Effective reaction rates of a thin catalyst layer

Abstract

The catalyst layer in a fuel cell can be described with the help of a system of reaction diffusion equations for the protonic overpotential and the oxygen concentration. The Tafel equation gives an exponential law for the reaction rate, the Tafel slope is a coefficient in this law. We present a rigorous thin layer analysis for two reaction regimes. In the case of thin catalyst layers, the original Tafel slope enters an effective boundary condition. In the case of large protonic overpotentials we derive the effect of a double Tafel slope, essentially a consequence of a thin active region within the catalyst layer.