Graphene in gas separation membranes—State-of-the-art and potential spoors

: Graphene and derivatives have been frequently used to form advanced nanocomposites. A very significant utilization of polymer/graphene nanocomposite was found in the membrane sector. The up-to-date overview essentially highpoints the design, features, and advanced functions of graphene nanocomposite membranes towards gas separations. In this concern, pristine thin layer graphene as well as graphene nanocomposites with poly(dimethyl siloxane), polysulfone, poly(methyl methacrylate), polyimide, and other matrices have been perceived as gas separation membranes. In these membranes, the graphene dispersion and interaction with polymers through applying the appropriate processing techniques have led to optimum porosity, pore sizes, and pore distribution, i.e., suitable for selective separation of gaseous molecules. Consequently, the graphene derived nanocomposites brought about numerous revolutions in high performance gas separation membranes. The structural diversity of polymer/graphene nanocomposites has facilitated the membrane selective separation, permeation, and barrier processes especially in the separation of desired gaseous molecules, ions, and contaminants. Future research on the innovative nanoporous graphene-based membrane can overcome design/performance related challenging factors for technical utilizations.


Introduction
For environmental remediation purposes, membrane technology has been widely adopted especially for the separation of desired or toxic gaseous species [1].Among membranes, polymeric membranes have durability, long functioning, and efficient performance so achieved significance for the separation applications.The graphene filled nanocomposite membranes possess superior characteristics for technical fields such as gaseous, water molecules, and chemical separations [2].The subsequent membranes were formed for large scale gas separation, water decontamination, fuel cells, and several other applied fields [3,4].Primarily, the graphene derived nanocomposite membranes have been developed with torturing pathways in the matrices to promote gaseous, water molecules, ions, or diffusion of other species [5].Consistent graphene dispersion in the membranes was found to improve the targeted impurities and toxic molecules from the medium of interest [6,7].The membrane processes studied for these nanocomposites include the ultrafiltration, microfiltration, nanofiltration, and reverse osmosis [8][9][10].The resultant membranes were competently applied for eliminating the pollutants [11].The graphene reinforced membranes revealed superior structural benefits than the pristine polymer designs due to facile manufacturing and performance advantages [12].Research developments have reported technical growth of these membranes for numerous sectors [13].
In the polymeric membranes, graphene, graphene oxide, and other modified graphene forms have been applied [14].It is worth mentioning that the thin layer neat graphene nanosheet has been designed for selective permeation of gaseous molecules [15].In polymeric matrices, graphene has revealed fine reinforcement effects relative to other carbon nanofillers (fullerene, carbon nanotube, etc.) [16].Including multilayered graphene or graphene oxide in the polymer membranes have been known to form two dimensional nanochannels for the selective permeation and barrier effects of gaseous molecules [17].Efficient and facile processing technologies have been applied such as solution casting, doctors blading technique, in situ method, phase inversion, infiltration, lift-off/float-on, etching, etc. have been applied [18,19].Mostly, thermoplastic matrices have been examined to form the graphene derived nanocomposites and membranes for gas separation [20][21][22].The pore sizes and graphene dispersion patterns directly affect the gaseous molecular permeability and diffusivity features of these membranes [23][24][25].Consequently, graphene scattering and layering in matrices have been known to develop percolation trails for the diffusing gaseous molecule [26].However, fine graphene dispersion, optimization of pore sizes and processing conditions have yet not been attained towards high performance commercial scale gas separation membranes.Applications of gas separation membranes for gaseous pollutants and desired molecules were found for the fields of fuel cells, gas sensors, chemical industries, etc. [27,28].
This review basically emphases on the design, development, and aspects of the graphene derived nanocomposite membranes for selective gas permeation application.Fine graphene dispersion, interface effects, and optimum pore formation in the membranes have broadened the potential of the gas partition membranes.This overview is groundbreaking to portray the methodical progressions of graphene resultant membranes for the gas separation.For the separation of gaseous species from mixtures, various polymer matrices have been filled with the graphene nanofillers to form the selectively permeable membranes.To the best of knowledge, this state-ofthe-art review is innovative to depict the advancements in gas separation membranes including the membrane designs, physical properties, and effect of graphene inclusion on the gas transportation features.This manuscript has been found indispensable for the future advances of gas separation graphene nanocomposite membranes, and so can be helpful guide for the interested field researchers.

Graphene
Two dimensional nanosheet like carbon nanostructure is referred as graphene [29].It is constituted of sp 2 hybridized carbon atoms, discovered in 2004 [30].Graphene was synthesized using numerous strategies like mechanical or liquid exfoliation of graphite, chemical vapor deposition, laser technique, plasma practice, and chemical synthesis methods [31][32][33].Graphene is a thin layered transparent nanostructure [34].Graphene has high electron mobilization of around 200,000 cm 2 V −1 s −1 and high thermal conductivity of 3000-5000 W/mK [35].Excellent mechanical properties of graphene include high Young's modulus of 1 TPa and strength of >200 times than steel [36].Graphene nanosheet has wrinkling effect due to the van der Waals interactions [37].To enhance the dispersion effects and final features, graphene nanosheet has been functionalized to introduce various surface functionalities such as hydroxyl, carbonyl, carboxylic, epoxide, etc. [38].Properties of graphene has been synergistically combined with other nanomaterial to form the nanocomposites.Graphene based nanocomposites revealed numerous superior electrical, mechanical, thermal, and physical features [39][40][41].Consequently, the graphene derived nanomaterials have been applied in wide ranging technological structure and applications like electronics, sensors, actuators, energy devices including fuel cells, batteries, membranes, engineering structure, and biomedical advanced devices [42].

Graphene and nanocomposites in gas separation
Graphene based nanoporous membranes have been applied for the gas molecule transport [43][44][45].The ultrathin graphene nanosheets have been designed for gas separation [46][47][48].Lee et al. [49] studied the selective separation of carbon dioxide CO2 molecules from CO2/CH4, CO2/O2, and CO2/N2 gas mixtures.Graphene nanosheets have affinity for CO2 molecules and pores in graphene nanosheet were suitable for the passage [50][51][52].Among the gas mixtures, high gas flux was observed for CO2/O2 as 0.43 [53].Jiang et al. [54] used first principles density functional theory to examine the permeability and selectivity of nanoporous graphene nanosheets.Figure 1 shows graphene nanosheet with hydrogen passivated pore.The nanopore width was 0.02 Å according to electron density isosurface Isovalue.The snapshot of gas molecules passage is given in Figure 2.  According to geometry optimization studies, H2 molecules entered through pores at 244 fs and molecules stayed there for 180 fs.Then molecules diffuse out through pores at 424 fs.High H2/CH4 permselectivity was observed, as per first principles molecular dynamics simulation studies on porous graphene.It has been observed that hydrogen atoms on graphene nanopores decreased the pore width to 2.5 Å, while the pore length remained same as 3.8 Å (Figure 3).Consequently, the interaction energy of incoming molecules to graphene nanosheet and diffusion barriers affected molecular adsorption or transportation.The resulting van der Waals density functional barrier for H2 and CH4 was observed as 0.22 and 1.60 eV, respectively.The graphene membranes having porous nanostructures were designed and studied aiming the gas separation [55][56][57].Graphene and graphene oxide membranes were designed having fine pores for the molecular sieving purposes.Koenig and colleagues [58] deposited the single layered graphene on silicon oxide substrate.Graphene layer was studied for the permeation of gas molecules.Etching process was applied for the separation of membrane from the substrate.Pristine graphene nanosheet is not permeable to gas molecules, however, the etched graphene membrane had porous nanostructure for gas molecule passage.Consequently, the etched graphene nanosheet was permeable to H2 and CO2 gas molecules [59][60][61].Li and researchers [62] designed the ultrathin porous graphene oxide membranes with pore size of −0.34 nm to 1 nm.The membranes were studied for permeability and selectivity properties of CO2, H2, N2, and gases.The H2/CO2 selectivity of 3400 and H2/N2 of 900 were observed [63,64].Smaller gas molecules revealed facile permeation, relative to the larger molecules through the porous membranes [65][66][67].
For gas separation applications, poly(methyl methacrylate) was applied for effective membranes thermoplastic material [68][69][70].For the formation of polymer/graphene nanocomposite membranes, facile methods have been used [71,72].Most commonly, solution cast procedure has been applied [73].In this method, polymer is dissolved in an appropriate solvent.The nanoparticles of interest are also dispersed in a solvent.Afterwards, both the dispersions are mixed to yield a consistent phase.The mixed solution is spread on an open surface to evaporate the solvent.Phase inversion method has also been used for the fabrication of graphene filled nanocomposite membranes [74].In this procedure, polymer is transformed from the liquid to solid phase.During this process, like controlled solution evaporation and immersion precipitation are involved.Additionally, interfacial polymerization has been used for the formation of graphene nanocomposite membranes [75].Interfacial polymerization consists of various steps such as oil phase formation, emulsification, and finally solvent evaporation.All the membrane formation methods have capabilities for fine dispersion of graphene nanofiller in the polymeric matrices.
Baldanza and co-workers [76] developed the graphene filled poly(methyl methacrylate) nanocomposite membranes by applying the wet deposition process.
Here, the 'lift-off/float-on' method was used for obtaining membrane [77][78][79].For the preparation of fine graphene nanosheets, the chemical vapor deposition practice was used.Figure 4 illustrates the lift-off/float-on procedure for the membrane formation.The poly(methyl methacrylate)/graphene nanocomposite membrane with 0.06% loading had thickness of 550 nm.According to the scanning electron microscopy images, graphene nanosheets were found to be sequentially layered in the polymeric membranes.According to permeability coefficients of humidified or pure O2 and CO2 measured for varying R.H. levels for poly(methyl methacrylate) and poly(methyl methacrylate)/graphene, the resultant membranes own lower permeability coefficient of 1.30 × 10 −17 and 0.21 × 10 −17 mol• m• m −2 • Pa −1 •s −1 , respectively, for CO2 and O2, than the unfilled polymeric membrane (Figure 5 and Table 1).The reduced permeability values of gases were attributed to the formation of better dispersion and development of more twisted gas diffusion paths for the gas molecule permeation [80].Nevertheless, few studies have reported the poly(methyl methacrylate) and graphene nanocomposite based gas separation systems and more concentrated future research efforts may lead to the formation of the high performance selective gas permeation membranes.Poly(dimethyl siloxane) was investigated towards essential material aiming membrane formation [81][82][83].The separation processes of carbon dioxide and other toxic gases have been studied using the poly(dimethyl siloxane) membranes.Here, membrane thickness has been found to affect the gas permeability and separation properties [84].To enhance the membrane features, nanofillers have been reinforced in the matrices for fine performance.Ha and co-workers [85] reported on the graphene oxide filled poly(dimethyl siloxane) membranes through solution processing.The kinetic diameters of CO2, O2, N2, and CH4 gases (in the range of 0.16 to 0.50 Å) affected the selectivity and permeability performance according to membrane porosity and microstructures.The membrane permeability was observed up to 99.9% by including 8 wt.% graphene oxide.Moreover, selectivity properties of the CO2/CH4, CO2/O2, and CO2/N2 have been observed.The gas transportation features were found to be reliant on the fine nanoparticle scattering in the polymer matrix.The microstructure and matrix-nanofiller interactions were also observe to be linked with the nanofiller alignment and scattering in the matrix for formation of gas transportation pathways.Koolivand and researchers [86] fabricated the poly(dimethyl siloxane) and graphene oxide derived membranes.Facile Hummer's method was used to form graphene oxide [87].For these membranes, the combination of solution and ultrasonication processing methods have been applied.Adding 5 wt.% graphene oxide loading, CO2 permeability and CO2/CH4 selectivity of 29% and 112%, respectively, were observed.Berean et al. [88] opted solution processing and ultrasonication for the formation of poly(dimethyl siloxane)/graphene nanocomposite membranes.Due to the interactions, graphene dispersion and matrix-nanofiller interactions have been perceived.Figure 6 shows change in the permeability behavior of the membranes with graphene loading.The membrane permeability was about 60% enhanced with the nanofiller loading for CO2, N2, Ar, and CH4 gases.Among these, CO2 had greater permeation with the 0.5 wt.% graphene than other gases showing permeation at 0.25 wt.%.The greater permeation of CO2 at higher nanofiller contents was observed due to its fine affinity towards graphene nanosheets.Figure 7 depicts the formation and behavior of diffusion pathways in poly(dimethyl siloxane) and graphene reinforced poly(dimethyl siloxane) nanocomposites.Aligned graphene nanosheets developed layered nanostructure with voids in the matrix.Formation of continuous gas diffusion trails were responsible for the passivation of the gaseous molecules through the matrix.Gas permeability of N2, CO2, Ar, and CH4 was enhanced up to 60% with just 0.2 wt.% graphene contents.Consequently, neat poly(dimethyl siloxane) had CO2/CH4 selectivity of 3.6, which was increased up to 4.2 in the poly(dimethyl siloxane)/graphene membrane.Polysulfone has been used as a popular matrix for membrane formation and also for the gas separation application [89][90][91].In this context, the mixed matrix membranes of polysulfone have been reported [92][93][94].The resulting polysulfone membranes have been observed functional for toxic gas separation such as carbon dioxide, nitrogen, and sulfur oxides [95].Zahri and co-workers [96] reported on polysulfone and graphene oxide based membranes through dry wet phase inversion process.The polysulfone based nanocomposite membranes revealed high CO2 permeability of 64-87 GPU.In addition, with the nanofiller loading CO2/CH4 selectivity was increased in the range of 19-25.The fine selectivity of the nanocomposite membranes was credited to the dispersal patterns in the polymer matrix [97].Sainath and co-worker [98] designed the mixed matrix gas separation polysulfone/graphene oxide nanocomposite membrane for gas separation using the solution method.As compared to pristine polysulfone membrane, graphene oxide filled system revealed >3 times higher selectivity for CO2/CH4.Fine selectivity was attributed to the homogeneous dispersion and formation of efficient diffusion trails in the matrix.Zhu and co-workers [99] opted the vacuum infiltration process to form graphene oxide filled nanocomposite membranes of the phosphotungstic acid grafted polyphenylsulfone-pyridine matrix.Transmission electron micrographs of polyphenylsulfone-pyridine, phosphotungstic acid, and graphene oxide based system is given in Figure 8.The nanofiller was observed to be homogeneously dispersed in the polymer matrix.
For He, CO2 and N2, the membrane permeability was found to slightly decrease with the nanofiller loading levels.The decreasing permeability was attributed to the increased nanofiller dispersion and membrane selectivity towards these gases.The dispersed graphene nanoplatelets were supposed to develop percolation pathways for the diffusion of gaseous species.Table 2 shows the permeability behavior of the membranes with different nanofiller loadings at 35 and 65 ℃.In this way, efficient graphene filled nanocomposite membranes have been designed for the selective gas separation or permeation properties [113][114][115].The selective permeability of the membranes was found to depend upon nanofiller scattering plus alignment in the matrix [116,117].Future studies on advanced graphene nanocomposite membranes may lead to better gas molecule separation from mixture of gases.Table 2. Permeability of the various gases in PPO and nanocomposite membranes [112].PPO = poly(1-trimethylsilyl-1-propyne).Reproduced with permission from MDPI.

Prospects, challenges and gaps
In the formation and application of graphene nanocomposites as high performance membrane materials, numerous challenges have been faced during the field research efforts.Generally speaking, not much efforts have been observed for various categories of polymer/graphene nanocomposite membranes such as poly(dimethyl sulfoxide)/graphene, polysulfone/graphene, poly(methyl methacrylate)/graphene, polyimide/graphene, polyamide/graphene, etc.The experimental designs of the polymer/graphene nanocomposite membranes have been reported using the matrices, graphene nanofillers, processing techniques (solution, phase inversion, infiltration, etc.), and related preparation parameters.
Table 3 outline the experimental design of the gas separation nanocomposite membranes used in important studies.Adding graphene in polymer matrices affected the membrane morphology, physical properties, permeability, selectivity, and separation properties.Polysulfone based nanocomposite membranes have efficient CO2/CH4 selectivity of 45%-74%.For gas separation membranes of poly(dimethyl siloxane) nanocomposites, N2, CO2, and other gases permeability was observed >99.9%.Similarly, higher selectivity values for gases like CO2/CH4 have been observed.Hence, there is huge scope for fabrication and investigations on graphene based air/water purification membranes.
Development and investigation of more designs definitely can lead to better analysis of optimum fabrication, selectivity, permeation, and gas separation performance along with better understandings on structure-property relationship and mechanism of innovative graphene membranes [118].Major challenges hindering the gas separation membrane performance have been observed as graphene dispersion depending upon nanofiller contents, functionality, matrix nanofiller interactions, and interface formation [7].Formation of interweaving pathways due to graphene dispersion in the polymer matrices has directly influenced the gas transportation properties.Controlled pore sizes, shape, and distribution in the matrices have also been found indispensable to promote the gas membrane performance.Important solutions to the nanofiller dispersion have been proposed depending upon the graphene modification as well as by applying appropriate processing techniques and steps with the optimized conditions [119].Further challenges have been observed regarding the fabrication of graphene based membranes on large scale and subsequent commercialization.Here, the appropriate fabrication techniques and processing parameters need to be implemented for the massive production of graphene nanocomposite membranes.In this case, development of nanofibrous polymer/graphene membranes must be developed with high surface area and well dispersed nanoparticle for separating the desired gaseous molecules [120].By controlling and overcoming all the above-mentioned graphene and graphene nanocomposite membrane design and processing challenges leading to the fine microstructure, robustness, permeability, selectivity, and barrier characteristics [121].Briefly speaking, further research on the mentioned line may lead to the proposition of high-tech future gas transportation membranes for commercial purposes.The research progress on the polymer/graphene nanocomposite membranes has led to several advances in the kinds, design, and applications to overcome the crucial foremost problems in this field.These separation membranes have been used for the efficient removal of gaseous pollutants with optimally high flux and permeation.For the purpose, microstructure and mechanical features like strength and flexibility have been considered important.For the enhancements in these properties, nanoparticle dispersion has been found significant for the matrix-nanofiller interactions to advance the ultimate membrane characters.In this context, compatibility of graphene nanoparticles with matrices must be enhanced for better miscibility and reinforcing effects.The pore shape, size, and distribution in the matrices have been found to affect the membrane selectivity/permeability features.Most important challenges of the graphene based gas separation membranes include the graphene nanosheet aggregation, phase separation, uncontrolled and undefined fabrication parameters.Such undefined conditions may lead to the different pore shapes, sizes, and random distribution in the matrices.The membranes with various pore sizes and shape may cause major hinderances towards the separation of particular gaseous molecules of specific types.The random pore distribution in membranes also affect the strength, durability, and life time of the membranes.In addition, poor membrane performance may result in the restricted cyclic uses.Consequently, the uncontrolled membrane features may cause poor barrier effects and selective molecular transportation.Hence, perfect membrane design features need to be identified before commercial scale production of these membranes.Investigations on the membrane separation mechanisms may be used to overcome the barrier, molecular selective diffusion, and performance challenges.In addition, advanced and facile fabrication methods need to be designed to form efficient membranes with controlled pore dimensions and essential features.Future research to resolve the stated challenging directions can be beneficial for the formation of high performance gas separation membranes.

Conclusions
In this state-of-the-art review article, the design, physical properties, and gas partition features have been scrutinized for important graphene and nanocomposite based membranes.Consequently, graphene has been filled in various polymeric matrices to form the efficient gas separation membranes.These membranes have been studied for the selective separation or permeation of various toxic or desired gas molecules such as O2, N2, CO2, CH4, etc. from the gas mixtures.Consequently, the membrane performance has been analyzed based on the microstructure, pore size, pore distribution, and specific tests related to the separation or permeation of the gaseous molecules.It has been observed that by varying the nanofiller contents, nanofiller functionalities, as well as polymer type, and fabrication methods, the resulting membrane performance has been rehabilitated.In addition, the graphene alignment and dispersion pattern in the polymer matrices resulted in advanced membrane performance with optimum porosity and tortuous pathway formation for the passage of gas molecules.In future, well-organized graphene based membrane need to be designed by overcoming the dispersion and processing challenges behind the development of high performance systems.

Table 3 .
Significant features of polymer/graphene nanocomposite membranes for gas separation.