Micromechanical Characterization of Ce_0.8Gd_0.2O_2‐δ–FeCo₂O₄ Dual Phase Oxygen Transport Membranes

Abstract

Oxygen transport membranes, which exhibit an almost 100% permeation selectivity regarding oxygen, [1] are a promising option to realize environmental-friendly, economically sound, and efficient processes for pure oxygen production, [1-4] oxycombustion, [5-7] and conversion of carbonaceous products. [8-12] These applications typically involve harsh environments, in particular high temperatures, and oxygen partial pressure gradients as well as the exposure to flue gas, which can be detrimental to the chemical and mechanical stability of the respective membrane components. [13,14] Ceramic dual phase oxygen transport membranes are promising candidates for long-term stable operation under application-relevant conditions. [15-19] These dual phase membranes consist of two separated phases responsible for ionic and electronic conductivity, respectively. [1] Flexible tailoring regarding chemical properties and stability is permitted by a choice of plenty of possible materials for the individual phases with sufficient ionic or electronic conductivity. [20-23] In fact, it has been indicated that the use of Ce 0.8 Gd 0.2 O 1.9 as an ionic conducting phase enables the formation of mechanically and chemically stable composites. [15,18,24-26] Furthermore, it appears that the performance of Ce 0.8 Gd 0.2 O 1.9-based dual phase membranes can benefit from phase interactions between the two phases in the composites. [27,28] To limit thermal stresses that might induce microcracking and failure, the selection of an electronic conducting phase for Ce 0.8 Gd 0.2 O 1.9-based dual phase membranes should be based not only on performance regarding conductivity, but also on a matching coefficient of thermal expansion (TEC) with that of Ce 0.8 Gd 0.2 O 1.9. Therefore, FeCo 2 O 4 is an excellent candidate because of its high electronic conductivity of 18 S cm À1 at 800 C. Furthermore, it possesses a TEC that is close to that of Ce 0.8 Gd 0.2 O 1.9 (%12 Â 10 À6 K À1). [13] In fact, composites of xCe 0.8 Gd 0.2 O 2Àδ : (1Àx)FeCo 2 O 4 (60 wt% ≤ x ≤ 90 wt%) (CGO20-FC2O) have been reported to possess a remarkable chemical stability under flue gas conditions. [19] These composites are multiphase systems consisting of a Ce 1Àx Gd x O 2Àδ (0 < x < 0.2) (CGO) fluorite phase, an Fe x Co 3Àx O 4 (0 < x < 1) (FCO) spinel phase, a (Fe,Co)O rock salt phase, and a Gd 0.85 Ce 0.15 Fe 0.75 Co 0.25 O 3 (GCFCO) perovskite phase formed via phase reaction between Ce 0.8 Gd 0.2 O 2Àδ (CGO20) and FeCo 2 O 4 (FC2O). [19,28] In this case, the newly formed electronic conductive phases, i.e., (Fe,Co)O and GCFCO, contribute to an improved ambipolar conductivity. [19,28] There are a number of reports on the mechanical properties of single phase oxygen transport membranes; [29-33] however, studies on the mechanical properties of dual phase membranes are limited. [18,34,35] Currently no study has been reported on the mechanical properties of CGO20-FC2O composites. Therefore, in this work, we report on the investigation of the mechanical

Advances in Electrochemical and Catalytic Performance of Nanostructured FeCo2O4 and Its Composites

“…Copyright 2019 Elsevier. [104] Furthermore, the same group reported that, at the optimum sintering temperature of 1200°C, Ce 0.8 Gd 0.2 O 2-δ -FeCo 2 O 4 microstructure exceeded 99% density with few surface defects and exhibited a high average flexural strength of~266 MPa. Sintering at this temperature contributes to the homogenization of the pore distribution and a higher density along with improved homogeneity of pore distribution result in enhanced mechanical strength.…”

“…Ceramic oxide transport membranes due to their 100% selectivity to oxygen, find wide range of applications in areas of pure oxygen production, carbon capture/ storage, petrochemical processes and oxy-combustion. [102][103][104][105][106] [102] for oxygen transport. [106] According to recent reports, Ce 0.8 Gd 0.2 O 1.9 as ionic conducting phase, promised mechanical and chemical stability in corrosive atmospheres (e. g., SO 2 , CO 2 ) at elevated temperature (~850°C).…”

“…Ceramic oxide transport membranes due to their 100% selectivity to oxygen, find wide range of applications in areas of pure oxygen production, carbon capture/ storage, petro‐chemical processes and oxy‐combustion [102–106] . Stability of single‐phase perovskite membranes such as La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3‐δ and Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3‐δ , are prone to degradation in CO 2 and SO 2 containing atmospheres.…”

“…Thereafter, this group studied the micromechanical properties (Elastic modulus (E) and Hardness (H)) of four different phases (Ce 1‐x Gd x O 2‐δ (0<x<0.2), Gd 0.9 Ce 0.1 Fe 0.8 Co 0.2 O 3 , Fe x Co 3‐x O 4 (0<x<1) and CoO) of Ce 0.8 Gd 0.2 O 2 : δ ‐FeCo 2 O 4 composites. At low indentation testing, the micromechanical properties were found to vary in the order‐ Gd 0.9 Ce 0.1 Fe 0.8 Co 0.2 O 3 >Ce 1‐x Gd x O 2‐δ ≈Fe x Co 3x O 4 >CoO [104] …”

Measurement of polarization effects in dual-phase ceria-based oxygen permeation membranes using Kelvin probe force microscopy

“…Dual-phase membranes with FeCo 2 O 4 , or its iron-rich pendant Fe 2 CoO 4 , and Gd-doped ceria as an ion conductor have already been successfully applied as oxygen permeation membranes with high permeability in the temperature range above 800 °C [10][11][12], and microstructural as well as mechanical investigations and optimizations have been performed [13][14][15]. Especially for FeCo 2 O 4 addition, the development of an additional electron-conductive phase (namely (Ce,Gd)(Fe,Co)O 3 ), whose composition and abundance depends on the ceria/spinel ratio, has recently been discussed [16].…”