Electrochemical proton-coupled electron transfer (PCET) reactions can proceed via an outer-sphere electron transfer to solution (OS-PCET) or through an inner-sphere mechanism by interfacial polarization of surface-bound active sites (I-PCET). Although OS-PCET has been extensively studied with molecular insight, the inherent heterogeneity of surfaces impedes molecular-level understanding of I-PCET. Herein we employ graphite-conjugated carboxylic acids (GC-COOH) as molecularly well-defined hosts of I-PCET to isolate the intrinsic kinetics of I-PCET. We measure I-PCET rates across the entire pH range, uncovering a V-shaped pH-dependence that lacks the pH-independent regions characteristic of OS-PCET. Accordingly, we develop a mechanistic model for I-PCET that invokes concerted PCET involving hydronium/water or water/hydroxide donor/acceptor pairs, capturing the entire dataset with only four adjustable parameters. We find that I-PCET is fourfold faster with hydronium/water than water/hydroxide, while both reactions display similarly high charge transfer coefficients, indicating late proton transfer transition states. These studies highlight the key mechanistic distinctions between I-PCET and OS-PCET, providing a framework for understanding and modelling more complex multistep I-PCET reactions critical to energy conversion and catalysis.

电化学质子耦合电子转移（PCET）反应可以通过外球电子转移到溶液（OS-PCET）或通过表面结合活性位点的界面极化的内球机制（I-PCET）进行。尽管 OS-PCET 已通过分子洞察进行了广泛研究，但表面固有的异质性阻碍了对 I-PCET 的分子水平理解。在这里，我们采用石墨共轭羧酸（GC-COOH）作为分子明确的 I-PCET 主体来分离 I-PCET 的内在动力学。我们测量了整个 pH 范围内的 I-PCET 速率，发现了 V 形 pH 依赖性，缺乏 OS-PCET 的 pH 独立区域特征。因此，我们开发了 I-PCET 的机械模型，该模型调用涉及水合氢离子/水或水/氢氧化物供体/受体对的协同 PCET，仅用四个可调参数即可捕获整个数据集。我们发现水合氢离子/水的 I-PCET 比水/氢氧化物的 I-PCET 快四倍，而两个反应都显示出相似的高电荷转移系数，表明晚期质子转移过渡态。这些研究强调了 I-PCET 和 OS-PCET 之间的关键机制区别，为理解和建模对能量转换和催化至关重要的更复杂的多步 I-PCET 反应提供了一个框架。