Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

Abstract We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non‐faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H 2 with cis ‐2‐butene‐1,4‐diol to form a mixture of 1,4‐butanediol and n ‐butanol with selectivity that is linearly dependent on the bulk solution pH value. We show that kinetic branching occurs from a common surface‐bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pH value within molecular length scales of the surface. We used the pH‐dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pH value can vary dramatically (>3 units) relative to the bulk value even at modest current densities in well‐buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner‐sphere electrocatalysis.