Hematite, the mineral form of iron oxide (or trivially, rust), is a promising anode material for photoelectro-chemical cells (PEC) because of its affordability, availability, high stability and good spectral match to the solar spectrum. Although it has the potential of a 15% solar-to-hydrogen energy conversion efficiency, its actual efficiency is lower than that of other metal oxides. This is due to hematite's electronic structure, which only allows for ultrashort electron-hole excited-state lifetimes.
Helpful holes in hematite