Role of mn2+ in the oxygen evolution reaction and the oxygen reduction reaction on birnessite

Abstract

An emphasis on the search for alternatives to fossil fuels which maintain energy storage capacities in performance similar to fossil fuels has driven research attempts at the development of batteries such as metal-air batteries, which require the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) to function. The focus of the thesis work was to determine if Mn2+ dissolution and redeposition during electrochemical polarization played an important role in catalytic OER and ORR processes on birnessite (δ-MnO2). It was our hypothesis that formation of Mn2+ occurred during negative (ORR) polarization. Given the high solubility and mobility of Mn2+, the as-formed Mn2+ will likely dissolve from δ-MnO2 electrode into the electrolyte. In contrast, positive polarization should result in the redeposition of dissolved Mn2+ back on the electrode surface, where the ions likely undergo oxidation to Mn3+. Therefore, we expected to see the concentration of Mn2+ in the electrolyte to show a potential-dependent trend. The results of our experiments agreed with the proposed hypothesis and showed an increase in the concentration of Mn2+ after the electrochemical charging of δ-MnO2 in 1 M KOH electrolyte at ORR potentials. This conclusion therefore provides insight and support to the possibility that Mn2+ acts as an active site during the catalysis of ORR on birnessite.