Ab-Initio and Experimental Techniques for Studying Non-Stoichiometry and Oxygen Transport in Mixed Conducting Oxides

Citation

Balaji Gopal, Chirranjeevi (2015) Ab-Initio and Experimental Techniques for Studying Non-Stoichiometry and Oxygen Transport in Mixed Conducting Oxides. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9TT4NWZ. https://resolver.caltech.edu/CaltechTHESIS:08292014-092737024

Abstract

The ability of cerium oxide (CeO_2-δ, also called ceria), to vary its oxygen stoichiometry in response to changes in temperature or oxygen activity is key to many of its applications in catalysis and electrochemical energy storage and conversion. This thesis explores ab initio and experimental approaches to study the fundamental thermodynamic and oxygen transport properties of ceria (M_xCe_1-xOO_2-δ), but the methods are applicable to other mixed conducting oxides as well.

In the first part of the thesis, a computational thermodynamics approach that integrates quantum mechanical and statistical ensemble-based simulations is used to calculate the reduction-oxidation thermodynamics of non-stoichiometric ceria entirely from first principles. This procedure is well understood and has been successfully implemented for metallic alloys, but has not been extended to correlated electron systems such as ceria, for which the physics of electronic structure calculations is significantly more complicated. Density functional calculations were used to obtain the ground state energies of ceria with vacancy concentrations ranging from fully stoichiometric up to δ=0.25$. For each δ, numerous vacancy configurations were sampled to capture the interactions between vacancies and other atoms. Using the frozen phonon method, lattice dynamical calculations of phonon density of states were performed for various δ. Based on the ground state energies of nearly 40 structures, a cluster expansion Hamiltonian was used to parametrize the energy as a polynomial in occupation variables. The vibrational energies were used to make the Hamiltoninan temperature dependent. Lattice Monte Carlo (MC) simulations using the cluster expansion Hamiltonian were then used to study, for the first time, the effect of temperature and chemical potential on the vacancy concentration in ceria from first principles. The temperature composition phase diagram constructed from the MC simulations successfully reproduced the experimentally reported miscibility gap. The inclusion of vibrational and electronic contributions to the entropy made the agreement quantitative. Further, the partial molar enthalpy and entropy of reduction as a function of δ were extracted and found to deviate significantly from those of an ideally behaved system. The deviations were quantified by calculating the Warren-Cowley short range order parameters. This was the first demonstration of an ab initio approach being used to accurately model the defect thermodynamics of a correlated electron system without resorting to experimental inputs. Using ceria as benchmark material, this project lays the groundwork for a computational approach to screen new oxides for thermochemical cycling.

The rest of the thesis describes experimental investigations of oxygen transport and non-stoichiometry in doped and undoped ceria. Oxygen transport studies were performed using electrical conductivity relaxation (ECR). In ECR, a small step change in _pO₂ forces the sample non-stoichiometry δ, and other dependent properties such as electrical conductivity, to equilibrate to a new value. The rate of this re-equilibration is governed by the bulk oxygen diffusivity, D_Chem, and surface reaction rate constant, k_S -- the two principal kinetic properties. By fitting the solution to Fick's second law, with the appropriate boundary conditions, to the conductivity relaxation profile, D_Chem and k_S can be extracted. The instrumental capability for performing electrical conductivity relaxation experiments was set up and a systematic data analysis procedure was developed to reliably and accurately extract D_Chem and or k_S. The experimental and data analytical methodologies were successfully benchmarked with 15 mol% Sm doped ceria, for which approximate values of the two principal transport properties, bulk oxygen diffusivity, D_Chem, and surface reaction rate constant, k_S, can be found in the literature. An unexpectedly high p-type electronic transference number enabled ECR measurements under oxidizing conditions. A systematic data analysis procedure was developed to permit reliable extraction of the kinetic parameters even in the general case of simultaneous bulk and surface limitation. When the surface kinetics were too sluggish compared to bulk diffusion, Pt catalyst nanoparticles were sputtered to catalyze the surface reaction and enable extraction of D_Chem. The D_Chem from this study showed excellent qualitative and quantitative agreement with expected values, falling in the range from ~ 2 x 10^-5 to 2 x 10^-4 cm²/s. The surface reaction constant under H₂/H₂O mixtures also showed good agreement with literature results. Remarkably, this value increased by a factor of 40 under mixtures of CO/CO₂ or O₂/Ar. This observation suggests kinetic advantages for production of CO rather than H₂ in a two-step solar-driven thermochemical process based on samarium doped ceria.

Using ECR, the effect of 20% Zr addition on the electrical conductivity and oxygen transport properties of ceria as a function of _pO₂ and T was investigated. Under oxidizing conditions, both CeO_2-δ and Zr_0.2Ce_0.8O_2-δ(ZDC20) showed n type, mixed conduction. The conductivity of ZDC20 was two orders of magnitude higher than that of undoped ceria. Contrary to previous studies, we found that Zr addition does not change the electronic mobility in this _pO₂ regime. The enhancement in conductivity is a consequence of higher vacancy concentration in ZDC20 under identical conditions compared to ceria. Under reducing conditions, while the n-type conductivity of ceria continued to increase with decreasing _pO₂, that of ZDC20 reached a broad maximum, eventually decreasing with _pO₂ (p-type) despite increasing carrier concentration. We show that the electronic mobility becomes strongly concentration dependent at high oxygen non-stoichiometry. This leads to a marked decrease in mobility with increase in δ, causing the conductivity to roll over from n to p type. The chemical diffusion coefficient and surface reaction rate constant of both ceria and ZDC20 showed strong dependence on _pO₂ under oxidizing conditions, decreasing by nearly an order of magnitude between 10^-2 atm and 10^-5 atm. The unexpectedly high sensitivity to _pO₂ was ascribed to the effect of extrinsic vacancies generated by trace quantities of lower valence cation impurities, that dramatically increase both the absolute value of the thermodynamic factor and its sensitivity to _pO₂ close to stoichiometry. Overall, the addition of Zr lowers the D_Chem and k_S of ceria in the temperature and oxygen partial pressure range of this study, the effect being more pronounced under reducing conditions. Beyond its relevance to ceria, this work demonstrates the potential of ECR to isolate the effect of kinetics from thermodynamics of the real thermochemical cycle, reveal the limiting transport parameters, and ultimately guide microstructure design for maximizing the rate of fuel production.

Lastly, we improve upon an existing formalism to calculate the oxygen non-stoichiometry in thin films of mixed conducting oxides using AC impedance spectroscopy. Cerium oxide was once again chosen as the benchmarking material, since it shows both ideal and non-ideal thermodynamic behavior under different conditions, and has been well studied in its bulk form. In this method, the impedance response of dense, thin films of CeO_2-δ deposited on a Y_0.84Zr_0.16O_1.92> (YSZ) substrate was measured using AC impedance spectroscopy. To explore potential grain boundary effects on bulk thermodynamic properties. A physically derived equivalent circuit model was fit to the impedance response to extract a quantity called the 'chemical capacitance', which was subsequently related to the non-stoichiometry. Previous studies employing this method were restricted to systems that can be described using ideal solution thermodynamics, which allows simplifications to the theoretical treatment of their capacitance. Apart from extending this technique to a non-ideally behaved oxide, we report excellent agreement between the non-stoichiometry of single crystal and polycrystalline films and that of bulk ceria. By virtue of using thin films, equilibration times are dramatically decreased, enabling faster measurements compared to bulk techniques like thermogravimetry and coulometric titration. Further, the electrochemical method is ideal for thin films, for which the mass changes are below the detection limits of bulk techniques.

Thesis Files