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Figure 40: Basic Principles of PEC
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In the PEC, a semiconductor photocatalyst harnesses solar energy to photolyse water in-situ. During the photoelectrochemical reaction, solar energy is absorbed by a semiconductor material, resulting in the formation of electronic charge carriers called electrons and holes. The holes produced at the photoanode oxidise water to form oxygen gas and hydrogen ions, while both electrons and hydrogen ions move to the cathode via an external circuit and through a PEM membrane, respectively. The hydrogen ions are reduced by the electrons to form hydrogen gas at the cathode. Figure 40 shows the basic principles of PEC.
The goal towards worldwide sustainability for the PEC is to develop efficient, stable and cheap semiconductor photocatalysts on a large scale. In order to ensure high efficiency of the water splitting process, the semiconductor material must have a small band gap (1.8–2.2 eV), appropriate band positions for redox reactions (the conduction band edge position of semiconductor should be at a more negative potential than the reduction potential of water while the valence band edge position to be at a more positive potential than the oxidation reaction), high photocorrosion resistance and good stability in the electrolyte.
However, the challenge in PEC water splitting is well-established semiconductors have either relatively low efficiency or low stability in aqueous solutions. A lot of research has been done to study PEC reaction by using metal oxide semiconductors such as TiO2, MoO3, ZnO, Fe2O3, In2O3, WO3, Cu2O, SrTiO3, SnO2 and others with reasonable photocatalytic properties. Improvements of these semiconductors and new materials are actively researched globally with the goal to seek the efficient and stable semiconductors that can be economically viable for PEC water splitting to produce green hydrogen.
