Solar Splitting of H₂O and CO₂ via Thermochemical Redox Cycles

Funding source: external page EU-ERC, external page Swiss Federal Office of Energy, external page Swiss National Science Foundation
Doctoral students: Remo Schäppi, Sebastian Sas

Background – A 2-step thermochemical cycle based on metal oxide redox reactions is developed to split H₂O and/or CO₂ and produce H₂ and/or CO. This redox cycle is schematically depicted in Fig. 1, and encompasses:

1^st step – reduction: the solar endothermic reduction of the metal oxide into a reduced-valence metal oxide and O₂ using concentrated solar energy as the source of high-temperature process heat. It is represented as:

MO_ox → MO_red + 0.5 O₂ (1)

2^nd step – oxidation: the non-solar exothermic oxidation of the reduced metal oxide with H₂O/CO₂ which yields syngas together with the initial metal oxide; the latter is recycled to the 1^st step. It is represented as:

MO_red + H₂O → MO_ox + H₂ (2a)

MO_red + CO₂ → MO_ox + CO (2b)

Schematic of a two-step solar thermochemical cycle for H2O/CO2 splitting based on metal oxide redox reactions. — Fig. 1: Schematic of a two-step solar thermochemical cycle for H₂O/CO₂ splitting based on metal oxide redox reactions. MO_ox denotes a metal oxide, and MO_red the corresponding reduced metal or lower-valence metal oxide. In the first, endothermic, solar step, MO_ox is thermally dissociated into MO_red and oxygen. Concentrated solar radiation is the energy source for the required high-temperature process heat. In the second step,MO_red reacts with H₂O/CO₂ to produce H₂/CO (syngas). The resulting MO_ox is then recycled back to the first step, while syngas is further processed to liquid hydrocarbon fuels.

The resulting net reactions are H₂O = H₂+½O₂ and CO₂ = CO+½O₂. In contrast to direct thermolysis, H₂/CO and O₂ are formed in different steps thereby eliminating the need for high-temperature gas separation.

The targeted solar fuel is syngas: a mixture of H₂ and CO. Syngas can be further processed to synthetic liquid hydrocarbons (e.g. diesel, kerosene, gasoline, methanol, and other alternative liquid fuels) via conventional Fischer-Tropsch or other catalytic reforming processes. Liquid hydrocarbon fuels offer exceptionally high energy densities and are convenient for the transportation sectors without changes in the current massive global infrastructure.

Links to projects on Solar Splitting of H2O and CO2

Project-related Publications

Solar Splitting of H2O and CO2 via Thermochemical Redox Cycles

Solar Splitting of H₂O and CO₂ via Thermochemical Redox Cycles