H2/CO production via high temperature electrolysis of H2O/CO2 coupling with solar spectral splitting at a tunable cut-off wavelength

Abstract

Solar driven CO2/H2O splitting is a promising path for large-scale and long-term solar energy conversion and storage. In this work, a thermodynamic model of solar driven high-temperature CO2/H2O electrolysis was established, with the full solar spectrum split at a tunable cut-off wavelength for meeting the electrical and thermal energy demands of the process at a high efficiency. Compared with the system without spectral splitting, the solar-to-H2 efficiency can be significantly improved from 36.0 % to 45.5 % by introducing spectral splitting. Besides, the optimal electrolysis temperature can be largely lowered, from 1623 K to 1323 K due to the significant reduction of the solar radiation input for electricity generation. The exergy efficiency of CO2 electrolysis was shown to be higher than that of H2O electrolysis due to the higher molar exergy of CO compared to H2, although the first-law efficiency of H2O electrolysis is higher at Tele < ∼1200 K. A high concentration ratio of the solar-thermal process can lead to both high solar-to-fuel efficiency and high optimal electrolysis temperature because of the reduction of reradiation loss. The electrolysis temperature and the use of excessive CO2/H2O were shown to have an important effect on both the efficiency and the optimal cut-off wavelength because they are closely related with the thermal and electrical energy demands of the whole process. The optimal cut-off wavelength can be as low as 540 nm at electrolysis temperature of 1873 K (Tele = 1873 K) and excessive H2O coefficient of 6 (r = 6), and increases to the working limit of the photovoltaic material (900 nm in this work) as the electrolysis temperature and excessive coefficient decrease to Tele < 1023 K and r < 2.