High‑index‑faceted and electron density-optimized Ni3S2 in hierarchical NiWO4-Ni3S2@NiO/NF nanofibers for robust alkaline electrocatalytic hydrogen evolution.

[Display omitted] • {1 2 ¯ 3} faceted NiWO 4 -Ni 3 S 2 @NiO/NF electrodes with tunable S vacancies were devised. • The facets and S vacancies on Ni 3 S 2 determined the overpotential (η) and TOF in HER. • A η 10 of 89 mV and a TOF of 1.5 × 10−3 s−1 were attained on NiWO 4 -Ni 3 S 2 @NiO/NF-3. • DFT calculations revealed the manipulation mechanism of facets and S vacancies. • An overall-water-splitting current density of 10 mA cm−2 at 1.64 V was achieved. Designing nonprecious electrocatalysts with outstanding performances in hydrogen evolution reaction (HER) via water splitting is attractive for renewable energy conversion. Here, a free-standing and binder-free electrode by in-situ growing a multi-level layered electrocatalyst consisted of NiWO 4 -coupled Ni 3 S 2 nanofibers with secondary NiO layer on Ni foam (NiWO 4 -Ni 3 S 2 @NiO/NF) is devised, in which NiWO 4 was recognized to boost the exposure of high-index {1 2 ¯ 3} facets on Ni 3 S 2 and NiO was identified to increase the electron density of Ni 3 S 2 via accumulating S vacancies. Such an integration of high‑index‑facet exposure and electron-density optimization endowed the resultant NiWO 4 -Ni 3 S 2 @NiO/NF-3 (3 represents the nominal Ni: S molar ratio in fabricating Ni 3 S 2) electrode with a more suitable H adsorption energy on the active Ni sites and a positive shift of the d-band center of Ni 3 S 2 toward the Fermi level, resulting in a strengthened Ni–H interaction. As a consequence, a low overpotential of 89 mV to deliver a current density of 10 mA cm−2 and a high turnover frequency of H 2 generation of 1.5 × 10−3 s−1 for HER in 1 mol l−1 KOH were acquired on NiWO 4 -Ni 3 S 2 @NiO/NF-3, both outperforming the Ni 3 S 2 /NF-1, NiWO 4 -Ni 3 S 2 /NF-1, and NiWO 4 -Ni 3 S 2 @NiO/NF-1 electrocatalysts. Remarkably, an electrolytic cell assembled by using NiWO 4 -Ni 3 S 2 @NiO/NF-3 as both anode and cathode delivered an overall-water-splitting current density of 10 mA cm−2 at 1.64 V, evidencing its robust bifunctional catalytic ability in water splitting. [ABSTRACT FROM AUTHOR]