Solar-driven H2O/CO2 conversion to fuels via two-step electro-thermochemical cycle in a solid oxide electrochemical cell.

• A two-step electro-thermochemical cycle is proposed to improve the solar-to-fuel efficiency. • The novel paths have significant advantages over pure two-step thermochemical cycle. • A thermodynamic model for CO 2 splitting based on ceria is established. • The reduction temperature could be lowered, thus improving the solar-to-fuel efficiency. • The oxygen carrier could be reduced in air instead of a low- p O 2 atmosphere. H 2 O/CO 2 splitting via two-step solar thermochemical cycles performed with metal oxides is a promising path for fuel production. However, currently the reduction temperature of two-step thermochemical cycles is still too high and the fuel productivity is relatively low, which significantly hinders the improvement of solar-to-fuel efficiency. In this work, a two-step electro-thermochemical cycle integrated with a solid oxide electrolysis cell (SOEC) is proposed to overcome the challenges faced by two-step thermochemical cycle. Meanwhile, the definition, classification and operating mode of this cycle are expounded in detail. Additionally, a thermodynamic model of electro-assisted reduction thermochemical cycle (ERTC) for CO 2 splitting is established and applied to investigate the effect of voltage (E), reduction temperature (T red), oxygen pressure (p O 2), heat recovery of the solid and gas phases on solar-to-fuel efficiency. The efficiency increases from 0.29 to 0.40 as E increases from 0 V to 0.6 V due to the required T red of ceria could be reduced from 1500℃ to 1000℃, with p O 2 = 10−6 bar. Moreover, the oxygen carrier in SOEC could even be reduced in air instead of a low- p O 2 atmosphere while maintaining high efficiency. The technological challenges of high T red and the oxygen carriers' high sensitivity to p O 2 can be mitigated in this innovative path. [ABSTRACT FROM AUTHOR]