EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Solid oxide fuel-assisted electrolysis cells (SOFEC) are electrochemical devices that perform conversion of low grade fuels, like biogas, into pure hydrogen, using low-grade fuel aggressive depolarization of the anode. As all solid oxide cells, SOFEC might operate at elevated temperatures, typically above 700°C. Contrary to the regular solid-oxide electrolysis cells (SOEC), SOFEC requires much lower voltages for effective operation. However, requirements to the SOFEC anode are different from ones, typical for SOEC or SOFC, so conventional materials do not demonstrate ample performance or stability. The major cause of this issue is specific conditions, namely an intermediate range of the oxygen chemical potential, which is typical for SOFEC anode: it is below acceptable for perovskites used in SOEC (LSC or LSCF) and too high for Ni-YSZ SOFC anode.
The use of molybdenum-doped ferrites with perovskite structure or Ruddlesden-Popper (RP) phases is a promising solution for this problem. While materials with a high Mo:Fe ratio are studied as SOFC anodes, materials with ratio below 1:2 might be considered as an option for SOFEC. Oxygen non-stoichiometry and conductivity of such solid solutions, namely perovskites SrFe(Mo)_3-δ, and RP phases Sr₃(Fe,Mo)₂O_7-δ and LaSr₃(Fe,Mo)₃O_10-δ, were studied in wide p(O2)-T domain. Experimental results confirmed that these materials possess sufficient electronic conductivity, as well as stability, both structural and thermodynamic. Analysis of the experimental data from the standpoint of the point defect interaction demonstrated, that interaction of the various oxidation states of the Mo and Fe allows preserve sufficient electronic transportation even in the region of p-n transition, where concentrations of the electrons and holes are typically very low.
SrFe_0.75Mo_0.25O_3-δ was used for the preparation of experimental SOFEC cells on Ni-YSZ fuel-side support with 8YSZ solid electrolyte. The performance of the cells will be discussed.
This work was partially supported by National Science Centre (NCN), Poland, grant 2018/30/M/ST8/00675.