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.

原子尺度的表面结构在催化剂的电化学行为中起着重要作用。许多相关反应的电催化活性，例如铂上的氧还原反应，表现出与步数的线性相关性，直到这种线性缩放在高步密度下被破坏。在这里，我们使用 Pt(111) 邻面和原位电化学扫描隧道显微镜表明，高台阶密度下的这种异常行为具有结构起源，并且归因于紧密间隔的台阶的聚集。虽然 Pt(554) 呈现出与其标称预期值相对应的平行单台阶和平台宽度，但 Pt(553) 上的大多数台阶都是聚集的。我们的研究结果挑战了电化学中的常见假设，即所有阶梯表面均由单原子高度的均匀间隔阶梯组成，并且可以成功解释文献中记录的将阶梯密度与零总电荷的活性和电势联系起来的异常趋势。