Selective Catalyst for Oxygen Evolution in Neutral Brine Electrolysis: An Oxygen-Deficient Manganese Oxide Film
Hikaru Abe, Ai Murakami, Shun Tsunekawa, Takuya Okada, Toru Wakabayashi, Masaaki Yoshida, Masaharu Nakayama
Abstract
Even though the oxygen evolution reaction (OER) is thermodynamically much more favored than the chlorine evolution reaction (CER), the former is much more sluggish. Thus, electrolysis of solutions containing a high concentration of chlorides yields preferentially chlorine. In this study, a thin film consisting of oxygen-deficient and disordered manganese oxide nanolayers on a fluorine-doped tin oxide (FTO) electrode exhibited a high selectivity for OER over CER in 0.5 M NaCl solution. The catalyst was fabricated by electrodeposition of layered manganese dioxide intercalated with Na+ ions (Na|MnO2) followed by heat treatment above 300 °C in air. X-ray photoelectron spectroscopy revealed that, at 200 °C, oxygen vacancies started to form, accompanied by a decrease in the valence state of Mn in the oxide. The X-ray diffraction and absorption fine structure data revealed that, above 300 °C, the multilayered structure of Na|MnO2 that had been constructed during electrodeposition was disordered to yield “oxygen-deficient” MnOx nanolayers, which could lead to an increase in catalytic activity and selectivity for OER. The Na|MnOx film heat-treated at 400 °C showed a Faradaic efficiency as high as 87% in galvanostatic electrolysis at 10 mA cm–2. The Mg2+-intercalated MnO2 electrodeposited similarly did not lose its multilayered structure and did not show a decrease in the Mn valence state during heat treatment, and its activity and selectivity for OER were much inferior to those obtained with Na+.