Oxygen Vacancy Migration Energies Research Articles

Praseodymium doped ceria: A new sight on coupled thermo-mechanical properties and oxygen vacancy diffusion via statistical moment approach

Applications of praseodymium doped ceria (PDC) in solid oxide fuel cells require a deep insight into the thermal expansion, stresses, and ionic conduction. Nonetheless, the investigation of thermo-mechanical properties and oxygen vacancy diffusion remains incomplete. Based on free Helmholtz energy, we formulate analytical expressions to directly connect thermal expansion coefficient, elastic moduli, oxygen vacancy-dopant binding and oxygen vacancy migration energies by employing statistical moment method (SMM). The SMM calculations capture the full anharmonicity of lattice thermal vibrations. Our analyses report the significant influences of temperature and dopant content on the thermal expansion coefficient, bulk and Young’s moduli, oxygen vacancy-dopant binding and oxygen vacancy migration energies, and ionic conductivity. The role of Pr dopant on the oxygen vacancy diffusion is fully described by combining the symmetric energy profiles related to blocking effect and asymmetric energy profiles generated from trapping effect. By switching van der Waals (vdW) forces on and off in the SMM calculations, we first show that the vdW forces cause a positive effect on the mechanical properties but reduce the thermal expansion and resist the oxygen vacancy diffusion. Our findings are discussed concerning the results from other calculations and experiments in the literature. The insights on the thermo-mechanical properties and oxygen vacancy diffusion for PDC crystal can be generalized and applied to other ceramic materials that are essential for solid state batteries.

Stabilizing a Li–Mn–O Cathode by Blocking Lattice O Migration through a Nanoscale Phase Complex

Among all intercalation cathodes for Li-ion batteries, Li–Mn–O layered oxides offer the highest initial energy density at the lowest cost, due to the joint contribution from cationic and anionic redox chemistry. However, the poor cycling capability, resulting from the continuous lattice O loss at high potentials (>4.5 V), hinders practical applications. Herein, we employed phase complex engineering to obtain a new Li–Mn–O nanohybrid cathode featuring the uniform and coherent integration of layered nanodomains and spinel nanodomains. The combination of DFT calculations, synchrotron-based transmission X-ray microscopy, in situ differential electrochemical mass spectrometry, in situ synchrotron XRD, and electrochemical tests demonstrated that the O migration path in layered nanodomains was blocked by the neighboring spinel nanodomains with a higher oxygen vacancy migration energy, thus effectively suppressing the irreversible lattice O loss at high potentials and enhancing the cycling stability in both capacity and average voltage. The strategy is experimentally demonstrated to be effective and it leads to a new path for developing stable high-energy-density cathode materials.

Study of Oxygen Vacancy Diffusion in Yttria-doped Ceria and Yttria-stabilized Zirconia by Statistical Moment Method

Oxygen vacancy diffusion in yttria-doped ceria (YDC) and yttria-stabilized zirconia(YSZ) are investigated using statistical moment method, including the anharmonicity effects of thermal lattice vibrations. The expressions of oxygen vacancy-dopant association energy and oxygen vacancy migration energy are derived in an explicit form. Calculation of the vacancy migration energy enable us to evaluate the important role of dopant cation on the oxygen vacancydiffusion. The dependences of the vacancy activation energies and diffusion coefficients in YDC and YSZ systems on dopant concentration are also discussed in detail. The calculated results are in good agreement with the other theoretical and experimental results.

Investigation of oxygen vacancy migration energy in yttrium doped cerium

Using the statistical moment method, the analytic expression of vacancy migration energy in yttria doped cerium (YDC) is derived within the fourth order moment approximation. This expression depends strongly on the interaction potentials between the diffusing oxygen ion and the surrounding cations. Our results show the predominant direction of oxygen vacancy migration and the influence of cation barriers on oxygen vacancy diffusion. The dependences of the migration energy on temperature and dopant concentration are evaluated in detail. Calculated results are compared with other theoretical results.

A study on the formation and migration of oxygen vacancies in Ba0.5Sr0.5Co0.75Fe0.25O3-δ perovskite surfaces by first-principles modelling

Abstract Mixed conducting perovskites such as Ba0.5Sr0.5Co0.75Fe0.25O3-δ (BSCF) are attracting interest for solid oxide fuel cells (SOFCs). Electrochemical behaviors of BSCF surfaces, compared to a bulk one, have not been investigated yet. In this paper, electrochemical properties of BSCF were investigated by density functional theory (DFT). We calculated barriers of the (011) BSCF terminated by Sr Co O, and compared them with the bulk values. We found that the oxygen vacancy migration energies within 6.87–9.59 A in depth in the slab are nearly similar to the bulk values. The surface oxygen vacancy migration energies in (011) and (001) slabs of BSCF were compared. They have two values in the (011) BSCF and vacancies migrate under two models, which are different in terms of their geometries. However, there is a single value for the oxygen vacancy migration energy in the (001) BSCF. The surface oxygen vacancy formation energy of the (011) BSCF terminated by the O O surface is much less than the bulk value. Oxygen vacancies can simply migrate to the O O surface, because the migration energy required for jumping to the surface is very small. Therefore, the presence and diffusion of vacancies in layers close to the O O surface are more probable.

Unlocking the nature of the co-doping effect on the ionic conductivity of CeO2-based electrolyte

Doped CeO2 is a very promising electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). To further improve the performance of the CeO2-based electrolyte, co-doping two different elements into CeO2 is a feasible method, however the co-doping effect on the ionic conductivity is not well understood and whether it is synergistic or average is even controversial. In order to gain a fundamental understanding of the co-doping effect, the microscopic properties of co-doped CeO2 are calculated using the DFT+U method. Density of states, band structures, oxygen vacancy formation energies, defect association energies, and oxygen vacancy migration energies are systematically calculated for In3+, Sm3+ single-doped and co-doped CeO2. Based on our calculations, we find that the coexistence of the two doped ions in the local structures of the doped CeO2 can suppress the reduction of Ce4+ to Ce3+, which is beneficial for the decrease of the internal short circuit current of the CeO2-based electrolyte. For In3+ and Sm3+ co-doped CeO2, when the distance between the two doped ions is the first nearest neighbor, the co-doping effect is average. However, when the distance between the two doped ions extends to the second nearest neighbor, the availability of the free oxygen vacancies is synergistically enhanced. Therefore whether the co-doping effect on the ionic conductivity is average or synergistic is highly dependent on the local structures of the co-doped CeO2 which are difficult to control in experiments, offering a reasonable explanation for controversial experimental results. Our work provides a new atomistic level insight into the co-doping effect in CeO2 which would be helpful for high performance electrolyte development.

DFT+U calculation of Sm3+ and Sr2+ co-doping effect on performance of CeO2-based electrolyte

Solid oxide fuel cells (SOFCs) have been attracting people's attention for their high energy conversion efficiency, good fuel compatibility, no precious metal catalysts, and pollution-free emissions. However, the high operating temperature (800-1200℃) of the traditional SOFC can reduce the long-term stability and cause the difficulties in either the selecting of material or the sealing of SOFC. Therefore, great efforts have been devoted to developing the intermediate temperature SOFC (IT-SOFC), which works at 600-800℃. In the IT-SOFC, the ionic conductivity of doped CeO2-based electrolyte has a significant advantage relative to that of the conventional yttria-stabilized zirconia (YSZ) electrolyte. For example, at 600℃, the ionic conductivity of Sm-doped CeO2 is 0.02 S/cm much higher than that of the traditional YSZ electrolyte (only 0.0032 S/cm). Therefore, the doped CeO2-based electrolyte is a very promising electrolyte for IT-SOFC.Recently, the co-doping of two different elements into CeO2 has become a hot research topic. The ionic conductivity of Sm3+ and Sr2+ co-doped CeO2 has proved to be nearly twice as high as that of Sm3+ doped CeO2 (SDC). However, the mechanism for the co-doping effect on the conductivity of CeO2 is not clear. In this paper, Sm3+ and Sr2+ co-doped CeO2 is systematically studied using the DFT+U method. The microscopic properties of the Sm3+ and Sr2+ co-doped CeO2 including electronic density of states, band structure, oxygen vacancy formation energy and oxygen vacancy migration energy and so on have been calculated and analyzed by comparing with those of the Sm3+ or Sr2+ single doped CeO2. The calculation results indicate that Sm3+ and Sr2+ co-doping has a synergistic effect on the performance improvement of CeO2-based electrolyte, which can not only suppress the electronic conductivity of doped CeO2 system, but also can reduce the oxygen vacancy formation energy on the basis of single doped CeO2. The existence of Sm3+ can help to reduce the trapping effect of Sr2+ on oxygen vacancies, meanwhile the addition of Sr2+ can further reduce the minimum oxygen vacancy migration energy on the basis of SDC. Calculations by the climbing image nudged elastic band (CINEB) method indicate that the oxygen vacancy migration energy of the co-doped system can reach as low as 0.314/0.295 eV, which is lower than the minimum oxygen vacancy migration energy of SDC. Our research reveals the synergistic mechanism for Sm3+ and Sr2+ co-doping effect on the conductivity of CeO2, which is of great instructive significance for the further research and development of other high-performance co-doped electrolyte materials in IT-SOFC.

Electro-Chemo-Mechanical Studies of Perovskite-Structured Mixed Ionic-Electronic Conducting SrSn1-XFexO3-x/2+d

High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive for conversion of fuels to electricity. Reduced operating temperatures, desirable for reduced costs and extended operation, however result in significant losses in efficiency. This has been traced primarily to slow cathode surface reaction kinetics. In this work, we extended previous studies on the promising mixed ionic and electronic conducting perovskite-structured SrTi1-xFexO3-x/2+δ (STF) materials system whose exchange kinetics were correlated with the minority electron charge density by replacing Ti with Sn, due to its distinct band structure and higher electron mobility. Oxygen nonstoichiometry and defect chemistry of the SrSn1-xFexO3-x/2+δ (SSF) system were examined by thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure while surface kinetics were examined by means of AC impedance spectroscopy. The electro-chemo-mechanical properties of SSF were found not to differ significantly from the corresponding composition in STF. Key thermodynamic and kinetic parameters for SrSn0.65Fe0.35O2.825+δ (SSF35) were derived including the reduction enthalpy, high electronic band gap, anion Frenkel enthalpy, oxygen vacancy migration energy, electron and hole mobilities. Though slightly shifted by Sn’s larger size, the defect equilibria and the cathode area specific resistance differed only in a limited way from those in STF. This was attributed to properties being largely dominated by Fe and not by the substitution of Ti with Sn.

Electrical conductivity and grain boundary composition of Gd-doped and Gd/Pr co-doped ceria

We characterize electrical conductivity, microstructure, nano-scale grain boundary structure and chemistry of ceria electrolytes with nominal compositions of Gd0.2Ce0.8O2-δ (GDC) and Gd0.11Pr0.04Ce0.85O2-δ (GPDC). The electrolytes are fabricated using mixed oxide nanopowders synthesized by spray drying. AC impedance spectroscopy was performed from 150°C to 700°C in air to determine grain-interior electrical conductivity. Grain-boundary conductivity was determined below 300°C. The grain-interior conductivity of the GPDC was higher than that of GDC by as much as 10 times, depending on the temperature. The GPDC specific grain-boundary conductivity was measured to be approximately 100 times higher than that of GDC. Energy dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) confirmed the grain-to-grain compositional uniformity of both materials following heat treatments. Grain boundaries were free of glassy intergranular phases; dopant concentration and Ce oxidation state were found to vary significantly near grain boundaries. Boundary core composition was estimated from STEM EELS to be Gd0.62Ce0.38O2-δ, and Gd0.29Pr0.16Ce0.55O2-δ in GDC and GPDC, respectively. Pr segregation to grain boundaries in the GPDC is hypothesized to enhance conductivity by both decreasing oxygen vacancy migration energy, and inducing mixed ionic–electronic conductivity in the near-boundary region.

Kinetic Monte Carlo model of defect transport and irradiation effects in La-doped CeO 2

A generalized Kinetic Monte Carlo code was developed to study oxygen mobility in UO 2 type nuclear fuels, using lanthanum doped CeO 2 as a surrogate material. Molecular Statics simulations were performed using interatomic potentials for CeO 2 developed by Gotte, Minervini, and Sayle to calculate local configuration-dependent oxygen vacancy migration energies. Kinetic Monte Carlo simulations of oxygen vacancy diffusion were performed at varying lanthanum dopant concentrations using the developed generalized Kinetic Monte Carlo code and the calculated configuration-dependent migration energies. All three interatomic potentials were found to confirm the lanthanum trapping effect. The results of these simulations were compared with experimental data and the Gotte potential was concluded to yield the most realistic diffusivity curve.

First principles study of oxygen vacancy migration in tantalum pentoxide

First principles total energy calculations were performed in order to determine oxygen vacancy migration energies in Ta2O5. A simplified version of the crystalline orthorhombic phase of Ta2O5 was used in this study. O vacancies in the chosen model of Ta2O5 can be broadly classified into ‘cap’ and ‘in-plane’ sites based on their location in the lattice. The cap type of vacancies display the largest barriers both for migration to a neighboring cap site or to a neighboring in-plane site, thus behaving as oxygen vacancy ‘sinks’. A lowering of the barriers to migration is generally seen when the vacancies are doubly positively charged. All calculations were performed within the local density approximation of density functional theory, and the elastic band method was used in the estimation of migration barriers.

Native vacancy migrations in zircon

Energy minimization methods were used to simulate the migration of Zr, Si, and O vacancies in zircon (ZrSiO 4). Two sets of interatomic potentials were employed for comparison: one with O–Si–O three-body terms for the SiO 4, and one without. Results for Si were inconclusive, but consistent with maintaining the integrity of the SiO 4 molecular units. Both Zr and O vacancies can migrate on three-dimensional sublattice networks, thus supporting the experimentally observed diffusional isotropy. The predicted Zr vacancy migration energy (1.16–1.38 eV) was in good agreement with experiment if supplemented by Zr vacancy formation via Schottky or Frenkel defects (6.21–12.28 eV/defect). Oxygen vacancy migration energies were predicted to be 0.99–1.16 eV, somewhat lower than the experimental value of 4.64 eV measured in natural zircons, which thus may include significant contributions from vacancy formation mechanisms at 3.31–6.52 eV/defect.

Electrical Conductivity in (Gdl-xCax)2Sn2O7±δPyrochlore System

AbstractElectrical conductivity measurements were performed on Gd2Sn207as a function of temperature, P02and Ca doping concentration. An effective frenkel constant and oxygen vacancy mobility were derived. The high level of intrinsic anion disorder found in this study is consistent with expectations based on the cation radius ratio (rA/rB) which was earlier found to be important in determining anion disorder in A2B207pyrochlore compounds [1]. The magnitude of the ionic conductivity in Gd2Sn2O7was found to be depressed relative to Gd2(Til-xZrx)207based systems due to a high oxygen vacancy migration energy.