Electrochemical water splitting, generating hydrogen, is crucial for energy harvesting and mitigating renewable energy intermittency. However, the oxygen evolution reaction (OER) at the anode is kinetically sluggish, involving multiple electron-proton transfer steps. Developing efficient OER electrocatalysts is therefore essential. Cobalt oxyhydroxides (CoOOH) are promising candidates due to their abundance, high activity, and tunable electronic structures. In conventional CoOOH, Co³⁺ is typically in a low-spin state (t2g⁶), with electron transfer occurring in face-to-face t2g orbitals. Most research focuses on improving electron transfer in low-spin Co³⁺ CoOOH through morphology engineering, doping, oxygen vacancy introduction, and heterojunction creation. Simulations suggest that high-spin Co³⁺ can significantly enhance OER activity. While high-spin Co ions have been reported in Co-based oxides, they often undergo irreversible reconstruction under alkaline conditions, forming CoOOH. This study provides experimental evidence of the effect of high-spin Co³⁺ on CoOOH's OER activity, achieved through a novel sulfurization and electro-oxidation synthesis route.

Numerous studies have explored enhancing the oxygen evolution reaction (OER) activity of cobalt oxyhydroxides (CoOOH) by focusing on the low-spin state of Co³⁺. Strategies such as morphology engineering, metallic ion doping, oxygen vacancy introduction, and heterojunction creation have been employed to improve electron transfer capabilities. However, theoretical simulations have indicated that incorporating high-spin state Co³⁺ ions could significantly enhance OER performance. While the incorporation of high-spin state Co ions has been reported in some cobalt-based oxides, these often undergo reconstruction under alkaline conditions, resulting in the formation of CoOOH. The influence of high-spin state Co³⁺ on the OER activity of CoOOH-based electrocatalysts has remained largely unexplored experimentally.

The study employed a novel two-step synthesis method to prepare CoOOH with high-spin Co³⁺ (S-CoOOH). First, cobalt sulfides were electrochemically synthesized on carbon cloth substrates. Following this, the sulfides were heated in a nitrogen atmosphere to form Co₃S₄. This precursor was then subjected to an electrochemical oxidation process in 1M KOH electrolyte to produce the final S-COOOH product. A reference sample of CoOOH (R-CoOOH) was synthesized via a hydrothermal and electrochemical oxidation approach. A range of characterization techniques were employed to investigate the structure and properties of the resulting materials. X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), scanning electron microscopy (SEM), and inductively coupled plasma (ICP) measurements confirmed the successful reconstruction of Co₃S₄ into CoOOH. Superconducting quantum interference device (SQUID) magnetometry, electron paramagnetic resonance (EPR) spectroscopy, and density functional theory (DFT) calculations were used to confirm the high-spin state of Co³⁺ in S-CoOOH and elucidate the mechanism for its formation. Electrocatalytic OER measurements were conducted using a three-electrode setup in 1 M KOH electrolyte (pH 13.92). Linear sweep voltammetry (LSV), Tafel analysis, electrochemical impedance spectroscopy (EIS), chronopotentiometry (CP), and pulse voltammetry (PV) were used to assess OER activity and stability, including both intrinsic activity and overall performance. The electrolyte was rigorously purified to remove iron impurities. DFT calculations provided insights into the electronic structure and magnetic properties of both S-CoOOH and R-CoOOH.

The synthesis of S-CoOOH with high-spin Co³⁺ was successfully demonstrated. SQUID magnetometry revealed ferromagnetic behavior in S-CoOOH at 300 K, which contrasts with the paramagnetic behavior of R-CoOOH. EPR data further corroborated the presence of unpaired electrons in the high-spin Co³⁺ in S-CoOOH. XAS analysis (Co K-edge, O K-edge, and Co L-edge) provided evidence for the non-degenerate 3d and 4p orbitals, and the average Co-O bond length in S-COOOH was longer (1.910 Å) than in R-COOOH (1.903 Å), further supporting the presence of high-spin Co³⁺. DFT calculations revealed that coordinatively unsaturated Co atoms at the edges of S-CoOOH are responsible for the emergence of high-spin Co³⁺. These unsaturated Co atoms result in a lower coordination number and induce a higher concentration of oxygen vacancies, resulting in π-donor oxygen ligands that increase octahedral field splitting energy and electron pairing energy. The calculated magnetic moment supported the ferromagnetic behavior observed in S-CoOOH. Analysis of the projected density of states (PDOS) indicated a significant increase in the electronic density of states around the Fermi level for both Co 3d and O 2p orbitals in S-CoOOH compared to R-CoOOH. This enhanced density of states facilitates faster electron transfer, evidenced by pulse voltammetry (PV) measurements showing a higher charge transfer ability for S-CoOOH. Electrocatalytic OER measurements demonstrated that S-CoOOH exhibits significantly improved activity with an overpotential of 226 mV at 10 mA cm⁻², which represents a 148 mV improvement over R-CoOOH. Tafel analysis revealed a much lower Tafel slope for S-CoOOH (28 mV dec⁻¹) than for R-CoOOH (77 mV dec⁻¹). The enhanced activity was consistent even after normalizing for electrochemical surface area (ECSA) and loading mass. Long-term chronopotentiometry (CP) measurements at 10 mA cm⁻² showed that S-CoOOH maintains its activity and structure with negligible decay over 200 h, confirming its excellent stability.

The superior OER activity of S-CoOOH is attributed to the presence of high-spin Co³⁺ and the resulting enhanced electron transfer capabilities. The electron transfer in the apex-to-apex eg orbitals of high-spin Co³⁺ is faster than in the face-to-face t2g orbitals of low-spin Co³⁺. The increased density of states around the Fermi level facilitates electron transfer from the electrocatalyst to the external circuit, accelerating the deprotonation process and improving OER kinetics. The high stability of S-CoOOH further underscores the potential for this material as a high-performance OER catalyst. This research highlights the importance of spin state engineering in designing highly efficient electrocatalysts and provides a new direction for enhancing the OER activity of other oxide-based materials.

This study successfully synthesized CoOOH with high-spin Co³⁺, demonstrating its superior OER activity and stability compared to its low-spin counterpart. The enhanced performance is attributed to the faster electron transfer kinetics in the apex-to-apex eg orbitals of high-spin Co³⁺. This work offers a novel strategy for designing efficient electrocatalysts for water splitting by focusing on spin state engineering. Future research could explore the optimization of synthesis methods to further enhance the proportion of high-spin Co³⁺ and investigate other transition metal oxides to explore similar spin-state effects.

The study primarily focuses on the effects of the high-spin Co³⁺ state. While the electrochemical measurements demonstrate superior performance, the precise quantification of the contribution of each factor affecting the overall OER performance, such as the number of active sites, is not fully elucidated. Further investigation is needed to fully quantify the impact of individual factors. The study also focused on one specific synthetic route, and additional research to explore other synthetic methods for generating high-spin Co³⁺ in CoOOH could provide further insights. The long-term stability testing was performed under a specific current density and it might be beneficial to investigate the stability under varying conditions.