Charge-Transfer Kinetics

Charge-transfer processes are among the least understood aspects of fuel cell chemistry. Computing the production rates from faradaic and nonfaradaic reactions requires the kinetic parameters for every reaction. In almost all cases, the charge-transfer kinetics are not known, and are instead used as fitting parameters to match model predictions with actual measured cell performance. Representing reactions in mass-action form does not suffer from restrictions that are inherent in the Butler-Volmer form. Only if the charge transfer is indeed rate limiting, such that the activities are established by much faster processes (e.g., adsorption/desorption), can the Butler-Volmer equation be used. To move away from a Butler-Volmer description of charge transfer, we are building a reaction database of single-electron-transfer reactions whose incorporation into patterned-anode and MEA models yields a good match to measured behavior of similar systems. This requires tuning the unknown electrochemical reaction kinetics so that model predictions are in best possible agreement with steady-state (Tafel) and time-variant (electrochemical impedance spectra, or EIS) responses. The multistep mechanism that is able to provide a good match to experimental data provides an understanding of the behavior at the TPB and the kinetics of that behavior.