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.     

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.     

Intermediate temperature fuel cell with a doped ceria-carbonate composite electrolyte

The performance of a composite electrolyte composed of a samarium doped ceria (SDC) and a binary eutectic carbonate melt phase has been examined. This material shows higher ionic conductivity than pure SDC in intermediate temperature region. SDC with different morphologies is obtained by co-precipitation, sol-gel and glycine-nitrate combustion preparation techniques. A tri-layer single cell is prepared with a cost-effective co-pressing and co-sintering technique. It is found that the surface properties of SDC and the electrolyte thickness have a great influence on the fuel cell performance. When the co-precipitated SDC is used as the electrolyte component and CO₂/O₂ gas mixture is adopted as the cathode oxidant gas, a fuel cell with an excellent performance is obtained, which has a peak power output of 1704 mW cm⁻² at a current density of 3000 mA cm⁻² at 650 °C. The influence of cathode atmosphere is examined with conductivity measurement and fuel cell performance test. The results support the concept of O²⁻/H⁺/CO₃²⁻ ternary conduction.     

A high performance composite ionic conducting electrolyte for intermediate temperature fuel cell and evidence for ternary ionic conduction

The performance of a composite electrolyte composed of a samarium doped ceria (SDC) and a ternary eutectic carbonate melt phase was examined. The formation temperature of a continuous carbonate melt phase is crucial to the high conductivity of this material. The electrolyte contains 30 and 50 wt% carbonate exhibited a sharp increase of conductivity at a temperature close to the melting point of the eutectic carbonate, ca 400 °C, which is more than 100 °C lower than those electrolytes using binary carbonate. At around 650 °C, and with CO₂/O₂ used as the cathode gas, the fuel cell gave a power output 720 mW cm⁻² at a current density 1300 mA cm⁻². Water was measured in both the anode and cathode outlet gases and CO₂ was detected in the anode outlet gas. When discharged at 800 mA cm⁻², a stable discharge plateau was obtained. The CO₂ in the cathode gas enhances the power output and the stability of the single cell. Based on these experimental facts, a ternary ionic conducting scheme is proposed and discussed.     

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.     

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.     

Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95

Ceria-based electrolytes have been widely investigated in intermediate-temperature solid oxide fuel cell (SOFC), which might be operated at 500–600 °C. Samarium doped (20 mol%) ceria (20SDC) one of the most promising material in this class of compounds. In this work we report effect of lattice substitution of 5 mol % Li on Sm in (20SDC). It was prepared by citrate–nitrate auto combustion synthesis having a powder of average particle size ∼50 nm. The sintered density of more than 98% of the theoretical density at 950 °C has been achieved. Increased ionic conductivity (lattice) at 500 °C has also been achieved in Ce_0.75Sm_0.2Li_0.05O_1.95 compare to that of Ce_0.8Sm_0.2O_1.95. Corresponding activation energy of conduction ∼0.7 eV has been calculated in the temperature range of 200–600 °C. In reducing atmosphere the electrical conductivity has not been altered much. Thus Ce_0.75Sm_0.2Li_0.05O_1.95 has been found to be quite promising in terms of reducing the processing temperature as well as operating temperature of SOFC.     

Thermal stability study of La0.9Sr0.1Ga0.8Mg0.2O2.85–(Li/Na)2CO3 composite electrolytes for low-temperature solid oxide fuel cells

Novel composite materials based on La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) and a binary eutectic carbonates (52 mol% Li₂CO₃:48 mol% Na₂CO₃) are potential electrolytes for low-temperature solid oxide fuel cells (LTSOFCs) operating at 400–600 °C. However, thermal stability of the LSGM–(Li/Na)₂CO₃ composites remains in doubt due to the molten state of the carbonates at elevated temperature. In this paper, XRD, SEM, TGA and EIS were employed for thermal ageing and cycling studies of the LSGM–(Li/Na)₂CO₃ composites. XRD and SEM results showed that ageing induced a slight effect on the structure and morphology of the composites. TGA and EIS results indicated that the composites had a good stability during cycling. The LSGM–20 wt% (Li/Na)₂CO₃ sample showed a relatively stable conductivity (7–9 × 10⁻² S cm⁻¹) during a 650 h measurement under air at 600 °C. Single cell based on the composite electrolytes was reported for the first time, a maximum power density of 617 W cm⁻² and the open circuit voltage (OCV) of 1.01 V were achieved at 600 °C for the composite containing 20 wt% carbonates. The notable thermal stability together with fairly high performance emphasize the promise of LSGM–(Li/Na)₂CO₃ composite electrolytes for long-term LTSOFCs.     

Role of gas-phase composition on the performance of ceria-based composite electrolytes

Ceria-based composites (ranging from 30 to 70 vol%) including a mixture of Li and Na carbonates (1:2 M ratio, respectively) were sequentially exposed to pure CO₂, O₂ and a mixture of 10 vol% H₂ diluted in N₂, at temperatures ranging from 300 to 580 °C, to study the role of composite/gas interaction on electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural and structural characterization. Changes in conductivity and electrode impedance when changing the atmosphere from CO₂ to O₂ and to H₂ confirmed changes in concentration and/or in mobility of charge carriers and electroactive species. Novel species identified either by X-ray diffraction (hydrated carbonates, only present at the surface of the samples), or by infrared spectroscopy (hydrogen carbonate ions), might be related to the observed conductivity enhancement under H₂.     

Potential low-temperature application and hybrid-ionic conducting property of ceria-carbonate composite electrolytes for solid oxide fuel cells

Ceria-carbonate composite materials have been widely investigated as candidate electrolytes for solid oxide fuel cells operated at 300-600 °C. However, fundamental studies on the composite electrolytes are still in the early stages and intensive research is demanded to advance their applications. In this study, the crystallite structure, microstructure, chemical activity, thermal expansion behavior and electrochemical properties of the samaria doped ceria-carbonate (SCC) composite have been investigated. Single cells using the SCC composite electrolyte and Ni-based electrodes were assembled and their electrochemical performances were studied. The SCC composite electrolyte exhibits good chemical compatibility and thermal-matching with Ni-based electrodes. Peak power density up to 916 mW cm⁻² was achieved at 550 °C, which was attributed to high electrochemical activity of both electrolyte and electrode materials. A stable discharge plateau was obtained under a current density of 1.5 A cm⁻² at 550 °C for 120 min. In addition, the ionic conducting property of the SCC composite electrolyte was investigated using electrochemical impedance spectroscopy technique. It was found that the hybrid-ionic conduction improves the total ionic conductivity and fuel cell performance. These results highlight potential low-temperature application of ceria-carbonate composite electrolytes for solid oxide fuel cells.     

One-step synthesis of composite electrolytes of Eu-doped ceria and alkali metal carbonates

Europium-doped ceria (EDC, Ce_0.9Eu_0.1O_2−δ)/alkaline carbonate (LNC, (Li,Na)₂CO₃) composite ceramics prepared through a one-step citrate-based route were analyzed by powder X-ray diffraction, infrared and laser Raman spectroscopies as well as scanning and transmission electron microscopy. The electrochemical behavior of the electrolyte material was studied by impedance spectroscopy in air, CO₂ and N₂ + H₂ (90/10 vol%, respectively) gas mixtures, in the temperature range 300–600 °C. The sub micrometric and even nanosized ceramic particles appeared as merged inside the mixed carbonates, with modest grain to grain necking. The EDC/LNC composite electrolytes showed a conductivity of 0.27 S cm⁻¹ at 600 °C in air, amongst the best ever reported, exceeding the usual requirements for fuel cell applications.     

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.     

Electrochemical oxidation of graphite in an intermediate temperature direct carbon fuel cell based on two-phases electrolyte

As a promising intermediate temperature fuel cell, Direct Carbon Fuel Cell (DCFC) with composite electrolyte composed of Samarium-Doped Ceria (SDC) and a binary carbonate phase (67 mol% Li₂CO₃/33 mol% Na₂CO₃) has a much higher efficiency compared with conventional power suppliers. In the present work, SDC powder has been synthesized by an oxalate co-precipitation process and used as solid support matrix for the composite electrolyte. Single cell with composite electrolyte layer is fabricated by a dry-pressing technique using LiNiO₂/Li₂Na₂CO₃/SDC as cathode and 1:9 (weight ratio) graphite mixture with 67 mol% Li₂CO₃/33 mol% Na₂CO₃ molten carbonate as anode. The cell is tested at 600–750 °C using electrolytical graphite mixture as fuel and O₂/CO₂ mixture as oxidant. A relatively good performance with high power density of 58 mW cm⁻² at 700 °C is achieved for a DCFC using 0.8 mm thick composite electrolyte layer. The sensibility of the 1 cm² DCFC single cell performance to the anode gas nature is also investigated. At temperatures higher than 700 °C, both carbon (C) and carbon monoxide (CO) can be considered as reacting fuel for the DCFC system.     

Investigation of La2NiO4+δ-based cathodes for SDC–carbonate composite electrolyte intermediate temperature fuel cells

Lanthanum nickelate based oxides, including La₂NiO_4+δ (LN), La₂Ni_0.8Co_0.2O_4+δ (LNC82) and La₂Ni_0.8Fe_0.2O_4+δ (LNF82), were investigated as cathodes for intermediate temperature fuel cells with samaria doped ceria (SDC)–carbonate composite electrolytes. These oxides were synthesized by glycine–nitrate process and characterized by XRD and SEM, showing that all samples annealed at 800 °C for 2 h exhibit a K₂NiF₄ phase and a foam-like structure. The electrochemical properties of these cathodes were evaluated by fabricating and testing fuel cells with two kinds of composite electrolytes, SDC-20 wt.% (0.53Li/0.47Na)₂CO₃ and SDC-30 wt.% (0.67Li/0.33Na)₂CO₃, referred to as SDC(53L47N)20 and SDC(67L33N)30, respectively. Among these three cathodes, LNC82 shows the best cell performances at 500–600 °C. Moreover, fuel cells with SDC(67L33N)30 composite electrolyte present much higher power output than those with SDC(53L47N)20 composite electrolyte. It reveals that cobalt doping greatly enhances the electrochemical property of lanthanum nickelate, and such cathodes are more compatible with the SDC(67L33N)30 composite electrolyte.     

Addition effects of erbia-stabilized bismuth oxide on ceria-based carbonate composite electrolytes for intermediate temperature-−solid oxide fuel cells

Highly conductive Er_0.2Bi_0.8O_1.5 (ESB) and rare-earth doped ceria solid oxide electrolytes (SOEs) at intermediate temperature (IT) continue to suffer disadvantages in terms of thermodynamic instability and significant electronic conduction, respectively, at low oxygen partial pressure for solid oxide fuel cell (SOFC) operations. It is therefore necessary to improve the low-temperature ionic conductivity in order to enhance the electrolytic domain of these materials and thereby mitigate cell efficiency dissipation by electronic conduction. In this work, an advanced multiphase carbonate composite material based on ceria has been developed to overcome this IT-SOE challenge. This advanced electrolyte is comprise of nanostructured neodymium-doped ceria (NDC) and 38 wt% (Li–0.5Na)₂CO₃ carbonate with a small amount of ESB phase. The addition of 2 wt% ESB in ceria-based materials decreases the grain boundary resistance of the SOEs in the IT range. Further, a small amount of highly conducting ESB phase in the NDC/[(Li–0.5Na)₂CO₃] composite electrolyte increases the overall conductivity of the composite SOEs. The NDC electrolyte containing 38 wt% carbonate shows the highest conductivity of 0.104 Scm⁻¹ at 600 °C, while the conductivity is increased to 0.165 Scm⁻¹ by the addition of 2 wt% ESB. In addition, the activation energy of the multiphase composite electrolytes (0.52 eV) is lower than that of the NDC/carbonates (0.65 eV) in the IT range. This is attributed to the effect of the physical properties of the NDC sample, induced by the light ESB doping, on the ionic conductivity, and this effect is closely associated with the grain boundary property. Furthermore, the interfacial effects of the multiphase materials also contribute to the improved conductivity of this advanced composite electrolyte.     

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.     

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.     

Electrical properties of SDC–BCY composite electrolytes for intermediate temperature solid oxide fuel cell

The electrolyte material Ce_0.85Sm_0.15O_1.92 (SDC) powders are synthesized by glycine–nitrate processes and BaCe_0.83Y_0.17O_3−δ (BCY) powders are synthesized by sol–gel processes, respectively. Then SDC–BCY composite electrolytes are prepared by mixing SDC and BCY. The SDC and BCY powders are mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as SB95, SB90 and SB85, respectively. The electrical properties of SDC and SDC–BCY composites are investigated. The experimental results show that SDC–BCY composites exhibit the excellent conductivity and could significantly enhance the fuel cell performances. The behavior that SDC–BCY composites display hybrid proton and oxygen ion conduction is substantiated. Among these electrolytes, the maximum power density reaches as high as 159 mW cm⁻² for the fuel cell based on SB90 composite electrolyte at 600 °C.     

Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells

Sm_0.2Ce_0.8O_1.9 (SDC)/Na₂CO₃ nanocomposite synthesized by the co-precipitation process has been investigated for the potential electrolyte application in low-temperature solid oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na₂CO₃ nanocomposite has been studied. The performance of 20 mW cm⁻² at 490 °C for fuel cell using Na₂CO₃ as electrolyte has been obtained and the proton conduction mechanism has been proposed. This communication demonstrates the feasibility of direct utilization of methanol in low-temperature SOFCs with the SDC/Na₂CO₃ nanocomposite electrolyte. A fairly high peak power density of 512 mW cm⁻² at 550 °C for fuel cell fueled by methanol has been achieved. Thermodynamical equilibrium composition for the mixture of steam/methanol has been calculated, and no presence of C is predicted over the entire temperature range. The long-term stability test of open circuit voltage (OCV) indicates the SDC/Na₂CO₃ nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.