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[Display omitted] • Cr dopant promotes the in situ reconstruction of Ni 3 N during HER and OER. • Cr-doping enhances OH– adsorption and boosts the formation of NiOOH. • Cr dopant facilitates the H 2 O activation and the generation of Ni 3 N/Ni(OH) 2. • Cr-Ni 3 N/NF displays high OER and HER activities along with good stability. • It gives a small voltage of 1.72 V at 100 mA cm−2 in overall water splitting. Understanding and utilizing the dynamic changes of electrocatalysts under working conditions are important for advancing the sustainable hydrogen production. Here, we for the first time report that Cr-doping can promote the in situ reconstruction of a self-supported Ni 3 N electrocatalyst (Cr-Ni 3 N/NF) during oxygen and hydrogen evolution reactions (OER and HER), and therefore improve the electrocatalytic water splitting performance. As identified by in situ measurements and theoretical calculations, Cr-doping enhances OH– adsorption during OER at anode and thereby boosts the transformation of Ni 3 N pre-catalysts to defect-rich nickel oxyhydroxide (NiOOH) active species. Meanwhile, it facilitates the generation of Ni 3 N/Ni(OH) 2 at cathodes due to effective H 2 O activation, leading to the fast HER kinetics on the Ni 3 N/Ni(OH) 2 interfaces. Notably, the optimal Cr-Ni 3 N/NF displays good OER and HER performance in 1.0 M KOH electrolytes, with low overpotentials of 316 and 188 mV to achieve the current density of ± 100 mA cm−2, respectively. Benefiting from its bi-functionality and self-supporting property, an alkaline electrolyzer equipped with Cr-Ni 3 N/NF as both anode and cathode affords a small voltage of 1.72 V at 100 mA cm−2, along with 100 h operation stability. Elucidating that Cr-doping can boost in situ reconfiguration and consequently the electrocatalytic activity, this work would shed new light on the rational design and synthesis of electrocatalysts via directional reconstructions. [ABSTRACT FROM AUTHOR]

[Display omitted] Bi-functional electrocatalysts are desired for both hydrogen and oxygen evolution reactions (HER and OER). Herein, facile O 2 -plasma activation is introduced to improve the bi-functionality via constructing nickel sulfide-oxide heterostructures. Ni 3 S 2 -NiO x supported by nickel foam delivers obviously elevated HER and OER activity in comparison with pristine Ni 3 S 2 and recently reported NiS x -based electrocatalysts, featured by the low overpotentials for HER (104 mV) and OER (241 mV) at ±10 mA cm−2 in 1.0 M KOH, as well as a voltage of 1.52 V for overall water splitting. As revealed by in-situ Raman spectroscopy, on the one hand, Ni(OH) 2 generated from Ni 3 S 2 during alkaline HER accelerates water dissociation toward the gradually improved performance; on the other hand, this heterostructure undergoes extensive oxidation during OER, leading to excessive NiOOH covering on Ni 3 S 2 and thereby declining activity. These changes are interpreted by the distinct thermodynamic relationship under specific electrochemical conditions via density functional theory calculations. [ABSTRACT FROM AUTHOR]