Harnessing cobalt coordination chemistry for water splitting: molecular design, electronic structure modulation, and catalytic mechanisms

The growing severity of climate change and rising energy demand for fossil fuels have increased interest in exploring clean energy and renewable sources. Electrochemical water splitting using renewable energy inputs has been considered the most promising approach to producing clean hydrogen (H₂). Water splitting involves oxygen evolution (OER) at the anode and hydrogen evolution reactions (HER) at the cathode. Common catalysts for OER and HER are composed of noble metals and non-metals, their alloys, and their compounds, but the high cost, scarcity, and stability of noble metals hinder wider application. To tackle this, cobalt-based complexes have been found to be homogeneous catalysts for both OER and HER, meeting the demand for low overpotentials. Cobalt complexes are highly promising, cost-effective electrocatalysts for electrochemical water splitting (EC-WS), and they can be precisely tuned both electronically and structurally in metal-organic frameworks (MOFs), layered double hydroxides (LDHs), phosphides, chalcogenides, and single-atom catalysts (SACs). Selective tuning of their σ-donors, π-acceptors, and redox-active (non-innocent) ligands gives rise to different and measurable shifts of the cobalt d-band center, of e^G orbital filling, and of the Co²⁺/Co³⁺/Co⁴⁺ redox accessibility, respectively. This review explores the cobalt coordination environment, its donor/acceptor properties, its geometry, its oxidation state, and its nuclearity, and how these affect the three key HER/OER catalytic metrics: overpotential, Tafel slope, and turnover frequency (TOF). An inter-study comparison of various systems reveals that low-cost cobalt catalysts, in particular with optimized coordination environments: multinuclear cubane architectures and dual-site phosphide systems, are as competitive (or more) as IrO₂ and RuO₂ systems in alkaline OER (best ɳ₁₀ = 157 mV). We further critically assess the synthesis routes, degradation mechanisms (ligand dissociation, cobalt leaching, phase reconstruction), and homogeneous vs. heterogeneous aspects of cobalt catalysts under operating conditions of water splitting. The review ends with quantitative electronic descriptor targets and actionable strategies for scalable synthesis and commercially viable development.