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.     

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.     

Novel structured gadolinium doped ceria based electrolytes for intermediate temperature solid oxide fuel cells

Novel three-layered intermediate temperature solid oxide fuel cell (SOFC) electrolytes based on gadolinium doped ceria (GDC) are developed to suppress the electronic conductivity of GDC, to improve the mechanical properties of the cell and to minimize power loss due to mixed conductive nature of GDC. Three different electrolytes are fabricated by sandwiching thin YSZ, ScSZ and ScCeSZ between two relatively thick GDC layers. An electrolyte composed of pure GDC is also manufactured for comparison. NiO/GDC and LSCF/GDC electrodes are then coated on the electrolytes by a screen printing route. SEM results show that it is possible to obtain dense and crack free thin layers of YSZ, ScSZ and ScCeSZ between two GDC layers without delamination. Performance measurements indicate that interlayered thin electrolytes act as an electronic conduction barrier and improve open circuit voltages (OCVs) of GDC based cells.     

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.     

Study on GDC-LSGM composite electrolytes for intermediate-temperature solid oxide fuel cells

The electrolyte materials Ce_0.9Gd_0.1O_1.9 (GDC) and La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) were synthesized by means of glycine-nitrate processes, respectively, then GDC-LSGM composite electrolytes were prepared by mixing GDC and LSGM. The GDC and LSGM powders were mixed in the weight ratio of 95:5, 90:10 and 85:15 and named as GL9505, GL9010 and GL8515. Their structures and ionic conductivities were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and AC impedance spectroscopy. The grain sizes of GDC-LSGM composites could be increased distinctly and the grain boundary resistance could be significantly decreased by small addition of LSGM. The experimental results show that the GDC-LSGM composites exhibit excellent ionic conductivity and could significantly enhance the fuel cell performances. The open circuit voltages are higher in the cell with composite electrolytes than in the cell with single GDC as electrolyte at the working temperature. Among these electrolytes, GL9505 has the highest ionic conductivity and the maximum power density.     

Electrochemical impact of the carbonate in ceria-carbonate composite for low temperature solid oxide fuel cell

Ceria-carbonate composite has been suggested as a promising electrolyte for solid oxide fuel cells operating at low temperatures. However, the roles of carbonate in the enhancement of the superionic conductivity and fuel cell performance of ceria-carbonate composite electrolytes are not yet confirmed. In this work, we look into the chemical and ionic state, transmission and segregation of carbonate and alkali cations under normal and electrochemical conditions. The XRD measurement confirms that there are not any carbonate crystals in the sample electrolyte. It is interesting to see that part of the carbonate and alkali ions are not formed into the stoichiometric carbonate in sample materials under the typical electrical field condition from the EDS and XPS analysis. Instead, carbonate (CO₃^2?) and alkali ions accumulate in the cathode side, which we believe, was caused by the electrochemical “catalyst” of CO₂ and alkali ions that accelerated the electrochemical oxygen reduction reaction, while the CO₃^2? ions as one of the charge carriers which diffuse from fuel cell cathode to anode are on account of the concentration gradient. Those together contribute to the excellent electrochemical performances of ceria-carbonate composite electrolyte for low temperature solid oxide fuel cell.     

3D printed Sm-doped ceria composite electrolyte membrane for low temperature solid oxide fuel cells

Three dimensional (3D) printing has attracted much more interest from the research community due to its ability to make complex structures with high resolution and simplified fabrication process. Here we constructed composited electrolyte with certain thickness for the application of low temperature of solid oxide fuel cells. The fabrication of a thin and dense Sm-doped ceria composite electrolyte layers with the thickness about 1200 μm utilizing paraffin-based slurry were investigated. To optimize the assembled cells, 1.7 wt. % glassfiber was introduced and an amazing electrochemical performance was observed. The maximum power density can reach 448 mW/cm², 20% higher than the one without glassfiber and the open-circuit voltage is approximately 1.0 V at 550 °C. It is of great potential for 3D printing technology to develop low temperature solid fuel cells with designed mini-structures.     

Geometrical modeling of microstructure of solid oxide fuel cell composite electrodes

A model based on random packing of electron and ion conductor particles is developed to study the microstructure performance relationship of solid oxide fuel cell electrodes. This three-dimensional model takes into account special variations of triple phase boundary (TPB) by keeping track of all particles in the packing. Porosity of the media can be controlled and is set to 30%. Effect of particle size, electrode thickness, electrode composition and particle size ratio on the length of TPB line has been studied. The study shows that unlike what models based on percolation theory suggest, the electrode media is not homogeneous for electrochemical reaction. While increasing the thickness increases the length of the TPB to some extent, beyond that little or no improvement was observed. The study also revealed that adding a current collector layer made of electron conductors can increase the TPB line by at least 4%. While for particles of the same size maximum length of TPB was observed at equal volume percent of electron and ion conductor particles, for size ratio of particles other than one the maximum TPB tends to occur above or below 50% depending on the size ratio.     

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.     

Reinforced composite sealants for solid oxide fuel cell applications

Glass-ceramic sealants are commonly used as joining materials for planar solid oxide fuel cells stacks. Several requirements need to be fulfilled by these materials: beside of electrical insulation and appropriate thermal expansion, a good adhesion on the ceramic and metallic components of a SOFC stack is necessary to form a gas-tight joint. Even though the joining process might have been successful, failures and leaks often occur during the stack operation due to fracture of the brittle material under thermal stresses or during thermal cycling of the components. This study focusses on composite materials consisting of a glass matrix based on the system of BaO-CaO-SiO₂ and various filler materials, e.g. yttria-stabilized zirconia fibres or particles and silver particles. In order to evaluate a possible reinforcing influence of the filler material of the composite, tensile strength tests were carried out on circular butt joints. The highest strength values were found for the composite material with addition of silver particles, followed by the glass matrix itself without any filler addition and the lowest values were measured for the composite with YSZ particles. SEM investigations of cross-sections of the joints elucidated these results by the microstructure of the glass-ceramic sealants.     

The heterogeneous electrolyte of CuFeO2 nano-flakes composited with flower-shaped ZnO for advanced solid oxide fuel cells

The flower-shaped ZnO was synthesized to form composite with the delafossite structure CuFeO₂. The composite heterojunction formed for the ZnO-CuFeO₂ composite material demonstrates a profound significance for exploring novel materials in solid oxide fuel cell (SOFC) field. At 550 °C, power outputs of 300 mW cm^?2 and 468 mW cm^?2 were achieved for SOFC devices using pure ZnO and composite with CuFeO₂ as the electrolytes, respectively. The composite showed a good performance at low temperatures, for instance, it showed a power output of 148 mW cm^?2 at 430 °C. The studies on photocurrent-time curves with visible light on/off irradiation provided an evidence for electron-hole separation. The heterojunctions separate holes and electrons, preventing short-circuiting while used in the SOFC device. These results demonstrate that introducing the heterojunctions in the electrolyte is an innovative approach for advanced SOFCs.     

Latest development of double perovskite electrode materials for solid oxide fuel cells: a review

Recently, the development and fabrication of electrode component of the solid oxide fuel cell (SOFC) have gained a significant importance, especially after the advent of electrode supported SOFCs. The function of the electrode involves the facilitation of fuel gas diffusion, oxidation of the fuel, transport of electrons, and transport of the byproduct of the electrochemical reaction. Impressive progress has been made in the development of alternative electrode materials with mixed conducting properties and a few of the other composite cermets. During the operation of a SOFC, it is necessary to avoid carburization and sulfidation problems. The present review focuses on the various aspects pertaining to a potential electrode material, the double perovskite, as an anode and cathode in the SOFC. More than 150 SOFCs electrode compositions which had been investigated in the literature have been analyzed. An evaluation has been performed in terms of phase, structure, diffraction pattern, electrical conductivity, and power density. Various methods adopted to determine the quality of electrode component have been provided in detail. This review comprises the literature values to suggest possible direction for future research.     

Performance evaluation of combined solid oxide fuel cells with different electrolytes

The performance of a hybrid system of solid oxide fuel cells with different electrolytes, i.e., an oxygen-ion conducting electrolyte (SOFC-O²⁻) and a proton-conducting electrolyte (SOFC-H⁺) is evaluated in this study. Due to an internal reforming operation, SOFC-O²⁻ can produce electrical power as well as high-temperature exhaust gas containing remaining fuel, i.e., H₂ and CO that can be used for SOFC-H⁺ operation. The remaining CO can further react with H₂O via water gas-shift reaction to produce more H₂ within SOFC-H⁺ and thus, the possibility of carbon formation in SOFC-H⁺ can be eliminated and overall system efficiency can be improved. The simulation results show that the performance of the SOFC-O²⁻–SOFC-H⁺ system provides a higher efficiency (54.11%) compared with the use of a single SOFC. Further, the SOFC hybrid system performance is investigated with respect to important operating conditions, such as temperature, pressure, degree of pre-reforming, inlet fuel velocity, and cell voltage.     

Perovskite-related intergrowth cathode materials with thin YSZ electrolytes for intermediate temperature solid oxide fuel cells

The electrochemical performances of solid oxide fuel cells with thin yttria-stabilized zirconia (YSZ) electrolytes and YSZ/Ni anodes were studied with two intergrowth oxides cathodes (Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and LaSr₃Fe_1.5Co_1.5O_10−δ) and the results compared to a related perovskite cathode (La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ). It was found that cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ exhibited peak power densities close to 0.75 W cm⁻², despite the relatively modest electrical conductivity of this compound. In contrast, cells produced with Sr_2.7La_0.3Fe_1.4Co_0.6O_7−δ and La_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ cathodes both exhibited peak power densities of less than 0.4 W cm⁻². The greater performance for the cells produced with LaSr₃Fe_1.5Co_1.5O_10−δ may be attributed to a higher catalytic activity for this compound or to an improved adhesion of the cathode to the interlayer/electrolyte.     

Ni-Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells with stabilized zirconia electrolytes

Ni-LnO_x cermets (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd), in which LnO_x is not an oxygen ion conductor, have shown high performance as the anodes for low-temperature solid oxide fuel cells (SOFCs) with doped ceria electrolytes. In this work, Ni-Sm₂O₃ cermets are primarily investigated as the anodes for intermediate-temperature SOFCs with scandia stabilized zirconia (ScSZ) electrolytes. The electrochemical performances of the Ni-Sm₂O₃ anodes are characterized using single cells with ScSZ electrolytes and LSM-YSB composite cathodes. The Ni-Sm₂O₃ anodes exhibit relatively lower performance, compared with that reported Ni-SDC (samaria doped ceria) and Ni-YSZ (yttria stabilized zirconia) anodes, the state-of-the-art electrodes for SOFCs based on zirconia electrolytes. The relatively low performance is possibly due to the solid-state reaction between Sm₂O₃ and ScSZ in fuel cell fabrication processes. By depositing a thin interlayer between the Ni-Sm₂O₃ anode and the ScSZ electrolyte, the performance is substantially improved. Single cells with a Ni-SDC interlayer show stable open circuit voltage, generate peak power density of 410 mW cm⁻² at 700 °C, and the interfacial polarization is about 0.7 Ω cm².     

Nanocrystalline doped lanthanum cobalt ferrite and lanthanum iron cobaltite-based composite cathode for significant augmentation of electrochemical performance in solid oxide fuel cell

Sr-doped lanthanum cobalt ferrite (La_0.54Sr_0.40Co_0.20Fe_0.80O_3−δ) and lanthanum iron cobaltite (La_0.54Sr_0.40Fe_0.20Co_0.80O_3−δ)-based mixed ionic and electronic conducting solid oxide fuel cell cathodes are synthesized by autocombustion technique. In order to examine the electrochemical activity including thermal matching with the adjacent cell components, a composite cathode comprising of both the ferritic and cobaltite system is prepared using mechanical mixing. Powder characterizations for cobaltite and ferritic-based perovskite revealed nanocrystallinity (15–30 nm) with particulate size ranging 50–100 nm. Anode-supported half cell with suitable doped ceria based interlayer on the top of the electrolyte and developed composite cathode augments the current density to 3.98 Acm⁻² at 0.7 V at 800 °C. The oxygen reduction reaction kinetics of such composite cathode shows high exchange current density of 1.16 Acm⁻² with relatively low electrode polarization of 0.02 Ωcm² at 800 °C. The electrochemical performance is clinically correlated with the cell microstructure exhibiting minimum SrO diffusion at the electrode-electrolyte interface.     

Development of trimetallic Ni–Cu–Zn anode for low temperature solid oxide fuel cells with composite electrolyte

Trimetallic alloys of Ni_0.6Cu_0.4−xZn_x (x = 0, 0.1, 0.2, 0.3, 0.4) have been investigated as promising anode materials for low temperature solid oxide fuel cells (SOFCs) with composite electrolyte. The alloys have been obtained by reduction of Ni_0.6Cu_0.4−xZn_xO oxides, which are synthesized by using the glycine–nitrate process. Increasing the Zn content x decreases the particle sizes of the oxides at a given sintering temperature. Fuel cells have been constructed using lithiated NiO as cathode and as-prepared alloys as anodes based on the composite electrolyte. Peak power densities are observed to increase with the increasing Zn addition concentration into the anode. The maximum power density of 624 mW cm⁻² at 600 °C, 375 mW cm⁻² at 500 °C has been achieved for the fuel cell equipped with Ni_0.6Zn_0.4 anode. A.c. impedance results show that the resistances dramatically decrease with increasing temperatures under open circuit voltage state. Both cathodic and anodic interfacial polarization resistances increase with the amplitude of applied DC voltage. Possible reaction process for H₂ oxidation reaction at anode based on composite electrolyte has been proposed for the first time. The stability of the fuel cell with Ni_0.6Cu_0.2Zn_0.2 composite anode has been investigated. The results indicate that the trimetallic Ni_0.6Cu_0.4−xZn_x anodes are considerable for low temperature SOFCs.     

Evaluation of porous platinum,nickel, and lanthanum strontium cobaltite as electrode materials for low-temperature solid oxide fuel cells

Porous Pt, Ni, and lanthanum strontium cobaltite (LSC) are evaluated as electrode materials for solid oxide fuel cells at the low temperature range under 500 °C. Porous metal electrodes 150 nm thick are prepared by sputtering. Porous LSC was deposited to a typical thickness of 1.5 μm by pulsed laser deposition as the cathode. In terms of fuel cell performance, we confirm that Pt is the best material for both the cathode and the anode under 400 °C, but LSC outperforms Pt as a cathode at temperatures over 450 °C in our configurations. Porous Ni anode is identified as being less effective than the porous Pt. It is determined that these results are closely related to the differences in electrode performance and to morphological changes during fuel cell operation.     

Enhanced ionic conductivity of yttria-stabilized ZrO2 with natural CuFe-oxide mineral heterogeneous composite for low temperature solid oxide fuel cells

We report for the first time that the commercial yttrium stabilized zirconia (YSZ) nanocomposite with a natural CuFe-oxide mineral (CF) exhibits a greatly enhanced ionic conductivity in the low temperature range (500–600 °C), e.g. 0.48 S/cm at 550 °C. The CF–YSZ composite was prepared via a nanocomposite approach. Fuel cells were fabricated by using a CF–YSZ electrolyte layer between the symmetric electrodes of the Ni_0.8Co_0.2Al_0.5Li (NCAL) coated Ni foam. The maximum power output of 562 mW/cm² has been achieved at 550 °C. Even the CF alone to replace the electrolyte the device reached the maximum power of 281 mW/cm² at the same temperature. Different ion-conduction mechanisms for YSZ and CF–YSZ are proposed. This work provides a new approach to develop natural mineral composites for advanced low temperature solid oxide fuel cells with a great marketability.