Development of co-sintering process for anode-supported solid oxide fuel cells with gadolinia-doped ceria/lanthanum silicate bi-layer electrolyte

Gadolinia-doped ceria (GDC) and lanthanum silicate (LS) are expected to be promising materials for electrolytes of solid oxide fuel cells (SOFCs) because of their high ionic conductivities at intermediate temperatures. However, performance degradation of SOFCs is caused by current leakage through GDC and poor densification of LS. In the present study, LS was used as a blocking layer for preventing the current leakage of GDC electrolyte. Thermal shrinkage measurements and scanning electron microscopy (SEM) observation suggested that the addition of Bi₂O₃ in LS electrolyte (LSB) contributed to the decrease in the sintering temperature of the LS and improved densification of the GDC/LS bi-layer electrolyte. Consequently, the open-circuit voltage for the cell with GDC/LS and GDC/LSB bi-layer electrolytes increased effectively in comparison with that of the cell with GDC single-layer electrolyte. The electrical conductivity and fuel cell characteristics were compared among the cells with GDC, GDC/LS, and GDC/LSB electrolytes.     

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.     

Interface engineering towards low temperature in-situ densification of SOFC

Electrolyte densification, which is often realized by high temperature sintering (i.e. >1000 °C), is an essential process for solid oxide fuel cell (SOFC). However, it is hard to achieve interfaces with high ionic conductivity because the interfaces between particles would be greatly eliminated during the sintering process. In this study, a novel interface engineering method is designed basing on capillary action to densify the electrolyte and at the meantime achieve ionic conductive interfaces at low temperature. A porous electrolyte layer is found to become dense during fuel cell operation when alkali metal hydroxide (AMH) is added to the NiO anode. The observation of improved open circuit voltage (OCV) indicates gas leakage has been eliminated after AMH modification. Raman images confirm that AMHs can be absorbed into the electrolyte layer when the operating temperature is higher than the melting point of AMH. In addition, using a lithiated metal oxide (i.e. LiNiO₂) as the anode, cell performance is further improved. EIS proves that the existing of AMH in the cell may affect gas diffusion in the electrode, but it significantly reduces ohmic resistance due to better interfacial ionic conductivity of the electrolyte. This in-situ electrolyte densification method not only enables us to simplify fuel cell fabrication process and lower the fabrication temperature but also provides ways for maintaining interface conductivity.     

A review of thin film electrolytes fabricated by physical vapor deposition for solid oxide fuel cells

The ohmic resistance in solid oxide fuel cells (SOFCs) mainly comes from the electrolyte, which can be reduced by developing novel electrolyte materials with higher ionic conductivity and/or fabricating thin-film electrolytes. Among various kinds of thin-film fabrication technology, the physical vapor deposition (PVD) method can reduce the electrolyte thickness to a few micrometers and mitigate the issues associated with high-temperature sintering, which is necessary for wet ceramic methods. This review summarizes recent development progress in thin-film electrolytes fabricated by the PVD method, especially pulsed laser deposition (PLD) and magnetron sputtering. At first, the importance of the substrate surface morphology for the quality of the film is emphasized. After that, the fabrication of thin-film doped-zirconia and doped-ceria electrolytes is presented, then we provide a brief summary of the works on other types of electrolytes prepared by PVD. Finally, we have come to the summary and made perspectives.     

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.     

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.     

Enhanced ionic conductivity in calcium doped ceria – Carbonate electrolyte: A composite effect

Recently, ceria-based nanocomposites, as a proton and oxygen ion conductor, has been developed as promising electrolyte candidates for low-temperature solid oxide fuel cells (LTSOFCs). Up to now, samarium doped ceria (SDC) was studied as a main oxide for nanocomposite electrolyte; while calcium doped ceria (CDC) is considered as a good alternative from both material performance and economical aspects. Yet the conduction behavior of CDC-based composite has not been reported. In the present study, calcium doped ceria was prepared by oxalate co-precipitation method, and used for the fabrication of CDC/Na₂CO₃ composite. The thermal decomposition process, structure and morphology of the samples were characterized by TGA, XRD, SEM, etc. The oxygen ion conductivity of single phase CDC sample was measured by electrochemical impedance spectroscopy (EIS), while the proton and oxygen ion conductivity of CDC/Na₂CO₃ nanocomposite sample were determined by four-probe d.c. measurements. The CDC/Na₂CO₃ samples show significantly enhanced overall ionic conductivity compared to that of single phase CDC samples, demonstrating pronounced composite effect. This confirms that the use of nanocomposite as electrolyte can effectively lower the operation temperature of SOFC due to improved ionic conductivity.     

Structural and electrochemical studies of microwave sintered nanocomposite electrolytes for solid oxide fuel cells

In this study, a nanocomposite electrolyte is synthesized using a microwave sintering technique, providing an alternative method and improving the conventional sintering technique. In this paper, samarium doped ceria (SDC) materials are synthesized by the co-precipitation method using carbonates as precipitating agents. The precursor of SDC-carbonate is sintered in a microwave (MW) oven and conventionally heated in a digital furnace at 900 °C. X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques are applied for structural studies. For electrochemical characterization, four probe conductivity and fuel cell performances are completed. The materials prepared using MW improved the densification and exhibited excellent sintering during compared to the conventional method of preparation for the same material. Excellent fuel cell performance (0.65 W cm⁻²) is achieved with microwave sintering. This method of sintering proves that the microwave process can save time and energy when compared to conventional sintering. This method can provide significant economic benefits compared to conventional sintered materials for applications in fuel cell technology.     

Enhanced performance of a direct methanol alkaline fuel cell (DMAFC) using a polyvinyl alcohol/fumed silica/KOH electrolyte

A novel polymer-inorganic composite electrolyte for direct methanol alkaline fuel cells (DMAFCs) is prepared by physically blending fumed silica (FS) with polyvinyl alcohol (PVA) to suppress the methanol permeability of the resulting nano-composites. Methanol permeability is suppressed in the PVA/FS composite when comparing with the pristine PVA membrane. The PVA membrane and the PVA/FS composite are immersed in KOH solutions to prepare the hydroxide-conducting electrolytes. The ionic conductivity, cell voltage and power density are studied as a function of temperature, FS content, KOH concentration and methanol concentration. The PVA/FS/KOH electrolyte exhibits higher ionic conductivity and higher peak power density than the PVA/KOH electrolyte. In addition, the concentration of KOH in the PVA/FS/KOH electrolytes plays a major role in achieving higher ionic conductivity and improves fuel cell performance. An open-circuit voltage of 1.0 V and a maximum power density of 39 mW cm⁻² are achieved using the PVA/(20%)FS/KOH electrolyte at 60 °C with 2 M methanol and 6 M KOH as the anode fuel feed and with humidified oxygen at the cathode. The resulting maximum power density is higher than the literature data reported for DMAFCs prepared with hydroxide-conducting electrolytes and anion-exchange membranes. The long-term cell performance is sustained during a 100-h continuous operation.     

Fabrication of anode-supported thin BCZY electrolyte protonic fuel cells using NiO sintering aid

A thin and fully dense BaCe_0.6Zr_0.2Y_0.2O_3-δ (BCZY) electrolyte for the use of anode-supported protonic fuel cells has been successfully prepared by spin coating using NiO sintering aid. The effects of NiO addition on the electrolyte microstructures and fuel cell performances are also investigated. An appropriate NiO addition has a significant positive contribution to the densification and grain growth of thin BCZY electrolytes. However, too much NiO addition gives rise to NiO aggregation in BCZY electrolyte and deteriorates the cell performance. The enhanced sintering mechanism can be mainly attributed to the oxygen vacancies generated from the NiO decomposition and bulk diffusion of Ni into BCZY perovskites. The fuel cell with a BCZY-3%NiO electrolyte exhibits the highest maximum power density of ~106.6 mW/cm² at 800 °C among all fuel cells in this study. The electrochemical impedance characteristics of thin BCZY electrolyte fuel cells are further discussed under open circuit conditions.     

Novel ionic plastic crystal-polymeric ionic liquid all-solid-state electrolytes for lithium ion batteries

离子塑性晶体作为一类新型的固态电解质材料,近年来受到研究人员的极大关注。本文合成了一种新型离子塑性晶体：N,N-二甲基吡咯双氟磺酰亚胺（P₁₁FSI）,并将其与吡咯阳离子离子液体聚合物-聚二甲基二烯丙基铵双氟磺酰亚胺（PILFSI）和锂盐（LiFSI）复合制备了P₁₁FSI-PILFSI-LiFSI全固态电解质。采用差示扫描量热法、热重分析、阻抗测试、线性扫描伏安法及对称锂电池测试等一系列表征技术对全固态电解质的热性能和电化学性能进行了系统研究。所制备的电解质膜具有好的柔韧性和热稳定性,高的离子电导率和电化学稳定性,以及与金属锂良好的界面相容性。将全固态电解质应用于Li/LiFePO₄电池中,在50℃、0.2 C充放电倍率时,电池放电比容量在60次循环后仍可达151.1 mA·h/g,容量保持率为96.8%;且在0.5 C、1.0 C倍率下放电比容量仍然高达138.1 mA·h/g和128.1 mA·h/g,展现出高的放电比容量,好的循环性能和倍率性能,有望应用于全固态锂离子电池中。     

Effect of ionic conductivity of a PVC–LiClO4 based solid polymeric electrolyte on the performance of solar cells of ITO/TiO2/PVC–LiClO4/graphite

This paper reports the effect of ionic conductivity of the solid polymeric electrolyte of polyvinylchloride–lithium perchlorate (PVC–LiClO₄) on the performance of a solar cell of ITO/TiO₂/PVC–LiClO₄/graphite. Titanium dioxide films have been used as a photoelectrochemical solar cells. The films were deposited onto a ITO-coated glass substrate by a screen printing technique. The electrolytes were prepared by solution casting. The ionic conductivity of the electrolytes was obtained with an impedance spectroscopy technique. ITO and graphite films were chosen as the front and counter electrode of the device, respectively. The graphite films were deposited onto a glass substrate by the electron-beam evaporation technique. The short-circuit current density and open-circuit voltage of the device were found to increase with increasing ionic conductivity of solid polymeric electrolyte of PVC–LiClO_4. The highest short-circuit current density and open-circuit voltage were 0.94 μA cm⁻² and 186 mV, respectively. The conversion efficiency was low.     

Anode supported micro-tubular SOFC fabricated with mixed particle size electrolyte via phase-inversion technique

Cerium-gadolinium oxide is a promising material for electrolytes of intermediate temperature solid oxide fuel cells (IT-SOFCs) due to its high electrical conductivity at relatively lower temperatures of 400–700 °C. However, a high sintering temperature of up to 1550 °C is typically required to produce dense CGO electrolyte, eventually leading to an interfacial interdiffusion between the electrolyte and electrode components as well as generate a highly resistive interface which reduces ionic conductivity. Lowering the sintering temperature of the electrolyte will greatly benefit the fabrication of SOFCs. This study examines the effectiveness of introducing nano size CGO particles as an approach to get dense CGO electrolyte at lower sintering temperature. A series of dope suspensions with 0–50% nano size loading were prepared to observe rheology and measure viscosity. Then, 30% loading was selected and casting into flat sheet via phase-inversion technique. The flat sheet was characterized by morphology, surface roughness and mechanical strength tests. The suspension was extruded into dual-layer hollow fiber (DLHF) as well. The electrolyte/anode dual-layer hollow fibers (DLHFs) half-cell of micro-tubular solid oxide fuel cells (MT-SOFCs) were prepared via phase inversion based co-extrusion/co-sintering technique. The developed half-cell was characterized by morphological and gas tightness tests which further compared them with fully micron ones. The results show that the incorporation of 30% nanoparticle yielded to dense and tight CGO layers sintered at temperature 1450 °C, which about 50 °C lower than those reported previously for 100% micron particles. The I–V measurements demonstrated the maximum power density of 0.66 Wcm^?2 at temperatures 500 °C using 100% H₂ as fuel. Therefore, this approach is able to reduce the energy cost for the microstructural control of the prepared fiber and thus is recommended for the fabrication of low-cost dual-layer hollow fiber micro tubular SOFCs.     

Phase transformation and electrical properties of bismuth oxide doped scandium cerium and gadolinium stabilized zirconia (0.5Gd0.5Ce10ScSZ) for solid oxide electrolysis cell

Scandium cerium and gadolinium stabilized zirconia (SCGZ) is among the zirconia-based electrolytes which exhibits high ionic conductivity. However, stabilization of high conducting cubic phase is needed to maintain the ionic conductivity during prolonged operation. In this study, 1–2 mol% of Bi₂O₃ is added in the SCGZ to enhance the stability of the cubic phase. The phase characterization, sintering behavior, and electrochemical performance of the electrolyte-supported cells are studied. Bi₂O₃ is found to act as both phase stabilizer and sintering aid. Doping of Bi₂O₃ results in a partial decrease of unwanted rhombohedral phase (Sc₂Zr₇O₁₇) which is a low conducting phase. Increasing Bi₂O₃ content also significantly increases electrolyte densification. The unwanted rhombohedral phase is found to form at high sintering temperature. Thus, the lowest sintering temperature which is able to provide sufficiently dense electrolyte is required. In the present work, adding 2 mol% of Bi₂O₃ in the SCGZ helps reduce sintering temperature to 1350 °C with sufficiently high relative density (>96.5%). The ionic conductivity of the electrolyte is also improved with adding Bi₂O₃.     

Fabrication,microstructure and properties of a YSZ electrolyte for SOFCs

Yttria stabilized zirconia (YSZ) has widely been used as an electrolyte in solid oxide fuel cell (SOFC) stacks. The microstructure and properties of YSZ related to the fabrication process are discussed in this paper. For the named two-step sintering process, uniform and hexagonal grains with a size of 1–4 μm were obtained from the adobe following tape calendaring (TCL). Elliptical and hexagonal grains with a size of 0.4–3 μm were obtained from the adobe of tape casting (TCS) using the three-step process. The electrical conductivities of YSZ with different grain sizes were measured via the four-probe DC technique and grain conductivities and grain boundary conductivities of YSZ were investigated by impedance spectroscopy. YSZ electrolytes with a grain size of 0.1–0.4 μm had the highest electrical conductivity in the range of 500–1000 °C, especially at medium and low temperatures 550–800 °C. As the YSZ grain size becomes small, the thickness of the intergranular region decreased greatly. The YSZ electrolytes with sub-micrometer grain sizes, high ion conductivity and low sintering temperatures are important to the electrode-supported SOFC, on which the dense YSZ electrolyte films are optimized at 10 μm.     

Application of sol gel spin coated yttria-stabilized zirconia layers for the improvement of solid oxide fuel cell electrolytes produced by atmospheric plasma spraying

Due to its high thermal stability and purely oxide ionic conductivity, yttria-stabilized zirconia (YSZ) is the most commonly used electrolyte material for solid oxide fuel cells (SOFCs). Standard electrolyte fabrication techniques for planar SOFCs involve wet ceramic techniques such as tape-casting or screen printing, requiring sintering steps at temperatures above 1300 °C. Plasma spraying (PS) may provide a more rapid and cost efficient method to produce SOFCs without sintering. High-temperature sintering requires long processing times and can lead to oxidation of metal alloys used as mechanical supports, or to detrimental interreactions between the electrolyte and adjacent electrode layers. This study investigates the use of spin coated sol gel derived YSZ precursor solutions to fill the pores present in plasma sprayed YSZ layers, and to enhance the surface area for reaction at the electrolyte-cathode interface, without the use of high-temperature firing steps. The effects of different plasma conditions and sol concentrations and solid loadings on the gas permeability and fuel cell performance have been investigated.     

Fuel cells with doped lanthanum gallate electrolyte

Single cells with doped lanthanum gallate electrolyte material were constructed and tested from 600 to 800°C. Both ceria and the electrolyte material were mixed with NiO powder respectively to form composite anodes. Doped lanthanum cobaltite was used exclusively as the cathode material. While high power density from the solid oxide fuel cells at 800°C was achieved. our results clearly indicate that anode overpotential is the dominant factor in the power loss of the cells. Better anode materials and anode processing methods need to be found to fully utilize the high ionic conductivity of the doped lanthanum galiate and achieve higher power density at 800°C from solid oxide fuel cells.     

Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

Insight into time-correlated structure and interfacial evolvement of nanocomposite and semiconductor heterostructure for advanced fuel cells

Super ionic channel or shuttle correlated with interfacial engineering of nanocomposite structure and semiconductor-based material electrolytes has shown great potential in electrochemical applications, such as photochemical water splitting or low temperature advanced fuel cell. By adjusting apriority composition between n and p components, the up-to-date semiconductor-ionic membrane fuel cells (SIMFCs) exhibit the improved performance, especially for high ionic conductivity and power outputs at lower temperature. Facing the commercialization of novel advanced ceramic cells, a combined literature survey and phenomenological analysis is proposed to interpret the difference between the current-voltage curve and stability performance of low temperature solid oxide fuel cell (LTSOFC). Meanwhile, it is experimentally determined that the time constant, which is closely related to the interfacial structure and heterostructure, has played a great role in the cell properties. Herein, steady state instead of transient characteristic is preferred, dedicating to provide much more reliable data. Among several prototype cells on semiconductor-based electrolyte, only the candidate that can resist the high current operation shows suppressing inside electronic leakage and survives in short-term test. The results illustrate that though semiconductor-based nanocomposite or heterostructure is an effective methodology in developing low temperature ceramic fuel cells, its durability and the risk of short-circuit should be taken with much care.     

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.