# Ferroelectric Materials: A Novel Pathway for Efficient Solar Water Splitting
## 2 Overview of Solar Water Splitting
### Solar Energy to Hydrogen Conversion System for Solar Water Splitting  
Among the previously mentioned hydrogen production systems, i.e., the PC, PEC, and PV-PEC, the PC offers reasonable solar-to-hydrogen efficiency, a low process cost, simplicity of design, and a large-scale method for water splitting. Moreover, the PC reaction could be performed in the homogeneous phase without transparent electrodes and directional illumination. However, a PC system should be inserted in a gas separator because both hydrogen and oxygen gases are produced during the water-splitting process. Therefore, an enclosed reaction system is required on a large-scale [10; 14; 15; 16]. However, the PEC systems do not require gas separation due to their structure, which entails one or two conductive electrodes and a small bias. The two different gases are sequentially generated and remain at the opposite electrodes.  
A PEC cell system is composed of an electrolyte, an n-type semiconductor (anode), and a p-type semiconductor (cathode) with a semiconductor/liquid junction, as shown in Figure 3. Photogenerated charge carriers are separated, and minor carriers (holes and electrons originated from p-/n-type semiconductor electrodes) move within the semiconductor-liquid interface. This allows for the separate release of oxygen and hydrogen gases during the water-splitting reaction.  
In electrode-liquid electrolyte systems, the photovoltaic and photoelectrochemical reaction behavior is mainly conducted in three steps: (1) light absorption, (2) water reduction and oxidation, and (3) the recombination of electrons and holes. When the system is exposed to photons with sufficiently high light energy, they are induced to form electron-hole pairs, resulting in chemical reactions such as oxidation-reduction reactions. During these reactions, photogenerated currents and voltages are observed inside the system. Such electrical phenomena can be observed in both photovoltaic and photoelectrochemical systems. The photovoltaic (PV) process is so-called because the conversion of light into electricity occurs in a photoelectrode through a process of conversion photons. In the case of a photoelectrochemical process, the generated photons induce a chemical reaction in the electrodes. Since both photovoltaic and photoelectrochemical processes are related to light absorbance and the conversion of photons to induce chemicals reactions, various semiconducting materials can be used to convert solar energy to hydrogen. They have been extensively used in the past for solar water splitting [17; 18].  
Until now, several different concepts have been proposed and reported from different research groups for solar water splitting. Figure 3 shows a schematic diagram of an enhanced PEC and PV-PEC hybrid water splitting system, which was proposed to improve solar energy to hydrogen efficiency. The PV/PEC hybrid cell consists of a photoelectrode PEC cell and a PV cell. The PV cells play an important role in supplying a potential to the PV cell. The PV cell assists the PEC cell  
Figure 3: Schematic diagram of photoelectrochemical (PEC) photovoltaic (PV)–PEC hybrid water splitting system; **(a)** PV/PEC hybrid cell, **(b)** A photoelectrode PEC cell with anode and cathode combined in parallel, and **(c)** Two photoelectrodes combined in series [3]. Copyright 2010, Royal Society of Chemistry.  
to develop a potential for photogenerated electrons, allowing water reduction at the surface of the photocathode [19; 20].