Dissociative Adsorption Of Oxygen Research Articles

Enhancing LSCF-based air electrode durability: Insights into sulfur poisoning and air purification

The electrochemical performance of perovskite-type air electrodes in solid oxide cells is strongly affected by a trace amount of SO2 as the air impurity. The present work aims to study air purification efficiency in suppressing the degradation rate for the LSCF-GDC air electrode while operating under different SO2 concentrations. For this purpose, the electrochemical performance of symmetrical cells was evaluated using impedance spectra measured over 320 h and compared with intrinsic electrode degradation in synthetic air. Accordingly, the intrinsic degradation rate was measured at 0.05 mΩ cm2 h−1 while exposing the cell to synthetic air containing 10 and 100 ppb SO2, causing a severe increase to 0.17 and 0.30 mΩ cm2 h−1, respectively. Nevertheless, the calculated degradation rate under purified air containing 100 ppb SO2 was similar to the LSCF-GDC intrinsic degradation rate, representing the high practicality of filter material to adsorb SO2 species. Interpreting the impedance spectra using a distribution of relaxation times (DRT) and a complex nonlinear least squares fit (CNLS) revealed that the intrinsic degradation mechanisms were the limitation of dissociative surface adsorption of oxygen and charge transfer to the adsorbed oxygen. At the same time, inhibition of the O2 adsorption caused by the deposition of SrO, SO32−, and SrSO4 phases and increasing surface diffusion resistance for the adsorbed oxygen atoms were the rate-determining steps in SO2-containing synthetic air. Lastly, the effective suppression of degradation rate by gas purification indicates an extensive use of filter material to extend the lifespan of SOCs.

Comprehensive study of O2 and H2O interaction with La0.9Sr0.1ScO3–δ oxide

In this paper we study the processes of O2 and H2O interaction with La0.9Sr0.1ScO3−δ. Using high temperature neutron powder diffraction (HT-NPD), the evolution of crystal structure and water uptake was investigated in the temperature range 25–700 °C at 21.3 kPa in O2 and O2 + D2O atmospheres. The results of HT-NPD show the influence of the water uptake by La0.9Sr0.1ScO3–δ bulk on the thermal expansion of the material. The kinetics of oxygen surface exchange was studied using pulsed isotope exchange technique. The measurements were carried out in the temperature range 300−900 °C at pO2 = 5.1, 12.2 and 21.3 kPa in dry and humid (pH2O = 2.6 kPa) conditions. For the first time the physical model describing the kinetics of oxygen surface exchange between the mixture of oxygen and water in the gas phase and a solid oxide has been suggested. The model includes following steps: oxygen dissociative adsorption, oxygen incorporation, hydrogen surface diffusion, water adsorption and dissociation. The rate-determining step of the exchange is considered. The influence of O2 humidification on the oxygen surface exchange is discussed.

Tuning the Magnetic and Catalytic Properties of Manganese Ferrite through Zn2+ Doping: Gas Phase Oxidation of Octanol

Spinel ferrites, ZnFe2O4, MnFe2O4, and ZnMnFe2O4, were synthesized using the sol–gel method and thoroughly investigated for their potential as catalytic and magnetic materials. Experiments unveiled that ZnMnFe2O4 exhibited excellent catalytic and magnetic properties, whereas the Density Functional Theory (DFT) calculations provided insight into the excellent performance of ZnMnFe2O4 compared with ZnFe2O4 and MnFe2O4. The catalytic efficiencies of the synthesized spinel ferrites were evaluated against a model reaction, i.e., the gas-phase oxidation of octanol to a corresponding aldehyde, utilizing molecular oxygen as an oxidant. The results indicated that the order of catalytic activity was ZnMnFe2O4 > MnFe2O4 > ZnFe2O4. The reaction was found to follow Langmuir Hinshelwood’s mechanism for dissociative adsorption of molecular oxygen. Owing to their superb catalytic and magnetic properties, mixed ferrites can be extended to a variety of organic transformation reactions.

Characterization of B‐Site Sc‐doped La2Ni1-xScxO4+δ (x=0, 0.05, 0.10, and 0.15) perovskites as cathode materials for IT-SOFCs

Cathode materials play a crucial role in enabling the widespread adoption of intermediate-temperature solid oxide fuel cells (IT-SOFC) for commercial applications. This study utilized the sol-gel method to prepare a range of perovskite oxides, specifically Sc-doped La2Ni1-xScxO4+δ (x = 0, 0.05, 0.10, and 0.15), without the use of cobalt. The investigation focused on making a study of how the introduction of Sc doping affected the thermal stability, electrochemical properties, and structure of cathode materials. XRD, FE-SEM, and electrochemical workstations were subjected to structural characterization and performance testing of materials. The findings indicate that all the prepared materials have a typical Ruddlesden-Popper structure, and an abundance of additional oxygen can be generated in close proximity to the interstitial by incorporating Sc into the cathode of La2Ni1-xScxO4+δ to increase the oxygen vacancy and improve the catalytic activity of ORR. La2Ni0.9Sc0.1O4+δ has the highest oxygen surface exchange kinetics, the dissociative adsorption of oxygen is the rate-limiting step at the cathode. At a temperature of 700 °C, the lowest polarization resistance of La2Ni0.9Sc0.1O4+δ is 0.1298 Ω cm2, while the single cell with La2Ni0.9Sc0.1O4+δ as the cathode achieves an impressive peak power density of 0.462 W cm−2, thus demonstrating exceptional long-term stability. La2Ni0.9Sc0.1O4+δ is expected to be a potential high-performance IT-SOFC cathode material.

Understanding the oxygen reduction reaction of one-dimensional Ca3Co2O6 cathodes for SOFC

Understanding the processes of different cathode materials is part of the journey toward the development of intermediate-temperature solid oxide fuel cells. In this work, the one-dimensional Ca3Co2O6 (C326) electrode and its composite with Ce0.9Gd0.1O2-δ (CGO) are systematically investigated as a function of sintering temperature using a symmetrical cell test. For both batches of samples, the optimum sintering temperature is 1000 °C. Interpretation of the impedance spectra reveals that this optimization is associated with improvements in the processes of surface diffusion and interfacial charge transfer due to the higher effective reaction area for ionic transfer at the electrode/electrolyte interface and better connectivity between the electrode grains. The temperature dependence of the individual processes indicates that the addition of CGO significantly improves the steps of dissociative oxygen adsorption and diffusion, which should occur first on its surface, with the charge transfer of oxygen species at the CGO/C326 boundary being the rate-limiting step.

Zn Modification of Pd/TiO2/Ti Catalyst for CO Oxidation.

The main goal of this study was to modify the activity of Pd/TiO2/Ti catalyst in the reaction of CO oxidation by the addition of Zn. Plasma electrolytic oxidation (PEO) of Ti wire was conducted to produce a uniform porous layer of TiO2. A mixture of Pd and Zn was then introduced by means of adsorption. After reduction treatment, the activity of the samples was examined by oxidation of 5% CO in a temperature range from 80-350 °C. Model catalysts with sufficient amounts of the metals for physico-chemical investigation were prepared to further investigate the reaction between Pd and Zn during CO oxidation. The structures and compositions of the samples were investigated using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), inductively coupled plasma mass spectrometry (ICP-MS), Fourier transform infrared (FTIR), and X-ray diffraction (XRD). Modification of Pd/TiO2/Ti catalyst by Zn with a Pd:Zn atomic ratio of 2:1 decreased the temperature of complete CO oxidation from 220 °C for Pd/TiO2/Ti to 180 °C for Pd-Zn/TiO2/Ti. The temperature of 50% CO conversion on Pd-Zn(2:1)/TiO2/Ti was around 55 °C lower than in the reaction on monometallic Pd catalyst. The addition of Zn to the Pd catalyst lowered the binding energy of CO on the surface and improved the dissociative adsorption of oxygen, facilitating the oxidation of CO. FTIR showed that the bridging form of adsorbed CO is preferred on bimetallic systems. Analysis of the surface compositions of the samples (SEM-EDS, TOF-SIMS) showed higher amounts of oxygen on the bimetallic systems.

Kinetics of Oxygen Exchange and N2O Decomposition Reaction over MeOx/CeO2 (Me = Fe, Co, Ni) Catalysts.

MeOx/CeO2 (Me = Fe, Co, Ni) samples were tested in an 18O2 temperature-programmed isotope exchange and N2O decomposition (deN2O). A decrease in the rate of deN2O in the presence of oxygen evidences the competitive adsorption of N2O and O2 on the same sites. A study of isotope oxygen exchange revealed dissociative oxygen adsorption with the subsequent formation of surface oxygen species. The same species, more probably, result from N2O adsorption and the following N2 evolution to the gas phase. We supposed the same mechanism of O2 formation from surface oxygen species in both reactions, including the stages responsible for its mobility. A detailed analysis of the kinetics of isotope exchange has been performed, and the rates of one-atom (RI) and two-atom (RII) types of exchange were evaluated. The rate of the stage characterizing the mobility of surface oxygen was calculated, supposing the same two-step mechanism was relevant for both types of exchange. The effect of oxygen mobility on the kinetics of deN2O was estimated. An analysis of the possible pathways of isotope transfer from MeOx to CeOx showed that direct oxygen exchange on the Me-Ce interface makes a valuable contribution to the rate of this reaction. The principal role of the Me-Ce interface in deN2O was confirmed with independent experiments on FeOx/CeO2 samples with a different iron content.

Surface kinetics and bulk transport in La2Ni0.5Cu0.5O4+δ membranes from conductivity relaxation

This work reports conductivity relaxation measurements on both uncoated (1.2 mm thick) and coated (2.0 mm thick) La2Ni0.5Cu0.5O4+δ membranes in the temperature range between 550 and 850 °C and oxygen partial pressures from 0.01 to 1.0 atm. The results show that surface kinetics has a significant effect on the relaxation profiles, especially at low temperatures and should not be neglected when extracting transport parameters. Oxygen chemical diffusion and surface exchange coefficients have been determined by transient conductivity with surface modification. Higher activation energy of surface exchange compared to bulk diffusion is observed for La2Ni0.5Cu0.5O4+δ, similar to that for La2NiO4+δ. Based on the oxygen partial pressure dependence of the surface exchange coefficient, it has been revealed that oxygen dissociative adsorption rate-limits the surface exchange.

Catalytic oxidation of propane over Pt-Pd bimetallic nanoparticles supported on TiO2

The oxidation of propane to CO2 was performed by means of Pt-Pd nanoparticles supported on TiO2. The catalyst activities were evaluated through conversion versus temperature, where propane conversion profiles and reaction rates followed the order Pt-Pd/TiO2 > Pt/TiO2 > Pd/TiO2. It is proposed that the oxidation of propane on Pt-Pd/TiO2 catalysts occurs not only on Pt0 and Pd0 sites but also on Pt2+, Pd2+ and Ti3+ones, improving the oxidation pathway of propane as shown by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and X-ray Photoelectron Spectroscopy (XPS). Propane uptake and Lewis and Brönsted acidity type on the Pt-Pd/TiO2 catalyst were four times higher than on Pt/TiO2 and Pd/TiO2 as observed by FTIR-pyridine thermo-desorption. Ab-initio calculations showed that Pt enhanced the Brönsted acidity of the interface favoring a dissociative oxygen adsorption, whereas the bimetallic system presented more acidic Lewis sites which correlate with the catalytic activity. Then, hydrogen adsorption could be considered as a chemical reactivity descriptor for the oxidation of propane. Likewise, the higher activity of the Pt-Pd/TiO2 catalyst was explained by considering that the Pt-Pd catalyzed reduction of Ti4+ to Ti3+created oxygen vacancies that enhanced the mobility of lattice oxygen of the TiO2 support and its transfer to the propane species adsorbed on the Pt-Pd bimetallic nanoparticles.

New insights into the interfacial interactions of O2 and H2O molecules with PuH2 (110) and (111) surfaces from first-principles calculations

The interfacial reaction of highly active plutonium hydride in humid circumstance is of great interest in nuclear safe handling and storage, but it is poorly understood so far. In this paper, we first studied the O2 adsorption on (110) and (111) surfaces of PuH2 by first-principles DFT + U method. The results show that there are dissociative and non-dissociative adsorption of oxygen on the surfaces. We analyze the vibrational frequencies of non-dissociative oxygen adsorbed on the surfaces. It is found that the corresponding frequency of oxygen with bond length of 1.330–1.340 Å is 1094.8–1098.2 cm−1. The corresponding frequency of oxygen with bond length of 1.448–1.500 Å is 726.3–905.2 cm−1. It shows that non-dissociative oxygen could be considered as superoxide (O2−) or peroxide (O22−) species. In order to expound the atomistic evolution process of oxidized surface exposed to moist air or corrosive solution, the interactions between H2O molecules and the strongest oxygen adsorption structures were further explored. The results indicate that H2O molecules could dissociate into OH groups and H atoms, then they were captured to create Pu–O and H–O bonds. This work could provide new insights into the adsorption morphology of oxygen on hydride surface and the interaction between oxide/hydride interface and water.

Tuning oxygen vacancy of the Ba0·95La0·05FeO3−δ perovskite toward enhanced cathode activity for protonic ceramic fuel cells

Innovation of highly active cathode is of great significance to the development of protonic ceramic fuel cells (PCFCs). Herein, tailoring oxygen vacancies in Zn-doped Ba0·95La0·05FeO3−δ (BLFZ) perovskite is proved to be beneficial for promoting the formation of proton defects. Hydration ability of the triple conducting BLFZ perovskites is confirmed by electrical conductivity relaxation (ECR). The results demonstrate that BLFZ exhibits a proton surface exchange coefficient of 1.34 × 10−3 cm s−1 at 600 °C, which greatly extends active sites from the electrolyte/cathode interface to the entire electrode. Mechanism and process elementary steps of the oxygen reduction reaction (ORR) of BLFZ-BaCe0.7Zr0·1Y0.1Yb0.1O3−δ (BCZYYb) are detailedly studied. It is found that the rate-determining step of ORR is surface dissociative adsorption of oxygen on BLFZ-BCZYYb cathode. A maximum power density of 673 mW cm−2 at 700 °C is achieved and BLFZ-BCZYYb based single-cell shows no obvious degradation at 600 °C for 200 h. The good performance is ascribed to the rapid proton diffusion of BLFZ-BCZYYb composite electrode by regulating the oxygen vacancies.

Self-sustained oscillations in oxidation of methane over palladium: Experimental study and mathematical modeling.

An experimental study of the catalytic oxidation of methane over Pd foil in a flow reactor revealed that regular temporal oscillations in the reaction rate can arise at atmospheric pressure under methane-rich conditions. CO, CO2, H2, and H2O were detected as products. The oscillations of partial pressures of products and reactants in the gas phase were accompanied by oscillations of the catalyst temperature. According to an operando x-ray diffraction and mass-spectrometry study, the oscillations originate due to spontaneous oxidation and reduction of palladium; the high active catalyst surface is represented by metallic palladium, and the transition to the low-active state is accompanied by the formation of the PdO phase. In addition, it was detected that carbon dissolves in near-surface layers of palladium to form the PdCx phase. To describe the oscillations in the oxidation of methane, a 17-step reaction mechanism and a corresponding kinetic model were developed. The mechanism considers direct dissociative adsorption of methane and oxygen, pyrolytic activation of methane, oxidation and reduction of palladium, and direct formation and desorption of CO, CO2, H2, and H2O. Numerical solutions from the mathematical model of the continuously stirred-tank reactor qualitatively reproduce experimentally observed oscillatory dynamics. We have also developed a model, which considers the reversible diffusion of adsorbed oxygen and carbon atoms into the Pd bulk that allows us to explain the long induction period preceding the appearance of the oscillations. Mathematical modeling shows that the concentrations of dissolved oxygen and carbon atoms also oscillate under reaction conditions.

Comparison of nanostructured composite cathodes synthesized by liquid self-assembly and nanosolid mechanical-mixing for solid oxide fuel cell

The composite perovskites are promising low/intermediate-temperature solid oxide fuel cell (LT/IT-SOFC) cathode materials, which can combine the complementary advantages of multi-components. Here, novel liquid self-assembly (SA) is compared with nanosolid mechanical-mixing (MM) for the high-performance nanostructured composite. The same composition of the active BaCo0.96Zr0.04O2.6+δ (12hBC, 38 mol%) with the stable BaZr0.82Co0.18O3-δ (BZC, 62 mol%) is designed to compare the effects of different syntheses on composite properties. The BZC-12hBC (SA) can achieve molecular-level contact of multiple phases, which exhibits superior electrical conductivity, surface and heterointerface activity to those of BZC-12hBC (MM). When further applied on LT/IT-SOFC, the cell with BZC-12hBC (SA) cathode achieves the remarkable peak power densities of 2.06–0.24 W cm−2 at 700–500 °C, while the cell with BZC-12hBC (MM) cathode only exhibits 0.63–0.11 W cm−2. The electrochemical analysis reveals that BZC-12hBC (SA) cathode possesses better oxygen dissociative adsorption and species transportation than those of the BZC-12hBC (MM) cathode.

Increase in the density of Sr2Fe1.5Mo0.5O6-δ membranes through an excess of iron oxide: The effect of iron oxide on transport and kinetic parameters

Due to its widespread application in modern industry, the ongoing development of all the most important technological directions in the field of sensors, oxygen membranes, and reforming is undoubtedly a promising trend in world research and development (R&D). The search for modern highly oxygen-conducting materials used as oxygen separation membranes continues today. In this work, we investigated the following well-proven oxygen-conducting material, such as strontium ferrite-molybdate Sr2Fe1.5Mo0.5O6-δ. As an alternative for obtaining high-density ceramics, we used a co-synthesis method with a sintering additive (Fe2O3). We investigated the influence of sintering additive on the structural characteristics by certifying the phase composition by X-ray phase analysis; matrix continuity and volume distribution of Fe2O3 by secondary electron microscopy using energy dispersive X-Ray analysis; and elemental analysis of ceramic surface by X-ray photoelectron spectroscopy. The influence of Fe2O3 on the electrical conductivity was investigated using the four-probe DC method in the temperature range 100–800 °C in air. The kinetic dependences of 18O isotope-labeled oxygen on time were also obtained in the temperature range 600–800 °C and absolute oxygen pressure of 10−2 atm. The rate of heterogeneous oxygen exchange, the oxygen diffusion coefficient, and the corresponding rates of elementary processes of dissociative adsorption and incorporation of oxygen are calculated. Furthermore, it was revealed that the diffusion coefficient in the near-surface area significantly differs from the bulk one. As a consequence, this paper evaluates the observed effect and carefully analyzes the results obtained. All considered material parameters were analyzed and the regularities of the influence of surface composition on the rate-determining step of oxygen exchange were revealed. This research has made it possible to obtain dense ceramics at low temperatures and to identify a step that determines the rate of the oxygen exchange process. Finally, how strontium molybdate based thin layer on the surface and between the grains of the polycrystalline strontium ferrite-molybdates influence on the oxygen surface exchange and diffusivity is considered. The presence of such a phase in the form of a nanosized layer on the surface was convincingly proven by X-ray photoelectron spectroscopy and a grazing incidence X-Ray Diffraction method. Furthermore, the corresponding layer thickness, as well as the oxygen diffusion coefficients of the near-surface layer and the interface layer, were estimated experimentally and by modeling.

Reaction kinetics and mechanism of n-hexane catalytic combustion over Co-ZSM-5 zeolites

In the present study, the Co-ZSM-5 catalysts with different Si/Al ratios of 23, 40 and 100 were studied by tests on the combustion of n-hexane. It was observed that the Co-ZSM-5 catalyst with ratio of Si/Al = 23 present the best catalytic performance, which is mainly attributed to higher concentration of active surface Co3+ species. The obtained data from the reaction kinetics experiments and the applied kinetics modeling lead to the suggestion that n-hexane complete oxidation on all catalysts proceeded via Langmuir-Hinshelwood mechanism by dissociative adsorption of oxygen, the reacting hydrocarbon and oxygen being adsorbed on different active sites.

Influence of Oxygen Partial Pressure on Cathodic Polarization of LaNiO3/GDC Composite Cathode

LaNiO3/GDC composite is one of the potential cathodic materials for intermediate temperature SOFCs. In order to evaluate its electrochemical performance, influence of oxygen partial pressure on cathodic polarization is examined with an electrolyte supported symmetric cell and a frequency response analyzer. An electrolyte disk made of GDC is used for the electrochemical symmetric cell. The operating temperature was controlled from 773K to 1073K, and the partial pressure was controlled between −4 < log(pO2) atm < 0 in this study.Polarization resistance increased with decreasing oxygen partial pressure at any temperature. In addition, the characteristic frequency (the summit frequency) of the impedance arc decreased with decreasing oxygen partial pressure. In order to search for the reaction mechanism, the rate-determining step approximation is applied to the experimental results. As a result, a rate-determining step is not dissociative adsorption of oxygen, but apparent oxygen reduction (charge transfer) reaction at temperature in the range from 773K to 973K. However, activation energy of the interfacial conductivity which is a reciprocal of polarization resistance is lower than 100 kJ/mol. Therefore, the rate-determining step is presumably a surface diffusion step, which is a migration step of dissociative oxygen from active point on the LaNiO3 surface to double phase boundary. At 1073K and higher oxygen partial pressure condition, the rate-determination step is still apparent oxygen reduction (charge transfer) reaction. Whereas at 1073K and lower oxygen partial pressure condition, the rate-determining step may be dissociative adsorption of oxygen.

A comparative density functional theory study of oxygen doping versus adsorption on graphene to tune its band gap

Graphene, having a perfect two-dimensional crystal structure, has many excellent features such as a high specific surface area, and extraordinary electrical, thermal and mechanical properties. However, its usage in electronic devices is possible only if band gap of desired value is induced in this gapless semi-metal. Therefore, first principle calculations have been carried out to investigate the role of oxygen (O) doping versus adsorption and, the impurity concentration and coverage to induce band gap in graphene employing PBE at GGA level. The band gap is induced owing to production of vacancies, dissociative adsorption of oxygen, subsituational doping and pre-dissociated oxygen adsorption. It is interesting to note that band gap is introduced by both the processes of doping and adsorption of O. The oxygen doping leads to induction of two energy gaps, smaller in value above and larger below the Fermi level; while adsorption irrespective of adsorption configuration produces single direct gap. Increase both in concentration and coverage leads to enhance band gap value maximum being 1.85 eV in case of hexagonal doping of 12.5% concentration with an exception in adsorption case. The results allow us to conclude that adsorption is as useful as doping to tune the band gap in graphene enabling its applications in designing high performance electronic devices.

CO Oxidation Efficiency and Hysteresis Behavior over Mesoporous Pd/SiO2 Catalyst

Carbon monoxide (CO) oxidation is considered an important reaction in heterogeneous industrial catalysis and has been extensively studied. Pd supported on SiO2 aerogel catalysts exhibit good catalytic activity toward this reaction owing to their CO bond activation capability and thermal stability. Pd/SiO2 catalysts were investigated using carbon monoxide (CO) oxidation as a model reaction. The catalyst becomes active, and the conversion increases after the temperature reaches the ignition temperature (Tig). A normal hysteresis in carbon monoxide (CO) oxidation has been observed, where the catalysts continue to exhibit high catalytic activity (CO conversion remains at 100%) during the extinction even at temperatures lower than Tig. The catalyst was characterized using BET, TEM, XPS, TGA-DSC, and FTIR. In this work, the influence of pretreatment conditions and stability of the active sites on the catalytic activity and hysteresis is presented. The CO oxidation on the Pd/SiO2 catalyst has been attributed to the dissociative adsorption of molecular oxygen and the activation of the C-O bond, followed by diffusion of adsorbates at Tig to form CO2. Whereas, the hysteresis has been explained by the enhanced stability of the active site caused by thermal effects, pretreatment conditions, Pd-SiO2 support interaction, and PdO formation and decomposition.

Effect of grain boundaries in La0.84Sr0.16CoO3-δ on oxygen diffusivity and surface exchange kinetics.

The single crystal and polycrystalline specimens of La0.84Sr0.16CoO3-δ oxide were synthesized and characterized by X-ray powder diffraction analysis, energy dispersive X-ray microanalysis, the electron backscatter diffraction technique, and X-ray photoelectron spectroscopy. A thin slab was prepared from the grown single crystal with its surface corresponding to the (110) plane. The kinetics of the oxygen exchange between the gas phase and a single crystal and a polycrystalline specimen was studied by means of 16O/18O oxygen isotope exchange at T = 750-850 °C and PO2 = 5.3 × 10-3-2.2 × 10-2 atm. Temperature dependencies of the oxygen heterogeneous exchange rate, the oxygen dissociative adsorption and incorporation rates, and oxygen diffusion coefficients were obtained. The relationship between the crystallographic orientation of oxides and the kinetic parameters of oxides has been established. Correlations between the surface state and the rates of individual stages of oxygen exchange as well as oxygen diffusion pathways in the single crystal compared with those in the polycrystalline specimen are considered.

Effect of Elemental Substitution in (Ba,Sr)6RE2Co4O15−BaCe0.5Pr0.3Y0.2O3-δ for Cathodes of Proton-Conducting Ceramic Fuel Cells

Solid oxide fuel cells are promising energy conversion devices because of high efficiency and high fuel flexibility, but their high-operating temperature (700-900 ºC) causes problems such as fast degradation of components and long start-up/shut-down time. To overcome these problems, low- temperature operation is preferable. Proton-conducting ceramic fuel cells (PCFCs) are promising energy conversion devices for intermediate-temperature operation, especially below 600 ºC. This is because the activation energy of proton migration in proton-conducting oxides, such as rare earth-doped barium cerate, is lower than that of the oxide ion. At the current state, however, the performance of PCFCs is mainly limited by the cathode materials.Recently, we have reported that the composite of Ba4Sr2Sm2Co4O15 (BSSC4224)–BaCe0.5Pr0.3Y0.2O3 (BCPY) showed higher activity below 600 ºC in 3% humidified synthetic air, as compared with high-performance cathodes reported1). In this composite, BSSC4224 mainly served as an active site for the dissociative absorption of oxygen. In this study, then, the effect of the elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)6 RE 2Co4O15, on the ASR was studied. Symmetric cells consisting of the BaCe0.8Y0.2O3 (BCY) disk electrolyte and Ba5SrRE 2Co4O15 (RE =Nd, La, Eu, Sm, Pr, and Gd), Ba4Sr2Gd2Co4O15 or Ba5SrGd2Co4O15 (BSGC5124)–BCPY electrodes were fabricated. The electrode performance was measured in 3% humidified synthetic air (21% O2–79% N2) at 450–700 ºC by impedance spectroscopy.In impedance analysis, BSGC5124 had the lowest ASR below 550 ºC among Ba5SrRE 2Co4O15 electrodes. Then, the composite of BSGC5124–BCPY was prepared and its performance was compared with that of BSSC4224–BCPY composite. We found that BSGC5124–BCPY has comparable area-specific resistance and activation energy to BSSC4224–BCPY although BSGC5124 itself had much lower area-specific resistance and activation energy than BSSC4224. Therefore, we concluded that the dissociative adsorption of oxygen on the BSGC5124 phase is not the rate limiting step for the oxygen reduction reaction in this composite system. [1] T. Matsui, K. Manriki, K. Miyazaki, H. Muroyama, K. Eguchi, J. Mat er. Chem. A, 6, 14188 (2018).