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| Fig. 1: Schematic of a typical PEC cell. Electrons on the VB of the photoelectrode get excited to CB after solar illumination, and flow to the counter electrode for Hydrogen evolution. The hole remaining at the photoelectrode oxidize water for oxygen evolution. (Source: X. Shi) |
Research on solar cells has been conducted for a long time. A solar cell converts solar energy to electrical energy, which is a clean and renewable energy. [1] Along with the solar cell, there has also been another energy conversion system known as the photoelectrochemical (PEC) cell, which has now been studied for a few decades as well. [2] The PEC cell, unlike the traditional solar cell, converts solar energy to chemical energy, and this chemical energy is embodied in a chemical bond.
A PEC cell consists of two sides, the anode and the cathode (Fig. 1). At least one side is made of photo-sensitive material, e.g. semiconductor. [3] Typically this is the anode, and in such cases the anode is referred to as the photoanode or photo electrode. Thus, the cathode part will typically also have another name, called the counter electrode. So far the most semiconductor material used for photoanode are metal oxide, for example iron oxide (Fe2O3), tungsten oxide (WO3), titanium oxide (TiO2), etc. [4] When light illuminates the anode, electrons on the valence band (VB) get excited to the conduction band (CB), and leaves a hole behind.
These excited electrons become free electrons. They will flow through the external circuit and reach the counter electrode. Both these excited electrons and the holes left behind in the photoelectrode will be involved in some chemical reactions, since they are reductive and oxidative, respectively. Using water splitting as an example, where the free electrons on the counter electrode will reduce protons (H+) to hydrogen (H2), while the holes at the photo electrode will oxidize water (or OH-, depending on the pH of the water used) to oxygen (O2) (Fig. 1). [5] Eventually the cell realizes the conversion of energy from solar to chemical, so that the energy is incorporated in the chemical bonds of H2 and O2.
The PEC cell has a many applications. For example, it may be used to split water to H2 and O2, which produces two kinds of useful chemical species. O2 can be used to support combustion, as well as medical purposes in a hospital. H2 is a flammable gas and can be used for combustion, fuel cells, and possibly in future hydrogen cars due to its high energy density.
In addition, the anode and cathode in a PEC cell can be used for oxidizing and reducing materials other than water. For example, if there is some organic material in the electrolyte, the holes at the anode could oxidize the organic material instead of water. [6] This provides the PEC cell with more potential applications, such as removing pollutants in a sample of water. Similarly, the PEC cell can do more work by oxidizing and reducing different materials in the electrolyte as well.
© Xinjian Shi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] B. O'Regan and M. Grätzel, "A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films," Nature 353, 737 (1991).
[2] M. Grätzel, "Photoelectrochemical Cells," Nature 414, 338 (2001).
[3] M. J. Kenney et al., "High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation," Science 342, 836 (2013).
[4] B. D. Alexander et al., "Metal Oxide Photoanodes for Solar Hydrogen Production," J. Mater. Chem. 18, 2298 (2008).
[5] M. G. Walter et al., "Solar Water Splitting Cells," Chem. Rev. 110, 6446 (2010).
[6] W. Zhao et al., "Efficient Degradation of Toxic Organic Pollutants with Ni2O3/TiO2-xBx under Visible Irradiation" J. Am. Chem. Soc. 126, 4782 (2004).