Unveiling Synergistic Effectiveness of Strategically Designed Cobalt Clusters for Efficient Water Electrolysis

Electrocatalytic water splitting is a challenging step toward hydrogen production to mitigate fossil fuel dependence. In nature, water oxidation is catalyzed by the Mn₄CaO_x cluster in photosystem-II, but the design of synthetic molecular catalysts still remains a challenge. A few catalysts with low-cost abundant cobalt metal ions have been previously reported, although with low durability and high overpotentials. Here, we report two cobalt cluster catalysts with very low overpotentials and high stability for electrochemical water splitting. These two highly efficient heterogeneous bifunctional (BF) electrocatalysts (ECs), formulated as [Co₃L₄(H₂O)₂]·2.5H₂O (Co3) and [Co₄L₄Cl₄] (Co4), (L^2– = ethyl-2-(picolinoylimino)propanoate), are readily prepared from economical and nontoxic starting materials. The distortions of the coordination geometry around the cobalt atoms, due to the steric effects of the bulky ligand (L), modify the electronic environment of the cobalt centers and facilitate water coordination and subsequent splitting. Furthermore, targeted molecular level modifications on previously reported clusters have provided insight into multimetallic cooperativity and structure–activity relationships. Interestingly, Co4, having a hitherto unknown Co₄O₄ core, acts as an efficient water splitting EC. Co4 shows a higher activity than Co3 and very low overpotentials (η) for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at 10 mA cm^–2 (η = 157 mV for the OER and 39.8 mV for the HER) and small Tafel slopes (40.0 mV dec^–1 for the OER and 40.4 mV dec^–1 for the HER). Additionally, Co4 also shows a high-performance alkaline H₂O electrolyzing capacity with a cell voltage of 1.486 V at 10 mA cm^–2 and exhibits remarkable long-term stability. Thus, our cheap BF molecular EC clearly opens up an innovative platform for scalable O₂ and H₂ production.