La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.     

Effect of oxygen deficiency reduction in Mg-doped Mn-spinel on its cell storage performance at high temperature

The effect of doping foreign metal ions into the LiMn₂O₄ spinel structure on the oxygen deficiency by high-temperature firing has been investigated. The degree of the oxygen deficiency in the Mg- and/or Al-doped spinel is very small even when fired at 1000 °C, and then the oxygen deficiency nearly disappeared after low temperature annealing. Mg-doped Mn-spinels (Li_1.05Mn_1.94−xMg_xCa_0.01O_4−z, 0 ≤ x ≤ 0.15) obtained at 1000 °C contained a small oxygen deficiency amount when the x-value reached 0.03 or more. The annealing at 800 °C then reduced the oxygen defect content in the Mg-doped spinels. The crystallinity of the spinels is improved by doping with a small amount of Ti. The storage performance of the coin-type cell at 60 °C is improved about 1.6 times using both Mg- and Ti-doping, which would be due to the oxygen stoichiometric spinels with a low solubility by firing at high temperatures and/or by Ti-doping to increase the crystallinity.     

Effects of manganese substitution at the B-site of lanthanum-rich strontium titanate anodes on fuel cell performance and catalytic activity

Sr_0.4La_0.6Ti_1−xMn_xO_3−δ with rhombohedral structure has been investigated in terms of their electrochemical performance, redox stability, and electro-catalytic properties for solid oxide fuel cell anodes. The performance of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes for solid oxide fuel cells strongly depends on the Mn substitution at the B-site of the perovskites. Electrical conductivity of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ increases with increasing Mn content. X-ray photoelectron spectroscopy analysis reveals that the amount of Mn³⁺ and Ti³⁺, which is an electronic charge carrier, increases with Mn doping. The reduced anode powders with high Mn/Ti ratio show oxygen storage capability and a low carbon deposition rate. Linear thermal expansion coefficients of Sr_0.4La_0.6Ti_1−xMn_xO_3−δ anodes range from 9.46×10⁻⁶ K⁻¹ to 11.3×10⁻⁶ K⁻¹. The maximum power densities of the single cell with the Sr_0.4La_0.6Ti_0.2Mn_0.8O_3−δ anode in humidified H₂ and CH₄ at 800 °C are 0.29 W cm⁻² and 0.24 W cm⁻², respectively.     

Studies on electrical properties of La0.8Sr0.2Ga0.8Mg0.2O2.80 (LSGM) and LSGM-SrSn1−xFexO3 (x = 0.8; 0.9) composites and their chemical reactivity

We report the electrical conductivity properties of solid-state synthesized perovskite-like La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.80 (LSGM) and LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites. LSGM exhibits both bulk and grain-boundary contribution in the ac impedance plots. The grain-boundary conductivity (σ_gb) is slightly (≤half-order of magnitude) higher than that of the bulk oxide ion conductivity (σ_bulk). Powder XRD study reveals that no chemical reaction occurs between LSGM and SrSn_1−xFe_xO₃ (1:1 wt.%) at 1000 °C (48 h) and forms a single-phase perovskite-like compound at 1300 °C (48 h) in air, while in hydrogen atmosphere, at 800 °C for 48 h, a growth of LaSrGaO₄ and LaSrGa₃O₇ impurity phases and formation of metallic Fe was observed. The LSGM-SrSn_1−xFe_xO₃ (x = 0.8; 0.9) composites show a single or part of semicircle in air at low-temperature regime. The electrical conductivity of the composites were found to be much higher compared to pure LSGM and lower about an order of magnitude than those of pure Sn-doped SrFeO₃ perovskite.     

Microstructures and dielectric properties of spark plasma sintered Ba0.4Sr0.6TiO3/CaCu3Ti4O12 composite ceramics

(1 − x)Ba_0.4Sr_0.6TiO₃/xCaCu₃Ti₄O₁₂ composite ceramics were prepared by spark plasma sintering. Sintering behavior, microstructures and dielectric properties of the composite ceramics were investigated by XRD, SEM, EDS and dielectric spectrometer. Dense composite ceramics consisting of Ba_0.4Sr_0.6TiO₃ phase and CaCu₃Ti₄O₁₂ phase were prepared at 800 °C for 0 min. The dielectric loss of the composite ceramic decreased with increasing amount of Ba_0.4Sr_0.6TiO₃, and the high dielectric constant were retained. Moreover, the better temperature stability of dielectric constant was obtained. These improvements of dielectric characteristics have great scientific significance for potential application.     

Synthesis and electrical conductivity of La-Sr-X-Mg-O (X = Ti, Zr, Al) perovskite solid solution

Perovskite solid solutions of (La_0.6Sr_0.4)(X_1−yMg_y)O_3−δ (X = Ti, Zr, Al) were prepared by a coprecipitation method using corresponding aqueous solutions and ammonium carbonate solution. The freeze-dried powders were sintered in air at 1000-1500 °C for 1-36 h. Single phase solid solutions were produced in the compositions of (La_0.6Sr_0.4)(Zr_0.6Mg_0.4)O_3−δ and (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ where (3 − δ) < 3. For the compositions of X = Ti and Zr for y = 0.1 where (3 − δ) > 3, two phases including perovskite solid solution were produced at 1400-1500 °C. The stability of perovskite solid solution was closely related to the fraction of lattice oxygen atom (3 − δ). A relatively high conductivity was measured for (La_0.6Sr_0.4)(Al_0.9Mg_0.1)O_3−δ (σ = 4.15 × 10⁻⁴ S/cm at 600 °C, activation energy 113.4 kJ/mol). The influence of fraction of oxide ion vacancy on the activation energy was small for δ = 0.1-0.3 of perovskite solid solution.     

Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni-MH batteries

La_(1−x)Sr_xFeO₃ (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La_(1−x)Sr_xFeO₃ perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La_(1−x)Sr_xFeO₃ oxide electrodes are reversible. The discharge capacities of La_(1−x)Sr_xFeO₃ oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g⁻¹ to 543.0 mA h g⁻¹ (when x = 0.2) and from 147.0 mA h g⁻¹ to 501.5 mA h g⁻¹ (when x = 0.4) at the current density of 31.25 mA g⁻¹, respectively. After 20 charge-discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La_(1−x)Sr_xFeO₃ oxide electrodes increase with the increase of the temperature. Compared with La_0.8Sr_0.2FeO₃, La_0.6Sr_0.4FeO₃ electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures.     

Synthesis and negative thermal expansion property of Y2−xLaxW3O12 (0≤x≤2)

Y_2−xLa_xW₃O₁₂ solid solutions were successfully synthesized by the solid state reaction method. The microstructure, hygroscopicity and thermal expansion property of the resulting samples were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and thermal mechanical analysis (TMA). Results indicate that the structural phase transition of the Y_2−xLa_xW₃O₁₂ changes from orthorhombic to monoclinic with increasing substituted content of lanthanum. The pure phase can form for 0≤x≤0.4 with orthorhombic structure and for 1.5≤x≤2 with monoclinic one. High lanthanum content leads to a low relative density of Y_2−xLa_xW₃O₁₂ ceramic. Thermal expansion coefficients of the Y_2−xLa_xW₃O₁₂ (0≤x≤2) ceramics also vary from −9.59×10⁻⁶ K⁻¹ to 2.06×10⁻⁶ K⁻¹ with increasing substituted content of lanthanum. The obtained Y_0.25La_1.75W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.66×10⁻⁶ K⁻¹ from 103 °C to 700 °C.     

Electrochemical and structural study of Ce0.8Sm0.2−xLaxO1.9 electrolyte materials for SOFC

Ce_0.8Sm_0.2−xLa_xO_1.9 powders, denoted as La_xSDC (for x=0, 0.01, 0.03, 0.05, 0.07 and 0.1), were synthesized via the mechanical milling reaction method. The La³⁺ doping content has a remarkable influence on structural and electrical properties. The phase identification and morphology were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Lattice parameters were calculated by the Rietveld method. It was observed that the lattice parameter values in Ce_0.8Sm_0.2−xLa_xO_1.9 systems obey Vegard's law. The pellets were then sintered at 1500 °C in air for 7 h. The relative densities of these pellets were over 93.7%.The electrical conductivity was studied using two-probe impedance spectroscopy and results showed that the conductivity of Ce_0.8Sm_0.2−xLa_xO_1.9 first increased and then decreased with La dopant content x. Results also showed that Ce_0.8Sm_0.17La_0.03O_1.9 had the highest electrical conductivity, σ₇₀₀ °C equal to 3.8×10⁻² Scm⁻¹ and an activation energy equal to 0.77 eV. It was therefore concluded that co-doping with the appropriate amount of La can further improve the electrical properties of ceria electrolytes.     

Synthesis and microstructure of (LaSr)MnO3 and (LaSr)FeO3 ceramics by a reaction-sintering process

(La_xSr_1−x)MnO₃ (LSMO) and (La_xSr_1−x)FeO₃ (LSFO) (x = 0.2–0.4) ceramics prepared by a simple and effective reaction-sintering process were investigated. Without any calcination involved, La₂O₃ and SrCO₃ were mixed with MnO₂ (LSMO) or Fe₂O₃ (LSFO) then pressed and sintered directly. LSMO and LSFO ceramics were obtained after 2 and 4 h sintering at 1350–1400 and 1200–1280 °C, respectively. Grain size decreased as La content increased in LSMO and LSFO ceramics.     

Co-doped ceria-based solid solution in the CeO2-M2O3-CaO, M = Sm, Gd system

The Pechinni method (A) as well as hydrothermal treatment (B) of co-precipitated CeO₂-based gels with NaOH solution were used to synthesise pure CeO₂, and CeO₂-based solid solutions with formula Ce_1−xM_xO₂, Ce_1−x(M_0.5Ca_0.5)_xO₂ M = Gd, Sm for 0.15 < x < 0.3 nanopowders. The thermal evolution of CeO₂-based precursors during heating them up to 1000 °C was monitored by thermal (TG, DTA) analysis and X-ray diffraction method. All nanopowders and samples sintered were found to be pure CeO₂ or ceria-based solutions with fluorite-type structure. The microstructure of CeO₂-based sintered samples at 1500 °C (A) or 1250 °C (B) was observed for 2 h under the scanning electron microscope. The electrical properties of singly Ce_1−xM_xO₂ or doubly doped CeO₂-based samples with formula Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm, 0.15 < x < 0.30 were investigated by means of the ac impedance spectroscopy method throughout the temperature range of 600-800 °C. It has been stated that partial substitution of calcium by samarium or calcium by gadolinium in the Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm solid solutions leads to ionic conductivity enhancement comparable with only samaria- or gadolina-doped ceria. The CeO₂-based samples with small-grained microstructures obtained from powders synthesised by hydrothermal method exhibited better ionic conductivity than samples with the same composition obtained from powders synthesised by the Pechinii method. The stability of the electrolytic properties of selected co-doped ceria sinters in fuel gases (H₂, CH₄) as well as exhaust gases from diesel engine was also investigated. The co-doped Ce_0.8(Sm_0.5Ca_0.5)_0.2O₂ or Ce_0.85(Gd_0.5Ca_0.5)_0.15O₂ dense samples would appear be to more adequate oxide electrolytes than Ce_1−xM_xO₂, M = Sm, Gd and x = 0.15 or 0.2 for electrochemical devices operating at temperatures ranging from 600 to 700 °C.     

Nano-sized Sm0.5Sr0.5CoO3−δ as the cathode for solid oxide fuel cells with proton-conducting electrolytes of BaCe0.8Sm0.2O2.9

Nano-sized Sm_0.5Sr_0.5CoO_3−δ (SSC) was fabricated onto the inner face of porous BaCe_0.8Sm_0.2O_2.9 (BCS) backbone by ion impregnation technique to form a composite cathode for solid oxide fuel cells (SOFCs) with BCS, a proton conductor, as electrolyte. The electro-performance of the composite cathodes was investigated as function of fabricating conditions, and the lowest polarization resistance, about 0.21 Ω cm² at 600 °C, was achieved with BCS backbone sintered at 1100 °C, SSC layer fired at 800 °C, and SSC loading of 55 wt.%. Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the migration of proton and the dissociative adsorption and diffusion of oxygen, respectively. There was an additional arc peaking at 1 Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions. With electrolyte about 70 μm in thickness, the simulated anode, cathode and bulk resistances of cells were 0.021, 0.055 and 0.68 Ω cm² at 700 °C, relatively, and the maximum power density was 307 mW cm⁻² at 700 °C.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Microstructure and dielectric properties of Ba0.6Sr0.4TiO3-MgAl2O4 composite ceramics

(1 − x)Ba_0.6Sr_0.4TiO₃-xMgAl₂O₄(x = 25, 30, 35 and 40 wt%) composite ceramics were prepared by conventional solid-state reaction method. The microstructures, dielectric properties and tunability of the composites have been investigated. The XRD patterns analysis reveals two crystalline phases, a cubic perovskite structure Ba_0.6Sr_0.4TiO₃ (BST) and a spinel structure MgAl₂O₄ (MA). SEM observations show that the BST grains slightly dwindle and agglomerate with increasing amounts of MA. A dielectric peak with very strong frequency dispersion is observed at higher MA content, and the Curie temperature shifts to a higher temperature with increasing MA content. The ceramic sample with 30 wt% MA has the optimized properties: the dielectric constant is 1503, the dielectric loss is 0.003 at 10 kHz and 25 °C, the tunability is 23.63% under a dc electric field of 1.0 kV/mm, which is suitable for ferroelectric phase shifter.     

Electrical conductivity of La1−xCaxFeO3−δ solid solutions

La_1−xCa_xFeO_3−δ solid solutions (x=0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) were investigated. The samples were prepared by the polymerizable complex route and characterized by X-ray diffraction and complex impedance spectroscopy techniques. Results reveal the formation of a single perovskite phase for the La_1−xCa_xFeO_3−δ (0≤x≤0.5) compositions. However, the La_0.4Ca_0.6FeO_3−δ sample is a mixture of many phases: perovskite, calcium ferrite and iron oxide. The unsubstituted lanthanum ferrite oxide, as well as the substituted samples, exhibits an orthorhombic symmetry. The direct current conductivity analyses reveal a typical negative temperature coefficient of the resistance behaviour for all the samples. The incorporation of calcium into the lanthanum ferrite lattice results in a significant improvement of the direct current conductivity. In fact, La_0.8Ca_0.2FeO_3−δ oxide shows the optimal conduction value. For all the studied compositions, a change in the activation energy is highlighted around 440 °C. This behaviour is attributed to the antiferromagnetic to paramagnetic transition of lanthanum ferrite. As for the alternating current conductivity, it obeys the Jonsher's power law. The correlated barrier hopping model is proposed to describe the transport mechanism in the studied matrix.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Hydrogen spillover phenomenon: Enhanced reversible hydrogen adsorption/desorption at Ta2O5-coated Pt electrode in acidic media

The current study is concerned with the preparation and characterization of tantalum oxide-loaded Pt (TaO_x/Pt) electrodes for hydrogen spillover application. XPS, SEM, EDX and XRD techniques are used to characterize the TaO_x/Pt surfaces. TaO_x/Pt electrodes were prepared by galvanostatic electrodeposition of Ta on Pt from LiF-NaF (60:40 mol%) molten salts containing K₂TaF₇ (20 wt%) at 800 °C and then by annealing in air at various temperatures (200, 400 and 600 °C). The thus-fabricated TaO_x/Pt electrodes were compared with the non-annealed Ta/Pt and the unmodified Pt electrodes for the hydrogen adsorption/desorption (H_ads/H_des) reaction. The oxidation of Ta to the stoichiometric oxide (Ta₂O₅) increases with increasing the annealing temperature as revealed from XPS and X-ray diffraction (XRD) measurements. The higher the annealing temperature the larger is the enhancement in the H_ads/H_des reaction at TaO_x/Pt electrode. The extraordinary increase in the hydrogen adsorption/desorption at the electrode annealed at 600 °C is explained on the basis of a hydrogen spillover-reverse spillover mechanism. The hydrogen adsorption at the TaO_x/Pt electrode is a diffusion-controlled process.     

Effect of Ca-substitution in La1 − xCaxFeO3 perovskites on the catalytic activity for soot combustion

La_1 − xCa_xFeO₃ (x = 0.1, 0.2, 0.3 and 0.4) perovskites, prepared by the citrate method and characterized by AAS, XRD, TPR-H₂ and TPD-O₂, were assayed, and its catalytic activity evaluated for soot combustion, using carbon black as a model compound for soot. The catalytic activity was determined by non-isothermal thermogravimetric analysis. A significant enhancement of the catalytic activity was found by increasing the degree of La³⁺ substitution by Ca²⁺ in the perovskites, which coincided with an increase of the α-oxygen concentration and thus, a probable enrichment of the CB surface with these oxygenated species, at the start of the combustion.     

La1−xSrxFeO3−δ perovskites as redox materials for application in a membrane reactor for simultaneous production of pure hydrogen and synthesis gas

This work reports on the preparation and characterization of perovskitic materials with the general formula La_1−xSr_xFeO₃ (x = 0, 0.3, 0.7, 1) for application in a dense mixed conducting membrane reactor process for simultaneous production of synthesis gas and pure hydrogen. Thermogravimetric experiments indicated that the materials are able to loose and uptake reversibly oxygen from their lattice up to 0.2 oxygen atoms per “mole” for SrFeO₃ with x = 1 at 1000 °C. The capability of the prepared powders to convert CH₄ during the reduction step, in order to produce synthesis gas, as well as their capability to dissociate water during the oxidation step, in order to produce hydrogen were evaluated by pulse reaction experiments in a fixed bed pulse reactor. The high sintering temperatures (1100-1300 °C) required for the densification of the membrane materials result in decreased methane conversion and H₂ yields during the reduction step compared to the corresponding values obtained with the perovskite powders calcined at 1000 °C. Addition of small quantities of NiO, by simple mechanical mixing, to the perovskites after their sintering at high temperatures, increases substantially both their methane decomposition reactivity, their selectivity towards CO and H₂ and their water splitting activity. Maximum H₂ yield during the reduction step is achieved with the La_0.7Sr_0.3FeO₃ sample mixed with 5% NiO and is 80% of the theoretically expected H₂, based on complete methane decomposition. In the oxidation - water splitting step, 912 μmol H₂ per gr solid are produced with the La_0.3Sr_0.7FeO₃ sample mixed with 5% NiO. The experimental results of this work can be equally well applied for the “chemical-looping reforming” process since they concern using the lattice oxygen of the perovskite oxides for methane partial oxidation to syngas, in the absence of molecular oxygen, and subsequent oxidation of the solid.     

Enhanced electrochemical stability of Al-doped LiMn2O4 synthesized by a polymer-pyrolysis method

LiAl_xMn_2−xO₄ samples (x = 0, 0.02, 0.05, 0.08) were synthesized by a polymer-pyrolysis method. The structure and morphology of the LiAl_xMn_2−xO₄ samples calcined at 800 °C for 6 h were investigated by powder X-ray diffraction and scanning electron microscopy. The results show that all samples have high crystallinity, regular octahedral morphology and uniform particle size of 100-300 nm. The electrochemical performances were tested by galvanostatic charge-discharge and cyclic voltammetry. The results demonstrate that the Al-doped LiMn₂O₄ can be very well cycled at an elevated temperature of 55 °C without severe capacity degradation. In particular, the LiAl_0.08Mn_1.92O₄ sample demonstrates excellent capacity retention of 99.3% after 50 cycles at 55 °C, confirming the greatly enhanced electrochemical stability of LiMn₂O₄ by a small quantity of Al-doping.