Energy Conversion Engineering Towards Low CO2 Power and Fuels

The performance of fuel cells at finite current, or finite power, is presented in this chapter, focusing on the sources of loss, how each loss mechanism is modeled, and how the design parameters and operating conditions contribute to each. In particular, we examine the role of chemical kinetics and transport processes in fuel cell efficiency. At finite current, fuel cells cannot achieve the ideal thermodynamic efficiency, corresponding to the maximum work or the Gibbs free energy of the overall reaction, due to a number of intrinsic loss mechanisms. These include: (1) non-electrochemical, or thermochemical, reactions, occurring on the surfaces or within the fuel channel; (2) potential loss associated with finite-rate electrochemical reactions; (3) decrease in reactants concentrations because of finite-rate transport processes; and (4) losses associated with the transport of ions and electrons across different elements. All of these mechanisms depend on the current drawn from the cell. Some small losses are observed even at open-circuit conditions, mostly due to electron and fuel leakage across the electrodes. Modeling these losses is tackled in some detail in this chapter.