Solar Splitting of H2O and CO2 via Thermochemical Redox Cycles

Funding source: external pageEU-ERCexternal pageSwiss Federal Office of Energy, external pageSwiss 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:

1st 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:

MOox  MOred + 0.5 O2     (1)

2nd 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 1st step. It is represented as:

MOred + H2O  MOox + H2    (2a)

MOred + CO2 → MOox + 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. MOox denotes a metal oxide, and MOred the corresponding reduced metal or lower-valence metal oxide. In the first, endothermic, solar step, MOox is thermally dissociated into MOred and oxygen. Concentrated solar radiation is the energy source for the required high-temperature process heat. In the second step,MOred reacts with H2O/CO2 to produce H2/CO (syngas). The resulting MOox 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.

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