GB2362500A - Method for activating a solid polymer electrolyte fuel cell. - Google Patents

Abstract

A solid polymer electrolyte fuel cell 1, comprising an electrolyte membrane 2, a cathode 3 having a porous substrate 5 and a catalyst layer 7, and an anode 4 having a porous substrate 6 and a catalyst layer 8, is assembled in a dry state and is activated prior to its operation by directing water vapour, in particular steam, over the fuel and/or oxidant electrodes and in particular parts 2, 7 and 8.

Description

2362500 METHOD FOR ACTIVATING SOLID POLYMER ELECTROLYTE FUEL CELLS The

invention relates to methods for activating solid polymer electrolyte fuel cells. In particular, it relates to activating such fuel cells by supplying a wet gas stream to a membrane ele ctrode assembly therein.

Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of economically delivering power with environmental and other benefits.

Fuel cells convert reactants, namely fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.

During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.

A broad range of reactants can be used in solid polymer electrolyte fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol 2 - fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein by means of fluid diffusion layers. In the case of gaseous reactants, these layers are referred to as gas diffusion layers.

Solid polymer electrolyte fuel cells employ a membrane electrode assembly ("MEA") which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Typically, the electrolyte is bonded under heat and pressure to the electrodes and thus such an MEA is dry as assembled. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, Nafionl'). The catalyst layer may also contain a binder, such as polytetrafluoroethylene (PTFE). The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Separator plates, or flow field plates for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA.

In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are typically placed at each end of the stack to hold it together and to compress the stack components together. Compressive force is generally needed for effecting seals and making adequate electrical contact between various stack components.) To be sufficiently ion-conductive, the membrane 3 electrolyte in a solid polymer fuel cell generally needs to be adequately hydrated. While water is generated by the electrochemical reactions in the fuel cell during operation, this product water is typically inadequately distributed for purposes of maintaining a sufficient state of hydration over the entire membrane. For instance, the inlet reactant streams as supplied may be relatively dry and thus may dry out the membrane in the vicinity of the reactant inlets. Thus, one or both inlet reactant streams are typically humidified.

Typically, solid polymer fuel cells are assembled in a dry state and the membrane electrolyte and the catalyst layers (including the catalyst) must be hydrated before normal electrical power producing operation can begin. The hydration of the membrane thus becomes part of an activating process in which an assembled fuel cell is first made ready for use. A similar hydrating process may be required if a previously operated fuel cell is allowed to dry out during prolonged storage.

A method for activating a solid polymer fuel cell or stack prior to normal power producing use involves operating it under controlled low load conditions until the membranes are suitably hydrated. This activation method however may be onerous in large scale manufacture since each stack must actually be supplied with reactants, connected to an electrical load, and operated to produce power as part of the assembly process, thus representing a relatively complex and expensive procedure. Further, the process may take hours to complete properly. Still further, cells in a stack are more likely to undergo voltage reversal during activation due to uneven hydration of the membrane, catalyst layer, or other electrode components. (Voltage reversal can occur when a cell in a stack is unable to sustain the current forced through it by other cells in the stack during operation. Voltage reversal events can cause damage to the stack and thus a low magnitude activation current would generally be used to prevent reversal.) An alternative to activating the - 4 assembled stacks as part of the manufacturing process is to activate the assembled stacks in the field. However, this undesirably shifts the burden of activating from the fuel cell manufacturer to the customer/user.

other methods may be used to hydrate the membrane and catalyst layers after dry assembly. As disclosed in European Patent Publication No. 0961334, liquid aqueous solutions may be used to activate a gas feed solid polymer fuel cell.

However, merely directing liquid water through reactant flow field channels may not hydrate the membrane in a timely fashion due to the highly water repellant nature of certain gas diffusion electrodes. (The liquid water does not readily access the membrane from reactant flow field channels. To permeate water repellant electrode substrates, liquid water would need to be provided at higher temperatures and/or pressures.) Instead, alternative activation methods disclosed in European Patent Publication No. 0961334 involve immersing the fuel cell in deionized, distilled water and boiling the immersed fuel cell, using a weak acidic aqueous solution such as sulfuric acid or hydrogen peroxide, or introducing an alcohol into the reactant stream (which has an affinity to the carbon materials in the fuel cell) prior to directing a liquid aqueous solution into a gas supply inlet.

In any method that involves introducing a liquid stream into the fuel cell, there is always a concern regarding contamination. The liquid stream must be free of metal ions and other impurities to avoid adverse effects on the membrane electrolyte or catalyst.

The present improved methods for activating solid polymer fuel cells apply generally to fuel cells having an anode, a cathode, a solid polymer electrolyte, a fuel diffusion layer for distributing fuel to the anode, and an oxidant diffusion layer for distributing oxidant to the cathode. The improved activation method comprises a hydrating step performed prior to operating the fuel cell to produce electric power comprising supplying a gas stream comprising water vapor to at least one of the fuel and oxidant diffusion layers and then exhausting the gas stream from that diffusion layer. The water vapor in the gas stream may more readily access the-solid polymer electrolyte and catalyst layers than liquid water would, particularly in gas feed solid polymer fuel cells where the gas diffusion layers tend to be hydrophobic. Thus, the method is particularly suitable for activating such gas feed solid polymer fuel cells. However, it may also be suitable for activating certain liquid feed solid polymer fuel cells in which one electrode may comprise a hydrophobic gas diffusion layer (for example, the cathode of a typical direct methanol fuel cell).

The hydrating gas stream can be supplied to either the fuel or oxidant diffusion layers or both. It may be advantageous to circulate the hydrating gas stream over the desired diffusion layer(s).

The fuel cell is not producing electrical power during the hydrating step and therefore does not need to be fed with reactants. Thus, the fuel cell may be in an open circuit condition during the hydrating step. Further, neither fuel nor oxidant is required and so the hydrating gas stream employed may contain no fuel or oxidant, and may consist essentially of water vapor. The hydrating gas stream for instance may simply be steam.

Preferably the vapor pressure of water in the gas stream is greater than about 31 kPa (the water vapor saturation pressure at 70oC). The temperature of the hydrating gas stream may be greater than about 70oC. Under conditions such as these, the hydrating step can typically be accomplished in less than about 5 minutes duration.

An advantage of the method is that the rate at which water in the gas stream is supplied to the desired diffusion layer(s) can exceed the rate at which product water would be generated inside the fuel cell from electrochemical reactions when the fuel cell is operating at its maximum normal current 6 density. Thus, in principle, water can be provided to the membrane electrolyte and catalyst layers faster via the present activation method than via operation of the fuel cell.

The hydrating gas stream may be supplied to any of the fuel or oxidant ports in the fuel cell (that is, the fuel or oxidant inlets or outlets) or to both a fuel and an oxidant port if desired. Both fuel and oxidant diffusion layers may be hydrated by a single hydrating gas stream (for example, by first supplying a hydrating.gas stream to a fuel port, exhausting from another fuel port, and then supplying to an oxidant port). Preferably, these ports are heated during the hydrating step such that water vapor does not condense from the gas stream prior to entering the fuel cell. However, the method may be used when the fuel cell is at ambient temperature.

Alternatively, the gas stream comprising water vapor can be generated inside the fuel cell itself by heating in-situ a liquid water stream supplied to the fuel and/or oxidant ports of the fuel cell. This may be accomplished, for example, simply by heating the entire fuel cell to a temperature above 100oC or by using a heater in a reactant manifold or the like to generate steam.

A preferred activation method involves hydrating the fuel cell after assembly into a fuel cell stack. Alternatively however, the membrane electrode assembly of the fuel cell may be hydrated first and then assembled into a fuel cell stack.

After adequately hydrating the solid polymer electrolyte and catalyst layers, the reactant inlets and outlets in the fuel cell are preferably sealed to prevent subsequent drying out of the electrolyte. The fuel cell may thus be stored without requiring further hydration later.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

- 7 FIG. 1 is a schematic diagram of a typical solid polymer fuel cell stack.

FIG. 2 is a plot of the AC impedance spectrum for the dry fuel cell described in Example 1 of the present 5 application.

FIG. 3 is a composite plot of AC impedance spectra taken at various times during fuel cell activation using a prior art method of initiating operation of the cell under controlled conditions, described in Example 2 of the present application.

FIG. 4 is a plot of fuel cell voltage as a function of time during activation using a conventional, prior art method of initiating operation of the cell under controlled conditions, described in Example 2 of the present application.

FIG. 5 is a composite plot of AC impedance spectra taken at various times during fuel cell activation in which the cell was heated and initially hydrated only by supplying wet reactant gases at 70oC and 1000 relative humidity (RH) to the electrodes, as described in Example 3 of the present application.

FIG. 6 is a plot of fuel cell voltage as a function of time under load after the cell was initially hydrated only by supplying steam at 110oC to the electrodes, as described in Example 5 of the present application.

A schematic diagram of a typical solid polymer fuel cell stack is depicted in FIG. 1. For simplicity, FIG. 1 shows only one cell in the fuel cell stack. Stack 1 comprises a membrane electrode assembly consisting of solid polymer electrolyte membrane 2 sandwiched between cathode 3 and anode 4. Cathode 3 comprises porous substrate 5 and catalyst layer 7. Anode 4 comprises porous substrate 6 and catalyst layer 8. Substrates 5, 6 serve as electrically conductive backings and mechanical supports for catalyst layers 7, 8. Substrates 5, 6 also serve as diffusion layers for fluid reactants supplied to flow field plates 9, 10. During operation, oxidant (typically air) and fuel (typically hydrogen) are supplied to flow field plates 9 and 10 respectively at inlets 11 and 13 respectively. The oxidant and fuel streams exhaust from stack I at outlets 12 and 14 respectively. During operation, power is delivered to a load depicted as resistor 15.

For simplicity, stack 1 is assembled in a dry state and membrane 2 and catalyst layers 7,8 are hydrated before normal operation is commenced. In the present activation method, a gas stream comprising water vapor is supplied to one or both flow field plates 9, 10 at a suitable port. (FIG. I shows steam being supplied to oxidant inlet 11 but it could instead be supplied at any of inlets 11, 13 or outlets 12, 14.) The gas stream is then exhausted from the flow field plate(s) from the appropriate port(s) (that is, outlet 12 as shown in FIG. 1). The gas stream may be circulated through the appropriate flow field plate(s) if desired. The gas stream may also be directed in series to both flow field plates if desired (for example, as shown in FIG. 1 except that the exhaust gas stream from oxidant outlet 12 may be directed to fuel inlet 13 and exhausted at fuel outlet 14). Unlike liquid water, which does not readily wet and permeate hydrophobic substrates 5, 6, the water vapor in the gas stream does not need to wet the porous substrates and can readily permeate them to access and hydrate membrane 2. Once stack 1 is hydrated, the stack inlets and outlets 11, 12, 13, 14 may then be sealed to prevent water loss, until the stack is to be used.

Preferably the concentration of water in the gas stream is substantial and thus the gas stream is generally at a high relative humidity and a relatively high temperature. In this way, a substantial amount of water can be delivered to the membrane and catalyst layers rapidly for hydration purposes.

Steam represents a convenient, essentially fully humidified, relatively high temperature gas stream for hydrating at 9 ambient pressure. Further, superheated steam at elevated temperature and pressure may be employed for introducing even greater amounts of water. Alternatively however, the gas stream may be, for example, heated and humidified air. The instant activation method is simpler than hydrating the membrane and catalyst layers by operating the fuel cell stack under load. In addition, the instant method can fundamentally supply water to the substrate(s) faster than it can be generated in the stack and hence faster than it can be supplied by operation at maximum normal current density.

In order to avoid condensing water vapor in the substrates and hence potentially blocking access to the membrane, the stack temperature is preferably at or above the temperature of the incoming hydrating gas stream. However, it has been found that hydration can still be carried out successfully when the stack temperature is initially much lower than that of the hydrating gas stream (for example, when the stack is at ambient temperature and the stream is just over 100oC). Under these conditions presumably the stack geometry, component heat capacities, and so on are such that whatever condensation may occur does not prevent water vapor from timely hydrating the membrane. (For instance, the substrates are relatively thin and are adjacent to the supplied gas stream. Thus, the substrates may warm quickly when a higher temperature gas stream is introduced.) Nonetheless, if the stack temperature is significantly less than the gas stream temperature, it may still be preferable to heat the port used for the supplied hydrating gas stream (inlet 11 in FIG. 1) to avoid condensing water vapor therein.

It may however be beneficial to havesome limited condensation occur at other-specific locations within the stack. For instance, it may be beneficial for water vapor to traverse the hydrophobic substrates but then condense in the area of the catalyst-membrane interface. The ionic conductivity of the membrane would then be expected to be higher since it is in contact with liquid water as opposed to - being surrounded by water vapor. It may also be advantageous to condense water in this way in order to reduce the length of time required for supplying the hydrating gas. For instance, instead of flowing hydrating gas until the membrane and catalyst layers are suitably hydrated, the hydrating gas stream may only need to be supplied for a shorter time if the water vapor introduced is deliberately allowed to condense appropriately. This may be accomplished by simply stopping the flow of the hydrating gas after a suitable period of time and allowing the fuel cell to cool.

Instead of supplying a gas stream containing water vapor to the stack, embodiments of the present method are contemplated in which liquid water is supplied to the stack but is converted into vapor before it reaches the flow field plates or diffusion layer substrates. This may be done by incorporating a heater in a fluid distribution manifold just inside the stack or by heating the entire stack (for example, above 100oC).

Further, instead of supplying a gas stream containing water vapor to the stack, embodiments of the present method are contemplated in which water vapor is supplied to a membrane electrode assembly separately or to MEA components, and then the hydrated membrane electrode is assembled into a fuel cell stack thereafter. Such embodiments may be employed to reduce the energy requirements associated with the heating plates or other stack components that do not require hydration. Such embodiments could also be integrated into a process line or may be employed for other reasons such as, for example, sealing or to accommodate a change in the dimensions of the membrane prior to compression.

A fuel cell stack can essentially be hydrated very quickly using the present activation method (for example, in less than 5 minutes, the membrane ionic conductivity can be increased an amount that is-within a few percent of the total increase required to achieve the conductivity observed under normal operating conditions). Because the stack is not operated during the hydration step, neither an electrical load nor reactants (fuel or oxidant), nor cooling are required.

The present method thus can be essentially equivalent to fuel cell operation under load insofar as fuel cell activation is concerned. A similar hydrating method may prove useful for other purposes, including product development or quality control. For instance, an individual solid polymer membrane, an electrode, or a membrane electrode assembly may be hydrated in a similar fashion in order to simulate conditions obtained in actual fuel cell operation. This may be useful for accelerated lifetime testing or factory component checking.

The following examples are provided to illustrate certain aspects and embodiments of the invention but should not be construed as limiting in any way.

Exam 1 e 1 A solid polymer electrolyte fuel cell was assembled in a dry state and was analyzed using AC impedance techniques.

The cell contained a 300 ce active area membrane -electrode assembly with platinum catalyzed electrodes and a NAFIONO N112 perfluorosulfonic acid membrane electrolyte. On both cathode and anode, carbon-supported Pt catalyst was employed on TGP060 grade (product of Toray) carbon fiber substrates at a total loading (both electrodes combined) of approximately 1 mg Pt/cm2. The cell employed serpentine flow field plates made of graphite clamped between end plates at a loading of

80 psig (552 kPa). Typical normal operation for this cell involves supplying 100% RH hydrogen and air, each at 700C and 3 bar absolute (bara), to the cathode and anode flow field plates respectively. The maximum normal operating current density for this cell is about 1 A/cm.

An AC impedance spectrum of the dry fuel cell at about - 12 70oC was taken while dry hydrogen (0% RH) and dry air at 70oC and 3 bara were supplied to anode and cathode respectively and with no applied load. The spectrum was obtained using a Solartron 1255B HF frequency response analyzer and a high power loadbank modulated by the frequency response analyzer. Current measurements were taken across a 0.5 milliohm shunt rated to 200A. Data points were taken over a frequency range from 65 kHz to 0.1 Hz using a 100 mV amplitude AC supply. FIG. 2 shows the AC impedance spectrum for this dry fuel cell. (In taking an AC impedance spectrum, a small amount of water is generated but by taking repeated spectra and comparing, the effect on the impedance was observed to be small.) Examnle 2 A similar fuel cell was activated according to a prior art method of operating it initially under controlled conditions. Instead of dry reactant streams, hydrogen and air were supplied at 70oC and 100-06 RH and thus the vapor pressure of water in the reactant streams was about 31.2 kPa. A load was applied such that the cell operated at a current density of 500,,A/CM2 of electrode area. AC spectra were taken at various times during the activation process and are shown in FIG. 3 (plots A, B, C, D and E show the spectra at 5, 15, 60, 180, and 1380 minutes respectively). A spectrum was also obtained after 1440 minutes of operation (plot F) and was essentially similar to that obtained at 1380 minutes (that is, the impedance had stabilized). The fuel cell voltage versus time during activation is shown in FIG. 4.

A comparison of FIGs. 2 and 3 shows a marked difference in impedance between the dry cell and the hydrated (activated) cell of this Example 2. The dry cell impedance is about two orders of magnitude larger than that of the hydrated cell. During activation, the greatest change in 13 impedance and in output voltage occurs within the first five minutes after which further.improvement is slow. The fuel cell is delivering current at an acceptable voltage (about 0.64 V) after about five minutes, but the fuel cell voltage continues to rise with time and stabilizes at about 0.68 V only after many hours of operation.

Example 3

A similar fuel cell was activated using the present activation method. Here, the cell was heated to 70oC and hydrogen and air streams were supplied at 70oC and 100% RH to the anode and cathode respectively in order to hydrate the membrane electrolyte and catalyst layers. No electrical load was applied initially. AC spectra were taken at various times during the activation process and are shown in FIG. 5 (plots A, B, C, D and E show the spectra at 5, 15, 60, 180, and 1260 minutes respectively). After 1260 minutes, an electrical load was applied such that the cell operated at a current density of 500 mA/cm. AC spectra were then obtained at various times during operation and are also shown in FIG. 5 (plots F and G show spectra at 5 and 1380 minutes after application of the load). The fuel cell voltage (open circuit) versus time during hydration (before applying the load) was about 0.93 V. Immediately upon application of the load, the cell voltage was 0.64 V. Thus, the fuel cell is immediately capable of delivering current at an acceptable voltage following hydration. After prolonged operation, the fuel cell voltage was again about 0.68 V.

The fuel cell initially activated by controlled operation in Example 2 and the fuel cell of Example 3 initially activated using wet reactant gas flow only (with no load) show similar impedance spectra during the initial activation period and their voltages under load are similar thereafter. Obtaining a stabilized output voltage appears to - 14 require operation for a longer period in both cases. However, both methods provide an acceptable output voltage in a short period of time. The present method represents a suitable activation method for manufacturing purposes.

Example 4

Another similar fuel cell was activated initially by supplying 70oC and 100-. RH hydrogen and air to the anode and cathode respectively and with no electrical load applied. In this case however, the cell was at ambient temperature (about 20oC) and thus water condensation in the fuel cell could potentially occur thereby affecting hydration of the membrane. The open circuit fuel cell voltage rose rapidly during hydration (before applying any load) and was over 0.9V within a few minutes. AC impedance spectra were taken at various times as above and were found to be essentially similar to the spectra taken in Example 3 at 70oC. Thus, this fuel cell activated at ambient temperature in Example 4 appeared to behave similarly to the cell activated at 70oC in Example 3. An electrical load was applied following hydration, but this time such that the cell operated at a lower current density of 100 mA/cm2. Immediately upon application of the load, the cell voltage was just a few millivolts below 0.79 V and after prolonged operation was just a few millivolts over 0.79 V.

Examn 1 e 5 Another similar fuel cell was activated initially by supplying steam (no reactants) to both the cathode and anode at 110oC. The cell was again at ambient temperature and no electrical load was applied. The inlets to the cell were heated in this case to prevent condensation of water prior to the stream entering the cell. AC impedance spectra were is - taken at various times as above and results were similar to those obtained in Examples 3 and 4. An electrical load was then applied such that the cell operated at a current density of 100 mA/cm2. FIG. 6 shows the cell voltage versus time during operation. The method used in this Example 5 also provided for an acceptable output voltage in a short period of time and thus represents a suitable activation method for manufacturing purposes. It requires no reactants or connection of an electrical load.

As mentioned above, a maximum normal operating current density for the cells used in the Examples is about 1A/cm2. At that current density, the water production rate from the electrochemical reactions therein would then be about 1.6 millimoles/second (or 2.2 slpm of water) per cell. However, the total flow rate of reactant gas supplied to cathode and anode together would typically be about 11.5 standard liters per minute (slpm) (based on typical air and hydrogen flow rates of about 1.8 and 1.2 times the rate at which they are consumed in the electrochemical reactions respectively). The total flow rate of water vapor supplied to cathode and anode together in these reactant gases would typically be about 1.3 slpm (assuming these reactant gases were saturated with water vapor at 70oC). If steam at 105oC was supplied at this total flow rate for hydrationpurposes, this would correspond to about 8.6 millimoles/second of water being delivered to the fuel cell. This is substantially more than the rate at which product water is typically supplied and generated inside the fuel cell when operating at maximum normal current density.

While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.

16 -

Claims (21)

Claims

1. A method for activating a solid polymer fuel cell, said fuel cell having an anode, a cathode, a solid polymer electrolyte, a fuel diffusion layer for distributing fuel to said anode, and an oxidant diffusion layer for distributing oxidant to said cathode, said method comprising performing a hydrating step prior to operating said cell to produce electrical power comprising supplying a gas stream comprising water vapor to at least one of said fuel and oxidant diffusion layers, and exhausting said gas stream from said at least one diffusion layer.

2. The method of claim 1 wherein said fuel cell is a gas feed solid polymer fuel cell and said diffusion layers are gas diffusion layers.

3. The method of claim 1 or 2 wherein said gas stream is circulated over said at least one of said fuel and oxidant diffusion layers.

4. The method of any preceding claim wherein gas streams comprising water vapor are supplied to both of said fuel and oxidant diffusion layers.

5. The method of any preceding claim wherein said fuel cell is open circuit during said hydrating step.

6. The method of any preceding claim wherein said gas stream contains neither fuel nor oxidant.

7. The method of claim 6 wherein said gas stream consists essentially of water vapor.

8. The method of any preceding claim wherein the vapor pressure of water in said gas stream is greater than about 31 - 17 kPa.

9. The method of any preceding claim wherein the temperature of said gas stream is greater than about 70oC.

10. The method of claim 9 wherein said gas stream is steam.

11. The method of any preceding claim wherein said hydrating step is less than about 5 minutes in duration.

12. The method of any preceding claim wherein the rate at which water in said gas stream is supplied to said at least one of said fuel and oxidant diffusion layers exceeds the rate at which water is generated inside said fuel cell from electrochemical reactions when said fuel cell is operating at the maximum normal current density.

13. The method of any preceding claim comprising supplying said gas stream to at least one of the fuel and oxidant ports in said fuel cell and exhausting said gas stream from another of said fuel and oxidant ports.

14. The method of claim 13 comprising heating said at least one port during said hydrating step such that water vapor does not condense from said gas stream prior to entering said fuel cell.

15. The method of claim 13 wherein said fuel cell is at ambient temperature at the commencement of said hydrating step.

16. The method of claim 1 comprising: supplying a liquid water stream to at least one of the fuel and oxidant ports in said fuel cell; heating said supplied liquid water stream to generate said gas stream comprising water vapor within said fuel cell; and exhausting said heated supplied stream from another of said fuel and oxidant ports.

17. The method of claim 16 wherein said fuel cell is at a temperature above 1000C at the commencement of said hydrating step.

18. The method of any preceding claim wherein said hydrating step is performed.on a membrane electrode assembly of said fuel cell prior to assembling said membrane electrode assembly into a fuel cell stack.

19. The method of any preceding claim further comprising sealing the ports for said fuel and said oxidant in said fuel cell after said hydrating step.

20. The method of any preceding claim further comprising operating said cell to produce electrical power after said hydrating step.

21. A method for activating a cell substantially as herein described with reference to the accompanying drawings.