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

Abstract

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 H 2. 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 H 2 as a fuel. Sizeable power densities were also observed with alternative fuels: 694 and 499 mW cm −2 with H 2 containing 5 parts per million of H 2S and CH 4, 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 H 2 at 750 °C and 14% at 850 °C in 50 cycles using H 2–H 2S. 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 pO 2). 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) Å 2) 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 SrMoO 3 and SrFe 0.6Mo 0.4O 2.7. The association of two perovskites oxides, SrMoO 3 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.