Effect of Contact between Electrode and Interconnect on Performance of SOFC Stacks

This work investigates the effect of contact between electrodes and alloy interconnects on output performance of solid oxide fuel cell (SOFC) stacks. The measured maximum output power density (p_max) of the unit cell increases from 0.07 to 0.1 W cm^–2 by increasing the tip area of the interconnect from 40 to 60 cm². The p_max increases from 0.07 to 0.15 W cm^–2 upon the addition of nickel foam and Ag mesh on the anode and cathode side, respectively. An additional (La_0.75Sr_0.25)_0.95MO_3–σ cathode current collecting layer is re‐printed on the original cathode current collecting layer, which aims to further improve the performance of the stack and individual cell. The performance of a 3‐cell short stack assembled by the cells with a new cathode current collecting layer is evaluated by measuring the current–voltage curve. The results indicate that the p_max values of the stack and individual cells are enhanced from 0.07 to 0.37 W cm^–2 and 0.15 to 0.5 W cm^–2 at 850 °C, respectively. The performance of the whole stack and individual cells is greatly improved due to the interconnect embedded in the re‐printed new cathode current collecting layer.     

In suit Investigation of Anode Support on Cell Performance Reduced under Various Temperatures for Planar Solid Oxide Fuel Cells

The performance of anode support of Ni‐YSZ reduced from room temperature (T_R) to working temperature (T_w) and at T_w in anode‐supported planar solid oxide fuel cell was investigated quantitatively in situ. A 2 μm thick Pt voltage probe was embedded at the interface between the anode support and the function anode in the cell. Results showed that the power densities of the stack that was reduced from T_R to T_w (stack 1) and stack reduced at T_w (stack 2) were 0.343 W cm⁻² and 0.583 W cm⁻² with the corresponding fuel utilization of 36.28% and 63.87%, respectively, under the operating voltage of 0.8 V. The degradation rate of stack 1 was 7.76 times more than that of stack 2 when the stack was discharged under a constant current of 0.476 Acm⁻² for 100 h. Ni particles agglomerated in the anode support of the cell inside stack 1, whereas Ni particles in the anode support of the cell inside stack 2 were evenly distributed. The performance of stack 1 was poor mainly because of the increasing ohmic and polarization resistances caused by Ni agglomeration and decreasing porosity of the anode support.     

Evaluation of a Non‐sealed Solid Oxide Fuel Cell Stack with Cells Embedded in Plane Configuration

A non‐sealed solid oxide fuel cell stack with cells embedded in plane configuration was fabricated and operated successfully in a box‐like stainless‐steel chamber. For a two‐cell stack, it demonstrated an open circuit voltage (OCV) of 2.13 V and a maximum power output of 569 mW at the flow rate of 67 sccm CH₄ and 33 sccm O₂. A fuel utilization of 4.16% was obtained. The cell performance was dominated by two different mechanisms, the polarization of the cathode at low current and the concentration polarization of the anode at high current. Finally, a scaled‐up stack with six cells in series generated an OCV of 6.4 V and a maximum power output of 8.18 W.     

Design and Optimization of Composite Electrodes in Solid Oxide Cells

The performance of solid oxide cells (SOCs) heavily relies on the population of three‐phase boundaries (TPBs) in the composite electrodes. In this study, SOC composite electrodes are described by percolating binary particle aggregates that are constructed from random loose packing models and classical sintering theories. Summed perimeters of the sintering necks represent the total TPB lengths. A case study has been carried out on lanthanum strontium manganite (LSM)–yttria‐stabilized zirconia (YSZ) composite electrodes. By employing three‐dimensional data that are converted from relevant two‐dimensional data, the TPB length of baseline LSM–YSZ electrodes investigated in this study is 35.4 μm μm^–3. The parametric and sensitivity analyses show the changes of TPB lengths in functions of the weight fraction of powders, particle size and particle size ratio of powders, void fraction of electrodes, and density of materials. In the case of baseline LSM–YSZ electrodes, proper electrode design and optimization would result in 2–3 times of the enlargement of TPBs. Technical guidelines on the design and optimization of SOC composite electrodes are proposed.     

固体氧化物燃料电池的研究进展

固体氧化物燃料电池(SOFC)是先进陶瓷材料的一种重要应用,可以通过电化学反应将燃料的化学能直接转换为电能。SOFC具有效率高、性能稳定、便携、低污染等优点,可以实现能源的有效清洁利用。本文结合国内外SOFC的研究情况,分析讨论了SOFC的技术优势、地方产业优势及产业难题,并结合国家政策方面对我国SOFC产业的未来发展做了展望。     

Investigation of Impactors on Cell Degradation Inside Planar SOFC Stacks

The impactors on cell degradation inside planar SOFC stacks were investigated using both coated and uncoated Fe–16Cr alloys as the interconnects under stable operating conditions at 750 °C and thermal cycling conditions from 750 to 200 °C. It was found that cell degradation inside the stack is primarily dependent on the interfacial contact between the cathode current‐collecting layer and the interconnect. Additionally, cell degradation is found to be independent of the high‐temperature oxidation and Cr vaporization of the interconnects during stack operation, as the stacks are well sealed. The coating on the interconnect can further improve the contact between the cell cathode and the interconnect when the latter is properly embedded into the current‐collecting layer.     

Analysis of Leakages in a Solid Oxide Fuel Cell Stack in a System Environment

A solid oxide fuel cell (SOFC) stack can exhibit both anodic and cathodic leakages, i.e. a fuel leak from the anode side and an air leak from the cathode side of the stack, respectively. This study describes the results of an in‐situ leakage analysis conducted for a planar SOFC stack during 2000 hours of operation in an actual system environment. The leakages are quantified experimentally at nominal system operating conditions by conducting composition analysis and flow metering of gases for both fuel and air subsystems. Based on the calculated atomic hydrogen‐to‐carbon ratio of the fuel and air gases, it is found that the fuel leakages are mostly selective by nature: the leaking fuel gas does not have the same composition as the fuel system gas. A simple diffusive leakage model, based on the leakage being driven by concentration differences weighted by diffusion coefficients, is applied to quantify the amount of leakages. The leakage model provides a good correspondence with the experimental results of the gas analysis.     

Doped Germanate‐Based Apatites as Electrolyte for Use in Solid Oxide Fuel Cells

Apatite ceramics, known for their good electrical conductivities, have garnered substantial attention as an alternative electrolyte for solid oxide fuel cells (SOFCs). However, studies focusing on the electrochemical performances of SOFCs with apatities as electrolytes remain rare, partly due to their high sintering temperature. In this study, the effects of Mg²⁺, Al³⁺, Ga³⁺, and Sn⁴⁺ dopants on the characteristics of La_9.5Ge₆O_{26 ± δ} are examined and their potential for use as SOFC electrolytes evaluated. The results indicate that La_9.5Ge_5.5Al_0.5O₂₆ is stabilized into a hexagonal structure, while the La_9.5Ge_5.5Sn_0.5O_26.25, La_9.5Ge_5.5Ga_0.5O₂₆, and La_9.5Ge_5.5Mg_0.5O_25.75 ceramics reveal triclinic cells accompanied with the second phase La₂Sn₂O₇ or La₂GeO₅. The study further demonstrates that a high sintering temperature is needed for both the La_9.5Ge_5.5Mg_0.5O_25.75 and the La_9.5Ge_5.5Sn_0.5O_26.25 ceramics, and the worst electrical conductivity among the examined systems appears in the La_9.5Ge_5.5Ga_0.5O₂₆ ceramic. The La_9.5Ge_5.5Al_0.5O₂₆ ceramic is accordingly selected for cell evaluation due to its ability to reach densification at 1,350 °C, its good electrical conductivity of 0.026 S cm^–1 at 800 °C, and its acceptable thermal expansion coefficient of 10.1 × 10^–6 K^–1. The maximum power densities of the NiO‐SDC/La_9.5Ge_5.5Al_0.5O₂₆/LSCF‐SDC single cell are found to be respectively 0.22, 0.16, 0.11, and 0.07 W cm^–2 at 950, 900, 850, and 800 °C.     

Development of Single Chamber Solid Oxide Fuel Cells (SCFC)

Single Chamber Solid Oxide Fuel Cells (SCFC) have been prepared using an electrolyte as support (Ce_0.9Gd_0.1O_1.95 named GDC). Anode (Ni‐GDC) and different cathodes (Sm_0.5Sr_0.5CoO₃ (SSC), Ba_0.5Sr_0.5Co_0.2Fe_0.8O₃ (BSCF) and La_0.8Sr_0.2MnO₃ (LSM)) were placed on the same side of the electrolyte. All the electrodes were deposited using screen‐printing technology. A gold collector was also deposited on the cathode to decrease the over‐potential. The different materials and fuel cell devices were tested under propane/air mixture, after a preliminary treatment under hydrogen to reduce the as‐deposited nickel oxide anode. The results show that SSC and BSCF cathodes are not stable in these conditions, leading to a very low open circuit voltage (OCV) of 150 mV. Although LSM material is not the more adequate cathode regarding its high catalytic activity towards hydrocarbon conversion, it has a better chemical stability than SSC and BSCF. Ni‐GDC‐LSM SCFC devices were elaborated and tested; an OCV of nearly 750 mV could be obtained with maximum power densities around 20 mW cm^–2 at 620 °C, under air–propane mixture with C₃H₈/O₂ ratio equal to 0.53.     

Effects of PH3 and CH3Cl Contaminants on the Performance of Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have been considered as one of the most efficient power generators that can directly convert chemical energy in the natural gas, biomass, or coal‐derived gas to electrical energy. Various contaminants in syngas are capable to cause catalyst malfunction and cell performance drop, limiting fuel cell to a wide application. The effects of PH₃ and CH₃Cl fuel impurities on the electrochemical performance of SOFCs are investigated at various testing conditions. Performance drop caused by the addition of 10 ppm PH₃ remains identical in pure hydrogen and simulated coal‐derived syngas at 750 °C, but a slight increase is observed when the cells are fueled syngas at 850 °C. The presence of CH₃Cl in syngas causes cell degradation to a larger extent at 850 °C. Moreover, the cooperative influences of PH₃ and CH₃Cl impurities in hydrogen are also studied at 750, 800, and 850 °C. The addition of CH₃Cl can stop and remove PH₃ poisoning behavior, which is associated with each contaminant concentrations and operational temperatures. The related mechanism has been deeply analyzed and diagnosed.     

Development of a Novel Ceramic Support Layer for Planar Solid Oxide Cells

The conventional solid oxide cell is based on a Ni–YSZ support layer, placed on the fuel side of the cell, also known as the anode supported SOFC. An alternative design, based on a support of porous 3YSZ (3 mol.% Y₂O₃–doped ZrO₂), placed on the oxygen electrode side of the cell, is proposed. Electronic conductivity in the 3YSZ support is obtained post sintering by infiltrating LSC (La_0.6Sr_0.4Co_1.05O₃). The potential advantages of the proposed design is a strong cell, due to the base of a strong ceramic material (3YSZ is a partially stabilized zirconia), and that the LSC infiltration of the support can be done simultaneously with forming the oxygen electrode, since some of the best performing oxygen electrodes are based on infiltrated LSC. The potential of the proposed structure was investigated by testing the mechanical and electrical properties of the support layer. Comparable strength properties to the conventional Ni/YSZ support were seen, and sufficient and fairly stable conductivity of LSC infiltrated 3YSZ was observed. The conductivity of 8–15 S cm^–1 at 850 °C seen for over 600 h, corresponds to a serial resistance of less than 3.5 m Ω cm² of a 300 μm thick support layer.     

A Novel Method for GDC Recovery During Solid Oxide Fuel Cell Manufacturing

There have been a tremendous research affects in recent years for alternative routes of electricity generation using some maximum yield technologies, increased reliability, and minimum pollution. From this point of view, the solid oxide fuel cells (SOFCs) are considered as the cleanest technologies for obtaining electrical energy generation. However, an important fraction of production is wasted during the manufacturing steps. From both economical and environmental point of view, recovery of waste GDC (Gd_0.1Ce_0.9O₃) materials is deemed important. Hence, it is the main purpose of the present study to develop a novel method to recover waste GDC materials and afterwards to produce a new SOFC (solid oxide fuel cell) from the recovered materials. The results showed that recovered GDC cell worked as efficient as the fresh materials, revealing the success of the recovery process proposed.     

Anode Current Collecting Efficiency of Tubular Anode‐supported Solid Oxide Fuel Cells

Anode current collection points (ACCPs) were fabricated on the outside surface of a tubular anode‐supported solid oxide fuel cell (SOFC). The ACCPs were distributed axially along the SOFC tube with the distance between every adjacent two ACCPs the same. The effect of collecting current with different number of ACCPs on the performance of the SOFC was studied. It was found that with the same effective area, using more ACCPs to collect the current leads to better performance, while with a SOFC with a determined total surface area, there is an optimum number of ACCPs to be made and used considering the area occupied by the ACCPs themselves.     

Modeling of Solid Oxide Fuel Cells with Particle Size and Porosity Grading in Anode Electrode

Solid oxide fuel cells (SOFCs) have the potential to meet the critical energy needs of our modern civilization and minimize the adverse environmental impacts from excessive energy consumption. They are highly efficient, clean, and can run on variety of fuel gases. However, little investigative focus has been put on optimal power output based on electrode microstructure. In this work, a complete electrode polarization model of SOFCs has been developed and utilized to analyze the performance of functionally graded anode with different particle size and porosity profiles. The model helps to understand the implications of varying the electrode microstructure from the polarization standpoint. The work identified conditions when grading can improve the cell performance and showed that grading is not always beneficial or necessary.     

Long‐Term Stability Studies of Anode‐Supported Microtubular Solid Oxide Fuel Cells

A long‐term stability study of an anode‐supported NiO/YSZ‐YSZ‐LSM/YSZ microtubular cell was performed, under low fuel utilization conditions, using pure humidified hydrogen as fuel at the anode side and air at the cathode side. A first galvanometric test was performed at 766 °C and 200 mA cm^–2, measuring a power output at 0.5 V of ∼250 mW cm^–2. During the test, some electrical contact breakdowns at the anode current collector caused sudden current shutdowns and start‐up events. In spite of this, the cell performance remains unchanged. After a period of 325 h, the cell temperature and the current density was raised to 873°C and 500 mA cm^–2, and the cell power output at 0.5 V was ∼600 mW cm^–2. Several partial reoxidation events due to disturbance in fuel supply occurred, but no apparent degradation was observed. On the contrary, a small increase in the cell output power of about 4%/1,000 h after 654 h under current load was obtained. The excellent cell aging behavior is discussed in connection to cell configuration. Finally, the experiment concluded when the cell suffered irreversible damage due to an accidental interruption of fuel supply, causing a full reoxidation of the anode support and cracking of the thin YSZ electrolyte.     

Effects of Microstructural Functional Layers on Flat‐Tubular Solid Oxide Fuel Cells

The functional layer of a flat‐tubular solid oxide fuel cell (SOFC) is examined using a three‐dimensional microscale electrode model. SOFC electrodes essentially include two types of layers: a structural layer and a functional layer. The structural layers, which are the anode support layer and the cathode current collector layer, are composed of large particles with a high porosity that facilitates gas diffusion. The functional layers consist of small particles with a low porosity that increases the triple phase boundary (TPB) reaction area and reduces the activation overpotential. In the model, the particle diameter and functional layer thickness are adjusted and analyzed. The effects of the two parameters on the performance of the functional layer are monitored in the contexts of several multilateral approaches. Most reactions occurred near the electrode–electrolyte interface; however, an electrode design that included additional TPB areas improved the electrode performance. The role of the functional layer in a flat‐tubular SOFC is examined as a function of the functional layer particle size and thickness. The performance of a cell could be enhanced by preparing a functional layer using particles of optimal size and thickness, and by operating the device under conditions optimized for these parameters.     

A Performance Prediction Tool for Solid Oxide Fuel Cells after Single Redox Cycle

The effects of anode support fabrication parameters on the cell performance and the redox behavior of the cell are investigated experimentally and theoretically. In the experimental program, an yttria stabilized zirconia based anode supported membrane electrode group (MEG) is developed via the tape casting, co‐sintering and screen printing methodologies. For comparison, various anode supported cells with different electrolyte thickness and anode support porosities are also fabricated. In the theoretical study, a mathematical model is developed to represent the fluid flow, the heat transfer, the species transport and the electrochemical reaction in solid oxide fuel cells. In addition, a redox model representing the mechanical damage in the electrochemical reaction zones due to redox cycling is developed by defining a damage function as a function of strain and a damage coefficient. The effects of anode support porosity and the electrolyte thickness on the cell performance and redox stability of the cells are numerically investigated. The experimental results are compared with the numerical results to validate the mathematical model. Finally, a predictive tool, which is valid for the ranges of the cell fabrication parameters investigated, is developed to estimate the electrochemical performance after single redox cycle.     

Thermodynamic Analysis of Solid Oxide Fuel Cell Gas Turbine Systems Operating with Various Biofuels

Solid oxide fuel cell–gas turbine (SOFC‐GT) systems provide a thermodynamically high efficiency alternative for power generation from biofuels. In this study biofuels namely methane, ethanol, methanol, hydrogen, and ammonia are evaluated exergetically with respect to their performance at system level and in system components like heat exchangers, fuel cell, gas turbine, combustor, compressor, and the stack. Further, the fuel cell losses are investigated in detail with respect to their dependence on operating parameters such as fuel utilization, Nernst voltage, etc. as well as fuel specific parameters like heat effects. It is found that the heat effects play a major role in setting up the flows in the system and hence, power levels attained in individual components. The per pass fuel utilization dictates the efficiency of the fuel cell itself, but the system efficiency is not entirely dependent on fuel cell efficiency alone, but depends on the split between the fuel cell and gas turbine powers which in turn depends highly on the nature of the fuel and its chemistry. Counter intuitively it is found that with recycle, the fuel cell efficiency of methane is less than that of hydrogen but the system efficiency of methane is higher.     

SrO‐Containing Diopside Glass–Ceramic Sealants for Solid Oxide Fuel Cells: Mechanical Reliability and Thermal Shock Resistance

A series of glasses and glass–ceramics (GCs) aiming at applications as sealants for solid oxide fuel cells (SOFCs) were synthesized by partial substitution of Ca for Sr in the diopside‐Ba disilicate composition. X‐ray diffraction in conjunction with the Rietveld‐RIR technique were employed to quantify the crystalline (diopside and Sr‐diopside) and amorphous phases in the glasses sintered/heat treated at 850 °C in humidified 10%H₂–90%N₂ gas mixture for 250 h. Weibull modulus varied in the range 11.6–34.4 implying toward good mechanical reliability of synthesized GCs. Thermal shock resistance of model electrochemical cells made of yttria‐stabilized zirconia, gadolinia‐doped ceria, and lanthanum gallate based solid electrolytes, hermetically sealed by one diopside‐based composition, was evaluated employing quenching from 800 °C in air and water. Suitable thermal expansion coefficient, mechanical reliability, and strong adhesion to stabilized zirconia and metallic interconnects, are all suggesting a good suitability of the sealants for SOFC applications.     

Evaluation of Using LaNi0.6Fe0.4O3–δ Contact Layer Between La0.6Sr0.4FeO3 Cathode and Crofer22APU Interconnect for Solid Oxide Fuel Cells

Interconnect‐cathode interfacial adhesion is important for the durability of solid oxide fuel cell (SOFC). Thus, the use of a conductive contact layer between interconnect and cathode could reduce the cell area specific resistance (ASR). The use of La_0.6Sr_0.4FeO₃ (LSF) cathode, LaNi_0.6Fe_0.4O_3–δ (LNF) contact layer and Crofer22APU interconnect was proposed as an alternative cathode side. LNF‐LSF powder mixtures were heated at 800 °C for 1,000 h and at 1,050 °C for 2 h and analyzed by X‐Ray power diffraction (XRD). The results indicated a low reactivity between the materials. The degradation occurring between the components of the half‐cell (LSF/LNF/Crofer22APU) was studied. XRD results indicated the formation of secondary phases, mainly: SrCrO₄, A(B, Cr)O₃ (A = La, Sr; B = Ni, Fe) and SrFe₁₂O₁₉. Scanning electron microscopy with energy dispersive X‐Ray spectroscopy (SEM‐EDX) and the X‐Ray photoelectron spectroscopy (XPS) analyzes confirmed the interaction between LSF/LNF and the metallic interconnect due to the Cr vaporization/migration. An increment of the resistance of ∼0.007 Ω cm² in 1,000 h is observed for (LSF/LNF/Crofer22APU) sample. However, the ASR values of the cell without contact coating, (LSF/Crofer22APU), were higher (0.31(1) Ω cm²) than those of the system with LNF coated interconnect (0.054(7) Ω cm²), which makes the proposed materials combination interesting for SOFC.