Hydrogen From Solar Via Light-Assisted High-Temperature Water Splitting Cycles

Hydrogen production from solar-driven thermochemical water splitting cycles (TCWSCs) provides an approach that is energy efficient and environmentally attractive. Of particular interest are TCWSCs that utilize both thermal (i.e., high temperature) and light (i.e., quantum) components of the solar resource, boosting the overall solar-to-hydrogen conversion efficiency compared to those with heat-only energy input. We have analyzed two solar-driven TCWSCs: (1) carbon dioxide $(CO2)$/carbon monoxide cycle; and (2) sulfur dioxide $(SO2)$/sulfuric acid cycle. The first cycle is based on the premise that $CO2$ becomes susceptible to near-ultraviolet and even visible radiation at high temperatures (greater than $1300K$⁠). The second cycle is a modification of the well-known Westinghouse hybrid cycle, wherein the electrochemical step is replaced by a photocatalytic step. At the Florida Solar Energy Center (FSEC), a novel hybrid photo-thermochemical sulfur-ammonia (S–A) cycle has been developed. The main reaction (unique to FSEC’s S–A cycle) is the light-induced photocatalytic production of hydrogen and ammonium sulfate from an aqueous ammonium sulfite solution. Ammonium sulfate product is processed to generate oxygen and recover ammonia and $SO2$ that are then recycled and reacted with water to regenerate the ammonium sulfite. Experimental data for verification of the concept are provided.