411 Fuel Cell Characteristics

Theoretically, fuel cells operate isothermally, meaning that all free energy in a fuel cell chemical reaction should convert into electrical energy. The hydrogen "fuel" in the fuel cell does not burn as in IC engines, bypassing the thermal to mechanical conversion. Also, because the operation is isothermal, the efficiency of such direct electrochemical converters is not subject to the limitation of Carnot cycle efficiency imposed on heat engines. The fuel cell converts the Gibbs free energy of a chemical reaction into electrical energy in the form of electrons under isothermal conditions. The maximum electrical energy for a fuel cell operating at constant temperature and pressure is given by the change in Gibbs free energy:

where n is the number of electrons produced by the anode reaction; F is Faraday's constant, equal to 96412.2 C/mol; and E is the reversible potential. The Gibbs free energy change for the reaction H2(g)+(1/2)O2g H2O(l) at standard condition of 1 atmospheric pressure and 25°C is -236 kJ/mol or -118 MJ/kg. With n=2, the maximum reversible potential under the same conditions is Eo=1.23 V, using Equation 4.1. The maximum reversible potential under actual operating conditions for the hydrogen-oxygen fuel cell is given by the Nernst equation, as follows:1

and are the concentrations where '/'is the temperature in Kelvin; R is the gas constant; and P\ P0, or partial pressures of the reactants and products.

The voltage-current output characteristic of a hydrogen-oxygen cell is illustrated in Figure 4.2. The higher potentials around 1 V per cell are theoretical predictions that are not achievable in a practical cell. The linear region where the reduction in cell potential is due to ohmic losses is where a practical fuel cell operates. The resistive components in the cell limit the practical achievable efficiency of a fuel cell. The working voltage of the cell falls with an increasing current drain, knowledge that is important in designing fuel-cell-powered EVs and hybrid vehicles. Because cell potential is small, several cells are stacked in series to achieve the desired voltage. The major advantage of fuel cells is lower sensitivity to scaling, which means that fuel cells in the kW range have similar overall system efficiencies up to the MW range.

FIGURE 4.2 Voltage-current relationship of a hydrogen/oxygen cell.
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