Perovskite-Based Materials As Alternative Fuel Electrodes for Solid Oxide Electrolysis Cells (SOECs)

Enhancing the lifetime of SOECs is a challenge to overcome regarding their commercialization. A major impact on the lifetime of a cell during electrolysis operation, particularly under thermoneutral potential and high current densities, is the degradation of the currently used electrode materials, mainly the Ni-based fuel electrode. Among other things, nickel migration, as well as agglomeration, is leading to a significant performance loss after a certain operating time. Hence, preventing degradation mechanisms of the fuel electrode during operation is a necessity to be tackled for using it commercially. Therefore the development of alternative materials which combine sufficient performance with the lowest possible degradation rate is needed. Perovskite-based materials have been investigated in the last years as all-ceramic possible substitutes. In this work, four perovskites (i.e., strontium-iron-niobate double perovskite (SFN), a strontium-iron-titanate material (STF), a lanthanum-strontium-titanate (LST) and a lanthanum-strontium-iron-manganese (LSFM)) were examined as alternative electrode materials. The aim is to substitute the active fuel electrode, at the moment commonly consisting of Ni cermets, with a perovskite-based electrode while at the same time using state-of-the-art materials for the remaining cell components. The first task here was to look at the chemical stability between the new electrode material and the electrolyte under the standard conditions used to manufacture fuel electrode-supported SOECs. Therefore, the compatibility between these perovskites with a yttria-stabilized-zirconia (8YSZ) electrolyte and how nickel inside the fuel electrode affected the chemical stability during sintering in air at 1400 °C for 5 h was investigated. At this point, SFN double perovskite shows the lowest interaction between the electrode and electrolyte after thermal treatment. A thorough evaluation of all preliminary tests (including compatibility, stability in reducing atmospheres and redox stability tests) indicates that SFN shows so far the best results of the four materials in terms of application as fuel electrode material, followed directly by STF. Thus SFN and STF were chosen to be evaluated in single cell tests. The tests of pure SFN and STF electrodes are carried out with electrolyte-supported single cells exhibiting an LSCF air electrode and symmetrical cells, respectively. CV-characteristics and impedance spectra are measured at varied operating conditions. Impedance spectra are evaluated by the distribution of relaxation times (DRT). These examinations are carried out to give an insight into the electrochemical properties of pure perovskite-based fuel electrodes in order to obtain a base for further optimization.