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

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.