Diagram showing the three main technologies for generating hydrogen from sunlight: a) photocatalytic, b) photoelectrochemical, and c) photovoltaic-electrolyzer systems. OEP and HEP in (a) correspond to the oxygen evolution photocatalyst and hydrogen evolution photocatalyst, respectively. The illustrations are representations of example systems of each technology, but other configurations are possible for each. For example, in the photocatalytic case represented here, the photogenerated electron on the HEP component reduces protons to make H2, while the photogenerated hole is consumed by a redox mediator to form the Ox species. Similarly, the photogenerated hole on the OEP oxidizes water, while the corresponding electron is consumed by the oxidized mediator to regenerate the Red species. This is referred to as a redox-mediated Z-scheme. Another possible device is a hybrid between PEC and PV-EC, with one of the photoelectrodes in (b) relying on a catalyst-coated PV cell, and the whole system being fully integrated and wireless. This is referred to as integrated PV-PEC tandem or artificial leaf.
