Al-doped SrTiO 3: Part II, unusual thermodynamic factor and chemical diffusivity

Variation of the chemical diffusivity of oxygen and viscosity of an andesite melt with pressure at constant temperature

Chemical diffusion has long been studied mostly on binary oxides, but practical interest in it is increasing with more complex oxides these days, e.g., perovskite oxides that are finding a variety of electroceramic and electrochemical applications. In this paper, we compile the chemical diffusivity data of oxygen as measured on a prototype perovskite BaTiO3 and review the connotation of their variation with oxygen activity in the light of the chemical or ambipolar diffusion theory. The chemical diffusivity of BaTiO3 is compared with those of other perovskite oxides of mostly electrochemical concern and of typical binary transition metal oxides. The chemical diffusivity is further compared with those of the compounds such as Ag2S, Ag2Te, YSZ and Fe3O4, that have dissimilar crystal structures, but with a similar defect structure to BaTiO3, in order to get a further insight into the inner workings of chemical diffusivity particularly in the stoichiometric regime of complex oxides. Finally, possible complication with nonstoichiometry re-equilibration kinetics due to cation mobility is pointed out with reference to what has been observed from LiNbO3 under an oxygen activity gradient.

Experimental evidence of hydrogen–oxygen decoupled diffusion into BaZr0.6Ce0.25Y0.15O3−δ

Electrochemical investigation of the chemical diffusion, partial ionic conductivities, and other kinetic parameters in Li 3Sb and Li 3Bi

Contribution a la theorie atomique de la diffusion chimique

Titanium self-diffusion and chemical diffusion in Ti 3Al

The rate of oxygen exchange across mixed conducting oxide surfaces, often described via measurements of the surface exchange coefficient k, is a key metric impacting high temperature electrochemical device efficiency. Continuous measurement over time of native surface k values in realistic operating conditions can be accomplished by the contact-free optical transmission relaxation (OTR) approach, which relies on changes in oxygen stoichiometry being proportional to changes in optical absorption that can be detected by in situ optical transmission measurements. Conventional approaches to measuring k may suffer from being non-continuous (e.g., tracer exchange with ex situ secondary ion mass spectrometry), and therefore unable to observe changes in k over time, or require metal current collectors (e.g., ac-impedance spectroscopy measurements of area-specific resistance, or electrical conductivity relaxation), which could modify the surface reaction mechanisms and/or kinetics being probed. It is therefore of interest to compare k values obtained by ac-impedance spectroscopy (AC-IS) using metal current collectors and those obtained by OTR on native surfaces to examine the impact of current collectors and different measurement approaches. While k values obtained by various methods are already available in the literature, comparisons of such values measured on different samples may not provide much insight, as k can vary orders of magnitude depending on surface chemical composition, which is influenced by measurement conditions, processing, and thermal history. Therefore, in the present study, the two techniques were compared simultaneously on the same sample. A thin film of the mixed conductor SrTi0.65Fe0.35O3-x (STF35) was prepared by pulsed laser deposition on a transparent, ionically conducting (Zr,Y)O2-δ single crystal substrate. Its k values were measured by AC-IS on one region and by OTR on another region of the film, at 600 °C as a function of oxygen activity, controlled by gas partial pressure and/or dc bias applied across the substrate. Upon changing the oxygen activity, optical transmission relaxations resulted, corresponding to changes in the film oxygen stoichiometry and corresponding concentration of optically absorbing oxidized Fe (~Fe4+) species; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry, to obtain kchem values. From AC-IS, the fitted resistance and capacitance of the low frequency arc were used to determine both kchem (from the time constant) and kq (from the resistance), and additionally kchem values were estimated by multiplying kq by the thermodynamic factor (γ). Thin film γ values, determined from the film’s apparent volumetric chemical capacitance, were typically higher than the bulk values, determined from thermogravimetric measurements of non-stoichiometry, and varied with current collector material used (porous Au vs. porous Pt), suggesting a possible interfacial capacitance contribution. The AC-IS-derived kchem values and kq values multiplied by the thermodynamic factor (both bulk and thin film) were consistently orders of magnitude higher than the kchem values obtained by OTR, regardless of current collector material (Au or Pt). The results suggest a possible enhancement in k by the metal current collectors, which has similarly been observed previously for films of the mixed conductor (Pr,Ce)O2-δ. Additionally, long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the current collector, while hardly any degradation was observed in that time period in the optically-derived kchem values. The results suggest that, while the current collector could modify surface exchange coefficient evaluation by AC-IS, the contact-free OTR method offers a continuous, in situ approach to evaluate native surface exchange behavior.

The chemical oxygen surface exchange and bulk diffusion coefficient determined by impedance spectroscopy of porous La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) cathodes

Synergistic effects of A-site deficiency and anion doping on Pr0.4Sr0.6Ni0.2Fe0.7Mo0.1O3 cathode for H2O electrolysis and CO2 electrolysis

The influence of SO2 in gas environments on the electronic transference number of a partially stabilized zirconia (PSZ: ZrO2 + 4.5 mol% Y2O3) was investigated. The total conductivity for PSZ was measured in air and in an SO2-O2 gas mixture of identical $$P_{{\text{O}}_{\text{2}} } $$ for temperatures in the range of 500–1000°C. The conductivity increased measurably with the presence of SO2. This increase is attributed primarily to the addition of electronic carriers in the presence of SO2. Two methods for estimating and measuring transference numbers in the SO2 environment gave electronic transference numbers ranging from ≈0.05 to ≈0.33 for the temperature range of 500–1000° C. These results indicate that zirconia electrolytes may be mixed conductors for other environments containing sulphur species and the Nernst equation may not be valid.

The electrical conductivity and thermal expansion of La2Mo2O9under low oxygen partial pressure were studied with the help of thermoelectric power and dilatometric measurements, respectively. The ionic conduction of La2Mo2O9was predominant with the electronic transference number less than 0.05 above an oxygen partial pressure of aboutp=10‐7Pa at 700°C, and below this pressure the electronic conduction became obvious. The defect reaction and small polaron hopping among molybdenum sites were proposed to explain the electronic conduction. Accompanying the phase transition, there was a sharp increase of thermal expansion, which became more serious under low oxygen partial pressure. The substitution of lanthanum by neodymium led to the increase of electrical conductivity but the decrease of phase stability.

Electrical conductivity of CeO 2-doped YSZ

Defect structure and electrical properties of LaSr 3Fe 3O 10–δ

NiAl2O4 spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 °C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to PO2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in PO2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10−4 cm2 s−1.

Important to all systems that utilize lithium-ion batteries is the characterization of the irreversibilities associated with solid-state lithium transport limitations that exist within the materials comprising the battery electrodes. Essential to this characterization is an accurate description of the flux of lithium, which necessarily involves the lithium diffusivity. The theory for an appropriate description of the lithium flux is detailed and ultimately used to demonstrate the importance of this description via comparisons of measured to calculated voltage responses in batteries specifically designed for hybrid electric vehicles. An analytic expression for the solid-state excess Gibb's free energy associated with a secondary reference state of full lithiation is developed, which is appropriate for present-day positive-electrode materials that cannot be operated at low degrees of lithiation. Expressions for the thermodynamic open-circuit voltage, the thermodynamic factor, and activity coefficients are also derived. The approach preserves thermodynamic consistency in dilute and in full occupancy and circumvents difficulties that can arise with the differentiation of discrete experimental data points. Variations in previously reported chemical diffusivities over a significant range of lithiation are shown to be explained by the inclusion of both the thermodynamic factor and the flux of lithium due to bulk motion of material.