Effect of reduced graphene oxide addition on cathode functional layer performance in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) operating at high temperatures are highly efficient electrochemical devices since they convert the chemical energy of a fuel directly into heat and electrical energy. The electrochemical performance of an SOFC is significantly influenced by the materials and microstructure of the electrodes since the electrochemical reactions in SOFCs take place at three/triple phase boundaries (TPBs) within the electrodes. In this study, graphene in the form of reduced graphene oxide (rGO) is added to cathode functional layer (CFL) to improve the cell performance by utilizing the high electrical properties of graphene. Various cells are prepared by varying the rGO content in CFL slurry (1–5 wt %), the number of screen printing (1–3) and the cathode sintering temperature (900–1100 °C). The electrochemical behavior of the cells is evaluated by electrochemical performance and impedance tests. It is observed that there is a ∼26% increase in the peak performance of the cell coated with single layer CFL having 1 wt % graphene and 1050 °C sintering temperature, compared to that of the reference cell.     

Factors of cathode current-collecting layer affecting cell performance inside solid oxide fuel cell stacks

Factors of cathode current-collecting layer (CCCL) affecting cell performance are studied by investigation of solid oxide fuel cell (SOFC) stacks with various (La_0.75Sr_0.25)_0.95MnO_3−δ (LSM) as CCCL in-suit. A larger real contact area between cathode and interconnect appears when the LSM is coated on cathode side as CCCL through characterization of a 2-cell stack. The result reveals that the real contact area depends on the surface roughness match (SRM) between CCCL and its neighboring components (active cathode and interconnect). A 6-cell stack using CCCLs with various levels of surface roughness is assembled and characterized further. The results show a higher electrical output performance of the stack repeating unit can be obtained when the surface roughness of the CCCL matches that of its neighboring components better, i.e. the surface roughness match (SRM) is the factor of cathode current collector affecting cell performance inside stack. Accordingly, the cell performance inside SOFC stack can be regulated by designing the SRM to its neighboring components.     

Cathode-side electrical contact and contact materials for solid oxide fuel cell stacking: A review

In a planar solid oxide fuel cell (SOFC) stack, a number of individual cells are stacked together to increase the voltage and power output. At both the cathode– and anode–interconnect interfaces, electrical contact layers are applied between the interconnect and electrodes during cell fabrication process or stack assembly to increase the electrode-interconnect contact area and to compensate for dimensional tolerance variation of the contacting components, thus minimizing ohmic contact resistance throughout the stack. As such, electrical contact is an essential component in SOFC stacks. In this paper, we review the cathode-side electrical contact design and contact materials for application in SOFC stacks. Following an introduction of the function and working principles of electrical contact, the material requirements for cathode-side contact layer in SOFC stacks are outlined. The current materials for the cathode–interconnect contact are thoroughly reviewed, including noble metals, conductive ceramics (e.g. perovskites and spinels), composites, and other more complex structures. Several potential directions for cathode–interconnect contact material research and development are also highlighted.     

Synthesis and characterization of aluminum-doped perovskites as cathode materials for intermediate temperature solid oxide fuel cells

(Ba_0.5Sr_0.5)(Fe_1-xAl_x)O_3-δ (BSFAx, x = 0–0.2) oxides have been synthesized as novel cobalt-free cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) using a sol-gel method. The BSFAx (x = 0–0.2) materials have been characterized by X-ray diffraction and scanning electron microscopy. The electrical conductivities and electrochemical properties of the prepared samples have also been measured. At 800 °C, the conductivity drops from 15 S cm⁻¹ to 5 S cm⁻¹ when the doping level of aluminum is increased to 20%. The aluminum-doping concentration has important impacts on the electrochemical properties of BSFAx materials. The BSFA0.09 cathode shows a significantly lower polarization resistance (0.26 Ω cm²) and cathodic overpotential value (55 mV at the current density of 0.1 A cm⁻²) at 800 °C. Furthermore, an anode-supported single cell with BSFA0.09 cathode has been fabricated and operated at a temperature range from 650 to 800 °C with humidified hydrogen (∼3vol% H₂O) as the fuel and the static air as the oxidant. A maximum power density of 676 mWcm⁻² has been achieved at 800 °C for the single cell. We believe that BSFA0.09 is a promising cathode material for future IT-SOFCs application.     

Novel materials for solid oxide fuel cell technologies: A literature review

This study aims to review novel materials for solid oxide fuel cell (SOFC) applications covered in literature. Thence, it was found that current SOFC operating conditions lead to issues, such as carbon surface deposition, sulfur poisoning and quick component degradation at high temperatures, which make it unsuitable for a few applications. Therefore, many researches are focused on cell performance enhancement through replacing the materials being used in order to improve properties and/or reduce operating temperatures. Most modifications in the anode aim to avoid some issues concerning conventionally used Ni-based materials, such as carbon deposition and sulfur poisoning, besides enhancing catalytic activity, once this component is directly exposed to the fuel. It was also found literature about the cathode with the aim of developing a material with enhanced properties in a wider temperature range, which has been compared to the currently used one: LSM perovskite (La_1-xSr_xMnO₃). Novel electrolyte materials can have ionic or protonic conductivity, thus performance degradation must be avoided at several operating conditions. In order to enhance its electrochemical performance, different materials for electrodes (cathode and anode) and electrolytes have been assessed herein.     

A short review of cathode poisoning and corrosion in solid oxide fuel cell

Solid oxide fuel cell (SOFC) has been recognized as a promising energy conversion device that is expected to play a critical role in solving the global energy and environmental challenges, however, the durability of SOFC under practical working conditions has limited its wide spread deployment and commercialization. Specifically, SOFC cathode often suffers from various contaminations such as Cr and Si arising from the interconnect and sealing materials, respectively, as well as humidity and CO₂ which are inherent in ambient air, resulting in serious issues in long-term performance degradation. In this review, the impacts of certain poisoning and corrosions on SOFC cathode are introduced, and the latest results of durability research on the corrosion resistant properties of cathode under CO₂, humidity, Cr and Si-containing conditions are reviewed. The poisoning and corrosion mechanism and durability of these aspects are systematically assessed and discussed.     

Enhancement of the cathode/electrolyte interface by a sintering-active barrier layer for solid oxide fuel cells

It's a critical issue for the successful commercialization of solid oxide fuel cells (SOFCs) to achieve long-term operation and thermal cycling stability without significant degradation. Current work reports an almost-dense, sintering-active and Sr-blocking cathode/electrolyte interface, fabricated through a cost-competitive and scalable method of modifying porous Gadolinia doped Ceria (GDC) barrier layer by in-situ grown GDC nanoparticles. Result show that the robust interface enables improved durability and thermal cycling stability. The hydrothermal modified anode supported cell performance only deteriorates by ∼0.5% after 20 times thermal cycles between 200 and 750 °C, which is more prominent enhancement than ∼16% for pristine cell. The modified symmetric cell shows smaller cathode polarization resistance and well-attached cathode/electrolyte interfaces after ∼1200 h of operation, including 4 times thermal cycles. While the pristine cell shows more obvious area specific resistances increases and the peeled-off cathode layer. It is discussed and concluded that the hydrothermal modified sintering-active barrier layer has contributed mainly to the construction of robust cathode/electrolyte interface, yielding the improved performance and prolonged operation.     

High performance praseodymium nickelate oxide cathode for low temperature solid oxide fuel cell

The praseodymium nickelate oxide Pr₂NiO_4+δ, a mixed conducting oxide with the K₂NiF₄-type structure, was evaluated as cathode for low temperature solid oxide fuel cells (T = 873 K). The electrochemical performance of the cathode has been improved by optimization of the microstructure of the porous cathode combined with the use of a ceria barrier layer in between the cathode and zirconia electrolyte. Both low polarization and ohmic resistances were obtained using Pr₂NiO_4+δ-powders with a median particle size of 0.4 μm, and sintering the screen printed layer at a sintering temperature of about 1353 K for 1 h. These manufacturing conditions resulted in a cathode microstructure with well established connections between the cathode particles and good adhesion of the cathode on the electrolyte. Full-sized anode supported cells have been manufactured using the same process conditions for the Pr₂NiO_4+δ cathode and tested. The best results were obtained when using a dense Ce_0.8Gd_0.2O_1.9 (20CGO) barrier layer. While a complete optimization of the cell preparation has not yet been achieved, the electrochemical performances of anode supported cells with Pr₂NiO_4+δ are higher than those with the well known state-of-the-art La_0.6Sr_0.4Fe_0.8Co_0.2O_3−δ (LSFC) material.     

Degradation behavior of anode-supported solid oxide fuel cell using LNF cathode as function of current load

We investigated the effect of current loading on the degradation behavior of an anode-supported solid oxide fuel cell (SOFC). The cell consisted of LaNi_0.6Fe_0.4O₃ (LNF), alumina-doped scandia stabilized zirconia (SASZ), and a Ni-SASZ cermet as the cathode, electrolyte, and anode, respectively. The test was carried out at 1073 K with constant loads of 0.3, 1.0, 1.5, and 2.3 A cm⁻². The degradation rate, defined by the voltage loss during a fixed period (about 1000 h), was faster at higher current densities. From an impedance analysis, the degradation depended mainly on increases in the cathodic resistance, while the anodic and ohmic resistances contributed very little. The cathode microstructures were observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).     

Electrochemical performance of a Ni0.8Co0.15Al0.05LiO2 cathode for a low temperature solid oxide fuel cell

The electrochemical performance of the Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL) cathode was investigated by comparing it with the traditional La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δ (LSCF) and LSCF/Ce_0.9Gd_0.1O_2-δ (GDC) cathodes with a GDC electrolyte-supported solid oxide fuel cell (SOFC). It is found that the electrochemical performance of the cells with the NCAL and NCAL/GDC cathode is better than that of the cells with the LSCF and LSCF/GDC cathode at 550 °C. The results of the electrochemical performance tests of the cells with different NCAL/GDC mass ratios (10/0, 9/1, 8/2, 7/3 and 6/4) show that the NCAL/GDC composite cathode with the mass ratio of 8/2 has the best electrochemical performance. XRD results show that when the sintering temperature is higher than 700 °C, the NCAL/GDC composite will undergo chemical reactions and generate new phases, reducing the performance of the composite cathode. XPS results show that a small amount of Li₂CO₃ was formed on the surface of NCAL during cathode preparation, forming a special interface between NCAL, Li₂CO₃ and GDC. At the NCAL-Li₂CO₃/GDC interfaces, due to the migration and aggregation of Li⁺ to the interface, a space charge region may be formed in which the Li⁺ enrichment may lead to the formation of the region with a high oxygen vacancy concentration. A very high oxygen vacancy concentration at the NCAL-Li₂CO₃/GDC interfaces will provide sufficient oxygen ion conductivity for oxygen reduction reaction (ORR) and reduce the activation energy of the reaction. NCAL will be a potential cathode material that can reduce the operating temperature of the traditional SOFC to 550 °C or lower.     

Fabrication of cathode supported solid oxide fuel cell by multi-layer tape casting and co-firing method

La_0.3Sr_0.7FeO_3-δ (LSF)/CeO₂ cathode supported Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte was prepared by a simple multilayer tape casting and co-firing method. SDC electrolyte slurry and LSF/CeO₂ cathode slurry were optimized and the green bi-layer tapes were co-fired at different temperature. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and Scan Electronic Microscopy (SEM). No additional phase peak line was observed in electrolyte and cathode support when the sintering temperature lower was than 1400 °C. The electrolytes were extremely dense with the thickness of about 20 μm. The cathode support was porous with electrical conductivity of about 4.21 S/cm at 750 °C. With Ni/SDC as anode, Open Current Voltage and maximum power density reached 0.61 V and 233 mW cm⁻² at 750 °C, respectively.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.     

Novel gradient porous cathode for intermediate temperature solid oxide fuel cell

As a mixed ion electronic conducting oxide, PrBaCo₂O_5+δ is regarded as a promising solid oxide fuel cell cathode. To further improve PrBaCo₂O_5+δ cathode's oxygen reduction reaction activity, porosity graded PrBaCo₂O_5+δ-based cathode is prepared by screen printing technology. With a porous top and a comparative denser base, oxygen ions concentration and oxygen gas concentration in the cathode can be graded distributed. This concentration gradient works as driven force which can enhance the cathode catalytic activity. And distribution of relaxation time analysis is carried out to investigate cathodes performance optimization mechanism, the result shows that gradient porous PrBaCo₂O_5+δ cathode's area specific resistance value is much lower than the traditional homogeneous porosity PrBaCo₂O_5+δ cathode. The scaffold porosity modification promotes the cathode oxygen ions transfer processes without obvious impact on the cathode oxygen surface processes.     

Novel perovskite-spinel composite conductive ceramics for SOFC cathode contact layer

Perovskite-spinel composite conductive ceramics are developed for solid oxide fuel cell (SOFC) cathode contact layer. The precursor of the composite materials includes micron-sized La_0.6Sr_0.4Co_0.2Fe_0.8O₃(LSCF) perovskite particles coated by the reduced nano-sized Mn_0.9Y_0.1Co₂O₄ (MYC) spinel material, and then it is sintered in-situ to obtain perovskite-spinel composites. The sintering activity of the composites is enhanced by using the reduced spinel powders. The conductive performance of the composite materials is effectively improved due to the high conductivity of perovskite LSCF particles utilized. Measured at 750^οC under constant current density of 400 mA/cm², after running 200 h, the area specific resistance (ASR) value of cathode contact layer remains relatively stable at around 5.4 mΩ cm². The developed LSCF-MYC composite as cathode contact layer presents good bonding strength with both cathode and interconnection, and shows obviously low contact resistance and high stability.     

CaO effect on the electrochemical performance of lanthanum strontium cobalt ferrite cathode for intermediate-temperature solid oxide fuel cell

The oxygen reduction reaction (ORR) on lanthanum strontium cobalt ferrite (LSCF) catalyst is critical for intermediate temperature solid oxide fuel cells (SOFCs). The reaction rate can be effectively improved by addition various nanoparticles including electrocatalysts such as Pd, Ag and mixed electronic-ionic conductors and electrolytes like samaria doped ceria (SDC). This work shows that ORR rate can also be improved by CaO, which is neither catalyst nor conductor. The CaO nanoparticles are deposited to porous LSCF electrodes using the infiltrating technique. No obvious reaction between CaO and LSCF is detected with X-ray diffraction analysis, indicating that CaO is chemically compatible with LSCF in the intermediate-temperature SOFC operation conditions. Impedance spectrum analysis demonstrates that the CaO nanoparticles can effectively reduce the interfacial polarization resistances for both single phase LSCF electrodes and LSCF-SDC composite electrodes. In addition, CaO nanoparticles can improve the peak power densities and reduce the total electrode resistances of single cells consisting of NiO-SDC anodes, SDC electrolytes, and LSCF based cathodes. Further, CaO can increase the oxygen surface exchange coefficient as demonstrated with electrical conductivity relaxation measurement. The improving factor is comparable to those for Rh and Pd catalysts, suggesting it is effective to increase ORR rate by infiltrating CaO nanoparticles.     

A novel catalytic layer material for direct dry methane solid oxide fuel cell

The perovskite structured oxide La_0.75Sr_0.25Cr_0.5−xFe_xMn_0.5O_3−δ (LSCFMx, x = 0.05, 0.1, 0.15, 0.2, 0.25) powder is prepared by the liquid phase method, using iron as dopant to replace the chromium. According to XRD patterns, perovskite-like LSCFMx are stable in pure H₂, except for LSCFM0.25. Thus the maximum content of Fe doping is 0.2. The calculated lattice volume increases along with the content of iron and the powders show excellent chemical compatibility with yttria-stabilized zirconia (YSZ). The electrical conductivities for LSCFM0.15 and LSCFM0.2 are very comparative, and they exhibit similar performance as catalytic materials. In contrast, the different sintered temperature with the LSCFM0.2 catalytic layer, at 1300 °C exhibits higher electrochemical performance. When dry methane is used as the fuel, the ohmic resistance and polarization resistance are 0.15 and 0.55 Ω cm², respectively, and the power density reaches 550 mW cm⁻².     

Fabrication and optimization of LSM infiltrated cathode electrode for anode supported microtubular solid oxide fuel cells

In this study, anode supported microtubular solid oxide fuel cells (SOFCs) with LSM (lanthanum strontium manganite) catalyst infiltrated LSM-YSZ (yttria stabilized zirconia) cathodes are developed to increase the density of triple/three phase boundaries (TPBs) in the cathode, thereby to improve the cell performance. For this purpose, two different porous YSZ layers are formed on the dense YSZ electrolyte, i.e., one is with co-sintering while the other one is not. Incorporation of LSM into these porous YSZ layers is achieved via dip coating of a sol-gel based infiltration solution. The effects of the fabrication method for porous YSZ, LSM solution dwelling time and the thickness of the porous YSZ layer on the cell performance are experimentally investigated and optimized in the given order. A reference cell having a conventional dip coated cathode prepared by mixing the commercial LSM and YSZ powders is also fabricated for comparison. The results show that among the cases considered, the highest peak power density of 0.828 W/cm² can be obtained from the cell, whose single dip coated porous electrolyte layer co-sintered with the dense electrolyte is impregnated with LSM for a dwelling time of 45 min. On the other hand, the peak power density of the reference cell is measured as only 0.558 W/cm². These results reveal that ～50% increase in the maximum cell performance compared to that of the reference cell can be achieved by LSM infiltration after the optimizations.     

An anode-supported micro-tubular solid oxide fuel cell with redox stable composite cathode

A NiO–YSZ anode-supported hollow fiber solid oxide fuel cell (HF-SOFC) has been fabricated with redox stable (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ–Sm_0.2Ce_0.8O_1.9–YSZ (LSCM–SDC–YSZ) composite cathode. The characterization of NiO–YSZ hollow fibers prepared by the phase inversion method is focused on the microstructure, porosity, bending strength and electrical conductivity. A thin YSZ electrolyte membrane (about 10 μm) can be prepared by a vacuum-assisted dip-coating process and is characterized in terms of microstructure and gas-tightness. The performance of the as-prepared HF-SOFC is investigated at 750–850 °C with humidified H₂ as fuel and ambient air as the oxidant. The peak power densities of 513, 408 and 278 mW cm⁻² can be obtained at 850, 800 and 750 °C, respectively, and the corresponding interfacial polarization resistances are 0.14, 0.29 and 0.59 Ω cm². The high performance at intermediate-to-high temperatures could be attributed to thin electrolyte and proper composite cathode with low interfacial polarization resistance. The low interfacial polarization resistance suggests potential applications of LSCM–SDC–YSZ composite oxides as the redox stable cathode. This investigation indicates that the redox stable LSCM–SDC–YSZ is a promising cathode material system for the next generation YSZ-based HF-SOFC. The results will be expected to open up a new phase of the research on the micro-tubular SOFCs.     

Selection of cathode contact materials for solid oxide fuel cells

The goal of this work is to identify suitable cathode contact materials (CCM) to bond and electrically connect LSCF cathode to Mn_1.5Co_1.5O₄-coated 441 stainless steel after sintering at the relatively low temperature of 900-1000 °C. A wide variety of CCM candidates are synthesized and characterized. For each, the conductivity, coefficient of thermal expansion, sintering behavior, and tendency to react with LSCF or Mn_1.5Co_1.5O₄ are determined. From this screening, LSCF, LSCuF, LSC, and SSC are selected as the most promising candidates. These compositions are applied to LSCF and Mn_1.5Co_1.5O₄-coated 441 stainless steel coupons and subjected to 200 h ASR testing at 800 °C. After area-specific resistance testing, the specimens are cross-sectioned and analyzed for interdiffusion across the CCM/LSCF or CCM/Mn_1.5Co_1.5O₄ interfaces. A relatively narrow band of interdiffusion is observed.     

A novel post-treatment to calcium cobaltite cathode for solid oxide fuel cells

The calcium cobaltite (CCO) cathodes are post-treated by dipping in the hydrogen peroxide (H₂O₂). The electrochemical properties are investigated by the electrochemical impedance spectra (EIS) and current-voltage test in the symmetrical cell and single cell, respectively. The phase structure and morphology of the cathodes are characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The experiment results show that the mesopores are created on the surface of the cathode particles and the pore channels of the cathode are cleaned up after leaching with 10 wt % H₂O₂, resulting in a remarkable decreasing of the area specific resistance (e.g. only 42.5% of that for the untreated cathode at 800 °C). The single cell with treated cathode is about 2 times the peak power density of the cell with untreated cathode, signifying the post-treating method may be promising.