Photoanode

A photoanode is the electrode in a photoelectrochemical cell that functions as the site of oxidation when illuminated. When light is absorbed by a semiconductor with an appropriate bandgap, electron–hole pairs are generated. The photogenerated holes migrate to the surface and oxidize species in contact with the electrolyte, typically oxidizing water to oxygen. The conduction-band electrons are extracted through the external circuit to drive reduction reactions at the counter electrode.

Photoanode materials are usually n-type semiconductors such as titanium dioxide (TiO2), tungsten trioxide (WO3), hematite (Fe2O3), and other oxides like BiVO4 or SrTiO3. They are often fabricated as nanostructured films—nanorods, nanotubes, or mesoporous layers—to maximize surface area and improve charge transport. In dye-sensitized configurations, a sensitizing dye absorbs light and injects electrons into the semiconductor, while surface cocatalysts or catalysts (for example IrO2, RuO2, Co–Pi, NiFeOx) enhance the oxygen evolution reaction.

Key design goals for photoanodes include reducing recombination of charge carriers, improving hole transfer to the electrolyte, and ensuring stability in aqueous environments. Challenges include limited light absorption for wide-bandgap materials and slow oxygen evolution kinetics, which can be addressed with protective coatings, doping, and advanced nanostructures.

Applications center on photoelectrochemical water splitting for solar fuel production, where the photoanode supplies holes to oxidize water, paired with a cathode to reduce hydrogen. Photoanodes are also used in dye-sensitized solar cells as the illuminated electrode. Characterization methods include photocurrent–voltage measurements, incident photon-to-current efficiency, and impedance spectroscopy.