Thermodynamic analysis of two-step electro-thermochemical cycle versus two-step thermochemical cycle for solar H2O splitting: A comparative study

Two-step thermochemical cycle (TSTC) is a promising approach for solar-driven H2O splitting. However, extremely high reduction temperature (Tred) is a key challenge limiting the development of TSTC. Two-step electro-thermochemical cycle (TSEC) has recently emerged to overcome this problem. However, the pros and cons of TSEC and TSTC are under debate. In this paper, a thermodynamic model was developed for studying the thermodynamic fundamentals and the variation rules of energy efficiency of TSEC and TSTC. The thermodynamic mechanisms of TSTC and TSEC were compared using the Gibbs free energy method and the Exergy & energy level analysis method. In comparison to TSTC, TSEC tends to operate at a lower Tred, enabling isothermal operation at moderate temperatures (<1473 K) while sustaining a high H2O conversion rate. In addition, energy efficiency analysis reveals that the introduction of 0.2 V auxiliary voltage can increase the energy efficiency from 0.25 in TSTC to 0.38 in TSEC, showing the advantage of TSEC for H2O splitting in terms of energy efficiency. This study analyses the advantages and disadvantages of TSTC and TSEC for H2O splitting, demonstrating a reference for further development of solar-driven H2O splitting.