Water management implications for ALD-modified polymer electrolyte membrane fuel cell catalysts

Device testing of polymer electrolyte membrane fuel cell catalysts is essential for electrocatalyst development, yet most reported catalysts fabricated with nanoceramic coatings via atomic layer deposition (ALD) have not been thoroughly examined in this fashion. Platinum nanoparticle catalysts supported on functionalized XC72 carbon black that had been modified with sub-monolayer WN films and subjected to various thermal treatments were incorporated into membrane electrode assemblies and tested for oxygen reduction reaction activity in a fuel cell. The mass activities of the catalysts calculated at low current densities had comparable trends to those previously recorded in aqueous half-cell testing; however, significant deviations in catalyst performance became apparent at high current densities in the mass transport region of the polarization curves. The poor performance of the ALD-modified catalysts was attributed to water flooding induced by the low hydrophobicity of these catalyst layers, as determined by contact angle measurements on the cathode surfaces. It was seen that hydrophobicity was increased by thermal treatment and removal of surface groups known to be prone to hydrogen bonding, thereby improving the catalyst performance in the mass transport region. A heat-treated WN-coated ALD catalyst was found to have superior performance to commercial Pt/C. The field of research on fuel cell catalyst design via ALD is rapidly expanding, and this work demonstrates the importance of understanding the impact of ALD-deposited nanostructures on materials properties beyond the pure electrocatalytic activity.