Photoelectrochemical hydrogen production from water using p-type and n-type oxide semiconductor electrodes

Photoelectrochemical hydrogen and oxygen production from water were demonstrated without external voltage using calcium iron oxide (p-type) and TiO ₂ (n-type) semiconductor electrodes. The calcium iron oxide electrode with the ratio Fe/Ca of 1.9, which consisted of two crystal phases (main phase: CaFe ₂O ₄, impurity phase: Ca ₂Fe ₂O ₅), showed the highest photocathodic current in 0.1 M NaOH aq. at potentials below +0.30 V vs. Ag/AgCl under a 500 W Xe lamp illumination, and TiO ₂ showed photo-anodic current in 0.1 M NaOH aq. at potentials over -0.78 V vs. Ag/AgCl. In the system where the two electrodes were connected under illumination, the open-circuit voltage was 1.09 V and the short-circuit current density was 550 μA cm ^-2. Hydrogen and oxygen were successfully generated from this present system without applying an external voltage. The ratio of hydrogen/oxygen evolved after 12-h reaction with both the electrodes short-circuited was around 3.7. This system is an ultimately artificial photosynthesis system where hydrogen and oxygen are generated separately under illumination. It is believed that this present system can form the basis of the future artificial photosynthesis system where hydrogen and oxygen are produced directly from water and sunlight.