Enhancing the Photoelectrochemical Water Oxidation Reaction of BiVO4 Photoanode by Employing Carbon Spheres as Electron Reservoirs

The rate-determining step of the photoelectrochemical (PEC) water splitting is the water oxidation reaction at the photoanode, which is 4 orders of magnitude slower than the water reduction reaction at the photocathode. In a conventional process to accelerate the water oxidation, oxygen evolution cocatalysts (OECs) are usually used on the surface of a photoanode. As an alternative strategy, we considered employing a composite photoanode made up of a semiconductor and carbon spheres, for it is expected that the photogenerated electrons on the semiconductor particles can be captured by the carbon spheres as electron reservoirs and leave the photogenerated holes on the surface of the semiconductor particles, which promotes the effective separation of photogenerated electrons and holes. More importantly, the holes accumulated in the valence band accelerate the water oxidation reaction rate with the rapid release of electrons stored on the carbon sphere. Therefore, the composite photoanode achieves a higher photocurrent at a lower applied bias. We provide a proof of concept for this strategy by preparing a composite photoanode by combining bismuth vanadium oxide BiVO4 with carbon spheres and find that the resulting photoanode displays a remarkable enhancement in the rate of the photoanode water oxidation.