Enhanced photoelectrochemical water splitting activity of porous anodic WO3 photoelectrodes decorated with FeWO4 nanoparticles

• Hydrothermally synthesized phase-pure FeWO 4 spin coated on anodic WO 3 electrode. • 1.5 times higher photocurrent from coating-optimized FeWO 4 -WO 3 than pristine WO 3 . • Efficient surface charge separation and transfer kinetics due to p-n junction formation. • IPCE% under visible light improved even if light absorption edge is unchanged. Only a few strategies have been reported to overcome the limitations of tungsten oxide (n-WO 3 ) photoanode fabricated via anodic oxidation of metallic tungsten foil for efficient photoelectrochemical (PEC) water splitting. This work presents a method for synthesizing pure iron tungstate (p-FeWO 4 ) nanoparticles (∼37 nm size) via a hydrothermal process, followed by their application as co-catalyst through spin coating onto porous anodic WO 3 (∼351 nm total thickness). A thin FeWO 4 layer was essential to ensure sufficient light exposure of the underlaying WO 3 during front illumination. Due to the high crystallinity of FeWO 4 and the favorable band alignment between these two semiconductors, the modified WO 3 exhibited suppressed charge carrier recombination and enhanced charge separation and transfer. As a result, the modified photoanode achieved a stable photocurrent density up to 1.5 times higher than that of pristine WO 3 under simulated solar illumination with practically unchanged onset potential. Notably, performance under visible light also improved, although no significant red shift of absorption edge (∼2.9 eV band gap) was noted. The results demonstrated high reproducibility, and we emphasize the significance of this approach for WO 3 photoanodes on opaque substrates, as it should be easily adaptable to WO 3 electrodes fabricated by any technique.