Size-Dependent Electrocatalytic Water Oxidation Activity for a Series of Atomically Precise Nickel-Thiolate Clusters

The development of renewable energy technologies is critical for reducing global carbon emissions. Water splitting offers a promising renewable energy mechanism by converting water into H₂ and O₂ gas, which can directly power fuel cells or be utilized as chemical feedstocks. To increase the efficiency of water splitting, catalysts must be developed for the water reduction and water oxidation half-reactions. To promote rational catalyst design, atomically precise metal clusters (APMCs) with earth-abundant metals provide a framework for developing both structure-activity relationships and cost-effective catalysts. Previous reports on the water oxidation activity of nickel-thiolate clusters [Ni(SR)₂] have not developed a systematic description of a possible size-activity relationship. Utilizing recent advancements in preparative chromatography for isolating APMCs, we have synthesized a series of Ni(SR)₂ (n = 4, 5, or 6) clusters as electrocatalysts for the oxygen evolution reaction. We discovered a clear size-activity and size-stability trend, with intrinsic activity and stability increasing with cluster size. Using density functional theory, we found that intrinsic activity is inversely correlated to intermediate binding energy, and by extension the oxidation potential of each cluster. Our work demonstrates the ability of APMCs to uncover previously unknown structure-activity relationships that can guide future catalyst design.