Oxygen Surface Diffusion Research Articles

Prevention of morphological change for (Ba,Sr)(Co,Fe)O3 cathodes by incorporating (Ce,Gd)O2 for intermediate-temperature solid oxide fuel cells

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) generally exhibits superior cathode activity compared to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the chemical stability of BSCF is inferior to that of LSCF. When BSCF cathodes are sintered at a high temperature of 1050 °C, performance was compromised due to the formation of (Ba,Sr)ZrO3 between the scandia-stabilized zirconia (ScSZ) electrolyte and gadolinia-doped ceria (GDC) interlayer. The oxide ionic conductivity of (Ba,Sr)ZrO3 was low, decreasing the cathode performance. Furthermore, the BSCF grains enlarged, and cobalt oxide formed due to BSCF decomposition, decreasing the active specific surface area. However, by incorporating GDC into the BSCF cathode, the morphology remained unchanged and cobalt oxide formation was prevented, despite (Ba,Sr)ZrO3 still forming. The performance of the cell with the BSCF-GDC composite cathode surpassed that with the BSCF cathode due to decreased polarization resistance attributed to the oxygen surface exchange and diffusion processes in the cathode.

Structure-electrochemical performance relationship in Sr2(Fe1-xVx)MoO6 anode

Structural property is becoming a powerful tool to manipulate oxygen vacancy concentration and electrochemical performance of electrode material. In this work, the structure-electrochemical performance relationship of Sr2(Fe1-xVx)MoO6 (x = 0, 0.1, 0.2 and 0.25) anode is investigated based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Energy Dispersive Spectrometer (EDS), BET surface area, DC four-point probe, I–V and electrochemical impedance spectra (EIS). XRD results show that all the compounds are of single phase, the tetragonal to cubic phase transition occurs at x = 0.1, which plays important role in its electrochemical performance. Firstly, the disappeared structural distortion in Sr2(Fe1.8V0.2)MoO6 and Sr2(Fe1.75V0.25)MoO6 anodes benefits electron hopping between Fe–O–Mo–O–Fe, so electron transfer processes (high-frequency semicircle) in their EIS are facilitated. Secondly, both XPS survey and EDS spectrum confirm the change in oxygen content with structural transition, Sr2(Fe1.9V0.1)MoO6 anode own higher concentration of oxygen vacancy, thus the oxygen surface exchange and diffusion process (low-frequency arc) of the corresponding cell is reduced significantly. Consequently, Sr2(Fe1.9V0.1)MoO6-based cell exhibits the highest power output (810 mW cm−2) at 850 °C. For Sr2(Fe1.8V0.2)MoO6 anode, BET surface area test reveals the slightly higher value, while its lower power density indicates that the influence of oxygen vacancy prevails over that of microstructure. Combining with EIS results, it can be deduced that Sr2(Fe1-xVx)MoO6 anodes with tetragonal symmetry own higher concentration of oxygen vacancy, thus the expected electrochemical performance is better than that of other cells. Understanding the critical role of structure is important for achieving highly active electrode material.

Oxidation of fine aluminum particles: thermally induced transformations in particle shells and kinetics of oxide nucleation.

This paper investigates the process of oxidation of fine aluminum powder, consisting of spherical Al particles of 'metal core/oxide shell' type, when heated in air at temperatures below 550 °C. The highly dispersed aluminum powder 'Alex' used in this work (particle size: 0.05-1.5 μm and number average particle diameter (DN): 0.11 μm) was produced by electric explosion of a thin Al wire in argon with subsequent passivation in an oxygen-containing atmosphere. For the first time, the influence of the particle size on the oxidation process has been identified. During the reaction, individual γ-Al2O3 oxide nuclei grow at the surface of Al particles with diameters less than 300 nm without laterally overlapping to form a protective passivating layer, as typically occurs during the oxidation of micron-sized particles or bulk metal. The localization of γ-Al2O3 nuclei is determined by the regions where peeling of the primary amorphous oxide film from the surface of the metal core of an Al particle occurs due to the thermal decomposition of aluminum hydroxides (T ≈ 350 °C) present within the particle shells. A significant increase in the contributions of the following factors leads to a high oxidation rate: the diffusion within the disordered structure of a particle core and the surface diffusion of cations (Ea = 80.6 ± 5.3 kJ mol-1, 400-450 °C), the surface and grain boundary diffusion of oxygen during the growth of γ-Al2O3 crystallites (Ea = 108.3 ± 11.2 kJ mol-1, 470-500 °C) and the surface and grain boundary transport of anions through an amorphous and underlying nanocrystalline oxide layer with a constant thickness (Ea = 205.1 ± 8.6 kJ mol-1, 520-550 °C). The results obtained make it possible to expand the theoretical understanding of the manifestations of the size effect in solid-phase reactions and take into account its influence in practice.

Electrochemical evaluation of Sr2Fe1.5Mo0.5O6-δ/Ce0.9Gd0.1O1.95 cathode of SOFCs by EIS and DRT analysis

Employing potent advanced materials to perform as the cathode of SOFC requires a profound investigation on the material’s electrochemical response within the oxygen reduction reaction. Accordingly, in this research, electrochemical characteristics of Sr2Fe1.5Mo0.5O6-δ/Gd0.1Ce0.9O1.5 (SFM/GDC) air electrode was studied using electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) technique. SFM/GDC composite electrode sintered at 1050 °C shows the lowest polarization resistance of 0.23 Ω.cm2 at 800 °C. Three DRT peaks determined, and an LRΩ(QRH)G equivalent circuit is proposed for the EIS response, in which the Gerischer element was related to the low-frequency part. Two of the peaks indicated by DRT were associated to the Gerischer response, which reveals simultaneous occurrence of oxygen surface reaction and diffusion sub-processes during ORR. By temperature rising from 650 °C to 800 °C, Gerischer resistance (RG) reduces by 91%. Introduction of Pd nanoparticles into the electrode reduces the RG by 48% and 10% at 650 °C and 800 °C, respectively. The results demonstrate that at the intermediate temperature range, the SFM/GDC composite electrode requires improvement by surface decoration due to its insufficient surface exchange kinetics; however, significantly improved surface exchange kinetics is observed at higher temperature range without any surface modification.

Influence of polarization on the electrochemical activity of La2–xCaxNiO4+δ electrodes in contact with Ce0.8Sm0.2O1.9 electrolyte

The effect of electrode polarization on the electrochemical activity of La2NiO4+δ and La1.9Ca0.1NiO4+δ electrodes in contact with the Ce0.8Sm0.2O1.9 electrolyte is studied by impedance spectroscopy. It is found that anodic polarization facilitates electrode reaction for both electrodes leading to significant decrease in the polarization resistance. The effect of cathodic polarization differs between the electrodes: the polarization resistance of La2NiO4+δ electrode slightly increases, while the polarization resistance of La1.9Ca0.1NiO4+δ electrode strongly decreases with the increase in the applied potential. It is established that in all cases the polarization mostly affects the low-frequency stage of the electrode reaction, connected with oxygen surface exchange and diffusion. The surface state of the samples after exposure under polarization is studied by X-ray photoelectron spectroscopy. Correlations between electrochemical activity of the electrodes and the changes in their surface composition under polarization are discussed.

Modulation of Structural, Electronic, and Optical Properties of Titanium Nitride Thin Films by Regulated In Situ Oxidation

Epitaxial titanium nitride (TiN) and titanium oxynitride (TiON) thin films have been grown on sapphire substrates using a pulsed laser deposition (PLD) method in high-vacuum conditions (base pressure <3 × 10-6 T). This vacuum contains enough residual oxygen to allow a time-independent gas phase oxidation of the ablated species as well as a time-dependent regulated surface oxidation of TiN to TiON films. The time-dependent surface oxidation is controlled by means of film deposition time that, in turn, is controlled by changing the number of laser pulses impinging on the polycrystalline TiN target at a constant repetition rate. By changing the number of laser pulses from 150 to 5000, unoxidized (or negligibly oxidized) and oxidized TiN films have been obtained with the thickness in the range of four unit cells to 70 unit cells of TiN/TiON. X-ray photoelectron spectroscopy (XPS) investigations reveal higher oxygen content in TiON films prepared with a larger number of laser pulses. The oxidation of TiN films is achieved by precisely controlling the time of deposition, which affects the surface diffusion of oxygen to the TiN film lattice. The lattice constants of the TiON films obtained by x-ray diffraction (XRD) increase with the oxygen content in the film, as predicted by molecular dynamics (MD) simulations. The lattice constant increase is explained based on a larger electrostatic repulsive force due to unbalanced local charges in the vicinity of Ti vacancies and substitutional O. The bandgap of TiN and TiON films, measured using UV-visible spectroscopy, has an asymmetric V-shaped variation as a function of the number of pulses. The bandgap variation following the lower number of laser pulses (150-750) of the V-shaped curve is explained using the quantum confinement effect, while the bandgap variation following the higher number of laser pulses (1000-5000) is associated with the modification in the band structure due to hybridization of O2p and N2p energy levels.

In-situ segregation of A-site defect (La0.6Sr0.4)0.90Co0.2Fe0.8O3-δ to form a high-performance solid oxide fuel cell cathode material with heterostructure

As a potential cathode material for solid oxide fuel cells, the commercial application of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) has to face the challenges in insufficient oxygen reduction reaction (ORR) activity, segregation of Sr element, and CO2 poisoning. Therefore, the effect of A-site non-stoichiometry on the electrochemical performance of (LS)1-xCFs (x = −0.05, 0, 0.05, 0.10, 0.15) from the aspect of microstructure/elemental surface chemical environment evolution is investigated in this paper. The results show that (LS)0.90CF has the best ORR activity and the highest electrochemical performance. The excellent electrochemical performance is attributed to the (LS)0.90CF/Co2FeO4/CoFe2O4-type heterostructure, formed by in-situ segregation induced by A-site defects, which improves the surface oxygen diffusion coefficient and chemical bulk diffusion coefficient of the cathode. At the same time, the A-site defect can effectively inhibit the segregation of Sr element in (LS)0.90CF, thereby improving the tolerance of CO2. At 800 °C, the area-specific resistance (ASR) of (LS)0.90CF (0.023 Ω cm2) is 66.7% lower than that of LSCF (0.069 Ω cm2), and the peak power density (1.57 Wcm−2) is 1.8 times higher than that of LSCF (0.87 Wcm−2). Therefore, perovskite A-site defect-induced B-site segregation to form heterostructures provides an effective strategy for the preparation of high-performance cathode materials.

Restrictions of nitric oxide electrocatalytic decomposition over perovskite cathode in presence of oxygen: Oxygen surface exchange and diffusion

Nitric oxide (NO) abatement from engine exhaust is of great significance to alleviate air pollution and haze. Compared with the traditional selective catalytic reduction (SCR) technology, electrocatalytic decomposition of NO simplifies the reductant supply system and therefore avoids secondary pollution. In this study, typical perovskite La0.6Sr0.4CoxFe1-xO3-δ (LSCF) infiltrated by different dosages of nano ceria Ce0.9Gd0.1O1.9 (GDC) was used as composite cathodes, in order to explore the critical factors to restrain NO conversion in excess of O2. The results show that electron as reactive species transfers among NO, ABO3-type cathode and oxygen vacancy. The maximum of NO removal efficiency can reach 96.27 % in absence of O2 and up to 80.55 % in presence of 1% O2 in case of LSCF infiltrated by moderate dosages LSCF-GDC(2), which is superior to those of LSCF, LSCF-GDC(4) and LSM-GDC(nano) composite cathode. Compared to oxygen storage capacity (OSC) caused by the infiltration of nano ceria, higher surface oxygen exchange coefficient (kδ) and chemical diffusion coefficient (Dchem) lead to the significant decrease in polarization resistance (Rp), and consequently to the enhancement of NO removal in presence of O2. No matter what kind of oxygen deriving from oxygen reduction reaction (ORR) and NO reduction reaction (NORR), GDC infiltration into LSCF improves oxygen transport property and however, the property of cathode in ORR is dominant over in NORR in presence of O2. Moderate GDC loading has the highest oxygen transport kinetics, and oxygen surface exchange is faster than chemical diffusion, due to lower activation energy. Over loading of GDC with greater ohmic resistance (Rs) inversely influences the NO removal.

Achieving High CO2 Electrocatalytic Activity By Tailoring Cation-Size Mismatch in Double Perovskite Oxides

Conversion of CO2 into useful chemical products by solid oxide electrolysis cells (SOECs) is a promising technology capable of reducing CO2 concentration for a carbon-neutral society [1]. This electrochemical device has several advantages such as high-energy efficiency and fast electrode kinetics due to its high operating temperature. Conventionally, Ni/YSZ cermet has been widely used as the cathode material of CO2 electrolysis system. However, they are prone to degrade under CO2 atmosphere due to the oxidation of nickel, particle agglomeration, and carbon deposition. Therefore, the development of alternative cathode materials with high electrocatalytic activity and good long-term stability for CO2 reduction reaction (CO2RR) is highly needed.The perovskite-type mixed ionic and electronic conducting (MIEC) oxides are widely investigated as the promising alternatives to the Ni/YSZ cermet cathode. Among them, double perovskite oxides PrBaCo2O5+d(PBCO) material is attracting attention because of high oxygen surface exchange, diffusion coefficients and adequate mixed ionic and electronic conductivity. However, this material is easily degraded in the presence of CO2 impurity, with the formation of BaCO3 nanoparticles [2]. To overcome this issue, doping the B-site Co cations with transition metals and tailoring the cation mismatch by controlling A-site dopant ratio in PBCO were selected as a novel strategy. As a result, it was proved that co-doping was an effective way to improve both electrochemical and surface chemical stability. Our design strategy could benefit the preparation of highly active and stable cathodes for direct CO2 reduction for SOECs.

An electrochemical assessment of pristine SrCoO3‐δ using the distribution of relaxation times analysis

AbstractStrontium cobalt oxide is an important perovskite‐type material in the research of solid oxide fuel cells (SOFCs), since a great number of derived cathode materials can be obtained through composition modification against SrCoO3‐δ (SCO). In this work, pristine SCO without any additional dopants was synthesized, and its electrochemical properties were evaluated according to the electrochemical impedance spectroscopies (EIS) analyses through the distribution of relaxation times (DRT) method on the symmetrical cells. The undoped SCO displayed low polarization resistances as well as a high oxygen transport ability, even comparable to the famous cathode, Sm0.5Sr0.5CoO3‐δ (SSC). Oxygen surface and bulk diffusion were found to be the dominating processes in the oxygen reduction reaction (ORR) of SCO. These results could broaden our knowledge about the ionic transport behavior of SCO‐based perovskite materials.

Impacts of A-site gadolinium doping on electrocatalytic performance of La0.33Ba0.62Gd0.05Co0.7Fe0.3O3-δ to realize promising cathode for intermediate-temperature solid oxide fuel cells

With respect to intermediate-temperature solid oxide fuel cells (IT-SOFCs), electrocatalytic activity of the cathodes dominates their output power. Herein A-site gadolinium-doped La0.33Ba0.62Gd0.05Co0.7Fe0.3O3-δ (LBG0.05CF) perovskite is synthesized and systematically evaluated as the cathode for IT-SOFCs. We demonstrate outstanding catalytic activity of LBG0.05CF toward oxygen reduction reaction (ORR). Benefiting from enhanced electrical conductivity, oxygen surface exchange and diffusion rates, the ORR activity of LBG0.05CF outperforms that of La0.33Ba0.67Co0.7Fe0.3O3-δ (LBCF), as evidenced by the lower polarization resistance (0.035 Ω cm2) at 700 °C. The Ce0.9Gd0.1O1.95 (CGO) electrolyte-supported fuel cell with the LBG0.05CF cathode delivers a peak power density (PPD) of 371 mW cm−2 at 700 °C, which surpasses the PPD (278 mW cm−2) for LBCF-based cell. Furthermore, LBG0.05CF exhibits an improved CO2 tolerance compared with pristine LBCF. These results highlight a promising strategy of A-site replacement of Ba by lanthanide for optimizing catalytic ORR activity of the perovskite materials.

The removal of N2O from gas stream by catalytic decomposition over Pt-alkali metal/SiO2

Using promoters is an efficient way to improve catalytic activity in many chemical and industrial applications. In this paper, the effects of the impregnation method and alkali metals in a series of 1%wt Pt-M/SiO 2 (M=Na, K, and Cs) catalysts were comprehensively examined on the decomposition of N2O. To reveal the physical and chemical properties of samples for analyzing obtained results of catalytic activity, catalysts were characterized by BET, XRD, ICP, H2-TPR, CO2-TPD, TGA, XPS and HRTEM. Catalysts were also synthesized by wet impregnation as a simple and efficient method. In order to find the effects of the synthesis method, different treatments of both sequential and co-impregnation were applied. The optimum results are acquired when Pt precursor was loaded on alkali metal oxide modified SiO 2. According to achieved results, Na and Cs prompters drop the conversion from 65% to 25% and 35%, respectively. Among the alkali metals, only K element has shown promotional characteristics despite 50% reduction of surface area and 75% growth of Pt particles. Also, the impact of alkali metal loading indicates that 5%wt KNO3 yielded an 18% promotion in conversion. Donating electron, facilitating oxygen surface diffusion and uniform distribution of KOx clusters are significant factors for the improvement of the PtK reactivity.

Study on the mechanical properties gradient in surface oxygen diffusion hardened layer of Ti6Al4V alloy

This paper studies the mechanical properties gradient in surface oxygen diffusion hardened (SODH) layer of Ti6Al4V alloy by four-point bending test, nano-indentation test and Vickers hardness measurement test. The relationships of hardness-oxygen content and hardness-yield stress were also investigated. Tests results show that the outmost ≈25 μm deep surface layer has the highest oxygen content, hardness and yield strength under compression monotonic loading but the lowest cracking strength before yielding under tension loading. However, from ≈50 μm deep position to substrate, the yield strengths under tensile and compressive loading are almost the same. Test result also shows that, with further decrease of oxygen content deeper down in the SODH layer, the hardness and yield strength decrease correspondingly. It is also found that there are linear relationships between oxygen content and Vickers hardness or nano-hardness. The above relationships can be used to estimate the oxygen concentration distribution in the SODH layer by the simple test method of hardness measurement. Since the yield stress also has a linear relationship with hardness, a valid estimate of the yield stress distribution in SODH layer can be obtained as well. The high hardness and compressive yield strength but low cracking tensile strength of the outmost ≈25 μm depth SODH layer can be employed to explain why surface oxygen diffusion treatment can improve greatly the wear resistance of titanium alloy but reduce its tensile mechanical properties and fatigue resistance.

Probing borophene oxidation at the atomic scale

Two-dimensional boron (i.e. borophene) holds promise for a variety of emerging nanoelectronic and quantum technologies. Since borophene is synthesized under ultrahigh vacuum (UHV) conditions, it is critical that the chemical stability and structural integrity of borophene in oxidizing environments are understood for practical borophene-based applications. In this work, we assess the mechanism of borophene oxidation upon controlled exposure to air and molecular oxygen in UHV via scanning tunneling microscopy andspectroscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. While borophene catastrophically degrades almost instantaneously upon exposure to air, borophene undergoes considerably more controlled oxidation when exposed to molecular oxygen in UHV. In particular, UHV molecular oxygen dosing results in single-atom covalent modification of the borophene basal plane in addition to disordered borophene edge oxidation that shows altered electronic characteristics. By comparing these experimental observations with density functional theory calculations, further atomistic insight is gained including pathways for molecular oxygen dissociation, surface diffusion, and chemisorption to borophene. Overall, this study provides an atomic-scale perspective of borophene oxidation that will inform ongoing efforts to passivate and utilize borophene in ambient conditions.

Selective oxidation of glycerol over different shaped WO3 supported Pt NPs

In this work, different shaped WO3 (rod-like, lamellar and cuboid) supported Pt catalysts were prepared in a facile routine and tested in the selective oxidation of glycerol in base-free aqueous solution. Characterizations indicated that rod-like WO3 supported Pt catalyst (Pt/R-WO3) possesses higher surface area because of the formation of vertical pore channels and highly exposed plane (100), the deposited Pt atoms combined strongly with the terminal WO in rod-like WO3. These properties promoted the adsorption, storage and surface diffusion of oxygen over Pt/R-WO3 which exhibited the excellent activity for the selective oxidation of glycerol. And the higher amount of acid sites on the surface of Pt/R-WO3 enhanced the selectivity of glyceric acid. The calculated turnover frequency of each Pt atom in Pt/R-WO3 reached 946 h–1 at 60 °C.

Enhancing Bifunctional Electrocatalytic Activities of Oxygen Electrodes via Incorporating Highly Conductive Sm3+ and Nd3+ Double-Doped Ceria for Reversible Solid Oxide Cells

Solid oxide cells (SOCs) are mutually convertible energy devices capable of generating electricity from chemical fuels including hydrogen in the fuel cell mode and producing green hydrogen using electricity from renewable but intermittent solar and wind resources in the electrolysis cell mode. An effective approach to enhance the performance of SOCs at reduced temperatures is by developing highly active oxygen electrodes for both oxygen reduction and oxygen evolution reactions. Herein, highly conductive Sm3+ and Nd3+ double-doped ceria (Sm0.075Nd0.075Ce0.85O2-δ, SNDC) is utilized as an active component for reversible SOC applications. We develop a novel La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)-SNDC composite oxygen electrode. Compared with the conventional LSCF-Gd-doped ceria oxygen electrode, the LSCF-SNDC exhibits ∼35% lower cathode polarization resistance (0.042 Ω cm2 at 750 °C) owing to rapid oxygen incorporation and surface diffusion kinetics. Furthermore, the SOC with the LSCF-SNDC oxygen electrode and the SNDC buffer layer yields a remarkable performance in both the fuel cell (1.54 W cm-2 at 750 °C) and electrolysis cell (1.37 A cm-2 at 750 °C) modes because the incorporation of SNDC promotes the surface diffusion kinetics at the oxygen electrode bulk and the activity of the triple phase boundary at the interface. These findings suggest that the highly conductive SNDC material effectively enhances both oxygen reduction and oxygen evolution reactions, thus serving as a promising material in reversible SOC applications at reduced temperatures.

Non-compact oxide-island growth induced by surface phase transition of the intermetallic NiAl during vacuum annealing

Crystal structure and composition are inter-dependent and decoupling their effects on surface reactivity is challenging. Using low-energy electron microscopy to spatially and temporally resolve the oxide film growth during the oxidation of NiAl(100), we differentiate such coupled effects by monitoring oxide growth while simultaneously fine-tuning the surface structure and composition during oxidation. We demonstrate that the oxidation of chemically ordered surfaces results in compact oxide island growth whereas non-compact oxide growth during the surface phase transition. By incorporating the surface phase transition induced chemical disordering into kinetic Monte Carlo simulations, we show that the non-compact oxide growth is induced by the composition effect on the surface diffusion of oxygen, which can be described by the concept of “ant in the labyrinth”.

Degradation evaluation by distribution of relaxation times analysis for microtubular solid oxide fuel cells

Distribution of relaxation times (DRT) analysis is a powerful assistant tool for electrochemical impedance spectroscopy (EIS) deconvolution. In the present work, electrode degradation is evaluated by estimation of polarization resistances using DRT analysis. Five or six DRT peaks are detected for anode-supported microtubular solid oxide fuel cells (SOFCs) at 600–650 °C. In a reduction-oxidation (redox) cycling test, the polarization resistance of charge transfer and ionic conduction processes in the anode increases due to the decrease in triple phase boundary (TPB) length. On the other hand, initial electrode degradation is also successfully evaluated using DRT analysis during a galvanostatic test. The rapid growth of nickel grains in the anode and the relatively slow growth of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) grains in the cathode are observed after the test, which corresponds with the increase in the polarization resistance of charge transfer and ionic conduction processes in the anode and that of oxygen surface exchange and diffusion processes in the cathode, respectively.

Oxygen isotope exchange in proton-conducting oxides based on lanthanum scandates

A method of the oxygen isotope exchange with equilibration of the gas phase was used in order to obtain the temperature dependences of the oxygen surface exchange and diffusion coefficients of the proton-conducting oxides La1–xSrxScO3–δ (x = 0; 0.04; 0.09) in the temperature range of 600–900 °C and oxygen pressure of 1.01 kPa. The diffusion and oxygen surface exchange coefficients increase with the strontium content in oxides. The rates of the individual stages of the oxygen exchange process on the surface of the oxides were determined. The oxygen incorporation was revealed to be rate-determining for the undoped LaScO3 oxide. However, for the strontium-doped oxides La1–xSrxScO3–δ (x = 0.04; 0.09), the difference between the rates of oxygen dissociative adsorption and oxygen incorporation decreases with increasing the strontium concentration, in a way that for the La0.91Sr0.09ScO3−δ oxide these stages become competing. The possible reasons for these differences in oxygen surface exchange kinetics were analyzed in this paper. Using the obtained oxygen diffusion coefficients, the oxygen-ionic conductivities in accordance with the Nernst-Einstein equation, and the contributions of the oxygen-ion and proton components of the total conductivity of the La1–xSrxScO3–δ (x = 0.04; 0.09) oxides in the wet reducing atmosphere (pH2O = 2.35 kPa, pO2 = 10−15 Pa) were calculated. The proton transference numbers were found to be close to unit at the wet reducing atmospheres in the temperature range of 500–600°С.

Measurement of Oxygen Potential Distribution on the Surface of a Mixed Conducting Electrode

Introduction Mixed ionic and electronic conductors (MIEC) are widely used as cathodes of intermediate temperature SOFCs. With a mixed conducting cathode, electrode reaction proceeds mainly through oxygen incorporation reaction at gas/electrode double phase boundary (DPB) followed by diffusion of oxide ion in the electrode particles. However, in certain conditions, the reaction through TPB (gas / electrode / electrolyte triple phase boundary) via surface diffusion may not be negligible. The contributions of TPB and DTB paths to the total process remain unclarified, but should be understood in a quantitative way to optimize the electrode design. An experimental method to evaluate the behavior of oxygen “on” and “in” the MIEC electrode is required. Kawada et al. proposed a potentiometric method to measure oxygen potential “on” the surface of a polarized electrode using a porous electrolyte that was made softly in contact with the surface of an electrode [1]. They explored the method to a position-resolved measurement to evaluate the surface diffusion of oxygen. Although it appeared promising, the reliability and reproducibility of the method was left unestablished. On the other hand, Hendriksen et al. used the similar technique to measure the local oxygen potential “in” the solid, by tightly pushing a pointed cone of an electrolyte on the surface of a MIEC [2]. The difference of those results could be from the way how the probe touches the sample surface, which makes the uncertainty of this technique. This paper revisits the experiments of oxide ion probes aiming at establishing the method and its interpretation. Discussion will be made on possibility of using this technique for studying surface reaction mechanism in detail. Methods 2.1 Oxide ion probe We made an oxide ion probe by coating a Pt wire with the porous Ce0.9Gd0.1O1.95 (GDC). The probe was made in contact with the MIEC electrode La0.6Sr0.4CoO3-δ (LSC64). In order to make the contact force as constant as possible, the probe was slowly brought into contact with the electrode surface and pressed until the error of the potentiostat disappeared and the measurement could be started. 2.2. Measurement condition The test cell was composed of GDC electrolyte, LSC64 thin film as Working Electrode (WE), LSC64 porous as Counter Electrode (CE), and Pt paste as Reference Electrode (RE). Measurements were made by three terminal electrochemical impedance spectroscopy method. Temperature dependence was measured from 723 K to 873 K. Oxygen partial pressure dependence was measured from 100ppm to 100%. The frequency response of the probe was evaluated by dividing the probe potential by the cell current. Results and Discussion Temperature and partial pressure dependences of the WE impedance and probe response are shown in Fig.1. At 873 K, the frequency response of the probe and WE were close to each other although the probe response did not include high frequency resistance which could be assigned to the contribution of the electrolyte. The WE and probe responses similarly increased as the oxygen partial pressure became lower. On the other hand, temperature dependence was different between WE and probe responses. WE resistance increased more remarkably than probe response when the temperature was decreased. A possible cause for the difference in the responses of the probe and WE is the resistance of oxide ion diffusion through the film. The oxide ion conductivity of LSC64 was estimated from the vacancy diffusion coefficient reported in [3] and vacancy concentration obtained from the chemical capacitance of the film. The roughly estimated diffusion resistance was 0.14 Ω at 873 K, 8.81 Ω at 773 K, and 32.7 Ω at 723 K, which are much smaller than the reaction resistance of WE. Additionally, the high frequency response of the impedance did not show 45° line in the Nyquist plot. So Thus, it can be said that the difference between the probe and WE is not due to the contribution of diffusion. Another possible factor is the oxygen incorporation resistance. Budiman et al. suggested the existence of a barrier layer just below the surface of the LaNi0.6Fe0.4O3-δ electrode [4]. A similar but less critical barrier layer could exist on LSC64. This also suggests the oxide ion probe measures at least the above the barrier layer, that is, almost the surface of the electrode.