Enhancing Photoelectrochemical Water Oxidation Efficiency of BiVO₄ Photoanodes by a Hybrid Structure of Layered Double Hydroxide and Graphene

Making solar fuels, e.g., hydrogen from water splitting, is one of the most critical pathways to developing a clean energy economy. The overall water splitting includes two half-reactions, i.e., water reduction and water oxidation, in which the latter is a speed-limiting step because of its multiproton-coupled four-electron process. It is highly desirable to improve the efficiency of the prevailing photoelectrochemical (PEC) anodes. We constructed an integrated BiVO₄ photoanode modified with a hybrid structure of CoAl-layered double hydroxides (LDHs) and graphene (G), i.e., G@LDH@BiVO₄. This triadic photoanode exhibited a remarkably enhanced performance toward PEC water oxidation, compared to LDH@BiVO₄ and pristine BiVO₄. The photocurrent density of G@LDH@BiVO₄ achieved 2.13 mA·cm^–2 (at 1.23 V vs reversible hydrogen electrode, RHE), 4 times higher than that of pristine BiVO₄. The oxidation efficiency is as high as 80% even at a low bias (<0.8 V vs RHE). The incident photon-to-current conversion efficiency (IPCE) of G@LDH@BiVO₄ reaches 52% at 400 nm, 2.5 times higher than that of BiVO₄. The photoconversion efficiency peaked at 0.55% at a bias of 0.72 V, a 25-fold increase over that of BiVO₄. The findings indicated that the improvement of charge separation efficiency is mainly ascribed to graphene. The enhanced charge transfer efficiency is a consequence of the synergy of graphene and an LDH, where the LDH is capable of expediting water oxidation kinetics and graphene promotes photogenerated charge transfer.