(571g) In Situ Interfacial pH Measurement during Electrocatalysis

The pH at the surface of an electrode where an electrochemical reaction occurs is expected to be quite different from the bulk pH due to the consumption or generation of protons. For example, protons are consumed during the carbon dioxide reduction reaction (CO₂RR), nitrate reduction reaction (NO₃RR), and hydrogen evolution reaction (HER) and protons are generated in the oxygen evolution reaction (OER). While a pH probe can measure the bulk pH, it cannot detect the pH gradient that forms at the electrode–electrolyte interface. Directly measuring the interfacial pH will lead to a better understanding of the reaction conditions and how to control them through electrolyte, reactor, or electrochemical engineering strategies.

We used in situ attenuated total reflectance–surface-enhanced infrared adsorption spectroscopy (ATR–SEIRAS) to quantitatively measure the interfacial pH within 5–10 nm of the electrode surface during electrocatalysis. A phosphate buffer was used to measure the local pH through the ratio of peak areas from the two dominant phosphate anions in each pH range. Three calibration ranges were established: pH 1–5, pH 5–9, and pH 9–13. The interfacial pH calibration data was gathered in 0.5 M phosphate buffer solutions during chronoamperometry at 0.1 V vs. the reversible hydrogen electrode (RHE) after 1 minute and the pH was adjusted in increments of 0.2 pH units by adding dilute sodium hydroxide or phosphoric acid.

We studied the repeatability of the technique and performed electrochemical nitrate reduction as a model reaction to observe the time evolution of the interfacial pH and the impact of pulsed electrolysis on reducing the pH gradient. By correlating the ATR–SEIRAS results with electrochemical experiments measuring NO₃RR selectivity, activity, and efficiency, we can relate bulk electrolyte properties with interfacial properties, and interfacial properties with electrocatalytic performance. We demonstrate how this correlation can lead to engineering strategies to optimize ammonia production in electrochemical NO₃RR, and discuss how the technique could be applied to other electrocatalytic reactions.