The Formation of Performance Enhancing Pseudo‐Composites in the Highly Active La_1–xCa_xFe_0.8Ni_0.2O₃ System for IT‐SOFC Application

Abstract

The La1–xCaxFe0.8Ni0.2O3–δ (0 ≤ x ≤ 0.9) system is investigated for potential application as a cathode material for intermediate temperature solid oxide fuel cells (IT‐SOFCs). A broad range of experimental techniques have been utilized in order to elucidate the characteristics of the entire compositional range. Low A‐site Ca content compositions (x ≤ 0.4) feature a single perovskite solid solution. Compositions with 40% Ca content (x = 0.4) exhibit the highest electrical and ionic conductivities of these single phase materials (250 and 1.9 × 10−3 S cm−1 at 800 °C, respectively), a level competitive with state‐of‐the‐art (La,Sr)(Fe,Co)O3. Between 40 and 50% Ca content (0.4 > x > 0.5) a solubility limit is reached and a secondary, brownmillerite‐type phase appears for all higher Ca content compositions (0.5 ≤ x ≤ 0.9). While typically seen as detrimental to electrochemical performance in cathode materials, this phase brings with it ionic conductivity at operational temperatures. This gives rise to the effective formation of pseudo‐composite materials which feature significantly enhanced performance characteristics, while also providing the closest match in thermal expansion behavior to typical electrolyte materials. This all comes with the advantage of being produced through a simple, single‐step, low‐cost production route without the issues associated with typical composite materials. The highest performing pseudo‐composite material (x = 0.5) exhibits electronic conductivity of 300–350 S cm−1 in the 600–800 °C temperature range while the best polarisation resistance (Rp) values of approximately 0.2 Ω cm2 are found in the 0.5 ≤ x ≤ 0.7 range.

Synthesis of Ba 0.3 Ca 0.7 Co 0.8 Fe 0.2 O 3-δ composite material as novel catalytic cathode for ceria-carbonate electrolyte fuel cells

“…The particle size varies in a certain range from 80 nm to 200 nm and the agglomeration of the particles can be widely observed. According to the literature, porosity level increases with Ca content that tends to cause agglomeration giving rise to the coarsening in the morphology [15]. Hence the composite material particles have been observed in the shape of clusters due to nano-particles agglomeration.…”

“…A number of studies for mixed perovskite conductors have reported the existence of secondary phases but mostly have been left unidentified [15][16][17]. Most of the cathode materials in SOFCs are composite in nature because no single material can fulfill every requirement for fuel cell activities [18].…”

Advances in Cathode Materials for Solid Oxide Fuel Cells: Complex Oxides without Alkaline Earth Metal Elements

“…Thus, the development of high-performance cathodes has become the REVIEW (3 of 34) 1500537 wileyonlinelibrary.com biggest obstacle to reduction of operational temperature of SOFCs and has attracted the most attention in current SOFC research. [ 35,36,[74][75][76][77] During the past several decades, numerous strategies have been investigated to increase the performance of SOFC cathode at reduced temperatures, from new materials development to advanced synthesis and microstructural optimization. [ 30,53,[78][79][80][81][82][83][84][85] To date, there are a number of good reviews that focus on electrodes for oxygen reduction reaction (ORR) in SOFCs, as listed in Table 1 .…”

“…[ 122 ] Normally, three ways can be used to introduce oxygen ionic conduction into the electrode: 1) partial reduction of an oxide to create oxygen vacancies (or other ionic defects); [ 123,124 ] 2) incorporation of an oxygen ion conducting phase to form a composite electrode; [ 122,125,126 ] 3) creation of mixed ionic and electronic conductivity through proper doping. [ 74,81 ] The La 0.8 Sr 0.2 MnO 3 (LSM) perovskite-type oxide is one of the state-of-the-art cathodes in SOFCs, which is an electronic conductor with poor ionic conductivity. [ 123 ] Under signifi cant polarization, the reduction of Mn to a lower oxidation state may introduce oxygen vacancies in LSM, thus improving the electrode performance.…”

Segregated Chemistry and Structure on (001) and (100) Surfaces of (La1–xSrx)2CoO4 Override the Crystal Anisotropy in Oxygen Exchange Kinetics

“…[12][13][14][15][16][17][18][19][20][21][22] In a number of recent studies the dominance of the AO termination of the perovskite ABO 3 (113) and the Ruddlesden-Popper (R-P) A 2 BO 4 (214) has been seen at the outermost surface layer. [23][24][25][26] These compositional changes can ultimately change the chemistry and phase composition, morphology and electronic structure of the surface and were often associated with degradation of the oxygen exchange kinetics at the surface. Furthermore, these changes are time, temperature and atmosphere dependent.…”