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quickly, especially when pure hydrogen fuel is available. It is a disadvantage in carbon monoxide-containing fuel streams, because carbon will attack the platinum catalyst sites, masking the catalytic activity and reducing cell performance.11 The effect is reversible by flowing a CO-free gas over the electrode. To minimize CO poisoning, operating temperatures must be greater than 120 °C, at which point there is a reduction in chemisorption and electro- oxidation. Due to CO affecting the anode, only a few ppm of CO can be tolerated at 80 °C. Because reformed and shifted hydrocarbons contain about one percent CO, a mechanism to eliminate CO in the fuel gas is needed. This can be accomplished with preferential oxidation (PROX) that selectively oxidizes CO over H2 using a precious metal catalyst. The low operating temperature also means that little, if any, heat is available from the fuel cell for endothermic reforming (19, 20). As this discussion suggests, there is a considerable advantage at the stack level to the use of pure hydrogen rather than reformate, but in most PEFC applications this must be traded off against the challenges in storing hydrogen and the limited availability of hydrogen. Although considerable effort has been expended to develop liquid-fueled PEFC for transportation applications, most believe that on-board storage of hydrogen will be necessary for practical vehicles (21). To overcome the challenges of operating on reformate, attempts have been made to develop so- called high-temperature PEFC, which would operate in the 120 °C to 160°C range. New or modified ion exchange membranes would be needed to allow this, because Nafion dehydrates rapidly at such temperatures unless high (greater than 10 bar) pressures are applied. One candidate material is polybenzimidizole (PBI) (22). The higher operating temperature eliminates CO poisoning by eliminating CO occlusion of the platinum sites. Also, this operating regime provides higher quality heat for possible use in stationary combined heat/power (CHP) applications. Because PBI requires significantly lower water content to facilitate proton transport, an additional benefit is that water management is dramatically simplified (23, 24). However, to achieve acceptable ionic conductivity, the membrane must be impregnated with phosphoric acid, which is apparently not very tightly bound to the polymer backbone. As a result, similar precautions are necessary as in a PAFC (avoiding liquid water, corrosion protection). The conductivity of PBI can approach the target of 10 S/cm set for high temperature membranes. Other approaches to high-temperature membranes are based on the modification of Nafion. Reports indicate that some of the modified materials achieve conductivities close to that of Nafion 112, while allowing operation up to 120 °C at low hydration levels (25,14 ,16 ,17) Both temperature and pressure significantly influence cell performance. Present cells operate at 80 °C over a range of 0.0010 to 1.0 MPa (~0.1 to 150 psig). Nominally, 0.285 MPa (25 psig) (18) is used for some transportation applications although some developers (26) pursue ambient- pressure technology. Using appropriate current collectors and supporting structure, polymer electrolyte fuel cells and electrolysis cells should be capable of operating at pressures up to 3000 psi and differential pressures up to 500 psi (27). 11. Referred to as poisoning in catalysis literature. 3-8PDF Image | Fuel Cell Handbook (Seventh Edition)
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