Effect of palladium oxidation state on the kinetics and mechanism of the charge transfer reaction taking place at the Pd/YSZ interface

Electrode polarization and conductivity measurements were carried out at the Pd/YSZ interface and at conditions close to the Pd–PdO thermodynamic equilibrium. The steady-state current–overpotential characteristics were analyzed with a Butler–Volmer type of equation. Both, apparent exchange current density, I_o, and anodic/cathodic charge transfer coefficients (α_a/α_c), were calculated. Based on the experimental results, it was concluded that the charge transfer at the electrode is rate-determining in the case of PdO during anodic operation and Pd during cathodic operation, while in the other case mass transport of adsorbed oxygen species along the electrode/solid electrolyte interface is in competition with the charge transfer process.     

Conductivity of RbCu4Cl3+xI2−x and copper electrode reaction computerized complex analysis

Attempts to measure the conductivity of solid electrolytes having the composition RbCu₄Cl_3+xI_2?x where x=0 or 0.25 were carried out using copper and Chevrel phase electrodes Cu₂Mo₆S₈. Complex impedance diagrams showed severe polarization effects at copper electrodes. Special programs were used with a computer coupled to the impedance meter allowing measurements at constant overpotential or constant current. In contrast to usual situations, the polarization at the copper electrode is a decreasing function of the ac signal voltage and a constant quantity of electrolysing current of ≈10^?2 C cm^?2 is required to disrupt the blocking effect at the interface. Results obtained with the Chevrel phases unambiguously show that the exchange of ions between two solid phases can take place at room temperature without any interfacial overpotential. The best conductivity value of RbCu₄Cl₃I₂ was 0.475 (Ω cm)^?1 at room temperature but all the measurements revealed a marked instability towards lower values. The conductivity of RbCu₄Cl_3.25I_1.75 at room temperature is 0.36 (Ω cm)^?1 and is stable. The activation energy is 0.045 eV.     

Oxygen transport model for layered MIEC composite membranes

Surface exchange resistance can reduce the oxygen transport through dense mixed ionic-electronic conducting (MIEC) membranes. Addition of an MIEC surface layer to a base substrate can reduce the surface exchange resistance. Existing oxygen transport relations that consider bulk diffusion and surface exchange resistance are extended to treat coated membranes formed by depositing a highly conductive, thin layer of MIEC on the surface of a dissimilar MIEC substrate and accounting for the solid/solid interfacial resistance. The oxygen flux through the coated membrane may exceed that through the bare membrane only if: 1) the surface exchange coefficient of the added layer is larger than the surface exchange coefficient of the bare membrane; and 2) the solid/solid interfacial resistance is sufficiently small. In general, deposition of the surface layer on the membrane tube surface exposed to lean gas leads to a larger oxygen flux than deposition of the layer on the oxygen rich side. A La_0.5Sr_0.5Fe_0.8Ga_0.2O_3-δ/SrCo_0.8Fe_0.2O_3-δ membrane achieved an oxygen outwards flux of 0.45 mL/min^⁎cm² at 1000 °C from an air/helium gradient. This was a ∼ 50% increase over that obtained using an uncoated LSFG tube.     

Oxygen transport in La2NiO4 + δ: Assessment of surface limitations and multilayer membrane architectures

The steady-state oxygen permeation through dense La₂NiO_4 + δ ceramics, limited by both surface exchange and bulk ambipolar conduction, can be increased by deposition of porous layers onto the membrane surfaces. This makes it possible, in particular, to analyze the interfacial exchange kinetics by numerical modelling using experimental data on the oxygen fluxes and equilibrium relationships between the oxygen chemical potential, nonstoichiometry and total conductivity. The simulations showed that the role of exchange limitations increases on reducing oxygen pressure, and becomes critical at relatively large chemical potential gradients important for practical applications. The calculated oxygen diffusion coefficients in La₂NiO_4 + δ are in a good agreement with literature. In order to enhance membrane performance, the multilayer ceramics with different architecture combining dense and porous components were prepared via tape-casting and tested. The maximum oxygen fluxes were observed in the case when one dense layer, ~ 60 μm in thickness, is sandwiched between relatively thin (< 150 μm) porous layers. Whilst the permeability of such membranes is still affected by surface-exchange kinetics, increasing thickness of the porous supporting components leads to gas diffusion limitations.     

Oxygen permeation in bismuth-based materials. Part I: Sintering and oxygen permeation fluxes

Oxygen permeation measurements were performed on two layered bismuth based oxide ceramics: a rhombohedral phase belonging to the Bi₂O₃–CaO system, (Bi₂O₃)_0.73–(CaO)_0.27 (BICAO) and a BICOVOX phase. Oxygen permeability for these systems was compared to permeability of the cubic fluorite type structure with composition (Bi₂O₃)_0.75(Er₂O₃)_0.25 (BE25). Low oxygen permeability was observed for the pure ceramic. As for BE25, permeability was considerably increased if 40 vol.% of silver was added to BICAO. In contrast, permeability was not improved by addition of gold to BICOVOX. For this latter phase, the oxygen molecular exchange at the surface is clearly the limiting step in the oxygen transfer.     

Microstructure and electrochemical properties of porous La2NiO4+δ electrodes spin-coated on Ce0.8Sm0.2O1.9 electrolyte

Porous La₂NiO_4+?? electrodes were prepared from superfine starting powder on dense substrates of Ce_0.8Sm_0.2O_1.9 electrolyte by a spin coating technique. The microstructure and electrochemical properties of the electrodes were investigated within the sintering temperature range of 1,000?C1,100?°C. An obvious effect of sintering temperature on the microstructure and electrochemical properties was detected. The variation of the electrochemical properties with sintering temperature was explained in relation to the microstructural evolution of the porous electrodes. It was detected that the electrode processes greatly depended on the microstructure of the electrodes. The polarization of surface oxygen exchange process was found to be the major contribution to the total electrode polarization. The electrode sintered at 1,050?°C showed the optimum electrocatalytic activity among the investigated electrodes. At 800?°C, the electrode exhibited a polarization resistance of 0.42????cm², an overpotential of 48?mV at a current density of 200?mA?cm^?2 and an exchange current density of 121?mA?cm^?2.     

Does power ultrasound (26 kHz) affect the hydrogen evolution reaction (HER) on Pt polycrystalline electrode in a mild acidic electrolyte?

In this study, we investigated the effects of power ultrasound (26 kHz, up to ∼75 W/cm², up to 100% acoustic amplitude, ultrasonic horn) on the hydrogen evolution reaction (HER) on a platinum (Pt) polycrystalline disc electrode in 0.5 M H₂SO₄ by cyclic and linear sweep voltammetry at 298 K. We also studied the formation of molecular hydrogen (H₂) bubbles on a Pt wire in the absence and presence of power ultrasound using ultra-fast camera imaging. It was found that ultrasound significantly increases currents towards the HER i.e. a ∼250% increase in current density was achieved at maximum ultrasonic power. The potential at a current density of −10 mA/cm² under silent conditions was found to be −46 mV and decreased to −27 mV at 100% acoustic amplitude i.e. a ΔE shift of ∼+20 mV, indicating the influence of ultrasound on improving the HER activity. A nearly 100% increase in the exchange current density (j_o) and a 30% decrease in the Tafel slope (b) at maximum ultrasonic power, was observed in the low overpotential region, although in the high overpotential region, the Tafel slopes (b) were not significantly affected when compared to silent conditions. In our conditions, ultrasound did not greatly affect the “real” surface area (A_r) and roughness factor (R) i.e. the microscopic surface area available for electron transfer. Overall, it was found that ultrasound did not dramatically change the mechanism of HER but instead, increased currents at the Pt surface area through effective hydrogen bubble removal.     

The influence of surface porosity on the stability and transition of supersonic boundary layer on a flat plate

In the present study, we examined, both experimentally and theoretically, the influence of surface permeability on the stability and laminar-turbulent transition of supersonic boundary layer at free-stream Mach number M_∞ = 2. A satisfactory agreement was obtained between the data calculated by the linear theory of stability and the data obtained in experiments with natural disturbances performed on models with different porous inserts.     

Magnetic and dynamic properties of Sm x Mn1 − x S solid solutions

The real and imaginary parts of the magnetic permeability at frequencies of 0.1, 1.0, and 10.0 kHz, as well as the electron paramagnetic resonance (EPR) line width and g-factor, have been measured in Sm _x Mn_{1 ? x} S (0.10 < x < 0.25) solid solutions in the temperature range 5–300 K. The logarithmic dependence of the maximum in the imaginary part of the magnetic permeability on the frequency and the power-law dependence of Imμ on the temperature have been determined. The mechanism of relaxation of the magnetic moment in the magnetically ordered and paramagnetic phases has been established. The experimental results have been explained in terms of the Heisenberg model with competing exchange interactions and the formation of the antiaspiromagnetic state at low temperatures.     

Electrode processes of LaCoO3+La2Zr2O7 cathodes deposited onto GCO

A combination of dc and ac results was used to study the electrode processes of LaCoO₃+La₂Zr₂O₇ cathodes deposited onto a GCO electrolyte, in air and in the range 600–800 °C. Steady state polarisation results showed that the overpotential values are very dependent on the electrode microstructures, which might be optimised by changes in the deposition and/or firing conditions of the electrodes. Tafel plots suggest different mechanisms under low and relatively high cathodic polarisation, with a transition at about −0.2 V at 700 °C. The transition tends to be displaced towards higher cathodic overpotentials with decreasing temperatures. The exchange current density is higher in the high polarisation regime (high values of |η|) than under low polarisation, and the exchange coefficient is lower in the range of high |η| The changes in pseudocapacitance and electrode resistance extracted from ac results also indicate a change in the regime. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Measurement of oxygen transport kinetics in epitaxial La2NiO4+δ thin films by electrical conductivity relaxation

The oxygen transport kinetics of mixed ionic and electronic conducting La₂NiO₄ thin films made by pulsed laser deposition (PLD) were measured using the electrical conductivity relaxation (ECR) technique. Since the film thickness is ∼ 3000 Å, the oxygen transport kinetics are controlled by the surface exchange rate. The experimental data are not well described by the usual single time constant model for oxygen surface exchange, but a good fit is obtained using two independent time constants. This model implies that the La₂NiO₄ film consists of two independent regions with different exchange rates that correspond to two different film microstructures.     

Surface effect on oxygen permeation through dense membrane of mixed-conductive LSCF perovskite-type oxide

La_0.1Sr_0.9Co_0.9Fe_0.1O_3−δ (LSCF1991) dense disks with different thicknesses and surface areas were prepared to investigate the contribution of surface reactions and bulk diffusion in oxygen permeation phenomena. The bulk diffusion controlled situation became significant with increasing the membrane thickness, and the surface reaction controlled situation prevailed at smaller surface area. The increase in surface area at the low P(O₂) (anode) side was more effective to increase the oxygen permeation flux than that at the high P(O₂) (cathode) side. The coating of a porous catalyst layer below the optimum thickness was also effective in enhancing the oxygen permeability due to the increase in surface area, but the coating with too thick layer deteriorated the permeability probably due to the increase in the gas diffusion resistance.     

Oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an oxygen-permeable membrane at high pressures

Oxygen permeation fluxes across the dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membrane disks were measured under an air/helium oxygen partial pressure gradient at high pressures (up to 10 atm) and various temperatures (973–1123 K). The fabricated BSCFO membrane exhibited good oxygen permeability with a high oxygen permeation flux of 2.01 ml min^− 1cm^− 2 (thickness: 1.37 mm) at 1123 K and 10 atm. Oxygen permeation results were analyzed theoretically using the surface exchange current model. The dependences of the oxygen permeation fluxes on the oxygen partial pressure gradient, suggested that the bulk oxygen ionic diffusion was the rate-limiting step for the overall oxygen permeation process across the BSCFO membrane. The ambipolar diffusion coefficients (D_a), the oxygen vacancy diffusion coefficients (D_v) and the oxygen ionic conductivities (σ_i) of the BSCFO material at different temperatures (973–1123 K) were calculated. It was found that BSCFO possessed high oxygen diffusion coefficients and ionic conductivities, which resulted in the good oxygen permeability of BSCFO. In addition, the BSCFO membrane exhibited good stability of oxygen permeation at 1123 K, while the deterioration of oxygen permeation stability was observed at 1098 K due to structural changes occurring at the surface of the BSCFO membrane disk as demonstrated by XRD.     

Microscopic derivation of Darcy's law

A microscopic derivation of Darcy's filtration law is given using Boltzmann's classical kinetic equation. This approach enables different functional relationships for the rate of filtration V_f to be obtained as a function of certain physical parameters and, in particular, as a function of the temperature. The relation between V_f and the frequency of a two-dimensional deformation wave propagating along the surface of a membrane is obtained from the solution of Boltzmann's equation and it is predicted that it ceases at high filtration frequencies. A general formula is derived for calculating the permeability K, which enables all the microscopic features of the process to be taken into account. N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 19–24, October, 1998.     

Inter- and intra-crystalline ion-exchange equilibria in the system FeCrAlO

The tie lines delineating ion-exchange equilibria between FeCr₂O₄FeAl₂O₄ spinel solid solution and Cr₂O₃Al₂O₃ solid solution with corundum structure have been determined at 1373 K by electron microprobe and EDAX point count analysis of oxide phases equilibrated with metallic iron. Activities in the spinel solid solution are derived from the tie lines and the thermodynamic data on Cr₂O₃Al₂O₃ solid solution available in the literature. The oxygen potentials corresponding to the tie-line composition of oxide phases in equilibrium with metallic iron were measured using solid oxide galvanic cells with CaOZrO₂ and Y₂O₃ThO₂ electrolytes. These electrochemical measurements also yield activities in the spinel solid solution, in good agreement with those obtained from tie lines. The activity-composition relationship in the spinel solid solution is analysed in terms of the intra-crystalline ion exchange between the tetrahedral and octahedral sites of the spinel structures. The ion exchange is governed by site-preference energies of the cations and the entropy of cations mixing on each site.     

Kinetics, thermodynamics and surface heterogeneity assessment of uranium(VI) adsorption onto cation exchange resin derived from a lignocellulosic residue

A new cation exchange resin (PGTFS-COOH) having a carboxylate functional group at the chain end was prepared by grafting poly(hydroxyethylmethacrylate) onto tamarind fruit shell, TFS (a lignocellulosic residue) using potassium peroxydisulphate-sodium thiosulphate redox initiator, and in the presence of N,N′-methylenebisacrylamide (MBA) as a crosslinking agent, followed by functionalisation. The adsorbent was characterized with the help of FTIR, XRD, scanning electron micrographs (SEM), and potentiometric titrations. The kinetic and isotherm data, obtained at optimum pH value 6.0 at different temperatures could be fitted with pseudo-second-order equation and Sips isotherm model, respectively. An increase in temperature induces positive effect on the adsorption process. The calculated activation energy of adsorption (E_a, 18.67 kJ/mol) indicates that U(VI) adsorption was largely due to diffusion-controlled process. The values of adsorption enthalpy, Gibbs free energy, and entropy were calculated using thermodynamic function relationships. The decrease in adsorption enthalpy with increasing U(VI) uploading on the adsorbent, reflects the surface energetic heterogeneity of the adsorbent. The isosteric heat of adsorption was quantitatively correlated with the fractional loading for the U(VI) ions adsorption onto PGTFS-COOH. The results showed that the PGTFS-COOH possessed heterogeneous surface with sorption sites having different activities.     

Influence of thickness and permeability of endothelial surface layer on transmission of shear stress in capillaries

The molecular coating on the surface of microvascular endothelium has been identified as a barrier to transvascular exchange of solutes. With a thickness of hundreds of nanometers, this endothelial surface layer(ESL) has been treated as a porous domain within which fluid shear stresses are dissipated and transmitted to the solid matrix to initiate mechanotransduction events. The present study aims to examine the effects of the ESL thickness and permeability on the transmission of shear stress throughout the ESL. Our results indicate that fluid shear stresses rapidly decrease to insignificant levels within a thin transition layer near the outer boundary of the ESL with a thickness on the order of ten nanometers. The thickness of the transition zone between free fluid and the porous layer was found to be proportional to the square root of the Darcy permeability. As the permeability is reduced ten-fold, the interfacial fluid and solid matrix shear stress gradients increase exponentially two-fold. While the interfacial fluid shear stress is positively related to the ESL thickness, the transmitted matrix stress is reduced by about 50% as the ESL thickness is decreased from 500 to 100 nm, which may occur under pathological conditions. Thus, thickness and permeability of the ESL are two main factors that determine flow features and the apportionment of shear stresses between the fluid and solid phases of the ESL. These results may shed light on the mechanisms of force transmission through the ESL and the pathological events caused by alterations in thickness and permeability of the ESL.     

Exploring the surface permeability of nanoporous particles by pulsed field gradient NMR

A new method to determine the surface permeability of nanoporous particles is proposed. It is based on the comparison of experimental data on tracer exchange and intracrystalline molecular mean square displacements as obtained by the PFG NMR tracer desorption technique with the corresponding solutions of the diffusion equation via dynamical Monte Carlo simulations. The method is found to be particularly sensitive in the "intermediate" regime, when the influence of intracrystalline diffusion and surface resistances of the nanoporous crystal on molecular transport are comparable and the conventional method fails. As an example, the surface permeabilities of two samples of zeolite NaCaA with different crystal sizes are determined with methane, as a probe molecule, at room temperature.     

A study of 3He-4He tunneling motion in solid 3He-4He mixtures by means of T1ϱ measurements

³He spin relaxation time in the rotating frame (T_1?) has been measured for 2% ³He in ⁴He solid samples of different molar volume at exchange plateau region (0.5 K). From the data, the characteristic frequency by which a ³He atom exchanges its position with a neighboring ⁴He atom is determined.     

Diffusion and conductivity properties of cerium niobate

The tracer diffusion coefficient (D⁎) and the surface exchange coefficient (k⁎) provide vital information for materials used in high temperature electrochemical devices (e.g. solid oxide fuel cells or oxygen permeation membranes). These values were established for the high temperature tetragonal scheelite structured CeNbO_4+δ (monoclinic fergusonite at room temperature), which is of interest due to its wide range of oxygen stoichiometries varying from stoichiometric CeNbO₄ to CeNbO_4.33. Measurements of D⁎ and k⁎ were performed by the isotopic exchange/line scan technique with SIMS (secondary ion mass spectrometry) used to determine ¹⁸O stable isotope depth distribution. This process was carried out between temperatures of 1073 K and 1173 K at 500 mbar of ¹⁶O/¹⁸O. These measurements were then correlated with oxide ion conductivity data previously determined from four probe d.c. and e.m.f. measurements using the Nernst–Einstein relation.