Modification Of The Double Perovskite Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells By Doping

The exploration of novel cathode materials with high catalytic activity for oxygen reduction reactions（ORR） in the intermediate-temperature range（600-800 oC） is of great importance for commercialization of solid oxide fuel cells（SOFCs）. The double perovskite-typed oxides with general formula LnBa Co₂O_5+δ（Ln = Lanthanide） are thought to be one of potential cathodes for intermediate-temperature solid oxide fuel cell（IT-SOFC） owing to their high electrical conductivities, rapid oxygen surface exchange and excellent catalytic activity for ORR. In this work, we focused on property optimization of the double perovskite cathodes Pr Ba_1-x Fe₂O_5+δ, PrBa_1-xCo₂O_5+δ and PrBa_1-x CaxCo₂O_5+δ via Ba²⁺ deficiency, K⁺ and Ca²⁺ doping. The main research contents and results are listed as below:（1）The A-site Ba²⁺ deficient cobalt-free double perovskite oxides Pr Ba_1-x Fe₂O_5+δ（x= 0.00–0.10, PB1-x F） were synthesized through a sol-gel method. The effects of Ba²⁺deficiency on structure and properties of PB1-xF oxides were evaluated. The results demonstrated that: Ba deficiency fraction（x） in PB1-x F was limited to approximately0.03. Minor Pr6O11 phase was detected for samples for which x ≥ 0.05. Ba deficiency resulted in a slight reduction of chemical state in transition metal Fe. At given temperature, Pr Ba0.97Fe₂O_5+δ exhibited a higher oxygen vacancy content than PrBaFe₂O_5+δ. An obvious decrease in electrical conductivity was obtained with higher x from 300-800 oC, which suggests that the formation of additional oxygen vacancies was the main charge compensation mechanism of Ba deficiency in PB1-x F. A slight decrease in TEC results from a Ba deficiency from 30-900 oC. The catalytic activity for the ORR of the cobalt-free double perovskites can be enhanced by introducing the appropriate amount of Ba deficiency, and is characterized by a decreased ASR compared with the parent oxide. ASR values as low as 0.119 Ω cm2 were obtained for Pr Ba0.97Fe₂O_5+δ at700 o C. The PrBa0.97Fe₂O_5+δ based single cell exhibited the maximum power densities of329, 256, 183 and 118 mWcm-2 at 800, 750, 700 and 650 oC, respectively.（2）PrBa_1-x Co₂O_5+δ（x = 0.00-0.10, PB1-x C） oxides were synthesized by introducing some K during sample synthesis, with the nominal formula PrBa_1-x KxCo₂O_5+δ. Theimpacts of K on the structure and properties of the PB1-x C double perovskite oxides were evaluated. The results showed that: A-site cation deficient oxides PB1-x C were synthesized by introduction of K during sample synthesis. Trace amounts of K were detected in the as-synthesized PB1-xC oxides. All of the samples crystallized to form orthorhombic Pmmm phase structures with lattice parameters of 2a ≈ 2b ≈ c and without any measurable impurity phases. The introduction of K can enhance the cation deficiency and transmission performance of oxygen ion in the double perovskite oxides.The presence of K during sample synthesis resulted in a decrease in the Co ion content at the surface of the PB1-xC（x = 0.00-0.10）, thereby causing inferior rates of oxygen surface exchange.（3）The influences of Ca on structure and properties of the double perovskite oxides PrBa_1-x CaxCo₂O_5+δ（x = 0.00-0.50, PB1-x CaxC） were evaluated. The related results indicated that: PB1-x CaxC oxides showed a structural transition from the tetragonal phase（x = 0.00） to the cubic phase（x = 0.50）. When x = 0.10-0.30, the co-existence of both phases in PB1-x CaxC oxides were observed. The doping of Ca in PB1-x CaxC had little influence on their average TEC at the temperature range of 30 to 900 oC. The electrical conductivity of PB1-x CaxC monotonously increased with higher x, because a better symmetry would be realized in PB1-x CaxC owing to the phase transition process.With higher Ca²⁺ content from x = 0.00 to 0.30, polarization resistance of PB1-x CaxC cathodes deceased greatly, while x = 0.30 to 0.50, polarization resistance of these cathodes showed a slight increase. Among those samples, PrBa0.7Ca0.3Co₂O_5+δ showed the lowest ASR value（0.034 Ω cm2, 700 °C）. Thus, Pr Ba0.7Ca0.3Co₂O_5+δ oxides is proved to be one of the IT-SOFC cathode materials with high ORR.