Defective Ni Perovskites as Cathode Materials in Intermediate-Temperature Solid-Oxide Fuel Cells: A Structure−Properties Correlation

Cathode material SmBaCoFeO5+δ (SBCF) was synthesized by a combined EDTA-citrate complexing sol-gel method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), AC impendence spectroscopy, respectively. The results showed that the SBCF cathode had an orthorhombic perovskite-structure and formed a good contact with the Ce0.9Gd0.1O1.95 (GDC) electrolyte after sintering at 950°C for 2h. Based on symmetric cell test, area specific resistance of SBCF on GDC electrolyte was as low as 0.1Ω cm2 at 800°C. The maximum power densities of a cell using the SBCF cathode are 431 mWcm−2, 313mWcm−2 and 200mWcm−2 at 800°C, 750°C and 700°C, respectively. Preliminary results demonstrate that SBCF is a very promising cathode material for application in intermediate-temperature solid oxide fuel cells (IT-SOFCs).

In the aim to stabilize novel three-dimensional perovskite oxides based upon SrCoO3−δ, we have designed and prepared SrCo1−xRexO3−δ phases (x = 0.05 and 0.10), successfully avoiding the competitive hexagonal 2H polytypes. Their performance as cathode materials in intermediate-temperature solid oxide fuel cells (IT-SOFC) has been investigated. The characterization of these oxides included X-ray (XRD) and in situ temperature-dependent neutron powder diffraction (NPD) experiments for x = 0.10. At room temperature, SrCo1−xRexO3−δ perovskites are defined in the P4/mmm space group, which corresponds to a subtle tetragonal perovskite superstructure with unit-cell parameters a = b ≈ ao, c = 2ao (ao = 3.861 and 3.868 Å, for x = 0.05 and 0.10, respectively). The crystal structure evolves above 380 °C to a simple cubic perovskite unit cell, as observed from in-situ NPD data. The electrical conductivity gave maximum values of 43.5 S·cm−1 and 51.6 S·cm−1 for x = 0.05 and x = 0.10, respectively, at 850 °C. The area specific resistance (ASR) polarization resistance determined in symmetrical cells is as low as 0.087 Ω·cm2 and 0.065 Ω·cm2 for x = 0.05 and x = 0.10, respectively, at 850 °C. In single test cells these materials generated a maximum power of around 0.6 W/cm2 at 850 °C with pure H2 as a fuel, in an electrolyte-supported configuration with La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) as the electrolyte. Therefore, we propose the SrCo1−xRexO3−δ (x = 0.10 and 0.05) perovskite oxides as promising candidates for cathodes in IT-SOFC.

Up to now, Solid Oxide Fuel Cell (SOFC) materials and processing does not meet the cost goals for commercialization. This resulted in a worldwide increase in R&D activities dealing with advanced materials and effective manufacturing methods. The present paper describes efforts to process novel SOFC materials as well as optimization of well known ones. The R&D trends are explained for key components such as anode, electrolyte, cathode, contact- and protective layers. Typical SOFC manufacturing methods include tape casting, extrusion, calendaring and axial pressing. Each of these techniques has advantages and limitations. Examples for the highly efficient use of these methods are given for electrolyte supported cells as well as anode and cathode supported designs. An evaluation in reference to automation, process complexity and costs is given under the present limiting factors. Exemplary the processing by tape casting and the micro structural fine tuning of an advanced anode-supported system is discussed in detail. To produce the layered components of an SOFC, techniques like screen printing, wet powder spraying, PVD and CVD are under development. While the layer properties are excellent, PVD and CVD are nowadays too expensive in some cases, due to the low deposition rates. If thin layers are required, these techniques become interesting under cost considerations. The effectiveness of a PVD interlayer between electrolyte and high power density cathodes is shown in comparison to a sintered layer. In thin electrolyte concepts, the cathode becomes the power limiting component at operating temperatures below around 750°C. Thus new cathode materials and adjusted processing parameters are under development. The possibilities to manufacture advanced cathode layers by screen printing, wet powder spraying and other wet chemical methods are discussed. As an example screen printing of LSCF is described which results in a high power density cathode layer for low temperature SOFC operation. Finally, future needs to achieve the technical and economic goals are summarized.

Fuel cells have developed of excessive interest as a probable economical, efficient, and clean candidate for alternative and environmental friendly power generation services. Solid Oxide Fuel Cell (SOFC) is an elevated temperature fuel cell, dealing with power generation as well as heat. Up to now many studies have been made to replace platinum, Pt, with a new cathode catalyst for intermediate temperature-solid oxide fuel cells (IT-SOFC) (500 °C<T) range but research has become inadequate. Since Pt sources are limited and very expensive, they could not meet the supply for the commercial fuel cells, the scientists started for searching new materials. There are two important aspects about SOFC cathodes, different cathode materials effect on the electrode electrochemical performance and the oxygen reduction reaction (ORR) kinetics. Understanding in these concepts would lead to improvements of SOFC systems. The production of novel and supreme cathode electrodes used in IT-SOFC is aimed to employ cheaper metals (Fe, Co, Cr, Mn, Gd, and V) by using superior properties of perovskite structure. The reduction of oxygen on metal oxide surface is achieved within the complicated mechanism. The completion of these steps depends on the nature of oxide ion carrier in cathode, atomic formation in crystal structure, and microstructure of cathode materials. The analysis of the impedances required the use of three to four (RQ) circuits in series in the equivalent circuit model. Of the four cathodes synthesized, the vanadium doped cathode on YSZ showed the highest area specific resistance.

Solid Oxide Fuel Cells (SOFCs) are a promising technology to produce electric energy from chemical energy. To achieve this, the development of new materials with enhanced properties are still needed. In this work, the synthesis of nanostructured powders to be used as components of an Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) was carried out through different synthesis methods. So, co-precipitation, sol-gel combustion, and hydrothermal synthesis methods were compared in the synthesis of the Gadolinium Doped Cerium G0,2C0,8O1,9 (GDC) used as electrolytic material. Cathodic material (La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) cobaltite type perovskite) was obtained by coprecipitation method. The materials were characterized by Fourier Transformed Infrared Spectroscopy (FTIR), Thermal Gravimetrical Analysis (TGA), Scanning Electron Microscopy (SEM-EDS), X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). The synthesized materials showed to be nanostructured. The chemical composition measured by EDS was constant in the entire sample and it has the expected stoichiometry in both materials. On the other hand, X-ray patterns exhibited well-defined peaks characteristic of ordered structures and with a high degree of crystallinity, corresponding with the information reported in the literature. From FTIR results, the characteristic peaks related to the presence of perovskites can be observed in the case of LSCF when it was compared to the results found in the literature. Nanostructured LSCF cathodic material was effectively synthesized by coprecipitation method. In the case of GDC highly crystalline material was obtain by sol-gel combustion method. Based on the results, the employed method allows to obtain homogeneous materials suitable to be used in IT-SOFC.

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].

Solid oxide fuel cells (SOFCs) which can directly covert the chemical energy of fuel to electric energy have received as one of alternative energy sources because of their high energy conversion efficiency, fuel flexibility and low emission of pollutant to the environments. However, there are still several technical issues for making commercialization of SOFCs such as limited durability and long start-up time. Hence, recent studies of the SOFCs have focused on improving performance of SOFCs in the intermediate temperature (IT) to achieve technical goals such as high stability, low-cost component materials, easier sealing and quicker start-up time. In particular, stable long-term performances of SOFCs are one of the essential factors for commercialization of fuel cell systems. However, the sudden unstable situations such as frequent electrical load on-off and high load can often occur during the SOFCs operations, and thus, these unexpected situations can affect the durability of SOFCs systems. Therefore, we need to investigate performance change of SOFCs under various electrical load conditions including the constant current, current on-off cycles and irregular current change. In addition, post-mortem analysis of SOFCs are necessary to find the main degradation mechanisms after various durability tests. In this work, Ni-cermet anode supported cells are fabricated by the tape-casting method with neodymium doped ceria and double-perovskite structure based cathode materials (NdBa0.5Sr0.5Co1.5Fe0.5O5+δ). The performance of the cells are investigated by I-V measurement and the durability test also conducted by using load trip (0~200 mAcm-2) and load cycle (120~200 mAcm-2) techniques with controlled current condition at 650 oC. For electrochemical analysis, electrochemical polarization and characterizations are conducted using an electrochemical impedance spectroscopy (EIS) with a Potentiostat/Galvanostat instrument using a frequency range from 1 MHz to 1Hz. The several physicochemical analyses are also investigated using field emission-scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). References E. D. Wachsman and K. T. Lee, Science 334 (2011) 935-939.S. B. Adler, Chem. Rev. 104 (2004) 4791-4843.E. Park, S. Taniguchi, T. Daio, J. T. Chou and K. Sasaki, Solid State Ionics 262 (2014) 421-427.B. C. H. Steele and A. Heinzel, Nature 414 (2001) 345-352.S. Omar, E. D. Wachsman and J. C. Nino, Solid State Ionics 177 (2006) 3199-3203.M. Kahlaoui, S. Chefi, A. Inoubli, A. Madani and C. Chefi, Ceram. Int. 39 (2013) 3873-3879. Keywords : Solid oxide fuel cells; intermediate-temperature; durability; perovskite.

Mixed oxygen ion and electron-conducting materials are viable cathodes for solid oxide fuel cells due to their excellent oxygen transport kinetics and mixed electrical conductivity, which ensure highly efficient operation at low and medium temperatures. However, iron-based double perovskite oxides usually exhibit poor electrocatalytic activity due to low electron and oxygen ion conductivity. In this paper, Ca is doped in PrBaFe2O5+δ A-site to improve the electrochemical performance of PrBaFe2O5+δ. Results show that replacing Pr with Ca does not change the crystal structure, and the Ca doping effectively increases the adsorbed oxygen content and accelerates the migration and diffusion rate of O2- to the electrolyte|cathode interface. The polarization resistance of the symmetric cell PC0.15BF|CGO|PC0.15BF is 0.033 Ω·cm2 at 800 °C, which is about 56% lower than that of PBF, confirming the enhancement of the mixed conduction of oxygen ions and electrons. In addition, the anode-supported single cell has a peak power density of 512 mW·cm-2 at 800 °C.

The high-temperature solid oxide fuel cells (SOFCs) are the most efficient and green conversion technology for electricity generation from hydrogen-based fuel as compared to conventional thermal power plants. Many efforts have been made to reduce the high operating temperature (>800 °C) to intermediate/low operating temperature (400 °C<T<800 °C) in SOFCs in order to extend their life span, thermal compatibility, cost-effectiveness, and ease of fabrication. However, the major challenges in developing cathode materials for low/intermediate temperature SOFCs include structural stability, catalytic activity for oxygen adsorption and reduction, and tolerance against contaminants such as chromium, boron, and sulfur. This research aims to provide an updated review of the perovskite-based state-of-the-art cathode materials LaSrMnO3 (LSM) and LaSrCOFeO3 (LSCF), as well as the recent trending Ruddlesden-Popper phase (RP) and double perovskite-structured materials SOFCs technology. Our review highlights various strategies such as surface modification, codoping, infiltration/impregnation, and composites with fluorite phases to address the challenges related to LSM/LSCF-based electrode materials and improve their electrocatalytic activity. Moreover, this study also offers insight into the electrochemical performance of the double perovskite oxides and Ruddlesden-Popper phase materials as cathodes for SOFCs.

Solid Oxide Fuel Cell (SOFC) cathode materials rely on the catalytic processes at their surfaces to reduce and incorporate oxygen. For many common cathode materials surface passivation occurs through 'A site' segregation at common operating temperatures [1]. These processes have a negative effect upon the oxygen reduction rates and therefore overall cell performance. Quantifying the true extent these segregation effects have on the exchange properties has not been fully explored but it is clear that the properties of the surface are essential. Many techniques have been employed to study cell properties in-situ, however, surface studies are typically restricted to post mortem or ex-situ analysis due to the necessity of a high vacuum. In this study High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) has been employed to analyse the morphology changes of common SOFC cathode materials in-situ. Samples of La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and La0.6Sr0.4CoO3-x (LSC) were annealed from room temperature to 1000oC at pressure ranges between 3 – 6mbar in atmospheres of Oxygen and Water. Using secondary electron imaging during the thermal annealing cycle morphological and compositional changes were able to be documented at temperature for the first time (Figure 1). This technique has enabled detailed analysis of the surface degradation that occurs during SOFC fuel cell operation. Secondary phase precipitation rate and formation behaviour has been quantified over a range of temperatures and time scales furthering the understanding of SOFC cathode surfaces significantly. [1] M. Kubicek, A. Limbeck, T. Fromling, H. Hutter, and J. Fleig, "Relationship between cation segregation and the electrochemical oxygen reduction kinetics of La0.6Sr0.4Co0.2Fe0.8O3 thin film electrodes" Journal of The Electrochemical Society, vol. 158, no. 6, pp. B727–B734, 2011. Figure 1 - Showing in-situ growth of secondary phase particulates on a polished surface of LSCF Figure 1

Bi 0.5Sr 0.5MnO 3 as cathode material for intermediate-temperature solid oxide fuel cells

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.

Two perovskite materials with SrMo1−xAlxO3−δ (x = 0.1, 0.2) compositions have been synthesized by reduction from the corresponding scheelite phases, with SrMo1−xAlxO4−δ stoichiometry; the pertinent characterization shows that the defective perovskites can be used as anode materials in solid oxide fuel cells, providing maximum output power densities of 633 mW/cm2 for x = 0.2. To correlate structure and properties, a neutron powder diffraction investigation was carried out for both perovskite and scheelite phases. Both perovskites are cubic, defined in the Pm-3m space group, displaying a random distribution of Mo and Al cations over the 1a sites of the structure. The introduction of Al at Mo positions produced conspicuous amounts of oxygen vacancies in the perovskite, detected by neutrons. This is essential to induce ionic diffusion, providing a mixed ionic and electronic conduction (MIEC), since in MIEC electrodes, charge carriers are combined in one single phase and the ionic conductivity can be one order of magnitude higher than in a conventional material. The thermal expansion coefficients of the reduced and oxidized samples demonstrated that these materials perfectly match with the La0.8Sr0.2Ga0.83Mg0.17O3−δ electrolyte, La0.4Ce0.6O2−δ buffer layer and other components of the cell. Scanning electron microscopy after the test in a real solid oxide fuel cell showed a very dense electrolyte and porous electrodes, essential requirements for this type of fuel. SrMo1−xAlxO3−δ perovskites are, thus, a good replacement of conventional biphasic cermet anodes in solid oxide fuel cells.

For the last few decades, alternative and sustainable energy sources have been approached which deplete the environmental issues and provides the clean energy conversion towards world energy demands. Fuel cells which convert chemical (Fuel) energy directly into electrical energy (electricity) via electro-chemical reaction mechanism are more advantageous when compared with the other conventional energy conversion technologies. Among the various available fuel cells, Solid oxide fuel cell (SOFC) has proven as most promising, efficient, eco-friendly energy conversion device due to its longer life time, variety choice of fuels and less environmental impact. The fundamentals of fuel cell technologies, background, working mechanism, different components of SOFC, brief note on traditional materials, and development of novel materials for the advanced SOFC were addressed. Working/operating temperature plays a key role in the development of SOFC because high operating temperature curbs the commercialization. Enormous efforts have been done in reducing the operating temperature by introducing various doped /co doped /triple-doped ceria based materials with enhanced electrical properties which led to the development of intermediate temperature SOFCs. On the way of reducing the operating temperature, R&amp;D introduced a composite approach using doped ceria and alkali carbonates which pave a way for the development of low operating temperature SOFC. The recent developments regarding ceria-carbonate composites with emphasis on the ionic conductivity have been discussed towards the electrolytes for advanced SOFC applications at relatively lower temperatures

Performance durability is one of the essential requirements for solid oxide fuel cell materials operating in the intermediate temperature range (500–700 °C). The trade‐off between desirable catalytic activity and long‐term stability challenges the development and commercialization of electrode materials. Here an oxygen cathode material, Ba0.5Sr0.5(Co0.7Fe0.3)0.69−xMgxW0.31O3−δ (BSCFW‐xMg), that exhibits excellent electrocatalytic performance through the addition of an optimized amount of Mg to the self‐assembled nanocomposite Ba0.5Sr0.5(Co0.7Fe0.3)0.69W0.31O3−δ (BSCFW) by simple solid‐state reaction is reported. Distinct from the bulk and surface approaches to introduce vacancies and defects in materials design, here the Mg2+ ions concentrate at the single perovskite/double perovskite interface of BSCFW with dislocations and Mg2+‐rich nanolayers, resulting in stressed and compositionally inhomogeneous interface regions. The interfacial chemistry within these nanocomposites provides an additional degree of freedom to enable performance optimization over single phase materials and promotes the durability of alkaline‐earth based fuel cell materials.