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

: 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 H2 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.