In Situ Visualization of Electrocatalytic Interfacial pH Changes via the Use of Phosphates as the Probe

The interfacial pH distribution throughout the electrode has a considerable influence on the electrocatalytic performance. However, a local pH mapping method that can be applied to various electrocatalytic electrodes over a full pH range is still lacking. Herein, we demonstrate in situ local pH visualization with phosphates as probes via Raman spectroscopy with wide universality. An automated procedure is developed to convert the Raman mapping spectra of phosphates to a pH distribution. The full pH range is divided into three segments: acidic, neutral and alkaline. A generalized logistic function is used and fitted in each segment. Nitrate reduction on CuO supported by Cu mesh in neutral is taken as the research model, and a cross on the mesh is chosen as the imaging area. The results show a rapid pH increase on the cross while a gentle increase in the adjacent electrolyte, as well as a nonuniform pH distribution on the electrode cross ranging from 5.6 to 10.0 at the same potential. A direct comparison between the distribution of the pH and the concentration of NO₃^- as the reactant is also demonstrated. Our work provides a method to obtain the in situ pH distribution for various electrocatalytic reactions in a wide pH, and is helpful for understanding the local microenvironment near an electrode.