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 Sr_0.9K_0.1FeO_3−δ (SKFO) and Sr_1.6K_0.4FeMoO_6−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 La_0.8Sr_0.2Ga_0.83Mg_0.17O_3−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 Sr_0.9K_0.1FeO_3−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 Sr_1.6K_0.4FeMoO_6−δ has been unveiled by neutron powder diffraction to consist of two main perovskite phases with the compositions SrMoO₃ and SrFe_0.6Mo_0.4O_2.7. The association of two perovskites oxides, SrMoO₃ with high electrical conductivity, and SrFe_0.6Mo_0.4O_2.7 with mixed ionic-electronic conductivity has resulted in an extraordinarily performing anode material for SOFCs.