Operating temperatures, PEFC

Due to the relative slow reaction kinetics for the oxygen reduction reaction (ORR) caused by the use of an acid reaction environment and the comparatively low operating temperatures, PEFCs are requiring the use of noble metal based catalysts. 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Polymer electrolyte fuel cells (PEFCs) have attracted great interest as a primary power source for electric vehicles or residential co-generation systems. However, both the anode and cathode of PEFCs usually require platinum or its alloys as the catalyst, which have high activity at low operating temperatures (<100 °C). For large-scale commercialization, it is very important to reduce the amount of Pt used in fuel cells for reasons of cost and limited supply. 

Besides using Pt-Ru or Pt-Co alloy anodes, CO poisoning can be mitigated by elevating the operating temperature. However, temperature dependencies of the HOR rates in the presence of CO with relevance to PEFC operation have been scarcely reported. One of the difficulties is correction of the change in H2 concentration [H2] in the... 

PEFC The PEFC, like the SOFC, has a solid electrolyte. As a result, this cell exhibits excellent resistance to gas crossover. In contrast to the SOFC, the cell operates at a low 80°C. This results in a capability to bring the cell to its operating temperature quickly, but the rejected heat cannot be used for cogeneration or additional power. Test results have shown that the cell can operate at very high current densities compared to the other cells. However, heat and water management issues may limit the operating power density of a practical system. The PEFC tolerance for CO is in the low ppm level. 

Operating temperature has a significant influence on PEFC performance. An increase in temperature lowers the internal resistance of the cell, mainly by decreasing the ohmic resistance of the electrolyte. In addition, mass transport limitations are reduced at higher temperatures. 

Improvements in solid polymer electrolyte materials have extended the operating temperatures of direct methanol PEFCs from 60 C to almost 100 C. Electrocatalyst developments have focused on materials that have higher intrinsic activity. Researchers at the University of Newcastle upon Tyne have reported over 200 mA/cm at 0.3 V at 80 C with platinum/ruthenium electrodes having platinum loading of 3.0 mg/cm. The Jet Propulsion Laboratory in the U.S. has reported over 100 mA/cm at 0.4 V at 60 C with platinum loading of 0.5 mg/cm. Recent work at Johnson Matthey has clearly shown that platinum/ruthenium materials possess substantially higher intrinsic activity than platinum alone (45). 

One particular application for which supported Au catalysts may find a niche market is in fuel cells [4, 50] and in particular in polymer electrolyte fuel cells (PEFC), which are used in residential electric power and electric vehicles and operate at about 353-473 K. Polymer electrolyte fuel cells are usually operated by hydrogen produced from methane or methanol by steam reforming followed by water-gas shift reaction. Residual CO (about 1 vol.%) in the reformer output after the shift reaction poisons the Pt anode at a relatively low PEFC operating temperature. To solve this problem, the anode of the fuel cell should be improved to become more CO tolerant (Pt-Ru alloying) and secondly catalytic systems should be developed that can remove even trace amounts of CO from H2 in the presence of excess C02 and water. 

These fuel cells are listed in the order of their approximate operating temperature, ranging from 80°C for PEFCs, 100°C for AFCs, 200°C for PAFCs, 650°C for MCFCs, and 800°C-1000°C for SOFCs. 

The two important drawbacks of PFSA membranes are the limited range of temperatures in which they can be effectively employed and their high cost at present. The first limitation typically forces operation of PEFCs at temperatures below 100 °C,... 

The high theoretical efficiency of a fuel cell is substantially reduced by the finite rate of dynamic processes at various locations in the cell. Substantial efficiency losses at typical operating temperatures occur already in the anodic and cathodic catalyst layers due to the low intrinsic reaction rates of the oxygen reduction and, in the case of the DMFC, of the methanol oxidation reaction. (The catalytic oxidation of hydrogen with platinum catalysts is very fast and thus does not limit PEFC performance.) In addition, at low temperatures, turnover may be limited by noble metal catalyst poisoning due to sulfur... 

The vulnerability of Pt and Pt alloy catalysts to poisoning by trace contaminants at operation temperatures typical for a PEFC is well documented and is of clear concern in the design of a power system based on a PEFC stack. Sources of contaminants include both fuel and air feed streams as well as processes derived from chemical instability of cell component(s). As to the feed streams, polishing of anode feed streams generated by fuel processing upstream the cell should leave very low levels of CO to be dealt with effectively within the cell (see Sect. 8.3.7.1), whereas any traces of sulfur or ammonia have to be perfectly eliminated upstream the anode... 

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