WO2008007108A1

WO2008007108A1 - Current distribution system for electrochemical cells

Info

Publication number: WO2008007108A1
Application number: PCT/GB2007/002629
Authority: WO
Inventors: Daniel Bentham
Original assignee: ITM Power Research Ltd
Current assignee: ITM Power Research Ltd
Priority date: 2006-07-12
Filing date: 2007-07-12
Publication date: 2008-01-17
Anticipated expiration: 2009-01-12
Also published as: GB0900434D0; GB2452465A; GB0613846D0

Abstract

An electrochemical half-cell comprises a non-conductive housing (3) having a fuel inlet and/or outlet and contains a membrane electrode assembly (4, 5) and an electrically conducting component (2, 6), wherein the electrically conducting component (2) is in contact with the membrane electrode assembly (4, 5), and wherein the housing (3) and the membrane electrode assembly (4, 5) define a chamber (1) for fuel.

Description

CURRENT DISTRIBUTION SYSTEM FOR ELECTROCHEMICAL CELLS

Field of the Invention

This invention relates to current distribution in electrochemical cells. Background of the Invention

Within an electrochemical cell the effective distribution of current is critical to its performance. For an electrolyser the volume of hydrogen produced is directly related to the current supplied to the three-phase contact interface between the catalyst material, ionic exchange membrane and the fuel. In order to produce large volumes of hydrogen for domestic and commercial use, significant electrical currents need to be employed.

Most conventional systems use a metal backplane or mesh with a carbon cloth, which is usually loaded with catalyst. The carbon cloth is in contact with the ion-exchange membrane. The backplanes and meshes are often stainless steel and can encompass a flow-field (i.e. they define the fuel chamber) as well as acting as the main current collector. Although this system has a widespread current distribution and can handle relatively large amounts of current, stainless steel is expensive, difficult to manufacture and heavy. Also, the carbon cloth can degrade over time reducing the performance of the cells. Therefore there is a need for a cheaper and preferably lighter system, which is able to deliver or receive large currents to or from the catalyst materials within an electrolyser or fuel cell respectively, whilst still maintaining a gas and watertight seal. Summary of the Invention The present invention is based on the realisation that by having two separate components, an electrically conducting component to act as the main current distributor/collector and a non-conducting material to define a fuel chamber, or to encompass a flow-field (which can be made of inexpensive nonconducting materials), the whole electrochemical cell will be lighter and cheaper and easier to manufacture as most of the cells can be injection-moulded using inexpensive plastics material. The cell can also deliver or receive large currents.

According to the present invention, an electrochemical half -cell comprises a non-conductive housing having a fuel inlet and/or outlet and containing a membrane electrode assembly and an electrically conducting component, wherein the electrically conductive component is in contact with the membrane electrode assembly, and wherein the housing and the membrane electrode assembly define a chamber for fuel. Description of the Drawings

The present invention will be illustrated with reference to the accompanying drawings.

Figures 1 and 2 each show a different schematic cross-sectional view of electrochemical half-cells embodying the present invention.

Figure 3 shows a schematic cross-section of an electrochemical cell embodying the invention. Figures 4 and 5 are schematic cross-sections of a multi-stack of electrochemical cells of the invention.

Figure 6A is a side view, and Figure 6B is a top view, of a star-shaped conducting component, suitable for use in the invention. Detailed Description of the Invention This invention is applicable to all electrochemical cells; electrolysers are used for illustration only.

The term "membrane electrode assembly", as used herein, is defined in the state of the art, and describes an assembly of an electrode, a catalyst and an ion exchange membrane. The catalyst and electrode may be collectively referred to herein, as a catalyst electrode.

The non-conductive housing and the membrane electrode assembly (MEA) together define the perimeter of the half-cell. Contained within the housing is an electrically conducting component. The, or each, conducting component is in contact with the catalyst in the MEA. The non-conductive housing may be constructed from ceramics, polypropylene or a glass-filled polyamide. The housing may contain gaps for a fuel inlet and/or outlet. Alternatively it may be porous or permeable in certain regions to allow entry/exit of reactant substances.

The non-conductive housing and MEA define a chamber for fuel. The fuel may be in direct contact with the housing or held within a non-conducting flow-field structure contained within the housing, to direct flow. This flow-field structure may be constructed from a non-metallic mesh or may be a porous plastic structure.

Figure 1 illustrates examples of the shape of the conducting components (electrical contacts) suitable for use in the present invention. Each cross-section shows a conducting component (2), a cell housing (3) and a fuel chamber (1). The conducting component (2) may be counter-sunk into a recess in the electrochemical half-cell and may consist of a variety of forms such as, but not limited to, a bar shape or a frame, which is annular, tetragonal, polyhedral or circular.

The conducting component (2) may be a point contact. However, to reduce the likelihood of a current "hot-spot" where it touches the catalyst, it is preferred that the conducting component is frame-shaped, or star-shaped. This increases the area of the conducting component that is in contact with the catalyst; the cell can therefore handle a larger current and will be more efficient.

Figure 6 illustrates a particularly preferred embodiment. In this embodiment, the conducting component (2) is star-shaped. Although the star may have any number of arms (preferably between 10 and 15), the shape illustrated has 12 arms, in which each of the arms extends radially outward into two separate end points. Preferably, the star-shape is also dished (as shown), meaning that only the end points of the star are in contact with a MEA (4,5). In a preferred embodiment, the dished, star-shaped conducting component (2) is malleable, and is essentially a spring electrical contact. This allows for a known and/or constant amount of contact pressure to be applied onto the MEA. This, combined with the high number of arms ensures good electrical contact, and therefore low electrical resistance.

There may be a plurality of conducting components. In a preferred embodiment there are nine conducting components, which are arranged in a 3x3 configuration. The conducting component (2) may be perforated. The conducting component (2) may be constructed from a number of materials, including but not limited to stainless steel, titanium alloys, nickel- chromium alloys and copper-nickel alloys, electrically conductive polymers and electrically conductive ceramics. This allows optimisation for the specific operational environments. Hence, an electrochemical cell may employ a single or multiple conducting component materials.

Figure 2 shows an electrochemical half-cell that is a preferred embodiment of the present invention. A catalyst electrode (5) is in physical contact with an ion exchange membrane (4), and lies over the conducting component (2). The conducting component (2) supplies the electrical current to the catalyst whilst providing good physical contact. In fact, the conducting component (2) may be used to secure the catalyst electrode (5) to the cell housing (3). The catalyst material may be flush with the half-cell, extend to allow a higher pressure to be put upon the membrane electrode assembly (4, 5), or recessed to allow a higher sealing pressure around the half-cell, depending on the membrane material properties.

As the fuel chamber (1) is defined by the cell housing (3) and the MEA (4, 5), there is good delivery of the reactants to the membrane electrode assembly (4, 5). The area of the fuel chamber (1) may be increased by perforating the conducting component (2), allowing the reactants to move through it. This allows the reactants to flow to the conducting component (2)/MEA (4, 5) interface, thereby increasing the effective active area of the cell. Flow of reactant substances may be directed or aided by having a flow-field structure in the fuel chamber (1). This may be in the form of a non-conducting porous structure within the housing. A non-conductive flow-field has advantages of reduced weight, reduced material costs and reduced manufacturing costs, compared to electrochemical cells with metal flow-field/electrode structures.

Figure 3 shows an electrochemical cell embodying the present invention. In this cell, the conducting components (2, 2a) in each half-cell are made from different materials. This may allow for optimisation for the specific operational environment.

In Figures 2 - 5, the conducting component extends through the housing, via a conducting member (6), bringing the current conduction point to the external face of the cell. The conducting component (2) and the conducting member (6) may be integral, or may be separate components, which connect together. The conducting member (6) may be an electrical connector. It may also be integrated with, or be connected to, an electrical plug or socket (not shown). This completes the current delivery, as well as aiding the retention of the conducting component (2). As the current conduction points are on the external face of the half -cell, multiple cells can be stacked back-to-back and supplied to run in electrical series (Figure 4), reducing the need for cables on the outside of the stack.

Figure 5 shows a multi-cell stack connected in parallel. For stacks that are required to run in parallel, rather than series, the conducting members (6) must not touch the adjoining half -cell. This may be achieved using the same half-cell module, but orienting the half-cells differently, and using a connection plate (7) to electrically connect the half-cells.

The conducting members (6) may either be continuous with each other between the two cell halves, or separate components that align when the cells are stacked. The conducting members (6) may be straight, or elbowed to allow access to the conducting components (2) from the external faces of the cells. This may allow a single cell to be by-passed, if necessary, in a multi-cell stack. The conductive members (6) may be, if required, connected by a push/fit mechanism into the conducting component (2). In a multi-cell stack, the housing (3) of one half-cell may be continuous with the housing (3) of an adjacent half-cell, i.e. they may be integral. This integral component may be a bipolar plate, i.e. one plate that is both the anode to one cell, and the cathode to an adjacent cell.

Such a plate may also be used in the series stack (Figure 4). In one embodiment, the plate is a switching device (physical or electrical) to bypass cells, either manually or automatically. In the case of an electrolyser, this may be allowed to occur if the voltage of the cell exceeds a predefined limit. In the case of a fuel cell, this may be allowed to occur if the voltage drops below a predefined limit. Alternatively, the switching devices may be located in the conducting members (6).

An advantage of a half-cell of the invention is that, as the cell-housing (3) is non-conducting, the whole cell can be injection-moulded using inexpensive plastics material. Further, if the cell includes a non-conductive flow-field (porous structure) in the fuel chamber (1), then the cell housing (3) can be injection- moulded with the integrated non-conducting flow-field.

The cell housing (3) may be injection-moulded with a conducting member (6) and/or conducting component (2) moulded into it. In a preferred embodiment, the conducting member (6) includes an electrical plug socket. Two conducting components (2) and/or two conducting members (6) may be moulded into either side of one piece of non-conductive housing, in order to be able to form two cell halves back-to-back. This single piece acts both as the cathode to one cell, and the anode to an adjacent cell.

A still further preferred embodiment uses a microcontroller device to monitor the operating characteristics of the cells in a stack. This device utilises a method of electrically removing cells from the stack if the monitored values pass outside predefined operating parameters, using either a transistor or relay-based system.

Claims

1. An electrochemical half-cell comprising a non-conductive housing having a fuel inlet and/or outlet and containing a membrane electrode assembly and an electrically conducting component, wherein the electrically conducting component is in contact with the membrane electrode assembly, and wherein the housing and the membrane electrode assembly define a chamber for fuel.

2. A half-cell according to claim 1 , wherein a surface of the conducting component that is in contact with the membrane electrode assembly is a bar shape.

3. A half-cell according to claim 1 , wherein a surface of the conducting component that is in contact with the membrane electrode assembly is circular.

4. A half-cell according to claim 1 , wherein a surface of the conducting component that is in contact with the membrane electrode assembly is annular, tetragonal or polyhedral.

5. A half-cell according to claim 1 , wherein the conducting component is a dished star-shape.

6. A half-cell according to any preceding claim, wherein the conducting component is perforated.

7. A half -cell according to any preceding claim, comprising, within said housing, a non-conducting porous structure.

8. A half-cell according to any preceding claim, wherein the conducting component extends through the housing.

9. An electrochemical cell comprising two electrochemical half-cells according to any preceding claim.

10. A stack of two or more electrochemical cells according to claim 9, connected in series.

11. A stack of two or more electrochemical cells according to claim 9, connected in parallel.

12. A stack according to claim 10 or claim 11 , wherein the housing of one half-cell and the housing of an adjacent half-cell, are integral, and form a bipolar plate, in use.

13. A stack according to any of claims 10 to 12, wherein the cells are electrically connected to each other by a switched control plate.