Fe-based perovskites as electrodes for intermediate-temperature solid oxide fuel cells

A solid-oxide fuel cell (SOFC) based upon Fe perovskites, has been designed and tested. Materials with nominal compositions Sr0.9K0.1FeO3−δ (SKFO) and Sr1.6K0.4FeMoO6−δ (SKFMO) with perovskite structure have been prepared and characterized as cathode and anode, respectively. The anode material exhibits high electrical conductivity values of 407–452 S cm−1 at 750–820 °C in pure H2. In the test cells, the electrodes were supported on a 300-μm-thick pellet of the electrolyte La0.8Sr0.2Ga0.83Mg0.17O3−δ (LSGM). The single SOFC cells gave a maximum power density at 850 °C of 937 mW cm−2 with pure H2 as a fuel. Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm−2 with H2 containing 5 parts per million of H2S and CH4, respectively, at 800 °C. Moreover, only a slight degradation of about 3.6% of the power density has been obtained after 65 different cycles of fuel-cell test in H2 at 750 °C and 14% at 850 °C in 50 cycles using H2–H2S. This remarkable behavior has been correlated to the structural features determined in a neutron powder diffraction experiment in the usual working conditions of a SOFC for a cathode (air) and an anode (low pO2). On the one hand, the cubic Pm-3m Sr0.9K0.1FeO3−δ cathode material is an oxygen deficient perovskite with oxygen contents that vary from 2.45(2) to 2.26(2) from 600 to 900 °C and high oxygen isotropic thermal factors (4.17(8) Å2) suggesting a high ionic mobility. On the other hand, the actual nature of the anode of composition Sr1.6K0.4FeMoO6−δ has been unveiled by neutron powder diffraction to consist of two main perovskite phases with the compositions SrMoO3 and SrFe0.6Mo0.4O2.7. The association of two perovskites oxides, SrMoO3 with high electrical conductivity, and SrFe0.6Mo0.4O2.7 with mixed ionic–electronic conductivity has resulted in an extraordinarily performing anode material for SOFCs.

Research highlights
► In this paper we have prepared Co-free, based upon Fe perovskites as electrode materials to be used in intermediate temperature solid oxide fuel cells (IT-SOFC).
► The single SOFC cells gave a maximum power density at 850 °C of 937 mW cm−2 with pure H2 as a fuel.
► Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm−2 with H2 containing 5 parts per million of H2S and CH4, respectively, at 800 °C.
► The structural features have been determined by a in situ neutron powder diffraction experiment of both, cathode and anode in their usual working conditions in a SOFC.