GB2461390A

GB2461390A - Composite Electrochemical Cell

Info

Publication number: GB2461390A
Application number: GB0911335A
Authority: GB
Inventors: Kristian Hyde; Rachel Louise Smith; Simon Bourne
Original assignee: ITM Power Ltd
Current assignee: ITM Power Ltd
Priority date: 2008-07-01
Filing date: 2009-06-30
Publication date: 2010-01-06
Anticipated expiration: 2029-06-30
Also published as: GB0911335D0; GB2461390B; GB0812017D0

Abstract

A membrane electrode assembly comprises an anode, a cathode and a polymer membrane, wherein the polymer membrane comprises a cationic electrolyte and an anionic electrolyte respectively on either side of a catalyst layer. The membrane electrode assembly is useful in a fuel cell and an electrolyser.

Description

Composite Electrochemical Cell

Field of the Invention

This invention relates to a membrane electrode assembly with controlled * water management/flow.

Background of the Invention

Many ionic polymer membranes used in electro-chemical cells are an electrolyte comprising only one active material. GB1463301 describes the use of composite SPE (solid polymer electrolyte) membrane materials in laminated layers, ideally with the join produced via an interpenetrated polymer system.

W003/023980 discloses hydrophilic polymers and their use in membrane electrode assemblies that can be used in or as electrochemical cells.

For liquid fuel cells (dual liquid or single liquid and gas), energy density is often a major limiting factor on the systems suitability for various applications.

For mobile applications, the weight and size of the entire system (cell and supply of fuel and oxidant) is critical. For example, military applications will require the system to support the electronic devices required for missions, and to be able to be carried by a soldier.

Current dual fuel cell systems use a standard cationic electrolyte Membrane Electrode Assembly (MEA) with dilute loops on fuel and oxidant; concentrated Fuel and Oxidant are fed into this. An example of a reaction occurring in such a cell is: Na+ NaBH4 + 8NaOH + 6H20 + NaBO2 + 8Na + 8e-> HO2 + 2Na+ + 2NaOH This system presents a number of problems to be solved. Firstly, the requirement to carry eight times as many moles of NaOH as NaBH4 leads to a heavier system and lowers the fuel density. Also, the reaction creates NaOH at the cathode. The reaction also creates 0.75 moles of water on the anode for every Na+ that goes through the membrane.

Electro-osmotic drag moves large volumes of water from the anode to the cathode, creating an imbalance in the solutions. Electro-osmotic drag also drags across reactants from the anode to the cathode, reducing energy efficiency and promoting gas evolution. If the gases are not utilised they must be removed from the system before a build of significant pressure occurs. This removal can be problematic, especially in military applications.

Electrolysers also suffer from osmotic-drag. Electro-osmotic drag occurs in all SPE cells. It is a process by which water is transported thought the membrane in the direction of ion transport. The degree of drag is directly related to the operational current of the electrolyser, the temperature of the system and the chemistry of the membrane.

Current electrolyser systems use a cationic electrolyte (CE) MEA with water circulating on both sides of the membrane. The balance of plant needs to ensure good flow of water on both sides of the system, to ensure separation of gases from liquids and provide cooling.

For gaseous fuel cell systems water is produced at the cathode (in the case of a Cationic Exchange (CE)) which makes this prone to flooding, the anode is prone to drying. This is further compounded by osmotic drag.

Summary of the Invention

The invention is based on the realisation that providing a catalyst layer or region inside a composite membrane may offer significant advantages to water management, and fuel and oxidant energy densities. Specifically, when used in dual fuel liquid fuel cell, hydration of the membrane is maintained, and energy densities improve. When used in an electrolyser, dry H2 and 02 are produced and the membrane is cooled.

According to a first aspect, the present invention is a membrane electrode assembly comprising an anode, cathode and polymer membrane, wherein the polymer membrane comprises a cationic electrolyte and an anionic electrolyte respectively on either side of a catalyst layer.

According to second and third aspects, the present invention is a fuel cell or an electrolyser comprising an assembly as defined above.

According to a fourth aspect, the present invention is a method of generating energy, comprising supplying water to the said catalyst layer of a fuel cell as defined above, supplying water to the catalyst layer of a fuel cell according to claim 3, supplying fuel to one of the electrodes, and supplying oxidant to the other electrode.

According to a fifth aspect, the present invention is a method of electrolysing water, comprising supplying water to the said catalyst layer, of an electrolyser as defined above, and electrolysing the water.

Description of Preferred Embodiments

An MEA of the invention can be used in a dual liquid fuel cell. The use of a composite membrane electrode assembly (cMEA) will reduce the problems associated with conventional cell, described above. In a preferred embodiment, a membrane is made from a laminated Anionic Electrolyte (AE) and cationic electrolyte (CE), with the provision of a catalyst layer (for example, platinum) at the junction between them. This enables water to be split at the catalyst layer into OH-and H+ ions, which can travel through the respective part of the composite membrane. Preferably, the cell is set up to have the AE at the cathode and the CE at the anode. This means that the OH-ion will travel to the anode, and the H+ will travel to the cathode, where it will produce water rather than NaOH. The reactions occurring in the above cell are: NaBH4 + 8OH-+ 6H20 + NaBO2 + 8e-H202+ 2H' + 2H20 The advantages of a fuel cell according to the invention are that energy densities are improved and that NaOH is removed from the system. Further, water is created at both the anode and cathode, maintaining membrane hydration levels. The water can also back-flow into the junction region (i.e. the catalyst layer). The use of hydrophilic polymer materials will facilitate this.

Another advantage is that the flow of electro-osmotic drag ions is against the reactant cross over. Therefore, reactant cross over will be reduced.

Although the system requires additional catalyst to be provided at the junction the additional system cost is considered to be low in relation to the benefits provided.

An MEA of the invention can be used in an electrolyser. The use of a composite AE:CE system with water provided at the join region provides scope for an electrolyser with no requirement for water at the gas production sites.

This removes significant balance of plant, such as the gas separating vessels, which can be a significant expense if the electrolyser is working at high pressures.

In a preferred embodiment, water is supplied only to the catalyst layer between the CE and AE membranes, i.e. the anode and cathode are substantially dry. This means that dry gas is produced.

Additional benefits are the cooling effect provided by the gas evolution at the anode and cathode. Further cooling will also be provided from the water carried across the membrane by osmotic drag, evaporating from the membrane surfaces. This can be collected via a condensation spot in the system.

Water may be introduced via manifold channels provided within the composite, or held within the hydrophilic polymer material itself. The polymers detailed in EP1428284 are sufficiently hydrophilic to contain significant amounts of water in the hydrated polymer material.

Claims

CLAIMS1. A membrane electrode assembly comprising an anode, a cathode and polymer membrane, wherein the polymer membrane comprises a cationic electrolyte and an anionic electrolyte respectively on either side of a catalyst layer.
2. An assembly according to claim 1, wherein the anionic electrolyte is adjacent to the cathode, and the cationic electrolyte is adjacent to the anode.
3. A fuel cell comprising an assembly according to claim I or claim 2.
4. An electrolyser comprising an assembly according to claim 1 or claim 2.
5. A method of generating energy, comprising supplying water to the said catalyst layer of a fuel cell according to claim 3, supplying fuel to one of the electrodes, and supplying oxidant to the other electrode.
6. A method according to claim 5, wherein the fuel is supplied to the anode, which is adjacent to the cationic electrolyte, and the oxidant is supplied to the said cathode, which is adjacent to the anionic electrolyte.
7. A method according to claim 5 or claim 6, wherein the fuel is sodium borohydride and the oxidant is hydrogen peroxide.
8. A method of electrolysing water, comprising supplying water to the said catalyst layer, of an electrolyser according to claim 4, and electrolysing the water.
9. A method according to claim 8, wherein the water is supplied to the said catalyst layer only, such that the anode and cathode are substantially dry.