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ISSN : 1225-0112(Print)
ISSN : 2288-4505(Online)
Applied Chemistry for Engineering Vol.35 No.2 pp.85-95
DOI : https://doi.org/10.14478/ace.2024.1011

Membrane-Based Direct Air Capture: A Review

Seong Baek Yang*, Kwang-Seop Im**, Km Nikita*, Sang Yong Nam*,**†
*Research Institute for Green Energy Convergence Technology, Gyeongsang National University, Jinju 52828, Republic of Korea
**Department of Materials Science and Convergence Technology, Gyeongsang National University, Jinju 52828, Republic of Korea
† Corresponding Author: Gyeongsang National University Research Institute for Green Energy Convergence Technology, Jinju 52828,
Republic of Korea Tel: +82-55-772-1657 e-mail: walden@gnu.ac.kr
February 28, 2024 ; March 24, 2024 ; March 24, 2024

Abstract


Direct air capture (DAC) technology plays a crucial role in mitigating climate change. Reports from the International Energy Agency and climate change emphasize its significance, aiming to limit global warming to 1.5 °C despite continuous carbon emissions. Despite initial costs, DAC technology demonstrates potential for cost reductions through research and development, operational learning, and economies of scale. Recent advancements in high-permeance polymer membranes indicate the potential of membrane-based DAC technology. However, effective separation of CO2 from ambient air requires membranes with high selectivity and permeability to CO2. Current research is focusing on membrane optimization to enhance CO2 capture efficiency. This study underscores the importance of direct air capture, evolving cost trends, and the pivotal role of membrane development in climate change mitigation efforts. Additionally, this research delved into the theoretical background, conditions, composition, advantages, and disadvantages of permeance and selectivity in membrane-based DAC.


Membrane , Direct air capture , Carbon dioxide , Selectivity , Permeance

막 기반 직접공기포집: 총설

양성백*, 임광섭**, 니키타 쿠마리*, 남상용*,**†
*경상국립대학교 그린에너지융합연구소
**경상국립대학교 나노·신소재융합공학과

초록


직접공기포집 기술은 기후 변화 완화에서 중요한 역할을 하고 있다. 국제에너지기구와 기후변화에 관한 보고서에서는 이러한 중요성을 강조하고 있고, 탄소의 지속적인 배출에도 불구하고 이를 감소시킴으로써 지구 온난화를 1.5 °C로 제한하는 것을 목표로 한다. 직접공기포집 기술은 초기 비용에도 불구하고 연구 및 개발, 운영 학습 및 규모의 경제를 통한 비용 절감의 가능성을 보여주고 있다. 최근 고투과도를 갖춘 고분자 막의 발전은 막 기반 직접공기포집 기술에 대한 잠재력을 제시하고 있으나, 효과적인 대기 중 CO2 분리를 위해서는 CO2에 대한 높은 선택성과 투과성을 갖춘 막을 필요로 한다. 현재 연구는 막 최적화 연구를 다수의 연구팀에 의하여 연구되고 있으며, CO2 포집 효율을 향상시 키는 데 중점을 두고 있다. 본 연구에서는 직접공기포집의 중요성, 발전 중인 비용 동향 및 기후 변화 완화에 있어서 막의 발전이 중요한 역할을 강조하고 있고, 덧붙여서 이 연구에서는 막 기반 DAC에서의 permeance와 selectivity의 이 론적 배경, 조건, 구성, 장단점에 대해 알아보았다.


    1. Introduction

    Direct air capture (DAC) is recognized as a key technology in combating climate change, as highlighted in the April 2022 report by the International Energy Agency (IEA)[1]. The latest Intergovernmental Panel on Climate Change (IPCC) report underscores efforts to limit global warming to 1.5°C, irrespective of carbon emissions. Despite the current high costs associated with DAC, advancements in research and development, operational learning, economies of scale, and other factors are expected to drive down costs in the future[2]. Additionally, private sector investment in DAC is on the rise. There is a growing recognition of the need to expand DAC facilities by 2050 to achieve carbon neutrality, as highlighted in the IEA's Net Zero Emissions report from 2021[3].

    The principle of DAC technology involves using a large fan to draw in ambient air and capture carbon dioxide from the drawn air. Carbon dioxide is separated and collected either by using filters coated with chemical agents capable of binding with carbon dioxide or through chemical bonding using solutions[4]. Wet absorbents, dry absorbents, amine-functionalized materials, ion exchange resins, and other materials are being researched primarily for CO2 capture[5]. Membrane filtration is another method proposed for carbon capture, which differs from solid or liquid absorbents in that the separation of CO2 is based on selective diffusion rather than selective binding with CO2 (Figure 1)[6].

    Membranes are selectively permeable, allowing CO2 to pass through while blocking other gases. Separation through selective permeability, unlike absorbent precipitates, is advantageous as membranes can be used permanently without the need for energy-intensive reabsorption steps[7]. According to Fujikawa et al., membranes offer advantages such as small installation space and relatively simple installation and operation compared to traditional CO2 capture methods[6]. However, membrane separation is only efficient when the membrane exhibits sufficient selectivity and permeability towards CO2 to create a significant concentration difference between the two sides of the membrane[8,9].

    Since the atmosphere contains only 400 ppm of CO2, membranes used for separating CO2 from ambient air must have higher permeability and selectivity compared to membranes used for exhaust gases[ 10,11]. Permeability is a key factor that enables membrane-based DAC (m-DAC)[12]. Membranes for separating CO2 from air should be no thicker than about 100 nm and must have a minimum permeance of 10,000 GPU (gas permeance units) to achieve effective separation.

    Recent advancements in polymer membrane materials with ultra-high gas permeability and selectivity have shown promise for potential application in m-DAC processes. The feasibility of m-DAC was explored based on process simulations considering the performance of advanced CO2 separation membranes. In DAC, while selectivity is important, only membranes with high permeability are considered attractive options for DAC. In practice, this means that exhaust gas flow must be compressed upstream of the membrane channel or suction provided downstream by a pump to maximize CO2 separation (Figure 2)[6,13]. Research teams worldwide are striving to optimize membranes to operate without complex accompanying systems. This paper will address various aspects of m-DAC, including variables, membrane composition, and theoretical calculation methods.

    2. Variables of membranes based direct air capture

    The effectiveness of direct air capture using membranes relies on various elements, encompassing both the characteristics of the membranes and the parameters of the process. Aside from the gas permeance and selectivity of membranes, the stage cut and the pressure ratio between the feed and permeate sides also play significant roles.

    2.1. Gas permeance parameter

    Gas permeance, which measures a material's ability to transport gas at a specific thickness, is influenced by both the permeability and thickness of the membrane. Membrane-based separation typically adheres to the Solubility-Diffusion model, which hinges on the solubility and diffusion rate of gases passing through the membrane[14,15]. Certain membranes employ facilitated transport mechanisms where carriers selectively react reversibly with CO2, while other gases primarily cross through the membrane via the Solubility-Diffusion mechanism but at a slower pace. Consequently, these membranes provide high selectivity and gas permeance[16]. Membranes exhibiting a strong Solubility-Diffusion correlation and minimal thickness are potential options for achieving enhanced permeance. Although selectivity remains highly important for DAC, membranes with high permeance are the primary focus for consideration as an attractive option. Research in membrane thinning is extensive, aiming to enhance permeance. Decreasing the thickness of DAC membranes to the nanometer scale is necessary to significantly boost membrane permeance, given that the required membrane area decreases with increase in permeance.

    2.2. Selectivity of membranes

    The m-DAC design finds significant interest in membrane gas selectivity, particularly in distinguishing between CO2 and N2 gases. This interest stems from the fact that nitrogen constitutes the majority of Earth's atmosphere, while CO2 concentration remains as low as 400 ppm. Insufficient selectivity results in increased migration of undesirable gases and reduced CO2 concentration in the permeate. Typically, it's advised to maximize permeance to minimize membrane area[6], yet it's crucial to optimize selectivity as excessively high selectivity in membranes demands more membrane area and offers minimal advantages in CO2 purity. Merkel et al. conducted a comparison between two membranes for point source CO2 capture: the "optimal" membrane and membrane B.

    The optimal membrane demonstrated a CO2 permeance of 1000 GPU and a CO2/N2 selectivity of 50, whereas membrane B exhibited a CO2 permeance of 1000 GPU with a CO2/N2 selectivity of 200. They observed that the optimal membrane resulted in a permeate containing 46% CO2 with a surface area of 2.1 MM m2, whereas membrane B achieved a CO2 concentration of 55% with an area almost tripled at 5.7 MM m2[17]. The CO2 concentration in the permeate experiences a significant increase with selectivity until approximately 30, after which it starts to level off. The selectivity also diminishes the energy needed for vacuuming, which likewise reaches a plateau. Fujikawa et al. demonstrated that employing a membrane possessing a CO2 permeance of 10,000 GPU and a CO2/N2 selectivity of 50 could attain a CO2 concentration of 66.7%. This achievement required only 4.63 m2/kg CO2/day and approximately 12.3~16 kWh/kg-CO2/day at a pressure ratio of 50, with a feed pressure of 110 kPa and a permeate pressure of 2 kPa. As selectivity increases by increments of 10, the membrane area needed rises nearly linearly, while the advantages in CO2 concentration, energy demand, and CO2 emission reduction linked to energy production decrease[6].

    While not as critical as the preceding two factors, it's worth taking into account the CO2/O2 selectivity, especially considering the downstream uses of captured CO2. Processes such as converting CO2 to CO and CH4 are often impeded by the presence of oxygen, making selectivity a relevant consideration.

    2.3. Pressure ratio of feed and permeate stream

    The performance of the process is influenced not only by the inherent properties of the membrane but also significantly by the conditions under which the process operates. The ratio of feed pressure (pf) and permeate pressure (pp) is defined as pressure ratio.

    φ = p f p p
    (1)

    X p ≤ X f × ϕ
    (2)

    Equation (2) illustrates that the mole fraction of CO2 on the permeate side, denoted by Xp, and the mole fraction in the feed, denoted by Xf, are related to the pressure ratio. This indicates that the permeate-side CO2 mole fraction is constrained by the pressure ratio, regardless of other factors such as selectivity[6,15]. To ensure a favorable pressure gradient across the membrane, the partial pressure of CO2 in the permeate must not surpass the partial pressure of CO2 in the feed for the flux to occur in the intended direction. As the pressure ratio increases, the concentration of CO2 in the permeate also increases because the mole fraction in the permeate is proportionally constrained by the pressure ratio.

    Fujikawa and colleagues investigated the impact of pressure ratio on m-DAC using process simulation, maintaining consistent CO2 retentate concentration (~300 ppm) and employing the same membrane specifications (permeance of 40,000 GPU and selectivity of 70) as outlined in Table 1 & 2. Notably, variations in the pressure ratio were observed. At a permeate side pressure of 4 kPa (φ = 25), the resulting CO2 concentration can surpass 40%. The membrane area and CO2 emissions associated with energy production decline from 3.19 to 2.6 square meters per kilogram of CO2 per day and from 0.6 to 0.54 kilograms of CO2 emitted per kilogram of CO2 captured, respectively, with an increase in the pressure ratio from 20 to 25.

    It's significant to highlight that at every subsequent stage, there's a remarkable decrease in the energy and membrane area needed compared to the initial stage. This demonstrates the significance of process parameters, as well as material properties such as permeance and selectivity.

    2.4. Stage cut

    The level of stage cut factor can be managed through adjusting the membrane area (with a consistent feed flow rate) and ought to be fine-tuned to reach a desired separation goal. Stage cut is defined as the ratio of permeate flow and feed flow[6,12]. Whenever compared to CO2, the driving forces for undesirable gases are less significant when there is an elevated feed flow because feed gas concentrations vary less. Since less CO2 passes at a low-stage cut, the outcome is a reduced recovery percentage but a greater purity of permeate. The quantity of CO2 that passes raises as the feed's flow rate is decreased, but doing so also raises the driving force for the remainder of the gases. Consequently, a greater stage cut results in a lower purity but a larger CO2 recovery percentage[6]. All these variables need to be adjusted precisely in m-DAC systems, depending on the ultimate objective, whether it's achieving complete recovery or maximizing purity for subsequent applications. Moreover, these parameters might fluctuate at every phase within a multi-stage separation setup. It's essential to take into account the energy demands for the setup, working, and production of membranes. Since m-DAC technology is still in its early stages, there is a limited amount of research on parameterization and gas module separation studies. A membrane material with high selectivity (referred to as HPM) featuring a CO2 permeance of 2500 GPU and CO2/N2 selectivity of 680 has demonstrated the potential to achieve a capture purity of approximately 20% in a single stage at a pressure ratio of approximately 0.02. Furthermore, commercial Polaris membranes, boasting a CO2 permeance of 2000 GPU and CO2/N2 selectivity of 30, can achieve a 50% capture purity with two stages[18]. The HPM exhibited a rapid rise in CO2 capture purity as the pressure ratio decreased from 0.02, displaying an exponential trend. In contrast, the Polaris membrane showed no notable impact, mainly due to the negligible influence of concentration polarization[18]. In the case of m-DAC, achieving purities of 20% and 2.5% required 3000 kWh/ton and 18,000 kWh/ton, respectively, for HPM and Polaris membranes at a single stage[7]. The research emphasized the significance of a lower stage cut to enhance the recovery ratio[18]. While m-DAC CO2 concentrations are typically not frequently examined, Lee et al. demonstrated that their ionic liquid/graphene oxide PIL/IL/GO facilitated transport membrane could achieve a purity of 32% with just one stage at 1 bar (1 bar = 100 kPa) of feed pressure, operating at 410 ppm CO2[19]. In these circumstances, the membrane exhibited a permeance of a CO2/N2 selectivity of 1180 and 3090 GPU[19]. However, there was no mention of the stage cut and pressure ratio in the report. The suggested models for multistage membrane separation have demonstrated enhanced CO2 separation in each subsequent stage. Particularly, they highlight that the initial stage incurs the highest energy, area, and emission values, with subsequent stages showing an exponential decrease thereafter. While there is limited literature available on membrane separation under m-DAC conditions, the aforementioned instances show considerable potential and should serve as encouragement for broader experimentation with membranes designed for atmospheric CO2 capture.

    3. Membrane composition for CO2 capture

    When designing membranes for CO2 capture, securing high permeability and selectivity is crucial. However, permeability and selectivity of the membrane are inversely related, constituting key determinants of membrane performance. Additionally, membrane durability is a critical consideration. To prevent mechanical damage and maximize permeability and selectivity, the composition of membrane components such as support layers, gutter layers, and selective layers is paramount[20, 21]. Permeability and selectivity typically trade off against each other, making it a significant challenge in membrane design to enhance selectivity while maintaining permeability. Various combinations of membrane materials and structures are being researched to address this challenge. Membrane durability is pivotal in determining its lifespan and performance. To prevent mechanical damage, membranes must be sufficiently robust and durable, while also considering resilience to environmental factors. Membrane components include support layers, gutter layers, and selective layers, each contributing to the membrane's mechanical support, functional properties, and separation characteristics( Figure 3). The right combination of materials and structures is crucial to optimizing membrane performance. In conclusion, designing membranes for CO2 capture involves optimizing membrane components and conditions considering permeability, selectivity, and durability. This is an essential step in ensuring efficient CO2 capture and processing processes.

    3.1. Support layer

    The support layer plays a crucial role as one of the components of the membrane structure. It provides physical support, ensuring that the membrane remains stable and undamaged, thus allowing for prolonged usage without deformation or breakage. Moreover, it not only functions as an individual component but also maintains the overall integrity of the membrane structure. Particularly, in the case of thin selective layers or chemically sensitive layers, the support layer shields the membrane, preserving its stability. Enhancing the membrane's resistance to external environmental conditions is another vital function of the support layer. This layer is not merely a constituent part but also a guardian of the membrane, safeguarding its structural integrity and ensuring its longevity amidst varying operational conditions.

    Common polymeric membrane materials mainly contain polyimides, polycarbonates, polysulfones, polyphenyl oxides, polyethylene oxide, and cellulose derivatives[27]. Support layers are being manufactured using strategies that involve nanostructures to enhance their mechanical and chemical properties[28-30]. These strategies aim not only to improve the support layer's mechanical and chemical characteristics but also to enable multifunctionality, allowing the support layer to not only provide structural support but also catalytic functions and promotion of chemical reactions[31,32]. There is also considerable focus on maintaining a balance between mechanical strength and flexibility. The use of nanostructures enhances the mechanical and chemical properties of the support layer while enabling multifunctionality, including catalytic roles and facilitation of chemical reactions. Moreover, there is significant interest in maintaining a balance between mechanical strength and flexibility in support layer design and fabrication.

    3.2. Gutter layer

    Gutter layers play a crucial role in the gas separation process, aiding in the selective separation and permeation of specific gases like CO2[33,34]. Additionally, they support the membrane structurally, preventing deformation and damage, thereby extending its lifespan and preventing potential harm during usage. Gutter layers help regulate and stabilize gas flow, contributing to efficient operation and enhancement of the gas separation process[35,36]. Moreover, by supporting the membrane's selective layers, they facilitate efficient gas permeation, thereby improving the membrane's separation efficiency and effectively separating desired gases.

    3.3. Selective layer

    Selective layer plays a pivotal role in gas separation processes by selectively allowing specific gases to permeate while blocking others[41, 42]. It ensures the separation of desired gases, such as CO2 and CH4, crucial for processes like CO2 capture and natural gas purification. Moreover, the selective layer controls gas permeation and selectivity, contributing to efficient gas separation processes. It also enhances membrane stability, prolonging its lifespan and ensuring resistance to external environmental factors. Ultimately, the selective layer maintains gas purity by controlling the composition of gas mixtures.

    In a selective layer, it is crucial to significantly increase selectivity. To achieve this, enhancing permeance is also important. To improve permeance, one needs to prepare thin films, and methods for thin film fabrication include spin coating, deep coating, solution casting, among others. Additionally, manufacturing multilayers and applying them onto the gutter layer can help secure selectivity (Figure 4).

    Recently, there has been a trend towards research aimed at enhancing the selectivity by increasing the adsorption capacity of CO2. This includes manufacturing films by incorporating inorganic nanoparticles to increase CO2 adsorption capacity or incorporating adsorptive liquids such as ionic liquids into the films to enhance selectivity[47-49]. Studies have also reported on improving selectivity through environmentally friendly manufacturing processes[50] and computer modeling and simulation[51,52].

    4. Constraints of CO2 separation using membranes

    4.1. Permeation mechanism of polymeric membranes

    Polymeric membranes have the advantages of good processability and easy mass production. However, due to differences in material selection and manufacturing methods, they can be classified into porous and non-porous membranes. To enhance selectivity, research mainly focuses on non-porous membranes. The permeation mechanisms through non-porous membranes are classified into three categories[53]. As shown in Figure 5, the permeation process involves the following steps: 1) Absorption or adsorption of the permeating gas onto the membrane surface[54,55], 2) diffusion through the polymeric membrane[ 56,57], and 3) desorption of the gas on the other side of the membrane. In this solution-diffusion mechanism, the properties of the membrane material are influenced not only by the physical properties but also by the thermodynamic activity of the respective gas[58,59]. The gas permeability of non-porous membranes varies depending on the diffusion coefficients of individual gas components or the flexibility of polymer chains and physicochemical interactions within the polymer membrane[53,60]. In Figure 5, the driving force for gas permeation through polymeric gas separation membranes is dominated by pressure and is separated based on differences in gas molecule permeation and diffusion coefficients within the membrane.

    4.2. Solubility and diffusivity

    The permeability of solution-diffusion within polymers can be expressed as follows[62-64]:

    P A = S A × D A
    (3)

    PA represents permeability, SA represents solubility, and DA represents diffusivity. And the gas permeability of polymeric membranes can be expressed as shown in Equation 3.

    P A = N A ⋅ l / A m ( p 2 , A − p 1 , A )
    (4)

    Here, NA [cm3(STP)s-1] represents the flow rate permeating through the membrane, Am denotes the membrane area (cm2), l stands for the membrane thickness (cm), and p2,A and p1,A represent the partial pressures of A in the feed and permeate, respectively, measured in cmHg. Gas permeability can be expressed in units of Barrer and GPU, as indicated in Equations 3 and 4[8].

    B a r r e r = 10 − 10 c m   ⋅   c m 3 c m 2 × c m   H g × s
    (4)

    G P U = 10 − 6 c m 3 c m 2 × c m   H g × s
    (5)

    Here, cm represents thickness, cm3 denotes the flow rate of permeated gas, cm2 stands for unit area, s represents unit time, and cmHg is unit pressure. Barrer and GPU differ significantly in terms of the thickness (cm)[65]. Barrer considers the thickness as a unit of permeability, while GPU represents the permeability of the membrane without considering thickness.

    The measured selectivity (αA,B) between gases A and B is distinguished by solubility selectivity ( S A S B ) and diffusivity selectivity ( D A D B ) and is utilized as an indicator for optimizing permeation per formance[66-69].

    α A , B = P A P B = S A S B × D A D B
    (6)

    The gas solubility of polymeric membranes is influenced by factors such as the condensability of the permeating gas, interactions between the gas and the polymer, and the chain packing density in the glassy polymer. Therefore, when designing gas separation membranes, the factors that need to be adjusted depending on the type of permeating gas vary[70,71]. Polymeric membranes manufactured by controlling solubility tend to exhibit high efficiency in separating gases with relatively high condensability and small gas molecules[72-74]. Additionally, gas permeability is primarily influenced by the kinetic diameter of the permeating gas (Table 6).

    In the case of diffusivity selectivity, besides the size-based separation of gas molecules, it encompasses the selectivity due to structural factors such as stiffness and density between polymer chains. Glassy polymers are primarily influenced by these structural factors. Moreover, one of the significant indicators in gas separation membranes, known as the upper bound, exhibits a trade-off relationship whereas permeability increases, selectivity decreases, and as permeability decreases, selectivity increases[76]. This upper bound, also called the critical performance curve, serves as an evaluation metric for the properties required for the commercialization of membranes (Figure 6a, b).

    5. Comparison of m-DAC with other potential DAC process

    Traditional CO2 capture technologies are classified into post-combustion capture, pre-combustion capture, and oxy-fuel combustion capture. However, DAC technology can be described as a process of removing CO2 directly from the ambient air. Depending on the materials used, it can be broadly categorized into solid direct air capture (s-DAC), liquid direct air capture (l-DAC), and m-DAC technologies. Among the specific chemical processes being explored, three are receiving significant attention: activation with alkali and alkaline earth hydroxides, carbonation, and organic/inorganic hybrid sorbents supported by porous adsorbents.

    DAC technology can be classified into five main categories: liquid scrubbing, solid adsorbents, electrochemical methods, low temperature, and membranes. Liquid scrubbing involves the absorption of CO2 from the atmosphere into a chemical solution for capture, commonly employed in chemical processes for subsequent treatment and capture. Solid adsorbents utilize materials such as waste, coal, and various plastics to adsorb CO2 from the atmosphere, often recycling byproducts in industrial processes for CO2 capture. Electrochemical methods capture CO2 through processes like electrolysis, electrocatalytic capture, and electrochemical reduction, employing electrical energy for chemical conversion and capture. Low-temperature methods capture and transform CO2 into liquid or solid states at reduced temperatures, commonly utilized in chemical processes and industrial applications. Membranes employ selective permeability to separate and capture CO2 from other gases, especially in areas where CO2 generation is prevalent, notably in chemical processes. The following figure illustrates the advantages and disadvantages of each DAC technology. Membrane-based DAC technology offers advantages over other methods in terms of selectivity, recyclability, adsorption stability, customization, and cost-effectiveness (Figure 7).

    In m-DAC, the thermodynamic barrier is typically higher compared to other types of DAC due to the low CO2 concentration in the air. Consequently, the direct energy requirement is higher. As a result, there are limitations regarding product purity, energy requirements, and productivity, as previously discussed. In solid amine adsorbent-based DAC, porous oxides are often used as supports. However, liquid absorbents consistently pose issues such as leakage, equipment corrosion, liquid degradation, and the risk of volatile organic compound emissions. Moreover, there are spatial limitations associated with the installation of facilities, imposing constraints on installation locations [79].

    6. Concluding remarks

    CO2 direct air capture plays a pivotal role in addressing climate change, supported by ongoing advancements and private sector investments, which are making it increasingly economically viable. Currently, membrane-based DAC offers significant advantages such as spatial constraints and simplified operation, achieving notable selectivity and permeability for CO2. Recent research underscores the importance of adjusting various parameters to enhance permeance and selectivity, closely tied to the characteristics of each layer. The pursuit of optimal physical and chemical properties, often through nanotechnology and novel materials, is a key focus. Moreover, membrane- based DAC must secure optimal parameters for gas mixture separation, comparing various membrane types. With these advantages and operational efficiencies, membrane-based DAC is poised for increased prominence in the near future.

    Acknowledgment

    This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R1A6A1A03038697).

    Figures

    ACE-35-2-85_F1.gif
    Schematic image of the four-stage membrane separation process able to increase CO2 concentration from pre-concentration levels in atmosphere of about 0.04% to up to 40%, which is sufficient for subsequent CO2 conversion by electrochemical or thermochemical means[6].
    ACE-35-2-85_F2.gif
    (a) Conceptual image of a carbon recycling society based on m-DAC CO2 capture systems combined with CO2 conversion, (b) schematic illustration of a thin-film composite membrane, and (c) state of the art: trade-off curve showing the CO2/N2 selectivity data for different membrane materials versus CO2 permeance for a 1-mm-thick membrane. ○ : polymeric membranes, ×: inorganic membranes, □ : facilitated transport membranes, ◇ : hybrid organic-inorganic[6,13].
    ACE-35-2-85_F3.gif
    Comparison of the three kinds of composite membranes for CO2 capture, the effect of the COF gutter layer thickness on the overall CO2/N2 separation performance of Pebax/COF-LZU1/PAN membranes and the Pebax concentration on the overall CO2/N2 separation performance of Pebax/COF-LZU1/PAN membranes[22].
    ACE-35-2-85_F4.gif
    Schematic illustration of (a) the preparation process of the membrane by interfacial polymerization with a gutter layer, (b) the solution-casting method, (c) spin-coating process, and (d) the dip-coating process[43].
    ACE-35-2-85_F5.gif
    (a) Membrane's solution-diffusion mechanism, (b) movement of gas molecules within the pores through a series of diffusional jumps[61].
    ACE-35-2-85_F6.gif
    Upper bound correlation for (a) O2/N2 separation and (b) CO2/N2 separation[77].
    ACE-35-2-85_F7.gif
    Radar chart for multidimensional materials selection criteria. The bold lines represent the average value of each property in the material category[78].

    Tables

    The Results of m-DAC Separation under Pressure Ratio 20 and Permeate Pressure 5 kPa[4,9]
    The Results of m-DAC Separation under Pressure Ratio 25 and Permeate Pressure 4 kPa[4,9]
    CO2 Permeance and Selectivity of Polymers for Support Layer
    CO2 Permeance and Selectivity of Polymers for Gutter Layer
    CO2 Permeance and Selectivity of Polymers for Selective Layer
    Kinetic Diameter of Gas[75]

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