# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 4 Ferroelectric Materials for Photoelectrochemical (PEC) Water-Splitting Devices
### Ferroelectric-Based Materials in Photocathodes for Hydrogen
#### 4.2.1 Ferroelectric Oxide Perovskites  
Ternary Metal Oxides  
**(1) BiFeO\({}_{3}\)**  
BiFeO\({}_{3}\) (BFO) is a well-known multiferroic material with a bandgap in the range of 2.2-2.7 eV, which has been researched extensively because it possesses a wide range of intriguing properties [42; 43; 44; 45; 46; 47; 48; 49; 50; 51]. Many studies have been reported on the significant effect of polarization on band bending in BFO, which plays a crucial role in the photocatalytic activity for PEC water splitting. This enhanced charge separation is attributed to the intimate relation between band bending and the separation of photoexcited electron-hole pairs in the space charge region [52]. Depending on the fabrication method, BFO can exhibit properties of either an n or p-type semiconducting material, thereby playing a role as either an anodic or cathodic electrode in PEC water splitting [42; 45; 47; 51]. Although BFO presents robust ferroelectricity and has attracted significant interest from researchers involved in PEC water-splitting research, there are two challenges that limit the tremendous potential of BFO as a photoelectrode material. The first issue is the low photocathodic current due to the rapid recombination of electron-hole pairs, which is required to achieve a better PEC performance. Another challenge is the rapid degradation during the PEC reaction [53; 54]. Undoubtedly, the noble metal Pt is by far the best-performing photocathode for H\({}_{2}\) evolution to date [55]. Theoretically, a combination of BFO and a noble metal such as Pt is considered a good choice to facilitate H\({}_{2}\) generation reaction. This can be explained by the reduced charge recombination and enhanced electric field that is attributed to the Schottky barrier due to the different work functions between BFO and Pt as well as the localized surface plasmon resonance (LSPR) effect of these noble metals [56; 57]. However, contact between Pt and BFO impedes the transfer of photoexcited electrons from an electrode surface to the electrolyte due to the Schottky upward barrier. Fortunately, the injection of a buffer layer between Pt and BFO has been discovered to overcome this shortcoming. [PERSON] et al. [53] inserted a porous carbon layer sandwiched between Pt and BFO to obtain a ITO/BFO/carbon/Pt photocathode in order to inhibit the formation of a BFO/Pt Schottky upward barrier. As a result, drastically enhanced photocathodic performance is achieved with a photocurrent density (\(J_{o}\)) and onset potential (\(V_{op}\)) up to \(-235.4\)\(\mu\)A/cm\({}^{2}\) and \(1.19\) V versus reversible hydrogen electrode (RHE), respectively, while the values for a photocathode without a carbon interlayer are only \(-61.6\)\(\mu\)A/cm\({}^{2}\) and \(0.83\) V versus RHE, respectively. More importantly, the BFO films were macroscopically poled to investigate the effect of the ferroelectric polarization on the PEC performance, and the results are reported in Figure 6. After poling, the \(J_{o}\) and \(V_{op}\) values increased significantly, which is attributed to the effective separation of the photogenerated carriers in the BFO films, thereby enhancing the PEC activity of the photocathodes. The novel configuration with carbon resulted in a reduced charge recombination and the facilitation of charge transfer by the removal of the upward barrier between Pt and BFO. Based on this result, an amorphous TiO\({}_{2}\) was chosen as a buffer layer to insert between BFO and Pt with the similar desire for the Schottky barrier [58]. As expected, the PEC yielded a \(J_{o}\) of \(-460\)\(\mu\)A/cm\({}^{2}\) at 0 V versus RHE and \(V_{op}\) of 1.25 V versus RHE under 100 mW/cm\({}^{2}\) Xe-lamp illumination. More importantly, the role of TiO\({}_{2}\) in stabilizing the photocathode is apparent with a 10-h continuous effective PEC reaction in an acid solution. Meanwhile, most of the BFO photocathodic performance is degradable within only 2 h of continuous reaction, even when measured using the neutral Na\({}_{2}\)SO\({}_{4}\) electrolyte [53; 54].