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Photoelectrochemical Cells (PEC), See Wan Hui (138725) - Coggle Diagram
Photoelectrochemical Cells (PEC)
What is PEC?
PEC consist of an anode and a cathode immersed in an electrolyte and connected in an external circuit.
The anode or the cathode consists of a semiconductor that absorbs sunlight, and the other electrode is typically a metal.
Photons with energies greater than the semiconductor band gap are absorbed by the semiconductor, creating electron-hole pairs.
The electron-hole pairs are then split by the electric field in the space-charge region between the semiconductor and the electrolyte.
The electric field reflects the band bending of the conduction and valence band edges at the semiconductor surface and is necessary to supply the free carriers to the appropriate electrode.
Water-splitting Reaction
At anode, water is oxidized if the hole potential at the anode is lower than the O2/H2O potential:
2H+ + H2O (l) -> ½O2 (g) + 2H+
H+ ions migrate through the electrolyte from anode to cathode
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At cathode, H+ ions are reduced to form hydrogen gas if the electron potential in cathode is greater than the H+/H2O potential:
2H+ + 2e- -> 2H2 (g)
Requirements of Materials
The bandgap of the semiconductor should be in the range of 1.8–2.2 eV.
The bandgap of the semiconductor must exceed the potential difference between the two half-reactions (1.23 eV) to drive the reaction.
3.The bandgap of the semiconductor must be small enough to allow appropriate visible light absorption.
4.The conduction and valence band edges must straddle the hydrogen and oxygen redox potentials unless an external bias is applied.
5.The semiconductor should be resistant to corrosion by the electrolyte.
Materials
Wide bandgap semiconductors: Zinc Oxide (ZnO), Tin Oxide (SnO2), Tungsten Oxide (WO3), Titanium Oxide (TiO2)
Properties: Fullfill requirements 2 and 5 but requirement 3 is not fulfilled. They can only absorb irradiation from the UV region, limiting the efficiency of PEC cells to less than 10%
Narrow and optimum bandgap semiconductors: Copper (I) Oxide (Cu2O) and Copper (II) Oxide (CuO)
Properties: Fullfill requirement 3 but requirement 5 is not fulfilled.
Modifications to improve efficiency of photoelectrode materials based on semiconductors
Doping, metal ion loading, swift heavy ion irradiation (SHI), composites of mixed oxides, dye-sensitization, bilayered systems
Apply external or internal voltage or bias to reduce electron-hole recombination when wide-bandgap semiconductors are used and when the band edges do not overlap with the water splitting potential.
Intermediate bandgap semiconductors: Iron Oxide (Fe2O3)
Properties: Fullfill requirements 1,2, and 3 but requirement 4 is not fulfilled.
GaN-based alloys
Properties: Have narrow band gap that could encompass almost entire light spectrum, but is still large enough to allow water splitting to occur at the surface. They are stable in chemical and electrochemical reaction.
Applications
Using the renewable solar energy to split water to H2 and O2, producing two kinds of useful chemical species.
O2 can be used to support combustion and medical purposes while 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.
Removing pollutants in water by oxidizing/degrading organic pollutants.
Convert carbon dioxide (CO2) into selective fuels in gaseous (e.g., methane, ethane, etc.) and liquid products (e.g., formate, methanol, ethanol, etc.) under solar light irradiation at ambient temperature and pressure.
See Wan Hui (138725)
