Jason D. Nicholas

Measurements and Modeling of SM0.5Sr0.5CoO3-x-Ce0.9Gd0.1O1.95 SOFC Cathodes Produced Using Infiltrate Solution Additives

Nanocomposite Sm 0.5 Sr 0.5 CoO 3-x (SSC)-Ce 0.9 Gd 0.1 O 1.95 (GDC) solid oxide fuel cell (SOFC) cathodes were produced by infiltrating SSC nitrate solutions into GDC scaffolds. A single infiltration of a concentrated solution resulted in a low polarization resistance of 0.1 Ω cm 2 at 600°C. Infiltrate solution additives slightly improved the SSC phase purity but did not significantly alter the SSC particle morphology/size or the infiltrated cathode polarization resistance. Polarization resistance predictions made using microstructural observations and a simple model were found to be within 35% of the experimentally measured values without the use of fitting parameters.

Use of the Simple Infiltrated Microstructure Polarization Loss Estimation (SIMPLE) model to describe the performance of nano-composite solid oxide fuel cell cathodes

Nano-composite Sm(0.5)Sr(0.5)CoO(3-δ) (SSC)-Ce(0.9)Gd(0.1)O(1.95) (GDC) and La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) (LSCF)-GDC Solid Oxide Fuel Cell (SOFC) cathodes with various infiltrate loading levels were prepared through multiple nitrate solution infiltrations into porous GDC ionic conducting (IC) scaffolds. Microstructural analyses indicated that the average SSC and average LSCF hemispherical particle radii remained roughly constant, at 25 nm, across multiple infiltration-gelation-firing sequences. Comparisons between symmetric cell polarization resistance measurements and Simple Infiltrated Microstructure Polarization Loss Estimation (SIMPLE) model predictions showed that the SIMPLE model was able to predict the performance of heavily infiltrated SSC-GDC and LSCF-GDC cathodes with accuracies better than 55% and 70%, respectively (without the use of fitting parameters). Poor electronic conduction between mixed ionic electronic conducting (MIEC) infiltrate particles was found in lightly infiltrated cathodes. Since these electronic conduction losses were not accounted for by the SIMPLE model, larger discrepancies between the SIMPLE-model-predicted and measured polarization resistances were observed for lightly infiltrated cathodes. This work demonstrates that the SIMPLE model can be used to quickly determine the lowest possible polarization resistance of a variety of infiltrated MIEC on IC nano-composite cathodes (NCCs) when the NCC microstructure and an experimentally-applicable set of intrinsic MIEC oxygen surface resistances and IC bulk oxygen conductivities are known. Currently, this model is the only one capable of predicting the polarization resistance of heavily infiltrated MIEC on IC NCCs as a function of temperature, cathode thickness, nano-particle size, porosity, and composition.

Finite-Element Modeling of Idealized Infiltrated Composite Solid Oxide Fuel Cell Cathodes

The polarization resistance of idealized, branched, composite cathodes was modeled using a two-dimensional finite element calculation. The model structures consisted of micrometer-scale columns and nanoscale branches of ionically conducting materials that were coated with a mixed-conducting material. The structures approximate an ionic-conductor matrix infiltrated first with the same ionic-conductor material and then with a mixed conductor. Increasing the length of the ionically conducting nanobranches, and hence, the surface area of the infiltrated mixed conductor, resulted in a factor of ∼ 10 polarization resistance decrease compared to mixed-conductor-coated columns without ionically conducting nanobranches. For many solid oxide fuel cell relevant temperatures (500-900°C), cathode geometries, and materials, the cathode resistance was limited by surface oxygen exchange and hence, was inversely proportional to the mixed-conductor surface area. However, for cathode columns or branches with large enough aspect ratios, ionic-conduction losses also limited the polarization resistance. The characteristic length of nanostructured cathodes was found to depend on the cathode surface area ratio in addition to the traditional bulk diffusion constant to surface exchange constant ratio (D/k). Lastly, the effects of materials properties, particularly ionic conductivity and surface resistance, were investigated and discussed for common cathode materials.

Polyamorphic transitions in vitreous B2O3 under pressure

We have studied the nature of structural transitions in B2O3 glass under pressure using molecular dynamics simulations, based on a newly developed coordination-dependent charge transfer potential, to complement the results from our earlier Brillouin and Raman scattering experiments and to interpret these findings. This interaction model allows for charges to re-distribute between atoms upon the formation and rupture of chemical bonds, and accommodates multiple coordination states for a given species in the course of the simulation. The macroscopic observables of the simulated vitreous B2O3, such as the variation of density and elastic modulus with pressure, agree well with those seen in experiments. The compaction of simulated structures is based on a polyamorphic transition that involves transitory four-coordinated boron atoms at high pressures. While the coordination of boron completely reverts to trigonal upon pressure release, without this transitory coordination increase permanent densification would not be manifest in the recovered glass. The response of vitreous B2O3 to pressure is virtually independent of the concentration of boroxol rings in the structure. In simulated glass, boroxol rings dissolve when subject to pressure, which explains the disappearance of the breathing mode in the Raman spectrum of compressed B2O3 glass. (Some figures in this article are in colour only in the electronic version)

Long-range charge transfer and oxygen vacancy interactions in strontium ferrite

The high oxygen non-stoichiometry (δ) in many mixed ionic conductors does not translate into high ionic conductivity due to strong interactions between oxygen vacancies. In this study, the charge states of Fe in various phases of SrFeO3−δ were determined by interpreting the density functional theory (DFT) predicted magnetic moment on Fe. The charge on Fe in square pyramidal (SP) was shown to always remain +4 while the Fe in distorted octahedral (Oh) varied from +4 to +3 with δ, due to the different d-orbital splitting of SP and Oh Fe. Furthermore, a new ‘long-range charge transfer’ mechanism was observed where the electrons left by the charge-neutral oxygen vacancy were transferred to the second nearest neighbor Fe (instead of the Fe directly connected to the vacancy) and resulted in extended lattice distortions that promoted strong oxygen vacancy interactions. A DFT-based thermodynamics model for interacting vacancies was also developed to predict the δ and oxygen vacancy site fraction (X) at high temperatures. The δ and X values calculated with this model showed good agreement with experiments. These calculations resolve a long-standing debate in the literature on the mixed charge states of Fe in SrFeO3−δ and identify a new mechanism for oxygen vacancy interactions.

Finite Element Modeling of Idealized Infiltrated Composite Solid Oxide Fuel Cell Cathodes

Using a two-dimensional finite element approach, the polarization resistance of idealized, branched, nano-particulate, composite cathodes was determined. Porous CGO, LSGM, or YSZ networks infiltrated with additional ionic conductor and subsequently infiltrated with LSCF or BSCF were modeled. For fixed mixed conductor particle size, dual nano-particle infiltrations (ionic + mixed conductor) resulted in an order of magnitude polarization resistance decrease, compared to single component mixed conductor infiltrations. For most SOFC relevant temperatures (500-900C), geometries, and material combinations, cathode performance was limited by the charge transfer reaction occurring at the mixed conductor interface and therefore scaled with the cathode surface area, as long as the cathode thickness was <~10microns. For cathodes thicker than ~10microns, losses within the ionic conducting network determined performance, resulting in a breakdown of the linear performance dependence on cathode surface area. The boundary between these regimes varied with ionic conducting cathode arm width, column width, and ionic conductivity.

Polaron size and shape effects on oxygen vacancy interactions in lanthanum strontium ferrite

Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry (δ) of mixed ionic and electronic conductors (MIECs). Unfortunately, it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here, the Fe charge state distribution was determined for various compositions and phases of the MIEC La1−xSrxFeO3−δ (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated, oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated, oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons, as in LaFeO3) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons, as in SrFeO3). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions, as indicated by an increase in the oxygen vacancy formation energy above a critical δ threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations, while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction (X), both δ and X were predicted as a function of temperature and oxygen partial pressure, for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.

In Situ Wafer Curvature Relaxation Measurements to Determine Surface Exchange Coefficients and Thermo-chemically Induced Stresses

The curvature relaxation technique is a new, electrode-free, in situ technique for simultaneously measuring the chemical oxygen surface exchange coefficient (k) and stress state \(\left( \lambda \right)\) of a thin or thick, dense or porous mechano-chemically active film atop a dense inert substrate. This chapter presents (1) an overview of the technique, (2) a detailed derivation of the fitting equations used to extract k from curvature relaxation experiments, (3) a discussion of the technique’s benefits and limitations, and (4) sample MATLAB code to predict the stress distributions found within dense thin or thick film bilayers as a function of temperature.

Mechanical, Thermal, and Electrochemical Properties of Pr Doped Ceria from Wafer Curvature Measurements

This work demonstrates, for the first time, that a variety of disparate and technologically-relevent thermal, mechanical, and electrochemical oxygen-exchange material properties can all be obtained from in situ, current-collector-free wafer curvature measurements. Specifically, temperature or oxygen partial pressure induced changes in the curvature of 230 nm thick (100)-oriented Pr0.1Ce0.9O1.95-x (10PCO) films atop 200 μm thick single crystal yttria stabilized zirconia or magnesium oxide substrates were used to measure the biaxial modulus, Youngs modulus, thermal expansion coefficient, thermo-chemical expansion coefficient, oxygen nonstoichiometry, chemical oxygen surface exchange coefficient, oxygen surface exchange resistance, thermal stress, chemical stress, thermal strain, and chemical strain of the model mixed ionic electronic conducting material 10PCO. The (100)-oriented thin film 10PCO thermal expansion coefficient, thermo-chemical expansion coefficient, oxygen nonstoichiometry, and Youngs modulus (which is essentially constant, at ∼200 MPa, over the entire 280-700 °C temperature range in air) measured here were similar to those from other bulk and thin film 10PCO studies. In addition, the measured PCO10 oxygen surface coefficients were in agreement with those reported by other in situ, current-collector-free techniques. Taken together, this work highlights the advantages of using a samples mechanical response, instead of the more traditional electrical response, to probe the electrochemical properties of the ion-exchange materials used in solid oxide fuel cell, solid oxide electrolysis cell, gas-sensing, battery, emission control, water splitting, water purification, and other electrochemically-active devices.

Anisotropic chemical strain in cubic ceria due to oxygen-vacancy-induced elastic dipoles

Accurate characterization of chemical strain is required to study a broad range of chemical-mechanical coupling phenomena. One of the most studied mechano-chemically active oxides, nonstoichiometric ceria (CeO2-δ), has only been described by a scalar chemical strain assuming isotropic deformation. However, combined density functional theory (DFT) calculations and elastic dipole tensor theory reveal that both the short-range bond distortions surrounding an oxygen-vacancy and the long-range chemical strain are anisotropic in cubic CeO2-δ. The origin of this anisotropy is the charge disproportionation between the four cerium atoms around each oxygen-vacancy (two become Ce3+ and two become Ce4+) when a neutral oxygen-vacancy is formed. Around the oxygen-vacancy, six of the Ce3+-O bonds elongate, one of the Ce3+-O bond shorten, and all seven of the Ce4+-O bonds shorten. Further, the average and maximum chemical strain values obtained through tensor analysis successfully bound the various experimental data. Lastly, the anisotropic, oxygen-vacancy-elastic-dipole induced chemical strain is polarizable, which provides a physical model for the giant electrostriction recently discovered in doped and non-doped CeO2-δ. Together, this work highlights the need to consider anisotropic tensors when calculating the chemical strain induced by dilute point defects in all materials, regardless of their symmetry.