Interfacial pH Measurements Using a Rotating Ring-Disc Electrode with a Voltammetric pH Sensor

Abstract

Electrochemical reactions in which H+ or OH− ions are produced or consumed, affect the pH near the electrode surface. Probing the pH locally is therefore highly desired to understand and model the reaction environment under operando conditions. We carried out interfacial pH measurements under mass transport control using a rotating ring-disc electrode (RRDE) coupled with our recently developed voltammetric pH sensor. The interfacial disc pH is detected by functionalizing the gold ring with a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. As protons only have to interact with a monolayer containing the 4-HATP/4-NSTP, the sensitivity and time resolution that can be achieved are superior to potentiometric sensors. We used hydrogen evolution as a model reaction and performed measurements in buffered and unbuffered electrolytes. The effects of the current density, potential, the buffer capacity of the electrolyte and rotation rate on the local pH were investigated. This work shows a reliable and sensitive method for accurately probing the reaction environment under well-defined mass transport conditions, over a wide pH range.