Improved electrical conductivity of Ce0.9Gd0.1O1.95 and Ce0.9Sm0.1O1.95 by co-doping

Solid electrolytes are the most important and indispensable part of a solid oxide fuel cell (SOFC) where hydrogen is used as one of the fuels to obtain electricity. Ce_0.9Gd_0.1O_1.95 and Ce_0.9Sm_0.1O_1.95 were chosen to be the base electrolytes. The effects of MgO and Nd₂O₃ as co-dopants on the electrical conductivity were investigated, respectively. For 4 mol% Mg-doped Ce_0.9Gd_0.1O_1.95 or Ce_0.9Sm_0.1O_1.95, MgO phases were detected by FESEM micrographs, which showed a very low solubility of Mg²⁺ in ceria lattice. The existence of MgO phases was observed to have negligible effect on the grain conductivity, but improve the grain boundary conductivity measured by ac impedance spectroscopy. However, when Nd₂O₃ was used as a co-dopant, XRD patterns and FESEM both indicated a pure cubic phase. Ce_0.9Gd_0.05Nd_0.05O_1.95 and Ce_0.9Sm_0.05Nd_0.05O_1.95 were found to exhibit higher grain conductivity, comparing with single-doped ceria.     

Structural,morphological and electrical properties of Gd0.1Ce0.9O1.95 prepared by a citrate complexation method

A variant of the sol–gel technique known as cation complexation is used to prepare a nanocrystalline Gd_0.1Ce_0.9O_1.95 (GDC) solid solution. A range of techniques including thermal analysis (TGA/DTA), X-ray diffraction, specific surface area determination (BET) and electron microscopy (SEM and TEM) are employed to characterise the GDC powders. GDC calcined at 500 °C is found to have an average crystallite size of 11 nm. Specific surface areas are found to be 29.7 m² g⁻¹ for the as-calcined powder and 57.5 m² g⁻¹ after ball milling at 400 rpm. Dense ceramic pellets are prepared from unmilled and ball-milled GDC powders employing different thermal treatments. Their electrical properties are studied by impedance spectroscopy. Those samples sintered at 1300 °C for 30 h (starting from ball-milled powders) exhibit the highest density (96% of theoretical density) and the highest total ionic conductivity (1.91 × 10⁻² S cm⁻¹ at 600 °C).     

Effect of titania concentration on the grain boundary conductivity of Ce0.8Gd0.2O1.9 electrolyte

The effect of titania (TiO₂) addition on the densification, crystal structure and electrical property of Ce_0.8Gd_0.2O_1.9 (GDC) are examined. TiO₂ addition reduces GDC sintering temperature by ∼200 °C and considerably improves grain boundary conduction. The minimum grain boundary resistivity is obtained at 0.8 mol% TiO₂ concentration. XRD analysis suggests that the solubility limit of TiO₂ in GDC is ∼0.6 mol%. The optimum doping level (0.8 mol%) is slightly higher than the solubility limit (0.6 mol%), which implies that a small amounts of TiO₂ (∼0.2 mol%) at the grain boundaries benefits the grain–boundary conduction.     

Formation of Ce0.8Sm0.2O1.9 nanoparticles by urea-based low-temperature hydrothermal process

The synthesis and formation mechanism of the nano-sized Ce_0.8Sm_0.2O_1.9 particles prepared by a urea-based low-temperature hydrothermal process was investigated in this study. From ex situ X-ray diffraction and induced coupled plasma-atomic emission spectroscopy investigations, it was found that large quantities of cerium hydroxide co-precipitated with some samarium hydroxide at the initial stage of the hydrothermal process. The remaining Sm³⁺ ions in the solutions were further hydrolyzed and deposited on the surface of the cerium hydroxide-rich precipitates to form a core–shell-like structure. During the hydrothermal process, the core–shell-like structure transformed to a single cubic fluorite phase which is due to the incorporation of the deposited samarium hydroxide into the cerium oxide-rich core. Further, the average grain size of the synthesized nanocrystalline Ce_0.8Sm_0.2O_1.9 was reduced with increasing the urea concentration in the solution. The density of the disk prepared with the synthesized Ce_0.8Sm_0.2O_1.9 powders was found to increase with a decrease in the grain size of Ce_0.8Sm_0.2O_1.9. The existence of SO₄²⁻ anions in the SDC powders prepared at low-urea concentration may result in the SDC disks with low density due to their decomposition during sintering process.     

Low-temperature sintering behaviors of nanosized Ce0.8Gd0.2O1.9 powder synthesized by co-precipitation combined with supercritical drying

Nanocrystalline Ce_0.8Gd_0.2O_1.9 (GDC20) powder was synthesized by ammonia co-precipitation combined with supercritical ethanol drying route, followed by characterizations with TG/DSC, XRD, BET, HR-TEM, and FESEM techniques. After calcination at 600 °C, the powder has a high specific surface area of 146.5 m² g⁻¹ and an average crystal size of 5 nm without hard-agglomeration. The nano-GDC powder showed excellent sinterability, where by pressureless sintering at 900 °C for 4 h, the relative density of more than 98% and average grain size of 84 nm have been attained, which is attributed to the powder's ultrafine nanocrystal size, weak agglomeration and high homogeneity. Investigations indicate that in the initial sintering stage, grain growth behavior is mainly controlled by volume diffusion mechanism with an activation energy of 5.4 eV.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

Analysis of the regenerative H2S poisoning mechanism in Ce0.8Sm0.2O2-coated Ni/YSZ anodes for intermediate temperature solid oxide fuel cells

Ceria is used as a sulfur sorbent due to its high affinity for sulfide at high temperatures. In addition, the ionic conductivity of ceria can be dramatically increased by doping with rare metals, including lanthanum, samarium, and gadolinium. Therefore, to enhance sulfur tolerance and improve anode performance, we modified an Ni-based anode with a thin layer coating of Sm_0.2Ce_0.8O_2−δ (SDC) on the pore wall surface of an Ni/YSZ anode. The anode-supported cells were tested with varying H₂S concentrations (0-100 ppm) at 600 and 700 °C. The cell performance was improved in the ceria- (by 20%) and in the SDC- (by 50%) modified anode by extending the additional TPB area in the anode. Under varying H₂S exposure, the polarization resistance was reduced by ceria and the SDC coating on the anode pore wall surface, which led to improved cell performance. A porous SDC layer on the Ni/YSZ anode pore wall acted as a sulfur sorbent as well as an additional TPB area. Otherwise, ceria mainly acted as a sulfur sorbent at high concentrations of H₂S (>60 ppm).     

Reducibility of Ce1−xGdxO2−δ in prospective working conditions

Nanocrystalline powders of CGO materials with different contents of Gd were prepared by a freeze-drying method and used to prepare dense ceramic samples. This method allowed one to obtain good quality samples to re-examine the reducibility of CGO materials. These materials were characterized by a combination of coulometric titration, impedance spectroscopy and ion blocking measurements to evaluate changes in point defect chemistry and mixed conducting properties. We have examined the onset of n-type conductivity as a function of temperature and oxygen partial pressure, and this information was used to re-examine the mixed transport properties in reducing conditions imposed by fuels, and also the OCV obtained with CGO solid electrolyte cells under air/CGO/fuel gradients. The polaron mobility was also evaluated and was found to depend slightly on temperature and to decrease with increasing oxygen deficiency. This confirmed that the electronic conductivity is mainly dependent on reducibility.     

Sm0.2(Ce1−xTix)0.8O1.9 modified Ni-yttria-stabilized zirconia anode for direct methane fuel cell

Sm_0.2(Ce_1−xTi_x)_0.8O_1.9 (SCTx, x = 0-0.29) modified Ni-yttria-stabilized zirconia (YSZ) has been fabricated and evaluated as anode in solid oxide fuel cells for direct utilization of methane fuel. It has been found that both the amount of Ti-doping and the SCTx loading level in the anode have substantial effect on the electrochemical activity for methane oxidation. Optimal anode performance for methane oxidation has been obtained for Sm_0.2(Ce_0.83Ti_0.17)_0.8O_1.9 (SCT0.17) modified Ni-YSZ anode with SCT0.17 loading of about 241 mg cm⁻² resulted from four repeated impregnation cycles. When operating on humidified methane as fuel and ambient air as oxidant at 700 °C, single cells with the configuration of SCT0.17 modified Ni-YSZ anode, YSZ electrolyte and La_0.6Sr_0.4Co_0.2Fe_0.8O₃-Sm_0.2Ce_0.8O_1.9 (LSCF-SDC) composite cathode show the polarization cell resistance of 0.63 Ω cm² under open circuit conditions and produce a peak power density of 383 mW cm⁻². It has been revealed that the coated Ti-doped SDC on Ni-YSZ anode not only effectively prevents the methane fuel from directly impacting on the Ni particles, but also enhances the kinetics of methane oxidation due to an improved oxygen storage capacity (OSC) and redox equilibrium of the anode surface, resulting in significant enhancement of the SCTx modified Ni-YSZ anode for direct methane oxidation.     

Preparation and characterization of nanocrystalline Ce0.8Sm0.2O1.9 for low temperature solid oxide fuel cells based on composite electrolyte

Nanocrystalline Ce_0.8Sm_0.2O_1.9 (SDC) has been synthesized by a combined EDTA–citrate complexing sol–gel process for low temperature solid oxide fuel cells (SOFCs) based on composite electrolyte. A range of techniques including X-ray diffraction (XRD), and electron microscopy (SEM and TEM) have been employed to characterize the SDC and the composite electrolyte. The influence of pH values and citric acid-to-metal ions ratios (C/M) on lattice constant, crystallite size and conductivity has been investigated. Composite electrolyte consisting of SDC derived from different synthesis conditions and binary carbonates (Li₂CO₃–Na₂CO₃) has been prepared and conduction mechanism is discussed. Water was observed on both anode and cathode side during the fuel cell operation, indicating the composite electrolyte is co-ionic conductor possessing H⁺ and O²⁻ conduction. The variation of composite electrolyte conductivity and fuel cell power output with different synthesis conditions was in accordance with that of the SDC originated from different precursors, demonstrating O²⁻ conduction is predominant in the conduction process. A maximum power density of 817 mW cm⁻² at 600 °C and 605 mW cm⁻² at 500 °C was achieved for fuel cell based on composite electrolyte.     

Microstructural improvements of the gradient composite material Pr0.6Sr0.4Fe0.8Co0.2O3/Ce0.8Sm0.2O1.9 by employing vertically aligned carbon nanotubes

Ionic–electronic conductor composite materials are widely employed to improve the ionic conductivity of solid oxide fuel cell electrodes, making them more compatible with the solid oxide fuel cell materials. The addition of an ionic conductor implies, however, a decrease of the cathode active area for the oxygen reduction reactions. In addition, the ionic conductor particles are usually dispersed and isolated, limiting the enhanced properties to certain areas. In order to avoid this drawback and improve the ionic conductivity of the electrode, vertically aligned carbon nanotubes (VACNTs) have been synthesized and employed as template for the deposition of Sm_0.2Ce_0.8O₂ (SDC) particles. Subsequently, these SDC-VACNTs have been coated with a Pr_0.6Sr_0.4Fe_0.8Co_0.2O₃ (PSFC) perovskite phase, leading to the formation of the desired composite material. Attending to the obtained results, it is proved that the new composite material exhibits a significantly better electrochemical performance, due to a better distribution of the ionic conductor and the burning of the carbon nanotubes (CNTs), which result in the improvement of the microstructure.     

Fabrication and characterization of a co-fired La0.6Sr0.4Co0.2Fe0.8O3−δ cathode-supported Ce0.9Gd0.1O1.95 thin-film for IT-SOFCs

A dense membrane of Ce_0.9Gd_0.1O_1.95 on a porous cathode based on a mixed conducting La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ was fabricated via a slurry coating/co-firing process. With the purpose of matching of shrinkage between the support cathode and the supported membrane, nano-Ce_0.9Gd_0.1O_1.95 powder with specific surface area of 30 m² g⁻¹ was synthesized by a newly devised coprecipitation to make the low-temperature sinterable electrolyte, whereas 39 m² g⁻¹ nano-Ce_0.9Gd_0.1O_1.95 prepared from citrate method was added to the cathode to favor the shrinkage for the La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ. Bi-layers consisting of <20 μm dense ceria film on 2 mm thick porous cathode were successfully fabricated at 1200 °C. This was followed by co-firing with NiO–Ce_0.9Gd_0.1O_1.95 at 1100 °C to form a thin, porous, and well-adherent anode. The laboratory-sized cathode-supported cell was shown to operate below 600 °C, and the maximum power density obtained was 35 mW cm⁻² at 550 °C, 60 mW cm⁻² at 600 °C.     

On the LiNi0.2Mn0.2Co0.6O2 positive electrode material

Layered LiNi_0.2Mn_0.2Co_0.6O₂ phase, belonging to a solid solution between LiNi_1/2Mn_1/2O₂ and LiCoO₂ most commercialized cathodes, was prepared via the combustion method at 900 °C for a short time (1 h). Structural, electrochemical and magnetic properties of this material were investigated. Rietveld analysis of the XRD pattern shows this compound as having the α-NaFeO₂ type structure (S.G. R-3m; a = 2.8399(2) ?; c = 14.165(1) ?) with almost none of the well-known Li/Ni cation disorder. SQUID measurements clearly indicate that the studied compound consists of Ni²⁺, Co³⁺ and Mn⁴⁺ ions in the crystal structure. X-ray analysis of the chemically delithiated Li_xNi_0.2Mn_0.2Co_0.6O₂ phases reveals that the rhombohedral symmetry was maintained during Li-extraction, confirmed by the monotonous variation of the potential-composition curve of the Li//Li_xNi_0.2Mn_0.2Co_0.6O₂ cell. LiNi_0.2Mn_0.2Co_0.6O₂ cathode has a discharge capacity of ∼160 mAh g⁻¹ in the voltage range 2.7-4.3 V corresponding to the extraction/insertion of 0.6 lithium ion with very low polarization. It exhibits a stable capacity on cycling and good rate capability in the rate range 0.2-2 C. The almost 2D structure of this cathode material, its good electrochemical performances and its relatively low cost comparing to LiCoO₂, make this material very promising for applications.     

Comparative study of CuO supported on CeO2, Ce0.8Zr0.2O2 and Ce0.8Al0.2O2 based catalysts in the CO-PROX reaction

CuO supported on CeO_2, Ce_0.8Zr_0.2O₂ and Ce_0.8Al_0.2O₂ based catalysts (6%wt Cu) were synthesized and tested in the preferential oxidation of CO in a H₂-rich stream (CO-PROX). Nanocrystalline supports, CeO₂ and solid solutions of modified CeO₂ with zirconium and aluminum were prepared by a freeze-drying method. CuO was supported by incipient wetness impregnation and calcination at 400 °C. All catalysts exhibit high activity in the CO-PROX reaction and selectivity to CO₂ at low reaction temperature, being the catalyst supported on CeO₂ the more active and stable. The influence of the presence of CO₂ and H₂O was also studied.     

Thermal properties of Li4/3Ti5/3O4/LiMn2O4 cell

The thermal properties of Li_4/3Ti_5/3O₄ and Li_1+xMn₂O₄ electrodes were investigated by isothermal micro-calorimetry (IMC). The 150-mAh g⁻¹ capacity of a Li/Li_4/3Ti_5/3O₄ half cell was obtained through the voltage plateau that occurs at 1.55 V during the phase transition from spinel to rock salt. Extra capacity below 1.0 V was attributed to the generation of a new phase. The small and constant entropy change of Li_4/3Ti_5/3O₄ during the spinel/rock-salt phase transition indicated its good thermal stability. Accelerated rate calorimetry confirmed that Li_4/3Ti_5/3O₄ has better thermal characteristics than graphite. The IMC results for a Li/Li_1+xMn₂O₄ half cell indicated less heat variation due to the suppression of the order/disorder change by lithium doping. The heat profiles of the Li_4/3Ti_5/3O₄/Li_1+xMn₂O₄ full cell indicated less heat generation compared with a mesocarbon-microbead graphite/Li_1+xMn₂O₄ cell.     

Effect of thickness of Gd0.1Ce0.9O1.95 electrolyte films on electrical performance of anode-supported solid oxide fuel cells

In the present paper, we investigated the electrical performance of anode-supported solid oxide fuel cells (SOFCs) composed of Gd_0.1Ce_0.9O_1.95 (GDC) electrolyte films of 1-75 μm in thickness prepared by simple and cost-effective methods (dry co-pressing process and spray dry co-pressing process), and discussed the effect of thickness of the GDC electrolyte films on the electrical performance of the anode-supported SOFCs. It was shown that reducing the thickness of the GDC electrolyte films could increase the maximum power densities of the anode-supported SOFCs. The increase of the maximum power densities was attributed to the decrease of the electrolyte resistance with reducing the electrolyte thickness. However, when the thickness of the GDC electrolyte films was less than a certain value (approximately 5 μm in this study), the maximum power densities decreased with the decrease in the thickness of the GDC electrolyte films. The calculated electron fluxes through the GDC electrolyte films increased obviously with reducing the thickness of the GDC electrolyte films, which was the reason why the maximum power densities decreased. Therefore, for anode-supported SOFCs based on electrolytes with mixed electronic-ionic conductivity, there was an optimum electrolyte thickness for obtaining higher electrical performance.     

Synthesis and electrochemical properties of LiNi0.8Co0.2O2 nanopowders for lithium ion battery applications

Nitrates of lithium, cobalt and nickel are utilized to synthesize LiNi_0.8Co_0.2O₂ cathode material through sol-gel technique. Various synthesis parameters such as calcination time and temperature as well as chelating agent are studied to determine the optimized condition for material processing. Using TG/DTA techniques, the optimized calcination temperatures are selected. Different characterization techniques such as ICP, XRD and TEM are employed to characterize the chemical composition, crystal structure, size and morphology of the powders. Micron and nano-sized powders are produced using citric/oxalic and TEA as chelating agent, respectively. Selected powders are used as cathode material to assemble batteries. Charge-discharge testing of these batteries show that the highest discharge capacity is 173 mAh g⁻¹ at a constant current of 0.1 mA cm⁻², between 3.0 and 4.2 V. This is obtained in a battery assembled with the nanopowder produced by TEA as chelating agent.     

Improvement of electrochemical properties of Li1.1Al0.05Mn1.85O4 achieved by an AlF3 coating

The electrochemical performance of AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ spinel was investigated. The morphology of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was observed by SEM and TEM, and the thickness of the coating layer was approximately 10 nm. Capacity retention and rate capability were substantially improved by the AlF₃-coating, as compared to pristine Li_1.1Al_0.05Mn_1.85O₄. Manganese dissolution was also dramatically reduced for the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄, which may reflect lower impedance for the coated spinel. The thermal stability of the AlF₃-coated Li_1.1Al_0.05Mn_1.85O₄ was improved, exhibiting an exothermic reaction at higher temperature with reduced heat generation, compared to the pristine Li_1.1Al_0.05Mn_1.85O₄.     

Compatibility of the Py24TFSI-LiTFSI ionic liquid solution with Li4Ti5O12 and LiFePO4 lithium ion battery electrodes

Ionic liquids are assuming a constantly growing importance as preferred media for the progress of various energy devices such as solar cells, supercapacitors and lithium batteries. However, their electrochemical properties are not yet fully recognized and in this paper we try to contribute to fill the gap by investigating the stability window of a sample IL-based solution, as well as its interfacial properties with two typical lithium ion battery electrodes, namely lithium titanate, Li₄Ti₅O₁₂ and lithium iron phosphate, LiFePO₄. The results of a detailed impedance spectroscopy analysis demonstrate that, although operating well within the stability domain of the selected IL-based electrolyte, both electrodes undergo passivation phenomena with the formation on their surface of a solid electrolyte interface, SEI, layer. The impedance spectra show that the resistance of the SEI is very low and stable, this suggesting that its occurrence is highly beneficial in terms of assuring reversible and safe electrode processes.