Experimental Framework for Evaluation of the Thermodynamic and Kinetic Parameters of Metal-Oxides for Solar Thermochemical Fuel Production

The two-step metal oxide redox cycle is a promising and thermodynamically attractive means of solar fuel production. In this work, we describe the development of a high-temperature tubular reactor in which the fundamental thermodynamic and kinetic behavior of thermochemical materials can be readily assessed. This reactor system is capable of operating at temperatures up to 1873 K, total pressures ranging from vacuum to ambient, and oxygen partial pressures (pO₂) as low as 10⁻²⁹atm. Compared to off-the-shelf systems like thermogravimetric analyzers or indirect conductivity-based measurement systems, this system has three inherent benefits: (1) the flexibility to control the sample morphology (e.g., powder, packed bed, reticulated porous ceramic, or pellet), (2) the potential for a well-developed and characterized flow, and (3) the ability to readily customize the system on demand (e.g., easy integration with a steam generator to control and operate at very low pO₂). The reactor system and experimental methods were validated by performing isothermal relaxation experiments with undoped ceria, wherein the sample environment was rapidly altered by stepwise changes in the delivered H₂O vapor concentration, and comparing measured oxygen nonstoichiometries with accepted data available in the literature. Data were measured at temperatures from 1173 to 1473 K and pO₂ from 4.54  × 10⁻¹⁸ to 1.02 × 10⁻⁹atm. The measured equilibrium data displayed strong agreement with the literature and the expected trends were preserved. Kinetic data were extracted by first transforming reactant concentrations measured downstream of the reaction zone using a tanks-in-series mixing model to account for gas dispersion. Next, a mechanistic kinetic model distinguishing surface and bulk species concentrations was fit to the data to extract pertinent thermodynamic and kinetic parameters. The model assumed a two-step reaction mechanism mediated by the formation of an intermediate hydroxyl species on the surface. Activation energies and defect formation enthalpies and entropies for the forward and reverse reactions were found to be in good agreement with previous modeling efforts, providing further validation of the use of this system to explore thermodynamic and kinetic behavior of emerging thermochemical materials.