Understanding the Influence of Structure on Activity and Stability in the Catalysis of the Oxygen Evolution Reaction (OER) Using Crystalline Oxides As a Platform

Oxides of manganese and cobalt have proved to be effective (electro)catalysts for the water oxidation half-reaction in water splitting. Recently, the research focus has mostly been on developing high surface area morphologies of catalysts with high specific activity that are potentially easily adaptable to photoelectrochemical cells for water splitting. However, it is still not completely understood why these catalysts show such high intrinsic activity, and many have amorphous structures, making it difficult to elucidate structure-function relationships. Using crystalline oxides as an experimental framework, we have elucidated the effects of structure on the activity and stability of cobalt and manganese oxides for the oxygen evolution reaction (OER). For instance, in two distinct polymorphs of lithium cobalt oxide (LiCoO₂), we have found the cubic structure (Fd-3m, spinel-like), with a Co₄O₄ oxo-metallic cube motif, is highly active for water oxidation, whereas the layered polymorph (R-3m) with slabs of CoO₂ (used for rechargeable Li ion batteries) is not. In fact, upon electrochemical processing, the layered polymorph undergoes a surface restructuring that we can monitor using high-resolution transmission electron microscopy (HRTEM). This has been shown to occur in both aqueous and non-aqueous electrolyte. After the transformation occurs, which can also be monitored by the electrochemical profile and corrosion of lithium, the OER activity of the layered material mirrors that of the cubic LiCoO₂. However, it is unstable and eventually diminishes with repeated electrochemical cycling or prolonged electrolysis. In this way, we show that the underlying bulk crystal structure has influence over not just activity, but also stability, both equally important for commercial applications. Parallel to this work, we isolated discrete Co₂O₂, Co₃O₃, and Co₄O₄ molecular clusters in the same ligand sets, and found only the complete Co₄O₄ cubanes were catalytically active for water oxidation. This difference in activity correlates to the accessibility of a formal Co⁴⁺ oxidation state, which occurs at modest potentials only for the cubane, which compensates by delocalizing the hole across all cobalt and bridging oxos. The singly oxidized cubane is sufficient to produce O₂ just in reaction with hydroxide. We believe this phenomenon relates back to the solid-state, and is in part the reason for the high activity in the cubic LiCoO₂.