Novel Processing of Cathodes for Solid Oxide Fuel Cells
Authors: Sažinas Rokas , Holtappels Peter , Hansen Kent Kammer 
Published: February 2020

Solid-oxide fuel cells (SOFCs) are electrochemical devices that efficiently convert chemical energy of fuels into electricity.[1] However, they typically operate at high temperature (800–1000 °C) causing substantial challenges in cost and material compatibility. SOFC that can work at intermediate temperature (IT) (500–750 °C) is thus more attractive.[2] SOFC is generally composed of anode, electrolyte and cathode. A practical cathode for SOFCs should possess sufficiently high thermo-mechanical stability, good thermal and chemical compatibility with the electrolyte, high chemical stability against the surrounding atmosphere, good electro-catalytic activity for oxygen reduction reaction (ORR), as well as high electrical conductivity.[1] However, the current widely used cathode, lanthanum strontium manganite (LSM), rapidly loses activity below 800 °C.[3]

According to numerical calculations, the efforts to optimize the oxygen surface exchange reaction are required while very high ionic conductivities are not necessary in order to achieve the goal of a highly active cathode.[4] Nano-sized palladium (Pd) and platinum (Pt) show very high activity towards oxygen activation, which can substantially increase the cathode electrochemical performance by improving the surface properties. However, precious metals are expensive and undergo sintering. Silver is a good alternative for its relatively low price and high electrocatalytic activity for oxygen activation, however more easily sintered than Pt and Pd resulting in the electrode deactivation.[5] As the electrodes and the dense electrolyte are sintered together in SOFC, the deactivated electrodes are normally neither regenerable nor replaceable, what brings the end of the SOFC. On the other hand, the Ag-doped perovskites have promoted catalytic oxidation of CO, CH4, n-hexane, and NO,[6] which was significantly improved by the partial substitution of Ag into the A-site of perovskite together with the additional formation of the oxygen vacancy and the metallic Ag on the surface of the perovskite forming composite materials. [7, 8] Among several ways to process the composites, infiltration has shown promising results bringing the possibility to tailor electronic, ionic and mixed ionic electronic conductivities in a porous backbone of proton conducting oxides. [9, 10] The exsolution of nickel, ruthenium, silver or other metal nanoparticles has been investigated in reducing conditions for the design of the electrodes for SOFC. [11-13] The development of highly electrochemically active cathodes for SOFCs requires the optimization of materials composition together with micro- and nanostructures in order to form stable and catalytically active composite electrodes.

Here we report on the novel heterostructured silver nanoparticle-decorated perovskite composites La0.95-xSrxMn1-y-z(Fe,Ni,Zn,Mg)zNbyO3-δ – 0.05Ag (exLSAMN) as highly active and durable cathodes for SOFCs, derived from single phase La0.95-xSrxAg0.05Mn1-y-z(Fe,Ni,Zn,Mg)zNbyO3-δ (LSAMN) perovskite precursors through an exsolution process. We report LSAMN as a novel precursor which can develop into high-performance nanosized silver modified LSM-based electrode under cathodic polarization or reducing atmosphere with improved stability and in situ electrochemical regeneration capability. The LSAMN materials were synthesized by solid state reaction and wet chemical synthesis method in order to compare the activity. The electrochemical intercalation/de-intercalation of metal catalysts is a conceptually attractive approach that is also applicable for the development of other metal-modified oxide electrodes. The composite formation and properties were tailored by changing the synthesis route and thermal treatment. A thorough description of the synthesis methods is presented as well as a careful characterization of the microstructure and phase composition of the resulting composite electrodes. The performance of the new composite cathodes with gadolinia-doped ceria (CGO) electrolyte is demonstrated. The exLSAMN electrode showed fairly high electrochemical activity and low area specific resistance (ASR). These unique features make the new materials highly promising cathodes for SOFCs at intermediate temperatures.

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