Optimization of Contact Cathode Composition Based on La0.8Sr0.2MnO3±δ for SOFC Stacks

Abstract

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 <950°C.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