Phase Interaction and Distribution in Mixed Ionic Electronic Conducting Ceria‐Spinel Composites

Abstract

Mixed Ionic electronic conductors find various applications as SOFC cathodes and oxygen transport membranes. Dual phase composites are a promising class of thermochemical stable materials, in which two ceramic phases are coupled to provide a pure electronic and ionic conducting pathway, respectively. Composites of 20 mol% Gadolinia doped ceria (GDC) and FeCo 2 O 4 spinel (FCO) are investigated. GDC-FCO 60:40 wt-% ratio showed reasonable oxygen permeation with ionic conductivity as the limiting factor. Spinel content was reduced to as low as 10 wt-% in the composite and their corresponding electrical conductivity and oxygen permeation were measured from which ambipolar conductivity was calculated. GDC-FCO 85:15 wt-% ratio shows the highest ambipolar conductivity comparable to standard single phase La 0.58 Sr 0.4 Fe 0.8 Co 0.2 O 3-δ (LSCF) at 850 °C. The Microstructure analysis showed reversible and thus temporary spinel decomposition at sintering temperature as well as phase interaction forming a Gd-and Fe-rich orthorhombic perovskite with traces of Ce and Co. To further investigate the phase interaction and secondary phase formation, Pulsed Layer Deposition of FCO layer (~400 nm) on a polycrystalline GDC substrate and annealing at varying temperatures and times from 1000 to 1200 °C were carried out. These samples were analyzed by XRD, STEM, and SIMS to understand the interlayer interaction of the phases.

Structural and chemical stability of high performance Ce0.8Gd0.2O2-δ – FeCo2O4 dual phase oxygen transport membranes

“…Above that point the permeation rate decreases again probably due to insufficient electronic conductivity in the composite as shown in Figure 4. A significant permeation rate can be measured although the nominative proportion of the electronically conductive spinel phase (15 wt% ≈ 18.5 vol%) is well below the percolation threshold [17], which is in agreement with the electrical conductivity measurements. This effect is already reported a few times without knowing the exact reason [10] , [21] , [22].…”

“…The as-prepared 60CGO-FCO and 85CGO-FCO composites are compared for the phases present using their X-ray diffractograms as shown in Figure 1 The decrease in spinel content can be observed from the decrease in intensity of the spinel peaks for the 85CGO-FCO composite in comparison to 60CGO-FCO composite particularly at 2θ = 38°. In addition, the presence of orthorhombic perovskite peaks of GCFCO [17] (between 24° and 33° 2θ) is confirmed in both compositions. Rietveld refinement of the diffractograms was used to obtain semi-quantification of the phases present in the both composites ( Table 1).…”

“…This phase is categorized as a predominant electronic conductor, and should therefore contribute to the electron transport in the CGO-FCO membrane. We also reported that the highest oxygen flux was achieved for 85CGO-FCO composite with the volume fraction of the minor phase, only 18.5 vol% [17], contrary to the hypothesis that the percolation threshold can be reached only if the volume fraction of the minor phase is no less than 30 vol% for a dual-phase material [18]. In this paper, the 60CGO-FCO and 85CGO-FCO composites have been categorically compared in depth based on their microstructure, performance and stability to identify the most prominent OTM composite composition.…”

“…66 kJ/mol indicating that the ionic conduction of CGO is the rate limiting step [23]. In the 15 wt% spinel containing composite, the activation energy decreases (55 kJ/mol) indicating a change in the limiting transport process [17].…”

“…Moreover, one can see some remaining closed pores (black). Semi quantitative determination of the elements from a STEM/EDS elemental mapping [17] provides an approximate composition of the perovskite phase that is formed in these dual phase composites during the sintering process. As Ceria is expected to Intensity replace Gd on A-sites and Co to replace Fe on B-sites of GdFeO 3 perovskite phase, the composition is estimated to be Gd 0.85 Ce 0.15 Fe 0.75 Co 0.25 O 3 (GCFCO).…”