High-Performance La0.5Ba0.5Co1/3Mn1/3Fe1/3O3−δ-BaZr1−zYzO3−δ Cathode Composites via an Exsolution Mechanism for Protonic Ceramic Fuel Cells

Abstract

A novel exsolution process was used to fabricate complex all-oxide nanocomposite cathodes for Protonic Ceramic Fuel Cells (PCFCs).The nanocomposite cathodes with La 0.5 Ba 0.−δ nominal composition were prepared from a single-phase precursor via an oxidation-driven exsolution mechanism. The exsolution process results in a highly nanostructured and intimately interconnected percolating network of the two final phases, one proton conducting (BaZr 1−z Y z O 3−δ ) and one mixed oxygen ion and electron conducting (La 0.5 Ba 0.5 Co 1/3 Mn 1/3 Fe 1/3 O 3−δ ), yielding excellent cathode performance. The cathode powder is synthesized as a single-phase cubic precursor by a modified Pechini route followed by annealing at 700 • C in N 2 . The precursor phase is exsolved into two cubic perovskite phases by further heat treatment in air. The phase composition and chemical composition of the two phases were confirmed by Rietveld refinement. The electrical conductivity of the composites was measured and the electrochemical performance was determined by impedance spectroscopy of symmetrical cells using BaZr 0.9 Y 0.1 O 2.95 as electrolyte. Our results establish the potential of this exsolution method where a large number of different cations can be used to design composite cathodes. The La 0.5 Ba 0.5 Co 1/3 Mn 1/3 Fe 1/3 O 3−δ -BaZr 0.9 Y 0.1 O 2.95 composite cathode shows the best performance of 0.44 Ω·cm 2 at 600 • C in 3% moist synthetic air.

Microstructural and compositional optimization of La0.5Ba0.5CoO3−δ—BaZr1−zYzO3−δ (z = 0, 0.05 and 0.1) nanocomposite cathodes for protonic ceramic fuel cells

“…Single phase Co0Y, Co5Y and Co10Y materials with a cubic structure (Pm 3 m) were prepared at 715°C by the modified Pechini route. The synthesis was optimized for the BaZrO 3 -based phase to accommodate Y doping up to 10 mol% on the B-site based on our previous work on Co0Y composition [19] and 3 M family compositions [20]. Different exsolution temperatures were required to achieve two-phase composites: 900°C for Co0Y and 1100°C for the 5% and 10% Y containing compositions.…”

“…The synthesis of Co5Y and Co10Y materials by exsolution driven decomposition of a single-phase material was chosen due to the demonstrated robustness, flexibility and adaptability of the method [19,20]. The reaction given in equation 1 is based on the hypothesis that the LB and BZ perovskite phases are not miscible at oxidizing conditions [19].…”

“…Ethylenediaminetetraacetic acid (EDTA, >99%) as well as citric acid (CA, >99%) were used as complexing agents. The synthesis of the materials was done as described in previous works [19,20] BaZr 0.9 Y 0.1 O 2.95 (BZY10) electrolyte supported symmetric cells with cathode materials were prepared as follows. BZY10 electrolyte powder was prepared by spray-pyrolysis (CerPoTech AS, Tiller, Norway.…”

“…The method is based on in situ driven decomposition of a single phase material by exsolution and the possibility to induce phase decomposition by red-ox chemistry has been demonstrated [19]. The flexibility and adaptability of the exsolution method was demonstrated by the addition of Mn, Fe and Y giving a total of 7 cations in La 0.5 Ba 0.5 Co 1/3 Mn [20]. Exsolution of nanoparticles such as nickel, ruthenium or other catalyst metals in SOFC and PCFC anodes under reducing conditions has been investigated [21][22][23][24].…”

“…On the contrary, cathodes operate at oxidizing conditions, and therefore, synthesis based on oxidationdriven exsolution from a single-phase material will produce suitable cathode materials at the operation conditions. Homogeneous composite cathodes with tailored chemical compositions can be prepared by the exsolution-based synthesis approach [19,20], but further in-depth studies of the effect of microstructure and morphology of the cathodes are necessary.…”