Biogenic Manganese Oxide Synthesized by a Marine Bacterial Multicopper Oxidase MnxG Reveals Oxygen Evolution Activity

Solar energy provides one major pathway to addressing global energy issues. Inspired by photosynthesis, nonbiological solar energy systems are designed for both absorbing light and "splitting water" to generate hydrogen fuel. However, during this process, the oxygen evolution reaction (OER) at the anode has a high kinetic barrier and overpotential, which reduces the overall efficiency. To improve the efficiency of the OER, significant efforts have been made to develop promising OER catalysts. Inspired by the highly efficient oxygen-evolving complex (OEC) in photosystem II in nature, manganese-oxide catalysts have garnered significant attention due to their low cost and minimal toxicity. However, the synthesis of most manganese-oxide catalysts requires strong oxidants, external high electric potentials, or highly basic conditions, which make large-scale production energy-consuming and less efficient. In this study, we present a natural and clean process for synthesizing manganese-oxide catalysts by using an oceanic bacterial manganese oxidase named MnxG. The biogenic manganese oxides, as generated under different conditions, have different morphologies and crystalline structures and are as effective as or even more effective than synthetic birnessite. Spectroscopic analyses, including XANES, XPS, and EPR, suggest that the monoclinic-birnessite component, together with the surface Mn(III) species, plays a crucial role in the OER activity of biogenic MnOx. This work provides insights into the development of efficient OER catalysts that can be produced by using a gentle and sustainable process.