Outstanding Properties and Performance of CaTi_0.5Mn_0.5O_3–δ for Solar-Driven Thermochemical Hydrogen Production

Variable valence oxides of the perovskite crystal structure have emerged as promising candidates for solar hydrogen production via two-step thermochemical cycling. Here, we report the exceptional efficacy of the perovskite CaTi_0.5Mn_0.5O_3–δ (CTM55) for this process. The combination of intermediate enthalpy, ranging between 200 and 280 kJ (mol-O)⁻¹, and large entropy, ranging between 120 and 180 J (mol-O)⁻¹ K⁻¹, of CTM55 create favorable conditions for water splitting. The oxidation state changes are dominated by Mn, with Ti stabilizing the cubic phase and increasing its reduction enthalpy. A hydrogen yield of 10.0 ± 0.2 mL g⁻¹ is achieved in a cycle between 1,350°C (reduction) and 1,150°C (water splitting) and a total cycle time of 1.5 h, exceeding all previous fuel production reports. The gas evolution rate suggests rapid material kinetics, and, at 1,150°C and higher, a process primarily limited by the magnitude of the thermodynamic driving force.