Solar thermochemical hydrogen production using ceria zirconia solid solutions: Efficiency analysis

In this work thermochemical hydrogen production using ceria zirconia redox materials of the form Ce1−xZrxO2, with x varied in the range 0-0.2, is investigated. A number of literature sources are used to fix the thermodynamic properties such as the partial molar entropies and enthalpies of reduction. From this, a full thermodynamic analysis of the fuel production is possible, where the heat required was assumed to be supplied by concentrated solar power. An efficiency model is presented and the materials are compared. The extent of reduction for a given temperature and oxygen partial pressure increases with increasing Zr concentration. However, the addition of Zr also has a negative effect on the oxidation reaction, requiring lower temperature oxidation and thus a larger temperature swing between the reaction steps. The thermodynamic analysis suggests that for cycles with a reduction temperature of 1773 K, the improvement in yield offered by Ce1−xZrxO2 does improve the efficiency, with Ce0.8Zr0.2O2 having the highest solar to fuel production efficiency of the materials considered. To achieve this efficiency the oxidation would need to be performed at lower temperature (≤800 °C for Ce0.8Zr0.2O2) than is common in experimental demonstrations, and may be kinetically limited.