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The impact of temperature on the ideal voltage, E, for the oxidation of hydrogen is also shown in Table 2-3 for the various types of fuel cells. Each case assumes gaseous products as its basis. Temperature Cell Type Ideal Voltage Table 2-3 Ideal Voltage as a Function of Cell Temperature 25°C 80°C 100°C 205°C 650°C 800°C (298K) (353K) (373K) (478K) (923K) (1073K) PEFC AFC PAFC MCFC ITSOFC 1.18 1.17 1.16 1.14 1.03 0.99 1100°C (1373K) TSOFC 0.91 The open circuit voltage of a fuel cell is also strongly influenced by the reactant concentrations. The maximum ideal potential occurs when the reactants at the anode and cathode are pure. In an air-fed system or if the feed to the anode is other than pure dry hydrogen, the cell potential will be reduced. Similarly, the concentration of reactants at the exit of the cell will be lower than at the entrance. This reduction in partial pressure leads to a Nernst correction that reduces the open circuit voltage locally, often by as much as 250 mV in higher-temperature cells. Because the electrodes should be highly conductive and the electrode within one cell consequently has close to uniform voltage, depressed open circuit voltage affects the operation of the entire cell. This significantly impacts the achievable cell operating voltage and consequently system efficiency of especially the higher-temperature fuel cells. The ideal performance of a fuel cell depends on the electrochemical reactions that occur between different fuels and oxygen as summarized in Tables 2-1 and 2-2. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electro-catalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and TSOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. While carbon monoxide severely poisons noble metal anode catalysts such as platinum (Pt) in low-temperature fuel cells, it is a reactant in high-temperature fuel cells (operating temperatures of 300 °C and higher) where non-noble metal catalysts such as nickel (Ni) can be used. Note that H2, CO, and CH4 are shown in Table 2-1 as potentially undergoing direct anodic oxidation. In actuality, direct electrochemical oxidation of the CO and CH4 usually represents only a minor pathway to oxidation of these species. It is common systems analysis practice to assume that H2, the more readily oxidized fuel, is produced by CO and CH4 reacting, at equilibrium, with H2O through the water gas shift and steam reforming reactions, respectively. A simple reaction pathway analysis explains why direct oxidation is rarely the major reaction pathway under most fuel cell operating conditions: • The driving force for anodic oxidation of CO and CH4 is lower than that for the oxidation of hydrogen, as reflected in the higher open circuit voltage of the hydrogen oxidation. • The kinetics of hydrogen oxidation on the anode are significantly faster than that of CO or CH4 oxidation. 2-6PDF Image | Fuel Cell Handbook (Seventh Edition)
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