Nonstoichiometric Perovskite Oxides for Solar-Driven Thermochemical Water Splitting

Qian, Xin

doi:https://doi.org/10.21985/n2-vh32-q106

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Nonstoichiometric Perovskite Oxides for Solar-Driven Thermochemical Water Splitting

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Two-step, solar thermochemical water splitting using nonstoichiometric oxides has emerged as an attractive approach for large-scale hydrogen production. Perovskite-structured oxides, with their wide tunability, offer the potential for high fuel productivity at moderate operating temperatures. Given the vast chemical space, the materials development effort is carried out here in combination with computational guidance. In the first step, computationally predicted enthalpies are validated against experimental data obtained by thermogravimetric analysis of twelve ABO3-δ oxides. We find it essential to use the ground state structure of the material rather than the assumed cubic structure for calculation. Two key observations further emerge: the entropy, which is not amenable to high throughput computation, is positively correlated with the enthalpy, and oxides bearing Mn, such as SrMnO3-δ and CaMnO3-δ, display an attractive enthalpy of reduction between 200 and 300 kJ (mol-O)-1. Guided by these insights, we then study experimentally the oxides SrTixMn1-xO3-δ (x = 0 – 0.80), with particular attention on the composition SrTi0.5Mn0.5O3-δ (STM55), and also CaTi0.5Mn0.5O3-δ (CTM55). Within the SrTixMn1-xO3-δ oxides, the compositions with x ≥ 0.50 are cubic perovskites at all conditions. Across this subseries, the entropy of reduction is approximately constant with x, whereas the enthalpy increases monotonically, from ~200 kJ (mol-O)-1 at x = 0.5 to ~240 kJ (mol-O)-1 at x = 0.8 (at δ → 0 limit), indicating surprising independent tunability of the thermodynamic parameters. CTM55 is a cubic perovskite under the thermochemical conditions (with δ ≥ 0.02) with an entropy that is slightly larger than that of the STM cubic materials. The functional form of the enthalpy differs, however, being either larger or smaller than that of STM55 depending on the value of δ. Comparison of measured gas production profiles to those experimentally predicted based on quasi-equilibrium behavior suggests that under most conditions the fuel production rate for all compositions is limited by thermodynamic rather than kinetic constraints. The highest fuel productivities are found for STM55 and CTM55. For CTM55 an outstanding hydrogen yield of 10.0 ± 0.2 mL g-1 is achieved with reduction at 1350 °C, oxidation at 1150 °C, and a total cycle time of 1.5 h.

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