What is a Photoelectrochemical Cell?

A photoelectrochemical cell (PEC) is a device that converts solar energy (light) into chemical energy or electricity. Light activates a semiconductor or photosensitizer component within the cell and either:

• Generates electrical energy, similar to how dye-sensitized solar cells work.
• Drives chemical reactions that store energy by forming chemical bonds, such as producing hydrogen through water splitting.

Components: Photoelectrochemical cells are composed of a working electrode, counter electrode, electrolyte and optionally a reference electrode. The working electrode is photoactive and can absorb light to generate charge carriers.

The working electrode, also referred to as photoanode, is normally an n- or p-type semiconductor, i.e. a polymer semiconductor on an ITO/FTO substrate, held by a platinum plate electrode holder. The counter electrode can be platinum or platinum free, i.e. multi-walled carbon nanotubes (MWCNT).

How does a photoelectrochemical cell work?

The critical component of the photoelectrochemical cell is the photosensitizer (semiconductor) on the working electrode. Electron-hole pairs are generated by the irradiation of photons with an energy level that is equal or greater than the bandgap (Eg) of the semiconductor. When light illuminates the photoanode, electrons on the valence band (VB) get excited to the conduction band (CB), and leave a hole behind.

For production of electricity: Photogenerated electrons are swept toward the conducting back contact. They then diffuse through the bulk semiconductor to the external circuit. The electrical energy produced and stored in this process is similar to the a photovoltaic dye-sensitized solar cell (DSSC). The holes are driven to the semiconductor surface and get scavanged by the reduced redox species to become oxidized. The separated electrons which have travelled to the counter electrode then reduce the oxidized species and so on. The regenerative cycle allows for continued electrical current generation.

For chemical reactions: Both the excited electrons and the holes left behind in the photoelectrodes will be involved in some form of chemical reactions, i.e. water splitting. At the counter electrode, the electrons reduce protons (H⁺) to form hydrogen (H₂) while the photogenerated holes at the photoanode oxidise water (or OH^-) to form oxygen (O₂).

What are the applications of photoelectrochemical cells

Photoelectrochemical cells are light or solar energy driven and they offer a promising potential applications in clean energy captivation, energy production and storage and light-emitting devices.

• Water splitting for hydrogen fuel production
• To reduce CO₂ into desirable products such as methane and methanol that can be directly used as fuel
• Photoelectrochemical dye-sensitized solar cells (DSSCs) by incorporating dyes such as N719 which absorb light and oxidize within the system
• Photoelectrochemical perovskite solar cells (DSSCs)

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References

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1. Hodes, G. et al. (2012).Photoelectrochemical Cell Measurements: Getting the Basics Right. J. Phys. Chem. Lett., 9. doi:10.1021/jz300220b
2. Hsu, CY. et al. (2024). Improvement of the photoelectric dye sensitized solar cell performance using Fe/S–TiO2 nanoparticles as photoanode electrode. Sci Rep, 14. doi:10.1038/s41598-024-54895-z
3. Fehr, A.M.K. et al. (2023). Integrated halide perovskite photoelectrochemical cells with solar-driven water-splitting efficiency of 20.8%. Nat Commun, 14. doi:10.1038/s41467-023-39290-y