6th International Conference
on Electroceramics

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Abstract

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Ceria and its derivatives as substrates for solar-driven thermochemical fuel production

Sossina M. Haile
California Institute of Technology

Perhaps the greatest challenge facing our planet is sustainable energy. Given the vast solar energy resource base available to modern society, key to addressing this challenge is the conversion of solar energy into a storable form suitable for on-demand utilization, i.e., the creation of solar fuels. We have developed a unique thermochemical approach to solar fuel generation using ceria as a reaction substrate. The approach relies on the large oxygen nonstoichiometry change that the material undergoes in response to variations in oxygen partial pressure (pO2) and temperature (T). Specifically, upon exposure to high temperatures ceria undergoes reduction without change in crystalline phase to release oxygen. On cooling in the presence of H2O (or CO2), the oxide is reoxidized, releasing H2 (or CO) [1]. The success of the method relies not only on favorable thermodynamics but also on facile kinetics, both in terms of surface reaction rates and bulk diffusion coefficient. Accordingly, we have undertaken a comprehensive study of ceria and its doped derivatives to assess both the equilibrium redox behavior by thermogravimetric methods and the kinetic response by conductivity relaxation methods. We find, for example, that introduction of Zr strongly increases the absolute non-stoichiometry of ceria, but at a penalty in terms of the sensitivity of the nonstoichiometry to changes in environmental conditions and in terms of bulk diffusivity. In another example, we find that the relaxation behavior of Sm-doped ceria is substantially more rapid than that of undoped and Zr-doped ceria, a result that is tentatively assigned to differences in species mobilities (Figure 1). The implications of these fundamental differences in material properties for thermochemical fuel production are discussed.

Figure 1. Chemical diffusivity of neutral oxygen through ceria, 15% Sm-doped ceria (SDC15), and 20% Zr-doped ceria (ZDC20) as functions of oxygen partial pressure and temperature.
References

  1. W. C. Chueh and S. M. Haile, Phil. Trans. R. Soc. A., 2010, 368, 3269–94.

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