Thermodynamic analysis of a novel two-step high temperature thermo-electrochemical water splitting cycle

Green hydrogen is a clean fuel aiming to revolutionize the transportation industry in the next decades. It can be produced by several processes with solar electrochemical (EC) and solar thermochemical hydrogen (SCTH) as the most attractive ones. High-temperature solid oxide electrolysis (SOE) is a relatively mature technology; however, these systems suffer low durability due to delamination, build-up of nonconductive phases and separation of metallic electrode contacts. Alternatively, two-step solar thermochemical hydrogen (STCH) exhibits ample durability and benefit from cheaper thermal energy, although solar-to-hydrogen efficiencies remain low. Herein, a novel electrochemically assisted solar thermochemical hydrogen production process (EC-STCH) is proposed, presenting a fundamental thermodynamic analysis of this novel route and a comparison with conventional SCTH and SOE, assuming a fixed H2O-to-H2 conversion of 10% and 50%. The thermodynamic model is based on fundamental thermodynamic principles and demonstrates that for materials which require a ΔT > 500 °C to conduct both reactions in STCH, could operate at lower temperatures and ΔT = 0 °C. Reaction conditions were evaluated showing that it may also be possible to increase the oxygen partial pressure while still achieving high fuel conversion, which would not be possible operating under the conventional SOE and STCH conditions.