Hierarchical W18O49/NiWO4/NF heterojunction with tuned composition and charge transfer for efficient water splitting

Highlights: • The hierarchical W₁₈O₄₉/NiWO₄/NF heterojunction was successfully synthesized. • The reconstruction process triggers the amorphization of W₁₈O₄₉ and enhances the interfacial interaction. • The redistribution of electrons promotes the formation of abundant active sites. • The activated electrocatalyst shows efficient electrocatalytic performance. Designing and synthesizing heterojunction electrocatalyst with abundant active sites is critical to optimize the activity of electrochemical water splitting. Herein, a typical hierarchical W₁₈O₄₉/NiWO₄ heterojunction grown on nickel foam was successfully synthesized via a multistep synthesis strategy. The electrochemical oxidation and reduction process triggers the amorphization of W₁₈O₄₉ and significantly enhances the interfacial interaction, thereby endowing the composites with enriched active sites. The W₁₈O₄₉/NiWO₄/NF is completely reconstructed to NiWO₄/NF and NiWO₄/NiOOH/NF with amorphous-crystalline interface (denoted as H-AM/NiWO₄/NF for HER and O-AM/NiWO₄/NiOOH/NF for OER). The valence of tungsten ions is well modulated to achieve a high proportion of tetravalent W (IV) active sites accompanied by abundant oxygen vacancies. The increased Ni (III) ions as OER active sites have strong orbital overlap with reaction intermediate, which stabilize the low-valent W (IV) by producing more NiOOH species during the oxygen evolution reaction. Benefiting from the amorphous–crystalline composites, unsaturated W (IV) sites, abundant oxygen vacancies, and highly active NiOOH species, the activated electrocatalyst exhibits excellent electrocatalytic activity, achieving a small overpotential of 1.53 V at 10 mA·cm⁻² for water-splitting and maintaining long-term stability at least 100 h.