WO2008073080A1

WO2008073080A1 - Method for operating a membrane electrode assembly to mitigate membrane decay

Info

Publication number: WO2008073080A1
Application number: PCT/US2006/046978
Authority: WO
Inventors: Thomas H. Madden; Sergei F. Burlatsky; Ned E. Cipollini; Mallika Gummalla; David A. Condit; Timothy W. Patterson; Dirk N. Weiss
Original assignee: Utc Fuel Cells, Llc
Priority date: 2006-12-11
Filing date: 2006-12-11
Publication date: 2008-06-19

Abstract

A method for operating a fuel cell having a membrane electrode assembly including an anode, a cathode, and a membrane between the anode and the cathode, the method including the steps of : feeding fuel to the anode at a fuel pressure; feeding oxidant to the cathode at an oxidant pressure, wherein the feeding steps create a transition plane between the fuel and the oxidant; and selectively maintaining the fuel pressure higher than the oxidant pressure sufficient to position the transition plane in the cathode or within 5% thickness of the membrane of the cathode. A protective layer can be included in the assembly, and the pressure can be manipulated to position a transition jplane (Xo) within the protective layer.

Description

METHOD POR OPERATING A MEMBRANE ELECTRODE ASSEMBLY TO

MITIGATE MEMBRANE DECAY

BACKGROUND OF THE DISCLOSURE

[0001] The; disclosure relates to fuel cells and, more particularly, to PEM fuel cells and reduction in degradation of the membrane of same.

[0002] In a PEM fuel cell, various mechanisms can cause peroxide to form or exist in the vicinity of the membrane . This peroxide can dissociate into highly reactive free radicals. These free radicals can rapidly degrade the membrane .

[0003] It is desired to achieve 40,000-70,000 hour and 5,000-10,000 hour lifetimes for stationary and transportation PEM fuel cells, respectively. Free radical degradation of the ionomer seriously interferes with efforts to reach these goals.

[0004] It is therefore the primary object of the present disclosure to provide a method for operating a fuel cell which further addresses these issues.

[0005] Other objects and advantages appear herein.

SUMMARY OF THE DISCLOSURE

[0006] In accordance with the present disclosure, the foregoing objects and advantages have been attained. [0007] According to the disclosure, a method is provided for operating a fuel cell having a membrane electrode assembly comprising an anode, a cathode, and a membrane between the anode and the cathode, wherein the method comprises the steps of feeding fuel to the anode at a fuel pressure; feeding oxidant to the cathode at an oxidant pressure; and selectively maintaining the fuel pressure higher than the oxidant pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] A detailed description of preferred embodiments of the present disclosure follows, with reference to the attached drawings, wherein:

[0009] Figure 1 schematically illustrates operation of a membrane electrode assembly in accordance with the present disclosure;

[0010] Figure 2 shows results of an example where membrane decay rate is compared as between a cell operated at elevated pressuring of the hydrogen (fuel reactant) and a cell operated at a balanced pressure;

[0011] Figure 3 further illustrates the position of the transition plane Xo for a standard MEA and anode overpressure and with no anode overpressure;

[0012] Figure 4 illustrates an MEA incorporating a protective layer on the cathode side, and shows that the position of the transition plane Xo can be maintained in the same location by the protective layer while on load and by the anode overpressure while the MEA is off load.

[0013] Figure 5 shows the position of the transition plane Xo for an MEA with a hydrocarbon protective layer.

DETAILED DESCRIPTION [0014] The disclosure relates to fuel cells and, more particularly, to polymer electrolyte membrane (PEM) fuel cells, and to a method for operating same which mitigates decay or degradation of such fuel cells. [0015] Figure 1 schematically illustrates a membrane electrode assembly (MEA) 10 in accordance with the disclosure. As shown, assembly 10 includes a membrane 12, a cathode 14, an anode 16, and gas diffusion layers 18, 20. According to one aspect of the disclosure, a protective layer 22 can also be provided, in this embodiment between membrane 12 and cathode 14. Cathode 14 and anode 16 are positioned to either side of membrane 12 as shown, with gas diffusion layers 18, 20 positioned to either side of the electrodes (cathode 14 and anode 16) .

[0016] As is well known to a person skilled in . the art, membrane electrode assembly 10 is operated by feeding oxygen in some form through gas diffusion layer 18 to cathode 14 and by feeding hydrogen in some form through gas diffusion layer 20 to anode 16. These reactants create a current across membrane 12 as desired.

[0017] Cathode 14 is a porous layer containing a suitable cathode catalyst that may contain ionomer material and typically having a porosity of at least about 30%. Anode 16 is similarly a porous layer containing suitable anode catalyst that may contain ionomer material, and also typically has a porosity of at least about 30%.

[0018] During operation of assembly 10, catalyst materials which are typically present within the electrodes, that is, cathode 14 and/or anode 16, can dissolve and then precipitate elsewhere in the assembly.

[0019] It has been found that during operation of assembly 10, there is a transition plane of sharp potential change between the electrodes, and this transition plane is referred to as Xo. At Xo, reaction potential abruptly shifts from a low value to a high value. The position of Xo depends heavily on the oxidant and reductant gas concentrations at locations on either side of Xo. If electrically isolated catalyst particles are present at Xo, this is a very likely position for formation of peroxide and/or generation of radicals which can have a deleterious effect upon membrane 12 and other ionomer present within assembly 10. The position of the Xo transition plane when pure hydrogen is fed to the anode, and air (21% O2 in N₂) is fed to the cathode is approximately 20% of the membrane thickness -from the cathode.

[0020] It has further been found that dissolved catalyst metal tends to precipitate or deposit at Xo, and that this deposited metal can increase the chance of formation of peroxide. Peroxide has been found to be directly responsible for degradation of membrane 12, because peroxide under certain conditions can break down to form radicals which react with the membrane and then carry portions of the membrane out of assembly 10 through exhaust from same. Also, radicals may form directly on such catalyst precipitates from the reaction of crossover^' gases and/or peroxide, which proceed to degrade the membrane. Therefore, there exists a degradation zone of finite thickness around Xo that represents a very likely position for formation of peroxide and/or generation of radicals which can have a deleterious effect upon the membrane. [0021] According to the disclosure, it has been found that by operating the assembly with a pressure difference between the anode or hydrogen fuel side and the cathode or oxidant side, the position of Xo can be influenced. Specifically, higher pressure from one reactant gas moves the transition plane from one reactant gas electrode towards the other, and hence the location of Xo can be influenced. In other words, higher pressure from one reactant gas stream moves the zone of transition from one reactant gas electrode towards the other, and hence the location of X₀. Thus, by manipulating the pressure of reactants en each side of assembly 10, the location of Xo can be controlled and, accordingly, can be caused to exist in a location where less membrane degradation Specifically, by overpressuring the anode reactant, the degradation zone around Xo can be forced to locate largely within the cathode (i.e. Xo can be forced to reside at only -5% of the membrane thickness towards the cathode with a hydrogen overpressure of 150 kPa_g) . Since the cathode contains a high catalyst to ionomer ratio, the cathode catalyst supports the decomposition of radicals and/or peroxide into benign byproducts, which reduces the rate of membrane attack.

[0022] In one particularly preferred embodiment, assembly 10 is operated with a higher pressure at the fuel or anode side, so that Xo is moved toward the cathode side. This in itself helps to mitigate deterioration of the membrane by causing an overlap of the degradation zone into the cathode. In addition, and as mentioned above, a protective layer can also be included in assembly 10 , and the pressure difference can be manipulated to position Xo within or near to the protective layer thereby also providing an overlap of the degradation zone into the protective layer. [0023] Operation in this mode is schematically illustrated in Figure 1 by arrows 26, 28. The relatively larger size of arrow 26 compared to arrow 28 schematically illustrates operation of assembly 10 at a pressure at gas diffusion layer 20 and anode 16 which is greater than the pressure at gas diffusion layer 18 and cathode 14. This is also referred to as operating under an anode over-pressure. [0024] Assembly 10 can be operated with different pressures at the anode and cathode side through numerous different mechanisms, including pumps at each side which can be controlled to increase and/or decrease the pressure as desired. Alternatively, or in addition, flow out of the exhaust of the anode and/or cathode can be selectively restricted in order to increase the pressure in that zone. Of course, other methods can be used in accordance with the disclosure to control pressure as desired, and all well within the broad scope of the present disclosure. [0025] During electrical load cycling of assembly 10, the rate of consumption of reactants changes and the position of Xo can move. When Xo moves, there is increased tendency toward dissolution of catalyst metal from the previous Xo location to the new Xo location. The increased deposition occurs because after a certain amount of operating time there will be sufficient metal deposits at Xo that there is less driving force for further dissolution and deposition at Xo. However, when Xo moves, additional dissolution of catalyst can take place from both the electrodes and from catalyst particles already deposited in the membrane. Thus, according to the disclosure, these are particular times during the operation of assembly 10 wherein manipulation of pressure can advantageously be used to keep Xo where desired, rather than allow it to move as would normally occur.

[0026] As set forth above, protective layer 22 can be included in assembly 10 and pressure can be manipulated to locate Xo within protective layer 22. Protective layer 22 in these circumstances serves to help assembly 10 to resist the degradation of the membrane 12 and/or electrode materials which normally occurs at Xo. [0027] Several embodiments of protective layer 22 are provided, each of which serves to restrict or greatly reduce migration of hydrogen and oxygen.

[0028] The protective layer 22 can be provided as one or more catalyzed layers containing catalyst to chemically scavenge oxygen and hydrogen, for example forming water. Protective layer 22 can also be provided as a layer which is substantially non-porous or impermeable to hydrogen and oxygen and which therefore physically restricts flow of such reactants and thereby accomplishes the same goal of controlling location of Xo. According to one embodiment this impermeable layer can be provided as a hydrocarbon layer. Each of these embodiments is discussed below. [0,029] Turning now to Figures 3 - 5, various configurations of assembly 10, and related operating steps and conditions are described.

[0030] Figure 3 schematically illustrates an assembly 10 with no protective layer 22, and thus illustrates only membrane 12, cathode 14 and anode 16. XoI in this Figure illustrates the location of the transition plane when this assembly is operated at a balanced pressure between the anode and cathode. Line Xo2 illustrates the position of the transition plane when operated at an anode overpressure, or a greater pressure of hydrogen within the anode than oxygen within the cathode. As shown in Figure 3, this results in positioning of the transition plane closer to the cathode, as desired, which is a position where the transition plane is less likely to cause issues with respect to longevity of assembly 10. [0031] Figure 4 schematically illustrates an assembly 10 which includes a protective layer 22. In this embodiment, protective layer 22 is provided as a catalyzed layer as discussed above.

E0032] In this embodiment, protective layer 22 is advantageously a layer of ionomer material preferably containing a catalyst, preferably in particulate form. The layer preferably has a porosity of less than about 10% by volume (most preferably non-porous) , contains between about 50% and about 80% vol ionoraer, and between about 10% and about 50% vol catalyst. Electrical connectivity between the catalyst particles is preferably between about 35% and about 75% . The catalyst is preferably selected to enhance reactions where hydrogen and oxygen are scavenged and reacted to produce harmless products, especially water. [0033] During normal operation of assembly 10 of this embodiment, protective layer 22 serves to scavenge any oxygen which would otherwise cross into membrane 12 and also scavenges hydrogen which has crossed through membrane 12. Becauεse of this, Xo is forced to reside within protective layer 22 during on-load operation. Protective layer 22 further serves to decompose any peroxide formed, for example; at cathode 14.

[0034] During off-load operation, little or no scavenging of crossover oxygen takes place in protective layer 22. In this situation, an air starvation protocol can be implemented, whereby the oxygen normally fed to gas diffusion layer 18 of cathode 14 is instead redirected, for example by being vented to ambient instead. While protective layer 22 under off-load conditions has reduced effectiveness at scavenging oxygen, the air or oxygen starvation protocol provides the same effect, which tends to keep Xo within protective layer 22 during off-load conditions as well. Such a protocol also limits the high potential that the cathode would otherwise experience. While the anode overpressure as discussed above already influences the Xo position, the air starvation protocol could also be implemented to further influence Xo position as desired.

[0035] Thus, with a catalyzed protective layer 22, anode over pressure can advantageously be used to keep Xo within protective layer 22 during off-load operation. [0036] Different types of ionomer and catalyst material can be used in this embodiment. As will be further discussed, the protective layer in this embodiment serves to scavenge crossover gasses by having a high gas reaction rate and a low gas diffusion rate-. The protective layer further serves to maximize selectivity to benign products, preferably water, from such crossover gasses. In addition, since the protective layer is intended, according to the disclosure, to contain Xo, the protective layer structure advantageously discourages the loss of catalyst from the electrodes as discussed below.

[0037] The catalyst in protective layer 22 is preferably largely electrically connected, and protective layer 22 therefore serves as a sink for deposition of dissolved catalyst metal, and the dissolution driving force is reduced or eliminated. Thus, keeping Xo within protective layer 22 minimizes or eliminates the driving force under both on and off-load operating conditions.

[0038] In accordance with this embodiment, the protective layer 22 comprises a catalyst, for example carbon supported platinum or alloy catalyst such as platinum alloy particles, the pores of which are filled with polymer electrolyte, or ionomer material. When alloys such as platinum alloys are used, the catalyst particles can advantageously be binary and/or ternary alloys, and can be supported, for example on carbon, or non-supported.

[0039] One suitable platinum alloy has the formula Pt_xYχ__x, wherein Y is selected from the group consisting of Co, Ni, V, Cu, Fe, Cr, Pd, Ti, W, Al, Ag, Cu and combinations thereof, and x is between 0.1 and 0.9.

[0040] According to a further embodiment of the disclosure, the platinum alloy can have the formula Pt_xM₂Yi- _x._Zι wherein: M is selected from the group consisting of Ir, Rh, Co, Ni and combinations thereof; Y is selected from the group consisting of Co, Ni, V, Cu, Fe, Cr, Pd, Ti, W, Al, Ag, Cu, Au and combinations thereof; and x+z is between 0.1 and 0.9.

[0041] According to a still further embodiment, the platinum alloy has the formula Pt_xZi-_x, wherein Z is selected from the group consisting of Ru, Mo, and combinations thereof, and wherein x is between 0.1 and 0.9.

[0042] Other suitable catalysts, including other metal alloy catalysts, can be utilized. Alternatives may be apparent to a person of skill in the art. While the foregoing embodiments represent preferred configurations, such alternatives are considered to be well within the broad scope of the present disclosure.

[0043] At the relatively high potential which will be present in protective layer 22, the four electron reduction of oxygen is predominately achieved so as to produce water and not produce peroxide. Thus, oxygen is scavenged by protective layer 22 at high potential as desired.

[0044] Protective layer 22 serves to consume such oxygen at high potential, most actively at the interface 21 between protective layer 22 and cathode 14. Protective layer 22 further serves to consume hydrogen at the interface 24 between membrane 12 and protective layer 22. Further, protective layer 22 also provides for benign decomposition of peroxide at interface 24 and throughout the thickness of the layer 22 if peroxide is generated in cathode 14 and/or at interface 24 and throughout the thickness of layer 22 if peroxide is generated in anode 16. These functions advantageously reduce a significant contributor: toward cell degradation.

[0045] In order to provide desirable results, protective layer 22 i.<3 advantageously electrically connected to cathode 14 through an electrically conducting phase, for example such as carbon support material, so as to ensure high potential and, therefore, consumption of crossover oxygen to produce water.

[0046] Protective layer 22 further preferably has substantially no porosity and a relatively high oxygen reduction rate. This will result in a maximized ratio of oxygen reduction rate to oxygen diffusion rate, and thereby will minimize oxygen escape from the cathode.

[0047] In this regard, protective layer 22 advantageously has a porosity of less than about 10%, and is preferably substantially non-porous (substantially 0% porosity) . Oxygen reduction rate per unit platinum surface area for protective layer 22 is also advantageously approximately the same as the cathode because of electrical connectivity to the cathode .

[0048] Any porosity of protective layer 22 should advantageously be flooded during operation, for example with water, so as to reduce the oxygen diffusion rate through the protective layer 22. A layer 22 having porosity which is flooded with water during normal operation is considered to be non-porous as used herein since the water-filled porosity is effectively non-porous to reactant gasses.

[0049] Provision of a protective layer 22 having these properties advantageously results in efficient oxygen consumption at interface 21 and throughout layer 22 and, therefore, proper conditions for keeping Xo within layer 22 during the on-load operating conditions.

[0050] Protective layer 22 in this embodiment can advantageously be provided as a non-porous, electrically connected and ionically conductive structure having a porosity of between about 0% and about 10%, and preferably substantially 0%. A catalyst can be present in an amount ' between about 10 and about 50% vol based upon volume of the layer. Ionomer is also present in an amount between about 50 and about 80% vol. based upon volume of the layer. Layer 22 also advantageously can be provided having particles selected from the group consisting of particles of carbon, particles of platinum and platinum alloy, and combinations thereof, and particularly preferred platinum alloys are described above.

[0051] Figure 5 illustrates an assembly 10 showing the position of the transition plane when the assembly includes protective layer 22 designed in the form of a physical barrier or impediment to hydrogen and oxygen reactants . In this particular embodiment, protective layer 22 is provided as a hydrocarbon ionomer material . As discussed below, this material has reduced permeability to hydrogen and oxygen reactant gases, and thereby serves to help position the transition plane as desired. This feature of protective layer 22 can be enhanced by anode overpressure as described herein. Figure 5 shows XoI positioned within protective layer 22 as desired. The combination of this type of protective layer with anode overpressure combines to even more positively ensure that the transition plane will reside in the protective layer as desired. [0052] Protective layer 22 being formed using hydrocarbon ionomer material is desirable because it has been found that these materials are highly effective at reducing catalyst dissolution and reactant gas crossover, and also have excellent mechanical strength. Hydrocarbon ionomer shows diminished oxygen permeability over a range of pressures. Hydrocarbon materials used as a protective layer 22 therefore produce a layer which is both resistant to the dis.3olution of catalyst materials, has reduced permeability to hydrogen and oxygen gasses . Such a layer thereby presents a physical restriction or impediment to the reactants, and through this mechanism serves to position Xo as desired. The mechanical strength properties are also useful in that hydrocarbon ionomer material can itself be the ionomer used in protective layer 22, or can be mixed into other desired ionomer materials during the preparation of protective layer 22.

[0053] When made from hydrocarbon ionomer, protective layer 22 preferably has a thickness of between about 0.01 and about 20 micrometers, depending upon whether protective layer 22 is also to serve as a reinforcement layer. If so, thicknesses between about 0.1 and about 20 micrometers are preferred.

[0054] Aεs used herein, hydrocarbon ionomers refer collectively to ionomers having a main chain which contains hydrogen and carbon, and which may also contain a small mole fraction of hetero atoms such as oxygen, nitrogen, sulfur, and/or phosphorus. Such hydrocarbon materials are fully set forth in co-pending and commonly owned PCT Patent Application Number PCT/US05/39196, filed October 27, 2005. The aforesaid application is incorporated herein in its entirety by reference. These hydrocarbon ionomers primarily include aromatic and aliphatic ionomers. [0055] Examples of suitable aromatic ionomers include but are not limited to sulfonated polyimides, sulfoalkylated polysulfones, poly (p-phenylene) substituted with sulfophenox.y benzyl groups, and polybenzimidazole ionomers. [0056] Non-limiting examples of suitable aliphatic ionomers are those based upon vinyl polymers, such as cross-linked poly(styrene sulfonic acid) , poly (acrylic acid) , poly (vinylsulfonic acid) , poly (2 -acrylamide- 2- methylpropcinesulfonic acid) and their copolymers. [0057] In accordance with this embodiment, a hydrocarbon protective layer can be used with or without catalyst. Since the primary mechanism of such a layer is to physically block gas flow or permeation, the catalyst is of less importance and can be avoided altogether if desired. Since such catalyst leads to extra cost, it may be preferred to use hydrocarbon protective layers which have no catalyst. The embodiment discussed above includes a protective layer 22 which includes catalyst particles. A separate or an additional hydrocarbon layer can be provided, with or without catalyst, to serve primarily as an oxygen barrier layer, and/or to supplement the oxygen barrier function.

[0058] As set forth above, applying anode over pressure on an assembly 10 having protective layer 22 between cathode 14 and membrane 12 advantageously serves to define and maintain Xo within protective layer 22 as desired, thereby allowing for reduced chance of catalyst driven generation of peroxide and catalyst driven formation of radicals, and also minimizing movement of Xo such that a sink of catalyst material can be initially provided in protective layer, or initially deposited in protective layer 22 during early operation, to thereby reduce or eliminate the driving force for catalyst dissolution during on-load operation. [0059] Protective layer 22 can be provided using various ionomer materials as discussed above, and advantageously serves to force Xo to stay within protective layer 22 as desired.

[0060] Hydrocarbon ionomer material has the added advantage when used in protective layer 22 of durability. This durability can be usefully incorporated into membrane 12 as well. Thus, according to the disclosure, membrane 12 can advantageously be provided as a blended per-fluorinated and hydrocarbon ionomer. Such a membrane could be used in any of a wide variety of fuel cell applications, including but not limited to those illustrated herein. [0061] In further accordance with the disclosure, protective layer 22 can be provided as a combined hydrocarbon and per-fluorinated ionomer based layer (such as Nafion) , for example by substantially homogeneously blending hydrocarbon in liquid ionomer or particulate form into the per-fluorinated ionomer-based material. [0062] During off-load operation, oxidant is re-directed away from cathode 14, and this serves to maintain oxygen depletion in the vicinity of protective layer 22 and thereby to keep Xo within protective layer 22 as desired. [0063] Still referring to Figure 1, a membrane electrode assembly 10 may be operated in such a way that the anode pressure is constantly or intermittently maintained above the cathode pressure. Such an assembly may optionally include one or more embodiments of protective layer 22 as discussed above. Operating the assembly according to the method of the present disclosure serves to position the transition plane within the cathode and/or protective layer 22. In so doing, the zone of membrane attack surrounding Xo is greatly mitigated.

[0064] Figure 2 shows membrane decay measurements for an assembly, in each case with no protective layer, operated at a balanced pressure, and operated with an anode overpressure of 100 kPag during a portion of the load cycle implemented.

^• [0065] As shown in Figure 2, the relative fluoride emission rate for the balanced pressure assembly is much higher than that of the anode over-pressure operated assembly. This clearly shows that the method of the present disclosure serves to mitigate the degradation of ^■ the assembly normally evidenced by fluoride emissions in the exhaust of the assembly. Figure 2 shows an approximate 6X reduction in membrane decay rate. [0066] It should be appreciated that the present disclosure, drawn to reduction of degradation due to pressure manipulation preferably in the form of anode overpressure during on-load and/or off-load operation, advantageously accomplishes the purpose of controlling location of the transition plane Xo. Furthermore, it should be appreciated that the disclosure is useful in connection with a wide variety of different types of ionomer in the protective layer, all within the broad scope of the present disclosure.

[0067] While the present disclosure has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.

Claims

WHAT IS CLAIMED

1. A method for operating a fuel cell having a membrane electrode assembly comprising an anode, a cathode, and a membrane between the anode and the cathode, the method comprising the steps of : feeding fuel to the anode at a fuel pressure; feeding oxidant to the cathode at an oxidant pressure, wherein the feeding steps create a transition plane between the fuel and the oxidant; and selectively maintaining the fuel pressure higher than the oxidant pressure sufficient to position the transition plane in the cathode or within- 5% thickness of the membrane of the cathode .

2. The method of claim 1 , wherein the membrane electrode assembly further comprises a protective layer between the membrane and the cathode, wherein the feeding steps create a transition plane between the fuel and the oxidant, and wherein the selectively maintaining step positions the transition plane within the protective layer.

3. The method of claim 2, wherein the protective layer comprises a catalyzed protective layer.

4. The method of claim 3, wherein the catalyzed protective layer contains catalyst having selectivity toward reactions which scavenge hydrogen and oxygen and produce water.

5. The method of claim 3 wherein the protective- layer has a porosity of less than about 10%.

6. The method of claim 3 wherein the catalyst in the protective layer is an alloy.

7. The method of claim 2, wherein the protective layer comprises a protective barrier layer having a permeability selected to substantially restrict flow of oxygen and hydrogen.

8. The method of claim 7, wherein the protective layer comprises a hydrocarbon layer.

9. The method of claim 1, wherein the selectively maintaining step comprises maintaining the fuel pressure higher than the oxidant pressure during startup of the fuel cell and during cycling of the fuel cell between on load and off load operation.

10. The method of claim 1, wherein the selectively maintaining step comprises maintaining the fuel pressure at least about. 50 kPa higher than the oxidant pressure

11. The method of claim 10, wherein the selectively maintaining step comprises maintaining the fuel preserve between about 50 kPa and about 200 kPa higher than the oxidant pressure.