Developing high performance and stable hetero-structured cathodes and fundamental understanding of oxygen reduction and reaction behavior

Abstract

Developing High Performance and Stable Hetero-structured Cathodes and Fundamental Understanding of Oxygen Reduction and Reaction Behavior ByXinxin Zhang New hetero-structured La2NiO4+δ (LNO)/ (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) SOFC cathodes have been successfully fabricated by the infiltration technique for long-term stability and oxygen reduction reaction (ORR) kinetic consideration. The current comparative study of LSCF and LNO-infiltrated LSCF shows a significant reduction in polarization resistance by one order of magnitude and a 67% increase in maximum power density by using LNO-infiltrated LSCF cathodes at 750 o C in air. The electronic conductivity relaxation (ECR) results demonstrate that the remarkable improved oxygen exchange properties of LNO/LSCF hetero-structured cathodes are attributed to not only the increase of surface reaction area, but also the introduction of LNO surface and LNO-LSCF interface with rapid oxygen exchange behavior. A trace amount of Co, Sr and more oxygen defects stabilized in the LNO layer have been identified by TEM, EDXS, XPS, XRD and iodine titration experiments, giving rise to the enhancement of stability and surface catalytic activity. A low degradation rate of 0.39% at a constant current density of 250 mA•cm-2 can be still achieved for the fuel cell using LNO-infiltrated LSCF cathodes after long-term durability of about 500 h at 750 o C, accompanied by nanoparticles growth/aggregation, delamination and slight lanthanum enrichment on LNO surfaces. The benefits from the presence of the LNO nanoparticles demonstrate that LNO-infiltrated LSCF materials can act as high active surface oxygen exchange cathodes with promising fast ORR behavior and stability. To further enhance the long-term stability of infiltrated (La0.6Sr0.4)0.95Co0.2Fe0.8O3-δ (LSCF) cathode, two kinds of new hetero-structured cathodes, CeO2 & LNO co-infiltrated LSCF and La2-xNiO4+δ-infiltrated LSCF, have been designed. La2NiO4+δ (LNO), as an superior performance promoter, and CeO2, as a particle growth retardant due to the high melting point and good wettability, were co-infiltrated into the LSCF backbone to mitigate the delamination and growth/aggregation of in-situ formed nano-sized infiltrants. A more advantageous microstructure was confirmed in the co-infiltrated sample relative to those infiltrated with LNO alone. The original morphology was largely retained in the coinfiltrated sample after heat treatment, showing finer surface particles, better connection between the infiltrant network and the LSCF backbone. XPS and EIS results reveal that the substitution of stoichiometric La2NiO4+δ by A-site deficient La2-xNiO4+δ can effectively suppress lanthanum enrichment and enhance the stability after cathodic polarization treatment observed by measurement. The advantages of both modified hetero-structured cathodes suggest that CeO2 & La2-xNiO4+δ co-infiltrated LSCF is a promising heterostructured cathode to mitigate the LSCF electrode degradation. iv ACKNOWLEDGMENTS My deepest gratitude goes first and foremost...

Intolerance of the Ruddlesden–Popper La2NiO4+δ Structure to A-Site Cation Deficiency

“…The results of calculations done assuming phase-pure La 2– x NiO 4+δ compositions for the entire series (Figure A) showed that the total oxygen content 4+δ decreases nearly linear with the nominal x . If the materials were indeed single-phase, this would indicate that the increase in the concentration of cation vacancies in the lanthanum sublattice is mainly compensated by a change in the concentration of oxygen defects  a decrease in the concentration of interstitial oxygen, followed by a transition to oxygen deficiency and an increase in the concentration of oxygen vacancies for x = 0.10, in accordance with eq and conclusions in selected literature sources. ,, …”

“…The noted trends in the relevant properties with increasing nominal A-site deficiency included a minor increase in electrical conductivity (shown only for selected compositions), a gradual decrease in the roomtemperature oxygen content with a transition to oxygen deficiency, and a progressive increase in the polarization resistance of La 2−x NiO 4+δ electrodes. 27,28 Similar inconsistencies can be found in available literature reports on A-site-deficient nickelates derived from Nd 2 NiO 4+δ 29−37 and Pr 2 NiO 4+δ . 38−47 The following communications are worth highlighting, among others, in the context of the present work.…”

“…If the materials were indeed single-phase, this would indicate that the increase in the concentration of cation vacancies in the lanthanum sublattice is mainly compensated by a change in the concentration of oxygen defects � a decrease in the concentration of interstitial oxygen, followed by a transition to oxygen deficiency and an increase in the concentration of oxygen vacancies for x = 0.10, in accordance with eq 1 and conclusions in selected literature sources. 21,22,28 However, the prepared La 2−x NiO 4+δ ceramic materials are not phase-pure, as confirmed by the XRD/ND and SEM/EDS results. Therefore, the oxygen content from the TGA data was recalculated assuming that the concentration of lanthanum vacancies is negligible and that excess nickel is precipitated as NiO.…”

“…25 Very similar results were obtained by Ferkhi 26 and Zhang. 27,28 Ferkhi et al 26 synthesized La 2−x NiO 4+δ (x = 0.01−0.05) powders by the modified Pechini (citrate) method with final thermal treatment at 1000 °C. The prepared powders were claimed to be of single phase, but available XRD patterns clearly showed the presence of a secondary phase (presumably La 3 Ni 2 O 7 ) at least for x = 0.03−0.05.…”

“…26 The values of polarization resistance of prepared La 2−x NiO 4+δ electrodes varied unsystematically with x and were substantially higher compared to those reported in the literature for La 2 NiO 4+δ . 26 Zhang et al 27,28 prepared La 2−x NiO 4+δ (x = 0−0.20) by the sol−gel citrate technique with final calcination at 1100 °C. Once again, the synthesized powders with x < 0.20 were discussed as phase-pure, although the presented XRD patterns of the x = 0.05−0.15 samples distinctly showed the reflections of a secondary phase (seemingly La 3 Ni 2 O 7 ).…”