Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt

Abstract

The effect of elastic strain on catalytic activity of platinum (Pt) toward oxygen reduction reaction (ORR) is investigated through dealloyed Pt–Cu thin films; stress evolution in the dealloyed layer and the mass of the Cu removed are measured in real-time during electrochemical dealloying of (111)-textured thin-film PtCu (1:1, atomic ratio) electrodes. In situ stress measurements are made using the cantilever-deflection method, and nanogravimetric measurements are made using an electrochemical quartz crystal nanobalance. Upon dealloying via successive voltammetric sweeps between −0.05 and 1.15 V vs standard hydrogen electrode, compressive stress develops in the dealloyed Pt layer at the surface of thin-film PtCu electrodes. The dealloyed films also exhibit enhanced catalytic activity toward ORR compared with polycrystalline Pt. In situ nanogravimetric measurements reveal that the mass of dealloyed Cu is approximately 210 ± 46 ng/cm2, which corresponds to a dealloyed layer thickness of 1.2 ± 0.3 monolayers or 0.16 ± 0.04 nm. The average biaxial stress in the dealloyed layer is estimated to be 4.95 ± 1.3 GPa, which corresponds to an elastic strain of 1.47% ± 0.4%. In addition, density functional theory calculations have been carried out on biaxially strained Pt(111) surface to characterize the effect of strain on its ORR activity; the predicted shift in the limiting potentials due to elastic strain is found to be in good agreement with the experimental shift in the cyclic voltammograms for the dealloyed PtCu thin film electrodes.