# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices
### Ferroelectric-Based Materials in Photoanodes for Water
#### 4.3.1 Ferroelectric Oxide Perovskites
##### (2) BiVO\({}_{4}\)  
Bismuth vanadate (BiVO\({}_{4}\)) is an n-type semiconductor (bandgap value \(\sim\)2.5 eV), which has been widely applied as a photoanode material in PEC cells because of its large absorbance range in the visible spectrum along with a suitable conduction band structure [118; 119]. Based on its bandgap energy, BiVO\({}_{4}\) is able to generate a photocurrent density of \(\sim\)7.5 mA/cm\({}^{2}\) and a theoretical solar-to-hydrogen conversion efficiency of 9.2% under AM 1.5 G conditions [120; 121]. However, BiVO\({}_{4}\) suffers from poor surface catalytic reactivity and substantial recombination losses, which prevent it from achieving the theoretical maximum. With the aim of addressing these drawbacks, many strategies such as doping, morphology control, and the construction of heterojunction structures have been investigated in PEC devices containing BiVO\({}_{4}\) photoelectrodes. [PERSON] et al. [122] prepared nanoporous BiVO\({}_{4}\) photoanodes with dual-layer oxygen evolution catalysts (OEC), which is reported to alleviate bulk carrier recombination at the BiVO\({}_{4}\)/OEC junction by creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. PEC devices containing BiVO\({}_{4}\)/FeOOH/NiOOH photoanodes have been shown to yield a photocurrent density of 2.73 mA/cm\({}^{2}\) at a potential as low as 0.6 V versus RHE without any extrinsic doping and composition tuning. Subsequently, this author's group performed further mild annealing treatment of nanoporous BiVO\({}_{4}\) under N\({}_{2}\) flow that produced nitrogen doping and the generation of oxygen vacancies [123]. This is attributed to the enhanced major carrier density as well as the major carrier mobility of BiVO\({}_{4}\). The photocurrent was drastically improved to 4.16 \(\pm\) 0.41 mA/cm\({}^{2}\), and the NiOOH/FeOOH/N-BiVO\({}_{4}\) photoanode manifested an applied biased photo-to-current efficiency (ABPE) of 2.0% under a bias of 0.6 V. In further investigations of the same BiVO\({}_{4}\)/Fe(Ni)OOH tandem structure, further enhancement of the photocurrent density up to 5.82 \(\pm\) 0.36 mA/cm\({}^{2}\) at 1.23 V versus RHE was achieved by Mo-doping with a concentration of 3% on a nanocone FTO/BiVO\({}_{4}\) substrate [124]. In other studies [125; 126; 127; 128], because WO\({}_{3}\) has well-known properties such as its low cost, high chemical stability, and good charge transport activity, BiVO\({}_{4}\) was coupled with this material to overcome the moderate charge transport feature of PEC cells fabricated from pure BiVO\({}_{4}\) to produce PEC cells with robust performance. Nevertheless, these studies at potentials as low as 0.6 V versus RHE still demonstrate charge separation efficiencies of less than 60%. [PERSON] et al. [129] reported on a non-doped nanostructured BiVO\({}_{4}\) photoanode with a bimetallic NiFe-(oxy)hydroxide/borate(NiFeO\({}_{x}\)-Bi\({}_{y}\)) oxygen evolution catalyst as an efficient oxidation co-catalyst to achieve a solar energy conversion efficiency in excess of 2%. More importantly, a very high of incident photo-to-current efficiency (IPCE) of approximately 80% was achieved at a potential as low as 0.6 V versus RHE under front irradiation up to 460 nm, which is nearly twice of that of a previous nanoporous BiVO\({}_{4}\) electrode [122] without compromising other beneficial properties. Recently, [PERSON] et al. [130] fabricated epitaxial BiVO\({}_{4}\) on a thin \(\gamma\)-WO\({}_{3}\) template layer deposited on a SrTiO\({}_{3}\) (001) substrate by pulsed laser deposition (PLD). Although PLD was applied in the fabrication of a BiVO\({}_{4}\) photoanode electrode in PEC cells for the first time by [PERSON] and [PERSON] [131] in the last decade, it drew less attention at the time because of its very low photocurrent values, regardless of the high-quality films.