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.