Investigating cation-substituted metal vanadate photoanodes for solar water splitting

Abstract

As a promising way to harvest solar energy, photoelectrochemical (PEC) water splitting has attracted intensive interest and attention in recent years. Some issues remain in the field of PEC water splitting and the choice of photoanode is an important problem. Recent decade has witnessed the rapid development of BiVO4 as one of the most successful photoanodes. While it has achieved 90% of its theoretical photocurrent, further improvement is limited by its relatively large bandgap 2.4 eV. Hence, to find alternative photoanode candidates with lower bandgap and comparable charge transport properties is urgently needed. This thesis aims to explore cation substituted metal vanadates which have lower bandgap than BiVO4 and comparable charge transport properties. First, the properties of BiVO4 and the role of catalyst have been explored to understand the limitation of BiVO4 photoanode. Next, Fe was incorporated into BiVO4 system to partially substitute Bi to create a mixture of BiVO4 and FeVO4. All the mixed metal vanadate photoanodes indeed have lower bandgap than BiVO4. PEC measurements show that Bi/Fe in 1:1 ratio has the highest performance and the bandgap has lowered to 2.2 eV. The heterojunction between BiVO4 and FeVO4 is also confirmed to be beneficial to charge separation. FeVO4 has a near-optimal bandgap around 2.07 eV, but its photocurrent achieved has been an order lower than BiVO4. The intrinsic properties of this compound are not available in the literature and hence limit its development as a potential photoanode. In this thesis, a series of characterizations have been carried out for the first time to determine the intrinsic properties and to understand the performance limiting factor. It is found that the performance of FeVO4 is limited by its poor charge carrier separation. With the help of microwave conductivity measurements, the origin of inefficient charge separation is attributed to the low carrier mobility. Doping has been applied to improve the carrier concentration and mobility, but the absolute photocurrent increase is limited and separation efficiency still remains low.