Artificial photosynthesis of solar fuels, such as hydrogen production via water splitting driven by solar energy, is an ideal solution for the sustainability of energy supply. Oxygen evolution reaction (OER) is considered to be the key primary step for achieving overall water splitting. Water oxidation reaction generates reductive electrons and H+ while deliberating gaseous molecular oxygen. The production of H+ makes the vicinity of water oxidation catalyst in acidic environment if these protons are not quickly consumed or transferred far away. The micro-acidic environment requires development of OER catalyst that is not only efficient but also acid-tolerant. At present, the only well-established electrocatalysts for the oxygen evolution reaction (OER: 2H2O → O2 + 4H+ + 4e-) are the oxides derived from iridium (Ir) under proten exchange membrane (PEM) acidic environment. Considering that the employment of PEM electrolyzers for water splitting at TW scale would require over ten years of Ir production, the development of acid-stable non-noble OER catalysts is the most important challenge. In this talk, I will present our recent research progress in the development of acid-stable transition-metal based OER catalysts. Typically, it was found that γ-MnO2 with intergrowth of pyrolusite (β-MnO2) and ramsdellite (R-MnO2) phases, can catalyze the OER for over 6000 hrs at 10 mA cm-2 withn a specific potential of 1.65-1.75 V in a pH 2 electrolyte. This is more than 100 times longer than any other earth-abundant metal oxides reported so far. The voltage efficiency of γ-MnO2 in an electrolyzer with a PEM was 70.12% at room temperature, showcasing how a material traditionally perceived to be unstable in acid has potential application as an industrial OER catalyst. This system is still operating today, showing the importance of precisely regulating electrochemical conditions with respect to the stable potential window. At the system scale, we expect that such potential windows are highly synergetic for water electrolysis using renewable electricity such as solar and wind energy. The spectroelectrochemical approach of identifying stable potential windows can readily be applied to other earth abundant metals, paving the way toward noble-metal-free PEM systems which can operate in the timescale of years.

Stable potential window of gamma-MnO2 for water oxidation in acidic electrolyte for more than 6000 hours

2019 Spring National Meeting

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