In Situ Measurement of Local pH at Working Electrodes in Neutral pH Solutions By the Rotating Ring‐Disk Electrode Technique

The pH is one of the most important factors for electrochemical reactions. The solution pH is often measured prior to the electrochemical measurements, but it is not sufficient to consider only bulk pH, especially for the measurements in neutral pH solutions. When electrochemical reactions occur that produce/consume protons or hydroxide ions, the local pH in the vicinity of an electrode becomes considerably different from the bulk pH. This local pH is important in considering electrodeposition and corrosion. Furthermore, it also has an critical effect on the operating voltage of aqueous batteries and capacitors, as local pH affects the electrochemical stability of water. 1 However it is difficult to measure the local pH, because the measurements have to be made in a small region adjacent to the electrode. Herein, rotating ring-disk electrode (RRDE) is a promising tool for the local pH measurements, though it has not been applicable to neutral pH solutions. 2 In this study, the theory of local pH measurements using RRDE was extended to apply to solutions over a wide pH range. 3 This extended theory was corroborated by measurements of the local pH change during hydrogen oxidation/evolution reactions on platinum disk electrode in solutions with near-neutral pH. This reversible reaction makes it possible to control the local pH on the disk electrode by the applied potential. The platinum ring electrode successfully detected the local pH changes by potentiometry, and the measured local pH was in good agreement with the theoretical predictions (Fig. 1a). In addition, using the RRDE modified with iridium oxide (IrOx) ring, we were able to evaluate the dynamic changes in the local pH of the working electrodes during oxygen evolution reaction in Ar-saturated neutral-pH solution (Fig. 1b). 4 Thus, RRDE with IrOx ring has the potential to be a widely applicable tool for measuring the local pH of the electrode during any electrochemical reactions. Reference Y. Yokoyama, T. Fukutsuka, K. Miyazaki, and T. Abe, J. Electrochem. Soc. , 165 , A3299–A3303 (2018). W. J. Albery and A. R. Mount, J. Chem. Soc. Faraday Trans. 1 , 85 , 1181–1188 (1989). Y. Yokoyama, K. Miyazaki, Y. Miyahara, T. Fukutsuka, and T. Abe, ChemElectroChem , 6 , 4750–4756 (2019). Y. Yokoyama, K. Miyazaki, Y. Kondo, Y. Miyahara, T. Fukutsuka, and T. Abe, Chem. Lett. , 49 , 195–198 (2020). Figure 1