(650f) CO2 Splitting Using Miec Membranes

Authors: 
Wu, X. Y., MIT
Ghoniem, A. F., Massachusetts Institute of Technology
In this presentation, we discuss fuel production using membrane-supported CO2 splitting. A mixed ionic-electronic conductive (MIEC) La0.9Ca0.1FeO3-δ (LCF-91) membrane is utilized. At intermediate temperatures (850 – 990oC), CO2 undergoes thermochemical reduction on the feed-side producing CO, while oxygen incorporates into the perovskite as lattice oxygen. Oxygen diffuses to the sweep side, driven by the chemical potential gradient across the membrane generated in this case by fuel (either H2 or CO in this study) oxidation on that side.

LCF-91 membranes have been examined in our laboratory before for air separation [1-3] and water splitting [4, 5]. In this work, we report a detailed study on CO2 reduction on the same membrane. We examine the dependence of CO2 splitting rate on various operating conditions such as CO2 concentration on the feed-side, fuel (H2 or CO) concentration on the sweep side and temperatures. CO2 splitting rate is proportional to the oxygen flux across, and the measured maximum flux is 0.19 μmol/cm2•s on a 1.3 mm membrane at 990oC, when 9.5% H2 is added to the sweep side (balance with argon gas). Long term study shows almost a constant flux, i.e., stable performances for 106 hours. Following the experiment, morphology changes, carbonate formation and calcium/iron enrichment are observed on the feed-side surface, while no obvious changes are found on the sweep-side surface as revealed by SEM, EDS and XRD. A resistance-network model is developed to describe the oxygen flux, in which CO2 direct-incorporation mechanism is used on the feed side, and the Mars-van Krevelen (MvK) mechanism for the oxidation reaction on the sweep side (for both H2 and CO) [6]. Using the flux measurements, surface kinetics parameters are derived, which shows that H2 has larger oxidation rate constant than CO. The model also shows that in CO2 splitting, the rate limiting step changes from the CO2 splitting reaction on the feed-side to the fuel (H2 or CO) oxidation reaction on the sweep-side at higher temperatures. This is different from the H2O splitting cases, where fuel oxidation is always the limiting step within the same temperature range tested. This is supported by the change in the slope of the Arrhenius plot of the oxygen flux. Moreover, the transition temperature for H2 is higher than that of CO (both used as fuels).

Porous layers made from the same materials are added on both side of the membrane to enhance the oxygen flux. Results shows that the flux is raised by about 0.02 μmol/cm2•s when CO is used as a fuel. And the same transition temperature is observed in the Arrhenius plot of oxygen flux with or without porous layers.

Reference

[1] Hunt, A., Dimitrakopoulos, G., and Ghoniem, A. F., 2015, "Surface oxygen vacancy and oxygen permeation flux limits of perovskite ion transport membranes," J. Membr. Sci., 489, pp. 248-257.

[2] Dimitrakopoulos, G., and Ghoniem, A. F., 2016, "A two-step surface exchange mechanism and detailed defect transport to model oxygen permeation through the La0.9Ca0.1FeO3−δ mixed-conductor," J. Membr. Sci., 510, pp. 209-219.

[3] Dimitrakopoulos, G., and Ghoniem, A. F., 2017, "Developing a multistep surface reaction mechanism to model the impact of H2 and CO on the performance and defect chemistry of mixed-conductors," J. Membr. Sci., 529, pp. 114-132.

[4] Wu, X. Y., Chang, L., Uddi, M., Kirchen, P., and Ghoniem, A. F., 2015, "Toward enhanced hydrogen generation from water using oxygen permeating LCF membranes," PCCP, 17(15), pp. 10093-10107.

[5] Wu, X. Y., Ghoniem, A. F., and Uddi, M., 2016, "Enhancing co-production of H2 and syngas via water splitting and POM on surface-modified oxygen permeable membranes," AlChE J., 62(12), pp. 4427-4435.

[6] McFarland, E. W., and Metiu, H., 2013, "Catalysis by Doped Oxides," Chem. Rev., 113(6), pp. 4391-4427.