Ruddlesden-Popper phase materials for solid oxide fuel cells: performance and structural studies
Creators: name: Yatoo, Mudasir
Publisher: name: Imperial College London
Year: 2019

description: This thesis investigated the three novel higher-order n = 3 Ruddlesden-Popper phase compositions, LaPr3Ni3O10-δ (L1P3N3) La2Pr2Ni3O10-δ (L2P2N3) and La3PrNi3O10-δ (L3P1N3) for their potential use as intermediate-temperature solid oxide fuel cell cathodes. This particular focus on the higher-order Ruddlesden-Popper phases was based on their superior electronic conductivity and phase stability, two major drawbacks associated with the lower-order Ruddlesden-Popper phase materials such as La2NiO4+δ and Pr2NiO4+δ. All the compositions were synthesised by the Pechini method followed by characterisation by X-ray diffraction. Initial half-cell impedance spectroscopy measurements of half-cells with L2P2N3 composition as electrode and La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) as electrolyte revealed promising area specific resistance (ASR) of 0.34 Ωcm2 at 800 ̊C. This is a major improvement over the previously reported performance of La4Ni3O10-δ material measured under similar conditions. An unexpected revelation of impedance data analysis was that the electron-transfer process between electrode and electrolyte was the major contribution to overall polarisation resistance. This was later evidenced by post-test microstructural analysis; cross-section secondary electron SEM images presenting open spaces as large as 2 micrometres at the electrode-electrolyte interface, thus showing significantly poor adherence of the electrode. Microstructure, as revealed by top-view SEM, further showed significant particle densification and limited particle interconnectivity. Optimisation in terms of electrode ink preparation and the addition of an LSGM ink interlayer prepared in the same way as the electrode ink afforded an ASR of 0.11 Ωcm2 at 700 ̊C for L2P2N3 composition. The ASR obtained for the analogous L3P1N3 and L1P3N3 cathodes were 0.14 Ωcm2 and 0.08 Ωcm2 respectively. Impedance analysis further revealed that the contribution emanating from the electron-transfer process was minimal and was almost negligible at high-temperatures. This was later corroborated by post-test microstructural analysis showing very strong adherence of electrode to the electrolyte. Particle interconnectivity further showed significant improvement, with no densification whatsoever. Single-cells with NiO-GDC (Gd0.1Ce0.9O2-δ) as the anode were built and the respective maximum power densities obtained for L1P3N3, L2P2N3 and L3P1N3 were 380 mW cm-2, 390 mW cm-2 and 400 mW cm-2. Total electrical conductivity measurements showed that all three compositions have significant electrical conductivity in the range of 200-300 Scm-1 in the intermediate temperature as defined for SOFCs. Anomalous behaviour was observed in the L1P3N3 and L2P2N3 compositions in the 250-350 ̊C temperature range. The structural origin of this anomaly was ruled out after high-temperature in situ XRD measurements. All three compositions were found to be understoichiometric by iodometric titrations, which was later confirmed by neutron diffraction measurements. The structural evolution with changing temperature revealed that the L2P2N3 and L3P1N3 compositions retained the orthorhombic Bmab structure while the L1P3N3 composition showed a transition to I4/mmm tetragonal symmetry at 800 ̊C. Lattice parameters showed a linear increase with the increase in temperature, and the thermal expansion coefficient calculated for all compositions was found to be of the order of 13.0 × 10-6 °C-1. In terms of the defect chemistry of the materials, oxygen vacancies were found to be confined to the perovskite layers, with a particular preference for equatorial oxygen sites. Contrary to the earlier reports in these materials, a limited quantity of oxygen vacancies was observed in the rock-salt layers of both L1P3N3 and L3P1N3 compositions. Further, a significant preference to the curved oxygen transport pathway of O(1)−O(1)−O(1) around the NiO6 octahedra was observed in all compositions.