Thermodynamic analysis of titanium dioxide-based isothermal methanothermal and carbothermal redox cycles for solar fuel production

Due to the less efficiency-penalty and avoidance of thermal stress resulting from temperature swings, isothermal thermochemical redox cycle has emerged as a promising way to produce solar fuels. However, the isothermal thermochemical cycles based on the currently reported metal oxides have not shown a competitive theoretical solar-to-fuel efficiency. Herein, titanium dioxide-based isothermal methanothermal redox cycle and carbo-thermal redox cycle for efficient solar fuel production have been proposed and thoroughly evaluated thermo-dynamically. The calculations suggest that the reactant ratio of 0.5 is beneficial for oxygen release and fuel selectivity in the reduction step. Excessive reducing agents could cause the generation of the by-product titanium carbide, resulting in the loss of redox capacity. Attributing to the considerable thermodynamic driving force for water splitting, the reduced titanium dioxide could be almost fully reoxidized even at high temperatures > 1600 K without excess water. As expected, the efficiency evaluation indicates that the studied isothermal redox cycles are more thermodynamically favorable than the non-isothermal ones. Furthermore, according to the efficiency optimization strategy, the methanothermal and carbothermal redox cycles should be performed at the reactant ratios of 0.4 (reduction step) and 0.5 (water-splitting step) under the isothermal operating at 1500 K and 1600 K, respectively, to achieve the highest solar-to-fuel efficiencies of 40.0% and 35.7% without heat recuperation. The results indicate that the titanium dioxide-based isothermal methanothermal and carbothermal redox cycles both have the potential of becoming competitive routes for solar fuel production.

Automated summary

This study proposes titanium dioxide-based isothermal methanothermal and carbothermal redox cycles as efficient methods for solar fuel production and evaluates them thermodynamically. The results suggest that these cycles have the potential to become competitive routes for solar fuel production.