Coupling the full solar spectrum with a two-step thermo-electrolytic cycle for efficient solar hydrogen production

Abstract

H2O splitting via redox cycles of metal oxides provides a promising path for solar hydrogen production. For various redox cycles with hybrid driving forces, the introduction of spectral splitting is expected to reduce the exergy degradation at the initial step and realize the cascade utilization of the full solar spectrum. A thermodynamic model coupling the full solar spectrum with a two-step thermo-electrolytic cycle was established in this work, based on which the cycling performance was theoretically examined. Solar spectral splitting at an adjustable cut-off wavelength was employed to allow for a flexible allocation of the solar spectrum to meet the thermal and electrical energy demands of the cycle. Solar-to-H2 efficiency can be significantly improved by the introduction of spectral splitting. It can be as high as 35.4% for the redox cycle performed with CeO2-δ, and the optimal reduction temperature can be lowered from 1323 K to 1123 K. Isothermal cycles have an efficiency advantage over non-isothermal cycles due to the elimination of the energy cost for oxygen carrier heating. However, they are also accompanied with lower H2O conversion rates and higher oxygen carrier demands, for which a compromise may be needed from a practical view. Oxygen carriers with lower oxygen affinity were shown to have a higher efficiency potential than pure ceria (CeO2-δ). However, as the reduction enthalpy drops close to or even lower than the enthalpy of direct H2O splitting, the efficiency decreases fast due to insufficient driving force for the oxidation reaction.