Enhancing Photoelectrochemical Water Oxidation Efficiency
of BiVO<sub>4</sub> 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 BiVO<sub>4</sub> photoanode
modified with a hybrid structure of CoAl-layered double hydroxides
(LDHs) and graphene (G), i.e., G@LDH@BiVO<sub>4</sub>. This triadic
photoanode exhibited a remarkably enhanced performance toward PEC
water oxidation, compared to LDH@BiVO<sub>4</sub> and pristine BiVO<sub>4</sub>. The photocurrent density of G@LDH@BiVO<sub>4</sub> achieved
2.13 mA·cm<sup>–2</sup> (at 1.23 V vs reversible hydrogen
electrode, RHE), 4 times higher than that of pristine BiVO<sub>4</sub>. 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@BiVO<sub>4</sub> reaches 52% at 400 nm, 2.5 times higher
than that of BiVO<sub>4</sub>. The photoconversion efficiency peaked
at 0.55% at a bias of 0.72 V, a 25-fold increase over that of BiVO<sub>4</sub>. 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