(174b) Experimental Framework for Evaluation of the Thermodynamic and Kinetic Parameters of Metal-Oxides for Solar Thermochemical Fuel Production

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

Richard J. Carrillo¹,
Jonathan Scheffe¹

¹Department of Mechanical & Aerospace Engineering,
University of Florida, Gainesville, USA 32611

Keywords:  solar, thermochemical, hydrogen, fuel

The two-step metal oxide redox cycle is a promising
and thermodynamically attractive means of solar fuel production. Experimental characterization
of the metal oxides used in these cycles is often performed under unrealistic
conditions (e.g. using excess oxidant) or with non-ideal control of reactant
gases (e.g. TGA), making comparison of the performance of candidate materials
difficult. To address this, we have developed a high-temperature tubular
reactor in which thermodynamic and kinetic experiments can be readily performed
that is capable of operating at temperatures up to 1873 K, total pressures
ranging from ultimate vacuum to ambient, and oxygen partial pressures as low as
about 10^-30 atm (precisely controlled using an H₂/H₂O
environment). The system was validated by performing isothermal relaxation
experiments (wherein the sample environment was rapidly changed by altering the
input vapor concentration) using ceria, and comparing measured oxygen
nonstoichiometries with the literature; see Figure 1. For this presentation, 
we will discuss the utility of this reactor configuration and experimental
scheme for producing accurate and repeatable measurements of the oxygen
exchange capacities, thermodynamic properties (namely partial molar enthalpy,
entropy, and Gibbs free energy changes), and kinetics of candidate redox
materials under a wide range of conditions akin to those expected in solar
thermochemical reactors.

Figure 1. Measured
nonstoichiometry of pure CeO2 versus reactor temperature and oxygen partial
pressure compared to measurements by Panlener et al. (1975) and model predictions by Tuller and Nowick (1979).