A simple equation for in situ measurement of the catalyst layer oxygen diffusivity in PEM fuel cell

•We derive a simple equation for the oxygen diffusivity D of the catalyst layer.•One has to measure two impedance spectra at different oxygen concentrations.•The difference of low-frequency cell resistivities determines D.•The method is direct, it does not require impedance curve fitting.

We report a method for in situ determination of the oxygen diffusion coefficient D in the cathode catalyst layer (CCL) of a PEM fuel cell. The method is based on our recent model equation for the CCL polarization potential. Two low-current impedance spectra of the CCL should be measured for the same cell current and different oxygen concentrations. We show that D   is given by D=(k-1)blt/(k12FδRcclc)D=(k-1)blt/(k12FδRcclc), where b   is the Tafel slope, ltlt is the CCL thickness, δRcclδRccl is the difference of the low-frequency CCL resistivities, c   is the smallest oxygen concentration used in experiments, and k>1k>1 is the ratio of two concentrations.