Oxygen Reduction Behavior of Thin‐Film Platinum and Platinum‐Rhodium Electrocatalysts in Sulfuric Acid

The objective of this work was to investigate the microstructural dependence of the oxygen reduction activity of carbon‐supported thin‐film Pt‐Rh catalysts in comparison with that of thin‐film Pt. Thin‐film rotating‐disk‐electrode (RDE) techniques were applied to arrive at diffusion‐corrected, i.e., true activity measurements at overpotentials typical of current generation. Thin‐films of Pt and Pt‐Rh [80:20 atomic percent ] were prepared by sputter deposition onto graphite, characterized using transmission electron microscopy (TEM), and tested in at 25°C. The surface of Pt‐Rh alloys becomes Pt rich when cycled repeatedly to potentials above ca. 1 V vs. reversible hydrogen electrode (RHE). The lattice parameter of thin‐film Pt has been shown to decrease with decreasing grain size while that of thin‐film Pt‐Rh increases. The latter effect has been attributed to the presence of oxygen in the Pt‐Rh lattice. The specific activity at 750 mVRHE of thin‐film Pt increases with decreasing particle size while that of thin‐film Pt‐Rh also shows a slight increase and is much lower than Pt. Correspondingly, the mass activity of thin‐film Pt‐Rh is also less than for thin‐film Pt. The lower specific activity of thin‐film Pt‐Rh is due to its lattice expansion and to the presence, at potentials of interest, of oxide on the surface. The activity of large‐grained or heat‐treated thin‐film Pt‐Rh is more like the bulk material which previously has been shown to be less than Pt.