In situ measurement of the oxygen diffusion coefficient in the cathode catalyst layer of a direct methanol fuel cell

We report an in situ measurement of the oxygen diffusion coefficient Dox in the cathode catalyst layer (CCL) of a direct methanol fuel cell (DMFC). The method is based on measuring two DMFC impedance spectra for the same cell current, but different oxygen concentrations in the cathode channel. The spectra are fitted using a transmission line model to extract the CCL resistivity. The difference of two CCL resistivities for different oxygen concentrations determines Dox. We found Dox=0.838×10−4·exp−EaR1T−1303 cm2 s−1 in the temperature range from 30 °C to 90 °C, with Ea ≃ 20.1 kJ mol−1. The activation energy is not far from that of the oxygen diffusion in Nafion; the value of this parameter is discussed.