Local Catalyst Support Degradation during Polymer Electrolyte Fuel Cell Start-Up and Shutdown

Abstract

Fuel cell start-up and shutdown can induce high potentials at the cathode, leading to carbon corrosion and platinum dissolution. In technical cells with large active area the degradation processes are inhomogeneous along the flow field, therefore a fundamental understanding in local corrosion is required. Local carbon corrosion is determined from CO2 emissions measured locally along the cathode channel by in-situ and online mass spectrometric analysis of the reactant gas. Mass spectrometry data is in line with local limiting current measurements and SEM imaging. For complete start/stop cycles carbon corrosion is highest at the hydrogen outlet, followed by the inlet and lowest in the center of the flow field. This spatial inhomogeneity can be explained by the combined effects of diffusive mass transport limitations in the gas diffusion electrode and the pseudo-capacity of the platinum electro-catalyst. A model is presented, explaining opposite findings in the literature by the relative importance of the mass transport effect in different flow field geometries. Cell temperature, relative humidity of the gases and the purging gas velocity influence the relative carbon loss differences along-the-channel. Inhomogeneities up to a factor of 4 are observed and are most prominent at begin of life.