2, Northwestern University, Evanston, Illinois, United States
3, Imperial College London, London, , United Kingdom
Solid acid fuel cells (SAFCs) based on the electrolyte CsH2PO4 have shown promising power densities at ~250C, a technologically attractive operating temperature. While SAFC development efforts have resulted in impressive performance gains in the last decade, electrocatalysis mechanisms in these devices remain unknown. Here, the hydrogen oxidation kinetics on Pt, Pd and Pt-Pd bimetallic thin film electrodes on proton-conductive CsH2PO4 have been evaluated. In particular, two types of cell geometries were studied. In the first case, a thin (10nm) film of Pt was embedded within CsH2PO4, which was in turn sandwiched between ‘standard’ composite electrodes. This cell configuration was employed to study hydrogen transport through Pt and across the Pt|CsH2PO4 interface. In the second case Pt, Pd, and Pt-Pd bilayer thin film electrodes with varying thicknesses were sputtered on both sides of CsH2PO4 electrolyte discs. These cells were used for studies of hydrogen electro-oxidation across the gas | metal | CsH2PO4 structure. Symmetric cells of both types were characterized by AC impedance spectroscopy under humidified H2 at ~248C. The bilayer films were additionally studied by ex-situ low energy ion scattering and scanning transmission electron microscopy. The results revealed: (1) 10 nm Pt films present negligible bulk resistance to proton transport; (2) the rate-limiting step for hydrogen electro-oxidation on Pt thin films occurs at the internal Pt | CsH2PO4 interface rather than the Pt | gas interface; (3) Pd is substantially more active than Pt for hydrogen electro-reduction; and (4) Pt and Pd undergo extensive intermixing at 250C. From these observations, it is concluded that the dramatic decrease in hydrogen electrooxidation resistance that occurs when Pd is deposited on Pt (on CsH2PO4) is a result of Pd diffusion to the metal | electrolyte interface, at which Pd serves to catalyze the rate-limiting step.