Regulating High Electron Spin State in Co3S4 for Enhanced Water Splitting

Implementing the hydrogen economy requires reducing the energy costs of electrocatalytic water splitting, thus making it crucial to design low-cost and high-efficiency electrocatalysts to minimize the needed overpotential for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Here, we propose a non-noble metal bifunctional electrocatalyst (HS Co₃S₄) with a high-spin state by adjusting the coordination structure of spinel sulfide (Co₃S₄). An analysis based on crystal field theory, molecular orbital theory, and density functional theory revealed that the unpaired electrons in the low-coordination Co in HS Co₃S₄ occupied the high-energy e_g* orbitals, resulting in a high-spin state. This unpaired electron in a high-spin state accelerates the transfer of electrons from the catalyst to the reaction intermediate, reducing the activation energy required for the electrocatalytic reaction and facilitating the HER and OER. The developed HS Co₃S₄ catalyst requires overpotentials of 70 and 222 mV to drive a current density of 10 mA cm^–2 for HER and OER, respectively. An anion exchange membrane water electrolyzer with this catalyst requires only 1.78 V to achieve an industrial-level current density of 1 A cm^–2, and it can operate stably for 1000 h. This work provides a promising strategy to regulate the electron spin state of low-cost catalysts for large-scale hydrogen production.