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” MnO
x
nanolayers, which could lead to an increase in catalytic activity
and selectivity for OER. The Na|MnO
x
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+.