Enhancing Performance of Photoanodes With a Dual‐Redox Mediator for Photoelectrocatalytic Water Splitting

Hydrogen production via photoelectrocatalytic (PEC) water splitting is a promising way, but great challenges remain in promoting charge separation and transfer, which has been a long‐standing bottleneck in PEC systems. Herein, we construct a BiVO 4 ‐based photoanode via successively introducing Fe 2 TiO 4 , NiOOH, GO, and Co‐complex cocatalyst, achieving an applied bias photon‐to‐current efficiency of 2.4%. Importantly, we found that Fe 2 TiO 4 featured with Fe 2+/3+ and Ti 3+/4+ redox states, eliminates the Fermi level pinning of BiVO 4 and may function as a dual‐redox electron mediator, which enhances charge separation and transfer. By in situ potential measurements, it is found that the interface layers can shift the hole quasi‐Fermi level positively and electron quasi‐Fermi level negatively, increasing the photovoltage and intrinsic driving force for PEC reaction. A tandem device composed of the BiVO 4 ‐based photoanode and a bulk heterojunction photocathode delivers a solar‐to‐hydrogen efficiency of as high as 4.7%. This work identifies Fe 2 TiO 4 as an efficient dual‐redox mediator, which promotes PEC water splitting via regulating the quasi‐Fermi levels of photoanodes.