Step bunching instability and its effects in electrocatalysis on platinum surfaces

The atomic-scale surface structure plays a major role in the electrochemical behaviour of a catalyst. The electrocatalytic activity towards many relevant reactions, such as the oxygen reduction reaction on platinum, exhibits a linear dependency with the number of steps until this linear scaling breaks down at high step densities. Here we show, using Pt(111)-vicinal surfaces and in situ electrochemical scanning tunnelling microscopy, that this anomalous behaviour at high step densities has a structural origin and is attributed to the bunching of closely spaced steps. While Pt(554) presents parallel single steps and terrace widths that correspond to its nominal, expected value, most steps on Pt(553) are bunched. Our findings challenge the common assumption in electrochemistry that all stepped surfaces are composed of homogeneously spaced steps of monoatomic height and can successfully explain the anomalous trends documented in the literature linking step density to both activity and potential of zero total charge. The electrocatalytic activity of metal catalysts commonly exhibits a positive linear correlation with the presence of steps, but this dependency breaks down for Pt catalysts with high step densities. Now, using in situ electrochemical scanning tunnelling microscopy, it is shown that this is due to the bunching of closely spaced steps, forming double and triple steps.