Structural ordering in Ceria-based suboxides applied for thermochemical water splitting

Abstract

Solar-thermochemical water splitting by means of suitable Redox materials is a promising way of future hydrogen supply. In the first step of the thermochemical process the Redox material is partially reduced at high temperatures (e.g. using concentrating solar facilities); in the second step which proceeds at lower temperatures, re-oxidation of the Redox material takes place by water vapor, resulting in splitting of H2O-molecules and subsequent H2 release. To avoid energy losses, the temperature gap between reduction and oxidation step should be low as possible. For this requirement the Redox entropy of the employed Redox material should be high. High redox entropies can be expected if structural disorder in the reduced condition is high in comparison to the oxidized state. Thus, sub-oxide forming materials like Ceria resulting in oxygen defect structures are suitable materials for thermochemical water splitting. Reduction temperature of pure Ceria, however, still is too high for technical standards. Lowering the reduction temperature can be achieved by adding suitable cations such as Zr. Zr addition, on the other hand, typically results in a drastical lowering of the oxidation temperature which is due to smaller Redox entropy with respect to the one of CeO2 => CeO2-d. This behavior can be explained intuitively by higher structural ordering of (Ce,Zr)O2-d in the reduced state. Actually Ce3+ and Zr4+ are known to form an ordered pyrochlore structure rather than a fluorite-type solid solution defect structure. Pyrochlore-forming species Ce3+ and Zr4+, however, are highly diluted in compositions such as Ce0.85 Zr0.15O2-d (d=0.03) and hence only clusters of pyrochlore-type short-range order can be expected distributed in a ceria-based solid solution. Direct detection of the presumed clusters with higher structural order is difficult but careful dilatometric studies provide indirect evidence of fluorite-pyrochlore transitions. In binary ceria-zirconia ceramics redox entropy and structural ordering depends significantly on processing temperature. A comparative Redox study using thermogravimetry, dilatometry and water splitting experiments reveals that samples synthesized at 1923 K develop structural ordering during reduction in contrast to samples processed below 1673 K. In general, a certain temperature is a precondition of pyrochlore-type ordering rather than the degree of reduction or the concentration of Ce3+, respectively. This finding is of high significance for processing conditions of ceria-zirconia ceramics and for the process conditions of thermochemical water splitting cycles.