Design Principle of Monoclinic NiCo2Se4 and Co3Se4 Nanoparticles with Opposing Intrinsic and Geometric Electrocatalytic Activity toward the OER.

Despite predictions of high electrocatalytic OER activity by selenide-rich phases, such as NiCo2Se4 and Co3Se4, their synthesis through a wet-chemical route remains a challenge because of the high sensitivity of the various oxidation states of selenium to the reaction conditions. In this work, we have determined the contribution of individual reactants behind the maintenance of conducive solvothermal reaction conditions to produce phase-pure NiCo2Se4 and Co3Se4 from elemental selenium. The maintenance of reductive conditions throughout the reaction was found to be crucial for their synthesis, as a decrease in the reductive conditions over time was found to produce nickel/cobalt selenites as the primary product. Further, the reluctance of Ni(II) to oxidize into Ni(III) in comparison to the proneness of Co(II) to Co(III) oxidation was found to have a profound effect on the final product composition, as a deficiency of ions in the III oxidation state under nickel-rich reaction conditions hindered the formation of a monoclinic "Co3Se4-type" phase. Despite its lower intrinsic OER activity, Co3Se4 was found to show geometric performance on a par with NiCo2Se4 by virtue of its higher textural and microstructural properties.