In Operando X-ray Spectroscopic and DFT Studies Revealing Improved H₂ Evolution by the Synergistic Ni-Co Electron...

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.