Photoanode Materials and Electrocatalysts
A photoanode is an n-type semiconductor, being responsible for the water oxidation in a PEC cell. The requirements of a promising effective photoanode include a favorable bandgap to absorb a wide range of solar spectrum, efficient charge separation and transportation, and fast OER activity [9,16,17,21,29]. More importantly, the VB edge of a photoanode material should be more positive than that of the oxygen evolution potential to drive the water oxidation. In addition, an optimal photoanode should have high stability in aqueous solution, low cost, and environmental friendliness [9,16,17,21,29]. To date, a large number of n-type semiconductors have been investigated as photoanodes for solar water splitting. Examples of promising photoanodes include monometallic oxides (TiO,, ZnO, W03, and a-Fe203), bimetallic oxides (BiV04), and metal (oxy)nitrides (Ta3N5 and TaON). Simultaneously, Co, Ni, and Fe-based OER electrocatalysts have also been widely used as cocatalysts to improve the oxygen evolution and water splitting performance of various photoanodes.
Titanium Dioxide (TiO2)
As one of the most promising PEC photoanodes, Ti02, with a bandgap of ~3.2 eV, was actually the first one reported by Honda and Fujishima in 1972 that ignited the photocatalysis field . Since then, it has been intensively investigated as a photoanode owing to many favorable properties, such as the suitable band edge positions straddling the water redox potentials, excellent photostability in aqueous solution, and high earth abundance . However, the large-scale application of TiO, photoanode in PEC water splitting is mainly limited by its wide band- gap, resulting in the absorbance of only 5% of the solar spectrum (predominantly ultraviolet [UV] light). Moreover, the severe charge recombination also leads to a very low PEC activity . In the past decades, many attempts have been made to improve the performance of Ti02 photoanodes. For example, Hoang reported an efficient TiO, photoanode by introducing N-doping to extend the light absorption into the visible region and loading a cobalt cocatalyst to improve the charge separation and utilization .