Effect of pH on the Selectivity of γ-MnO₂ Electrocatalysts Towards Oxygen Evolution Reaction in the Presence of Chloride Ions in Alkaline Environment

This paper reports on progress in development of electrocatalysts intended for use in seawater, which optimize the balance of competing reactions between chlorine evolution and oxygen evolution. The oxygen evolution reaction (OER) from H 2 O is thermodynamically more favorable due to a lower equilibrium potential [1]. However, chlorine evolution successfully competes with the OER due to faster kinetics. One of a few catalysts that demonstrates high selectivity towards the OER in the presence of chloride ions in the alkaline environment (i.e. pH 12) is γ-MnO 2 [2]. In this presentation, we (1) compare activities and selectivities towards the OER of electrodeposited and chemically synthesized γ-MnO 2 and (2) determine how pH affects selectivity in the pH range from 11 to 13. g-MnO 2 powder was synthesized using hydrothermal method. The synthesis resulted in fabrication of material with a flower-like morphology, characterized by a crystallite size of 5-15 nm and BET surface area of 75 m 2 /g. In parallel, MnO 2 films were prepared by electrodeposition on a Pt RDE substrate, following the procedure outlined by Tsagareli et al. [3]. The electrocatalytic activity of both γ-MnO 2 powder and electrodeposited MnO 2 was investigated by the thin-film RDE method. The thin films of γ-MnO 2 powder were obtained by drop-casting MnO 2 /VC ink [1] on glassy carbon substrates. We found that the shape of polarization curves on MnO 2 /VC films is strongly dependent on the Nafionä loading. At low Nafion loadings, we observe diffusion-limited currents, which we attribute to OH - diffusion through the Nafion. At higher Nafion loadings, neither diffusion-limited currents, nor the currents’ dependence on the rotation rate are observed. Our data indicate that at low Nafion loadings, MnO 2 /VC films selectively produce O 2 in 0.5M NaCl at pH 12 and 13 and current densities up to 15 mA/cm 2 . Because of the potential effect of Nafion on the permeability of Cl - ions to the catalyst’s active sites, a set of parallel experiments is being carried out on the electrolytically deposited MnO 2 films. The results of the latter experiments will be compared to those obtained on MnO 2 /VC films in the presentation. References [1] F. Dionigi, T. Reier, Z. Pawolek, M. Gliech, P. Strasser, Chemsuschem, 9 (2016) 962. [2] K. Fujimura, T. Matsui, K. Izumiya, N. Kumagai, H. Habazaki, A. Kawashima, K. Asami, K. Hashimoto, Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 267 (1999) 254. [3] G. Tsagareli, S. Makhatadze, M. Soselia, N. Maisuradze, Journal of the Indian Chemical Society, 99 (2022).