Effect of temperature and dopant concentration on the conductivity of samaria-doped ceria electrolyte

The conductivity, σ, of a samaria-doped ceria electrolyte is studied as a function of temperature and dopant concentration, x, which was from 5 to 30 mol%. It is shown that a maximum in σ versus x corresponds to a minimum in activation energy. It is found that the conductivity is completely due to oxygen vacancy conduction. The conductivity increases with increasing samaria doping and reaches a maximum for (CeO₂)_0.8(SmO_1.5)_0.2, which has a conductivity of 5.6×10^–1 S/cm at 800 °C. A curvature at T=T _c, the critical temperature, has been observed in the Arrhenius plot. This phenomenon may be explained by a model which proposed that, below T _c, nucleation of mobile oxygen vacancies into ordered clusters occurs, and, above T _c, all oxygen vacancies appear to be mobile without interaction with dopant cation. In addition, the composition dependences of both the critical temperature and the trapping energy are consistent with that of the activation energy. Electronic Publication     

A concerted migration mechanism of mixed oxide ion and electron conduction in reduced ceria studied by first-principles density functional theory

Ceria based oxides are regarded as key oxide materials for energy and environmental applications, such as solid oxide fuel cells, oxygen permeation membranes, fuel cell electrodes, oxygen storage, or heterogeneous catalysis. This great versatility in applications is rendered possible by the fact that rare earth-doped ceria is a pure oxygen ion conductor while undoped ceria, CeO(2-δ), is a mixed oxygen ion-electron conductor. To get deeper insight into the mixed conduction mechanism of oxygen ions and electrons from atomistic and electronic level viewpoints we have applied first-principles density functional theory (DFT + U method). The calculation results show that oxygen vacancies strongly attract localized electrons, forming associates between them. The migration energy of an oxygen vacancy in such an associate is substantially lowered compared to the unassociated case due to the simultaneous positional rearrangement of localized electrons during the ionic jump process. Accordingly, we propose a concerted migration mechanism of oxygen vacancies and localized electrons in reduced ceria; this mechanism results in an increased diffusivity of oxygen vacancies supported by localized electrons compared with that in pure oxide ion conductors.     

Kinetic Monte Carlo simulations of oxygen vacancy diffusion in a solid electrolyte: Computing the electrical impedance using the fluctuation–dissipation theorem

We present a new method for computing the electrical impedance of solid oxide electrolyte from kinetic Monte Carlo simulations of oxygen vacancy diffusion. The impedance values at all frequencies are obtained from a single equilibrium simulation based on the fluctuation–dissipation theorem, leading to a significant gain of efficiency over existing methods.This allows us to systematically examine the effect of dopant concentration. Increasing dopant concentration is found to decrease the infinite-frequency impedance, which is attributed to the increasing density of oxygen vacancies. The difference between the impedance values at zero- and infinite-frequency, on the other hand, shows the opposite trend, and is linked to dopant–vacancy interactions. Hence the two competing mechanisms, previously proposed to explain the existence of an optimal doping concentration, are separately quantified.Our model also predicts a significant effect of the arrangement of dopant cations on the electrolyte conductivity.     

Special quasirandom structures for gadolinia-doped ceria and related materials

Gadolinia doped ceria in its doped or strained form is considered to be an electrolyte for solid oxide fuel cell applications. The simulation of the defect processes in these materials is complicated by the random distribution of the constituent atoms. We propose the use of the special quasirandom structure (SQS) approach as a computationally efficient way to describe the random nature of the local cation environment and the distribution of the oxygen vacancies. We have generated two 96-atom SQS cells describing 9% and 12% gadolinia doped ceria. These SQS cells are transferable and can be used to model related materials such as yttria stabilized zirconia. To demonstrate the applicability of the method we use density functional theory to investigate the influence of the local environment around a Y dopant in Y-codoped gadolinia doped ceria. It is energetically favourable if Y is not close to Gd or an oxygen vacancy. Moreover, Y-O bonds are found to be weaker than Gd-O bonds so that the conductivity of O ions is improved.     

A DFT+U study of defect association and oxygen migration in samarium-doped ceria

A specialized genetic algorithm (GA) is used to search the structural space of samarium-doped ceria (SDC) for the most energetically stable configurations which will predominate in low temperature fuel cells. A systematic investigation of all configurations of 3.2% SDC and a GA investigation of 6.6% SDC are presented for the first time at the DFT+U level of theory. It was found that Sm atoms prefer to occupy the nearest neighbor (NN) position relative to the oxygen vacancy at 3.2%, while at 6.6%, a balance exists between various Sm-vacancy interactions and the vacancies prefer to be separated by ～6 ?. Also, the migration barriers for oxygen diffusion are calculated amongst the best structures in 3.2% and 6.6% SDC and are found to be comparable to the barriers in Gd-doped ceria at the DFT+U level of theory. While the migration calculations provide insight on the oxygen diffusion mechanism in this material, the favored configurations from our GA enable future research on concentrated SDC and contribute to the atomistic understanding of the influence of dopant-vacancy and vacancy-vacancy interactions on ionic conductivity.     

An insight into the structure,conductivity and ion dynamics of Sr-Sm co-doped ceria oxygen ion conductors: Effect of defect interaction

In this work, we investigate the structure, conductivity and ion dynamics of mixed di and tri-valent doped Ce_0.8Sm_0.2-xSr_xO_2-δ (x = 0–0.2) oxygen ion conductors. The lattice parameter and root mean square strain are significantly affected by the ionic radius of dopants and their solubility into ceria lattice. Due to the solubility limit of Sr²⁺ ions, SrCeO₃ phase increases with the doping concentration of Sr²⁺. The increase of Sr²⁺ ions into ceria lattice promotes the formation of large defect clusters by expense of formed oxygen vacancies. The coulombic interaction between oxygen vacancies with substituted dopant cations enhances with Sr²⁺ ions due to decrease of the value of dielectric constant of the compositions. The defect interaction significantly affects the conductivity values by means of increase of SrCeO₃ phase and defect clusters. The conductivity values are found to be consistent with the migration and association energy. The scaled spectra of dielectric tangent loss and real part of complex conductivity confirm the temperature and defect interaction independent nature of hoping mechanism in the compositions.     

CeO2基固溶体氧缺位拉曼光谱表征的研究进展

总结了拉曼光谱表征CeO2基固溶体中氧缺位的研究成果,评述了氧缺位的生成和影响氧缺位浓度观察值的因素,并提出了亟待解决的问题.CeO2基固溶体的拉曼谱图中出现三个重要的特征拉曼峰(465、560、600cm-1),一般分别归属于CeO2的F2g振动模式、氧缺位和MO8型缺陷物种.研究发现氧缺位的产生与掺杂金属离子价态有关,而MO8型缺陷物种的产生与掺杂金属离子半径有关.CeO2基固溶体中氧缺位浓度观察值(AD/AF2g)与样品吸光度和表面富集有关.原位拉曼光谱研究表明:气氛及温度影响CeO2基固溶体的吸光度变化,从而影响拉曼光谱采样深度,导致氧缺位浓度观察值的变化.     

碱土金属氧化物掺杂氧化铈基电解质材料中的晶格缺陷

基于能量最小化算法，对碱土金属氧化物(MgO、CaO、SrO、BaO)掺杂的氧化铈基电解质缺陷进行模拟计算. 研究了掺杂离子与空位缺陷形成能和氧空位跃迁能之间的关系. 结果说明，在碱土金属氧化物掺杂氧化铈的固溶反应中，氧空位缺陷是电荷补偿缺陷的首选形式，CaO和SrO较MgO和BaO 易溶于CeO₂; Ca²⁺掺杂离子与氧空位缺陷对[CaCe″•VO^••]×的结合能最高；复合缺陷[VO•••MCe″•VO••]••在CeO₂中的状态不稳定；氧空位在次近邻间的跃迁能最低，因此最容易实现跃迁.     

Ce0.8Gd0.2-xPrxO1.9固溶体的合成与表征

利用X射线衍射(XRD)、拉曼光谱(Raman)、X射线光电子能谱(XPS)和交流阻抗谱对溶胶-凝胶法制备的稀土双掺杂固溶体Ce0.8Cd0.2-xPrxO1.9(x=0,0.02,0.10)的结构和导电性进行了研究.XRD结果表明,经800℃焙烧所得样品都形成了单相立方萤石结构,平均晶粒尺寸在23～30 nm之间;X...     

Doping and defect association in AZrO(3) (A = Ca, Ba) and LaMO(3) (M = Sc, Ga) perovskite-type ionic conductors

Computer simulation techniques have been used to investigate the defect chemistry of perovskite-structured ionic conductors based upon AZrO(3)(A = Ca, Ba) and LaMO(3)(M = Sc, Ga). Our studies have examined dopant site-selectivity, oxide ion migration and dopant-defect association at the atomic level. The energetics of dopant incorporation in AZrO(3) show strong correlation with ion size. We predict Y(3+) to be one of the most favourable dopants for BaZrO(3) on energetic grounds, which accords with experimental work where this cation is the commonly used acceptor dopant for effective proton conduction. Binding energies for hydroxy-dopant pairs in BaZrO(3) are predicted to be favourable with the magnitude of the association increasing along the series Y < Yb < In < Sc. This suggests that proton mobility would be very sensitive to the type of acceptor dopant ion particularly at higher dopant levels. Oxygen vacancy migration in LaScO(3) is via a curved pathway around the edge of the ScO(6) octahedron. Dopant-vacancy clusters comprised of divalent dopants (Sr, Ca) at the La site have significant binding energies in LaScO(3), but very low energies in LaGaO(3). This points to greater trapping of the oxygen vacancies in doped LaScO(3), perhaps leading to higher activation energies at increasing dopant levels in accord with the available conductivity data.     

Theoretical Study on the Structural and Electronic Properties of the Reduced SnO2 （110） Surface

The reduced SnO2(110) surface has been investigated by using first-principles method with a slab model. By examining the vacancy formation energy of three kinds of reduced SnO2(110) surfaces, the most energetically favorable defect surface is confirmed to be the surface with the coexistence of bridging and in-plane oxygen vacancies, which is different with the traditional model by only removing bridging oxygen. The results of band structure calculations indicate that the electronic structure of this defect surface is similar to the SnO surface.     

Effects of deposited Pt particles on the reducibility of CeO2(111)

The interaction of Pt particles with the regular CeO(2)(111) surface has been studied using Pt(8) clusters as representative examples. The atomic and electronic structure of the resulting model systems have been obtained through periodic spin-polarized density functional calculations using the PW91 exchange-correlation potential corrected with the inclusion of a Hubbard U parameter. The focus is on the effect of the metal-support interaction on the surface reducibility of ceria. Several initial geometries and orientations of Pt(8) with respect to the ceria substrate have been explored. It has been found that deposition of Pt(8) over the ceria surface results in spontaneous oxidation of the supported particle with a concomitant reduction of up to two Ce(4+) cations to Ce(3+). Oxygen vacancy formation on the CeO(2)(111) surface and oxygen spillover to the adsorbed particle have also been considered. The presence of the supported Pt(8) particles has a rather small effect (～0.2 eV) on the O vacancy formation energy. However, it is predicted that the spillover of atomic oxygen from the substrate to the metal particle greatly facilitates the formation of oxygen vacancies: the calculated energy required to transfer an oxygen atom from the CeO(2)(111) surface to the supported Pt(8) particle is only 1.00 eV, i.e. considerably smaller than 2.25 eV necessary to form an oxygen vacancy on the bare regular ceria surface. This strongly suggests that the propensity of ceria systems to store and release oxygen is directly affected by the presence of supported Pt particles.     

Phase transition in ceria alumina

Al-doped CeO₂ samples were prepared by conventional solid state reaction. The electrical conductivity of CeO₂ doped with Al₂O₃ has been studied at different temperatures for various molar ratios. The isothermal conductivity increases with dopant concentration due to the vacancy migration phenomenon induced by doping. It has been found that the conductivity increases and shows a jump from 450 to 520°C due to the phase transition of ceria from cubic to orthorhombic type. A slight deflection is seen for 0.5 and 0.6 moles of alumina at about 250°C due to its phase transition from γ to α type. AC impedance measurements proved that the oxide ion conductivity predominantly arises from the grain and grain boundary contribution as two well defined semi-circles are clearly seen. The sample characterization and the study of phase transition changes were done by using X-ray diffraction analysis, Fourier transform infrared spectral and differential scanning calorimetry (DSC) measurements. On increasing the concentration of dopant, the transition temperature shifts towards lower side which is confirmed by DSC as well as conductivity measurements.     

氧缺陷TiO2-B作为可充电锂离子电池负极材料的第一性原理研究

利用对氧缺陷的TiO₂-B材料进行密度泛函理论的计算,阐述了氧空穴对于TiO₂-B材料的电化学性质的影响。计算研究主要聚焦于缺陷材料的锂离子迁移和电子导电性等基本问题。计算结果表明在低锂离子浓度下(x(Li/Ti)≤ 0.25),相比于无缺陷的TiO₂-B,氧缺陷TiO₂-B有着更高的插入电压和更低的b轴方向迁移活化能,意味着锂离子的嵌入也更容易,这对于可充电电池的充电过程是有利的。而在高浓度下(x(Li/Ti) = 1),锂饱和的氧缺陷TiO₂-B相较于无缺陷的TiO₂-B有着较低的插入电压,更有利于锂离子的脱嵌过程,这对于可充电电池的放电过程也是有利的。电子结构计算表明缺陷材料的禁带宽度在1.0-2.0 eV之间,低于无缺陷的材料的3.0 eV。主要态密度贡献者是Ti-Ov-3d,并且随着氧空穴的增加它的强度也变得更强。这就表明氧缺陷TiO₂-B有更好的电子导电性。     

氧化锡多晶电导的氧空位控制模型

SnO_2、ZnO等金属氧化物的气敏特性通常以它们在不同气氛中电导值的变化来体现。为弄清这类多晶材料及气敏元件电导变化的基本规律,本文根据表面势垒控制模型和晶界势垒控制模型讨论了氧空位密度对SnO_2多晶材料及气敏元件电导值的重要影响,并根据SnO_2多晶电导的氧空位控制模型,讨论了在不同烧结条件下元件电导的变化规律,并用X光电子能谱(XPS)对结果进行了分析。     

Defect clustering and local ordering in rare earth co-doped ceria

Defect clustering and local ordering in rare earth co-doped ceria were studied by computer simulation and electron diffraction, respectively. The simulation of electrically neutral defect clusters containing up to four oxygen vacancies revealed that the permutation of different dopant cations in a co-doped cluster could have a significant influence on the binding energy of the cluster. Moreover, the growth of larger clusters (number of oxygen vacancies ≥ 3) could be restrained by a co-doping effect. The selected area electron diffraction study indicated that the restrained growth of larger clusters will further lead to a suppression of the local ordering of oxygen vacancies in co-doped ceria. The correlation between defect clustering, local ordering of oxygen vacancies and ionic conduction in co-doped ceria was discussed.     

Optimization of ionic conductivity in solid electrolytes through dopant-dependent defect cluster analysis

Atomistic simulation based on an energy minimization technique has been carried out to investigate defect clusters of R(2)O(3) (R = La, Pr, Nd, Sm, Gd, Dy, Y, Yb) solid solutions in fluorite CeO(2). Defect clusters composed of up to six oxygen vacancies and twelve accompanied dopant cations have been simulated and compared. The binding energy of defect clusters increases as a function of the cluster size. A highly symmetric dumbbell structure can be formed by six oxygen vacancies, which is considered as a basic building block for larger defect clusters. This is also believed to be a universal vacancy structure in an oxygen-deficient fluorite lattice. Nevertheless, the accurate positions of associated dopants depend on the dopant radius. As a consequence, the correlation between dopant size and oxygen-ion conductivity has been elucidated based on the ordered defect cluster model. This study sheds light on the choice of dopants from a physical perspective, and suggests the possibility of searching for optimal solid electrolyte materials through atomistic simulations.     

Ordered structures of defect clusters in gadolinium-doped ceria

The nano-domain, with short-range ordered structure, has been widely observed in rare-earth-doped ceria. Atomistic simulation has been employed to investigate the ordering structure of the nano-domain, as a result of aggregation and segregation of dopant cations and the associated oxygen vacancies in gadolinium-doped ceria. It is found that the binding energy of defect cluster increases as a function of cluster size, which provides the intrinsic driving force for the defect cluster growth. However, the ordered structures of the defect clusters are different from the chain model as previously reported. Adjacent oxygen vacancies prefer to locate along <110>/2 lattice vector, which results in a unique stable structure (isosceles triangle) formation. Such isosceles triangle structure can act as the smallest unit of cluster growth to form a symmetric dumbbell structure. This unique dumbbell structure is hence considered as a building block for the development of larger defect clusters, leading to nano-domain formation in rare-earth-doped ceria.     

Defect and transport properties of the NdCoO3 catalyst and sensor material

The NdCoO₃ perovskite has been investigated using a combination of atomistic simulation and experimental techniques to examine its possible use as an oxidation catalyst and/or sensor material. The sensing properties of NdCoO₃ and Nd_0.8Sr_0.2CoO₃ towards CO have been investigated by employing thin films deposited by means of radio-frequency (RF) magnetron sputtering onto polycrystalline Al₂O₃. The response of the films was monitored by performing four-probe DC-conductivity measurements. The conductivity variation induced by switching between a CO-free atmosphere (air) and a CO-rich one with the same composition of residual gas was recorded and analysed as a function of temperature; results are compared for the two samples. Simulation studies focussed on the dopant, transport and redox properties of the pure material; the results indicate that Sr and Ca on the Nd site are the most soluble dopants and that when divalent dopants are incorporated in the structure, charge compensation occurs via oxygen ion vacancies. The low activation energy for oxygen vacancy migration suggests high oxide-ion mobility through the lattice. Particular attention is paid to the electronic processes because of their importance with respect to practical applications of the material.     

Nitrogen/gold codoping of the TiO2(101) anatase surface. A theoretical study based on DFT calculations

The interaction between implanted nitrogen atoms, adsorbed gold atoms, and oxygen vacancies at the anatase TiO(2)(101) surface is investigated by means of periodic density functional theory calculations. Substitutional and interstitial configurations for the N-doping have been considered, as well as several adsorption sites for Au adatoms and different types of vacancies. Our total energy calculations suggest that a synergetic effect takes place between the nitrogen doping on one hand and the adsorption of gold and vacancy formation on the other hand. Thus, while pre-implanted nitrogen increases the adsorption energy for gold and decreases the energy required for the formation of an oxygen vacancy, pre-adsorbed gold or the presence of oxygen vacancies favors the nitrogen doping of anatase. The analysis of the electronic structure and electron densities shows that a charge transfer takes place between implanted-N, adsorbed Au and oxygen vacancies. Moreover, it is predicted that the creation of vacancies on the anatase surface modified with both implanted nitrogen and supported gold atoms produces migration of substitutional N impurities from bulk to surface sites. In any case, the most stable configurations are those where N, Au and vacancies are close to each other.