Effects of lanthanide doping on the electrode properties and exsolution behavior of A-site deficient Sr1.9Fe1.3Ni0.2Mo0.5O6-δ double perovskites

Abstract

Double perovskite oxides of the Sr2Fe1.5Mo0.5O6-δ class are promising electrodes for Solid Oxide Fuel Cells (SOFCs) and Electrolyzers (SOECs) due to their outstanding electrochemical properties and durability. Additionally, their physicochemical properties can be tuned by B-site cation doping, namely, by partially substituting Fe cations with other transition metals such as Ni, Co, Mn, or Cu. This also allows the grain-surface functionalization with metallic alloyed nanocatalysts upon reduction driven by the exsolution phenomena, leading to higher electrocatalytic activity and resistance against sintering and coking. Traditionally, A-site deficiency has been employed to improve the amount of exsolved nanoparticles and, thereby, increase the number of Triple Phase Boundaries in the electrode. However, in Ni-doped non-stoichiometric double perovskite materials (e.g., Sr1.9Fe1.3Ni0.2Mo0.5O6-δ), A-site deficiency results in undesired NiO phase segregations. This, in turn, lowers the number of exsolvable cations, decreasing the number of metallic nanocatalysts compared to the stoichiometric compound. In this work, the impact of A-site doping with lanthanides (viz. Gd, La, Nd, and Pr) on the exsolution ability and electrochemical properties of non-stoichiometric Sr1.9Fe1.3Ni0.2Mo0.5O6-δ was evaluated. The results indicate that lanthanide doping enhances the conductivity of the double perovskites both in oxidizing and reducing atmospheres. In addition, lanthanide dopants that slightly destabilize the cubic structure also improve the exsolution dispersion, which decreases with increasing the double perovskite cell volume. These results illustrate how to design more efficient and durable electrocatalysts by modifying the double perovskite structure with A-site dopants.