Oxygen Evolution Reaction by Silicate-Stabilized Manganese Oxide

Abstract

Oxygen evolution reaction (OER) through water oxidation under acidic conditions catalyzed by first-row transition-metal-based compounds remains a challenge in artificial photosynthesis. A critical issue for OER under acidic conditions is the solubility of first-row transition-metal compounds during the reaction. This study investigates the silicate-stabilized Mn oxide for OER under acidic conditions (H 2 SO 4 (0.10 M)). Compared to Mn oxide, the silicate-stabilized Mn oxide is significantly more stable under acidic conditions, with an overpotential of 457 mV for the onset of OER. The Mn oxides forming in the absence and presence of silicate groups are α-Mn 2 O 3 and α-MnO 2, respectively. For the Mn oxides forming in the presence of silicate groups, the corresponding current densities of 1 and 10 mA/cm 2 are recorded at 527 and 640 mV, respectively. Silicate-stabilized Mn oxide was characterized by several methods before and after OER. The 29 Si NMR spectrum for silicate-stabilized Mn oxide shows that the Si–O groups chemically bonded to Mn ions. The scanning transmission electron microscopy shows small 2–10 nm particles of Si–O compound in silicate-stabilized Mn oxide, especially Si–O, to stabilize the higher-indices facets of the Mn oxide crystallites. X-ray absorption spectroscopy confirms that the predominant structure for silicate-stabilized manganese oxide is α-MnO 2, with di- and mono-μ-oxo-bridged Mn atoms. After prolonged oxygen evolution, a certain fraction of the mono-μ-oxo bridges disappears for silicate-stabilized manganese oxide. Adding silicate to Mn oxide is a low-cost and environmentally friendly procedure to increase the stability of Mn oxide toward OER under acidic conditions. Thus, our procedure is a clear improvement on current methods to stabilize Mn oxide for OER under acidic conditions.

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