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Temperature and Pressure: The effect of temperature and pressure on the ideal potential (E) of a fuel cell can be analyzed on the basis of changes in the Gibbs free energy with temperature and pressure. (2-38) (2-39) Because the entropy change for the H2/O2 reaction is negative, the reversible potential of the H2/O2 fuel cell decreases with an increase in temperature (by 0.84 mV/°C, assuming reaction product is liquid water). For the same reaction, the volume change is negative; therefore, the reversible potential increases with an increase in pressure (with the square root of the pressure, assuming pressure is equal on both electrodes). However, temperature has a strong impact on a number of other factors: • Electrode reaction rates. Typically, electrode reactions follow Arrhenius behavior. As a consequence, these losses decline exponentially with increasing temperature, usually more than off-setting the reduction in ideal potential. The higher the activation energy (and hence usually the losses) the greater the impact of temperature. The impact of total pressure depends on the pressure dependence of rate-limiting reaction steps. • Ohmic losses. The impact of temperature on cell resistance is different for different materials. For metals, the resistance usually increases with temperature, while for electronically and ionically conductive ceramics it decreases exponentially (Arrhenius-form). For aqueous electrolytes, the impact is limited though high temperatures can lead to dehydration of the electrolyte (e.g. PEFC) and loss of conductivity. As a rule of thumb, for high-temperature fuel cells, the net effect is a significant reduction in resistance, while for low-temperature fuel cells the impact over the operating range is limited. Mass transport processes are not strongly affected by temperature changes within the typical operating temperature and pressure ranges of most fuel cell types. An increase in operating pressure has several beneficial effects on fuel cell performance because the reactant partial pressure, gas solubility, and mass transfer rates are higher. In addition, electrolyte loss by evaporation is reduced at higher operating pressures. Increased pressure also tends to increase system efficiencies. However, there are compromises such as thicker piping and additional expense for pressurization. Section 8.1.1 addresses system aspects of pressurization. The benefits of increased pressure must be balanced against hardware and materials problems, as well as parasitic power costs. In particular, higher pressures increase material problems in MCFCs (see Section 6.1), pressure differentials must be minimized to prevent reactant gas leakage through ⎛∂E⎞ =∆S ⎜∂T⎟ nF ⎝ ⎠P or ⎛∂E⎞ =−∆V ⎜∂P⎟ nF ⎝ ⎠T 2-20PDF Image | Fuel Cell Handbook (Seventh Edition)
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