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A stack based on planar solid oxide fuel cells (SOFCs) is an assembly of ceramic plates - membrane-electrode assemblies (MEA) and metal current collectors, which play the role of the distribution of gas mixtures over the electrode area, as well as current collection. In order to organize the high-quality current collection it’s necessary to provide a reliable electrical contact between the MEA’s electrodes and contact surfaces of the current collectors. Electrical contact between MEAs’ anode electrode and the collector is achieved by pressing the contact nickel meshes. Metal-based conductive adhesives are also used to reduce the contact resistance at the anode electrode. Despite this in order to ensure contact between the MEA’s cathode electrode and the collector this approach is not applicable due to the rapid oxidation of contact meshes in air atmosphere at high temperatures. In turn, contact pastes based on conductive ceramics do not have such a drawback and make it possible to ensure reliable contact under the conditions of the SOFC cathode chamber. Materials for the contact adhesive for the SOFC cathode chamber must be chemically and thermomechanically compatible with the materials of the SOFC cathode and bipolar plate along with high electrical conductivity. These requirements are met by materials based on lanthanum strontium manganite oxide (LSM) materials, which are widely used as cathode materials for SOFCs. It should be noted that the contact adhesive must also correspond to the limitations of technological processes of SOFC stack manufacturing. SOFC stacks are sealed with glass in a temperature range of 930-950°C, so it’s necessary to use a material that forms a conductive contact at temperatures not higher than 950°C, which is significantly lower than typical sintering temperatures of LSM-based cathode. The synthesis of single-phase highly dispersed powders with the composition of La0.8Sr0.2MnO3±δ (LSM) and (La0.8Sr0.2)0.97MnO3±δ (LSM-d) was carried out by the glycine-nitrate method. After synthesis some of the obtained materials were ball-milled in ethanol using 5 mm ceramic milling bodies at a speed of 600 rpm for 300 and 600 min. A mixture of polyvinyl butyral, toluene, and butanol was used as the basis for the contact paste. Tests of the electrical conductive properties of contact pastes were carried out in air in temperature range of 600-950°C. Samples were kept for 2 hours before cooling at the maximum temperature. Figure shows the results of resistivity measurements: the dashed lines represent the values obtained when the sample was cooled. The maximum values of the resistivity at a temperature of 850°C were obtained for contact adhesive samples based on non-activated stoichiometric (LSM) and A-deficient (LSM-d) powders. Low electrical conductivity can be associated with weak intergranular contact due to the strong agglomeration of the material after synthesis. Grinding for 300 minutes results in a noticeable reduction in the resistivity of the contact adhesive (LSM-300). Change in resistivity after holding at maximum temperature, as well as the change in the slope of the temperature dependence of the resistivity of the adhesive based on LSM-300 indicates the sintering of the contact adhesive powder into a ceramic compact, which is confirmed by SEM results. In turn, holding at a temperature of 950°C of contact adhesive based on LSM-600 (600 min grinding pre-activated) powder does not lead to its sintering. Grinding of the initially single-phase LSM powder results in the appearance of additional reflections in the XRD-spectrum corresponding to the Sr(OH)2 and MnCO3 phases. An increase in the grinding time leads to an increase in the proportion of impurity phases that prevent the contact adhesive from sintering at temperatures Thus, the best results were shown by a contact adhesive based on stoichiometric LSM powder, pre-activated by grinding for 300 minutes. Holding at a temperature of 950°C leads to sintering of LSM-300 powder into ceramics, which makes it possible to form a reliable electrical contact under the conditions of the SOFC cathode chamber. We have proposed a method for the manufacture and deposition of a contact cathode composition based on La0.8Sr0.2MnO3±δ. It is shown that the initial powders obtained by the glycine-nitrate method according to the Pechini modification are preliminary prepared. The optimal grinding parameters have been determined. The sintering modes of the contact paste are selected, which are consistent with those for the sealing glass. An experimental SOFC assembly was made using a contact cathode paste. It is shown that the electrochemical characteristics of individual MEAs correspond to the declared ones and the specific power reaches 0.25W/cm2. This work was carried out with financial support from the Russian Scientific Foundation, grant no. 17-79-30071. Figure 1

Ferritic stainless steel with high chromium content (>16 wt%) are regarded as promising interconnects materials for solid oxide fuel cell applications. However, particularly in oxidizing atmospheres, the formation of chromia scales leads to high contact resistances harmful to SOFC performance. At the same time, up to now one of main challenges in SOFC technology developments is to suppress the degradation processes, often associated with the stainless-steel interconnect materials, and to provide low contact resistivity in oxidizing atmospheres. In previous works [1,2] we proposed a new approach for forming barrier layers impeding Cr diffusion to the metallic current-collector surface. The deposition of Ni-based layers with subsequent thermal treatments was shown to result in unique interfacial microstructures, where the formation of an internal oxidized Cr layer prevents the segregation of insulating Cr2O3 phase on the stainless steel surface and reduces electrical resistivity [2]. The present work, focused on the analysis of near-surface interdiffusion phenomena in Crofer 22 APU ferritic steel interconnects with Ni-based protective layers, summarizes our developments in this field. Particular emphasis is centered on the studies of time dependencies of the area-specific resistance (ASR) between the current collector and standard La0.8Sr0.2MnO3(LSM) cathodes, depending on the protective inter-layer composition. A theoretical model describing the contact resistivity behavior was proposed and validated. The deposition of protective layers onto the Crofer 22 APU interconnect plates was described elsewhere [2]. The microstructural and compositional analysis of the junctions “current collector – LSM cathode” was performed using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS, Supra 50/VP instrument). The studies of element distribution profiles across the interfaces were carried out after prolonged isothermal treatments at 850°C in air, coupled with electrical measurements as a function of time. In order to detect trace separation of new phases such as Cr2O3, micro Raman scattering spectroscopy was also employed. The area-specific resistance of the “current collector – LSM cathode” junctions was studied over long periods of time under the SOFC cathode operation conditions (atmospheric oxygen pressure, 850ºC, current density of 0.5 A/ cm2). The results showed that the electrical resistance variations of the assemblies “Crofer 22 APU | LSM” with and without surface modification of the metallic plates can be quantitatively described in framework of the Schottky barrier model [3,4] for metal– semiconductor interfaces. The microscopic mechanism governing these changes involves metal interdiffusion between the cell components leading, in particular, to the formation of essentially immobile Cr2O3grains at the boundary between Crofer 22 APU and deposited Ni-based layer. These interfacial alterations make it possible to preserve low contact resistances during, at least, 20000 h. The junction between the current collector and SOFC cathode should be considered as forward-biased. In the case of relatively thin blocking layer, the current-voltage relationship for such a junction is J=AT2(Ve/kT)exp( -F/kT), where the Richardson constant A=4 π emk2/h3 comprises the Planck ( h ) and Boltzmann ( k ) constants as well as electron charge ( e ) and mass ( m ); V is the voltage drop across the forward-biased junction; F = Φ metal – Χ LSM is the difference between the metal work function and electron affinity of the LSM cathodes. Time dependencies of the contact ASR between Crofer 22 APU and LSM can be adequately described by the Schottky barrier changes, originating from changing the current-collector work function due to metal interdiffusion between the Ni-based coatings and stainless steel. This work was supported by the Ministry of Education and Science of the Russian Federation (project 14.610.21.0007) References [1]. N. Ledukhovskaya, E. Frolova, G. Strukov, D. Matveev, S. Bredikhin “New Type of Current Collectors with Modified Near-Surface Layer” ECS Transactions, 25 (2) 1523-1528 (2009). [2]. N. Demeneva, S.Bredikhin “Improvement of Oxidation Resistance of Crofer 22 APU with Modified Surface for Solid Oxide Fuel Cell Interconnects” ECS Transactions, 57 (1) 2195-2201 (2013) [3]. S. Bredikhin, T. Hattori, M. Ishigame “Schottky barriers and their properties in superionic crystals” Rhys. Rev. B, 50, p. 2444 – 2449 (1994) [4]. S.I. Bredikhin, V.N. Bondarev, A.V. Boris, P.V. Pikhisa, W. Weppner “Electronic conductivity and current instability in superionic crystals” Solid State Ionics 81. 19 -28 (1995).

Anode cermets of metallic Ni and stabilized zirconia are among the most common SOFC materials. The electrochemical performance of the composite anodes is essentially governed by the triple phase boundary formed by electronic conductor, solid electrolyte and gaseous phase. One important approach to optimize the anode morphology relates to decreasing particle size of the initial powders. On the other hand, a drastic reduction of the component particle size may lead to dramatic changes in the material properties. The tendencies to agglomeration, a presence of modified surface layers and fast rates of microstructural alterations under SOFC operating conditions are all expected to considerably affect the electrode preparation procedure and anode behavior. The present work is centered on the analysis of key relationships between the morphology of initial NiO powders used to form Ni-containing cermets and overall quality of the composite SOFC anodes. The commercial Nickel (II) oxide nanopowder (99.8% purity, Sigma-Aldrich) was employed as a model starting material for these studies. A large series of NiO powders were prepared by annealing of initial NiO at different temperatures (RT-1100 oC). Scanning and transmission microscopy, X-ray diffraction, thermogravimetry, X-ray photoelectron and Raman spectroscopic analyses were employed for morphological, chemical and structural characterization. Then the powders were mixed with highly dispersed solid electrolyte of 10 mol.% Sc2O3 and 1% mol.% CeO2 stabilized zirconia (10Sc1CeSZ) and an organic solvent to obtain screen-printable pastes, used to deposit anode layers onto 10 mol.% Sc2O3 and 1% mol.% Y2O3 stabilized ZrO2 membranes for further microstructural and electrochemical studies. The results showed, in particular, that starting NiO nanopowder (Fig. 1) possesses a core-shell structure, with the crystalline core covered by amorphous shell rich of excessive oxygen and adsorbed water. These surface layers, having critical impact on the integrity and performance of resultant anodes, can be eliminated by thermal pre-treatment. Fig. 1. TEM image of starting NiO powder. Figure 1

Kinetics of redox reactions and the variations of real charge-carrier concentrations across the SOFC electrode / electrolyte interfaces have critical importance for the fuel cell performance. The microscopic mechanisms of electrochemical processes in the vicinity of triple-phase boundary (TPB) can be assessed employing impedance spectroscopy, current-voltage measurements and various pulse techniques. Except for the micro-electrode approaches, however, these methods are cumulative. Moreover, unambiguous interpretation of the electrochemical measurement results requires, as a rule, to introduce complimentary experimental methods and/or to use simplified models. The present work is centered on the developments of a new combined technique for in-situ Raman spectroscopy studies of local chemical and electrochemical reactions, phase transitions, strains and morphological alterations in the SOFC electrodes under working conditions. The Raman spectroscopy was already successfully used to investigate both cathodic [1,2] and anodic [3] processes in SOFC, but the resultant information was mainly related to outer boundaries of the model electrochemical cells, primarily electrode surface, due to low penetration depth of the excitation radiation. The most important zones of the electrode systems, where the electrochemical reactions occur and ionic charge carriers are generated, cannot be achieved viewing the surface and edge areas. In the present work, this problem was solved by employing optically transparent, single-crystal membranes made of 10 mol.% Sc2O3 and 1% mol.% Y2O3 stabilized zirconia (10Sc1YSZ). In order to provide simultaneous Raman and electrochemical measurements of the cell placed under an oxygen chemical potential gradient, such as air/H2, a special controlled-atmosphere chamber was elaborated and tested. An appropriate selection of the electrodes geometry makes it possible to directly collect Raman spectra from the TPB zone, passing the beam through single-crystal solid electrolyte onto the interface, as a function of temperature, atmosphere, current density and/or overpotential. The results of case studies focused on redox kinetics of Ni-containing cermet anodes, are presented. This work was supported by grant 14-29-04031 of Russian Foundation for Basic Research. References S. Loridant, L. Abello, E. Siebert, G. Lucazeau, Solid State Ion. 78 (1995) 249-258 K. Blinn, H. Abernathy, M.Liu, Advances in Solid Oxide Fuel Cells V. (2010) 63-73 W. Bessler, M.Vogler, H. Stoermer, D. Gerthsen, A. Utz, A. Weber, E. Ivers-Tiffee, Phys. Chem. Chem. Phys., 12 (2010) 13888-13903

Modeling and subsequent examination of ambipolar conductivity and surface reaction resulted in a noticeable optimization of the total anode conductivity. The electrochemical properties of solid oxide fuel cells (SOFCs) were investigated as a function of the Ni volume content in composite Ni/YSZ anodes. The dependence of experimentally measured SOFC characteristics on the anode thickness agreed well with the numerical calculation results. The maximum power output was achieved for the SOFC with an anode thickness 10-20 µm, and the part of the electrode exceeding this thickness mainly operated as a current collector for that functional layer.