GB2534254A

GB2534254A - Operation of a fuel cell system

Info

Publication number: GB2534254A
Application number: GB1514976.8A
Authority: GB
Inventors: Akhtar Naveed
Original assignee: AFC Energy PLC
Current assignee: AFC Energy PLC
Priority date: 2014-08-28
Filing date: 2015-08-24
Publication date: 2016-07-20
Also published as: GB201415222D0; GB201514976D0

Abstract

A fuel cell system 10 includes a liquid electrolyte fuel cell stack 20 comprising at least one fuel cell (80, fig 2) consisting of a liquid electrolyte chamber (82, fig 2) between an anode (83, fig 2) and a cathode (84, fig 2), and gas chambers (85, 86; fig 2) adjacent to the electrodes, the electrodes separating the liquid electrolyte chamber from the gas chambers. In normal operation a fuel gas 22 is supplied to the anodic gas chamber (85, fig 2) and an oxidising gas 30 is supplied through a humidification chamber 52 to the cathodic gas chamber (86, fig 2). There is also a heater 49, 63 to raise the temperature of the fuel cell stack 20, which during normal operation would be off. However, if the supply of fuel gas or oxidising gas ceases, the method of operation comprises the steps of: (a) supplying a flow of inert gas 70 through the anodic gas chamber (85, fig 2); (b) continuing a flow of gas through the humidification chamber 52 to the cathodic gas chamber (86, fig 2), by supplying the inert gas if the supply of oxidising gas has ceased; and (c) energising the heater 49, 63 to maintain the temperature of the fuel cell stack 20. Two methods of heating the fuel cell stack 20 are provided. In a first method, the heater comprises an electrical heater 63 associated with the fuel cell stack 20, while in a second method, electrolyte 12 passing through the fuel cell stack 20 can be heated by a heat exchanger 49. The process may be automated using control means 77. This method allows the fuel cell temperature to be maintained during periods of inactivity, which allows a faster restarting of the fuel cell.

Description

Operation of a Fuel Cell System The present invention relates to liquid electrolyte fuel cell systems, preferably but not exclusively incorporating alkaline liquid-electrolyte fuel cells, and to methods of 5 operating such fuel systems, in particular when shutdown is required.

Background to the invention

Fuel cells have been identified as a relatively clean and efficient source of electrical power. Alkaline fuel cells with a liquid electrolyte 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, typically nickel, that provides mechanical strength to the electrode, and the electrode also incorporates a catalyst coating which may comprise activated carbon and a catalyst metal, typically platinum.

In operation, chemical reactions occur at each electrode, generating electricity. For example, if a fuel cell is provided with hydrogen gas and with air, supplied respectively to an anode chamber and to a cathode chamber, the reactions are as follows, at the anode: H2 + 2 OH- -> 2 H20 + 2 e-; and at the cathode: 1/2 02 + H20 + 2 e--> 2 oft so that the overall reaction is hydrogen plus oxygen giving water, but with simultaneous generation of electricity, and with diffusion of hydroxyl ions from the cathode to the anode through the electrolyte. It has been found that in practice there are a variety of detrimental effects that occur during operation in the vicinity of the electrodes, such as the potential formation of crystals, so that over a prolonged period the electrical output of a fuel cell or fuel cell stack gradually declines.

Discussion of the invention The fuel cell system of the present invention addresses or mitigates one or more

problems of the prior art.

According to the present invention there is provided a method of operating a fuel cell system that comprises an aqueous liquid electrolyte fuel cell stack comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and anodic and cathodic gas chambers adjacent to the anode and the cathode respectively, so the electrodes separate the liquid electrolyte chamber from the respective gas chambers; wherein during normal operation a fuel gas is supplied to the anodic gas chamber and an oxidising gas is supplied through a humidification chamber to the cathodic gas chamber; and the fuel cell system also comprises a heater to raise the temperature of the fuel cell stack; wherein if the supply of fuel gas or of oxidising gas ceases, the method of operation comprises the steps of: (a) supplying a flow of inert gas through the anodic gas chamber; (b) continuing a flow of gas through the cathodic gas chamber, by supplying the inert gas through the humidification chamber if the supply of oxidising gas has ceased; and (c) energising the heater to maintain the temperature of the fuel cell stack, wherein the heater: (i) is an electrical heater integral with components that form the fuel cell stack, or 25 incorporated within the fuel cell stack, or in thermal contact with the fuel cell stack; or (ii) is arranged to heat the aqueous liquid electrolyte, the heated electrolyte being supplied to the fuel cell stack.

This is particularly pertinent to an alkaline liquid electrolyte fuel cell system. The term "inert gas" in this context refers to a gas that does not undergo any chemical reactions when passed through the anodic or cathodic gas chambers. A suitable inert gas is nitrogen. It will be appreciated that the supply of fuel gas or of oxidising gas may cease as a consequence of a malfunction, or it may cease because an operator determines that a shutdown of the fuel cell system is required for another reason.

In the circumstances that the supply of fuel gas ceases, the output voltage from the fuel cell stack will become zero. Consequently the method of operation may also comprise disconnecting the fuel cell system from any load. If the normal method of operation involves circulation of the electrolyte through the liquid electrolyte chamber of each fuel cell, then the method of operation, when the supply of fuel gas ceases, may involve continuing to circulate the electrolyte through the liquid electrolyte chamber of each fuel cell, and in this case it is desirable to heat the electrolyte. The fuel cell stack may be provided with an electrical heater as specified above, to maintain its temperature, in addition to heating the electrolyte; or alternatively the heated electrolyte may maintain the temperature of the fuel cell stack, so no additional heating of the fuel cell stack is required.

In conventional systems, the fuel cell stack would be shut down if there is a malfunction so the supply of fuel gas or of oxidising gas ceases. This shutdown would be achieved by ceasing to pass gases through the gas chambers, ceasing to pass the electrolyte through the electrolyte chambers, and allowing the temperature to decrease. It has been found that if the fuel cell stack is shut down in this way, then when the fuel cell stack is started up again its performance is likely to be worse in two respects: the output voltage is likely to be slightly lower, and the deterioration in electrical output with time is likely to be worse than before. In contrast, the method of the invention suppresses both aspects of this deterioration in performance, and may indeed lead to a slight increase in the output voltage.

Preferably the heater is arranged and operated such that, when it is energised, the temperature of the fuel cell stack varies from its normal operating temperature by no more 25 than 10°C, more preferably by no more than 5°C.

The humidification of the inert gas supplied to the cathodic gas chamber prevents the drying out of the electrode structure, which would be detrimental to subsequent cell performance. Humidification ensures that the electrode structure remains in a damp state, and in some cases has been found to provide sufficient moisture to lead to dissolution of crystals that have formed within the electrode structure; hence this can provide improved performance when the fuel cell stack starts operating again.

In a modification, if the supply of fuel gas or of oxidising gas ceases, the inert gas 35 supplied to the anodic gas chamber is also humidified. For example the fuel cell system may also comprise a secondary chamber through which the fuel gas is supplied to the anodic gas chamber, and with means to supply an aqueous liquid to the secondary chamber if the supply of fuel gas ceases, so that the inert gas supplied to the anodic gas chamber is humidified. Humidification of the inert gas supplied to the anodic gas chamber prevents drying out of the electrode structure, which would be detrimental to subsequent cell performance.

The invention also provides a liquid electrolyte fuel cell system whereby this method may be performed.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of the fluid flows of a fuel cell system of the invention; 15 Figure 2 shows a sectional view of a fuel cell for use in the fuel cell system of figure 1; and Figure 3 shows a schematic diagram of a modification to the fuel cell system of figure 1.

Referring to figure 1, a fuel cell system 10 includes a fuel cell stack 20 (represented schematically), which uses an aqueous solution of potassium hydroxide as electrolyte 12, for example at a concentration of 6 moles/litre, although the electrolyte might be a different aqueous alkaline solution. The fuel cell stack 20 is supplied with hydrogen gas as fuel, air as oxidant, and electrolyte 12, and operates at an electrolyte temperature typically between 50°C and 80°C, for example about 50° or 60°C or about 65° or 70°C. Hydrogen gas is supplied to the fuel cell stack 20 from a hydrogen supply 22 (represented diagrammatically) through a supply valve 24 leading to a duct 25, and an exhaust gas stream emerges through a first gas outlet duct 28. The hydrogen supply pressure may be monitored by a pressure gauge 26 upstream of the supply valve 24. Air is supplied by a blower 30, and any CO2 is removed by passing the air through a scrubber 32 and a filter 33 before the air flows through a duct 35 to the fuel cell stack 20, and spent air emerges through a second gas outlet duct 38. Downstream of the filter 33 is a supply valve 34, and the pressure of the air may be monitored by a pressure gauge 36 upstream of the supply valve 34.

The fuel cell stack 20 is represented schematically, but it consists of a stack of fuel cells. For example the stack 20 may contain up to a hundred or more fuel cells, although it 35 may have fewer, such as five, ten or twenty fuel cells, or any other convenient number of fuel cells to provide a desired electrical output. The individual fuel cells are normally connected electrically in series, so that the output voltage from the fuel cell stack 20 is the sum of the voltages produced by each fuel cell.

Referring now to figure 2 there is shown a sectional view through a single cell 80 which would be suitable for incorporation within the fuel cell stack 20. The fuel cell 80 comprises a liquid electrolyte chamber 82 between opposed electrodes, an anode 83 and a cathode 84. Each anode 83 has one surface facing the liquid electrolyte chamber 82, and an opposite surface facing hydrogen in a gas chamber 85. Similarly each cathode 84 has one surface facing the liquid electrolyte chamber 82, and its opposite surface facing air in a gas chamber 86. In each cell 80 within the fuel cell stack 20, air flows through the gas chamber 86 adjacent to the cathode 84, to emerge as the spent air in the outlet duct 38. Similarly, in each cell 80, hydrogen flows through the gas chamber 85, and emerges as the exhaust gas stream in the outlet duct 28.

The anode 83 and the cathode 84 each consists of a metal current collector 87 and a catalyst-containing coating 88. The metal current collector 87 may be a metal mesh or a perforated metal sheet. The catalyst-containing coating 88 faces the electrolyte chamber 82, so there is catalyst adjacent to the electrolyte, and the coating 88 is hydrophobic at least in the portions further from the electrolyte to suppress migration of aqueous electrolyte through the catalyst-containing coating 88. The catalyst-containing coating 88 must also be electrically conducting, and may comprise a particulate conductor such as carbon and a hydrophobic binder such as polytetrafluoroethylene (PTFE). Indeed the coating 88 may consist of a plurality of different layers, for example a layer without catalyst which allows for gas diffusion, covered by a catalyst-containing layer. The catalyst material in the anode 83 may be different from that in the cathode 84.

Such an electrode structure is known. In use there is a gas/electrolyte interface within the catalyst-containing coating 88, and electrochemical reactions take place at the 30 gas/electrolyte/catalyst interface.

Referring again to Figure 1, operation of the fuel cell stack 20 generates electricity. The fuel cell stack 20 has output terminals 60 connected to a circuit containing a switch 61 and a load 62, so the electricity from the fuel cell stack 20 can be supplied to the load 62. 35 The normal operating temperature of the fuel cell stack 20 is above ambient temperature, and the fuel cell stack 20 is provided with an electrical heater 63 (shown schematically) connected through a switch 64 to an electrical power supply 65. The electrical heater 63 may be integral with components that form the stack 20; or may be incorporated within the stack 20; or may be outside the stack 20, but in thermal contact with the stack. The electrical heater 63 is energised when operation of the fuel cell stack 20 is initiated from cold, in order to raise the temperature to the required operating temperature. During normal operation the electrical heater 63 is switched off, because heat is generated by the passage of electricity through the fuel cell stack 20, and indeed the excess heat is removed from the fuel cell stack system 10 using a heat exchanger 49, as explained below. By way of example, during normal operation the electrolyte may be supplied to the fuel cell stack 20 at 50°C and may emerge from the fuel cell stack 20 at 55°C or may be supplied at about 65°C and may emerge at about 70°C.

Operation of the fuel cell stack 20 also generates water by virtue of the chemical reactions described above. In addition water evaporates in both the anode and cathode gas chambers 85, 86 so both the exhaust gas stream and the spent air contain water vapour. The overall result may be a steady loss of water from the electrolyte 12; the lost water can be partly recovered by condensing water vapour from the spent air in the outlet duct 38 (or from the exhaust gas), for example by providing a condenser 39.

The electrolyte 12 is stored in an electrolyte storage tank 40 provided with a vent 41. A pump 42 circulates electrolyte from the storage tank 40 into a header tank 44 provided with a vent 45, the header tank 44 having an overflow pipe 46 so that electrolyte returns to the storage tank 40. This ensures that the level of electrolyte in the header tank 44 is constant. The electrolyte is supplied at constant pressure through a duct 47 to the fuel cell stack 20; and spent electrolyte returns to the storage tank 40 through a return duct 48. The storage tank 40 includes the heat exchanger 49 to remove excess heat during normal operation, or to provide heat to the electrolyte 12 if required. The heat exchanger 49 may define a flow path for a coolant fluid which is cooled externally, if it is removing heat from the electrolyte 12. If the heat exchanger 49 is to provide heat to the electrolyte 12, it may define a flow path for a hot fluid which is heated externally, or it may be an electrical heater. The heat exchanger 49 may therefore perform two roles: means to define a flow path for a coolant fluid, and an electrical heater. These two roles may be performed by separate components.

In the duct 35 the air stream passes through a heat exchanger 50, and then a humidification chamber 52. Within the humidification chamber 52 the air stream is contacted with water, i.e an aqueous liquid, for example in the form of distilled water, or in the form of electrolyte, for example electrolyte from the header tank 44. The humidification chamber 52 may contain one or more nozzles through which aqueous liquid may be sprayed into the air stream in the form of small droplets, or alternatively the humidification chamber 52 may contain multiple surfaces such as parallel plates over which the aqueous liquid is arranged to flow continuously, so there is a large area of contact between the aqueous liquid and the air stream. The humidification chamber 52 significantly raises the 10 level of humidity in the air stream. The excess aqueous liquid emerging from the humidification chamber 52 is discharged through a duct 55 to waste (or, if the aqueous liquid is electrolyte, then the excess may be discharged back to the storage tank 40).

It has been found that in operation of a fuel cell system, despite the provision of the humidification chamber 52, there is a net evaporation of water from the electrodes, in particular from the cathode, which may lead to the formation of crystalline potassium hydroxide or potassium carbonate in pores in the electrode if the electrolyte is an aqueous solution of potassium hydroxide. This may cause delamination of the electrode, and may hinder mass transport for the gaseous reactants. In any event, over a period of time which may be months, the electrical (ohmic) resistance of each cathode 84 has been found to gradually increase. This is detrimental to the electrical output.

During operation of the fuel cell system 10, other processes may occur that lead to a deterioration in electrical output. For example there may be an increase in concentration of 25 KOH in the vicinity of the catalyst layer of the cathode 84, leading to a reduction in the solubility of oxygen, and so a decline in the rate of mass transport of oxygen to the catalyst.

Thus, it has been observed that during prolonged operation of the fuel cell system 10 there is a gradual and slight deterioration in the electrical output over a period of months.

However, it has also been observed that the deterioration is made worse if operation of the fuel cell system 10 is stopped for a period of time, and then subsequently started again. In conventional systems, the fuel cell stack 20 would be shut down if the supply of fuel gas ceases, by ceasing to pass gases through both the gas chambers 85 and 86, ceasing to pass the electrolyte 12 through the electrolyte chambers 82, and allowing the temperature to decrease. It has been found that if the fuel cell stack is shut down in this way, then when the fuel cell stack is started up again its performance is likely to be worse in two respects: the output voltage is likely to be slightly lower once a steady state at full load has been reestablished, and the deterioration in electrical output with time is likely to be worse than before.

Malfunction: Loss of Hydrogen In accordance with the present invention, if the supply of hydrogen from the hydrogen supply 22 ceases, a number of operations are continued. However, it will be appreciated that if there is no supply of hydrogen there will no longer be any electricity generated by the fuel cell stack 20, and it may therefore be necessary to switch off the switch 61 in the circuit leading to the load 62. Since there is no longer any electricity being generated, the heating associated with the electricity generation within the fuel cell stack 20 will also cease.

A supply 70 of an inert gas such as nitrogen is connected into the duct 25 carrying the hydrogen to the fuel cell stack 20, the inert gas supply 70 having an output valve 72. If the hydrogen supply 22 ceases, then the supply valve 24 would be closed and the output valve 72 opened so that the inert gas flows through the duct 25 and so through the anodic gas chambers 85. The air flow through the cathodic gas chambers 86 is continued, and the electrolyte flow through the electrolyte chambers 82 is continued. In addition, the switch 64 is closed so that the heater 63 maintains the temperature of the fuel cell stack 20. Thus the temperature of the fuel cell stack 20 can be held constant, and the gas and electrolyte flows can remain constant, so ensuring minimal change in conditions within the fuel cell stack 20.

When the hydrogen supply 22 becomes available again, it is then only necessary to open the supply valve 24 and close off the output valve 72 to recommence the supply of hydrogen to the anodic gas chambers 85. The provision of electricity to the load 62 can then recommence, closing the switch 61 if necessary. The switch 64 can then be opened, as the heater 63 will no longer be required.

Malfunction: Loss of Air Supply Similarly, in accordance with the present invention, if the supply of air from the 35 blower 30 ceases, a number of operations are continued. But if there is no supply of air there will no longer be any electricity generated by the fuel cell stack 20, and it may therefore be necessary to switch off the switch 61 in the circuit leading to the load 62.

Firstly, the supply of hydrogen is shut off using the supply valve 24, and the output 5 valve 72 is then opened so that the inert gas flows through the anodic gas chambers 85. The supply valve 34 is closed off, and a supply of inert gas is provided to take the place of the air from the blower 30. This may be provided from the flow of inert gas from the inert gas supply 70, by opening a valve 74 in a duct 73 that interconnects the duct 25 and the duct 35 so some of the inert gas from the duct 25 can flow into the duct 35; alternatively, a 10 separate supply (not shown) of the inert gas may be provided, equivalent to the supply 70, but connected to the duct 35. Thus the inert gas is also supplied to the cathodic gas chambers 86.

The electrolyte flow through the electrolyte chambers 82 is continued. Since there is no electricity generated in the fuel cell stack 20, the heating associated with the electricity generation in the fuel cell stack 20 ceases. The switch 64 is therefore closed so that the heater 63 maintains the temperature of the fuel cell stack 20. Thus the temperature of the fuel cell stack 20 can be held constant, and the gas and electrolyte flows can remain constant, so ensuring minimal change in conditions within the fuel cell stack 20.

When the air supply from the blower 30 becomes available again, it is then only necessary to close off the output valve 72 (and, where applicable, to close off the valve 74) to cease supplying inert gas. The hydrogen flow from the supply 22 can then be recommenced by opening the valve 24, and the air flow restarted by actuating the blower 30 and opening the supply valve 34. This returns the fuel cell stack 10 to its original operating state. The provision of electricity to the load 62 can then recommence, closing the switch 61 if necessary. The switch 64 can then be opened, as the heater 63 will no longer be required.

Discussion It has been found that this method of operating the fuel cell system 10, when a malfunction occurs leading to a loss of hydrogen or a loss of air supply, is beneficial. Not only does it prevent the small decrease in cell voltage when the fuel cell stack 20 recommences generation of electricity, but it also ensures that the subsequent rate of deterioration is no greater than before. Indeed in some cases it has been found that ceasing the supply of hydrogen for a short period of time, in this fashion, and then starting operation again, has been observed to provide an enhancement in the electrical output of the fuel cell stack 20. This may be the result of discharging product water from the anode, and clearing the active catalyst sites.

Although described above in the context of a malfunction, the method is applicable in any context in which the supply of one or more of the gases ceases. In particular, it is therefore applicable if the fuel cell system 10 is to be temporarily shut down for any reason.

In one modification the electrical heater 63 is not provided, and instead the temperature of the fuel cell stack 20 is maintained using the heat exchanger 49 to supply heat to the electrolyte 12, and recirculating the electrolyte 12 through the fuel cell stack 20. This has been found to be a very satisfactory and efficient way of maintaining the temperature of the fuel cell stack 20; in normal operation the electrolyte is at an elevated temperature for example of 65°C, as mentioned above, and there is a significant quantity of heat available in the hot electrolyte, which would otherwise be gradually lost to the environment during a shut-down. The hot electrolyte is aqueous, and so has a large heat capacity per unit volume, and provides good heat transfer by conduction as it flows through the stack 20.

It will also be appreciated that the arrangement of valves may differ from that shown. For example the hydrogen supply valve 24 and the inert gas supply valve 72 may be combined into a three-way valve; and similarly the air supply valve 34 and the inert gas 25 supply valve 74 (where applicable) may also be combined into a three-way valve.

As mentioned above the fuel cell stack 20 may typically operate at an electrolyte temperature of about 50° or 60°C, or at about 65° or 70°C. If the supply of hydrogen or of air ceases, as mentioned above, the temperature of the fuel cell stack 20 is maintained either by the electrical heater 63 or by supplying heat to the electrolyte, and in either case the temperature of the fuel cell stack 20 preferably does not vary by more than 10°C, more preferably by not more than 5°C. Where heat is supplied to the electrolyte, this may make use of the heat exchanger 49 within the electrolyte storage tank 40, or alternatively heat may be supplied to the electrolyte at another place in the electrolyte circuit.

Thus, in the event of a malfunction that leads to loss of the hydrogen supply or of the air supply, flows of gas through the anodic and cathodic gas chambers 85 and 86 and flow of electrolyte through the electrolyte chamber 82 are continued. The flow rates may be different from those during normal operation, and the flows are preferably regulated so as to minimise any degradation of the electrodes 83 and 84 during the shutdown.

It will also be appreciated that the shutdown processes described above may be operated automatically, without the need for intervention by an operator. This would necessitate a control system, represented schematically as 77, which would be connected to the pressure sensors 26 and 36, and would provide control signals to the valves 24, 34, 72 and 74 and to the switches 61 and 64.

For example, considering the process that occurs when the malfunction is the loss of hydrogen, the hydrogen pressure in normal operation may for example be 10-15 mbar (1.0-1.5 kPa) above atmospheric pressure. This pressure may be monitored by the pressure sensor 26. If at any time the hydrogen inlet pressure monitored by the pressure sensor 26 drops to below 5 mbar (0.5 Pa), then this may be detected by the control system 77. The control system 77 would therefore close the hydrogen supply valve 24 and open the inert gas supply valve 72 to start the purge process, i.e. flowing nitrogen through the anodic gas chambers 85; and also switch off the switch 61 supplying electricity to the load 62, and switch on the switch 64 to energise the heater 63. When hydrogen gas again becomes available at an acceptable pressure level, for example at least 10 mbar (1.0 kPa), this would be detected by pressure sensor 26, so that the control system 77 would close the supply valve 72 and open the supply valve 24 to restart hydrogen flow through the anodic gas chambers 85; and also switch on the switch 61 to supply electricity to the load 62, and switch off the switch 64 as the heater 63 would no longer be required. All of these steps can readily be automated.

In the other type of malfunction, involving loss of the air supply, the operation would be automated in substantially the same way. The air inlet pressure, as monitored by the sensor 36, would normally be at a pressure such as 20-25 mbar (2.0-2.5 kPa) above atmospheric pressure; the control system 77 would detect the malfunction if the air inlet pressure drops below a preset threshold, which might for example be set at 15, 10 or 5 mbar (1.5, 1.0 or 0.5 kPa). If the malfunction is detected, the control system 77 would then close the supply valves 24 and 34, and then open the valves 72 and 74, so as to supply the inert gas to both the ducts 25 and 35, so purging both the anodic and cathodic gas chambers 85 and 86 with the inert gas. As in the previously-described case, the control system 77 would also switch off the switch 61, and switch on the switch 64.

When the air supply is restored, as can be detected by the pressure sensor 36 detecting a pressure significantly above the preset threshold, the above steps would be reversed: the control system 77 would close the valves 72 and 74, and open the supply valves 24 and 34, so as to restart the flows of hydrogen and air to the fuel cell stack 20; the control system 77 would then switch off the switch 64 and switch on the switch 61.

Referring now to figure 3, this shows a fuel cell system 110 which is identical in most respects to the fuel cell system 10 described above, and so in most respects operates in exactly the same way; identical features are referred to by the same reference numerals. Hydrogen gas is supplied to the fuel cell stack 20 from a hydrogen supply 22 (represented diagrammatically) through a supply valve 24 leading to a duct 25, and air is supplied by a blower 30, and flows through a duct 35 to the fuel cell stack 20. The fuel cell system 110 differs only in that the duct 25 includes a secondary chamber 152.

The secondary chamber 152 is connected via a duct 153 and a control valve 154 to a source of an aqueous liquid, for example in the form of distilled water, or in the form of electrolyte, for example electrolyte from the header tank 44. If the control valve 154 is open the secondary chamber 152 acts as a humidification chamber. The secondary chamber 152 may contain one or more nozzles through which aqueous liquid may be sprayed into the gas stream in the form of small droplets, or alternatively the secondary chamber 152 may contain multiple surfaces such as parallel plates over which the aqueous liquid is arranged to flow continuously, so there is a large area of contact between the aqueous liquid and the gas stream. The secondary chamber 152 is also provided with a discharge duct 155, which may either discharge excess aqueous liquid to waste, or, if the aqueous liquid is electrolyte, then the excess may be discharged back to the storage tank 40.

During normal operation, with hydrogen gas flowing through the duct 25, the control valve 154 is closed, and so this secondary chamber 152 does not have any effect on the hydrogen gas.

During a temporary shutdown or malfunction situation, as described above, the supply valve 24 would be closed and the output valve 72 opened so that inert gas flows through the duct 25 and so through the anodic gas chambers 85. In addition, the control valve 154 would be opened so that the inert gas flowing through the secondary chamber 152 is humidified. This significantly raises the level of humidity in the inert gas stream supplied to the anodic gas chambers 85. This has been found to be beneficial in subsequent operation of the fuel cell stack 20.

Claims

Claims 1. A method of operating a fuel cell system that comprises an aqueous liquid electrolyte fuel cell stack comprising at least one fuel cell, each fuel cell comprising a liquid 5 electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and anodic and cathodic gas chambers adjacent to the anode and the cathode respectively, so the electrodes separate the liquid electrolyte chamber from the respective gas chambers; wherein during normal operation a fuel gas is supplied to the anodic gas chamber and an oxidising gas is supplied through a humidification chamber to the cathodic 10 gas chamber; and the fuel cell system also comprises a heater to raise the temperature of the fuel cell stack; wherein if the supply of fuel gas or of oxidising gas ceases, the method of operation comprises the steps of: (a) supplying a flow of inert gas through the anodic gas chamber; (b) continuing a flow of gas through the cathodic gas chamber, by supplying the inert gas through the humidification chamber if the supply of oxidising gas has ceased; and (c) energising the heater to maintain the temperature of the fuel cell stack, wherein the heater: CO is an electrical heater integral with components that form the fuel cell stack, or 20 incorporated within the fuel cell stack, or in thermal contact with the fuel cell stack; or 00 is arranged to heat the aqueous liquid electrolyte, the heated electrolyte being supplied to the fuel cell stack.
2. A method as claimed in claim 1 wherein the inert gas is supplied through a 25 secondary chamber to the anodic gas chamber, the secondary chamber being provided with an aqueous liquid so that the inert gas supplied to the anodic gas chamber is humidified.
3. A method as claimed in claim 1 or claim 2 wherein the normal method of operation involves circulation of the electrolyte through the liquid electrolyte chamber of each fuel cell, and wherein the method of operation, if the supply of fuel gas or of oxidising gas ceases, also comprises continuing to circulate the electrolyte through the liquid electrolyte chamber of each fuel cell.
4. A method as claimed in claim 3 wherein the heater is arranged to provide heat to the electrolyte, and the electrolyte is circulated to the fuel cell stack to maintain the temperature of the fuel cell stack.
5. A method as claimed in any one of the preceding claims wherein the heater is an electrical heater.
6. A method as claimed in any one of the preceding claims wherein the heater maintains the temperature of the fuel stack within 10°C of its normal operating temperature.
7. A method as claimed in any one of the preceding claims wherein the fuel cell system incorporates control means for automating performance of the requisite steps of the method, so the steps are carried out automatically.
8. A fuel cell system comprising an aqueous liquid electrolyte fuel cell stack comprising at least one fuel cell, each fuel cell comprising a liquid electrolyte chamber between opposed electrodes, the electrodes being an anode and a cathode, and anodic and cathodic gas chambers adjacent to the anode and the cathode respectively, so the electrodes separate the liquid electrolyte chamber from the respective gas chambers; wherein during normal operation a fuel gas is supplied to the anodic gas chamber and an oxidising gas is supplied through a humidification chamber to the cathodic gas chamber; and the fuel cell system also comprises a heater; wherein the system also includes pressure sensors to sense the pressures of the fuel gas and of the oxidising gas, and a control system for performing a method as claimed in claim 7.
9. A fuel cell system as claimed in claim 8 also comprising means to achieve humidification of the inert gas supplied to the anodic gas chamber if the supply of fuel gas or of oxidising gas ceases.
10. A fuel cell system as claimed in claim 9 wherein the fuel cell system also comprises a secondary chamber through which the fuel gas is supplied to the anodic gas chamber, with means to supply an aqueous liquid to the secondary chamber if the supply of fuel gas ceases, so that the inert gas supplied to the anodic gas chamber is humidified.