Durability Study of Intermediate Temperature-Solid Oxide Fuel Cells with Excellent Cathode Materials

Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediate-temperature SOFC

Planar solid oxide fuel cells (SOFCs), which are intended to be used in the intermediate temperature range (600–800 °C) and whose supporting anode contains various amount of Cu in Li‐doped gadolinia‐doped ceria matrix, are produced and characterized in the present work. Intermediate temperature solid oxide fuel cell (IT‐SOFC) performances are investigated by recording polarization and power density at different temperatures. Electrochemical impedance spectroscopy (EIS) measurements are carried out to analyze how internal resistances change as function of temperature and, above all, for variable Cu content. The gadolinia‐doped ceria electrolyte microstructure and the cell integrity after sintering are also shown to depend on the initial CuO content in the anode. The electronic conductivity increases with Cu content within the anodic cermet. When the anode contains more than 25 vol.% CuO, a dense Li‐doped gadolinia‐doped ceria electrolyte can be obtained by sintering at 900 °C. Conversely, CuO load in excess to 50% in the anode is detrimental because of the formation of macroscopic cracks within the electrolyte. Intermediate temperature solid oxide fuel cells produced with an anodic layer containing 50 vol.% CuO show the lowest impedance parameters.

Solid oxide fuel cells (SOFCs) are emerging as energy conversion devices for large-scale electrical power generation because of their high energy conversion efficiency, excellent ability to minimize air pollution, and high fuel flexibility. In this context, this critical review has focussed on the recent advancements in developing a suitable electrolyte for SOFCs which has been required for the commercialization of SOFC technology after emphasizing the literature from the prior studies. In particular, the significant developments in the field of solid oxide electrolytes for SOFCs, particularly zirconia- and ceria-based electrolytes, have been highlighted as important advancements toward the production of sustainable and clean energy. It has been reported that among various electrolyte materials, zirconia-based electrolytes have the potential to be utilized as the electrolyte in SOFC because of their high thermal stability, non-reducing nature, and high mechanical strength, along with acceptable oxygen ion conductivity. However, some studies have proved that the zirconia-based electrolytes are not suitable for low and intermediate-temperature working conditions because of their poor ionic conductivity to below 850°C. On the other hand, ceria-based electrolytes are being investigated at a rapid pace as electrolytes for intermediate and low-temperature SOFCs due to their higher oxygen ion conductivity with good electrode compatibility, especially at lower temperatures than stabilized zirconia. In addition, the most emerging advancements in electrolyte materials have demonstrated that the intermediate temperature SOFCs as next-generation energy conversion technology have great potential for innumerable prospective applications.

Solid oxide fuel cell (SOFC) is well known as power and heat generation device which converts chemical energy directly from fuel into electricity. SOFC operate at high temperature becomes obstacle for SOFC which reducing ionic conductivity material of current electrolyte, reduce lifetime of cell components, high fabrication cost, limited durability and performance issues. This introduce to environment pollution and decrease the SOFC lifetime. The fabrication of durability and stability composite cathode are comprised from mixing of perovskite La0.6Sr0.4CO0.2Fe0.8 (LSCF) powders with nanoscale ionically conducting ceramic electrolyte materials, SDC-carbonate (SDCc) was overcome this problems. Powder preparation and composite cathode fabrication must consider which as main factors in the development of durability and stability of LSCF-SDCc composite cathode. Powders must in nanoscale to enhance the conductivity and decrease the interfacial polarization resistance and the composite cathode should in nanoporous morphology for achieve high power density over than 500 h and remarkable durability. Calcination also plays in important role and its operations will effects to the SOFC durability and performance. The necessary to prolong the lifetime and increase the SOFC performance has lead to development of durability and stability of SOFC. This paper reviews the durability and stability of the composite cathode and focus on the challenges in material technology.

Influence of carbon deposition on the anode performance of solid oxide fuel cells

Solid oxide fuel cells (SOFCs) offer direct conversion of hydrogen or hydrocarbon fuels to electricity with high energy conversion efficiency (>50%). Impurities that block active reaction sites on the electrode surface can markedly degrade the oxygen exchange rate, and lead to a reduction in long term durability of such devices. In this work, we propose to develop electrodes that are tolerant to Si which is one of the most prevalent cathode impurities. This work draws on our recent method to fully recover oxygen exchange kinetics following Si induced aging of ceria containing cathodes. We take advantage of elements, which over time during SOFC operation, are released by exsolution to actively trap (i.e. getter) Si impurities in a self-regulating chemical fashion. The objective is to evaluate impurity scavenging by added reactive elements that are intentionally exsolved from the SOFC cathode electrode during operation. In this study, we chose to focus our effort on one cathode material: Pr doped ceria (PCO). This material has the advantage of not containing strontium, an element known to segregate to the surface of the well-known perovkite based cathodes materials, thereby degrading their performance. As for the Si getter, we choose to focus on lanthanum, which is added in PCO in solid solution.

Microarchitectured solid oxide fuel cells with improved energy efficiency (Part II): Fabrication and characterization

Implementation of nano-catalyst materials into solid oxide fuel cell (SOFC) electrodes to improve performance and stability has been widely studied. Addition of the nano-catalysts into an electrode structure serves to enhance the electrochemical performance of the SOFC by increasing the Triple Phase Boundary (TPB) area, improving redox stabilization, and modifying reaction kinetics of hydrocarbon gases that cause anode degradation due to carbon deposition.Typical Ni-based cermet anodes suffer from anode deactivation due to carbon build up under hydrocarbon flows. Carbon builds up and covers the TPB area which results in poor electrochemical performance. Larger carbon deposits can block pores within the anode microstructure which leads to gas diffusion issues. Carbon buildup also causes mechanical stresses to the electrode due to volumetric changes which can lead to fracture and complete failure of the cell.This work studied nano-catalyst decoration of the Ni-based cermet anodes with catalysts that promote internal reforming to protect against coking. The addition of active metal components and ceramic reforming promoters were investigated. Multi-component systems with several catalysts were also examined. Uniform incorporation of nano-catalyst into the anode microstructure was achieved through a patented liquid phase surfactant assisted process (using various catechol surfactants). Deposition loading densities and distribution of nanoparticles was controlled by altering the surfactant and catalyst solution concentrations.This work defined initial morphology and studied the coarsening/annealing kinetics of nano-catalysts used for enhanced solid oxide fuel cell (SOFC) operation. Nano-catalyst depositions were performed on flat single crystal surfaces using a bio-inspired surface modification technique. 3D profiles of the nanoparticles at various coarsening stages were obtained using atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) was used to track the changes in composition. Figure 1 shows AFM imaging of single component cerium oxide and cobalt oxide depositions on flat single crystal yttrium stabilized zirconia (YSZ) before and after 24 h of annealing in reducing conditions.Impregnated SOFCs were tested for 200+ h in constant load while operating under high methane fuel streams. Fuel compositions were selected to have high hydrocarbon content to accelerate degradation. Deposition techniques were optimized for multi-component internal reforming catalysts to obtain sustained performance enhancement. Current-voltage-power (I-V-P) measurements and electrochemical impedance spectroscopy (EIS) results were used for evaluation. Post-mortem microstructure and chemistry characterizations were used in analysis.Impregnated SOFCs demonstrated greater than 50% sustained improvement in harsh environments in long-term tests where the anode was subjected to 40% CH4 for 200+ h. Acknowledgements: Work supported by a subcontract under the GE Aerospace’s US DOE- ARPA-E project (DE-AR0001344) entitled “FueL CelL Embedded ENgine (FLyCLEEN)”. Material characterization and imaging work was made possible with the support of the West Virginia University Shared Research Facilities. Figure 1

A dynamic infiltration technique to synthesize nanolayered cathodes for high performance and robust solid oxide fuel cells

Solid Oxide Fuel Cells (SOFCs) are one of the most attractive energy conversion and storage devices, which can be applied for decentralized energy applications. However, the high operating temperature making SOFCs still unmarketable, which indicates that lowering the working temperature of SOFCs while still maintaining a high-power output is very crucial. For Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs), new cathode materials with high electrocatalytic activity and stability are necessary to provide a stable and high performance of cells. Perovskite-structured oxide (ABO3−δ) is one group of the most interesting material candidates for IT-SOFCs, presenting great potential in chemical composition modifications, yielding the design and gain of desired physicochemical and electrochemical properties [1]. The simple perovskite LaCuO3 is one of the well-studied Cu-content oxides with a high conductivity (106 S·cm−1), but it can be hardly obtained and suffers with stability issues in air [1, 2]. To stabilize the perovskite structure and to improve electrochemical properties of Cu-content oxides A-site and B-site modification strategy can be applied. It has been noted that Cu- and Ni-containing LaNi0.5Cu0.5O3−δ simple perovskite possesses low cathodic polarization of 0.056 Ω·cm2 at 800 °C, and a high power output of 870 mW·cm−2 at 900 °C [1, 3]. The generation of oxygen vacancies by strontium doping contributes to the increase of ionic conductivity component, and the Sr dopant can reduce the oxygen vacancy formation energy and increase the ionic conductivity of La1−xSrxMO3−δ (M = Fe, Mn) perovskites [1,4]. Therefore, in this work, Cu-content La1−xSrxNi1−yCuyO3−δ oxides with strontium doping at the A-site were evaluated as very promising Co-free cathode material candidates for IT-SOFCs.In this work, all La1−xSrxNi1−yCuyO3−δ compounds have been successfully obtained using soft chemistry. The room-temperature crystal structure of all oxides was assigned to the R-3c trigonal system. The substitution of La with Sr lowers the phase transition temperature from the R-3c space group to Pm-3m simple perovskite while increases the oxygen non-stoichiometry and the thermal expansion coefficient of studied materials. All investigated materials are chemically compatible with GDC-10, while unstable in contact with LSGM and 8YSZ. The manufactured anode-supported IT-SOFC with the La0.95Sr0.05Ni0.5Cu0.5O3−δ cathode presents an excellent power density of 445 mW·cm−2 at 650 °C in humidified H2. The results confirmed that La1−xSrxNi1−yCuyO3−δ perovskites can be potentially qualified as promising cathode candidates for IT-SOFCs, yielding promising electrochemical performance in the intermediate-temperature range [1].

Long term durability was demonstrated on high-performance direct ethanol solid oxide fuel cell operating in gradual internal reforming. Anode supported single cells, fabricated at Forschungszentrum Jülich, composed of (La,Sr)MnO3 cathode, 8 mol% yttria-stabilized zirconia (YSZ) electrolyte, and YSZ-Ni anode were used for long term testing on anhydrous (direct) ethanol. Gadolinia-doped ceria (CGO), with 0.1 wt.% of Ir, catalyst was produced by the impregnation of Ir salt in commercial CGO (Praxair) and mixed with organic additives. The CGO-Ir suspension was deposited onto the surface of the YSZ-Ni anode support with an air brush. Such a catalytic layer acts both as a physical barrier to avoid direct contact of the fuel with Ni-based anode and to promote the gradual internal reforming of ethanol. The CGO-Ir catalyst was designed for the reforming of the main compounds resulting from the thermal decomposition of ethanol at high temperature and to inhibit the formation of ethylene, which is a known source of carbon deposits. After deposition the sample was heated at 850ºC in inert atmosphere to ensure good adhesion between the catalytic layer and the anode. Gold grid and paint were used as current collectors in both cathode and anode sides. The single fuel cells were mounted in the test apparatus, sealed with gold rings and alumina cement and heated to the measuring temperature (850°C) under inert atmosphere. After gas tightness was checked by flowing air on the cathode side and argon on the anode side, the anode was reduced under hydrogen. The fuel cell was kept at 0.6 V while monitoring the current drained from the cell at 850 °C. Samples were initially operated in hydrogen for a few hours and fuel was switched to dry ethanol, carried by nitrogen. It is important to mention that no oxidizing agent was added to the fuel and that water produced by the electrochemical reaction of hydrogen at the anode promoted the reforming of the ethanol in the CGO-based layer, in the so-called gradual internal reforming. In such conditions the current output was recorded and observed to not be significantly different form that under hydrogen. Moreover, the direct ethanol fuel cell was continuously operated for 650 hours without major losses due to carbon formation. The experimental results demonstrate that the water released by the electrochemical oxidation of reformate hydrogen at the electrolyte/anode active interfaces is sufficient for the reforming of the primary fuel in the catalytic layer, provided that a minimum fuel utilization is respected. After the durability tests, energy dispersive X-ray spectroscopy and scanning electron microscopy analyses of the samples revealed no carbon deposit formation in the anode. The experimental results provided compelling evidence for the viability of gradual internal reforming of ethanol in high performance solid oxide fuel cells.

As global energydemands shift toward a sustainable alternative,hydrogen-powdered solid oxide fuel cells (SOFCs) offer a high-efficiency,low-emission solution for electrical energy conversion. However, performancelimitations at intermediate temperatures (600–800 °C)necessitate advancements in electrolyte and electrode design. Thepresent work presents the fabrication of a trilayer (porous/dense/porous)La0.8Sr0.2Ga0.8Mg0.2O3‑δ electrolyte using a tape casting method, yieldinga sintered structure with ∼55 μm thick, porous layers(∼55% porosity) and a ∼20 μm dense electrolytesupported by La0.8Sr0.2Ga0.8Mg0.2O3‑δ rings. The porous La0.8Sr0.2Ga0.8Mg0.2O3‑δ backbone is infiltrated with nominal chemical composition NdBaCoFeO5+δ (NBCF) and a Ni–Gd-doped-Ce (Ni-GDC) anode.Electrochemical impedance spectroscopy, distribution functions ofrelaxation times, and equivalent circuit modeling identified an optimalNBCF loading of 1.58 mg/cm2, which minimizes charge transferand diffusion resistance, reducing the area-specific resistance to0.025 Ω cm2 at 800 °C. Full cell testing underSOFC conditions achieves a peak powder density of 400 mW/cm2 at 750 °C with low ohmic (0.11 Ω cm2) andpolarization (0.33 Ω cm2) resistances.

One of the most promising next-generation energy conversion technologies is the solid oxide fuel cell (SOFC), which is more efficient and less polluting than conventional internal combustion engines (ICEs). However, in order to commercially compete with ICEs, SOFCs must be cost-competitive and operation-reliable. To achieve this goal, lowering the operating temperature of a SOFC from the current 700-1000oC to ≤ 600oC is vitally important. The realization of such intermediate-temperature SOFCs (IT-SOFCs) depends critically upon the availability of high-conductivity electrolytes and high-activity electrode catalysts. In recent years, significant progress has been made in the area of cathode catalysts through new material discovery and microstructural optimization, which has demonstrated a polarization resistance (area-specific resistance) as low as 0.1 W×cm2 even at 500oC. 1 2 In contrast, the development of high-conductivity oxide-ion conductors significantly lags behind that of IT-SOFC cathodes. Therefore, identification of new IT solid-state fast oxide-ion conductors is in high demand. A practical criterion for an electrolyte to be suitable for an IT-SOFC is its oxide-ion conductivity so ≥0.01 S/cm at an operating temperature < 600oC. None of the existing chemically stable oxide-ion conductors can satisfy this requirement at a temperature ≤500oC. Recently, the Goodenough group at the University of Texas at Austin discovered a new class of oxide-ion conductors that can meet this conductivity-temperature requirement. These new oxide-ion conductors have a generic formula Sr3-3xNa3xSi3O9-1.5x (or SNS hereinafter) and layered structure as shown in Fig.1 (a). 3 4 The oxide-ion conductivity of SNS is so»0.01 S/cm at 500oC, Fig.1 (b), the highest among all known chemically stable oxide-ion conductors. More importantly, it is stable over a broad pO2 range of approximately from 10-30 to 1 atm and an extended lifetime, making it an ideal electrolyte for IT-SOFCs. 5 Despite the exciting discovery of this superior ion conductor, so far very little is known about the transport mechanisms governing the high oxide-ion conductivity in the SNS. This study investigates the origin of high oxide-ion conductivity by probing the local structure and dynamics of ionic motion with high-temperature multinuclear (17O, 23Na, and 29Si) solid-state NMR techniques. It reveals the inhomegeneity of chemical phases in this electrolyte. The increased level of Na-doping has led to increased degree of amorphorization in structure, which may be correlated to the enhancement in ionic conductivity. The foundational knowledge gained from this research may have significant impact on understanding the critical structure-conductivity relationship and designing new solid-state fast ion conductors for advanced electrochemical energy conversion and storage systems. References (1) Liang, F. L.; Zhou, W.; Li, J.; Zhu, Z. H.: Microwave-plasma induced reconstruction of silver catalysts for highly efficient oxygen reduction. J Mater Chem A 2013, 1, 13746-13749. (2) Zhu, Y. L.; Chen, Z. G.; Zhou, W.; Jiang, S. S.; Zou, J.; Shao, Z. P.: An A-Site-Deficient Perovskite offers High Activity and Stability for Low-Temperature Solid-Oxide Fuel Cells. Chemsuschem 2013, 6, 2249-2254. (3) Singh, P.; Goodenough, J. B.: Monoclinic Sr1-xNaxSiO3-0.5x: New Superior Oxide Ion Electrolytes. J Am Chem Soc 2013, 135, 10149-10154. (4) Singh, P.; Goodenough, J. B.: Sr1-xKxSi1-yGeyO3-0.5x: a new family of superior oxide-ion conductors. Energ Environ Sci 2012, 5, 9626-9631. (5) Wei, T.; Singh, P.; Gong, Y. H.; Goodenough, J. B.; Huang, Y. H.; Huang, K.: Sr3-3xNa3xSi3O9-1.5x (x=0.45) as a superior solid oxide-ion electrolyte for intermediate temperature-solid oxide fuel cells. Energ Environ Sci 2014, 7, 1680-1684.

Understanding and optimizing electrochemically active zone for oxygen reduction reaction in solidoxide fuel cells (SOFCs) cathodes are indispensable to maximize device’s performance. For mixed ionicand electronic conductors, the range of this zone depends on the oxygen surface exchange and solid-stateoxygen/electron transport properties of materials [1]. This aspect is also critical to realize validcharacterization of the electrode’s electrochemical activities. For instance, when a material’s electronicconductivity is low or the electrical contact points are spaced beyond a critical distance, the electrodepolarization resistance measured by electrochemical impedance spectroscopy will be controlled by the sheetresistance not the oxygen exchange resistance [2]. The situation becomes more complicated as the electricalconductivity of porous layer, which is an advantageous structure with large surface area and facile gas phasetransport, considerably deviates from the material’s intrinsic property. It is expected that porosity,connectivity, and impurity contents can have significant impact on porous electrode’s electronic conduction[3].In this context, we have studied effective in-plane electronic conductivity of lanthanum strontiumcobalt ferrite (LSCF) porous layers. The in-plane conductivity was measured by four-line probeconfiguration [4], modified from the widely used four-point probe technique to be applied for a porouslayer. We have confirmed that the in-plane electrical conductivity of screen-printed LSCF porous layers isone or two orders of magnitude lower than the material’s bulk property in ambient air, depending ontemperatures. This electronic conductivity deviation was taken into account to rationally designcombination of electrode thickness and current collector spacing and, in turn, to guarantee a validassessment of electrode performance. In addition, we found that electrical conductivity of LSCF porouslayers decreases by 20~30 %, within relatively short period of time (250~400 hours) upon repeated thermalcycles (500 ℃ to 700 ℃). Furthermore, we found that in-plane conductivity of thin (<10 µm) electrodes isstrongly dependent on the measurement time and the local atmospheric environment relative to thickerelectrodes. Special attention is required to use a thin electrode to evaluate electrode performance.This work was supported by the US Department of Energy (DOE), Office of Fossil Energy, (Officeof Clean Coal & Carbon Management), “Solid Oxide Fuel Cell Manufacturing in Support of Office ofFossil Energy” program through Argonne National Laboratory under FWP No. 27327.1.[1] S.B. Adler et al. J. Electrochem. Soc. 143 (1996) 3554-3564; S.B. Alder Solid State Ionics 111 (1998)125-134.[2] B.A. Boukamp et al. Solid State Ionics 192 (2011) 404-408; B.A. Boukamp et al. Solid State Ionics 283(2015) 81-90.[3] L. Holzer et al. J. Mater. Sci. 48 (2013) 2934-2952.[4] S.W. Peterson et al. J. Electrochem. Soc. 161 (2014) A2175-A2181.

Integrated gasification fuel cell (IGFC) systems combining coal gasification and solid oxide fuel cells (SOFC) are promising for highly efficient and environmentally friendly utilization of coal for power production. Most IGFC system analyses performed to-date have used nondimensional thermodynamic SOFC models that do not resolve the intrinsic constraints of SOFC operation. In this work a quasi-two-dimensional (2D) finite volume model for planar SOFC is developed and verified using literature data. Special attention is paid to making the model capable of supporting recent SOFC technology improvements, including the use of anode-supported configurations, metallic interconnects, and reduced polarization losses. Activation polarization parameters previously used for high temperature electrolyte-supported SOFC result in cell performance that is much poorer than that observed for modern intermediate temperature anode-supported configurations; thus, a sensitivity analysis was conducted to identify appropriate parameters for modern SOFC modeling. Model results are shown for SOFC operation on humidified H2 and CH4 containing syngas, under coflow and counterflow configurations; detailed internal profiles of species mole fractions, temperature, current density, and electrochemical performance are obtained. The effects of performance, fuel composition, and flow configuration of SOFC performance and thermal profiles are evaluated, and the implications of these results for system design and analysis are discussed. The model can be implemented not only as a stand-alone SOFC analysis tool, but also a subroutine that can communicate and cooperate with chemical flow sheet software seamlessly for convenient IGFC system analysis.