  The effect of elastic strain on catalytic activity of platinum (Pt) towards
oxygen reduction reaction (ORR) is investigated through de-alloyed Pt-Cu thin
films; stress evolution in the de-alloyed layer and the mass of the Cu removed
are measured in real-time during electrochemical de-alloying of (111)-textured
thin-film PtCu (1:1, atom %) electrodes. In situ stress measurements are made
using the cantilever-deflection method and nano-gravimetric measurements are
made using an electrochemical quartz crystal nanobalance. Upon de-alloying via
successive voltammetric sweeps between -0.05 and 1.15 V vs. standard hydrogen
electrode, compressive stress develops in the de-alloyed Pt layer at the
surface of thin-film PtCu electrodes. The de-alloyed films also exhibit
enhanced catalytic activity towards ORR compared to polycrystalline Pt. In situ
nanogravimetric measurements reveal that the mass of de-alloyed Cu is
approximately 210 +/- 46 ng/cm2, which corresponds to a de-alloyed layer
thickness of 1.2 +/- 0.3 monolayers or 0.16 +/- 0.04 nm. The average biaxial
stress in the de-alloyed 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.
