Lattice Oxide Ions Research Articles

Elucidation of the origin of voltage hysteresis in xLi2MnO3∙(1−x)LiCoO2 using backstitch charge-discharge method

Lithium-excess (LEX) materials with a composition of xLi2MnO3∙(1−x)LiMO2 (M = Fe, Co, or Ni) exhibit large reversible capacities and are therefore promising positive electrode materials for Li-ion batteries. However, the widespread application of these materials is hindered by their large voltage hysteresis, cycling-induced voltage decay, and long initial-cycle voltage plateau. Herein, Co-LEX materials with the composition of xLi2MnO3∙(1−x)LiCoO2 are synthesized and electrochemically characterized using the backstitch charge-discharge method to elucidate the origin of voltage hysteresis. A trade-off relationship between reversible capacity and energy efficiency is observed, and both reversible capacity and voltage hysteresis extent increase with increasing Mn content. Quantitative analysis of voltage hysteresis by the backstitch charge-discharge method reveals that the energy loss per cycle due to the difference in reversible potentials during charge and discharge is proportional to oxide redox reaction-derived capacity, which indicates that voltage hysteresis is caused by the solid-phase redox reactions of lattice oxide ions. Thus, electrode potentials of solid-state oxide redox reactions are necessary to be matched in order to achieve both high reversible capacity and low voltage hysteresis for LEX materials.

Contrasting anion disorder behavior in Sm2Zr2O7 by simultaneous aliovalent cation substitutions and its structural and electrical properties

In the present study, the effect of simultaneous aliovalent cation substitutions on the B site of the pyrochlore type compositions has been studied to understand the anion disorder and its influence on the electrical properties. In this regard, the Sm2Zr2 – x(YNb)x/2O7 (x = 0, 0.25, 0.5, 0.75, 1) compositions were prepared via solid-state reaction route, and the structural and electrical properties of the prepared compositions were analyzed using advanced techniques. The structural analysis studies reveal that the concentration of aliovalent cation substitution has a distinct effect on the structure; the lower concentration of Y and Nb (x ≤ 0.25) destabilizes the system, lowering the distortion, and at higher concentrations (x ≥ 0.5), it stabilizes the system. As a consequence, the distortion of BO6 increases at higher substitutions; thus, the lattice strain of the system increases, manifesting the higher anion disorder in the system. The degree of the anion disorder is further evaluated by the intensity ratio of the Raman intense Eg mode and the T2g mode around 800 cm−1 from the Raman spectrum whose trend is in line with the oxygen x-parameter variation with different concentrations of the aliovalent cation substitution. The electrical properties follow the same trend of the anion disorder and the lattice strain in the system. The maximum conductivity of 4.44 × 10−4 S/cm is obtained for the composition (x = 1) having the maximum anion disorder and lattice strain. The present study demonstrates the interrelationship among the lattice strain, anion disorder, and oxide ion conductivity, which will be helpful in designing and developing new oxide ionic conductors.

The Role of Oxygen Release from Li- and Mn-Rich Layered Oxides during the First Cycles Investigated by On-Line Electrochemical Mass Spectrometry

In the present work, the extent and the role of oxygen release during the first charge of lithium- and manganese-rich Li1.17[Ni0.22Co0.12Mn0.66]0.83O2 (also referred to as HE-NCM) was investigated with on-line electrochemical mass spectrometry (OEMS). HE-NCM shows a unique voltage plateau at around 4.5 V in the first charge, which is often attributed to a decomposition reaction of the Li-rich component Li2MnO3. For this so-called “activation”, it has been hypothesized that the electrochemically inactive Li2MnO3 would convert into MnO2 while lattice oxide ions are oxidized and released as O2 (or even CO2) from the host structure. However, qualitative and quantitative examination of the O2 and CO2 evolution during the first charge shows that the onset of both reactions is above the 4.5 V voltage plateau and that the amount of released oxygen is an order of magnitude too low to be consistent with the commonly assumed Li2MnO3 activation. Instead, the amount of released oxygen can be correlated to a structural rearrangement of the active material which occurs at the end of the first charge. In this process, oxygen depletion from the HE-NCM host structure leads to the formation of a spinel-like phase. This phase transformation is restricted to the near-surface region of the HE-NCM particles due to the poor mobility of oxide ions within the bulk. From the evolved amount of O2 and CO2, the thickness of the spinel-like surface layer was estimated to be on the order of ≈2–3 nm, in excellent agreement with previously reported (S)TEM data.

Eliminating Evolved Oxygen through an Electro-flocculation Efficiently Prompts Stability and Catalytic Kinetics of an IrOx·nH2O Colloidal Nanostructured Electrode for Water Oxidation

A facile approach which harnesses dual-potential pulsed amperometric technique to expedite the water oxidation and oxygen reduction reactions intermittently through an electro-flocculation is reported. The new strategy yields discriminated physical morphology and chemical composition of as-prepared IrOx·nH2O, compared with the previous approach, which lead to an alteration of kinetic mechanism and enhancement of stability in alkaline solution (pH 12). By the virtue of the catalyst architecture efficiently producing electroactive Ir sites (Γea=∼4 × 10−8molmol/cm2), η for water oxidation was measured to be 0.22V (at current density of 1.2mA/cm2). The tafel slope was estimated to be 29±1mV per decade, suggesting the recombination of S−Oads as the rate determining step. Contrast to the considerably flocculated structure reported previously, the IrOx·nH2O construction herein was well-defined sphere-like clusters (5-10nm) and resides separately. Furthermore a XPS examination revealed that the oxygen-containing constituents of the prepared IrOx, regardless of preparation techniques, were hydroxides (dominate) and hydrated water, instead of the lattice oxide ions. IrOx·nH2O formed using the present strategy yielded O-to-Ir ratio to be 2.9, which was substantially lower than 6.4 of a hydroxide-abundant constituent prepared without intermittent elimination of oxygen.

Understanding the influence of surface chemical states on the dielectric tunability of sputtered Ba0.5Sr0.5TiO3 thin films

The surface chemical states of RF-magnetron sputtered Ba0.5Sr0.5TiO3 (BST5) thin films deposited at different oxygen mixing percentage (OMP) was examined by x-ray photoelectron spectroscopy. The O1s XPS spectra indicate the existence of three kinds of oxygen species (dissociated oxygen ion O2−, adsorbed oxide ion O− and lattice oxide ion O2−) on the films’ surface, which strongly depends on OMP. The presence of oxygen species other than lattice oxygen ion makes the films’ surface highly reactivity to atmospheric gases, resulting in the formation of undesired surface layers. The XPS results confirm the formation of surface nitrates for the films deposited under oxygen deficient atmosphere (OMP ≦̸ 25%), whereas the films deposited in oxygen rich atmosphere (OMP ≧̸ 75%) show the presence of metal-hydroxide. The influence of a surface dead layer on the tunable dielectric properties of BST5 films have been studied in detail and are reported. Furthermore, our observations indicate that an optimum ratio of Ar:O2 is essential for achieving desired material and dielectric properties in BST5 thin films. The films deposited at 50% OMP have the highest dielectric tunability of ~65% (@280 kV cm−1), with good ϵr-E curve symmetry of 98% and low tan δ of 0.018. The figure of merit for these films is about 35, which is promising for frequency agile device applications.

Hole conductivity in oxygen-excess BaTi1−xCaxO3−x+δ

BaTiO3 containing Ca substituted for Ti as an acceptor dopant, with oxygen vacancies for charge compensation and processed in air, is a p-type semiconductor. The hole conductivity is attributed to uptake of a small amount of oxygen which ionises by means of electron transfer from lattice oxide ions, generating O(-) ions as the source of p-type semiconductivity. Samples heated in high pressure O2, up to 80 atm, absorb up to twice the amount expected from the oxygen vacancy concentration. This is attributed to incorporation of superoxide, O2(-), ions in oxygen vacancies associated with the Ca(2+) dopant and is supported by Raman spectroscopy results.

Role of Bond Strength on the Lattice Thermal Expansion and Oxide Ion Conductivity in Quaternary Pyrochlore Solid Solutions

Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 Å). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.

Non-lattice surface oxygen species implicated in the catalytic partial oxidation of decane to oxygenated aromatics

The one-step transformation of C(7)-C(12) linear alkanes into more valuable oxygenates provides heterogeneous catalysis with a major challenge. In evaluating the potential of a classic mixed-metal-oxide catalyst, we demonstrate new insights into the reactivity of adsorbed oxygen species. During the aerobic gas-phase conversion of n-decane over iron molybdate, the product distribution correlates with the condition of the catalyst. Selectivity to oxygenated aromatics peaks at 350°C while the catalyst is in a fully oxidized state, whereas decene and aromatic hydrocarbons dominate at higher temperatures. The high-temperature performance is consistent with an underlying redox mechanism in which lattice oxide ions abstract hydrogen from decane. At lower temperatures, the formation of oxygenated aromatics competes with the formation of CO(2), implying that electrophilic adsorbed oxygen is involved in both reactions. We suggest, therefore, that so-called non-selective oxygen is capable of insertion into carbon-rich surface intermediates to generate aromatic partial oxidation products.

Synthesis and characterization of dense SnP2O7–SnO2 composite ceramics as intermediate-temperature proton conductors

Dense SnP2O7–SnO2 composite ceramics were prepared by reacting a porous SnO2 substrate with an 85% H3PO4 solution at elevated temperatures. At 300 °C and higher, SnO2 reacted with H3PO4 to form an SnP2O7 layer on exterior and interior surfaces in the substrate, the growth rate of which increased with increasing reaction temperature. Finally, at 600 °C, the pores of this composite ceramic were perfectly closed and its electrical conductivity became several orders of magnitude higher than that of the SnO2 substrate alone. Proton conduction was demonstrated in this composite ceramic using electrochemical measurements and various analytical techniques. Comparison of the observed electromotive force with the theoretical value in two gas concentration cells demonstrated that this composite ceramic is a pure ion conductor, wherein the predominant ion species are protons. Fourier transform infrared (FT-IR) and proton magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses revealed that the protons interacted with lattice oxide ions in the SnP2O7 layer to form hydrogen bonds. An H/D isotope effect suggested that proton conduction in this composite ceramic was based on a proton-hopping mechanism. The proton conductivity in this material reached ∼10−2 S cm−1 in the temperature range of 250–600 °C.

Energy Materials: Layered LaSrGa3O7‐Based Oxide‐Ion Conductors: Cooperative Transport Mechanisms and Flexible Structures (Adv. Funct. Mater. 22/2010)

AbstractNovel melilite‐type gallium‐oxides are attracting attention as promising new oxide‐ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic‐scale investigation of the LaSrGa3O7‐based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner‐sharing GaO4 tetrahedra. A major finding is the first indication that oxide‐ion conduction in La1.54Sr0.46Ga3O7.27 occurs through an interstitialcy or cooperative‐type mechanism involving the concerted knock‐on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.

Layered LaSrGa3O7‐Based Oxide‐Ion Conductors: Cooperative Transport Mechanisms and Flexible Structures

AbstractNovel melilite‐type gallium‐oxides are attracting attention as promising new oxide‐ion conductors with potential use in clean energy devices such as solid oxide fuel cells. Here, an atomic‐scale investigation of the LaSrGa3O7‐based system using advanced simulation techniques provides valuable insights into the defect chemistry and oxide ion conduction mechanisms, and includes comparison with the available experimental data. The simulation model reproduces the observed complex structure composed of layers of corner‐sharing GaO4 tetrahedra. A major finding is the first indication that oxide‐ion conduction in La1.54Sr0.46Ga3O7.27 occurs through an interstitialcy or cooperative‐type mechanism involving the concerted knock‐on motion of interstitial and lattice oxide ions. A key feature for the transport mechanism and high ionic conductivity is the intrinsic flexibility of the structure, which allows considerable local relaxation and changes in Ga coordination.

Partially Proton-Exchanged WP[sub 2]O[sub 7] with High Conductivity at Intermediate Temperatures

Proton conduction is reported in partially proton-exchanged WP 2 O 7 at intermediate temperatures. Electrochemical measurements demonstrated that proton conduction occurs in this material. Proton magic angle spinning nuclear magnetic resonance and Fourier transform IR analyses revealed that the ion-exchanged protons interact with the lattice oxide ions in WP 2 O 7 to form hydrogen bonds. The H/D isotope effect suggests that proton conduction in this material is based on the hopping mechanism. Consequently, the electrical conductivity of this material reached 1.1 x 10 -3 S cm -1 at 250°C and 1.7 × 10 -2 S cm -1 at 500°C.

The role of rusts in corrosion and corrosion protection of iron and steel

The processes of atmospheric corrosion of iron and steel and the properties of corrosion products (rusts) are modeled based on a quantitative evaluation of the chemical reactions pertaining to corrosion to elucidate the conditions with which corrosion-protective rust films form. Based on the model, it is suggested that in the initial stage of corrosion, in the rusts, the pH of the aquatic system is maintained at 9.31 owing to an equilibrium with iron(II) hydroxide and the rate of air-oxidation at this pH is very fast, and that dense, self-repairing rust films form, protecting the underlying iron and steel. However, after corrosion stops, the rust film deteriorates due to the dissolution and shrinkage by aging, and the deteriorated rust film separates the anode and cathode reaction products (Fe 2+ and OH − ions) to cause crevice corrosion. The air-oxidation of iron(II) in anode channels without the presence of OH − ions results in strongly acidic solutions (pH 1.41), causing acid-corrosion. It is proposed that good catalysts (e.g. copper(II) and phosphate ions) accelerate the air-oxidation at low pH, delaying the crevice- and acid-corrosion stages. Further, it is argued that iron compounds with negative charges due to the non-stoichiometric proportions of the lattice oxide ions and metal ions (solid oxoanions of iron) exhibit stable cation-selective permeability even with a drop in pH. Rust films including such compounds would stop the passage of aggressive anions and act to protect iron and steel.

EPR Investigation of TiO2 Nanoparticles with Temperature-Dependent Properties

An in situ electron paramagnetic resonance (EPR) study has been carried out for anatase (Hombikat UV100) and rutile TiO(2) nanoparticles at liquid helium (He) temperature (4.2 K) under UV irradiation. Rutile titania was synthesized by ultrasonic irradiation with titanium tetrachloride (TiCl(4)) as the precursor. XRD and Raman results evidence the crystallinity of titania phases. The nature of trapped electrons and holes has been investigated by EPR spectroscopy under air and vacuum conditions. Illumination of TiO(2) powder (anatase and rutile) at 4.2 K resulted in the detection of electrons being trapped at Ti(4+) sites within the bulk and holes trapped at lattice oxide ions at the surface. The stability of electron traps was very sensitive to temperature in both phases of TiO(2). The annealing kinetics of the EPR detected radicals has been studied from 4.2 K to ambient temperature and also for calcined titania particles from 523 to 1273 K.

Nanocluster iron oxide-silica aerogel catalysts for methanol partial oxidation

Nanostructured pure and silica-supported iron oxide materials have been prepared by the aerogel approach. Pure iron oxide powder derived from sol-gel ferric acetylacetonate formed agglomerates of 5–30 nm small crystallites of hematite and maghemite according to TEM identification of crystal faces. Depositing ferric species to mesoporous silica aerogels generated 1–5 nm particles in the amorphous matrix. They were evaluated for methanol oxidation in an ambient fixed-bed flow reactor from 225 to 300 °C. Product selectivity and oxidation activity were dependent upon iron dispersion and reactor operation. The formation of dimethyl ether was mainly related to the bulk phase and Lewis acidity of iron oxide. Active catalysts that were selective to formaldehyde and methyl formate required appropriate iron dispersion on the silica surface, including a strong electronic interaction. Methoxy transformation to formaldehyde and formate species was found to be a function of surface temperature based on a Fourier transform infrared (FT-IR) analysis. Low to moderate reactor temperature and short catalyst contact time favored methanol conversion to formaldehyde. The formation of methyl formate was found to compete with that of formaldehyde. The dependence of response time on oxygen feed attenuation suggests that mobile lattice oxide ions participate in the surface reaction and that oxygen molecules help to maintain surface iron sites highly oxidized for Lewis chemisorption and redox electron transfer. A good correlation between microstructures and reaction characteristics is proposed.

Manganese–lanthanum oxides modified with silver for the catalytic combustion of methane

The characterization of manganese–lanthanum oxides modified with silver has been performed in order to identify factors responsible for the variation of their activity in the oxidation of methane. A significant increase in the activity per unit surface area in silver-containing catalysts occurred above 800 K, where a new source of surface oxygen appeared. It is probably oxygen released from filled oxygen vacancies, more weakly bound in the oxides structure in comparison with lattice oxide ions, more mobile, and therefore easily accessible to methane oxidation. Such oxygen is probably neighboring with silver ions. The remaining part of the catalyst may constitute a reservoir of oxygen ions with which the vacancies are filled and which is supplemented with the gaseous oxygen. A consequence of filling up oxygen vacancies is the appearance of a larger number of manganese ions in the unstable oxidation state Mn 4+. The rate of methane oxidation is a function of the Mn 4+/Mn 3+ surface ratio which is a parameter characterizing the intrinsic properties of the manganese–lanthanum oxides, influencing their activity.

X-ray photoelectron spectroscopic study on α- and γ-bismuth molybdate surfaces exposed to hydrogen, propene and oxygen

In order to shed light on the redox behavior of γ- and α-bismuth molybdates (γ- and α-BiMos), the oxide surfaces time-dependently reduced by hydrogen and propene at 723 K and subsequently reoxidized by oxygen at 373–723 K were measured by using XPS. On the oxide surfaces reduced either by hydrogen or by propene, simultaneous generation of Mo 5+ and Mo 4+ was observed. At the initial stage, the generation of Mo 5+ occurred faster than Mo 4+. With exposure time, each of the cations increased up to almost equivalent amounts: 34–37 and 7–14% of total Mo atom for the γ- and α-BiMos, respectively, while no reduced bismuth species were found except for the formation of metallic bismuth on the γ-BiMo reduced by hydrogen for a long time at 723 K. In reoxidation of the reduced surfaces by oxygen, a part of the Mo 5+ was oxidized to Mo 6+ faster than the Mo 4+ below 373 K, while above 473 K, the Mo 4+ was oxidized preferentially. The results indicated that the lattice oxide ion bridging Bi 3+ and adjacent Mo 6+ was more active and mobile than the oxide ion doubly bonded to Mo 6+. The surface reduction accompanied the diffusion of lattice oxide ions from the bulk to the surface, while in the reoxidation, the incorporated lattice oxide ions migrated into bulk oxide ion vacancies and to the doubly bonded oxide ion vacancies on the Mo 4+ through the bulk.

Mechanism of Hydroxylation of Metal Oxide Surfaces

Surface hydroxyl groups on metal oxides are sites for adsorption of cations or anions from water, which in turn release protons or hydroxide ions to solution resulting in ion-exchange reactions. It is important to know the mechanism of the formation of surface hydroxyl groups to understand the ion-exchange capacities and properties of metal oxides. The hydroxyl groups are formed by dissociative chemisorption of water molecules, and it is generally considered that hydration and hydroxylation occur at exposed lattice metal ion sites on the surface as the lattice metal ions are strong Lewis acids. With this mechanism, the surface hydroxyl site density would be different for different oxide samples as the number of metal ions exposed is different depending on the metal ion valency and the lattice planes exposed. In the present study, the Grignard reagent method was applied to determine the surface hydroxyl site densities of a variety of metal oxide samples. The measured surface hydroxyl site densities were similar for different oxides with di-, tri-, and tetravalent metals. By considering this, a different mechanism of hydroxylation is proposed: Lattice oxide ions (rather than lattice metal ions) are exposed to aqueous solutions because of their large size and low polarizing power, and the surface oxide ions, which are strong Lewis bases because of insufficient coordination to the lattice metal ions, take up protons from water to form acid and base type hydroxyl groups. The maximum possible surface hydroxyl site density was calculated from the hydroxide ion size with the assumption that metal oxides have the structure of the closest packing of oxide ions. The measured values could be explained with the calculated value.

X-Ray Photoelectron Spectroscopic Study on γ-Bismuth Molybdate Surfaces on Exposure to Propene and Oxygen

γ-Bismuth molybdate (γ-Bi2MoO6) surfaces reduced by C3H6, reoxidized by oxygen, on exposure to C3H6-jet and subsequently O2-jet were time- or temperature-dependently measured by X-ray photoelectron spectroscopy (XPS). The C3H6 molecule interacted simultaneously with two different lattice oxide ions to result in almost equivalent amounts of Mo5+ and Mo4+, but reduced species of bismuth were not found. The Mo5+ appeared likely to be formed by oxygen abstraction from Bi3+–O-Mo6+ and the Mo4+ from those of O=Mo6+ along with the elimination of two oxide ions bound on both sides of the [MoO2]2+ layer. A part of Mo5+ was reoxidized faster than Mo4+, below 373 K, while above 473 K Mo4+ was reoxidized preferentially. On exposure to C3H6-jet and O2-jet under ultrahigh vacuum (UHV), only Mo5+ was detected on the surface and the core spectrum intensity of Mo5+ oscillated with respect to exposure time. The oscillation phenomena were interpreted by the time lags between the surface reduction rate with C3H6 and the diffusion rate of oxygen from the oxide bulk, and by the difference between the surface reoxidation (oxygen incorporation) rate and the diffusion rate of newly incorporated oxygen into the bulk anion vacancies. It was also clarified that incorporation of oxygen occurred on the lattice oxide ion vacancies bridging Bi and adjacent Mo atom.

Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene

Kinetic equations of propene oxidation along the nondestructive (nucleophilic oxidation to allylic products) and the destructive (electrophilic oxidation to degraded oxygenated and total oxidation products) pathways have been determined for three types of oxide catalysts: Bi2O3/MoO3, giving only allylic products, Co3O4, showing only total oxidation, and SnO2/MoO3, giving both types of products. In all cases the reaction order of the nucleophilic oxidation was 1 with respect to propene and 0 with respect to oxygen, whereas that of electrophilic oxidation was close to 1 with respect to oxygen. The model of surface interactions is discussed in which propene reacts either with surface lattice oxide ions to give nucleophilic oxidation products or with transient surface oxygen species to give electrophilic oxidation. These transients result from the dynamic equilibrium between nonstoichiometric transition metal oxides and gas phase oxygen, so that the two kinetic equations are coupled by the equation expressing the equilibrium of surface oxygen vacancies. The rate constants of the homomolecular isotopic exchange of oxygen may be taken as a measure of the surface concentration of the transient oxygen species.