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... Show moreElectrochemical 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. Show less
The electrocatalytic reduction of carbon dioxide is widely studied for the sustainable production of fuels and chemicals. Metal ions in the electrolyte influence the reaction performance, although... Show moreThe electrocatalytic reduction of carbon dioxide is widely studied for the sustainable production of fuels and chemicals. Metal ions in the electrolyte influence the reaction performance, although their main role is under discussion. Here we studied CO2 reduction on gold electrodes through cyclic voltammetry and showed that, without a metal cation, the reaction does not take place in a pure 1 mM H2SO4 electrolyte. We further investigated the CO2 reduction with and without metal cations in solution using scanning electrochemical microscopy in the surface-generation tip-collection mode with a platinum ultramicroelectrode as a CO and H2 sensor. CO is only produced on gold, silver or copper if a metal cation is added to the electrolyte. Density functional theory simulations confirmed that partially desolvated metal cations stabilize the CO2– intermediate via a short-range electrostatic interaction, which enables its reduction. Overall, our results redefine the reaction mechanism and provide definitive evidence that positively charged species from the electrolyte are key to stabilize the crucial reaction intermediate. Show less
