Investigation of the Stability of SOC Stacks By Thermo Cycles

Abstract

In recent years different research institutes and companies have developed anode supported cells (ASCs) with an LSC ((La,Sr)CoO3δ) oxygen electrode for solid oxide cells (SOCs), which showed an improved performance in comparison to ASCs with an LSCF electrode. Until now, the SOC-stacks by Forschungszentrum Jülich GmbH (FZJ) used ASCs with LSCF on the oxygen side. To minimize undesired effects which might occur due to the inter-diffusion of the constituting cations between adjacent layers, the contact layer on the oxygen side was made of the same LSCF composition.One mayor concern accompanying the use of LSC layers in ASCs and stacks is the thermal expansion coefficient (TEC) of the LSC material, which is almost a factor two higher than the TECs of the other materials of the cells and in the stacks (La0.6Sr0.4CoO3: 20.5·10-6 K-1 [1], La0.6Sr0.4Co0.2Fe0.8O3: 17.5·10-6 K1 [1], 8mol% Y2O3 stabilized ZrO2 (8YSZ): 9.3·10-6 K-1 [2], 30vol% Ni – YSZ cermet: 11.6·10-6 K-1 [3], CroFer 22 APU: 11.6·10-6 K-1 [4]). This large difference may lead to cracking in the layers or in the worst case to delamination between the layers. To meet this concern and demonstrate the stability and robustness of ASCs with LSC on the oxygen side of SOC-stacks performance tests were done with three F-10 stacks by FZJ, each consisting of four repetitive units (RU). The first stack (stack#1) used ASCs with LSCF as electrode and contact layer on the oxygen side and was taken as reference. Then, replacing LSCF by LSC was performed step wise. First only the cells with LSCF were replaced by cells with LSC, leaving all other components of the stack the same, including the LSCF contact layer on the oxygen side (stack#2). In the second step also the contact layer on the oxygen side was manufactured from the same LSC composition (stack#3). Thereby, commercially available cells were used resulting in additional differences between stack#1 (30 µm oxygen electrode, 5 µm oxygen barrier, 10 µm electrolyte, 7 µm fuel electrode and 300 µm fuel electrode support layer) and stack#2 as well as stack#3 (30 µm oxygen electrode, 2 µm oxygen barrier, 3 µm electrolyte, 10 µm fuel electrode and 400 µm fuel electrode support layer). Each stack was characterized in fuel cell mode and investigated by thermo cycles.The performance enhancement of stack#2 due to the introduction of LSC electrodes can be reported. At an operating temperature of 700 °C with 20% humidified hydrogen as fuel, current densities well over 1.2 A/cm² could be obtained at a cell voltage of 700 mV. This is a substantial increase in performance compared to the reference stack, stack#1, with LSCF electrodes on the oxygen side for which only 0.7 A/cm² at 700 mV could be reached. An initial analysis of impedance spectra recorded showed that this improvement could be attributed a lower ohmic resistance, mainly due to the thinner electrolyte layer, and a lower polarization contribution of the ASCs with LSC electrode.After this initial characterization all three stacks were subjected to thermo cycles from the operating temperature of 700 °C down to 200 °C and backwards. After 21 initial thermo cycles both, stack#2 and stack#3 were subjected to even more severe thermo cycles down to below 100 °C and back to 700 °C. From OCV, IV and EIS measurements during each cycle the stability of all three stacks could be shown for up to 50 thermo cycles with voltage losses from 0.2 to 0.5 mV/cycle despite the introduction of LSC layers in two of these stacks.Compared to ASCs with LSCF electrodes on the oxygen side an enhancement in performance was achieved by using ASCs with LSC electrodes and LSC contact layers on the oxygen side in the FZJ F10-design. Also, the stability of performance was demonstrated for up to 50 thermo cycles despite the higher TEC of LSC compared to the other stack materials.