Ultrathin gutter layer for high-performance thin-film composite membranes for CO2 separation
Introduction
Research on membrane science and technology is being actively pursued in both academic and industrial contexts because of a broad range of potential membrane applications for simple, energy-efficient chemical separation [1], [2], [3]. For high energy efficiency in separation, a multilayer composite membrane is usually used, consisting of an ultrathin selective layer mechanically supported by a microporous substrate. High energy efficiency requires a high-permeance gas membrane and minimum thickness of the selective layer, of which have been continual challenges for membrane technologies. These challenges arise because of the low concentration solution needed for the thin selective layers, which usually causes pore penetration in a porous substrate, thus resulting in surface defects.
Many thin-film composite membranes use a gutter layer to prevent such pore penetration [4], [5], [6], [7], [8], [9]. Gutter layer materials must have high permeability to minimize transport resistance. Therefore, high-permeability materials such as poly(1-trimethylsilyl-1-propyne) and polydimethylsiloxane (PDMS) are usually preferred for gutter layer materials, but because of the rapid physical aging of the former, the latter has been the most widely used gutter layer material so far [10].
Although PDMS is widely used in gas-separation membranes, there is still significant room to enhance ultrathin film composite (TFC) membranes just by increasing the gas transport properties of the gutter layer. Teflon AF, especially AF2400, is more attractive as a gutter layer material than PDMS because it offers higher gas permeance, no additional crosslinking, harmless dissolution, a high tendency to form uniform thin films, etc. For these reasons, the gas transport properties of AF2400 have been widely studied for membrane applications [11], [12], [13], [14]. However, the superhydrophobic nature of AF2400 and its low wettability by most solvents have conspired to hinder the use of a selective layer on top of an AF2400 gutter layer, so that, despite the significant potential of AF2400, no reports exist of its use as gutter layer material.
Although oxygen-plasma treatment is an ideal non-chemical method to overcome the superhydrophobicity of AF2400, the ultrathinness (< 100 nm) of the AF2400 gutter layer makes it very susceptible to plasma-induced damage. Minimizing such plasma damage is essential to apply AF2400 as a gutter layer material in TFC membranes. Capacitively coupled plasma (CCP) sources can generate plasma by generating a strong static electric field between the electrode and the ground. This type of plasma delivers high energy via ion bombardment, which damages membrane surfaces, especially those of thin membranes. In general, ion bombardment damage decreases with increasing pressure, so dielectric barrier discharge plasma has become popular owing to its less severe damage under atmospheric pressure. However, the plasma and the membrane must be placed sufficiently close together to allow proper plasma treatment, which inevitably increases the ion-bombardment damage [15]. Contrastingly, unlike CCP sources, inductively coupled plasma (ICP) sources generate plasma via an electric field induced by a time-varying magnetic field. ICP is characterized by weaker ion bombardment and higher plasma density than CCP [16].
We used ICP to minimize the damage of plasma treatment while increasing its impact, producing appropriate gutter layer surface properties without damage, even for gutter layers thinner than 100 nm. By using this method, we fabricated ultrathin film composite CO2/N2 separation membranes thinner than 100 nm for use as both gutter layer and selective layer, thereby attaining high-performance TFC membranes.
This study compares the gas permeances of PDMS and AF2400 membranes as gutter layers and discusses how the gas transport properties of the gutter layer affect the TFC membrane permeance and selectivity as functions of gutter layer permeance and selective layer thickness. We used the floating harmonic method [17], [18] to measure the electron temperature and ion flux, which are two parameters related to membrane damage. The AF2400 membrane fabricated for use as a gutter layer was tested under mixed gas conditions, and the experimental gas transport properties were compared with calculations based on a series resistance model.
Section snippets
Materials
Teflon AF2400 was procured from DuPont (USA), and Sylgard184 base and curing agent were procured from Dow Corning (USA). Galden HT110 as a solvent for AF2400 was procured from Solvay (Brussels, Belgium), and n-hexane (95%) as a solvent for Sylgard184 was procured from Daejung Chemicals (Gyeonggi-do, South Korea). The PAN350 membrane for the substrate was procured from Nanostone (Waltham, USA). Mixed gas (CO2 14 mol%, N2 86 mol%) was procured from AirKorea (Gyeonggi-do, South Korea). PEA-GO for
Gutter layer function in the thin-film composite membrane
Membranes used industrially usually consist of multilayer composite structures, as shown in Fig. 2. To obtain high efficiency, the selective layer must be thin (< 1 µm), so this layer is usually too fragile for direct use. Accordingly, a porous substrate with the requisite mechanical strength is used to support the membrane. The mass transport resistance of the porous substrate should be minimized so as to maximize the pore size and porosity of the substrate. However, such large pores allow
Conclusions
A gutter layer is usually used in TFC membranes with ultrathin selective layers, and PDMS has been the most widely used material for gutter layers. However, the theory developed herein shows that a PDMS gutter layer is not the best option in terms of TFC membrane performance due to its low permeance and relatively complex preparation process. Teflon is much more attractive as gutter layer material than PDMS because it offers up to sixfold the gas permeance of PDMS and exhibits easy
Acknowledgements
This work was supported by the Korea CCS R&D Center (Korea CCS 2020 Project) grant funded by the Korea government (Ministry of Science and ICT) in 2016 (2014M1A8A1049307).
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These authors contributed equally to this work.