Solar thermochemical splitting of water to generate hydrogen

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 114; no. 51; pp. 13385 - 13393
• Main Authors: Rao, C. N. R., Dey, Sunita
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 19.12.2017
• Subjects: Carbon dioxide; Commercialization; Fuel production; High temperature; Hydrogen; Kinetics; Low temperature; Materials selection; Nuclear fuels; Photochemicals; Photosynthesis; Physical Sciences; Rare earth elements; Reaction kinetics; Redox properties; Splitting; Synthesis gas; Temperature effects; Thermodynamics; Yttrium; perovskites; metal oxides; thermochemical CO2 splitting; thermochemical H2O splitting
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1700104114

Solar photochemical means of splitting water (artificial photosynthesis) to generate hydrogen is emerging as a viable process. The solar thermochemical route also promises to be an attractive means of achieving this objective. In this paper we present different types of thermochemical cycles that one can use for the purpose. These include the low-temperature multistep process as well as the high-temperature two-step process. It is noteworthy that the multistep process based on the Mn(II)/Mn(III) oxide system can be carried out at 700 °C or 750 °C. The two-step process has been achieved at 1,300 °C/900 °C by using yttrium-based rare earth manganites. It seems possible to render this high-temperature process as an isothermal process. Thermodynamics and kinetics of H2O splitting are largely controlled by the inherent redox properties of the materials. Interestingly, under the conditions of H₂O splitting in the high-temperature process CO₂ can also be decomposed to CO, providing a feasible method for generating the industrially important syngas (CO+H₂). Although carbonate formation can be addressed as a hurdle during CO₂ splitting, the problem can be avoided by a suitable choice of experimental conditions. The choice of the solar reactor holds the key for the commercialization of thermochemical fuel production.