Solar thermal decoupled water electrolysis process I: Proof of concept

Abstract

A new concept for a solar thermal electrolytic process was developed for the production of H2 from water. A metal oxide is reduced to a lower oxidation state in air with concentrated solar energy. The reduced oxide is then used either as an anode or solute for the electrolytic production of H2 in either an aqueous acid or base solution. The presence of the reduced metal oxide as part of the electrolytic cell decreases the potential required for water electrolysis below the ideal 1.23 V required when H2 and O2 evolve at 1 bar and 298 K. During electrolysis, H2 evolves at the cathode at 1 bar while the reduced metal oxide is returned to its original oxidation state, thus completing the H2 production cycle. Ideal sunlight-to-hydrogen thermal efficiencies were established for three oxide systems: Fe2O3–Fe3O4, Co3O4–CoO, and Mn2O3–Mn3O4. The ideal efficiencies that include radiation heat loss are as high or higher than corresponding ideal values reported in the solar thermal chemistry literature. An exploratory experimental study for the iron oxide system confirmed that the electrolytic and thermal reduction steps occur in a laboratory scale environment.