(Invited) Ruddlesden-Popper-Type Oxides As Air Electrodes for Solid Oxide Electrolyzer Cells

Abstract

Ruddlesden-Popper-type oxides are promising air electrode materials for solid oxide fuel cells (SOFCs) and electrolyser cells (SOECs). Within the Ruddlesden-Popper (RP) series Lnn+1BnO3n+1, first order (n=1) RP-type rare earth nickelates with Ln=La, Nd, Pr and B=Ni show high oxygen diffusivities, high catalytic activity for the oxygen reduction reaction as well as good electronic and ionic conductivities [1]. In order to further improve the surface oxygen exchange kinetics, the effect of partial substitution of Ni by Co on the B-site in Pr2NiO4+δ was studied, similar to an earlier report for the La2NiO4+δ system [2].Structure-composition-property–relationships were examined for the Pr2NiO4+δ system. The effect of A-site substitution of Pr with La as well as B-site substitution of Ni with Cobalt was investigated with respect to crystal structure, thermodynamic stability, oxygen non-stoichiometry, electronic conductivity as well as oxygen surface exchange and transport properties. Moreover, differences in functional properties between Ruddlesden-Popper phases of different order within the same compositional system were studied. In cases where material characterisation was problematic due to difficulties in obtaining densely sintered samples with high phase purity (including of third-order RP-Phases), electrochemical impedance spectroscopy (EIS) measurements on microelectrodes were applied in order to obtain reliable results for oxygen surface exchange rates. Oxygen surface exchange coefficients kq and kδ could be calculated from surface resistances and chemical capacitances of the thin-film electrodes between 550 and 850°C. Substitution of Ni by 10% of Co in Pr2NiO4+δ results in increased oxygen surface exchange rates expecially at lower oxygen partial pressures [3]. Performance tests with Pr2Ni0.9Co0.1O4+δ SOEC air electrodes for water electrolysis were conducted on anode supported full cells at 800°C at OCV conditions and in electrolyser mode up to current densities of 1000 mA cm-2. For anode supported cells, an electrode-electrolyte composite was used as functional anode layer in order to improve adhesion and avoid delamination issues. Moreover, the long-term stability and resistance against Cr-poisoning was examined by electrochemical impedance spectroscopy and current-voltage measurements on symmetrical cells at 800°C in dry and humid atmospheres. A three-electrode configuration allowed to analyse the time dependence of the air electrode polarization resistance during the SOFC as well as SOEC operation mode. Compared to La2NiO4+δ, a reduced susceptibility for Cr-poisoning was found for Pr2Ni0.9Co0.1O4+δ over a period of 1000 hours in SOEC operation.The development of high-performance SOEC air electrodes is part of the Austrian integrated project HydroMetha. This project combines high-temperature co-electrolysis of CO2 and H2O by solid oxide cells with catalytic methanation in order to enable storage of electrical energy from fluctuating renewable sources with high overall efficiency.