US20160312370A1

US20160312370A1 - Electrochemical cell without an electrolyte-impermeable barrier

Info

Publication number: US20160312370A1
Application number: US15/103,026
Authority: US
Inventors: Gerhard Frederick Swiegers; Michael Leigh ANGLISS
Original assignee: Aquahydrex Pty Ltd
Current assignee: Aquahydrex Inc
Priority date: 2013-12-10
Filing date: 2014-12-10
Publication date: 2016-10-27
Also published as: WO2015085363A1

Abstract

In one aspect there is provided an electrochemical cell without an electrolyte-impermeable barrier. In another aspect there is provided an electrochemical cell comprising a liquid electrolyte, a cathode and at least one cathode product able to be produced at the cathode, and an anode and at least one anode product able to be produced at the anode. The at least one anode product and the at least one cathode product are substantially separated, and the cell is without an electrolyte-impermeable barrier positioned between the cathode and the anode. There is a relatively low ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m³)/electrode surface area (m²)). The cell can be operated at a relatively low current density. Optionally, an electrolyte-permeable separator may be employed.

Description

TECHNICAL HELD

The present invention relates to electrochemical cells. In one form, the present invention more specifically relates to the elimination of the need for an electrolyte-impermeable barrier in electrochemical cells, where in a conventional electrochemical cell an electrolyte-impermeable barrier would be needed to ensure products from the anode and cathode are kept separate while allowing ion transport through the electrolyte-impermeable barrier for example as is the case in commercial water electrolysers.

BACKGROUND

In many electrochemical processes different products are generated at the anode and the cathode electrodes. Because the electrodes are most advantageously located in the closest possible proximity to each other, the products (e.g. gases, in the form of bubbles) may mix, contaminating each other. The product generated at one electrode may also be converted back to its reactant or destroyed if it contacts the opposite electrode. To avoid this possibility, electrochemical cells of this type typically employ an electrolyte-impermeable barrier. The electrolyte-impermeable barrier is a physical barrier that lies between the electrodes, either partially or fully. Being impermeable to liquid electrolyte, the electrolyte-impermeable barrier stops or hinders the products of the anode from mixing with the products of the cathode immediately after their formation (e.g. the mixing of dissimilar gas-supersaturated electrolyte solutions generated at the anode and the cathode). The electrolyte-impermeable barrier is, nevertheless, also designed so as to allow for electrical communication between the anode and cathode. This usually occurs in the form of an ion current between the electrodes. Thus, for example, the electrolyte-impermeable barrier may be a polymeric, ion-exchange membrane that allows ions to move from one side of the electrolyte-impermeable barrier across the electrolyte-impermeable barrier to the other side of the electrolyte-impermeable barrier (thereby closing the electrical circuit between the anode and the cathode), but not liquid electrolyte nor associated reaction products of the ions. An electrolyte-impermeable barrier of this type is sometimes referred to as a “diaphragm”. Alternatively, the electrolyte-impermeable barrier may be an impermeable solid material which partially but not completely partitions the anode from the cathode, and around whose side's ions may migrate between the electrodes to thereby close the electrical circuit. An electrolyte-impermeable barrier of this type is sometimes referred to as a “skirt”, a “partition wall”, or a “chamber divider”.
To illustrate the need for and role of an electrolyte-impermeable barrier, one may consider the representative case of water electrolysis. In this process, water is electrochemically split into oxygen gas at the anode and hydrogen gas at the cathode as per the half-reactions below


At the anode:	2 H₂O →O₂+ 4H⁺ + 4 e⁻	E⁰ _ox= 1.23 V
At the cathode:	4 H⁺ + 4 e⁻→ 2 H₂	E⁰ _red= 0.00 V
Overall reaction:	2 H₂O → O₂+ 2 H₂	E⁰ _cell= 1.23 V

As can be seen, hydronium ions (H⁺; also called ‘protons’) are generated at the anode and must migrate to the cathode in order to close the electrical circuit. Thus, the electrolyte-impermeable barrier in a water electrolyser must allow H⁺ions to move from the anode to the cathode but stop the water electrolyte and associated gas bubbles from moving between the anode and cathode compartments.
In modern-day water electrolysers, the electrolyte-impermeable barrier used is most typically a diaphragm comprising sulfonated tetrafluoroethylene based fluoropotymer-copolymer material, sold under the trade name Nafion™, which is a “proton-exchange membrane” (or “PEM” ). Protons (H⁺) are readily able to migrate across such a PEM and thereby move from one electrode to the other. Liquid water electrolyte, and associated gas bubbles/molecules are, however, blocked from passing through the PEM polymer. In alkaline electrolysers, asbestos woven cloths have traditionally been used as electrolyte-impermeable diaphragms in the past.
According to an authoritative scientific review of water electrolysis issued by the Danish government lab, Riso, entitled “Pre-Investigation of Water Electrolysis” (PSO-F&U; (2008), Pre-Investigation of Water electrolysis, NE1-DK-5057, p. 39-49), the electrolyte-impermeable diaphragm in such an electrolysis cell must fulfil multiple roles, including the following:

- (1) The electrolyte-impermeable barrier must prevent mixing of gas-filled electrolyte from the cathode with gas-filled electrolyte from the anode. Gas evolution at an electrode in an electrochemical cell typically generates a two-phase mixture of liquid electrolyte with dispersed bubbles. Mixing of the anode and cathode electrolyte will result in mixing of the gases, precluding the attainment of high gas purities and electrical efficiencies.
- (2) The electrolyte-impermeable barrier must form an effective diffusion barrier for the gas molecules formed at each of the anode and cathode, so as to thereby avoid contamination of the gases by molecular diffusion across the electrolyte-impermeable barrier.
- (3) In the case of an elastic electrolyte-impermeable barrier, the electrolyte-impermeable barrier may also be useful in preventing the formation of an electrically insulating gas bubble curtain at the front side of the electrodes. This is achieved by locating the electrodes physically close to the electrolyte-impermeable barrier, such that the bubbles are rapidly swept off the face of the electrode.
- (4) Most importantly, in order to avoid an uncontrolled increase in the electrical resistance of the electrolysis cell, it is critical that the pores of the electrolyte-impermeable barrier should not become clogged with gas bubbles. This may occur when mechanical forces drive gas bubbles into the mouths of the pores, or when a gas-supersaturated electrolyte solution spontaneously forms new bubbles inside the pores. In such cases, bubbles may only form in small cavities of radius r if a certain degree of supersaturation is established according to the equation:

$P - P_{sat} \geq \frac{2 σ}{r}$

- At 30-60 bar, the supersaturation pressures of hydrogen and oxygen are believed to be no more than a few bars. Thus, for electrolyte surface tensions of ca. 200 dyn cm⁻¹, pore diameters of 1-2 micrometers will reliably avoid gas clogging of the electrolyte-impermeable barrier.
- (5) The electrolyte-impermeable barrier must also provide a sufficiently high hydrodynamic resistance of more than ca. 5 cm³centipoises (cm²bar s)⁻¹so as to avoid mixing of oxygen saturated electrolyte from the anode with hydrogen saturated electrolyte from the cathode due to occasional, operational pressure differences between the cathodic and anodic compartments.
- (6) The electrolyte-impermeable barrier must display a low electrical surface specific resistance when immersed in the electrolyte, ideally not exceeding 0.2 cm²so as to avoid high ohmic potential drops within the electrolyte-impermeable barrier at current densities around 1 A cm⁻².

Thus, their is a conventional understanding that electrolyte-impermeable barriers are required in electrochemical cells. A key challenge experienced in the water electrolyser industry, by way of example, is the high cost of the most widely used eleetrolyte-impermeable barrier material, Nation™, which may routinely retail for prices of US$500-$1500 per square meter at the present time. The excessive cost of the electrolyte-impermeable barrier is beaten, in many water electrolysers only by the still higher cost of the precious metal catalysts that must be used; for example, platinum, which is used in electrolysers with acidic electrolytes, currently trades for around US$1,300 per ounce on world markets. In water electrolysers employing basic electrolyte, the electrolyte-impermeable barrier is often the highest cost component.
Complicating this challenge is the fact that alternative electrolyte-impermeable barrier materials, which may be less costly, generally display higher resistance to ion (H⁺) transport when used in a cell. This means that such alternative electrolyte-impermeable barrier materials increase the energy requirement to drive the electrochemical process.
The key limitation at the present time in respect of water electrolyser electrolyte-impermeable barriers, is that many commercial water electrolysers operate most efficiently at current densities of 1500-3000 mA/cm²at voltages of <3 V. At the present time however, only expensive electrolyte-impermeable barrier materials like Nation™ membranes are capable of facilitating such current densities at these voltages.
It is for these reasons that the US Department of Energy (DOE) have, over many years, instituted well-funded and wide-ranging programs seeking to identify suitable, low-cost, low-energy, alternative materials for use as electrolyte-impermeable barriers in water electrolyser cells.
The DOE has also funded extensive programs aimed at reducing the high cost of the catalysts used in water electrolysers, most particularly the platinum employed in acidic electrolysers and the iridium oxide used in alkaline electrolysers. These two components comprise, by far, the major and overwhelming cost of water electrolyser stacks.
Very similar challenges exist in a wide range of other industrial electrochemical processes, including, for example, the chlor-alkali process for manufacturing chlorine, which is one of the most widely used electrochemical reactions in the world. The obvious way to reduce the capital cost of the cells in such cases, is to use a simpler, less expensive electrolyte-impermeable barrier.
In summary, the challenge of finding cheaper and more energy efficient alternatives to the electrolyte-impermeable barriers used in current electrochemical cells remains a problem, for which a solution is still needed.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Examples. This Summary is not intended to identify all of the key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
In one form, the present invention provides an electrochemical cell without an electrolyte-impermeable barrier positioned between the electrodes (i.e. between the anode(s) and the cathode(s)) of the electrochemical cell. This is contrasted to a conventional electrochemical cell, where an electrolyte-impermeable barrier is required to be present to ensure products from the anode(s) and the cathode(s) are kept separate while allowing ion transport through or around the electrolyte-impermeable barrier.
In another form there is provided an electrochemical cell comprising a liquid electrolyte, a cathodic and at least one cathode product able to be produced at the cathode, and an anode and at least one anode product able to be produced at the anode. The at least one anode product and the at least one cathode product are substantially separated, and the cell is without an electrolyte-impermeable barrier positioned between the cathode and the anode.
The inventors have discovered that re-configuration of the components and/or their operating conditions within electrochemical cells, like exemplar water electrolyser cells, provides for the advantageous elimination of any need for an electrolyte-impermeable barrier between electrodes. This may be achieved without incurring a substantial energy and/or a cost penalty. Indeed, an overall energy and/or cost benefit may instead be realised.
Contrary to current practice, the inventors have recognised that at lower current densities, and preferably with appropriate, improved or ideal electrolytes, there may be a relatively small energy penalty associated with increasing the inter-electrode gap. That is, with use of a strongly ion-conductive electrolyte, the anode and cathode may be located relatively far apart from each other in a cell, without creating an excessive ion-conduction resistance and thereby incurring a large energy penalty to operating the cell.
Moreover, at lower current densities, each of the anode and cathode will typically generate a relatively small stream of products (e.g. gas bubbles) per unit area. In the specific case of product streams comprising gas bubbles that rise to the surface of a liquid electrolyte, two such well separated and small product streams can, additionally, be collected in different parts of the cell, thereby avoiding mixing of the gases. Cells may be specifically designed to separately collect the small and distinct streams of gas bubbles.
Alternatively and optionally, two such well separated and small product streams can be directed to different locations within a cell for collection, by ensuring that electrolyte which flows or is pumped through the cell, sweeps the products (e.g. gas bubble streams) away from each other, or otherwise maintains a separation between the product gas bubble streams, and to different compartments within the cell where they are separately collected.
This need not involve an additional cost, since virtually all such electrochemical cells already require circulating pumps. The only additional cost, in this non-limiting example, is then incurred in designing the cell so as to ensure that the electrolyte is pumped or flows along pathways that sweep the product streams to different locations for collection. Collection may involve; (i) in cases where the products are gases: coalescence of the gas bubbles and drawing off of the gas through a suitable gas outlet or valve; or (ii) in cases where the products are in the liquid phase: physical removal of the electrolyte stream containing the products through a suitable outlet or valve, for isolation or use of the products elsewhere. Various other means of collection may also be used.
In effect, and referring to an example only, the inventors have unexpectedly realised that in the case where:

- i. a small product stream is generated at the anode and/or the cathode, and
- ii. at least one of the product streams involves the generation of as bubbles, and
- iii. the anode and cathode are well separated,
  then the physical features noted above that are associated with the optimum electrolyte-impermeable barrier (e.g. maximum pore diameter, supersaturation pressure, hydrodynamic resistance and surface specific electrical resistance) are such that the electrolyte-impermeable barrier is not required or may be replaced with a barrier or structure that is electrolyte-permeable. That is, there is no substantive need for an electrolyte-impermeable barrier between the electrodes at all. Alternatively, an electrolyte-permeable separator, or structure, that is wholly or substanially permeable (e.g. porous) to the liquid electrolyte may be located between the electrodes in place of an electrolyte-impermeable barrier. For example, a porous plastic sheet, that allows free movement of the liquid electrolyte through the porous plastic sheet, provides an electrolyte-permeable separator and may be used instead of an electrolyte-impermeable barrier in electrochemical cells of the present invention.

The distinction between an electrolyte-impermeable barrier, that still allows ion transport, and an electrolyte-permeable separator can be very significant when considered from the viewpoint of cost. There are a very wide variety of available electrolyte-permeable separators that are porous to liquids; many of these are commodity materials that are already manufactured in high volume and at low cost. By contrast, there is a much more limited number of electrolyte-impermeable barrier materials available and only a relatively small fraction of those have ion-exchange or other properties that make them suitable as an electrolyte-impermeable barrier in an electrochemical cell. Thus, there generally will be a substantial cost advantage to using an electrolyte-permeable separator in an electrochemical cell, if a separator is used at all, rather than an electrolyte-impermeable barrier. Still more inexpensive would be to not have any electrolyte-permeable separator between the electrodes.
Thus, in an example embodiment there is provided an electrochemical cell without an electrolyte-impermeable barrier and with an electrolyte-permeable separator between the electrodes. In another example embodiment there is provided an electrochemical cell without an electrolyte-impermeable barrier and without an electrolyte-permeable separator between the electrodes. Preferably, the electrochemical cell is an electro-synthetic cell (i.e. a commercial cell having industrial application) or an electro-energy cell (e.g, a fuel cell). In another example, the cell utilizes abiological manufactured components.
In noting the above, the inventors recognised that there is, of course, a larger trade-off in cost, in that a cell of the above alternative design needs electrodes with a substantially greater surface area than does a conventional cell, in order to generate the same overall quantity of products. For example, a cell based on the above alternative approach and operating at a low current density of 10 mA/cm²would, in one example, have to employ about 180-times more electrode surface area than a conventional cell operating at 1800 mA/cm², in order to generate the same overall quantity of products (assuming no changes in microscopic pore structure of the electrode material).
Furthermore, the inventors have recognized that another benefit of operating at low current density is that, at low current densities, one may make use of inexpensive catalysts and electrodes, and still facilitate the electrochemical transformation with high energy efficiency. For example, in the case of water electrolysis, one may avoid using very expensive precious metal catalysts, like platinum or iridium/ruthenium oxide, which are essential to achieving high energy efficiencies at high current densities. Instead, one may instead use cheaper, Earth-abundant materials, like nickel or manganese/cobalt oxides. At low current densities, the cheaper catalysts may readily achieve or even surpass the energy efficiencies achieved by the expensive catalysts at high current densities.
Alternatively, one may still make use of precious metal catalysts but at low current density operation, one would typically require orders of magnitude less of the precious metal catalysts per unit area than is conventionally required. Low current density operation may, in this way, also result in lower overall costs.
Thus, example cells of the alternative designs and operation, may, in fact, achieve better overall cost and energy efficiencies than existing, conventional electrochemical cell technology.
The inventors have further recognised that a cell having a large geometric electrode surface area can only be operated viably, i.e. commercially, at a low current density if the ratio of the electrolyte volume (unit: m³) to the electrode surface area (unit: m²) of either the cathode or the anode, is relatively low. The electrode surface area, of either the cathode(s) or the anode(s) separately, refers to the geometric surface area of the cathode(s) or the anode(s). The geometric surface area is the macroscopic surface area of the cathode(s) or the anode(s) (i.e. not including microscopic pores that might provide a higher electrochemically active surface area). For example, if the ratio of electrolyte volume to the geometric surface area of one of the electrodes (electrolyte volume:electrode surface area) is 1:1, or in fractional notation electrolyte volume/electrode surface area is 1 m (unit: metres), then a conventional cell operating at 1800 mA/cm²with an electrode, surface area of 1 m²and an electrolyte volume of 1 m³cannot be adapted to low current density operation at 10 mA/cm²to achieve the same overall output, since increasing the electrode surface area by 180-fold will require an increase in the electrolyte volume to 180 m³, which would be impractical and unviable. For this reason, a cell operating at a low current density can only do so practically if it has a relatively low ratio of electrolyte volume to electrode surface area expressed in fractional notation (unit: m), i.e. a relatively low ratio of electrolyte volume:electrode surface area. If there is an array of cathodes, then the electrode surface area is the geometric surface area of the cathodes in the array of cathodes. If there is an array of anodes, then the electrode surface area is the geometric surface area of the anodes in the array of anodes.
In one example, the ratio of electrolyte volume to electrode surface area is less than or about 0.1 m (or 100 mm) (i.e. 1 m³:10 m²). In another example, the ratio is less than or about 0.01 m (or 10 mm). In another example, the ratio is less than or about 0.001 m (or 1 mm). In another example, the ratio is less than or about 0.0001 m (or 100 μm). In another example, the ratio is less than or about 0.00001 m (or 10 μm). In another example, the ratio is less than or about 0.000001 m (or 1 μm). In another example, the ratio is less than or about 0.0000001 m (or 0.1 μm). In another example, the ratio is less than or about 0.00000001 m (or 0.01 μm). In another example, the ratio is less than or about 0.000000001 m (or 0.001 μm).
In another example form, there is provided an electrochemical cell, comprising a cathode located in a cathode compartment and an anode located in a physically separated anode compartment, and at least two fluid passages allowing an electrolyte to flow between the cathode compartment and the anode compartment.
In another example form, there is provided an electrochemical cell, comprising a cathode that in operation may produce a cathode product, and an anode that in operation may produce an anode product. The cell also includes an electrolyte, and the cathode and the anode are separated within the cell. Preferably, at least one product from the cathode, if any are produced, and/or at least one product from the anode, if any are produced, are directed to different locations.
In another example form, there is provided for the partial or complete elimination of a need for an electrolyte-impermeable barrier between the electrodes in electrochemical cells, in which dissimilar products are generated at the anode and the cathode. In one form, this can be achieved by:
locating the anode(s) and the cathode(s) in substantially separate locations within the cell, whereby:
a product stream from the anode(s), if present, and a product stream from the cathode(s), if present, are directed to different locations; and,
wherein the cell is operated at a relatively low current density.
Optionally, but not essentially, the cell may be so configured that circulating electrolyte separately sweeps the product stream(s) and/or intermediate ion(s) from the cathode(s) and/or the amode(s) to different locations within the cell, from where the products may be separately collected in pure or near-pure form.
Optionally, but not essentially, an electrolyte-permeable separator through which liquid electrolyte is able to move freely, may be positioned between or partially between the electrodes, such as being located in the inter-electrode gap between the anode and cathode to assist with the complete separation of the product streams originating from the anode and the cathode. The electrolyte-permeable separator is distinguished from an electrolyte-impermeable barrier in that the electrolyte-permeable separator permits free liquid electrolyte movement across the thickness of the electrolyte-permeable separator, whereas a electrolyte-impermeable barrier does not. An example of an electrolyte-permeable, separator is a plastic sheet that is freely permeable by a liquid electrolyte. Examples of such sheets include, for example, woven polymer or natural fabrics having large, liquid-permeable holes/pores through the full thickness of the sheets.
Preferably, but not exclusively, the cell employs an electrolyte that has a high ionic conductivity to thereby ensure a low overall resistance to the electrical current.
Preferably, but not exclusively, the cell operates at a low current density. This is preferably, but not exclusively, less than or about 10 mA/cm². In an alternative embodiment, this is preferably, but not exclusively, less than or about 20 mA/cm². In an alternative embodiment, this is preferably, but not exclusively, less than or about 70 mA/cm². In a still further alternative embodiment, this is preferably, but not exclusively, less than or about 250 mA/cm². In additional embodiments, this is preferably, but not exclusively, less than or about 500 mA/cm², or less than or about 1000 mA/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments will now be described solely by way of non-limiting examples and with reference to the accompanying figures. Various example embodiments will be apparent from the following description, given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures.

FIGS. 1(a) and 1(b) illustrate example tank electrochemical cells (e.g. electrolysers) without an electrolyte-impermeable barrier between the anode(s) and cathode(s).

FIG. 2 schematically illustrates an example electrochemical cell (e.g. electrolyser) without an electrolyte-impermeable barrier between the anode(s) and cathode(s) and having circulating electrolyte.

EXAMPLES

The following modes, features or aspects, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.
In one example there is provided an electrochemical cell, comprising a cathode that in operation produces a cathode product, and an anode that in operation produces an anode product. The electrochemical cell, comprising the cathode, the anode and an electrolyte, is without an electrolyte-impermeable barrier positioned between the cathode and the anode. The cell also includes an electrolyte, and the cathode and the anode are separated within the cell, and the cathode product and the anode product are directed to different locations. The cell can be operated at a low current density, due to the ratio of a relatively small electrolyte volume to a relatively large electrode geometric surface area. The ratio of electrolyte volume to electrode geometric surface area can be expressed as electrolyte volume (m³):electrode surface area (m²), or preferably the ratio can be expressed in fractional notation as electrolyte volume (m³)/electrode surface area (m²). To be clear, reference to the electrode surface area refers to either: the macroscopic geometric surface area of the cathode if there is one cathode; the macroscopic geometric surface area of the cathodes if there is more than one cathode; the macroscopic geometric surface area of the anode if there is one anode; or the macroscopic geometric surface area of the anodes if there is more than one anode. Hence, a relatively low ratio may apply to the cathode(s) and not the anode(s), to the anode(s) and not the cathode(s), or to both the cathode(s) and the anode(s).
For example, the ratio is less than or about 0.1 m, less than or about 0.01 m, less than or about 0.001, less than or about 0.0001, less than or about 0.00001, less than or about 0.000001 m, less than or about 0.0000001 m, less than or about 0.00000001 m, or less than or about 0.000000001 m. The electrochemical cell is without an electrolyte-impermeable barrier. That is, the electrochemical cell does not require or include a partial or full electrolyte-impermeable barrier between the cathode and the anode. In another form, the cell may incorporate a partial or full electrolyte-permeable separator between the anode and the cathode.
In a general example, there is provided an electrochemical cell comprising a liquid electrolyte, a cathode and an anode. At the cathode at least one cathode product is able to be produced. At the anode at least one anode product is able to be produced. The at least one anode product and the at least one cathode product are substantially separated, or most preferably separated, after being produced. The cell is without an electrolyte-impermeable barrier positional between the cathode and the anode. By between, is also meant partially between, i.e. there is no electrolyte-impermeable barrier positioned between, wholly or in part, the cathode and the anode, or between part of the cathode and anode or cathode(s) and anode(s) and anode(s) is electrode arrays are used).
In an example, the electrolyte flows past either the cathode or the anode, and/or the electrolyte exits the cell after flowing past the cathode or the anode. In another example the electrolyte circulates between the cathode and the anode. The electrolyte can sweep an ion species away from the cathode or the anode. This means the cell does not require or include an electrolyte-impermeable barrier between the cathode and the anode.
In another example, the cathode and/or the anode have some degree of porosity to enable electrolyte to pass through the cathode and/or the anode. For example, the cathode and/or the anode can be a series of ribbons of thin metallic foil, and the thin metallic foil can be of the order of about 0.025 mm thick. A spacing between the ribbons can be in the range of about 1 mm to about 20 mm. A spacing between the cathode and the anode can be greater than 10 mm, greater than 35 mm, or greater than 90 mm. In other examples, the ribbons are coated with nano-particulates of a metal and a binder; the cathode and/or the anode are made at least partly from nickel; the cathode and/or the anode are made at least partly from titanium; or the cathode and/or the anode are made all least partly from manganese or cobalt oxides. In further examples, the cathode is located in a cathode compartment and the anode is located in an anode compartment, and the cathode compartment and the anode compartment are physically separated.
Preferably, but not exclusively, the cell can be configured and operated in a manner that maximises the energy and cost savings that can be achieved. Alternatively, the cell can preferably, but not exclusively, be configured and operated in a manner that achieves some energy and cost savings. Alternatively, the cell can preferably, but not exclusively, be configured and operated in a manner that is suitable in respect of the energy and cost savings that can be achieved. Preferably, but not exclusively, the separation of the anode(s) and cathode(s) is limited to the minimum required for a reliable and complete separation of the products in more than 99.99% purity each.
In the representative case of a water electrolyser, where the products are streams of hydrogen or oxygen bubbles, the anode(s) and cathode(s) can preferably, but not exclusively, be separated by more than 10 mm. In an alternative embodiment, the anode(s) and cathode(s) can preferably but not exclusively, be separated by more than 35 mm. In a still further alternative embodiment, the anode(s) and cathode(s) can preferably, but not exclusively, be separated by more than 90 mm. Preferably, but not exclusively, the bubble streams from each of the anode and cathode can be collected in separate compartments within the cell, within which the gas babbles can be allowed to coalesce to form a bulk gas phase that will then be collected, dried and stored.
Preferably, but not exclusively, low-cost. Earth-abundant catalysts and conductors can be used at the anode(s) and cathode(s). For example, in the example case of a water electrolysis cell, cheap, Earth-abundant materials, like manganese or cobalt oxides can be used for the anode catalyst and nickel used for the cathode catalyst. Preferably, but not exclusively, the cell can be fabricated out of low-cost materials. For example, in the case of a water electrolysis cell, the cell may be fabricated out of low-cost polymeric materials which may be manufactured using low-cost manufacturing techniques, such as injection moulding or extrusion.
In one, non-limiting example embodiment, the cell is a water electrolyser of a tank design, containing near its base, at least one water inlet regulated by a suitable valve, and containing near its top, at least two gas outlets regulated by suitable gas valves. In this example embodiment, the tank is separated at a defined height, for example about two thirds of the way up, into two physically-distinct compartments, each of which acts as the gas collection receptacle for gas bubbles from either the cathode or the anode electrodes of the electrolyser, respectively. Each compartment is open at its base to the electrolyte in the cell. Each compartment contains near its top, a gas outlet regulated by a suitable gas valve. Directly below each compartment, are located closely-packed arrays of electrically-connected, conductive sheets or foils, coated with suitable, inexpensive catalysts that serve as either the anodes or the cathodes of the cell (depending on which compartment they lie beneath). The purpose of having in each array, large numbers of very closely packed conductive sheets or foils coated with suitable catalysts, is to maximize the surface area of that particular electrode at the lowest possible cost.
Preferably but not exclusively, the cathode and anode arrays are, at their closest, physically separated by about 35 mm, or greater, this being the minimum separation that ensures that bubble streams created at each of the arrays only rise into the compartments for which they are designated.
Preferably, but not exclusively, the electrodes have a large surface area and, even more preferably, the electrodes have some degree of porosity so that the electrolyte is made to pass through the electrodes (to thereby ensure near to complete reaction of a chemical species at the electrode before the electrolyte is circulated back to the other electrode).
In particular examples, the cell has a relatively small volume of electrolyte, giving rise to a relatively low ratio of electrolyte volume to electrode geometric surface area. For example, the ratio of electrolyte volume to electrode surface area, expressed in fractional notation as electrolyte volume (m³)/electrode surface area (m²), is less than or about 0.1 m (or 100 mm). In one example, the ratio is less than or about 0.01 m (or 10 mm). In another example, the ratio is less than or about 0.001 m (or 1 mm). In another example, the ratio is less than or about 0.0001 m (or 1.00 μm). In another example, the ratio is less than or about 0.00001 m (or 10 μm). In another example, the ratio is less than or about 0.000001 m (or 1 μm). In another example, the ratio is less than or about 0.0000001 m (or 0.1 μm). In another example, the ratio is less than or about 0.00000001 0m (or 0.01 μm). In another example, the ratio is less than or about 0.000000001 m (or 0.001 μm)
In other examples, the ratio of electrolyte volume to electrode surface area, expressed in fractional notation as electrolyte volume (m³)/electrode surface area (m²), is in the range of, inclusively, from about 0.001 μm to about 0.1 m, or from about 0.001 μm to about 0.01 m, or from about 0.001 μm to about 1 mm, or from about 0.001 μm to about 100 μm, or from about 0.001 μm to about 10 μm, or from about 0.001 μm to about 1 μm, or from about 0.001 μm to about 0.1 μm, or from about 0.001 μm to about 0.01 μm.
Preferably, there is no ion-conductive electrolyte-impermeable barrier between the electrodes/electrode arrays, either fully or in part. Optionally, but not essentially, there may be an electrolyte-permeable separator located, either fully or in part, between the electrodes/electrode arrays. An example of such an electrolyte-permeable separator includes a polymer or natural fabric that allows free transport of the electrolyte through the electrolyte-permeable separator.
In one example, the electrode arrays are configured to not completely fill the compartment above them, but to leave a headspace near the top of the chamber for the collection of gases. Preferably, the tank and the supports for the conductive sheets or foils in each array, are made of durable, economic polymers. The polymers may, optionally, be transparent.
Optionally, there may be a water circulation system in the tank that is configured to separately sweep the bubble streams off each of the cathode and anode arrays in such a way as to ensure that the bubbles end up in their correct, designated compartment. In such a case, the minimum separation between the anode and cathode arrays may be smaller than 35 mm.
Optionally, the anode array and the cathode array may be in separate tanks, connected by suitable piping such that there may be additional means to facilitate the removal of gaseous reaction product from the electrolyte in one tank before the electrolyte is circulated back to the following tank. Such additional means includes the use of reduced pressure, use of a media to facilitate the formation of gas bubbles, or the use of a gas-liquid contactor.
Other configurations, involving other reactions, can be provided without an electrolyte-impermeable barrier. For example, flat-sheet, plate and frame configurations involving circulating electrolyte that separately sweeps bubbles off multiple, distinct cathode or anode electrodes and directs them into channels that are exclusively plumbed for hydrogen or oxygen bubble stream collection respectively.
In a particular example, the anode and/or cathode could be constructed according to the electrode examples discussed in International Patent Publication No. WO 2012/075546 for “Multi-Layer Water-Splitting Devices”, the disclosures of which are incorporated herein.
The following examples provide a more detailed discussion of particular embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention.

Example 1

A Tank Electrolyser Without an Electrolyte-Impermeable Barrier Which Separates Gaseous Products

FIG. 1(a) schematically depicts an example tank electrolyser 5 without an electrolyte-impermeable barrier. The electrolyser 5 includes a polymer tank containing a water inlet 10 near the base of electrolyser 5, and two gas outlets—a first gas outlet 20 (for hydrogen collection) and a second gas outlet 3 (for oxygen collection) at the top of the electrolyser 5. The gas outlets 20, 30 sit atop a cathode compartment 200 and an anode compartment 300, respectively. For a top region, for example the top one third of the tank electrolyser 5, the compartments 200, 300 are separated by a solid polymer wall 40, as shown in FIG. 1. Note that, in this example, the wall 40 does not extend to any point that is directly between the anode and cathode.
Immediately below the cathode compartment 200 is located the cathode electrode array 50. Immediately below the anode compartment 300 is located the anode electrode array 60. The arrays 50, 60 are placed so that gas bubbles emanating from them rise into the respective compartment 200, 300 immediately above the electrode array 50, 60 and not into the neighbouring compartment.
Each of the anode and cathode arrays 50, 60 include a series of closely-spaced ribbons of thin metallic foil. The foil is typically titanium or nickel, which is optimally around 0.025 mm thick. The ribbons are typically coated with nano-particulate metal, such as nickel, and a binder, for example a polymer such as a fluoropolymer-copolymer (e.g. Nafion™) (typically 5% of the coating by weight). The ribbons are physically and electrically attached at one or both ends to a metallic, 3D mounting bracket comprising numerous thin arms. The metal of the mounting bracket may be titanium, nickel, or metal-coated stainless steel. Various types of mounting brackets may be used. For example, the mounting bracket may be a series of closely-spaced, parallel, thin, rails from which the ribbons are made to hang in haphazard arrangements (much like clothing on hangers may hang from the racks of a clothing store). The spacing between ribbons may fall in the range of about 1 mm to about 20 mm. Ideally, but not necessarily, the ribbons are able to move during the formation, release and buoyant rise of generated gas bubbles, to thereby ensure that bubbles do not become blocked in the narrow spaces between the ribbons.
The minimum spacing between the cathode and anode arrays 50, 60 is preferably about 35 mm at their nearest separation, this being as close as they can be located in this configuration without bubbles of hydrogen ending up in the anode compartment and bubbles of oxygen in the cathode compartment.
Each mounting bracket and thereby also all of the ribbons which are attached in each electrode array 50, 60, are electrically connected to an external terminal. The cathode array 50 is attached to the external electrical terminal 500. The anode array 60 is attached to the external electrical terminal 600. The cathode and the anode each extend in a substantially vertical direction. The cathode and the anode are also substantially parallel to each other.
In order to operate the electrolyser 5, the tank is filled from water inlet 10 with an electrolyte solution. The tank is filled up to the fill-line 70. The electrolyte solution can be 6 M KOH in the case of an alkaline electrolyser, where the electrode arrays 50, 60 comprise of nickel strips. Alternatively, the electrolyte solution may be a strongly acidic electrolyte in the case of an acid electrolyser, where the electrode arrays 50, 60 comprise of titanium or stainless steel strips.
A direct electrical current is now applied over the external terminals 500 and 600. A voltage of 1.8 V would typically be applied such that a low current density of between, inclusively, from about 2 to about 20 mA/cm²is achieved. The ratio of electrolyte volume to electrode surface area is, in this example, less than 0.027 m. The cell can also be operated at other low current density values, for example less than or about 1000 mA/cm², less than or about 500 mA/cm², less than or about 250 mA/cm², less than or about 70 mA/cm², less than or about 20 mA/cm², or less than or about 10 mA/cm².
As a result of the applied electrical current, bubble streams of hydrogen rise from the cathode array 50, into exclusively, the cathode compartment 200. In the cathode compartment 200, the bubbles coalesce and pure hydrogen gas is collected at the gas outlet or valve 20. At the same time, bubble streams of oxygen rise from the anode array 60 into, exclusively, the anode compartment 300. In the anode compartment 300, the bubbles coalesce and pure oxygen gas is collected at the gas outlet or valve 30.
During operation, hydronium ions (H⁺; protons) freed by the oxidation of water molecules migrate from the anode array 60 to the cathode array 50. This migration is unimpeded in any way by the presence of any sort of ion-permeable and electrolyte-impermeable barrier (e.g. diaphragm) between the anode array 60 and the cathode array 50. Electrons released from the oxidation of water molecules on the catalytic surface of the anode array 60 travel through the external electrical circuit to the cathode array 50.
Thus there is provided an electrochemical cell 5, comprising a cathode 50, an anode 60 and an electrolyte, without an electrolyte-impermeable barrier positioned between the cathode 50 and the anode 60.
Hence, in this example embodiment, the tank electroyser operates as follows.

- Using the water valve(s), the tank is fed with water containing suitable ion-conductive electrolyte up to the level of the headspace in each chamber. A sensor may be used to detect and maintain the water level in the electrolyser.
- The water in the tank fills the spaces between the closely packed conductive sheets or foils coated with catalysts that make up each of the anode and/or cathode arrays.
- A suitable current is passed through the anode and cathode arrays, such that, while the overall current may be large, only a relatively small current density is created at any one point on the conductive sheets or foils coated with catalysts that make up the arrays.
- The dissimilar gases thereby generated at each of the arrays (hydrogen at the cathode array and oxygen at the anode array), form bubbles that rise in streams between the closely packed sheets or foils coated with catalyst, to thereby fill the headspace directly above the water in each compartment.
- The gas collected in the anode compartment will then be pure oxygen, while the gas collected in the cathode compartment will then be pure hydrogen.
- The accumulated gas bubbles in the headspaces atop each array are separately allowed to coalesce and the pure gases are collected by being drawn through the gas valves in each compartment in the tank.

FIG. 1(b) illustrates an alternative embodiment of the electrolyser in FIG. 1(a). The only difference with FIG. 1(a) is that an electrolyte-permeable separator 45 is present between the anode and the cathode in FIG. 1(b). The electrolyte-permeable separator 45 may be a fine metal mesh (eg. a 150 LPI stainless steel mesh), a porous plastic sheet, or a fine polymer net or fabric (e.g. a polypropylene mesh fabric). The electrolyte-permeable separator 45 is porous and readily allows transport of the electrolyte through its thickness. In so doing, the electrolyte-permeable separator 45 does not impede electrolyte movement or block the movement of ions from the anode to the cathode, or vice versa. However, the presence of the electrolyte-permeable separator 45 acts to diminish turbulence in the liquid electrolyte. In so doing, the electrolyte-permeable separator 45 helps ensure that the bubbles of gas from each of the anode and cathode rise correctly into their respective collection areas or collection chambers. The tank electrolyser 5, providing an electrochemical cell, can be used an electro-synthetic cell (i.e. a commercial cell having industrial application) or an electro-energy cell (e.g. a fuel cell). The tank electrolyser 5 utilizes abiological manufactured components.

Example 2

A Tank Electrolyser Without an Electrolyte-Impermeable Barrier in Which Circulating Electrolyte is Used to Separately Collect Gaseous Products

FIG. 2 depicts in a schematic form, another example tank electrolyser 15 of similar design to the electrolyser 5 shown in FIG. 1, except that the cathode and anode compartments have been physically separated into two distinct chambers—an anode chamber 310 (i.e. anode compartment) and a cathode chamber 210 (i.e. cathode compartment). The cathode array 50 has also been moved to the front of its chamber, while the anode array 60 has been moved to the rear of its chamber. The two chambers are connected by passages, pipes, conduits or channels which allow the liquid electrolyte to circulate from one chamber to the other—via a first or front pipe 80 and a second or rear pipe 90.
The operation of the electrolyser 15 shown in FIG. 2 differs from that in Example 1 only in that the electrolyte is pumped between the two chambers 210, 310 in such a way that the electrolyte circulates from the anode chamber 310 along pipe 80 to the cathode chamber 210, and then back again along pipe 90. In so doing, the electrolyte sweeps hydronium ions (H⁺; protons) that are generated at the anode array 60, to the cathode array 50, thereby facilitating and improving the necessary ion-conduction between the electrodes 50, 60. Indeed, if the anode array 60 and cathode array 50 were located the same distance apart in a single tank filled with electrolyte, then the rate of ion migration between them would be slower than the case where the pump was running and driving the electrolyte through pipes 80 and 90. That is, all else being equal, pipes 80 and 90 represent the shortest pathway for ion migration by the protons between the anode array 60 and cathode array 50 when the pump is running. One effect of the circulating electrolyte is, arguably, to facilitate and speed up ion transport between the electrodes.
That sweeping motion of the circulating electrolyte also acts to facilitate bubble formation and dislodgement at each of the anode array 60 and the cathode array 50. Because these arrays are relocated in their respective chambers toward the inlet for the circulating electrolyte and away from the outlet for the circulating electrolyte, any bubbles swept off each array have no option but to rise into the collection area directly above their respective electrode array. That is, the action of pumping the circulating electrolyte around the cell acts to direct or release the bubbles into their correct collection area, thereby facilitating complete separation of the gases. This is done without need for an electrolyte-impermeable barrier between the electrodes. Indeed, the shortest pathway for ion-conduction between the electrodes when the pump is running, along pipe 80 or 90, is entirely free of any electrolyte-impermeable barrier—that is, there is no electrolyte-impermeable barrier present or required.
Thus there is provided an electrochemical cell 15 comprising a cathode 50 located in a cathode compartment 210 and an anode 60 located in a physically separated anode compartment 310, and at least two fluid passages 80, 90 allowing an electrolyte to flow between the cathode compartment 210 and the anode compartment 310.
The schematic illustration of the electrolyser 15 in FIG. 2 is intended to illustrate how circulating electrolyte may be harnessed and directed with the intention of eliminating the need for an electrolyte-impermeable barrier in a device like an electrolyser. As such, the separation that may be needed between the anode chamber 310 and the cathode chamber 210 is exaggerated and is not to scale. This has been done purely for the purpose of illustration. In fact, with the assistance of carefully directed circulating electrolyte, it is possible to locate the cathode and anode arrays in very close proximity to each other.
In another example, the electrode arrays need not be located in physically distinct chambers. The electrode arrays could be positioned apart in an integrated single compartment or chamber that allows liquid electrolyte to flow or be pumped between or past the electrode arrays. For example the integrated single compartment or chamber could be torus or doughnut-shaped, and could have a variety of cross-sectional geometries such as circular, square or rectangular. The electrode arrays can be located to be diametrically opposite, and their respective gas collection areas, sections or chambers can be located above the electrode arrays. The electrolyte can be caused to flow in one direction around the integrated single compartment or chamber. A variation in cross-section may be provided at different locations about the integrated single compartment or chamber, for example in regions between the electrode arrays the cross-sectional area may be smaller.
In a still further example, electrolyte-permeable separators, such as fine metal meshes (e.g. a 150 LPI stainless steel mesh) or fine polymer nets or fabrics (e.g. a polypropylene mesh fabric) may be placed at the entrance to pipe 90 (in the cathode chamber) and/or at the entrance to pipe 80 (in the anode chamber). The electrolyte-permeable separators allow free movement of the circulating electrolyte through them, but act to facilitate the bubbles from each of the anode and cathode rising in their correct respective chambers for collection.

Example 3

An Electrochemical Cell Without an Electrolyte-Impermeable Barrier in Which a Continuous Flow of Electrolyte is Used to Separate and Collect Products in the Liquid Phase

In strongly alkaline (caustic) environments (e.g. 1 M NaOH), hydrogen peroxide may be manufactured electrochemically. The process uses a gas-diffusion electrode as the cathode and a conventional solid-state electrode as the anode. Oxygen is typically fed into the gas-diffusion cathode, thereby inducing the following half reactions when a suitable voltage and current are applied (with suitable peroxide-forming catalysts):


Cathode:	2O₂+ 2H₂O + 4 e⁻→2HO₂ ⁻ + 2OH⁻	. . . (1)
Anode:	4 OH⁻ → O₂+ 2 H₂O + 4 e⁻	. . . (2)
OVERALL:	O₂+ 2 OH⁻→ 2 HO₂ ⁻ E^o _cell0.476 V	. . . (3)

As can be seen, this overall reaction consumes base, OH,⁻, and oxygen, O₂, to produce the hydroperoxide ion, HO₂ ⁻, which is the natural form of hydrogen peroxide under basic conditions.
A critical feature of this electrochemical process is that the hydroperoxide ion thus formed, is not allowed to contact the anode. If it does contact the anode, then the anode half-reaction changes to that shown below;


Cathode:	O₂+ H₂O + 2 e⁻→HO₂ ⁻ + OH⁻	. . . (1)
Anode:	HO₂ ⁻ + OH⁻→ O₂(pure) + H₂O + 2 e⁻	. . . (4)
OVERALL:	O₂(air) → O₂(pure)	. . . (5)

That is, a suitable mechanism is needed in such a cell to keep the hydroperoxide ion formed at the cathode away from the anode, whilst still allowing OH⁻ ions formed at the cathode to migrate to the anode, where they are consumed.
In other words, if the hydroperoxide ion generated at the cathode migrates to the anode, the cell will effectively waste the applied electrical energy to simply convert oxygen pumped in at the cathode into oxygen generated at the anode (equation (5) above).
A solution to this problem is to separate the anodes and the cathodes as discrete arrays similar to those described in Example 1, with a continuous stream of 1 M KOH electrolyte pumped over and/or through one or both of the electrode arrays. As a result, the hydroperoxide ions generated at the cathode are swept away with the electrolyte and do not have the possibility of contacting the anode. The four equivalents of hydroxide ion (OH⁻) consumed at the anode in equation (2)) are provided by the continuous stream of 1 M NaOH, while the two equivalents of OH⁻ produced at the cathode (equation (1)) are swept away with the hydroperoxy ions to thereby replace two of the four equivalents consumed at the anode.
Thus, before being passed over the electrode arrays, the electrolyte solution contains only 1 M KOH. After having been swept over the electrode arrays, the electrolyte solution now also contains hydrogen peroxide. The resulting solution may, typically be used directly in a pulp and paper mill. That is, the electrolyte is not circulated. Instead, the electrolyte solution is manufactured as a 1 M NaOH solution, which is then treated by being passed through an electrochemical cell which imparts the electrolyte solution with high concentrations of hydrogen peroxide. The resulting solution is used directly for pulp and paper treatment.
This example therefore describes a situation in which a product in the liquid phase in an electrochemical cell is swept away from one electrode by a continuous stream of electrolyte to prevent the electrolyte from reaching the other electrode. For example, this may be achieved by appropriately positioning an inlet area and an outlet area for the electrolyte in the cell, such as near one of the electrodes.
The cathode array in this example is preferably a set of closely packed, high surface area gas-diffusion electrodes, while the anode array preferably comprises of conductive ribbons of the type described in Example 1. A cell voltage of 1.6 V is preferably applied, resulting in a low current density of from about 2 to about 10 mA/cm².
Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
Optional embodiments may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.
Although a preferred embodiment has been described in detail, it should be understood that many modifications, changes, substitutions or alterations will be apparent to those skilled in the art without departing from the scope of the present invention.

Claims

1. An electrochemical cell comprising,:

a liquid electrolyte;

a cathode, at least one cathode product able to be produced at the cathode; and

an anode, at least one anode product able to be produced at the anode;

wherein, the at least one anode product and the at least one cathode product are substantially separated, and without an electrolyte-impermeable barrier positioned between the cathode and the anode.

2. The cell of claim 1, where the cell is an electro-synthetic or an electro-energy cell.

3. The cell of claim 1 or 2, wherein the cell utilizes abiological manufactured components.

4. The cell of any one of claims 1 to 3, wherein the ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m³)/electrode surface area (m²)) is in the range of, inclusively, from about 0.001 μm to about 0.1 m.

5. The cell of any one of claims 1 to 3, wherein the ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m³)/electrode surface area (m²)) is less than or about 0.1 m (or 100 mm).

6. The cell of any one of claims 1 to 3, wherein the ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m³)/electrode surface area (m²)) is selected from the group of:

less than or about 0.01 m (or 10 mm):

less than or about 0.001 m (or 1 mm);

less than or about 0.0001 m (or 100 μm);

less than or about 0.00001 m (or 10 μm);

less than or about 0.000001 m (or 1 μm);

less than or about 0.0000001 m (or 0.1 μm);

less than or about 0.00000001 m (or 0.01 μm); and

less than or about 0.000000001 m (or 0.001 μm).

7. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about 1000 mA/cm².

8. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about 500 mA/cm².

9. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about. 250 mA/cm².

10. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about 70 mA/cm².

11. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about 20 mA/cm².

12. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of less than or about 10 mA/cm².

13. The cell of any one of claims 1 to 6, wherein in operation the cell has a low current density of between, inclusively, about 2 to about 20 mA/cm².

14. The cell of any one of claims 1 to 13, wherein an electrolyte-permeable separator is positioned at least partially between the cathode and the anode.

15. The cell of any one of claims 1 to 13, wherein an electrolyte-permeable separator is positioned on the shortest pathway for ion-conduction between the cathode and the anode.

16. The cell of any one of claims 1 to 15, wherein the electrolyte flows past the cathode or the anode.

17. The cell of claim 16, wherein the electrolyte exits the cell after flowing past the cathode or the anode.

18. The cell of any one of claims 1 to 16, wherein the electrolyte circulates between the cathode and the anode.

19. The cell of any one of claims 1 to 18, wherein the cathode and the anode are separated within the cell, and wherein the cathode product produced at the cathode and the anode product produced at the anode are directed to different collection areas.

20. The cell of any one of claims 1 to 19, wherein the cathode and the anode each extend in a substantially vertical direction.

21. The cell of any one of claims 1 to 20, wherein the cathode and the anode are substantially parallel to each other.

22. The cell of any one of claims 1 to 21, wherein the cathode and/or the anode are an array of electrodes.

23. The cell of any one of claims 1 to 22, wherein the cathode and/or the anode have some degree of porosity to enable electrolyte to pass through the cathode and/or the anode.

24. The cell of any one of claims 1 to 23, wherein the cathode and/or the anode are a series of ribbons of thin metallic foil.

25. The cell of claim 24, wherein the thin metallic foil is of the order of about 0.025 mm thick.

26. The cell of claim 24, wherein a spacing between the ribbons is in the range of about 1 mm to about 20 mm.

27. The cell of claim 24 or 26, wherein the ribbons are coated with nano-particulates of a metal and a binder.

28. The cell of any one of claims 1 to 27, wherein a spacing between the cathode and the anode is greater than 10 mm.

29. The cell of any one of claims 1 to 27, wherein a spacing between the cathode and the anode is greater than 35 mm.

30. The cell of any one of claims 1 to 27, wherein a spacing between the cathode and the anode is greater than 90 mm.

31. The cell of any one of claims 1 to 30, wherein the cathode and/or the anode are made at least partly from nickel.

32. The cell of any one of claims 1 to 30, wherein the cathode and/or the anode are made at least partly from titanium.

33. The cell of any one of claims 1 to 30, wherein the cathode and/or the anode are made at least partly from manganese or cobalt modes.

34. The cell of any one of claims 1 to 33, wherein the cathode is located in a cathode compartment and the anode is located in an anode compartment.

35. The cell of claim 34, wherein the cathode compartment and the anode compartment are physically separated.

36. The cell of claim 34 or 35, wherein the cathode compartment has an associated cathode product outlet or valve.

37. The cell of any one of claims 34 to 36, wherein the anode compartment has an associated anode product outlet or valve.

38. The cell of any one of claims 1 to 37, wherein the cell is a water electrolyser and the cathode product is hydrogen gas and the anode product is oxygen gas.

39. The cell of any one of claims 1 to 37, wherein the cell is used to manufacture hydrogen peroxide and the electrolyte flows past the cathode and sweeps hydroperoxide ions formed at the cathode so that the hydroperoxide ions exit the cell.

40. An electrochemical cell, comprising a cathode, an anode and an electrolyte, wherein the ratio of electrolyte volume to electrode geometric surface area of the cathode or the anode (electrolyte volume (m³)/electrode surface area (m²)) is in the range of, inclusively, from about 0.001 μm to about 0.1 m.

41. An electrochemical cell, comprising a cathode located in a cathode compartment and an anode located in a physically separated anode compartment, and at least two fluid passages allowing an electrolyte to flow between the cathode compartment and the anode compartment.