Sputter-Deposited Pt/CrN Nanoparticle PEM Fuel Cell Cathodes: Limited Proton Conductivity Through Electrode Dewetting

Platinum-coated chromium nitride electrodes are deposited onto gas diffusion layers by normal and glancing angle deposition and are tested as cathodes for proton exchange membrane (PEM) fuel cells. X-ray diffraction and scanning electron microscopy show that the CrN forms 111-oriented nanoparticles with {100} facets that are covered by , independent of Pt loading 0.05 to . Polarization curves exhibit slopes and −150 mV/dec for high and low potentials, respectively, but show an anomalous drop with in the intermediate voltage range. This is attributed to poor proton conduction associated with a reversible dewetting of electrode pores during low current operation. Quantitative analyses of rate-dependent polarization curves and electrochemical impedance spectra show that the time scale for pore filling by process water is , and that the ionic resistance within the electrode increases by a factor of 4, from to , as increases from 0.5 to 0.8 V. The increasing electrode resistance is attributed to a low water production rate at low current, which allows the relatively hydrophobic CrN to expel water from the electrode pores, resulting in a higher resistance for ionic transport. These results show that even ultrathin sputtered catalyst layers can exhibit incomplete flooding.