Defective Ni Perovskites as Cathode Materials in Intermediate-Temperature Solid-Oxide Fuel Cells: A Structure−Properties Correlation

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Abstract

Four oxides belonging to the perovskite series of nominal stoichiometry LaNi1−xMoxO3, x = 0.10, 0.15, 0.20, 0.25 (LNMO), have been tested as cathode materials for solid oxide fuel cells (SOFC). The electrodes were supported on a 300-μm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM) with Sr2MgMoO6 (SMMO) as the anode and LNMO as the cathode. The test fuel cells gave a maximum power density of 660 mW/cm2 at 850 °C and 565 mW/cm2 at 800 °C for the x = 0.25 cathode material and exhibited high cyclability and low cathodic overpotential losses with pure H2 as fuel. All the cathode materials were characterized by X-ray diffraction (XRD) and electrical conductivity measurements while the test fuel cell cross-section was examined by scanning electron microscopy (SEM). The X-ray data indicate that all the cathode materials exhibit a perovskite phase with an additional impurity phase present in LNMO x = 0.20, 0.25 materials. Molybdenum (Mo)-poor LNMO x = 0.10 exhibits a purely metallic behavior in the 120−800 °C temperature range, while Mo-rich LNMO, x = 0.15, 0.20, and 0.25, are semiconducting. Additionally, analysis of in situ temperature-dependent neutron powder diffraction (NPD) data from LNMO x = 0.20 at 25 and 800 °C in air reveal that it possesses a rhombohedral structure (R3c̅ space group) with a crystallographic formula La0.98(1)Ni0.88(3)Mo0.12(3)O2.78(3) and a nominal oxidation state for Ni of 2.16+, which points to the presence of a Ni(III)/Ni(II) redox couple in the cathode materials at the working temperature of the cell (800 °C). The Ni(III)/Ni(II) redox energy at the top of the O 2p bands accounts for a good electronic conductivity of polaronic nature of approximately 30 S·cm−1 at 800 °C. NPD data also showed a measurable oxygen deficiency of approximately 0.2 oxygen atoms per formula unit exhibiting large thermal factors, which accounts for the good oxygen mobility expected in a mixed ion−electronic conductor (MIEC) oxide. The excellent properties of LaNi1−xMoxO3−δ cathodes, especially for x = 0.25, with long-term stability and low cathodic overpotential losses make them potential electrode materials for the first generation of cobalt-free intermediate-temperature SOFCs, with power densities exceeding the target of 500 mW/cm2 at 800 °C in pure H2 as a fuel.

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