Ultrasmall Manganese Nanospinels Produced via an Alcohol Reduction Method and Their Electrocatalytic Oxygen Evolution Reactivity

Green hydrogen production via electrochemical water splitting extensively demands the development of cost-effective and highly efficient electrocatalysts for the anodic oxygen evolution reaction (OER). Nanosized spinel nanoparticles (nanospinels) are potential candidates as electrocatalysts for the OER because of their very high specific surface areas. This work systematically investigated the influence of the A-site metals in the Mn-based nanospinels, i.e., LiMn₂O₄, MgMn₂O₄, ZnMn₂O₄, NiMn₂O₄, CuMn₂O₄, and CoMn₂O₄. Evaluation of their OER activities then indicated that higher OER activities for NiMn₂O₄ and CoMn₂O₄ than those of ZnMn₂O₄, LiMn₂O₄, CuMn₂O₄, MgMn₂O₄, and NiMn₂O₄ nanospinels possessed the best OER activity among the Mn-based nanospinels. Additionally, the NiMn₂O₄ nanospinel exhibited a dramatically improved OER performance compared with NiMn₂O₄ synthesized by the conventional sol-gel process with a much larger particle size, which indicated the advantage of employing nanospinels as an OER electrocatalyst. Also, the NiMn₂O₄ nanospinel was one of the best OER electrocatalysts among the previously reported bimetal spinel oxides. Finally, operando XAFS measurements using an in-house electrochemical cell unveiled that the surface of the NiMn₂O₄ nanospinel was electrochemically transformed to Mn-Ni hydroxide under an OER potential, and the generated compound was preferable for the OER process. This work uncovered that nanospinels are promising candidates as OER electrocatalysts and provided a guideline for the selection of metal components for nanospinel design.