GB2508817A

GB2508817A - Fuel Cell System

Info

Publication number: GB2508817A
Application number: GB1222214.7A
Authority: GB
Inventors: Martin Thomas
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2012-12-11
Filing date: 2012-12-11
Publication date: 2014-06-18
Also published as: GB201222214D0

Abstract

5 A fuel cell system (100) comprises at least one fuel cell, each fuel cell comprising an electrolyte, and comprising two electrodes, one on either side of the electrolyte, one an anode and the other a cathode. The anode is next to a fuel gas chamber and the cathode is next to an oxidant gas chamber. The system (100) includes means (42) to supply a fuel gas, and means (50) to supply an oxidant gas such as air to the fuel 10 cell. A recirculation means (59, 72) recirculates an exhaust gas which has passed through the oxidant gas chamber back to the oxidant gas chamber. The recirculation assists in ensuring that the oxidant gas supplied to the fuel cell is humid. The electrolyte may be a liquid electrolyte, for example aqueous potassium hydroxide solution, and the system (100) may also provide means (62, 64) to supply a liquid 15 electrolyte to the fuel cell.

Description

Fuel Cell System The present invention relates to fuel cells, preferably but not exclusively alkaline fuel cells, and to a system that includes such fuel cells.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Fuel cells with an alkaline aqueous electrolyte are of particular interest because they operate at relatively low temperatures, are efficient and mechanically and electrochemically duiable. Fuel cells with an aqueous acidic electrolyte are also of interest. Such fuel cells typically comprise an electrolyte chamber separated from a fuel gas chamber (containing a fuel gas, typically hydrogen) and a further gas chamber (containing an oxidant gas, usually air). The electrolyte chamber is separated from the gas chambers using electrodes. Typical electrodes for alkaline fuel cells comprise a conductive metal mesh, typically nickel, that provides mechanical strength to the electrode, and a catalyst.

During operation, the oxygen electrode of a fuel cell is continuously supplied with an oxidant gas (usually air or oxygen), a proportion of which leaves the fuel cell.

The oxidant gas leaving the fuel cell carries away water in the form of moisture from the fuel cell. This can lead to drying out of the electrode which can affect the ionic conductivity (in the case of Polymer Electrolyte Membrane (PEM) fuel cells) of the membrane, can change the properties of a gas diffusion layer, can lead to the formation of crystals (potassium hydroxide and potassium carbonate in the case of alkaline fuel cells), or a change in the pH value within the gas diffusion layers and near the catalyst, at the electrolyte/gas interface (often referred to as the triple phase boundary). Depending on the type of fuel cell and the operating regime, it can therefore be desirable to reduce the evaporation rate inside the fuel cell. This can be achieved by increasing the humidity of the oxidant gas entering the fuel cell. An improved way of increasing the humidity of the oxidant gas stream without a significant consumption of energy would be desirable.

Discussion of the invention The present invention addresses or mitigates one or more problems of the prior art. In particular, provision of a humid oxidant gas stream is achieved or assisted by recirculating some of the oxidant gas that has passed through the fuel cell.

Accordingly the present invention provides a fuel cell system comprising at least one fuel cell, each fuel cell comprising an electrolyte, and two electrodes, one electrode on either side of the electrolyte, one electrode being an anode and the other electrode being a cathode, and the system comprising means to supply a fuel gas to a fuel gas chamber adjacent to the anode, and means to supply an oxidant gas to an oxidant gas chamber adjacent to the cathode; wherein the system comprises a recirculation means to recirculate at least part of the exhaust gas which has passed through the oxidant gas chamber back to the oxidant gas chamber.

Typically the oxidant gas is air, which is readily available from the environment. The fuel cell typically operates at an elevated temperature, for example above 50°C, and it is usually advantageous to be able to control both the humidity and the temperature of the oxidant gas. Although this may be achieved by separate devices such as membrane humidifiers, or by bubbling the oxidant gas through a tank of water or electrolyte at the operating temperature, this would impose an additional pressure drop and therefore energy would be consumed in causing the oxidant gas to flow through the tank. This also increases the complexity or cost of the system.

The electrolyte may be an aqueous solution, for example of potassium hydroxide; in this case the electrolyte would be within an electrolyte chamber, and may be caused to flow through the electrolyte chamber. The fuel gas may be hydrogen. The oxidant gas may be oxygen, but it is usually more convenient to use air. Recirculating the oxidant gas will reduce the proportion of oxygen in the gas stream supplied to the oxidant gas chamber, and will therefore decrease the power output from the fuel cell, or its efficiency. However, for moderate reductions of the oxygen level, the impact on fuel cell performance may be acceptable in certain applications.

The exhaust gas which has passed through the oxidant gas chamber is preferably recycled without any treatment to remove moisture from it. The recycled gas may however be treated to reduce or increase its temperature, if desired. The recirculation will require a pump, but the pressure drop in the oxidant gas passing through the fuel cell stack may be less than 5 kPa (50 mbar), for example 2 kPa (20 mbar), so the pump does not have to provide a high pressure to bring about recirculation.

Humidification of the oxidant stream is thus assisted by recirculating some of the oxidant gas by diverting it from the fuel cell oxidant exhaust to the fuel cell oxidant inlet. In addition to increasing the humidity of the oxidant stream entering the fuel cell, this recirculation may also raise the temperature of the oxidant stream at the inlet, which may be desirable during system start-up in order to reduce heat losses through the oxidant exhaust stream or during steady state operation in order to reduce thermal gradients.

In one example such a recirculation system may be controlled in response to measurement of humidity, the proportion of exhaust oxidant gas that is recirculated being adjusted to achieve a desired humidity of the gas supplied to the fuel cell.

Hence the system may include a sensor to monitor the humidity of the oxidant gas supplied to the fuel cell, and means to adjust the proportion of the recycled exhaust gas in accordance with the measured humidity.

A fuel cell system would typically include multiple fuel cells arranged as a fuel cell stack, in order to provide a larger output voltage and power. Typically all the cells are supplied or provided with the same electrolyte, fuel gas and oxidant gas. The fluids may flow in parallel through all the cells.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawing in which: Figure 1 shows a schematic flow diagram of a liquid electrolyte fuel cell system incorporating the invention.

A liquid electrolyte fuel cell comprises electrodes on either side of an electrolyte chamber. Each electrode may for example consist of mesh or perforated sheet of a metal such as nickel or ferritic stainless-steel. The metal sheet or mesh ensures electrical conductivity, and provides strength. This may be covered on one surface with a fluid-permeable layer which provides electrical conduction and catalytic properties. For example such a fluid-permeable layer may comprise carbon particles, fibres or nanotubes, which may be bonded together by a polymer binder, which may be hydrophobic to suppress passage of liquid through the fluid-permeable layer. Catalyst may be incorporated into such a fluid-permeable layer, or coated onto such a fluid-permeable layer. The electrode may alternatively include a first fluid-permeable layer without catalyst, and a second such fluid-permeable layer that does contain catalyst.

In any one fuel cell, one electrode is an anode and the other electrode is a cathode, which separate the electrolyte from gas chambers, one for fuel gas and the other for an oxidant gas. The anode may have the same structure as the cathode, although the catalytic material may be different. It will be appreciated that the term "anode" refers to the electrode at which electrochemical oxidation takes place, and is the negative electrode of the fuel cell; the term "cathode" refers to the electrode at which electrochemical reduction takes place, and is the positive electrode of the fuel cell. Byway of example the electrode used as the anode may incorporate 10% palladium or 10% palladium/platinum on activated carbon; and the electrode used as the cathode may incorporate activated carbon combined with a spinel MnCoO4.

Referring now to figure 1, this shows the flows of fluids to and from a fuel cell stack 10, within a fuel cell system 100. The fuel cell system 100 includes the fuel cell stack 10 (represented schematically), which uses aqueous potassium hydroxide as electrolyte 40, for example at a concentration of 6 moles/litre. The fuel cell stack 10 is supplied with hydrogen gas as fuel, air as oxidant, and electrolyte 40, and operates at an electrolyte temperature of about 65° or 70°C.

Hydrogen gas is supplied to the fuel cell stack 10 from a hydrogen storage cylinder 42 through a regulator 44 and a control valve 46, and an exhaust gas stream emerges through an exhaust gas outlet duct 48. Air is supplied by a blower 50, and any CO2 is removed by passing the air through a scrubber 52, and the air is also passed through a filter 54 before the air flows through a duct 55 to a humidification chamber 56, and then through a duct 57 to the fuel cell stack 10. Spent air emerges through an air outlet duct 58.

The electrolyte 40 is stored in an electrolyte storage tank 60 provided with a vent 61. A pump 62 circulates electrolyte from the storage tank 60 into a header tank 64 provided with a vent 65, the header tank 64 having an overflow pipe 66 so that electrolyte returns to the storage tank 60. This ensures that the level of electrolyte 40 in the header tank 64 is constant. The electrolyte 40 is supplied at constant pressure through a duct 67 to the fuel cell stack 10; and spent electrolyte returns to the storage tank 60 through a return duct 68. The storage tank 60 includes a heat exchanger 69 to remove excess heat.

Operation of the fuel cell stack 10 generates electricity, and also generates water by virtue of the chemical reactions that occur at the electrodes: hydrogen reacting with hydroxyl ions to form watei (and elections) at the anodes 25a, and oxygen reacting with water (and electrons) to form hydroxyl ions at the cathode 25b.

The humidification chamber 56 ensures the air is humid as it is supplied to the fuel cell stack 10, and indeed that the air is almost saturated with water vapour. In the system 100 this is achieved by bringing the air into contact with electrolyte 40.

Electrolyte from the storage tank 64 is ted through a line 70 to the top of the humidification chamber 56, within which the air flow is exposed to the electrolyte, and remaining electrolyte flows out of a return line 71 which returns the electrolyte to the storage tank 60 through the return duct 68.

In addition water evaporates in both anode and cathode gas chambers within the fuel cell stack 10, so both the exhaust gas stream in the outlet duct 48 and the spent air in the air outlet duct 58 contain water vapour. The rate of evaporation depends on the electrode surface area exposed to reactant gases, the flow rate of the reactant gases, and the operating temperature. It also depends on the partial pressure of water vapour in the anode and cathode gas chambers. The overall result would be a steady loss of water from the electrolyte 40; the loss of water can be prevented by condensing water vapour from the exhaust gas in the outlet duct 48 by providing a condenser 49. The condensed water may then be returned to the electrolyte storage tank 60.

Furthermore, in this fuel cell system 100, the air outlet duct 58 communicates via a three-way valve 59 to a pump 72. The valve 59 allows a proportion of the spent air to flow to a discharge vent 73. The remaining spent air is compressed by the pump 72 to the pressure at which the air is supplied, and is recycled into the air stream at a three-way junction 75 in the duct 55 upstream of the humidification chamber 56. By adjusting the three-way valve 59, the proportion of spent air which is recycled to the junction 75 to be combined with fresh air from the blower 50 can therefore be adjusted. It will be appreciated that the recycled spent air is already at the operating temperature of the fuel cell stack 10, and is substantially saturated with moisture at that temperature.

The fuel cell system 100 may also include a humidity sensor 76, which in this example is arranged to monitor the humidity of the air in the duct 55, but in a modification might be arranged to monitor the humidity of the air in the duct 57. The value of humidity as detected by the humidity sensor 76 may be used in controlling the position of the three-way valve 59.

The system 100 thus includes a recirculation path for at least part of the spent air, constituted by the three-way valve 59, the pump 72 and the junction 75.

Recirculating the spent air without removing moisture, and without significantly altering its temperature, helps to ensure that the air stream supplied to the fuel cell stack 10 is humid. It will be appreciated that the pump 72 must raise the pressure of the recirculated air by an amount equivalent to the pressure drop through the humidification chamber 56 and the fuel cell stack 10. So preferably the pressure drops across these components amount to no more than 5 kPa, and more preferably no more than 2 kPa. In some circumstances the air in the duct 55 may be sufficiently humid as a result of the recirculation of spent air, so that the humidification chamber 56 may be omitted. Where a humidification chamber 56 is provided, the recirculation of the spent air will increase the humidity of the air in the duct 55, which may enable satisfactory humidification of the air to be achieved using a simpler or smaller humidification chamber 56 than would be required without the recirculation; in particular it may be possible to use a humidification chamber 56 that produces a significantly smaller pressure drop than would be required without the recirculation.

It will be appreciated that recycling spent air in this manner will inevitably reduce the percentage of oxygen in the air stream supplied to the fuel cell stack 10.

This may reduce the power output of the fuel cell stack 10. It will be appreciated that normal air contains about 24% oxygen; recycling spent air in such a way that the oxygen concentration supplied to the fuel cell stack 10 is no less than 18%, preferably no less than 20%, will lead to only a small reduction in power, of about 5%. This may be acceptable, taking into account the simplification of the system 100.

In a modification, the fuel cell system 100 also includes a condenser (which may be equivalent to the condenser 49) in the duct between the three-way valve 59 and the vent 73. In another modification, the proportion of air in the air outlet duct 58 that is recirculated may be adjusted by adjusting the operation of the pump 72, rather than by adjusting the three-way valve 59.

Claims

Claims 1. A fuel cell system comprising at least one fuel cell, each fuel cell comprising an electrolyte, and comprising two electrodes, one electrode on either side of the electrolyte, one electrode being an anode and the other electrode being a cathode, means to supply a fuel gas to a fuel gas chamber adjacent to the anode, and means to supply an oxidant gas to an oxidant gas chamber adjacent to the cathode; wherein the system comprises a recirculation means to recirculate at least part of the exhaust gas which has passed through the oxidant gas chamber back to the oxidant gas chamber.
2. A fuel cell system as claimed in claim 1 wherein theoxidant gas is air.
3. A fuel cell system as claimed in claim 1 or claim 2 wherein the exhaust gas recirculation means does not comprise means to remove moisture, and does not comprise means to reduce its temperature.
4. A fuel cell system as claimed in any one of the preceding claims, wherein the flow path of the air through the fuel cell stack is such that in operation the oxidant gas experiences a pressure decrease of less than 5 kPa (50 mbar).
5. A fuel cell system as claimed in any one of the preceding claims, also comprising means to adjust the proportion of the exhaust gas from the oxidant gas chamber that is recycled.
6. A fuel cell system as claimed in claim 5, also comprising a sensor to measure the humidity of oxidant gas supplied to the fuel cell, wherein the proportion-adjusting means are controlled in accordance with the measured humidity.
7. A liquid electrolyte fuel cell system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.