually expands to a 4 mm width in the downstream CE section. The active section of the WE is therefore 2 mm wide, with a flow-through depth of 0.5 mm. The CE is sized with a flow-through depth of 4 mm to provide an order of magnitude larger ESA than the WE. Once the electrodes are placed within their respective grooves, the cell is capped by a glass slide and pressure sealed by a customized clamp. Contact is made with the electrodes which extend beyond the cell via copper clips. The co-laminar flow cell on the other hand has two identical electrodes 1 mm thick and with an active length of 10 mm compressed to a similar 150 μm channel height through which the reactants are pumped. The cell is more permanently bonded by plasma treatment of the PDMS and glass slide cap layer.
Characterization
Two syringe pumps (Harvard Apparatus MA1 70-2209) are operated in unison at flow rates ranging from 2 to 400 μL·min-1 to inject the reactants into the inlets and remove them from the outlets of either cell. All polarization measurements are made with a potentiostat (Gamry Reference 3000) operating at voltammetric scan rates slow enough to ensure steady state conditions. Electrochemical impedance spectroscopy (EIS) measurements are conducted at 10 mV perturbation from the open circuit potential (OCP) to determine the impedance of the cells. All Tafel plots have been corrected for the effect of IR in order to display only the relevant kinetics, whereas IR effects have been preserved for all polarization data during discharge of the co-laminar flow cell to demonstrate genuine overall performance improvements. The capacitance of the WE in the analytical cell was determined via fitting of a Randles circuit. The pressure drop across the enhanced electrode was measured by a pressure transducer (Honeywell HSCMRRN001PDAA5) placed in parallel with the fluidic circuit with the output read by a source meter (Keithley 2400).
Results and Discussion
The following study presents a novel flow-through deposition technique for CNTs on carbon paper electrodes and quantifies this deposition rate with a known the microfluidic analytical method¹⁶and supports these measurements with corroborating evidence from EIS. The performance of a co-laminar flow cell for discharge of vanadium reactants is assessed by both EIS and polarization data before and after in situ deposition of CNTs on the on the anode of the cell. Lastly, the effect of the added CNTs on the pressure drop through the enhanced electrode is measured via pressure transducer. ¹⁶Supra, at 15
Flowing CNT Deposition
Without hydrophilic surface functionalization, carbon nanoparticles such as carbon nanotubes do not form stable suspensions in aqueous solutions, but rather phase separate into macroscopic agglomerates. It has been found that the addition of solvents (other than water) interferes with the desired reactions. In addition, and of importance to the claimed invention, the omission of solvents reduces the complexity and cost of the deposition and satisfies the growing need for ‘greener’ chemical processes. The flowing deposition process of the invention is based on the idea of maximizing the time before agglomeration such that the deposition procedure can be completed. This is accomplished by preparation of a very low weight percent suspension of carbon nanotubes in deionized water to decrease the probability of carbon nanotube interactions. Sonication of the mixture leads to a temporary suspension in which CNT fiber bundles can be visually observed to agglomerate over the course of an hour. When the temporary suspension is injected into the analytical flow cell depicted in FIG. 1-a), some of the CNTs become lodged within and at the entrance to the carbon paper working electrode, while others pass through the electrode and are either deposited at the thicker counter electrode or may potentially reach the outlet. As such, the upstream working electrode acts as a first filter for the suspension. As more of the suspension is injected, a dark layer forms at the entrance to the working electrode which is composed of agglomerated CNTs as seen in FIG. 2-a).
The scanning electron microscope (SEM) images in FIG. 2 depict the different structures formed by the CNTs during the deposition. The top down view in FIG. 2-a) shows the overall structure of a 500 μm wide carbon paper electrode after the deposition process. The carbon paper, with 7 μm diameter carbon fibers connected by sections of pyrolyzed binder, serves as a supporting scaffold for the deposited CNTs primarily distributed at the upper side of the paper. A closer view of the upper section is seen in FIG. 2-b) which shows the CNTs blanketing the surface of the carbon paper which had been cross-sectional to the upstream flow of the CNTs. A close-up of one of the carbon fibers in FIG. 2-c) illustrates the two primary structures encountered during these experiments, namely randomly amalgamated amorphous clumps of CNTs and self-assembled CNT fiber bundles which seem to attach themselves to the larger carbon paper fibers. It is highly likely that these physically adhered, naturally occurring CNT bundles provide the mechanical support required for this deposition process to provide stable electrode enhancement during electrolyte flow without the need for ionomer binders. It is preferred to use a thorough rinsing step after each deposition step involving fast flow (1000 μL·min-1) of deionized water in order to retain only the most firmly attached CNTs. It is noted that the amorphous section seen in FIG. 2-d) which is the primary form of the blanketing layer at the inlet of the electrode has a nanoporous structure. The enhancement in charge transfer resistance and exchange current density seen in the next section can reasonably be attributed to this type of high surface area porous structure through which the electrolyte passes.
Demonstration of Electrode Enhancement in an Analytical Flow Cell
In order to accurately evaluate the performance enhancement of the flowing deposition process of the invention, the carbon paper is placed within the three electrode analytical flow cell in FIG. 1-a) to characterize the impedance and polarization of the electrode under both quiescent and convective electrolyte conditions. As noted above, the deposition process of the invention comprises injecting a temporary suspension of CNTs through the working electrode, followed by a rinsing step before testing the EIS and Tafel behavior of the electrode in a V2+/V3+ electrolyte. The process is repeated by adding an additional 0.1 ml of suspension at each step until a total volume of 0.4 ml was used.
The Nyquist plots in FIG. 3-a) clearly demonstrate the decrease in charge transfer resistance (Rct) after each deposition step until the final step when no significant change is observed. It should be noted that the high frequency ohmic resistance of the electrode, which is used for post-run IR correction of polarization data, remained constant throughout the testing. The improvements in reaction rate measured via the EIS perturbation method are further supported by standard Tafel analysis with the analytical flow cell technique. After each EIS measurement in quiescent electrolyte, the cell is operated at a high flow rate of 400 μL·min-1 in order to minimize mass transport effects. In conjunction with a small electrode depth, the high flow rate used leads to conversion below 5% for the V2+/V3+ reactants in this experiment, thereby ensuring mostly kinetic control. This is also substantiated by the fact that the linear Tafel slopes in FIG. 3-b) are within 30 mV/dec of the theoretical value for a one electron transfer reaction in the absence of diffusion. In agreement with the EIS data, the Tafel measurements display similar improvements in reaction rate as seen by the increase in exchange current determined by the y-intercept of the Tafel slopes. As with the EIS data, the largest relative improvement occurs during the first step, with diminishing returns as the procedure is repeated. It should also be noted that the deposition process was continued beyond 0.4 ml with little measurable effect. This could be due to the deposition process reaching a saturation point in the stability of adhered CNTs, with additional CNTs being too weakly bound to the carbon paper scaffold to resist being washed out during the rinsing step. Nevertheless, as seen in FIG. 4 both Tafel analysis and EIS indicate that the method is capable of enhancing the baseline reaction rate of this electrode by over an order of magnitude.
With no significant change in the Tafel slopes seen in FIG. 3-b) there is no reason to believe that the mechanism for the V2+/V3+ reaction changes during the deposition procedure. Indeed, the data in FIG. 4-a) shows a clear linear relationship between the capacitance and charge transfer resistance, which suggests that the increase in reaction rate is solely due to an increase in ESA, rather than any change in the intrinsic kinetics of the reaction. Similarly, the exchange current measured from the data in FIG. 3-b) should also increase proportionally to the ESA. In addition, the slope of proportionality between the charge transfer resistance and the exchange current should follow the low-field approximation of the Butler-Erdey-Gruz-Volmer (BE-GV) expression for a one electron transfer:
I_0=RT/FR_ct̂(−1)
where R is the universal gas constant, T is the temperature and F is Faraday's constant. This theoretical relationship has been added to FIG. 4-b) to highlight the results from the deposition. The exchange current measured for the baseline carbon paper and the first deposition step fall directly on the theoretical line, indicating that the EIS and Tafel techniques are consistent and interchangeable.
These studies demonstrate the capability of the process of the invention to considerably increase the electrochemically active surface area of flow-through porous electrodes. From the Rct-1 and Cdl data, this increase was roughly measured to be 15×, whereas the exchange current increased by 13×.
In Situ Fuel Cell Enhancement
The following in situ experiment is designed to enhance the surface area of only one electrode of the co-laminar flow cell depicted in FIG. 1-b). This is achieved by pumping the temporarily suspended CNTs through that electrode, while pumping only deionized water through the other electrode. The high flow rate (1000 μL·min-1) ensures that the two streams meet in the center channel and flow towards the outlet without crossing over to the other side¹⁷. After rinsing both sides of the cell with deionized water, the CLFC is then operated as a fuel cell or discharging flow battery by pumping 90% V2+ through the anode and 90% VO2+ through the cathode. As a two electrode cell with kinetically different reactants on each side, the Nyquist plot of the EIS data exhibits a slightly doubled semicircle profile, with the less visible higher frequency smaller semicircle corresponding to the faster VO2+/VO2+ redox couple; while the lower frequency greater semicircle relates to the sluggish V2+/V3+ reaction in the red curve of FIG. 5. ¹⁷M.-A. Goulet, E. Kjeang, Reactant recirculation in electrochemical co-laminar flow cells, Electrochim. Acta. 140 (2014) 217-224.
As noted above, the inverse of the charge transfer resistance is proportional to the rate of reaction and therefore inherently linked to the ESA of the electrode with the limiting reaction. By increasing the area of the anode, it is expected that charge transfer resistance of the cell will decrease,
A more incremental test is portrayed in FIG. 5 in which the Nyquist plots illustrate the effect of additional CNT deposition steps. As before, the cell is rinsed with water between each deposition step and then operated with charged reactants to measure both polarization and EIS behavior. As expected, the larger V2+/V3+ semicircle which determines the Rct of the cell decreases with each deposition step until the point where the Nyquist plot resembles a single semicircle, indicating similar kinetics between both electrodes. This demonstrates that the deposition treatment can effectively compensate for the sluggish kinetics of the V2+/V3+ reaction. As in the ex situ case, the CNT deposition has no effect on the high frequency intercept of the Nyquist plot, indicating that the ohmic resistance of the cell has not changed.
Increasing the surface area of the limiting electrode reduces the activation or kinetic overpotential of this reaction and therefore the polarization behavior of the entire cell improves. The trend observed in the EIS data can also be seen in the polarization curves of FIG. 6-a). In the same way that the charge transfer resistance improves incrementally, the kinetic region in the polarization plots improve incrementally as well, leading to greater overall current at a given cell voltage, i.e., a higher electrochemical efficiency of the cell reaction. With no effect on the ohmic resistance of the cell, the linear section of the polarization curve should not change appreciably but there is a slight improvement in this section which indicates that the mass transport of reactants may have improved as well. Indeed, the nanoporosity of the deposited CNT layer may reduce the concentration overpotential of the electrode as well, by making the pore size distribution more favorable towards fast diffusion of reactants to the electrode surface. Overall, the enhancement of the anode and compensation for the sluggish kinetics of the V²⁺/V³⁺ reaction produces a substantial increase in the maximum power density of the cell, with 9 ml of injected temporary CNT suspension leading to a 23% increase in the maximum power density as shown in FIG. 6-b). At 100 μL·min⁻¹this enhanced cell produced a maximum power of 10.4 mW, equivalent to roughly 700 mW·cm⁻²when normalized by the electrode area cross-sectional to the flow and 2330 mW·cm⁻³when normalized by the total volume of the electrochemical chamber (both electrodes and center channel). It was also observed that further deposition of CNTs on the anode had no measurable effect, indicating that the cathode was now limiting the reaction rate.
The data displayed in FIG. 7 shows the pressure versus flow rate relationship for the incremental stages of electrode enhancement discussed thus far. As expected from Darcy's law, the increase in pressure is linear in all cases with respect to the change in flow rate. For the baseline cell with no enhancement, the experimental values for the pressure are in good agreement with a previous modeling study on a similar cell design with flow-through porous electrodes¹⁸. Comparing the required pressures at each flow rate, the fully enhanced electrode with 9 ml of deposition treatment consistently requires 3× the pumping pressure of the baseline electrode. When these pressures are converted into pumping power however, the power lost is insignificant to the power gained. For the single electrode measured the pumping power P=Q·Δp, which equates to roughly 0.001 mW for the baseline case and 0.003 mW for the fully enhanced case at a flow rate of 100 μL·min-1. This negligible change is less than 1% of the power gained. ¹⁸D. Krishnamurthy, E. O. Johansson, J. W. Lee, E. Kjeang, Computational modeling of microfluidic fuel cells with flow-through porous electrodes, J. Power Sources. 196 (2011) 10019-10031.

Claims

We claim:

1. A process for increasing the energy conversion and electrochemical efficiency of a scaffold material using a deposition material, which comprises:

flowing by at least one surface of the scaffold material a solution which comprises the deposition material;

forming agglomerations of the deposition material with at least one surface of the scaffold material, wherein the deposition material fills pores on the at least one surface of the scaffold material (“scaffold pores”) thereby increasing the surface area of the scaffold material; and

electrically connecting deposition material to the scaffold material via the formation of agglomerations, wherein said scaffold material is conductive and flow-through and wherein deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.

2. The process of claim 1 wherein scaffold material is porous, electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase.

3. The process of claim 1 wherein scaffold material is selected from the group consisting of carbon paper, carbon/graphite felt, carbon cloth, carbon foam, metal foam, carbon aerogel, carbon nanofoam and packed bed electrodes.

4. The process of claim 1 wherein scaffold material is an electrode.

5. The process of claim 1 wherein the deposition material is electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase.

6. The process of claim 1 wherein the deposition material is selected from the group consisting of carbon nanotubes, graphene, carbon nanoparticles, graphite flakes, carbon black, fullerenes, carbon oxides, graphene oxide, functionalized carbon materials, doped carbon materials, catalysts, catalyst nanoparticles, metal nanoparticles, metal nanorods, metal supported catalyst materials, carbon supported catalyst materials, and metal particles.

7. The process of claim 1 wherein the solution is water.

8. The process of claim 1 wherein the solution comprises an electrolyte which comprises at least one reactant for an electrochemical reaction.

9. The process of claim 1 wherein the solution is sulfuric acid.

10. A process for increasing the energy conversion and electrochemical efficiency of a scaffold material using a deposition material, which comprises

i) breaking existing bonded agglomerates of the deposition material to form broken agglomerates;

ii) diluting in a solution the broken agglomerates of the deposition material to mitigate interaction and re-agglomeration of the deposition material during flow through the scaffold material; and

iii) flowing the solution through at least one surface of a scaffold material such the broken agglomerates bond and re-agglomerate with the at least one surface of the scaffold material.

11. The process of claim 10 wherein sonication is used to break existing bonded agglomerates of the deposition material to form broken agglomerates.

12. The process of claim 10 wherein the solution is water.

13. The process of claim 10 wherein the solution comprises an electrolyte which comprises at least one reactant for an electrochemical reaction.

14. The process of claim 10 wherein the solution is sulfuric acid.

15. The process of claim 10 wherein scaffold material is porous, electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase.

16. The process of claim 10 wherein scaffold material is selected from the group consisting of carbon paper, carbon/graphite felt, carbon cloth, carbon foam, metal foam, carbon aerogel, carbon nanofoam and packed bed electrodes.

17. The process of claim 10 wherein scaffold material is an electrode.

18. The process of claim 10 wherein the deposition material is electrically conductive and facilitates electrochemical reactions when contacted with reactant in liquid or gas phase.

19. The process of claim 10 wherein the deposition material is selected from the group consisting of carbon nanotubes, graphene, carbon nanoparticles, graphite flakes, carbon black, fullerenes, carbon oxides, graphene oxide, functionalized carbon materials, doped carbon materials, catalysts, catalyst nanoparticles, metal nanoparticles, metal nanorods, metal supported catalyst materials, carbon supported catalyst materials, and metal particles.

20. A scaffold material with increased energy conversion and electrochemical efficiency properties which comprises a base scaffold material which is electrically conductive and flow-through, coupled with a deposition material deposited in pores on at least one surface of the scaffold material (“scaffold pores”) in the form of agglomerations thereby increasing the surface area of the scaffold material and wherein the deposition material has a pore size (“deposition material pore size”) which is no larger than the scaffold pore size.