Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
  • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
  • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
  • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
  • H01M8/04126—Humidifying
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
  • H01M8/04492—Humidity; Ambient humidity; Water content
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
  • H01M8/04537—Electric variables
  • H01M8/04604—Power, energy, capacity or load
  • H01M8/04619—Power, energy, capacity or load of fuel cell stacks
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
  • H01M8/04537—Electric variables
  • H01M8/04604—Power, energy, capacity or load
  • H01M8/04626—Power, energy, capacity or load of auxiliary devices, e.g. batteries, capacitors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04746—Pressure; Flow
  • H01M8/04753—Pressure; Flow of fuel cell reactants
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04858—Electric variables
  • H01M8/04895—Current
  • H01M8/0491—Current of fuel cell stacks
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04858—Electric variables
  • H01M8/04925—Power, energy, capacity or load
  • H01M8/0494—Power, energy, capacity or load of fuel cell stacks
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M8/1016—Fuel cells with solid electrolytes characterised by the electrolyte material
  • H01M8/1018—Polymeric electrolyte materials
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04313—Processes for controlling fuel cells or fuel cell systems characterised by the detection or assessment of variables; characterised by the detection or assessment of failure or abnormal function
  • H01M8/0432—Temperature; Ambient temperature
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates to fuel cells and in particular to proton-exchange membrane type fuel cells in which hydrogen is supplied to the anode side of the fuel cell, oxygen is supplied to the cathode side of the fuel cell and water byproduct is produced at and removed from the cathode side of the fuel cell.
• Such fuel cells comprise a proton exchange membrane (PEM) sandwiched between two porous electrodes, together comprising a membrane-electrode assembly (MEA).
• the MEA itself is conventionally sandwiched between: (i) a cathode diffusion structure having a first face adjacent to the cathode face of the MEA and (ii) an anode diffusion structure having a first face adjacent the anode face of the MEA.
• the second face of the anode diffusion structure contacts an anode fluid flow field plate for current collection and for distributing hydrogen to the second face of the anode diffusion structure.
• the second face of the cathode diffusion structure contacts a cathode fluid flow field plate for current collection, for distributing oxygen to the second face of the cathode diffusion structure, and for extracting excess water from the MEA.
• the anode and cathode fluid flow field plates conventionally each comprise a rigid, electrically conductive material having fluid flow channels in the surface adjacent the respective diffusion structure for delivery of the reactant gases (e.g. hydrogen and oxygen) and removal of the exhaust gases (e.g. unused oxygen and water vapour).
• This water must be exhausted from the MEA via the cathode diffusion structure at the same time that oxygen is transported to the cathode face of the MEA.
• the MEA remains suitably hydrated to ensure that the internal electrical resistance of the cell remains within tolerable limits.
• the cathode fluid flow field plates are open to ambient air, usually assisted by a low pressure air source such as a fan, which provides the dual function of stack cooling and oxygen supply.
• a low pressure air source such as a fan
• the dual purpose of the air flow may lead to a conflict in air flow requirements.
• a very high stoichiometric air flow across the cathode electrodes is required for cooling and, depending on ambient conditions and stack temperature this may result in a low membrane water content (resulting in low performance) or in extreme cases a continual net water loss from the fuel cell stack over time which will eventually result in the stack ceasing to function. This is because for a set level of stack power output (current density) a balance will be achieved between the water content of the fuel cell polymer membranes and the rate of water removal by the flow of air. A lower current, high air flow and wanner stack will tend to reduce the membrane water content and conversely a higher current, lower air flow and cooler stack will increase the membrane water content.
• the present invention provides an electrochemical fuel cell assembly comprising: a fuel cell stack comprising a number of fuel cells each having a membrane-electrode assembly and fluid flow plates for delivery of fuel and oxidant thereto, and an electrical output for delivery of current from the stack; and a stack power controller for periodically and temporarily increasing the current drawn from the fuel cell stack, in addition to or instead of independent current demand external to the fuel cell assembly, during rehydration intervals to increase the hydration level of the fuel cells.
• the present invention provides an electrochemical fuel cell assembly comprising: a fuel cell stack comprising a number of fuel cells each having a membrane-electrode assembly and fluid flow plates for delivery of fuel and oxidant thereto, and an electrical output for delivery of current from the stack; a stack controller for modulating air flow through the fuel cell stack on a periodic basis independent of current demand on the fuel cell stack assembly to provide rehydration intervals that increase the hydration level of the fuel cells; and means for mamtainuig the current demand to a load external to the fuel cell • assembly during the rehydration intervals.
• the present invention provides a method of operating an electrochemical fuel cell assembly having a fuel cell stack comprising a number of fuel cells each having a membrane-electrode assembly and fluid flow plates for delivery of fuel and oxidant thereto, and an electrical output for delivery of current from the stack, the method comprising the steps of: periodically and temporarily increasing the current drawn from the fuel cell stack independent of current demand external to the fuel cell assembfy during rehydration intervals to increase the hydration level of the fuel cells; and maintaining the current demand to a load external to the fuel cell assembly during the rehydration intervals.
• the present invention provides a method of operating an electrochemical fuel cell assembly having a fuel cell stack comprising a number of fuel cells each having a membrane-electrode assembly and fluid flow plates for delivery of fuel and oxidant thereto, and an electrical output for delivery of current from the stack, the method comprising the steps of: modulating air flow through the fuel cell stack on a periodic basis independent of current demand on the fuel cell stack assembly to provide rehydration intervals that increase the hydration level of the fuel cells while maintaining the current demand to a load external to the fuel cell assembly during the rehydration intervals.
• the invention provides a fuel cell assembly in which one or more operating parameters, such as electrical current flow from, and air flow to, a fuel cell stack within the assembly is periodically modulated during rehydration intervals to intermittently increase hydration levels of the fuel cell stack independently of an electrical current demand on the fuel cell assembly from an external load to the fuel cell assembly. During the rehydration interval, the electrical current delivery to the external load is maintained.
• one or more operating parameters such as electrical current flow from, and air flow to, a fuel cell stack within the assembly is periodically modulated during rehydration intervals to intermittently increase hydration levels of the fuel cell stack independently of an electrical current demand on the fuel cell assembly from an external load to the fuel cell assembly.
• Figure 1 is a graph showing cell potential as a function of time for conventional steady state operation of a fuel cell compared with pulsed current operation of the fuel cell for two different pulse repetition intervals
• Figure 2 is a graph showing instantaneous and averaged cell potential as a function of time for conventional steady state operation of a fuel cell compared with pulsed current operation;
• Figure 3 is a schematic diagram of an electrochemical fuel cell power unit for implementing a pulsed current operation scheme
• Figure 4 is a graph showing stack voltage and stack current as a function of time before, during and after a stack hydration pulse.
• Figure 5 is a graph showing the comparative improvement in fuel cell performance for the pulsed current operation in (i) a fuel cell "with single layer cathode diffuser, and (ii) a fuel cell with multi-layer cathode diffuser.
• the invention proposes temporarily disrupting the equilibrium (as would be determined by existing operating conditions of a fuel cell stack) of membrane water content and rate of water removal to achieve a higher stack and system efficiency.
• the procedure involves producing excess water at the fuel cell cathode for short periods of time and subsequently operating the stack with a higher performance while the equilibrium with a lower water content is gradually reestablished.
• the process can be repeated at certain interval frequencies as required.
• 'rehydration intervals' which expression is intended to indicate a period of time in which the fuel cell assembly actively controls its operating environment to purposively increase hydration levels above a level that would otherwise prevail based on the external electrical load on the fuel cell and its environmental operating conditions such as temperature.
• This rehydration process can be achieved by one or both of the following techniques:
• the main benefit of this procedure is an improved performance during the normal operating conditions due to a higher cell voltage, giving improved energ ⁇ ' conversion efficiency. This results in a lower stack operating temperature that can extend the life expectancy of the membrane.
• the beneficial effects of the rehydration procedure described here is most evident during start-up of a fuel cell assembly. This is particularly the case for conditioning a newly-built fuel cell where the rehydration process can improve the response of an unconditioned fuel cell stack to immediate large load demands.
• a secondary benefit is allowing air-cooled (open cathode) fuel cell stacks to operate in a wider range of environments, especially hotter and drier ambient conditions.
• a fuel cell system utilising an open cathode stack conventionally the only way to adjust the fuel cell membrane water content is to change the air flow delivered by the cooling fan, where an increase in air flow will result in a lower stack temperature and conversely a lower air flow wall heat the stack.
• the invention proposes integrating a fuel cell stack into a system with hardware and an operational controller to provide for rehydration not directly dependent on operating ambient and external load conditions so as to maintain a more optimum performance. .
• the effect of periodically and temporarily increasing the current drawn from a fuel cells above its base load to a higher current is shown in figure 1.
• the stack was operating at a base load of 320 mA/cm 2 .
• the base load may be considered as that which is determined by the external current demand on the fuel cell assembly, together with any continuous parasitic load on the fuel cell stack by the fuel cell assembly itself (i.e. control circuits, fans, etc).
• the periodic and temporary increased current demand on the fuel cell stack occurred as current pulses in which the stack current was increased to 900 mA/cm 2 for a rehydration interval of approximately 5 seconds at two and five minute intervals respectively.
• the overall improvement in efficiency was based on the lower heating value (LHV) of H 2 at 50 degrees C and assumes zero efficiency during the 5 second rehydration pulse at high load.
• LHV lower heating value
• a stead ⁇ ' state base load of 320 mA/cm" results in a cell voltage of just over 0.65 V as shown by line 10 in figure 1 and an LHV of 52.4%.
• the cell voltage when operated with rehydration intervals of 5 seconds duration every 2 minutes is given by line 11 in figure 1. This corresponds to a rehydration duty cycle of about 4.2% and gives an LHV of 57.6%.
• the cell voltage when operated with rehydration intervals of 5 seconds duration every 5 minutes is given by line 12 in figure 1. This corresponds to a rehydration duty cycle of about 1.7% and gives an LHV of 57.2%.
• the scale of the immediate increase in cell voltage after the rehydration intervals and the following decay is clear in figure 1.
• the improvement in performance will depend on the water retention factors of the fuel cell, in particular the characteristics of the polymer membrane and any gas diffusion layers incorporated therewith as well as the temperature and air flow through the cell stack.
• the hydration intervals are especially effective when used in conjunction with advanced diffusion media adjacent to the MEA which assist in control and retention of water levels at the membrane.
• the invention is therefore particularly advantageous when used in conjunction with multi-layer diffusion structures that assist in water trapping, such as the arrangement for open cathode stacks described in UK patent application 0501598.7 and the corresponding international patent application PCT/GB2006/000074.
• Figure 5 shows a comparison between the improvements in fuel cell performance observed for the pulsed current operation in (i) a fuel cell with a single layer cathode diffuser, and (ii) a fuel cell with a multilayer cathode diffuser.
• the upper trace shows the cell voltage for configuration (ii) and the lower trace shows the cell voltage for configuration (i).
• the rehydration pulses occur every ten minutes.
• Figure 2 illustrates the effect on real time and averaged cell voltage of a fuel cell with and without rehydration current pulses.
• the cell voltage axis represents the average cell voltage across an entire stack, i.e. the stack voltage divided by the number of cells in the stack.
• the upper straight line 20 shows the time-averaged, cell voltage of just over 0.69 V, and the upper trace 21 shows the instantaneous cell voltage, both when the stack is operated with rehydration intervals.
• the lower straight line 22 shows the time-averaged cell voltage of just over 0.65 V, and the lower trace 23 shows the instantaneous cell voltage, both when the stack is operated without rehydration intervals.
• the lower trace 23 exhibits some periodicity at a different frequency than that of the upper trace 21 because there is, in both cases, a periodic anode purge to clear water build up on an otherwise closed-ended anode configuration, and this dominates the lower trace 23.
• the closed-ended anode Periodically, the closed-ended anode is switched to open-ended configuration to purge water from the anode for about 1 second.
• the effects of the rehydration intervals are very clear from the significant increase hi average and instantaneous voltages 20, 21 over the equivalent voltages 22, 23 without rehydration intervals.
• An electrochemical fuel cell assembly 30 comprises a fuel cell stack 31 having a number of fuel cells 32 connected in series. Each fuel cell 32 includes a membrane-electrode assembly and fluid flow plates for delivery of fuel and oxidant thereto, hi accordance with conventional fuel cell stack design.
• An electrical output 33 provides for delivery of electrical current from the stack 31.
• a cooling system 34 such as a fan, provides both cooling air flow and oxygen to the flow plates. Power from the fuel cell assembly 30 is delivered to an external load 41 by external power output terminals 35 via relays 42 and 43.
• An internal electrical load 36 is switchable, by switch 37 under the control of a power controller 38, to periodically and temporarily increase the current drawn from the fuel cell stack 31.
• An auxiliary or 'reservoir' power source 39 is connected to the output terminals 35, through relay 43, to supply power to the power output terminals 35 at times when the fuel cell stack 3.1 is switched to supply the internal load 36.
• the load control circuit 40 and cooling sj'stern 34 may also be under the control of the power controller 38.
• the reservoir power source 39 is preferably a rechargeable battery but any other form of suitable charge storage device may be used, such as supercapacitors.
• the load control circuit 40 is preferably a DC/DC converter.
• the fuel cell stack 31 is normally switched to supply the external load 41, and the internal load 36 and reservoir power source 39 are both electrically isolated from the fuel cell stack 31 and the power output terminals 35.
• the power controller 38 opens relay 42 and operates switch 37 so that the reservoir power source 39 is isolated from the fuel cell stack 31 and current from fuel cell stack 31 is diverted to the internal load 36.
• the power controller 38 maintains relay 43 in a closed condition to maintain electrical continuity between the reservoir power source 39 and the power output terminals 35 thereby suppfying the external load 41.
• the controller 38 operates switch 37 and relay 42 to isolate the internal load 36 from the fuel cell stack 31 and to reconnect the fuel cell stack to the output terminals 41.
• the reservoir power source 39 preferably remains connected so that it can be recharged by current from the fuel cell stack 31.
• the load control circuit 40 may operate to isolate the reservoir power source 39 using a third relay 44. Alternatively, the reservoir power source 39 could simply remain connected at all times.
• the fuel cell stack 31 is the main power source but during the rehydration interval, the battery 39 is the sole provider of power to the external load 41.
• the fuel cell stack 31 comes back on line, it is able to fully recharge the battery 39 and as the battery approaches full state of charge the current into it will diminish.
• the switch 37 need not be of the double throw variety if it is not necessary to isolate the fuel cell stack 31 and internal load 36 frorn the external load 41 during the rehydration interval.
• the internal load 36 can simply be added to the external load 41 in parallel during the rehydration interval.
• a reservoir power source 39 might not be strictly necessary since current is maintained from the fuel cell stack 31 to the external load 41 even during the rehydration interval.
• the power output terminals 35 could be directly connected to the fuel cell stack 31; the internal load 36 switched in and out in .a first parallel circuit as required; and the reservoir power source switched in and out with charge control in a second parallel circuit, as required.
• the stack power controller 38 may use the internal load 36 for periodically and temporarily increasing the current drawn from the fuel cell stack in addition to or instead of independent current demand external to the fuel cell assembly during the rehydration intervals. If necessary, a power control device could be used to switch in the internal load 36 on a controlled basis to avoid large switching transients.
• Rehydration intervals may also be implemented using a periodic and temporary reduction in air flow to the cathodes of the fuel cell stack 31.
• the power controller 38 may be configured to reduce power to the cooling fan 34 during a rehydration interval.
• the cooling fan is switched off during the rehydration interval.
• the stack power controller 38 may modulate air flow through the fuel cell stack 31 on a periodic basis independent of electrical current demand on the fuel cell stack to provide rehydration intervals that increase the hydration level of the fuel cells.
• the expression 'independent' in this context is intended to indicate independence from immediate or transient changes in the external electrical load 41 on the fuel cell assembly 30. Both air flow modulation and increased load may be used for the purposes of implementing rehydration intervals.
• the graph of figure 4 illustrates current and voltage profiles for this operation.
• the upper trace 50 represents stack voltage as a function of time
• the lower trace 51 represents stack current as a function of time.
• normal operation of the fuel cell is illustrated.
• the cooling fans 34 providing the cathode air flow are turned off causing a rise in stack temperature.
• a small reduction in cell voltage is observed, with a corresponding small rise in current to maintain constant power, due to mass transport limitations.
• the voltage input to the DC/DC converter 40 approaches that of the terminal voltage of the battery 39, current from the fuel cell is reduced towards zero.
• power delivery to the output terminals 35 will be supplemented by the battery 39.
• the output from the fuel cell stack 31 is then isolated by opening relay 42 under minirnum electronic load leaving the battery 39 to provide the continuous power supply to the application (e.g. external load 41), evidenced by the current falling to zero in time period 54 (t ⁇ 10 - 11 seconds).
• the internal load resistor 36 is switched across the terminals 33 of the fuel cell stack 31, as evidenced by the spike 55. This electrically loads the fuel cell stack 31 further for a controlled period of time, namely a high current interval 56 (t ⁇ 11 — 12 seconds).
• the fuel cell recharges the battery as well as supplying power to the external load 41 during the following time period 61.
• the current gradually falls as the battery 39 approaches full state of charge.
• time interval e.g. between 2 and 5 minutes
• the next rehydration operation is initiated (not shown in figure 4).
• Any suitable time interval ma3 ⁇ be used that is effective to provide a useful average increase in cell voltage.
• the time interval might be as short as 1 minute or as long as 2 hours, for example.
• the optimum frequency of the rehydration operations can depend on a number of factors, including atmospheric conditions such as temperature and humidity.
• the rehydration operations are implemented automatically on a fixed periodic basis.
• a further control algorithm may be used to switch the fuel cell assembly 30 between a normal mode in which no rehydration operations take place, and a rehydration mode in which the periodic and temporary rehydration operations are performed.
• the periodicity of the rehydration operations may be controlled according to some measurable stack operating parameter, such as average temperature, humidity, voltage profile, current profile and power demand etc.
• the duty of the rehydration intervals may be controlled according to some measurable stack operating parameter such as average temperature, humidity, voltage profile, current profile and power demand etc.
• zero current is drawn during the isolation interval 57, but it will be understood that a low current might be drawn, In some embodiments, the isolation interval might not be required.

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