Effect of the A Cation Size Disorder on the Properties of an Iron Perovskite Series for Their Use as Cathodes for SOFCs

Abstract

In order to develop a cathode that can be used in intermediate temperature solid oxide fuel cells, a family of iron perovskites with the general formula Ln0.5M0.5FeO3–δ (A = Ln1–xMx; Ln = La, Nd and/or Pr; M = Sr, Ca or/and Ba) with fixed divalent dopant concentration x to 0.5, mean A cation radius 〈rA〉 to 1.25 Å but with different A cation size disorder σ2(rA) has been prepared by the conventional solid state reaction. These compounds have been characterised by power X‐ray diffraction (XRD), high‐resolution scanning electron microscopy (SEM), electrical conductivity, I–V measurements and AC impedance spectroscopy. All the samples have a rhombohedral crystal structure (space group: R‐3c). SEM images show that the average grain size increases with increasing σ2(rA) in the bulk samples. The DC four‐probe measurements indicate that electrical conductivity is strongly dependent on the σ2(rA). Electrochemical impedance spectra at 700 and 800 °C revealed that better electrochemical cathode performances are for samples with smaller values of σ2(rA).

Effect of the synthesis conditions on the properties of La0.15Sm0.35Sr0.08Ba0.42FeO3−δ cathode material for SOFCs

“…(Table 4) The electrical conductivities obtained are comparable with those previously reported for this type of perovskite material with high σ 2 (r A ) values [7,11]. However, in this study, a slight decrease in electrical conductivity was observed for the sample with G/N = 2 when a current of 100 mA was applied.…”

“…As have been studied for similar compositions with high values of σ 2 (r A ) [11,12], the Rp values are very sensitive to changes in applied currents. Figure 13 shows the typical Nyquist diagram of the impedance spectroscopy for Samples with G/N = 1 and 2 at 800°C at OCP and under a current of 50 and 100 mA.…”

“…Our group has studied the effect of this last parameter by impedance spectroscopy and a significant decrease in polarization resistance (Rp) with the application of the current was observed for a perovskite compound with a high value of σ 2 (r A ) [11,12].…”

Effects of using (La0.8Sr0.2)0.95Fe0.6Mn0.3Co0.1O3 (LSFMC), LaNi0.6Fe0.4O3−δ (LNF) and LaNi0.6Co0.4O3−δ (LNC) as contact materials on solid oxide fuel cells

“…Then, sintered pellets were cut in ~1 x 3 x 7 mm bars; the conductivity measurements were carried out with the standard dc four-point method on the rectangular sintered bars, from room temperature to 1000 ºC in air using a heating rate of 2 ºC•min -1 , using a power source controlled by PC using Lab Windows/CVI field point system. The measured conductivity values were corrected taking into account the porosity of the samples [21]. Thermal expansion measurements (TEC) for the contact layers, cathode and interconnect were carried out from room temperature to 1000 ºC in air with a heating rate of 5 ºC•min -1 by using a Unitherm Model 1161 dilatometer.…”

Oxygen Permeation Measurements: An Alternative Tool to Select New Intermediate Temperature Solid Oxide Fuel Cell Cathodes

“…The proposed way to verify the goodness of a material as cathode can be carried out in addition to the conventional methods (such as the electrical measurements). [36][37] The oxygen redox process is affected by a lot of features; Adler, in his widely cited review, 38 summarizes the main ones.…”