GB2508649A - Fuel Cell System - Google Patents

Abstract

A liquid electrolyte fuel cell system (100) comprising at least one fuel cell, each fuel cell comprising means to define an electrolyte chamber (18), and comprising two electrodes (25), one on either side of the electrolyte chamber (18), one an anode (25a) and the other a cathode (25b). The anode is next to a fuel gas chamber (17) and the cathode is next to an oxidant gas chamber (19). The system (100) includes means (62, 64) to supply a liquid electrolyte, means (42) to supply a fuel gas, and means (50) to supply an oxidant gas to the fuel cell. A recirculation means (48, 74) recirculates an exhaust gas which has passed through the fuel gas chamber (17) back to the fuel gas chamber (17), passing through a first heat exchanger (70) to cool the exhaust gas and to separate condensed water, and then a second heat exchanger (72) to reheat the recirculated gas. The cooling may utilise the oxidant gas being supplied to the fuel cell, while the heating may utilise electrolyte (68) that has passed through the fuel cell. The recirculation can ensure that the fuel gas is dry when supplied to the fuel cell.

Description

Fuel Cell System The present invention relates to liquid electrolyte 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. Alkaline fuel cells are of particular interest because they operate at relatively low temperatures, are efficient and mechanically and electrochemically durable. Acid fuel cells and fuel cells employing other liquid electrolytes 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.

In a fuel cell in which the fuel is hydrogen, water is formed at the hydrogen electrode, which is the anode, while water reacts at the oxygen electrode, which is the cathode. The net result is the production of water. Some of the water evaporates at one or other of the electrodes, while some of the water combines with the aqueous electrolyte. Ensuring water balance is important to satisfactory operation of the fuel cell. The operation of the cell also generates heat, which must be dissipated.

Discussion of the invention The present invention addresses or mitigates one or more problems of the

prior art.

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

The recirculation means may include a first heat exchanger to cool the exhaust gas before it is recirculated to the fuel gas chamber. The recirculation means may also include a liquid separation unit to separate any water that condenses from the exhaust gas.

The first heat exchanger may enable heat to be exchanged between the exhaust gas from the fuel gas chamber, and oxidant gas being supplied to the oxidant gas chamber. Alternatively the first heat exchanger may enable heat to be exchanged between the exhaust gas from the fuel gas chamber and another fluid at a lower temperature, for example a refrigerant.

The recirculation means may also include a second heat exchanger to heat the exhaust gas after its passage through the liquid separation unit. The second heat exchanger may also be used to heat fuel gas supplied to the fuel gas chamber. The second heat exchanger may be arranged to enable heat to be exchanged between the recirculated exhaust gas and liquid electrolyte that has flowed through the electrolyte chamber.

The first heat exchanger, the liquid separation unit, and the second heat exchanger together ensure that the recirculated exhaust gas is of low humidity when it is returned to the fuel cell, because water vapour has been separated from it in the liquid separation unit, and the recirculated exhaust gas has then been reheated to a temperature close to that within the fuel cell. This reduces the risk of any water vapour condensing from the gases supplied to the fuel gas chamber.

It will be appreciated that heat generated within the fuel cell or fuel cells during operation of the fuel cell system is primarily transferred to the electrolyte.

Preferably the fuel cell system includes an electrolyte storage tank, and a heat exchanger to extract heat from the electrolyte in the storage tank.

A fuel cell system would typically include multiple fuel cells arranged as a fuel cell stack, in order to provide a larger output voltage. Typically all the cells are supplied with the same electrolyte, fuel gas and oxidant; these 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 diagram of fuel cells in a fuel cell stack; and Figure 2 shows a flow diagram of a fuel stack 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. By way 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.

CELL STACK STRUCTURE

Referring now to figure 1, there is shown a cross-sectional view through the structural components of a cell stack 10 with the components separated for clarity.

The stack 10 consists of a stack of moulded plastic plates 12 and 16 arranged alternately. Each plate 12 defines a generally rectangular through-aperture surrounded by a frame 14; the apertures provide electrolyte chambers 18, and immediately surrounding the electrolyte chamber 18 is a 5mm wide portion 15 of the frame which projects 0.5 mm above the surface of the remaining part of the frame 14.

The plates 16 are bipolar plates; each defines rectangular blind recesses on opposite faces to act as gas chambers 17 and 19, each recess being about 3 mm deep, surrounded by a frame 20 generally similar to the frame 14, but in which there is a 5 mm wide shallow recess 21 of depth 1.0 mm surrounding each recess.

It will thus be appreciated that between one bipolar plate 16 and the next in the stack 10 (or between the last bipolar plate 16 and an end plate 23), there is an electrolyte chamber 18, with an anode 25a on one side and a cathode 25b on the opposite side; and there are gas chambers 17 and 19 at the opposite faces of the anode 25a and the cathode 25b respectively. These components constitute a single fuel cell.

Electrodes 25a and 25b locate in the shallow recesses 21 on opposite sides of each bipolar plate 16, with the catalyst-carrying face of the electrode 25a or 25b facing the adjacent electrolyte chamber 18. Before assembly of the stack components, the opposed surfaces of each frame 14 (including that of the raised portion 15) may be provided with a resilient sealing element 26. The components are then assembled as described, so that the raised portions 15 locate in the shallow recesses 21, securing the electrodes 25 in place. The sealing element 26 ensures that electrolyte in the chambers 18 cannot leak out, and that gases cannot leak in, around the edges of the electrodes 25a and 25b, and also ensures that gases cannot leak out between adjacent frames 14 and 20. The gas-permeable layer with the catalyst is on the face of the electrode 25 closest to the adjacent electrolyte chamber 18.

It will be appreciated that this cell stack 10 is shown by way of example only, and is a schematic view. For example the sealing element 26 may be arranged in a somewhat different fashion from that shown. Within the stack 10 several fuel cells are arranged so as to be electrically in series, to provide a greater voltage than is available from a single cell.

The flows of fluids to the fuel cells follow respective fluid flow ducts, at least some of which may be defined by aligned apertures through the plates 12 and 16.

Only one such set of apertures 27 and 28 is shown, which would be suitable for carrying electrolyte to or from the electrolyte chambers 18 via narrow transverse ducts 30. The flows of the gases to and from the gas chambers 17 and 19 may similarly be along ducts defined by aligned apertures through the plates 12 and 16. In a modification, the cell stack is arranged so the aligned apertures 27 and 28 are at the bottom of the cell stack, for supplying electrolyte; and electrolyte leaves the electrolyte chambers 18 through ducts (not shown), similar to the transverse ducts 30. but leading to the outer surface of the cell stack. Although the transverse ducts are shown as being within the plates 12 they may instead be defined by grooves at the surface of the plates 12.

At one end of the stack 10 is an end plate 23 which is a polar plate; it defines a recess for a gas chamber 19 on one face but is blank on the outer face. Outside this is an end plate 31, which also is moulded of polymeric material, and which defines apertures 32 which align with the apertures 27 and 28 in the plates 12 and 16; at the outside face the end plate 31 also defines ports 34 communicating with the apertures 32 and so with the fluid flow ducts through which the gases and electrolyte flow to or from the stack 10. At the other end of the stack 10 is another polar plate (not shown) which defines a blind recess for a gas chamber 17. There is then another end plate (not shown) which define no through apertures; alternatively it may define through apertures for one or more of oxidant gas, fuel gas and electrolyte.

After assembly of the stack 10 the components may be secured together for example using a strap 35 (shown partly broken away) around the entire stack 10.

Other means may also be used for securing the components, such as bolts.

FLUID FLOW SYSTEM

Referring now to figure 2, this shows the flows of fluids to and from the 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 a resulting exhaust gas stream emerges through a first 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 a filter 54 before the air flows through a duct 56 to the fuel cell stack 10, and spent air emerges through a second gas outlet duct 58. The duct 56 may include, as shown, a humidification chamber 57 to ensure the air is humid as it is supplied to the fuel cell stack 10; for example this may bring the air into contact with electrolyte 40 or with water.

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 water (and electrons) at the anodes 25a, and oxygen reacting with water (and electrons) to form hydroxyl ions at the cathode 25b.

Since the chemical reactions form water at the anodes, but remove water from the cathodes, it is desirable to ensure that the hydrogen stream is dry when it is supplied to the fuel cell stack 10, whereas the air stream is desirably humid when it is supplied to the fuel cell stack 10.

In addition water evaporates in both the anode and cathode gas chambers 17, l9so both the exhaust gas stream in the outlet duct 48 and the spent air in the 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 17, 19. 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 spent air in the outlet duct 58 (or from the exhaust gas), for example by providing a condenser 59. The condensed water may then be returned to the electrolyte storage tank 60.

Furthermore, in this fuel cell system 100, the first gas outlet duct 48 supplies the exhaust gas through a valve 49 to a heat exchanger 70. The valve 49 allows a proportion of the exhaust gas to flow to a discharge vent 71. The remaining exhaust gas flows to the heat exchanger 70 in which it exchanges heat with inflowing air in the duct 56, so cooling the exhaust gas stream and condensing water (which may be returned to the electrolyte storage tank 60). and at the same time warming the air supplied to the fuel cell stack 10.

The cooled and partly dehumidified exhaust gas stream that has passed through the heat exchanger 70 is then heated back to operating temperature by passing through a heat exchanger 72 in which it exchanges heat with outflowing electrolyte in the return duct 68. The exhaust gas stream is then pumped by a pump 74 to the pressure at which the hydrogen gas is supplied, and is recycled into the hydrogen stream at a three-way junction 75 upstream of the control valve 46.

It will be appreciated that the heat exchanger 72 may be either upstream or downstream of the pump 74. In a further modification, the heat exchanger 72 may instead be arranged downstream of the three-way junction 75, so that both the recycled exhaust gas and also the hydrogen gas from the storage cylinder 42 are heated by the outflowing electrolyte in the return duct 68, before being supplied to the fuel cell stack 10.

The system 100 thus includes a recirculation path for at least part of the exhaust gas, constituted by the heat exchanger 70, the heat exchanger 72 and the pump 74. Recirculating the exhaust gas, while removing moisture, ensures that the hydrogen stream supplied to the fuel cell stack 10 is dry.

Claims (8)

1. Claims 1. A liquid electrolyte fuel cell system comprising at least one fuel cell, each fuel cell comprising means to define an electrolyte chamber, and comprising two electrodes, one electrode on either side of the electrolyte chamber, one electrode being an anode and the other electrode being a cathode, the anode separating the electrolyte chamber from a fuel gas chamber and the cathode separating the electrolyte chamber from an oxidant gas chamber; and the system comprising means to circulate a liquid electrolyte through the electrolyte chamber, means to supply a fuel gas to the fuel gas chamber, and means to supply an oxidant gas to the oxidant gas chamber; wherein the system comprises a recirculation means to recirculate an exhaust gas which has passed through the fuel gas chamber back to the fuel gas chamber.
2. 2. A fuel cell system as claimed in claim 1 wherein the recirculation means includes a first heat exchanger to cool the exhaust gas before it is recirculated to the fuel gas chamber.
3. 3. A fuel cell system as claimed in claim 2 also comprising a liquid separation unit to separate any water that condenses from the exhaust gas.
4. 4. A fuel cell system as claimed in claim 2 or claim 3 wherein the first heat exchanger enables heat to be exchanged between the exhaust gas from the fuel gas chamber, and oxidant gas being supplied to the oxidant gas chamber.
5. 5. A fuel cell system as claimed in any one of claims 2 to 4 wherein the recirculation means also includes a second heat exchanger to heat the exhaust gas after its passage through the liquid separation unit.
6. 6. A fuel cell system as claimed in claim 5 wherein the second heat exchanger is also used to heat fuel gas supplied to the fuel gas chamber.
7. 7. A fuel cell system as claimed in claim 5 or claim 6 wherein the second heat exchanger is arranged to enable heat to be exchanged between the recirculated exhaust gas and liquid electrolyte that has flowed through the electrolyte chamber.
8. 8. A fuel cell system substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.