In Operando X-ray Spectroscopic and DFT Studies Revealing Improved H2 Evolution by the Synergistic Ni–Co Electron Effect in the Alkaline Condition

The different electrolyte conditions, e.g., pH value, for driving efficient HER and OER are one of the major issues hindering the aim for electrocatalytic water splitting in a high efficiency. In this regard, seeking durable and active HER electrocatalysts to align the alkaline conditions of the OER is a promising solution. However, the success in this strategy will depend on a fundamental understanding about the HER mechanism at the atomic scale. In this work, we have provided thorough understanding for the electrochemical HER mechanisms in KOH over Ni- and Co-based hollow pyrite microspheres by in operando X-ray spectroscopies and DFT calculations, including NiS₂, CoS₂, and Ni_0.5Co_0.5S₂. We discovered that the Ni sites in hollow NiS₂ microspheres were very stable and inert, while the Co sites in hollow CoS₂ microspheres underwent reduction and generated Co metallic crystal domains under HER. The generation of Co metallic sites would further deactivate H₂ evolution due to the large hydrogen desorption free energy (-1.73 eV). In contrast, the neighboring Ni and Co sites in hollow Ni_0.5Co_0.5S₂ microspheres exhibited the electronic interaction to elevate the reactivity of Ni and facilitate the stability of Co without structure or surface degradation. The energy barrier in H₂O adsorption/dissociation was only 0.73 eV, followed by 0.06 eV for hydrogen desorption over the Ni_0.5Co_0.5S₂ surface, revealing Ni_0.5Co_0.5S₂ as a HER electrocatalyst with higher durability and activity than NiS₂ and CoS₂ in the alkaline medium due to the synergy of neighboring Ni and Co sites. We believe that the findings in our work offer a guidance toward future catalyst design.