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    Metallic NiPS<sub>3</sub>@NiOOH Core–Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction

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    We report metallic NiPS<sub>3</sub>@NiOOH core–shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS<sub>3</sub> nanosheets are obtained by exfoliation of bulk NiPS<sub>3</sub> in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS<sub>3</sub>@NiOOH core–shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS<sub>3</sub>@NiOOH core–shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS<sub>3</sub> nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS<sub>3</sub> nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS<sub>3</sub>@NiOOH core–shell heterostructure, where the presence of P and S modulates the surface electronic structure of Ni in NiPS<sub>3</sub>, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS<sub>3</sub>@NiOOH core–shell electrode materials for the electrochemical water oxidation
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