Abstract:
A method of shutting down operation of a fuel cell system comprising a fuel cell stack, the method comprising the sequential steps of: i) ceasing a supply of fuel to the fuel cell stack; ii) closing a shut-off valve on an exhaust line in fluid communication with a cathode system of the fuel cell system, the cathode system comprising a cathode fluid flow path passing through the fuel cell stack; iii) pressurising the cathode system with an air compressor in fluid communication with a cathode air inlet port in the fuel cell stack; and iv) ejecting water from the cathode flow path.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the operation of, and apparatus relating to, a fuel cell system, and in particular though not exclusively to a strategy for shutting down a fuel cell system. 
       BACKGROUND 
       [0002]    Water is integral to the operation of a fuel cell system, for example in the form of the system described herein comprising a fuel cell stack based around a proton exchange membrane (PEM). Reaction of protons (hydrogen ions) conducted through the PEM from an anode flow path, with oxygen present in a cathode flow path, produces water. Excess water needs to be removed from the fuel cell stack to avoid flooding and causing a consequent deterioration in performance. An amount of water, however, needs to be present in at least the cathode flow path to maintain hydration of the PEM, so as to achieve optimum performance of the fuel cell. Managing this water, by deliberate injection and removal, can also provide a useful mechanism for removing excess heat from the fuel cell stack. 
         [0003]    To optimise performance, water can be employed deliberately in such fuel cell systems through injection into the cathode flow path of the stack. Such water injection fuel cell systems have potential advantages of reduced size and complexity, as compared with other types of fuel cell systems employing separate cooling channels. Water may be injected directly into the cathode flow path through water distribution manifolds, as for example described in GB2409763. 
         [0004]    For water injection systems, it is important that any water fed back into the cathode flow path is of high purity, so as to avoid contamination of the PEM and consequent degradation of stack performance. This requirement for high purity, however, means that additives to lower the freezing point of water cannot be used. For automotive applications in particular, typical requirements include starting up from below freezing, typically as low as −20° C. to replicate environments in which the fuel cell may be used in practice. Since high purity water has a freezing point of 0° C. (at 1 bar pressure), any water left in the fuel cell system will, given sufficient time, freeze after shut-down of the fuel cell. 
         [0005]    Ice in the fuel cell system, and in particular within the cathode flow path, can prevent the stack from operating properly, or even at all. If any part of the cathode flow path is blocked with ice, air cannot be passed through the cathode and the fuel cell may not be capable of self-heating to above freezing point. Other methods of heating the whole stack will then be necessary, which will require consumption of external power before the fuel cell can begin supplying electrical power and heat by itself. 
         [0006]    A purging operation can be used on shut-down of a fuel cell stack, such as that described in U.S. Pat. No. 6,479,177. This document discloses a fuel cell stack having water cooling passages separate from the cathode flow path. A pressurised dry nitrogen feed is used to purge water from the stack before allowing the temperature of the stack to fall below freezing. This method, however, requires a supply of pressurised nitrogen, which might not be available or even desirable in an automotive environment. 
         [0007]    It is an object of the invention to address one or more of the above mentioned problems. 
       SUMMARY 
       [0008]    In a first aspect, the invention provides a method of shutting down operation of a fuel cell system comprising a fuel cell stack, the method comprising the sequential steps of
       i) ceasing a supply of fuel to the fuel cell stack;   ii) closing a shut-off valve on an exhaust line in fluid communication with a cathode system of the fuel cell system, the cathode system comprising a cathode fluid flow path passing through the fuel cell stack;   iii) pressurising the cathode system with an air compressor in fluid communication with a cathode air inlet port in the fuel cell stack; and   iv) ejecting water from the cathode flow path.       
 
         [0013]    In a second aspect, the invention provides a fuel cell system comprising:
       a fuel cell stack;   a cathode system having a cathode fluid flow path comprising a cathode air inlet line, a cathode volume within the fuel cell stack and a cathode exit line connected in series and configured to allow passage of air through the fuel cell stack;   an air compressor in fluid communication with the cathode air inlet line;   a thermally insulated containment vessel configured to receive water through a water return line from the cathode flow path,   wherein the fuel cell system is configured to eject water from the cathode flow path into the containment vessel through the water return line upon shutting down operation of the system.       
 
         [0019]    In a third aspect, the invention provides a reverse flow relief valve comprising:
       a first feed port;   a second feed port;   a non-return valve within a main fluid passage extending between the first and second feed ports, the non-return valve configured to allow fluid to pass from the first to the second feed ports and to block passage of fluid in the reverse direction;   a bypass fluid passage in fluid communication with the main fluid passage;   a sealing valve biased against an end of the bypass passage between the bypass passage and a purge port,   wherein the sealing valve is configured to maintain a seal against the bypass passage when fluid pressure in the first feed port exceeds fluid pressure at the second feed port to prevent fluid flow from the main fluid passage to the purge port through the bypass fluid passage, and to allow fluid flow from the second feed port to the purge port through the bypass fluid passage when fluid pressure at the second feed port exceeds fluid pressure at the first feed port.       
 
         [0026]    In a fourth aspect, the invention provides a fuel cell system comprising:
       a fuel cell stack;   a cathode system having a cathode fluid flow path comprising a cathode air inlet line, a cathode volume within the fuel cell stack and a cathode exit line connected in series and configured to allow passage of air through the fuel cell stack;   a heat exchanger connected in series with a water separator to the cathode exit line of the cathode fluid flow path,   wherein a water ejection outlet line of the water separator is connected to a water containment vessel by a first water return line.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The invention will now be described by way of example only, and with reference to the appended drawings in which: 
           [0032]      FIG. 1  illustrates a schematic diagram of the arrangement of various components within an overall fuel cell system; 
           [0033]      FIG. 2  illustrates a schematic diagram of an exemplary water containment vessel; 
           [0034]      FIG. 3  illustrates a cutaway perspective view of an exemplary reverse flow relief valve; 
           [0035]      FIGS. 4   a  and  4   b  illustrate schematically the operation of the reverse flow relief valve of  FIG. 3 ; and 
           [0036]      FIGS. 5   a  and  5   b  illustrate schematically two alternative configurations for cathode exit stream liquid separation. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  shows a schematic diagram of an exemplary fuel cell system  100  comprising a fuel cell stack  110  and other associated components. The fuel cell stack  110  has a cathode flow path passing through it, the cathode flow path comprising an air inlet  124  leading to an air inlet line  123  and into the stack at the cathode air inlet  126 . After passing through an internal cathode volume (not shown) within the fuel cell stack  110 , the cathode flow path exits the fuel cell stack  110  into the cathode exit line  121 , through the cathode exhaust line  122  and an exhaust shut-off valve  120 . During normal operation, the exhaust shut-off valve  120  is partially or fully open. Various components such as a heat exchanger  130 , with associated cooling fan  139 , and a water separator  131  may be connected to or part of the cathode exit line  121  and exhaust line  122  in the cathode flow path. Temperature sensors TX 1 , TX 2 , TX 3 , TX 5  and pressure sensors PX 2 , PX 3  may also be present, connected at appropriate places to monitor the inlet line  123  and exit line  121  of the cathode flow path. 
         [0038]    The expression ‘cathode system’ in the present context is intended to encompass those parts of the fuel cell system  100  that are associated with the cathode volume within the fuel cell stack. These include the various internal components of the fuel cell such as the inlets, outlets, the internal flow path and water distribution structures, as well as components in fluid communication with the cathode volume such as the various inlet, outlet, recirculation and exhaust lines for both liquids and gases. The term ‘cathode flow path’ is intended to encompass a subset of the cathode system that includes a fluid flow path from the air inlet  124  through an air compressor  133 , the inlet line  123 , the cathode volume of the fuel cell stack  110 , and the cathode exit line  121 . The terms ‘anode system’ and ‘anode flow path’ are to be interpreted similarly, with reference to the various components of the fuel cell system  100  associated with the anode volume. 
         [0039]    The air compressor  133 , connected to the cathode air inlet line  123 , provides compressed air to the cathode flow path. Other components such as an air inlet heat exchanger  134 , a flow meter  135 , one or more air filters  136 ,  137  and an air heater  138  may be present in the cathode inlet line  123  between the air inlet  124  and the fuel cell stack  110 . The air inlet heat exchanger  134  may be used in conjunction with a coolant line  141 , a three-way valve  142  and a temperature sensor TX 7  to pre-heat air from the air compressor  133  with coolant from the coolant line  141  during operation of the fuel cell system  100 . The coolant line  141  passing through the air inlet heat exchanger  134  forms a separate cooling circuit configured to extract heat from the air stream after the compressor  133 . This coolant line  141  is preferably operated after the fuel cell stack  110  reaches a normal operating temperature, in order to avoid extracting heat from the air inlet stream in the cathode air inlet line  123  during start-up of the system  100 . Diversion of coolant in the line  141  may be achieved through use of the valve  142 , allowing control over whether coolant is delivered to the heat exchanger  134 . Since the coolant line  141  is separate from water fed into the cathode system, the requirement for high purity water is not the same. The coolant used in the coolant line  141  may therefore comprise additives such as glycol to lower the freezing point of the coolant used. 
         [0040]    Fuel, typically in the form of gaseous hydrogen, enters the fuel cell system via a pressure-reducing valve  151  and an actuated valve  152 , preferably in the form of a normally-closed solenoid-actuated valve. The fuel supply  150 , when in the form of hydrogen gas, is typically located remotely from the fuel cell system, for example in the form of a pressurised tank towards the rear of a vehicle. A further solenoid-actuated valve  153  and a pressure-reducing valve  154  may be provided closer to the fuel cell stack  110  in the fuel inlet line  155  of the anode flow path between the fuel source  150  and the anode inlet  156  of the fuel cell stack  110 . Two separate sets of valves are therefore provided leading to the anode inlet  156 , one set  151 ,  152  near to the tank and the other set  153 ,  154  closer to the fuel cell stack  110 , with an intermediate pressurised fuel line  119  in between. The pressure-reducing valve  154  regulates the pressure of the dry fuel gas to a level suitable for introduction to the fuel cell stack  110 . The pressure-reducing valve  154  is preferably a passive device which has a preset pressure setting applied, although an actively controlled device may be used. A fuel heater  145  is optionally provided, for example in the pressurised fuel line  119  before the valve  153 , as shown in  FIG. 1 , or alternatively in the fuel inlet line  155  either before or after the pressure-reducing valve  154 . 
         [0041]    A further actuated valve  161  is provided on the anode exit line  165 . Each actuated valve  152 ,  153 ,  161  may be provided with a local heater element to defrost the valve as required, although activation of the valves  152 ,  153 ,  161  through passage of current through the solenoid will provide a certain degree of heating. Preferably each of the actuated valves  152 ,  153 ,  161  is configured to be fail-safe, i.e. will only open when actuated by current passing though the solenoid. 
         [0042]    To monitor and to relieve pressure of fuel within the anode flow path, a pressure sensor PX 1  and/or pressure relief valve  157  may be provided. The pressure relief valve  157  is preferably set to open and exhaust fluid from the anode flow path through a pressure relief exhaust line  158  when the pressure in the anode flow path exceeds a safe operating level. 
         [0043]    A further manually operable valve  162  in the anode exit line  165  may be present, this valve  162  being for used for example during servicing to ensure depressurisation of the anode flow path. Water build-up in the anode flow path in the fuel cell stack  110  may occur, for example as a result of diffusion of water through the PEM from the cathode side. Consequently, an anode exhaust water separator  163  may be provided in the anode exhaust line  164  to separate any water present in the exhaust line  164 . This water can be exhausted or optionally recirculated. During operation of the fuel cell stack  110 , the valve  161  is typically held closed, and only opened intermittently to exhaust any built-up water from the anode fluid path. 
         [0044]    A cathode water injection inlet  127  is provided in the fuel cell stack  110 , the inlet  127  connected to a cathode water injection line  125 . The cathode water injection line  125  may be heated along a part or the whole of its length, and extends between a water containment vessel  140  and the cathode water injection inlet  127 . A heater  129  may be provided to apply heat to a specific region of the line  125  to heat water passing through the injection line  125  towards the cathode water injection inlet  127 . A further pressure sensor PX 4  may be provided on the cathode water injection line  125  in order to monitor the back-pressure on the line  125  during operation. 
         [0045]    Water from the cathode exit line  121  is pumped with a water pump  132 , optionally provided with a heater  143 , through a water return line  128  towards the water containment vessel  140 , further details of which are provided below with reference to  FIG. 2 . Excess water is ejected from the fuel cell system  100  out of the water containment vessel  140  through a water overflow line  144 . 
         [0046]    Shown in  FIG. 2  is a schematic cross-sectional view of the water containment vessel  140  of  FIG. 1 . The vessel  140  comprises a thermally insulating wall  210  and a lid  211 , which may also be thermally insulated. Preferably the wall  210  of the vessel  140  is of a double wall construction, having a vacuum or other thermally insulating layer such as air or expanded polystyrene between the two walls. The inner surface  215  of the vessel  210  is preferably made from a material having a resistance to corrosion, such as stainless steel, to prevent contamination of water  212  within the vessel. 
         [0047]    The purpose of the lid  211  is to allow connections to the various elements housed within the vessel  140 , whilst also maintaining a good degree of insulation. Typically the lid  211  is manufactured from glass-reinforced nylon with an additional layer of insulating foam. Ports in the lid to accommodate passage of the lines  125 ,  144 ,  128  are preferably configured such that when the system is shutdown, any residual water runs back into the vessel. This involves using pipe of a suitably large diameter such that beads of water do not form to span the internal bore of the pipe and hang up in the line. Preferably, no fittings are used in the lid so that pipes passing through the lid  211  contain no sharp bends. In a general aspect, therefore, the water injection line  125  extending between the water containment vessel  140  and the cathode water injection inlet  127 , and an exhaust line  121 ,  128  extending between a cathode exit line  121  and the water containment vessel  140  both comprise piping having an internal bore such that beads of water do not span the bore after ejection of water from the cathode system. 
         [0048]    A thermostatic heating element  236  within the vessel  140  is provided to maintain the temperature of water  212  within the vessel  140  above freezing point. A level sensor  233  provides a signal indicating the level of water  212  within the vessel. A heater  237  is provided in addition to the thermostatic heating element  236  in order to provide faster heating to defrost the water  212  if frozen. Due to the energy requirement of changing the phase of water from solid to liquid, this heater  237  is typically of a higher power rating than the thermostatic heating element  236 , for example around 180 W or higher. The thermostatic heater  236  is configured to ensure that the temperature of the water  212  in the vessel  140  remains above a set point. This set point is typically 5° C., in order to prevent the water from freezing. The thermostatic heater  236  may be powered by a 12V battery supply, and set to operate for a prescribed period. Hence, during this period, liquid water in the vessel can be guaranteed. For longer periods at sub-zero ambient temperature, the thermostatic heater  236  is disabled to save on battery power. The water  212  may then freeze, and will require defrosting with the higher power heater  237 . The thermostatic heater is typically of a power rating such that a maximum heat output is slightly larger than the maximum rated losses from the vessel. A typical power rating is in the range of 2 to 4 W. 
         [0049]    A temperature sensor TX 4 , preferably comprising a submerged thermistor, is installed in order to allow the temperature of the water  212  in the vessel  140  to be monitored. 
         [0050]    An overflow line  144  is provided to eject excess water from the containment vessel if a level of water in the vessel exceeds a preset amount. 
         [0051]    Water from the water return line  128  enters the vessel  140  through a filter  234 . A pump  230 ,  231 ,  240  pumps the water  212  from the vessel  140  through a further filter  214 , a reverse flow relief valve  213  and into the cathode water injection line  125 . A flow meter  235  is configured to monitor the amount of water passing through the cathode water injection line  125 . 
         [0052]    The pump is preferably constructed such that a motor portion  231  is located outside the containment volume of the vessel  140  and therefore avoids being in contact directly with water  212  within the vessel. A shaft  240  between the motor  231  and a pump head  230  allows the motor  231  to drive the pump head  230 . The pump head, comprising at least the inlet, outlet and impeller, is preferably of a construction such that, after being submerged in frozen water the pump is able to operate again once the water is defrosted. The motor  231  is preferably rated for operation at sub-zero temperatures. 
         [0053]    The pump head  230  is located so as to be submerged by water  212  in the vessel  140 . This has the advantage of there being no requirement for the pump head  230  to be purged during shutdown or heated during startup, particularly when water  212  is maintained within the flask after shutdown. The pump head  230  is preferably configured to have a small thermal mass. Thawing of any ice within the pump head  230  is achieved via heat transferred from the surrounding water as it defrosts. The pump head  230  is also preferably configured to accommodate expansion due to ice formation. On thawing, the pump head  230  then returns to its original shape without compromising its operation. 
         [0054]    The reverse flow relief valve  213  is constructed such that water is allowed to pass from the pump head  230  through the cathode water injection line  125  towards the cathode water injection inlet  127  when the pump is operational, creating a pressure drop across the valve in the direction of flow. However, when the pump is stopped and pressure in the cathode water injection line  125  is increased, the valve  213  allows water to flow back into the vessel  140  through a purge port  238 . 
         [0055]    The purpose of the reverse flow relief valve  213  is to allow water to be back flushed into the vessel  140  from the fuel cell stack  110  and connecting lines during shutdown of the system  100 . Closing the cathode exhaust valve  120  allows water in the fuel cell stack  110  to be forced under pressure from the air compressor  133  back through the cathode water injection inlet  127 , out of the stack  110  and through the water injection line  125  towards the water containment vessel  140 . However, if a gear pump is used in the containment vessel, without the reverse flow relief valve  213  no water would flow due to the pressure required to push water back through the pump head  230 . Therefore, the reverse flow relief valve  213  is configured such that in normal operation it allows water to pass through it from the pump to the fuel cell stack  110 . When subjected to a small back pressure (for example in the region of 300 mBar·g) when the pump head  230  is not being operated, a diaphragm opens and allows water to flow back into the flask through the purge port  238 . 
         [0056]    An exemplary embodiment of the reverse flow relief valve  213  is shown in  FIG. 3  in cutaway form. In normal operation, water flows from the containment vessel  140  through the reverse flow relief valve  213  in the directions indicated by arrows  301 . Water flows through a first feed port  314 , through a non-return valve  316  and out of the valve  213  through a second feed port  320  towards the cathode water injection line  125 . The pressure of water in the first feed port  314  is transmitted via a connecting passage  313  and transfer passage  312  to a cavity  311  sealed by a sealing valve, for example in the form of a diaphragm  321 , and enclosed by a cover face  323 . The pressure maintains a sealing face  317  of the diaphragm  321  against a face of a bypass passage  318 , and thus prevents fluid from passing between the second feed port  320  and a low pressure cavity  315  behind the diaphragm  321 . 
         [0057]    Once the water containment vessel pump  230  is disabled, a loss of pressure in the first feed port  314  and an increase in pressure in the second feed port  320  caused by an increased pressure in the cathode volume of the fuel cell stack  110  causes the non-return valve  316  to close. The increased pressure in the second feed port causes the diaphragm  321 , which is preferably composed of a resilient material such as a rubber, to flex and open up a passage between the bypass passage  318  and the low pressure cavity  315 . Water is then allowed to flow from the second feed port  320  through the bypass passage  318 , into the low pressure cavity  315 , through a purge passage  322  and out of the valve  213  through the purge port  238 . The overall direction of flow in the reverse direction is indicated by arrows  302 . 
         [0058]    The configuration of the reverse flow relief valve allows water to be ejected from the cathode volume in the fuel cell stack and the cathode water injection line while allowing the pump head  230  in the containment vessel  140  to remain primed with water. Provided the water  212  in the vessel is not frozen, the pump head  230  then remains in a state ready to immediately begin pumping water for injection into the cathode volume of the fuel cell stack  110 . 
         [0059]      FIGS. 4   a  and  4   b  show schematically the two different modes of operation possible with the reverse flow relief valve  213 . In  FIG. 4   a , the purge operation is shown, in which low pressure air from the fuel cell stack enters the valve  213  through the second feed port  320 , and there is no flow into the first feed port  314 . This low pressure air causes the diaphragm  321  to deflect and allow flow through the bypass passage  318  and out of the valve through the purge port  238  for delivery to the water reservoir in the water containment vessel  140 . The non-return valve  316  prevents flow through to the first feed port  314 . 
         [0060]    In  FIG. 4   b , the water delivery operation of the reverse flow relief valve  213  is shown, in which high pressure water pumped from the reservoir in the water containment vessel  140  enters the valve  213  through the first feed port  314 . The pressure, transmitted via the connecting passage  313  to the high pressure cavity  311 , maintains the diaphragm  321  in a closed position against the bypass passage  318 . Flow of water passes through the non-return valve  316  and out of the reverse flow relief valve  213  via the second feed port  320 . No flow occurs through the purge port  238 . 
         [0061]    Two alternative arrangements of the cathode exit, exhaust and water ejection lines are shown schematically in  FIGS. 5   a  and  5   b . In  FIG. 5   a , the water separator  131  is connected first in line with the cathode exit line  121 , in series with and before the heat exchanger  130 , and two water ejection lines  128   a ,  128   b  are used to send water via pumps  132   a ,  132   b  to the containment vessel  140 . In  FIG. 5   b , the water separator  131  is connected in series with, but after, the heat exchanger  130 , and a single pump  132  is used to pump water through the water return line  128  to the containment vessel  140 . 
         [0062]    In the configuration of  FIG. 5   a , the cathode exit stream is passed through the cathode exit line  121  to a cyclonic water separator  131 , which removes the liquid content before redirecting saturated air to a heat exchanger  130 . The heat exchanger  130  cools the saturated air stream, which results in a proportion of the entrained water changing to liquid phase. Two pumps  132   a ,  132   b  are used to transfer recovered water to the containment vessel  140 , one pump  132   a  connected via water return line  128   a  to the base of the separator  131  and the other  132   b  connected via water return line  128   b  to the heat exchanger  130  exit manifold box. During normal fuel cell operation the variable shut off valve  120  is held open. However, during fuel cell system shutdown, this valve is closed to back pressure the cathode flow path. In this case the compressor  133  (or other device which supplies air to the fuel cell according to a defined flow rate set point) works harder to maintain a fixed air flow rate. The separator  131  contains a pressure relief valve (not shown) which, during this back pressured shutdown phase, opens allowing ejection of water from the water separator  131  through a water purge line  510  (typically to atmosphere). Prior to back pressuring the cathode flow path at system shutdown, the pump  132   a  is typically allowed to run for a few seconds to clear most of the water from the separator  131 . 
         [0063]    In the configuration of  FIG. 5   b , operation is similar to that of  FIG. 5   a , except that the cyclonic separator  131  is positioned after the heat exchanger  130 . As such, the liquid element of the cathode exit stream is passed through the heat exchanger  130 . In addition, only one pump  132  is required to transfer the recovered water back to the containment vessel  140 . One advantage of this configuration is that as the inlet to the heat exchanger  130  is part way from the bottom, any residual water that remains in the heat exchanger  130  after shutdown will fall to occupy the lower part of the heat exchanger  130 . In the event of the heat exchanger  130  then being subjected to sub-zero temperature, this water will freeze. However, flow will still be possible through the cathode flow path through the rest of the heat exchanger, which will then heat up and defrost the frozen water in the lower part. 
         [0064]    The purge port  510  in each case allows the cyclonic separator to be cleared dry, allowing subsequent storage at low temperatures. 
         [0065]    Preferably, the thermal inertia of the cyclonic separator  131  is low such that when a small amount of liquid water enters the separator when the separator  131  is below 0° C., the liquid water does not freeze. 
         [0066]    Optionally, a pressure relief valve may be used to increase the back pressure of the system to such an extent that the transfer pumps  132 ,  132   a ,  132   b  are not required. However, in this instance the cyclonic relief valve and purge port  510  might not be fitted, to ensure that the internal pressure in the cathode water exit line  121  forces water through the water ejection line towards the water containment vessel  140 . If no transfer pump  132  is fitted, the arrangement shown in  FIG. 5   b  is more preferable, to ensure that water ejected from the cathode exit line passes through a single water ejection line  128 . 
         [0067]    During operation of the fuel cell system  100  (with reference to  FIGS. 1 and 2 ), water from the containment vessel  140  is pumped through the cathode flow path via the cathode water injection line  125  and the cathode water injection inlet  127 . After passing through the cathode volume within the fuel cell stack  110 , water passes out of the stack  110  via the cathode water exit line  121  and into the heat exchanger  130 . A mixture of exhaust gas and condensed water passes through the water separator  131 . Condensed water then passes through the water return line  128  and into the water containment vessel  140 . Any excess water is ejected through the water overflow line  144 . Exhaust gases are ejected through the exhaust shut-off valve  120 , which is held at least partially open to control the pressure within the cathode flow path. 
         [0068]    With reference to  FIG. 1 , fuel gas is fed into the anode inlet  156  and into the anode volume (not shown) within the fuel cell stack  110 . Valves  153 ,  161  are operated to maintain a desired pressure within the anode volume. The manual valve  162  connected to the anode outlet  159  remains closed. Optionally, water ejected from the anode exhaust stream is separated into liquid and gas phases with a further water separator  163 . 
         [0069]    On shutdown of the fuel cell system  100 , the fuel supply to the fuel cell stack  110  is first shut off by closing the solenoid actuated valve  153  on the fuel supply line  155 . The shut-off valve  120  on the cathode exhaust line  122  is then closed, while the air compressor  133  continues operation. In practice a time may be required to flush through the cathode flow path with air before the shut-off valve  120  is closed. The pressure in the cathode flow path then rises. The water ejection line pump  132 , if present, preferably continues operation for a time after the shut-off valve is closed, to allow water to continue passing through to the containment vessel  140 . The containment vessel pump  230  ceases operation, and water consequently stops being fed into the cathode water injection line  125 . 
         [0070]    A short purging operation of the anode flow path may also be used during shutdown to eject water present in the anode flow path, with water being forced through opened anode exit valve  161 , followed by depressurisation of the anode volume in the fuel cell stack  110 . 
         [0071]    Air being flushed through the cathode flow path forces residual water out of the cathode volume in the fuel cell stack  110  and through the cathode exit line  121 , through the heat exchanger  130  to the water separator  131 . The pump  132  pumps water from the separator  131  through the water return line  128  and into the water containment vessel  140 . When the shut-off valve  120  closes the cathode air pressure will rise, forcing water out of the cathode volume in the fuel cell stack  110  through the cathode water injection inlet  127  and through the cathode water injection line  125  and the purge port  238  of the reverse flow relief valve  213  towards the water containment vessel  140 . 
         [0072]    The ability to purge water from the cathode flow path through to the water containment vessel  140  allows water, which may be trapped in internal features and water distribution galleries, to be removed. For a typical size of fuel cell stack for automotive applications, this may result in around 30 ml of water being removed from the cathode flow path. The use of the air compressor  133  rather than a nitrogen purge feed reduces the number of components required and avoids excessive drying out of the fuel cell stack. The membranes in the stack can then be kept in a more suitable state for a subsequent start-up operation. Timed operation of the air compressor on shutdown can be optimised to provide a balance between removing sufficient water to prevent adverse effects from sub-zero conditions and dehydration of the membranes. For a typical fuel cell system, the air compressor may be operated around 1 to 2 minutes after closing the exhaust valve  120 . Hydrogen gas may also be used to purge excess water from the anode flow path. 
         [0073]    Other embodiments of the invention are intended to be within the scope of the invention, as defined by the appended claims.