Ultrathin , graphene oxide-based membranes for co2 capture from flue gas

Abstract

Membrane technology for CO2 capture from flue gas exhibits the superior separation performance and attractive levelized cost of energy to achieve the post combustion capture goal set by the US Department of Energy (DOE), 90% capture of CO2 with purity of >95% with less than 35% increase in energy cost. Novel membrane materials with potential to greatly improve membrane performance, such as 2-dimensional graphene and graphene oxide (GO) and its derivatives, have attracted great attention as a new membrane building block, primarily owing to their potential to make the thinnest possible membranes and thus provide the highest permeance for separation. My research work is focused on ultrathin GO-based membranes for CO2 capture from flue gas. Previous studies have shown that free-standing GO laminates in flat-sheet has potential for selective transport of gas molecules. However, the relatively poor separation performance and lack of scalable membrane fabrication methods limit the development of practical use of GO-based membranes. We developed facile and scalable methods for depositing ultrathin GO-based membranes on both hollow fiber and flat sheet substrates. We firstly synthesized GO by modified Hummers method and further modified or functionalized GO material by ultra-sonication or amine-treatment. To deposit GO-based membranes on polymeric hollow fiber substrate, specifically on the inner surface of hollow fiber, we developed a novel, two-step modified vacuum-assisted coating method. Uniform and high-quality GO membranes were successfully deposited on hollow fibers according to our characterizations by field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD). Gas separation was conducted with the lab-designed permeation system, and the results on base-GO hollow fiber membranes showed a moderate CO2 separation performance. We then functionalized our GO membranes by incorporating CO2-philic agent, amines (including primary and secondary amine molecules), and verified the successful functionalization by further characterizations. The amine-functionalized GO hollow fiber membranes offered a predominant facilitated CO2 transport mechanism as an addition to solution-diffusion mechanism, and therefore presented a highly-efficient CO2 capture performance under elevated temperature and wet feed condition. Zero-dimensional material, graphene oxide quantum dots, was firstly used as membrane building blocks by a smart strategy that to deposit a carbon framework skeleton with single-walled carbon nanotubes, and then to fill the carbon frame layer by nitrogen-doped graphene oxide quantum dots (N-GOQDs). Membrane was prepared in both hollow fiber and flat sheet substrates to demonstrate its potential on different separation purposes. Characterizations indicated the unique membrane structure and carbon-based chemical compositions, and the gas permeation and water treatment tests suggested the excellent performance for molecular separations. Consequently, the N-GOQD membranes showed superior CO2 capture performance from model flue gas, and exhibited high rejection for various dye molecules and divalent salt. We also developed a scalable printing method for depositing large-area (>100 cm2) GO-based membranes on polymeric flat-sheet support. Membrane quality was characterized and improved by modifying the membrane printer, the GO ink composition and the printing methods. Preliminary results on gas permeation with different single gases indicated a great potential of the printed GO membranes for separating smaller gas molecules. Further study by adding CO2-philic agent as a second printing ink demonstrated a good CO2 capture performance.