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

Abstract

H2O/CO2 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 CO2 splitting is established and applied to investigate the effect of voltage (E), reduction temperature (Tred), oxygen pressure (pO2), 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 Tred of ceria could be reduced from 1500℃ to 1000℃, with pO2 = 10−6 bar. Moreover, the oxygen carrier in SOEC could even be reduced in air instead of a low-pO2 atmosphere while maintaining high efficiency. The technological challenges of high Tred and the oxygen carriers’ high sensitivity to pO2 can be mitigated in this innovative path.