Study on GDC-KZnAl composite electrolytes for low-temperature solid oxide fuel cells

Development of low-temperature solid oxide fuel cells (LTSOFC) is now becoming a mainstream research direction worldwide. The advancement in the effective electrolyte materials has been one of the major challenges for LTSOFC development. To further improve the performance of electrolyte, composite approaches are considered as common strategies. The enhancement on ionic conductivity or sintering behavior ceria-based electrolyte can either be done by adding a carbonate phase to facilitate the utilization of the ionic-conducting interfaces, or by addition of alumina as insulator to reduce the electronic conduction of ceria. Thus the present report aims to design a composite electrolyte materials by combining the above two composite approaches, in order to enhance the ionic conductivity and to improve the long-term stability simultaneously. Here we report the preparation and investigation of GDC-KAlZn materials with composition of Gd doped ceria, K₂CO₃, ZnO and Al₂O₃. The structure and morphology of the samples were characterized by XRD, SEM, etc. The ionic conductivity of GDC-KAlZn sample was determined by impedance spectroscopy. The composite samples with various weight ratio of GDC and KAlZn were used as electrolyte material to fabricate and evaluate fuel cells as well as investigate the composition dependent properties. The good ionic conductivity and notable fuel cell performance of 480 mW cm⁻² at 550 °C has demonstrated that GDC-KAlZn composite electrolyte can be regarded as a potential electrolyte material for LTSOFCs.     

Ceria-carbonate composite for low temperature solid oxide fuel cell: Sintering aid and composite effect

In this study, the effect of carbonate content on microstructure, relative density, ionic conductivity and fuel cell performance of Ce_0.8Sm_0.2O_1.9-(Li/Na)₂CO₃ (SDC-carbonate, abbr. SCC) composites is systematically investigated. With the addition of carbonate, the nano-particles of ceria are well preserved after heat-treatment. The relative densities of SCC pellets increase as the carbonate content increases or sintering temperature rises. Especially, the relative density of SCC2 sintered at 900 °C is higher than that of pure SDC sintered at 1350 °C. Both the AC conductivity and DC oxygen ionic conductivity are visibly improved compared with the single phase SDC electrolyte. Among the composites, SDC-20 wt% (Li/Na)₂CO₃ (SCC20) presents high dispersion, relative small particle size, and the dense microstructure. The optimized microstructure brings the best ionic conductivity and fuel cell performance. It is hoped that the results can contribute the understanding of the role of carbonate in the composite materials and highlight their prospective application.     

Periodic Nafion-silica-heteropolyacids electrolyte t for PEM fuel cell operated near 200 °C

Periodic ordered Nafion-silica-heteropolyacids electrolyte is synthesized through a facile multiphase self-assembly between the positively charged silica, negatively charged HPW acids (H₃PW₁₂O₄₀) and Nafion ionomers. The results exhibit uniform nanoarrays with long-range order of the electrolyte. The well-ordered proton conducting sites make the proton move through the membrane freely with low humidity dependence of proton transportation through the electrolyte. The Nafion-Silica-HPW electrolyte displays desirable conductivity at both low and elevated temperature. The proton conductivity of Nafion-Silica-HPW electrolyte at absolutely dry condition of 200 °C is 0.044 Scm⁻¹. At low temperature of 75 °C, the proton conductivities of the electrolyte were 0.029 Scm⁻¹ and 0.093 Scm⁻¹ at absolute dry (0 RH%) and full humidifying condition (100 RH%), respectively.     

Heavily neodymium doped ceria as an effective barrier layer in solid oxide electrochemical cells

10 mol% gadolinium doped ceria (GDC10) is widely used as a barrier layer between oxygen electrode and electrolyte to prevent interfacial reactions. A 50 mol% neodymium doped ceria (NDC50) barrier layer has been proposed and studied in this paper. Symmetrical cells with NDC50 and GDC10 barrier layers, Nd₂NiO_4+δ(NNO)–Ce_0.5Nd_0.5O_2-δ(NDC50) electrode, and YSZ electrolyte have been systematically studied using impedance spectroscopy (EIS) at various temperatures and oxygen partial pressure (pO₂). The NDC50 barrier layer has significantly decreased polarization resistance across a wide temperature and pO₂ range compared to the GDC10 barrier layer. The rare earth C-type structure of the NDC50 barrier layer causes barrier free migration of oxygen ions resulting in improved ionic conductivity compared to GDC10. Distribution of relaxation time (DRT) modeling has been used to obtain insights into the electrode processes.     

A strategy for improving the sinterability and electrochemical properties of ceria-based LT-SOFCs using bismuth oxide additive

The introduction of an appropriate amount of Bi₂O₃ is advantageous for improving the sinterability of SNDC, resulting in a decrease in the sintering temperature to 1150 °C of the half-cell with a high density electrolyte. The sintering behavior and electrochemical properties is investigated systematically. The ionic conductivity of 5ESB-SNDC is increased significantly compared to that of the conventional SNDC due to the fast oxygen ion transport along the ESB. Series xESB-SNDC electrolyte is applied to anode supported single cells with the structure of NiCuO-SNDC|xESB-SNDC|ESB-LSM from 450 to 650 °C. A higher maximum power densities (MPD) and lower ohmic resistance (R_o) are obtained with the addition of ESB. The power output for composite electrolyte cell is very encouraging，reaching 129 mW cm⁻² at 450 °C for 5ESB-SNDC(20 μm). The results demonstrate that ESB addition is an effective strategy to optimize ceria-based low-temperature SOFCs (LT-SOFCs).     

Effects of salt composition on the electrical properties of samaria-doped ceria/carbonate composite electrolytes for low-temperature SOFCs

Samaria-doped ceria (SDC)/carbonate composite electrolytes were developed for low-temperature solid oxide fuel cells (SOFCs). SDC powders were prepared by oxalate co-precipitation method and used as the matrix phase. Binary alkaline carbonates were selected as the second phase, including (Li–Na)₂CO₃, (Li–K)₂CO₃ and (Na–K)₂CO₃. AC conductivity measurements showed that the conductivities in air atmosphere depended on the salt composition. A sharp conductivity jump appeared at 475 °C and 450 °C for SDC/(Li–Na)₂CO₃ and SDC/(Li–K)₂CO₃, respectively. However, the conductivities of SDC/(Na–K)₂CO₃ increase linearly with temperature. Single cells based on above composite electrolytes were fabricated by dry-pressing and tested in hydrogen/air at 500–600 °C. A maximum power density of 600, 550 and 550 mW cm⁻² at 600 °C was achieved with SDC/(Li–Na)₂CO₃, SDC/(Li–K)₂CO₃ and SDC/(Na–K)₂CO₃ composite electrolyte, respectively, which we attribute to high ionic conductivities of these composite electrolytes in fuel cell atmosphere. We discuss the conduction mechanisms of SDC/carbonate composite electrolytes in different atmospheres according to defect chemistry theory.     

Studies of modified lithiated NiO cathode for low temperature solid oxide fuel cell with ceria-carbonate composite electrolyte

In this work, the effect of copper, iron and cobalt oxides on electrochemical properties of lithiated NiO cathodes was reported in low temperature solid oxide fuel cell (LT-SOFC) with ceria-carbonate composite electrolyte. The modified lithiated NiO cathodes were characterized by XRD, DC conductivity, SEM and electrochemical measurements. In spite of lower conductivities of modified cathodes, Li–Ni–M (M = Cu, Fe, Co) oxides with the order of Li–Ni–Co oxide > Li–Ni–Fe oxide > Li–Ni–Cu oxide, compared with that without modification, the catalytic activities of all the Li–Ni–M oxides were improved. In particularly, cobalt oxide modification favors both charge transfer and gas diffusion for O₂ reduction reaction as confirmed by AC impedance measurements. SEM micrographs show that grains aggregate with the modification of copper oxide or iron oxide, which may be responsible for the increased gas diffusion resistance. The results indicate that the lithiated NiO modified by cobalt oxide as cathode is an alternative to improve LT-SOFC performance with ceria-carbonate composite electrolyte.     

Durability study of an intermediate temperature fuel cell based on an oxide–carbonate composite electrolyte

It was reported that ceria–carbonate composites are promising electrolyte materials for intermediate temperature fuel cells. The conductivity stability of composite electrolyte with co-doped ceria and binary carbonate was measured by AC impedance spectroscopy. At 550 °C, the conductivity dropped from 0.26 to 0.21 S cm⁻¹ in air during the measured 135 h. At a constant current density of 1 A cm⁻², the cell performance keeps decreasing at 550 °C, with a maximum power density change from 520 to 300 mW cm⁻². This is due to the increase of both series and electrode polarisation resistances. Obvious morphology change of the electrolyte nearby the cathode/electrolyte interface was observed by SEM. Both XRD and FT-IR investigations indicate that there are some interactions between the doped ceria and carbonates. Thermal analysis indicates that the oxide–carbonate composite is quite stable at 550 °C. The durability of this kind of fuel cell is not good during our experiments. A complete solid oxide-carbonate composite would be better choice for a stable fuel cell performance.     

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.     

A novel dual phase BaCe0.5Fe0.5O3-δ cathode with high oxygen electrocatalysis activity for intermediate temperature solid oxide fuel cells

A novel iron-based perovskite BaCe_0.5Fe_0.5O_3-δ (BCF) powders were successfully fabricated and the phase composition, lattice structure, oxygen surface exchange coefficient and electrochemical performance were investigated. The ultrafine BCF powder with grain size of about 200 nm consisted of dual phase BaCe_0.15Fe_0.85O_3-δ and BaCe_0.85Fe_0.15O_3-δ. Electrical conductivity relaxation measurement illustrates the low conductivity activation energy and the high oxygen surface exchange kinetics with oxygen surface exchange coefficient of 3.8 × 10⁻⁵ cms⁻¹ at 600 °C. BCF cathode exhibits 1.04 Ωcm² on doped ceria electrolyte and remains stable in 400 h long term test at 600 °C. A single cell based on doped ceria electrolyte with BCF cathode shows a maximum power density of 228 mW cm⁻² at 650 °C. The preliminary results indicate that the dual phase BCF can be applied as cathode material for oxygen ion conductive solid oxide fuel cells.     

Development of solid oxide fuel cell materials for intermediate-to-low temperature operation

The commercialization of solid oxide fuel cell (SOFC) needs the development of functional materials for intermediate-to-low temperature (400-700 °C, ILT) operation. Recently, we have successfully developed new electrolyte materials for ILT-SOFCs, including Ce_0.8Sm_0.2O_1.9 (SDC), BaCe_0.8Sm_0.2O_2.9 (BCSO) and SDC-carbonate composites. Compared with the state-of-the-art yttria-stabilized zirconia (YSZ), these materials exhibit much higher ionic conductivity at ILT range. Especially, SDC-carbonate composites show an ionic conductivity of 10⁻² to 1 Scm⁻¹ between 400 and 600 °C in fuel cell environment. Some new cathode materials were investigated for above electrolyte materials and showed promising performance. Alternative anode materials were developed to directly utilize alcohol fuels. A dry-pressing and co-firing process was employed to fabricate thin SDC and BCSO electrolyte membranes as well as thick SDC-carbonate composite electrolyte with acceptable density on anode substrate. Many efforts have also been made on fabrication of larger-size planar cells and exploitation of reliable sealing materials.     

Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd_0.2Ce_0.8O_2−d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd_0.2Ce_0.8O_2−d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.     

La0.84Sr0.16MnO3−δ cathodes impregnated with Bi1.4Er0.6O3 for intermediate-temperature solid oxide fuel cells

La_0.84Sr_0.16MnO_3−δ–Bi_1.4Er_0.6O₃ (LSM–ESB) composite cathodes are fabricated by impregnating LSM electronic conducting matrix with the ion-conducting ESB for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The performance of LSM–ESB cathodes is investigated at temperatures below 750 °C by AC impedance spectroscopy. The ion-impregnation of ESB significantly enhances the electrocatalytic activity of the LSM electrodes for the oxygen reduction reactions, and the ion-impregnated LSM–ESB composite cathodes show excellent performance. At 750 °C, the value of the cathode polarization resistance (R_p) is only 0.11 Ω cm² for an ion-impregnated LSM–ESB cathode, which also shows high stability during a period of 200 h. For the performance testing of single cells, the maximum power density is 0.74 W cm⁻² at 700 °C for a cell with the LSM–ESB cathode. The results demonstrate the ion-impregnated LSM–ESB is one of the promising cathode materials for intermediate-temperature solid oxide fuel cells.     

High conductive (LiNaK)2CO3Ce0.85Sm0.15O2 electrolyte compositions for IT-SOFC applications

Composite electrolytes of lithium, sodium, and potassium carbonate ((LiNaK)₂CO₃), and samarium doped ceria (SDC) have been synthesized and the carbonate content optimized to study conductivity and its performance in intermediate-temperature solid oxide fuel cell (IT-SOFC). Electrolyte compositions of 20, 25, 30, 35, 45 wt% (LiNaK)₂CO₃–SDC are fabricated and the physical and electrochemical characterization is carried out using X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscope, and current–voltage measurements. The ionic conductivity of (LiNaK)₂CO₃–SDC electrolytes increases with increasing carbonate content. The best ionic conductivity is obtained for 45 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.72 S cm^?1 at 600 °C) followed by the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.55 S cm^?1 at 600 °C). The symmetrical cell of the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte with lanthanum strontium cobalt ferrite (LSCF) electrode in air gives an area specific resistance of 0.155 Ω cm² at 500 °C. The maximum power density of the fuel cell using 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte, composite NiO anode and composite LSCF cathode is found to be 801 mW cm^?2 at 550 °C.     

Impedance spectroscopy studies of Gd-CeO2-(LiNa)CO3 nano composite electrolytes for low temperature SOFC applications

Electrical properties of 20 mol % Gd doped CeO₂ with varying amounts of (LiNa)CO₃ have been investigated by employing AC-impedance spectroscopic technique. The impedance spectra show a high frequency depressed arc, represents the bulk composite and low frequency incomplete semicircle representing electrode contribution. The bulk resistance of the composites decreases with increasing carbonate content up to 30 wt% (LiNa)CO_3, thereafter the resistance increases, whereas all the compositions show a decrease in resistance with increasing temperature. The typical nature of the impedance spectra of the composite shows the possibility of coexistence of multi ionic transport or existence of space charge effect at the interface of Gd-CeO₂ and carbonate phase. The composite containing 25 wt% (LiNa)CO₃ shows the highest ionic conductivity of 0.1757 S cm⁻¹ at 550 °C and lowest activation energy of 0.127 eV in the temperature range 550-800 °C. A symmetric cell is fabricated with GDC-25 wt% (LiNa)CO₃ electrolyte, NiO-GDC(LiNa)CO₃ anode and lithiated NiO-GDC(LiNa)CO₃ cathode. Pure H₂ and air are used as fuel and oxidant. The cell delivers a maximum power density of 45 mW/cm², 58 mW/cm² and 92 mW/cm² at 450, 500 and 550 °C, respectively.     

Structural, thermal and electrochemical properties of layered perovskite SmBaCo2O5+d, a potential cathode material for intermediate-temperature solid oxide fuel cells

The synthesis, conductivity properties, area specific resistance (ASR) and thermal expansion behaviour of the layered perovskite SmBaCo₂O_5+d (SBCO) are investigated for use as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The SBCO is prepared and shows the expected orthorhombic pattern. The electrical conductivity of SBCO exhibits a metal–insulator transition at about 200 °C. The maximum conductivity is 570 S cm⁻¹ at 200 °C and its value is higher than 170 S cm⁻¹ over the whole temperature range investigated. Under variable oxygen partial pressure SBCO is found to be a p-type conductor. The ASR of a composite cathode (50 wt% SBCO and 50 wt% Ce_0.9Gd_0.1O_2−d, SBCO:50) on a Ce_0.9Gd_0.1O_2−d (CGO91) electrolyte is 0.05 Ω cm² at 700 °C. An abrupt increase in thermal expansion is observed in the vicinity of 320 °C and is ascribed to the generation of oxygen vacancies. The coefficients of thermal expansion (CTE) of SBCO is 19.7 and 20.0 × 10⁻⁶ K⁻¹ at 600 and 700 °C, respectively. By contrast, CTE values for SBCO:50 are 12.3, 12.5 and 12.7 × 10⁻⁶ K⁻¹ at 500, 600 and 700 °C, that is, very similar to the value of the CGO91 electrolyte.     

Role of carbonate phase in ceria–carbonate composite for low temperature solid oxide fuel cells: A review

Ceria–salt composites represent one type of promising electrolyte candidates for low temperature solid oxide fuel cells (LT‐SOFCs), in which ceria–carbonate attracts particular attention because of its impressive ionic conductivity and unique hybrid ionic conduction behavior compared with the commonly used single‐phase electrolyte materials. It has been demonstrated that the introduction of carbonate in these new ceria‐based composite materials initiates multi new functionalities over single‐phase oxide, which therefore needs a comprehensive understanding and review focus. In this review, the roles of carbonate in the ceria–carbonate composites and composite electrolyte‐based LT‐SOFCs are analyzed from the aspects of sintering aid, electrolyte densification reagent, electrolyte/electrode interfacial ‘glue’ and sources of super oxygen ionic and proton conduction, as well as the oxygen reduction reaction promoter for the first time. This summary remarks the significance of carbonate in the ceria–carbonate composites for low temperature, 300–600 °C, SOFCs and related highly efficient energy conversion applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantifying multi-ionic conduction through doped ceria-carbonate composite electrolyte by a current-interruption technique and product analysis

A composite electrolyte consisting of a samarium doped ceria and a binary eutectic carbonate phase is investigated in this work. It has been found that O²⁻/H⁺ conductions take place when H₂ and O₂ used as the reactants. The presence of CO₂ in the cathode gas leads to the appearance of CO₃²⁻ conduction. The overall conductivity of the composite electrolyte is measured with a current-interruption technique and the ions transferred by O²⁻/H⁺/CO₃²⁻ respectively are obtained by a quantitative measurement of the reaction products, i.e. H₂O and CO₂. The change of the carbonate content in the composite electrolyte presents a great influence on the conductivity of each ion. According to these experimental facts, the pathways for the individual ionic conductions are proposed.     

Electrical conductivity and related properties of molten carbonates coexisting with ceria-based oxide powder for hybrid electrolyte

The electrical, thermal and structural properties of composite electrolyte containing Ce_0.9Gd_0.1O_1.95 (GDC) powder and (Li_0.52Na_0.48)₂CO₃ eutectics are investigated by AC impedance, differential thermal analysis and polarized Raman scattering spectroscopy. The system shows a dependence of the electrical conductivity upon the temperature. The transition point varies with the apparent average thickness of the liquid phase, while the activation energy, ΔE_a, remains constant at any distance from the solid phase. Higher electrical conductivity was obtained for the GDC/(Li_0.52Na_0.48)₂CO₃ composite than that for α-Al₂O₃/(Li_0.52Na_0.48)₂CO₃. Even in the N₂ or Air gas flow, the weight loss caused by decomposition of CO₃²⁻ ion based on Lux–Flood equilibrium was rarely observed. The symmetric stretching mode of the polarized Raman spectra shows that carbonate ion maintains its D_3h symmetry in the presence of ceria. A constant value of the depolarization ratio of the ν₁(A′₁) mode with regard to the apparent average thickness confirms that the symmetry of carbonate ions in the molten state is not altered by the presence of ceria powder. It was more stable than that for the system containing α-Al₂O₃ as a reference sample. These findings contribute to the understanding of the properties of ceria-based carbonate electrolyte for intermediate temperature solid oxide fuel cells.     

A high performance intermediate temperature fuel cell based on a thick oxide–carbonate electrolyte

A high performance intermediate temperature fuel cell (ITFC) with composite electrolyte composed of co-doped ceria Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) and a binary carbonate-based (52 mol% Li₂CO₃/48 mol% Na₂CO₃), 1.2 mm thick electrolyte layer has been developed. Co-doped Ce_0.8Gd_0.05Y_0.15O_1.9 was synthesized by a glycine–nitrate process and used as solid support matrix for the composite electrolyte. The conductivity of both composite electrolyte and GYDC supporting substrate were measured by AC impedance spectroscopy. It showed a sharp conductivity jump at about 500 °C when the carbonates melted. Single cells with thick electrolyte layer were fabricated by a dry-pressing technique using NiO as anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ or lithiated NiO as cathode. The cell was tested at 450–550 °C using hydrogen as the fuel and air as the oxidant. Excellent performance with high power density of 670 mW cm⁻² at 550 °C was achieved for a 1.2 mm thick composite electrolyte containing 40 wt% carbonates which is much higher than that of a cell based on pure GYDC with a 70 μm thick electrolyte layer.