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

In this manuscript, we explore the effect of pH on the selectivity of a hydrothermally synthesized nanostructured gamma-MnO2 electrocatalyst in the alkaline environment. Selectivity of electrodeposited gamma-MnO2 toward oxygen evolution reaction (OER) in 0.5 M NaCl at pH 12 has been demonstrated by Fujimura et al (Mat. Sci. Eng., A267 (1999) 254-259). Herein, we extend the pH region from pH 8.2 to pH 13 and demonstrate by thin-film rotating disk electrode (RDE) method that the catalyst is selective toward the OER at current densities up to ca 15 mA/cm(2) in the entire range of pH, if buffer is utilized to mitigate the effect of local pH. In synthetic seawater, at pH 8.2, the catalyst is not selective toward the OER. The analysis of the OER Tafel slopes at low current densities (<3 mA/cm(2)) shows that the slopes were not affected by pH in 0.5 M NaCl solutions, which suggests the same OER mechanism. At the same time, reaction rate decreased with decrease in pH. Nafion ionomer in the catalyst layer may adversely affect the catalyst's performance at pH <= 12 by limiting diffusion of OH- ions through the Nafion film. Catalyst layers need to be carefully designed to avoid negative effects of Nafion.