One-Dimensional Co(OH)F as a Noble Metal-Free Redox Mediator and Hole Extractor for Boosted Photoelectrochemical Water Oxidation in Worm-like Bismuth Vanadate

Abstract

In this work, a noble metal-free one-dimensional Co(OH)F and hierarchical BiVO₄ combination as a model system is proposed for an efficient photoelectrochemical catalyst. BiVO₄, known for its superior theoretical current density of 7.5 mA/cm², suffers from poor photoelectrochemical performance due to the sluggish water oxidation kinetics, recombination of photogenerated carriers, and results in reduced photoelectrochemical water oxidation. BiVO₄/Co(OH)F photoanode exhibits an enhanced photocurrent and a cathodic shift of 160 mV in onset potential as compared to the pristine BiVO₄ photoanode. Cyclic voltametric studies reveal that cobalt exists in the mixed valence state. The presence of fluorine by virtue of its high electronegativity induces facile positive charge on the metal center, i.e., on cobalt, thereby Co²⁺ ions as an active site accept holes more efficiently from the semiconductor and oxidized to Co³⁺ or/and Co⁴⁺. Subsequently, these active species, Co³⁺ or/and Co⁴⁺, deliver the positive charge to produce O₂ and recover to the initial state to regenerate redox couple. A mechanistic study reveals that Co(OH)F nanorods as an efficient hole extractor by virtue of its redox ability, suppresses the recombination of photogenerated electron–hole at the electrolyte/semiconductor interface and accelerates the water-oxidation kinetics. Co(OH)F modification of the BiVO₄ surface is able to utilize a higher number of holes, which have reached the semiconductor/electrolyte interface for water oxidation that resulted in a photocurrent of 3.4 mA/cm². Investigations on the hole transfer efficiency reveals a faster oxidation kinetics, resulting in improved charge injection in the presence of Co(OH)F. Electrochemical impedance measurements shows a low interfacial charge transfer resistance leading to better photoelectrochemical performance. A Faradaic yield of ∼95% suggests that the generated charge carriers (anodic photocurrent) in the BiVO₄/Co(OH)F photoanode is predominantly due to water oxidation.