Influence of the Transition Metal Doping on the Properties of Sr2Fe1.5Mo0.5O6 Double Perovskite for Solid Oxide Fuel Cells
Authors: Ducka Agata , Błaszczak Patryk , Kąkol Wiktoria , Bochentyn Beata 
Published: December 2024

The increase in environmental awareness has led to a significant worldwide trend for the development of alternative energy sources other than conventional use of the fossil fuels. One of the alternatives is the use of Solid Oxide Fuel Cells (SOFCs), which are capable of converting chemical energy of the fuel into electricity. In SOFCs, dense electrolyte is covered with two porous electrodes. So far, the most widely used fuel electrode material is Ni-YSZ composite, which degrades significantly when is fed with the fuels other than hydrogen. Thus, the development of a substation for Ni cermet is desirable. Among many materials, one may find perovskite families, which are known for their high stability and ionic conductivity. Unfortunately, their catalytic performance is rather low and needs to be increased. This can be done by introducing catalytically active materials as dopants and force them to form nanoparticles via the exsolution phenomenon. Main advantages of exsolving nanoparticles, rather than their deposition, are better connection with the surface and lower tendency for agglomeration.

Perovskites may be divided into several sub-groups and one of them are double perovskites with general formula of A2BB’O6. Strontium ferrite molybdate (SFM – Sr2Fe1.5Mo0.5O6) is one of the most prominent compounds in this family. It is well known for its high ionic and electronic conductivity as well as stability in both reducing and oxidizing atmospheres. Those properties make it a promising material for not only the anode, but also a cathode, hence SFM-based compounds can be considered prominent candidates for symmetrical fuel cells. On the other hand, our previous work has shown that, in highly reducing atmospheres, this double perovskite can partially transform into the Ruddlesden-Popper layered perovskite. This transition could increase the amount of exsolved nanoparticles due to internal strain.

Herein, the SFM-based compounds were co-doped with La at A-site and one of transition metals - Co or Ni, (with x being doping level within 5-20 mol.%) - giving the composition of La0.3Sr1.7Fe1.5-0.75xMo0.5-0.25xMexO6. Fixed Fe:Mo ratio was maintained to prevent formation of either Fe-, or Mo-rich phases. Based on our previous studies, La-doping was found to suppress the phase transition and to simultaneously increase the electrical conductivity. Transition metals were incorporated during the synthesis to boost the catalytic performance of the compound. After the reduction at 800 °C in hydrogen, the surface of the grains was almost fully covered with nanoparticles of one metal (Co or Ni) or bimetallic alloys with co-exsolved iron. Even small amount (10 mol.%) of introduced transition metal resulted in enormous amount of nanoparticles. The electrical conductivity measurements were performed in both air and hydrogen. It was found that the mechanism of electrical conductivity is different in both atmospheres: in the air, the electrical conduction occurs by the polaron hopping mechanism through Fe/Mo-O-Fe/Mo bonds. Selected samples underwent several cycles of electrical measurements in air and hydrogen, alternately, to determine their stability for several redox cycles and to study the degradation. To better understand how Fe3+/4+-Mo5+/6+ redox pairs are influenced by transition doping, the XPS technique was used for as-synthesized and reduced compounds. After the reduction, both iron and transition metals were present also in Me0 form, hence the amount of Mo6+
cations increased. Finally, TPR/TPO measurements were performed to understand the reducibility of the compounds and to have a better understanding of the formation of exsolved nanoparticles.

Acknowledgements:

The research project was supported by the National Science Center under grant No. NCN 2022/45/N/ST5/02933.