Ruddlesden-Popper Phases as Solid Oxide Fuel Cell Cathodes: Electrochemical Performance and In Situ Characterisation

摘要

The aim of this work was to develop oxide fuel cell (SOFC) cathodes made from (LaNiO3)nLaO Ruddlesden-Popper (R-P) phases, and to investigate novel in situ characterisation techniques for SOFC cathodes.udCathodes were developed from La2NiO4+δ (L2N1) and La4Ni3O10-δ (L4N3), R-P phases known to have attractive conductivities at SOFC temperatures. These phases were shown to be chemically stable, both with each other and with the common electrolyte material La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM). LSGM-supported symmetrical cells were fabricated with electrodes of single phase L2N1 and L4N3, and a range of L2N1+L4N3 composites. The performance of these was tested from 500 – 700 °C with the composites giving the lowest area-specific resistance (ASR); a 50:50 wt.% L2N1:L4N3 composition being optimal.udFunctionally graded electrodes were developed consisting of a thin compact L2N1 layer deposited onto the LSGM, topped by a thicker porous L2N1+L4N3 composite layer, completed by a thin porous L4N3 current collector. These gave a lower ASR than the ungraded electrodes. Using a 50:50 composite was optimal with ASRs of 15.59, 2.29, and 0.53 Ωcm2 at 500, 600, and 700 °C respectively; amongst the best-in-class for electrodes made from this type of material.udX-ray absorption near-edge spectroscopy was chosen as a method to gain in situ information on the redox chemistry of elements within SOFC materials. Initial studies were carried out on powder samples of L2N1 and L4N3; the nickel oxidation state in these was found to reduce on heating to SOFC operating temperatures. Bespoke equipment was developed to enable such studies to be carried out on symmetrical cells under polarisation and with simultaneous AC impedance spectroscopy. The bulk nickel redox chemistry was correlated with the changing concentration of ionic charge carriers in the materials, and was found to be dominated by thermal effects. These techniques were then used to explore in situ chromium poisoning of state-of-the-art perovskite cathodes.udThe surface chemistry of SOFC materials is key to performance. Low-energy ion scattering was used to find the composition of the outer monolayer for the entire (LaNiO3)nLaO R-P series; lanthanum termination was found for each phase.