The use of alkaline electrolyte in regenerative fuel cells makes it possible to use transition metal oxides as catalysts for electrode reactions instead of traditional expensive catalysts. Recently, it was shown that among the perovskite-like oxides of the general composition ABO3-y, the materials containing end-of-series 3d-metal cations (Fe, Co, Ni) in the B-sublattice exhibit the largest catalytic activity in the process of oxygen evolution in an alkaline medium. The aim of this work was to search for new oxygen evolution catalysts based on cobalt and iron oxofluorides with a perovskite structure.
Oxides SrFe1-xCoxO3- (x = 0, 0.4) with the structure of perovskite were obtained by co-precipitation of oxalates. Then by performing reductive fluorination using PVDF, we synthesized oxofluorides SrFe1-xCoxO2F (x = 0, 0.4).
In case of SrFeO3-y the X-ray powder diffraction pattern revealed splitting of a number of diffraction reflections, which indicates that the real symmetry of the obtained phase is lower than cubic. As a result, the X-ray diffraction pattern was completely indexed in the orthorhombic unit cell with the following lattice parameters a = 5.478 (2) Å, b = 5.465 (2) Å, c = 3.850 (1) Å. Fluorination resulted in a material with cubic perovskite structure, with lattice parameter of a = 3.9550(3) Å.
The X-ray powder diffraction patterns recorded from SrFe0.6Co0.4O3 and its fluorinated derivative showed, that the sample of SrFe0.6Co0.4O2F is not single-phase and contains poorly crystallized SrF2 (about 5–7%) as an impurity. The reflections of SrF2 phase are indicated with asterisks. Reflections belonging to the perovskite phase were indexed in a cubic perovskite cell with a = 3.963 (1) Å.
According to Mössbauer spectroscopy data, we found that the values of chemical shifts in the spectrum indicate the presence of Fe3 + exclusively in Sr2Fe2O5 and in SrFeO2F, which implies that fluorine occupies exactly 1/3 of the positions of oxygen atoms in the crystal lattice. We also concluded that the real oxygen content in SrFeO2F is close to stoichiometric.
SEM images showed, that the particle size of SrFeO2F is 2-5 microns. Both SrFe0.6Co0.4O2F and SrFeO2F were obtained with a surface area of 50 m2/g according to BET. The electrocatalytic activity of SrFe0.6Co0.4O2F in oxygen evolution reaction was measured; according to preliminary data, the compound shows activity in the OER.
To conclude by using three-stage synthesis, a sample of SrFeO2F was obtained. The sample contains exclusively Fe3+ cations, as follows from the Mössbauer spectroscopy data. The particle size of the SrFeO2F was 2-5 microns. Also, a new oxofluoride SrFe0.6Co0.4O2-yF was obtained, which crystallizes in the cubic structure of perovskite, and shows activity in the OER.
|Affiliation of speaker||Lomonosov Moscow State University|
|Publication||Impact Factor journals|
|Position of speaker||Student|