Incorporation Of Oxygen Ions Research Articles

Local structure of an oxygen impurity and fluorine vacancy complex in LiYF4

Crystal-growth defects and impurities are among the key factors limiting the performance of fluorine-based optical materials. This study reports the local structure of an oxygen impurity centre in LiYF4 single-crystal. Experimental electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy investigations were combined with first principle theoretical modelling to elucidate the defect structure. It was determined that, after irradiation with X-rays, an electron spin S=1/2 paramagnetic centre is generated. It is stable at room temperature and anneals above 150°C. The alignment of the g-tensor and magnitudes of hyperfine interactions suggest that the centre is an oxygen impurity and fluorine vacancy complex. Highly anisotropic hyperfine interaction with three fluorine atoms within 3 Å of the oxygen defect is observed. The results provide novel insights into oxygen ion incorporation in LiYF4 and are crucial to understanding and minimising radiation-induced degradation of the material.

Validation of and Simulations using a High-Temperature Electrolysis Model

A continuum mechanistic, real-scale model with the most important reactions of high-temperature electrolysis is proposed. IV-characteristics and impedance spectra serve as comparison to experimental data for steam, CO2 and co-electrolysis. The exchange current-density i0 can be determined to the same order of magnitude as observed in experiments for steam electrolysis. The IV-characteristics for CO2 electrolysis are well described by an i0 one order of magnitude lower as compared to steam electrolysis. Parameter variations identify contributing variables determining the shapes of characteristics. Limitations in prediction are revealed for low current densities up to 0.5 A∙cm-2. EIS calculations demonstrate the dominant behavior of charge transfer or oxygen ion incorporation into the lattice as function of i0 or temperature. The model forms a solid basis for various future investigations like a more detailed reaction system with possible intermediates or degradation and poisoning effects.

Highly active cobalt-free perovskites with Bi doping as bifunctional oxygen electrodes for solid oxide cells

Cobalt-containing perovskite materials have been intensively investigated as bifunctional oxygen electrodes for solid oxide cells because of their mixed ionic-electronic conducting nature. However, the direct application of these materials to the most widely used zirconia-based electrolytes has been limited because of their thermo-chemical incompatibility. Herein, we introduce a cobalt-free manganite-perovskite with significantly improved electrocatalytic activity by doping with Bi which possesses stereochemically active lone pair electrons. Replacing Bi with Pr in Pr0.8Sr0.2MnO3−δ increases the oxygen vacancy concentration and promotes oxygen ion incorporation and charge transfer processes at the electrode. Consequently, we apply Pr0.5Bi0.3Sr0.2MnO3−δ, to the interlayer-free yttria-stabilized zirconia electrolyte-based cells, which exhibits record high performance in the fuel cell (1.91 W∙cm−2) as well as electrolysis (1.90 A∙cm−2) modes at 700 °C along with remarkable long-term stability (450 h). Thus, our findings demonstrate that Pr0.5Bi0.3Sr0.2MnO3−δ is a highly promising bifunctional catalyst for solid-state energy storage and conversion applications.

Interrelationship between Extended Transport Pathways of Mixed Conducting Electrode and Delamination in Solid Oxide Electrolyzer Cells

Generating electricity using renewable energy such as solar and wind rather than fossil fuels like coal and natural gas reduces CO2 emissions and addresses climate change. However, due to the intermittency nature of renewables, highly efficient and massive scale energy storage method is the major enabler for its integration into the grid. Solid oxide electrolyzer cell (SOECs) (1), which is an electrochemical device to convert electricity into chemicals (such as hydrogen, syngas, ammonia or even methane, ethane, and ethylene), have received widespread attention. There have been tremendous improvements of the technology during the past 10 to 15 years. On the other hand, to drive the technology for commercialization, durability for long-term operation is still facing significant challenges (2). One degradation mechanism that has been widely reported is the delamination of the oxygen electrode/electrolyte interface. Pure ionic conductors such as 8 mol% Y2O3 doped ZrO2 (YSZ) and Gadolinium-doped ceria (GDC) are usually used as the dense electrolyte; mixed electronic and ionic conductors (MIEC) such as La1-xSrxMnO3-d(LSM), La1-xSrxCo1-yFeyO3-d (LSCF), Sm0.5Sr0.5CoO3 (SSC) are widely used as the oxygen electrode (3).The most popular delamination mechanism (2, 4) is attributed to the buildup of oxygen partial pressure at the LSM/YSZ interface and growth of microcracks caused by exceeding the fracture strength of the electrolyte. Another interesting explanation (5) from atomic level is that under high temperature the inter-diffusion of different cations across the LSM/YSZ interface significantly affect oxygen transport which develops pressure buildup and delamination. A mechanism (6) from a different angle has been proposed by analyzing SEM images of the LSM/YSZ interface at different stages of polarization. It is shown that the formation of nanoparticles caused by the migration or incorporation of oxygen ions from the electrolyte into the electrode, leading to the shrinkage of LSM lattice.Meanwhile, there have been modeling and experimental investigations of extended charge transport pathways from triple phase boundary (3PB) to two phase boundary (2TP) (3, 7) in MIEC electrodes, such as LSM and LSCF, as well as the multi-step charge transfer reaction (8-10) at gas/MIEC interface. It has been demonstrated that there is a transition voltage between two different transport pathways, and it can cause significant oxygen vacancy concentration variation at the electrode/electrolyte interface. In this work, we explore the correlation between the transition of transport pathways and the delamination phenomenon. We assume that large oxygen vacancy concentration variation will cause chemical-expansion-mismatch at the electrode/electrolyte interface, which leads to delamination of oxygen electrode from the electrolyte.Figure 1 shows a schematic of model domain constructed with details at the interface. A bilayer oxygen electrode structure consists of a commercial LSCF-GDC porous backbone coated by a thin SrCo0.9Ta0.1O3- d (SCT10) film. The new oxygen electrode has an inherently fast oxygen evolution reaction (OER) electrokinetics (or high oxygen evolution rate) and is capable of mitigating the delamination problem by minimizing the accumulation of oxygen in OER electrode lattice and thus chemical stresses. To understand the fundamental Electro-Chemical-Mechanical coupling mechanism and its contribution to SOEC performance degradation, we develop a rigorous Multiphysics model to explore the interplays between reaction rates and transport for two different pathways in delamination probability of the pristine and bilayer OER electrodes. The work will lead to a better understanding of the delamination mechanisms and effective mitigation plan and provide guidelines for the experimental optimization of bilayer OER electrode processing with improved electrochemical and mechanical performance. References A. Hauch, R. Küngas, P. Blennow, A. B. Hansen, J. B. Hansen, B. V. Mathiesen and M. B. Mogensen, Science, 2020, 370, eaba6118.B.-K. Park, Q. Zhang, P. W. Voorhees and S. A. Barnett, Energ Environ Sci, 2019, 12, 3053-3062.Y. Li, R. Gemmen and X. Liu, J Power Sources, 2010, 195, 3345-3358.A. V. Virkar, Int J Hydrogen Energ, 2010, 35, 9527-9543.S. N. Rashkeev and M. V. Glazoff, Int J Hydrogen Energ, 2012, 37, 1280-1291.K. Chen and S. P. Jiang, Int J Hydrogen Energ, 2011, 36, 10541-10549.M. Gong, R. S. Gemmen and X. Liu, J Power Sources, 2012, 201, 204-218.J. Fleig, Phys Chem Chem Phys, 2005, 7, 2027-2037.D. S. Mebane and M. Liu, J Solid State Electr, 2006, 10, 575-580.D. S. Mebane, Y. Liu and M. Liu, J Electrochem Soc, 2007, 154, A421. Figure 1

Effect of Zn/S non-stoichiometric ratio on the structural, optical and electronic properties of nano-ZnS

Non-stoichiometric ZnS nanomaterials were prepared using a thermolysis procedure by decreasing the stoichiometric amount of thiourea relative to the amount of zinc acetate as starting precursors: Zn(Ac)/(1−x) thiourea (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5). The high-resolution transmission electron microscope revealed the nanonature of the obtained samples. The X-ray diffraction analysis applying Rietveld method was carried out to explore the influence of Zn/S non-stoichiometry on the structural and microstructural properties of the crystalline phases in the samples. ZnO phase appeared for x ≥ 0.2 forming ZnS1–x/ZnO heterostructures; its percentage increased with increasing non-stoichiometric parameter (x). Incorporation of oxygen ions into the ZnS lattice compensating the sulfur deficiency was manifested by Fourier transform infrared spectroscopy. The UV–Vis analysis revealed the decrease in the bandgap of ZnS from 3.42 eV for x = 0.0 to 2.71 eV for x = 0.5, making these materials suitable for new applications. The influence of non-stoichiometric parameter (x) on the photoluminescence emissions of formed samples was examined; results indicated the spectrum shift towards higher wavelengths. Density function calculations were performed in order to compare the electronic and optical properties of sulfur-deficient ZnS0.9 (single phase) with the ZnS sample.

Solid oxide fuel cells using Sm0.075Nd0.075Ce0.85O2−δ electrolyte coupled with an electron-blocking interlayer

We applied Ba-containing anode (Ni-BaZr0.1Ce0.7Y0.2O3-δ (NiO-BZCY)) to the Nd doped Sm0.15Ce0.85O2−δ(SNDC) electrolyte layer, aiming to solve the electron-leakage problem for CeO2-based electrolyte as well as to use the high conductivity of SNDC and the good interfacial oxygen incorporation. The NiO-BZCY anode not only acts as a catalysis for anode reactions but also offers barium source to form an electron-blocking layer which contains Ba for single cells based on SNDC electrolyte. The high conductivity of SNDC allows a reduction of electrolyte resistance and also provides a better interfacial oxygen-ion incorporation ability compared with the traditional Sm-doped CeO2 (SDC), facilitating the interfacial reactions. As a result, the open circuit voltage and the maximum power density of the cell using SNDC electrolyte exhibits are 1.03 V and 1024 mW/cm2 at 650 °C, which is much higher than that for traditional SDC-based cell even using the same barium-diffusion strategy, which is only 610 mW/cm2 at 650 °C.

Structural and optical characteristics investigations in oxygen ion implanted GaN epitaxial layers

This paper presents the lattice disorder-induced effect by oxygen ion implantation in GaN epitaxial layers. Oxygen ion implantation in GaN epitaxial layers is done with fluencies 5 × 1014 cm−2 and 5 × 1015 cm−2 at 120 keV energy on two different pieces of a GaN sample. Unimplanted and implanted GaN layers are investigated by High-resolution X-ray diffraction (HRXRD), Optical absorption and Raman spectroscopy. The in-plane and out of plane lattice parameters (i.e. a and c) of the distorted and undistorted region in the GaN samples are determined from the respective XRD peak positions. Due to the incorporation of oxygen ions in GaN layers up to implantation depth, lattice parameters are extended, as a result, the samples are under hydrostatic strain. Raman spectroscopic studies on the unimplanted and oxygen ion-implanted GaN samples are done by green and ultra-violet (UV) lasers. Raman spectroscopy results indicate that the broadening of the E2 (High) peak and intensity of A1 (LO) peak increases with increasing fluence. A doublet in A1 (LO) peak is found in UV excited Raman spectra in unimplanted and implanted GaN samples. The occurrence of the A1 (LO) doublet is due to surface defects and phonon-plasmon coupled-mode generated due to nonequilibrium photoexcited carriers. In this article, the origin of doublet has been studied and reported.

Performance of La0.5Sr1.5MnO4±δ Ruddlesden-Popper manganite as electrode material for symmetrical solid oxide fuel cells. Part A. The oxygen reduction reaction

The electrochemical performance of La0.5Sr1.5MnO4±δ (L5S15 M) as SOFC cathode was studied using Electrochemical Impedance Spectroscopy (EIS) measurements. The influence of the sintering temperature on the electrochemical performance was examined in air for Au/L5S15 M/GDC/YSZ/GDC/L5S15 M/Au symmetrical cell. For the oxygen reduction reaction, the electron transfer between the electrode and oxygen, and the incorporation of oxygen ions into the electrode are the main controlling processes of oxygen reduction reaction for electrodes sintered at 1150 and 1200 °C. Increasing the sintering temperature up to 1250 °C increases the Sr diffusion through GDC interlayer, which leads to Sr accumulation at the GDC/YSZ interface with SrZrO3 formation and subsequent cell performance degradation. The lowest area specific resistance (ASR) values was obtained for the L5S15 M electrode sintered at 1150 °C.

Nanoscale oxygen ion dynamics in SrFeO2.5+δ epitaxial thin films

A variety of functional properties in transition metal oxides are often underpinned by oxygen vacancies. While the oxygen vacancy concentration and arrangements are well-known to have strong influence on the physical properties of oxides, the oxygen dynamics in oxides—including oxygen ion incorporation and movements during redox reactions—remain elusive. Performing conductive AFM studies of epitaxial thin films of oxygen-deficient SrFeO2.5 treated by air-annealing at various temperatures, we observe oxidation-induced enhancement of local electronic conduction on the higher terraces near the outer step edges at which oxygen ions are preferably incorporated and diffuse into the films. We also show that the local conduction can be reversibly controlled by electric-field-induced redox reactions at room temperature. These results highlight the importance of the nanoscale oxygen dynamics in redox reactions in SrFeO2.5 films.

High performance zirconia-bismuth oxide nanocomposite electrolytes for lower temperature solid oxide fuel cells

We develop a novel nanocomposite electrolyte, consisting of yttria-stabilized zirconia (YSZ) and erbia-stabilized bismuth oxide (ESB). The 20 mol% ESB-incorporated YSZ composite (20ESB-YSZ) achieves the high density (>97%) at the low sintering temperature of 800 °C. The microstructural analysis of 20ESB-YSZ reveals the characteristic nanocomposite structure of the highly percolated ESB phase at the YSZ grain boundaries (a few ∼ nm thick). The ionic conductivity of 20ESB-YSZ is increased by 5 times compared to that of the conventional YSZ due to the fast oxygen ion transport along the ESB phase. Moreover, this high conductivity is maintained up to 580 h, indicating high stability of the ESB-YSZ nanocomposite. In addition, the oxygen reduction reaction at the composite electrolyte/cathode interface is effectively enhanced (∼70%) at the temperature below 650 °C, mainly due to the fast dissociative oxygen adsorption on the ESB surface as well as the rapid oxygen ion incorporation into the ESB lattice. Thus, we believe this ESB-YSZ nanocomposite is a promising electrolyte for high performance solid oxide fuel cells at reduced temperatures.

Correlation of Local Structure and Diffusion Pathways in the Modulated Anisotropic Oxide Ion Conductor CeNbO(4.25).

CeNbO4.25 is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long-range commensurately modulated structural motif. Here we show that CeNbO4.25 forms with a unit cell ∼12 times larger than the stoichiometric tetragonal parent phase of CeNbO4 as a result of the helical ordering of Ce(3+) and Ce(4+) ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species, creating interlayer "NbO6" connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intralayer; however, when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.

Modeling Water Reduction on Doped Ceria Thin Films

In recent years, there has been an increasing interest in understanding electrochemical reactions on oxide surfaces. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better understanding of aspects such as the nature of the surface1-2 and the species involved in the reactions3-4. However, a quantitative connection between macroscopic rates and the surface structure and dynamics remains elusive. Among these reactions, water electrolysis has attracted interest due to favorable kinetics when compared to low temperature electrolysis5. Doped ceria, a mixed ionic and electronic conductor, is a potential candidate for this reaction due to higher stability and more efficient use of the electrode area compared to currently available materials, such as nickel/yttria-stabilized zirconia cermet6. Recent spectroscopic studies of water reduction/oxidation on doped ceria have shed valuable new insights about this reaction, but uncertainties remain in the interpretation of these measurements (in terms of a rate-determining step) and the connection to macroscopic rates3,4. In order to better connect spectroscopic observations to macroscopic rates, we have developed a thermodynamically self-consistent elementary kinetic model for H2/H2O exchange on the surface of ceria. This model considers both the bulk defect thermodynamics, as well as the effects of cation reconstruction/segregation at the surface and space charge effects, both of which are expected to alter the defect chemistry and concentrations of reactive intermediates. We have used this model to explore a variety of mechanisms and rate-limiting steps proposed in the literature3,4, and to predict the expected nonlinear electrochemical rate law. This includes prediction of the nonlinear electrochemical impedance (NLEIS) response of thin film ceria electrodes, in an approach similar to our previous studies of O2 reduction kinetics on perovskite thin films8. References Mueller, D. N.; Machala, M. L.; Bluhm, H; Chueh, W. C. Redox activity of surface oxygen anions in oxygen-deficient perovskite oxides during electrochemical reactions. Nature Comm. 2015, 6, 6097Chueh, W.C.; McDaniel, A. H.; Grass, M. E.; Hao, Y; Jabeen, N; Liu, Z; Haile, S. M.; McCarthy, K. F.; Bluhm, H; El Gabaly, F. Highly Enhanced Concentration and Stability of Reactive Ce3+ on Doped CeO2 Surface Revealed in Operando. Chem. Mater. 2012, 24, 1876-1882Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013 , 135, 11572-15579Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014 , 5, 4374Brisse, A; Chefold, J; Zahid, M. High Temperature Water Electrolysis in Solid Oxide Cells. International Journal of Hydrogen Energy. 2008, 33, 5375-5382Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402Wilson, J.R.; Sase, M; Kawada, T; Adler, S. B. Measurement of Oxygen Exchange Kinetics on Thin-Film La0.6Sr0.4CoO3-δ Using Nonlinear Electrochemical Impedance Spectroscopy. Electrochemical and Solid State Letters. 2007, 10, B81-B86

Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes

Currently, the most widely considered material for solid oxide fuel and electrolysis cells is the nickel/yttria-stabilized zirconia (Ni/YSZ) cermet1. However, issues such as low resistance to poisoning, coking, and changes in the microstructure have pushed for the use of new materials. Mixed ionic conductors2 alleviate the shortcomings of Ni/YSZ and can achieve more efficient use of the electrode material, since a three-phase boundary is not required for reaction. Among these materials, doped ceria has shown promise, yet little is known regarding the reaction mechanism for water electrolysis. The advent of techniques such as ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) has helped to provide a better picture of the surface, as well as the species involved in the reaction. Unfortunately, there is still controversy regarding the nature of the rate-determining step3,4. In addition, most work has made use of electrochemical impedance spectroscope (EIS), a linear technique that cannot differentiate among rate determining scenarios. The use of nonlinear techniques such as non-linear EIS (NLEIS) has been shown to be a powerful tool that is sensitive to different reaction mechanisms5. In order to elucidate these rate limiting phenomena, porous 10% doped gadolinia-doped ceria /YSZ electrochemical cells were analyzed at 750 and 800 °C under different H2-H2O gas environments using both EIS and NLEIS. The experimental results were then analyzed using a macro-homogeneous model6 involving oxygen vacancies and the chemical potential of electrons7. Multiple modeling scenarios were considered including one-dimensional and multidimensional transport, as well as the presence or absence of surface diffusion of hydroxyl species in the surface. References Nakamura, T; Kobayashi, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Mizusaki, J; Kawada, T. Electrochemical Behaviors of Mixed Conducting Oxide Anodes for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B563-B569Nakamura, T; Yashiro, K; Kaimai, A; Otake, T; Sato, K; Kawada, T; Mizusaki, J. Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell. J Electrochem Soc. 2008, 155, B1244-B1250Zhang, C; Yu, Y; Grass, M. E; Dejoie, C; Ding, W; Gaskell, K; Jabeen, N; Hong, Y. P; Shavorskiy, A; Bluhm, H; Li, W. X; Jackson, G. S; Hussain, Z; Liu, Z; Eichhom, B. W. Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells. J. Am. Chem. Soc. 2013 , 135, 11572-15579Feng, Z. A; El Gabaly, F; Ye, X; Shen, Z. X; Chueh, W. C. Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface. Nature Comm. 2014 , 5, 4374Wilson, J. R; Schwartz, D. T; Adler, S. B. Nonlinear electrochemical impedance spectroscopy for solid oxide fuel cell cathode materials. Electrochimica Acta. 2006, 51, 1389-1402Adler, S. B; Lane, J. A; Steele, B. C. H. Electrode Kinetics of Porous Mixed-Conducting Oxygen Electrodes. J. Electrochem Soc. 1996, 143, 3554-3564Wang, S; Kobayashi, T; Dokiya, M; Hashimoto, T. Electronic and Ionic Conductivity of Gd-Doped Ceria. J Electrochem Soc. 2000, 147, 3606-3609

Investigation of humidity-dependent size control of local anodic oxidation on graphene by using atomic force microscopy

We demonstrate nanoscale local anodic oxidation (LAO) patterning on few-layer graphene by using an atomic force microscope (AFM) at room temperature under a normal atmosphere. We focus on the humidity dependency of nanoscale oxidation of graphene. The relations between the oxidation size and the setting values of the AFM, such as the set point, tip speed, and the humidity, are observed. By changing these values, proper parameters were found to produce features with on-demand size. This technique provides an easy way for graphene oxide lithography without any chemical resists. We obtained oxidation sizes down to 50 nm with a 6-nm-high oxide barrier line by using a 0.1-μm/s tip scanning speed. We also obtained micrometer-sized symbols on a graphene flake. We attribute the bumps of oxidized graphene in the graphene layer to local anodic oxidation on graphenes surface and to an incorporation of oxygen ions into the graphene lattice.

Fast vacancy-mediated oxygen ion incorporation across the ceria-gas electrochemical interface.

Electrochemical incorporation reactions are ubiquitous in energy storage and conversion devices based on mixed ionic and electronic conductors, such as lithium-ion batteries, solid-oxide fuel cells and water-splitting membranes. The two-way traffic of ions and electrons across the electrochemical interface, coupled with the bulk transport of mass and charge, has been challenging to understand. Here we report an investigation of the oxygen-ion incorporation pathway in CeO2-δ (ceria), one of the most recognized oxygen-deficient compounds, during hydrogen oxidation and water splitting. We probe the response of surface oxygen vacancies, electrons and adsorbates to the electrochemical polarization at the ceria-gas interface. We show that surface oxygen-ion transfer, mediated by oxygen vacancies, is fast. Furthermore, we infer that the electron transfer between cerium cations and hydroxyl ions is the rate-determining step. Our in operando observations reveal the precise roles of surface oxygen vacancy and electron defects in determining the rate of surface incorporation reactions.

Electrochemical characteristics of nano-structured PrBaCo 2O 5+ x cathodes fabricated with ion impregnation process

Nano-structured electro-catalyst of layered-structure cobaltite PrBaCo 2O 5+ x (PBC) has been developed as cathode for solid oxide fuel cells (SOFCs) and excellent electrochemical activity towards oxygen reduction has been achieved. PBC nano-particles are deposited into porous samaria-doped ceria (SDC) backbones with an impregnation method. The fabrication processing parameters including composition of precursor solution, PBC loading, and firing temperature have been investigated to optimize the cathode microstructure and further to minimize the cathode interfacial polarization resistance, leading to a cathode interfacial polarization resistance of only 0.082 Ω cm 2 at 600 °C, much lower than those reported for pure PBC electrode (0.86 Ω cm 2), PBC–SDC composite cathode (0.25 Ω cm 2) or various other impregnated cobaltite cathodes such as Sm 0.5Sr 0.5CoO 3− δ (0.25 Ω cm 2), La 0.5Sr 0.5CoO 3− δ (0.31 Ω cm 2), and La 0.6Sr 0.4Co 0.2Fe 0.8O 3− δ (0.24 Ω cm 2). The novel nano-structured PBC electrochemical reaction mechanism has found to be similar to that of a conventional PBC cathode. However, both oxygen ion incorporation and charge transfer steps are greatly accelerated for the novel nano-structured PBC cathode, suggesting that the impregnation process is very effective in fabricating layered-structure cobaltite electro-catalyst for intermediate-temperature SOFC cathode with enhanced electrode performance.

Failure mechanism of (La,Sr)MnO 3 oxygen electrodes of solid oxide electrolysis cells

The delamination behavior of La 0.8Sr 0.2MnO 3 (LSM) oxygen electrode of solid oxide electrolysis cell (SOEC) is studied in detail under anodic current passage of 500 mA cm −2 and 800 °C. The delamination or failure of LSM oxygen electrode is observed after the current passage treatment for 48 h, and is accompanied by the significant increase in the electrode polarization and ohmic resistances. The delaminated electrode and electrolyte interface is characterized by the formation of nanoparticles within LSM contact rings on the electrolyte surface. SEM analysis of the interface at different stages of the polarization indicates that the formation of these nanoparticles is caused by the localized disintegration of the LSM grains at the electrode/electrolyte interface. The formation of nanoparticles is most likely due to the migration or incorporation of oxygen ions from the YSZ electrolyte into the LSM grain, leading to the shrinkage of LSM lattice. The shrinkage of the LSM lattice will create local tensile strains, resulting in the microcrack and subsequent formation of nanoparticles within LSM particles at the electrode/electrolyte interface. The formation of nanoparticle clusters weakens the anode/electrolyte interface, eventually leading to the delamination and failure of the LSM oxygen electrode under high internal partial pressure of oxygen at the interface.

Electrochemical characteristics of samaria-doped ceria infiltrated strontium-doped LaMnO3 cathodes with varied thickness for yttria-stabilized zirconia electrolytes

Samaria-doped ceria (SDC) infiltrated into strontium-doped LaMnO3 (LSM) cathodes with varied cathode thickness on yttria-stabilized zirconia (YSZ) were investigated via symmetrical cell, half cell, and full cell configurations. The results of the symmetrical cells showed that the interfacial polarization resistance (RP) decreased with increasing electrode thickness up to ∼30μm, and further increases in the thickness of the cathode did not cause significant variation of electrode performance. At 800°C, the minimum RP was around 0.05Ωcm2. The impedance spectra indicated that three main electrochemical processes existed, possibly corresponding to the oxygen ion incorporation, surface diffusion of oxygen species and oxygen adsorption and dissociation. The DC polarization on the half cells and characterization of the full cells also demonstrated a similar correlation between the electrode performance and the electrode thickness. The peak power densities of the single cells with the 10, 30, and 50-μm thick electrodes were 0.63, 1.16 and 1.11Wcm−2, respectively. The exchange current densities under moderate polarization are calculated and possible rate-determining steps are discussed.

Identification of O2 reduction processes at yttria stabilized zirconia|doped lanthanum manganite interface

Abstract Oxygen reduction processes at a yttria stabilized zirconia|doped lanthanum manganite (LSM|YSZ) interface are investigated by ac impedance spectroscopy. Three semi-arcs are clearly observed at an oxygen partial pressure ( p O 2 ) of 0.001 atm. The constant-phase element of frequency arc and the p O 2 -dependence of 1/ R ( R : resistance) are correlated to interpret the reaction processes associated with the frequency arcs. It is found that at least five elementary steps are involved in oxygen reduction at the LSM|YSZ interface. Three of these steps are identified as the rate-determining steps, namely: (i) gas diffusion through the porous LSM electrode from the bulk to the reaction sites; (ii) surface diffusion of oxygen intermediate species along the LSM surface; (iii) incorporation of oxygen ions from the three phase boundary into the YSZ electrolyte lattice. The p O 2 -dependent exchange current density of the rate-determining step predicted by model is consistent with the experimental data, which further confirms the validity of the proposed oxygen reduction processes at the LSM|YSZ interface.

Effects of ion energy and arrival rate on the composition of zirconium oxide films prepared by ion-beam assisted deposition

Thin zirconium oxide films were grown using the ion-beam assisted deposition method. Zirconium metal was evaporated by an electron beam and condensed on a Si substrate, while oxygen ions were directed simultaneously onto the substrate, allowing the fundamental deposition parameters of ion energy and arrival rate ratio ARR(O/Zr) to be measured and controlled easily. X-ray photoelectron spectroscopy (XPS) was used to study the oxidation and the composition of the films. XPS analyses indicated the presence of four oxidation states of zirconium (Zr4+−Zr1+) in Zr 3d spectra and two peaks in O 1s spectra; Zr4+ is a predominant ion in all the films and the two peaks in O 1s spectra are related to the oxide and to hydroxyl groups and/or carbonates, respectively. Composition analyses of the films suggested that these oxygen-associated species may be bound to zirconium. The variation of composition as a function of ion energy (from 2 to 20 keV) and ARR(O/Zr) (at 0.54 and 1.09) could be explained with the preferential sputtering of zirconium from the growing film by incoming oxygen ions and the incorporation of oxygen ions into the film.