Nanoarchitectonics of carbon molecular sieve membranes with graphene oxide and polyimide for hydrogen purification

<jats:p>Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the CMS membrane exhibited ultrahigh H<jats:sub>2</jats:sub>/N<jats:sub>2</jats:sub> selectivity (117) and H<jats:sub>2</jats:sub> permeability, which have bright prospects for hydrogen purification.</jats:p>

section_content: Global warming is one of the most important challenges today that all mankind is facing and needs to be solved urgently. The key to meeting this challenge is to consistently expand the renewable energy sources. With the growing demand of global energy and environmental protection, hydrogen energy has been considered as one of the most promising alternatives to traditional fossil fuels and offers the potential to effectively alleviate the serious energy crisis and green-house effect owing to its distinct advantages such as abundant sources, high energy density and clean combustion without carbon emission. 1, 2 owever, the produced H 2 stream usually contains a signicant amount of N 2, CH 4, and CO 2 that should be removed to produce high purity hydrogen. Purication of hydrogen from this gas stream requires novel separation technologies to improve energy efficiency as it consumes 60% of the total energy requirement in the whole hydrogen production process. 4] [5] [6] Various materials such as polymeric membranes, 7, 8 inorganic membranes such as graphene oxide (GO), 9 MoS 2, 10 metal-organic frameworks (MOFs), 11, 12 zeolite imidazolate framework (ZIF), 13, 14 2D covalent organic frameworks, 15 and carbon membranes 16 have been explored for H 2 separation. However, the main disadvantages of the above membranes are low gas permeability as well as poor selectivity, since they usually exhibit a trade-off constraint. 17, 18 n addition, the physical aging problem and CO 2 plasticization must also be solved during the long-term service under adverse conditions (e. g., high temperature and pressure in the steam methane reforming process). 19, 20 2] [23] [24] [25] [26] [27] CMS membranes with strong carbon structures are typically fabricated by controlled carbonization of polymeric precursors at high temperature. During the thermal treatment process, the disordered polymeric structures tend to form ordered slit-like carbon structures as the pathway for H 2 gas transport. 28, 29 ifferent kinds of polymeric materials, including polyimide and its derivatives, [30] [31] [32] [33] [34] cellulose and its derivatives, 35 poly(arylene ether ketone), [36] [37] [38] poly(phenylene oxide) 39, 40 and polybenzimidazole, 41 have been employed as the precursors to fabricate CMS membranes for H 2 purication. Compared to pristine polymeric membranes, CMS membranes have excellent gas separation performance for hydrogen purication, which can exceed the Robeson upper bounds of polymeric membranes that plot a trade-off relationship between the gas permeability and selectivity. Furthermore, CMS membranes exhibit higher thermal stability and better plasticization resistance. Previous research has provided guidance for the structural design of polymeric precursors, the polymeric chain properties such as conformation, rigidity and packing efficiency also have a great effect on the gas separation performance of the CMS membrane. Therefore, great efforts have been devoted to exploring novel polymeric precursors to prepare CMS membranes with high gas permeability and good selectivity. High permeability is benecial to reduce investment costs and good selectivity will meet the demand for high purity. Tseng et al. 42 found that the functional groups of precursors (polyimide versus polyetherimide) and their thermal stability could be optimized to enhance the H 2 separation properties of CMS membranes. Cao et al. 37 reported that the chemical structure (sulfone/ketone unit ratio) of copolymer precursors had a signicant inuence on the H 2 separation performance. Ma et al. 31 believed that the conguration of monomer prominently affected the polyimide properties, leading to great differences among the CMS membranes. Koros et al. 43 and Lee et al. 44 reported that reducing the chain packing (or enhancing the FFV) of polymeric precursors by introducing monomers with bulky side groups could enhance the gas permeability. Pinnau et al. 45 found that the rigid and contorted aromatic structure was benecial for reducing the shrinkage of micropores when the membranes were pyrolyzed at high temperatures. These previous works investigated the effect of chemical structure on H 2 permeability and selectivity of CMS membranes. 46 ntil now, the incorporation effect of 2D nanollers into polymer matrix has not been fully explored. Graphene-based 2D materials, such as graphene and graphene oxide, have been considered promising membrane materials, because they are only one carbon atom thick and, thus, may offer a wide range of opportunities for membrane with minimize transport resistance and maximize ux. Additionally, they have good stability and strong mechanical strength. Therefore, in this work, the CMS membranes were fabricated via the in situ incorporation of GO into the polymeric membranes. The aims of this study were to explore the science and engineering that 2D GO in the polymer matrix can (1) regulate the packing of polymer chains, (2) restructure the architecture, (3) manipulate the free volume with enhanced gas separation performance, and (4) improve the resistance to physical aging and CO 2 plasticization problems.
section_name: Introduction

section_name: Experimental

section_content: The monomer 4,4 ′ -(hexauoroisopropylidene)diphthalic anhydride (6FDA) was obtained from Clariant and 2,3,5,6tetramethyl-1,4-phenylenediamine (Durene diamine) was purchased from Sigma-Aldrich. The graphene oxide (GO) aqueous solution of 0. 5 wt% was purchased from Angstron Materials Inc. The analytical grade N,N ′ -dimethylformamide (DMF, >99. 5%) and N-methyl-2-pyrrolidone (NMP, >99. 5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. The methanol (MeOH, $99. 9%), ethanol (EtOH, $99. 9%), and other solvents were ordered from Sinopharm Chemical Reagent Co. Ltd. and used as received. The high purity gases H 2 (>99. 999%), CO 2 (>99. 999%), N 2 (>99. 999%), and CH 4 (>99. 999%) were supplied by Nanjing Shangyuan Industrial Gas Factory (Jiangsu, China). Petri dishes and glass sheets were purchased from Shanghai Glass Instrument Factory (Shanghai, China).
section_name: Materials

section_content: The typical procedure for the fabrication of 6FDA-durene membrane was carried out as follows: a stoichiometric amount of 6FDA was added to NMP solution consisting of durene at an equal molar ratio in a moisture free ask with stirring and nitrogen inlet at room temperature. Aer reaction for 24 h, a high molecular weight polyamic acid was formed, and then the polyamic acid solution was cast in Petri dishes at ambient temperature in fume hood. The thin layer of the solution was then covered by a tin foil to allow the slow evaporation of NMP. The thermal imidization of polyamic acid was performed in vacuum under the stepwise heating at 100, 200 and 250 °C for 60, 60 and 30 min, respectively. A polyimide lm was formed aer most of the solvent had evaporated in the vacuum oven. The nascent membrane was then peeled off and the average thickness of the membranes was measured from 10 different points using a Digimatic indicator with an accuracy of 1 mm. The thicknesses of polyimide membranes were approximately 50 ± 5 mm. The synthetic scheme and chemical structure of polyimide are shown in Fig. 1.
section_name: Fabrication of 6FDA-durene membrane

section_content: To improve the compatibility between GO and the polymer substrate, a stoichiometric amount GO nanosheets was rstly interact with durene in NMP solution for 24 h. Aer that the 6FDA in NMP solution was added partially 3 times. The mixture consisting of GO, durene, and 6FDA at stoichiometric molar ratio was stirred in a moisture free ask, and the whole system was purged with N 2 at room temperature. Aer reaction for 24 h, a high molecular weight polyamic acid-GO solution was formed, and then the polyamic acid-GO solution was cast in a Petri dish at ambient temperature in a fume hood. The thin layer of the solution was then covered by a tin foil to allow the slow evaporation of NMP. The thermal imidization process of polyamic acid was performed in a vacuum oven under the stepwise heating at 100, 200 and 250 °C for 60, 60 and 30 min, respectively. A polyimide-GO-1. 0% lm was formed aer most of the NMP solvent evaporated in the vacuum oven. The nascent membrane was then peeled off and the average thickness of the membranes was measured from 10 different points using a Digimatic indicator with an accuracy of 1 mm. The thicknesses of PI-GO-1. 0% membranes were around 50 ± 5 mm. The synthetic scheme and chemical structure of PI-GO are shown in Fig. 1.
section_name: In situ fabrication of 6FDA-durene-GO membranes

section_content: The chemical structures of 6FDA-durene and 6FDA-durene-GO were analyzed by the attenuated total reection Fourier transform infrared (ATR-FTIR) spectroscopy 8400 spectrometer in the range of 400-4000 cm -1. The spectra were obtained with an average of 32 scans at a resolution of 4 cm -1 and the samples for testing were membranes. The interchain spacings of the polymeric membranes were investigated by a Bruker D8 Advance Xray diffractometer (XRD), and general area detector diffraction system (GADDS), utilizing Cu K-a as the X-ray radiation source with a wavelength of 1. 54 Å. The X-ray diffraction angles between 5 and 30°were studied. The average d-space was evaluated based on the Bragg's law (nl = 2d sin q), where n is an integer (1, 2, 3), l represents the X-ray wavelength, d represents the intersegmental spacing between two polymer chains and q denotes the X-ray diffraction angle. The thermal stability was characterized under a nitrogen atmosphere with a ow rate of 50 mL min -1 by thermogravimetric analysis (TGA). A Shimadzu Thermal Analyzer (DTG-60AH/TA-60WS/FC-60A) was used for these characterizations with a heating rate of 10 °C min -1 from 50 °C to 800 °C. The mechanical properties were tested with at least ve lm specimens (30 mm long, 5 mm wide, and about 0. 05 mm thick) by an Instron model 5542.
section_name: Characterizations

section_content: Both pure gas and mixed gas permeation of the membranes were conducted on a variable-pressure constant-volume gas permeation cell. The lm of dense membrane was mounted onto the permeation cell and vacuumed at 35 °C for 12 h before the gas permeation test was carried out. Pure gas permeability was tested following the order of H 2, O 2, N 2, CH 4, and CO 2. The cell temperature was kept constant at 35 °C. The gas permeability coefficient through the membrane was calculated according to the steady state pressure increment (dp/dt) in the following eqn (4):

where P denotes the gas permeability coefficient in Barrer (1 Barrer = 1 × 10 -10 cm 3 (STP) cm cm -2 s -1 cmHg -1 ), V refers to the volume of the downstream reservoir (cm 3 ), A is the effective membrane area (cm 2 ), l represents the membrane thickness (cm), T is the testing temperature (K) and p 2 is dened as the upstream pressure of the system (atm). The permeability tests were repeated at least three times and the average deviation obtained was less than 5%. 

The ideal selectivity (a A/B, gas A over gas B) between two different gases in a polymeric membrane is the ratio of the permeability of single gas as described in the equation as follows:

where p A and p B refer to the permeability coefficients of gases A and B, respectively. According to the solution-diffusion model, permeability is the product of diffusivity (D) and solubility (S). Therefore, the ideal permeability selectivity (a P ) can be dened as the product of diffusivity selectivity (a D ) and solubility selectivity (a S ) as follows:

where D A and D B are the diffusivity coefficients of gases A and B, respectively (cm 2 s -1 ); S A, and S B are the solubility coefficients of gases A and B, respectively (cm 3 (STP) cm -3 polymer atm). The mixed gas permeation properties were evaluated using a binary 50 mol% H 2 and 50 mol% N 2 mixtures. For binary gas separation measurements, the pure gas permeation cell was modied for the continuous owing feed by connecting to a needle valve to control the upstream pressure. Besides, it is also equipped with a retentate channel to avoid the accumulation of feed at the upstream. Then the receiving volume of permeation cell was connected to a gas chromatograph (GC) by a valve. Before permeation testing, GC was calibrated by one set of Messer gas mixtures of H 2 and N 2 with known composition to obtain the GC peak area as a function of gas mole fraction. The gas permeabilities of CO 2 and CH 4 are expressed by the following equations:

where P H 2 and P N 2 are the permeability (barrer) of H 2 and N 2, respectively. P 2 denotes the upstream feed gas pressure (atm), x refers to the mole fraction in the feed gas and y is the mole fraction in the permeate. The other symbols retain the same meanings as described earlier.
section_name: Measurements of gas permeation

section_content: Gas sorption behaviors of the 6FDA-durene and 6FDA-durene-GO membranes were measured via the dual-volume pressure decay method using XEMIS microbalance system. The membrane samples weighing about 50-60 mg was placed into the sample chamber and a vacuum was maintained for 3 h before sorption tests were conducted. Both the reference and sample chambers were immersed in the same water bath ((Techne FTE-10DE) from Techne Cambridge LTD (UK) with an accuracy of ±0. 01 °C) to maintain the desired temperature. The sorption isotherm of each gas was obtained at 35 °C up to a maximum pressure of 10 atm. Since CO 2 would plasticize the membranes at high pressure, sorption tests were conducted in the order of H 2, O 2, N 2, CH 4 and then CO 2. The solubility coefficient was calculated from the slope of sorption isotherm since the sorption behavior obeyed the Henry's law. Aer that, the diffusion coefficient was calculated from solubility coefficient based on the equation of permeability (P) = diffusion coefficient (D) × solubility coefficient (S).
section_name: Measurements of gas sorption

section_name: Results and discussion

section_content: As presented in Fig. 1, a novel multi-permselective mixed matrix membrane was developed by incorporating versatile graphene oxide (GO) via in situ polymerization, which can improve the dispersion of GO into polyimide matrix. Here, the Lerf-Klinowski model GO has hydroxyl groups (-OH) and epoxy groups (-O-) on its surface, and carboxylic acid groups (-COOH) on the periphery of the basal graphitic platelets, as shown in Fig. 1. The Lerf-Klinowski model GO was a good candidate as a nanoller in mixed matrix membranes because of its high aspect ratio, easy surface functionalization, and high thermal and mechanical properties. In order to improve the compatibility between GO and the polymer, the GO nanosheet was rst interacted with durene, the carboxylic acid groups (-COOH) on the edge of GO nanosheets reacted with amine groups of durene. Aer that the stoichiometric amount of 6FDA was added partially 3 times. The polyamic acid (PAA) consisting of GO solution was prepared under N 2 purge without moisture at room temperature. Aer reaction for 24 h, the viscous PAA-GO solution was cast on a Petri dish in the fume hood, as presented in Fig. 1. The thermal imidization process from PAA-GO to PI-GO was conducted in an oven under the heating stepwise at 100, 200 and 250 °C for 60, 60 and 30 min, respectively. The gas separation performance of PI-GO membranes was evaluated in the following session. The CMS membranes were prepared from PI-GO precursor at the carbonization temperatures from 500 °C to 600 °C. The structural change and fabrication process from PAA-GO-0. 10% to PI-GO-0. 10%, and thermally carbonized PI-GO-0. 10%-550 °C were fully presented in Fig. 1. 

The series of polyimide-graphene oxide membranes (PI-GO) were prepared via in situ polymerization. The TEM image in Fig. 1 shows the microstructure and high aspect ratio nanosheet. As shown in Fig. 2, GO was well dispersed in the polymer via in situ polymerization, with the increase of GO content, the viscous PAA-GO showed a signicant change in color, and the color of PI-GO lm is consistent with that of viscous PAA-GO solution. The chemical structures of PI were conrmed by FTIR spectra, as shown in Fig. 3 The incorporation of GO will effectively restructure the architecture and have an immediate effect on polymer chain packing. Therefore, XRD was employed to investigate the evolution of the pore size as a function of the thermal treatment temperature. As revealed in Fig. 3 (b), the original PI and PI-GO membranes had a peak at approximately 6. 55 Å (micropore), which was probably attributed to the loosely packed polymer chains derived from the kinked backbone of polyimide. Here, the micro-pores were formed among the polymer chains. The next peak at 3. 91 Å (ultra-micropore) was assigned to the dspacing of the efficiently packed polymer chain-to-chain distance that contributes directly to the conformation of ultramicropores. Aer thermal treatment at 500 °C, a new peak around 2. 97 Å (ultrane-micropore) gradually appeared, especially when the GO content increased from 0. 5% to 1. 0%. This means the polymer chain tends to have a much denser packing, at the same time, the partial carbonization or methyl (-CH 3 ) group degradation might occur in the membrane. In this process, the GO sheets help to create small ultrane micropores. Clearly, the density of the other two peaks at 6. 55 and 3. 91 Å increase with increasing the GO content. The ultranemicropores (2. 97 Å) play an important role for gas selectivity, while the micropores (6. 55 Å) and ultra-micropores (3. 91 Å) dominate the gas permeability. Here, the GO nanosheet could regulate the polymer arrangement via chemical bonds, and create many more micro-pores for gas transport. These XRD observations suggest that, the crystallites in carbon membranes Fig. 2 The preparation of polyimide-graphene oxide membranes via in situ polymerization. 

have intermediate structures between graphite and amorphous states, also known as turbostratic structures or random layer lattice graphite structures. 

As presented in Fig. 3 (c), the thermal decomposition behaviors of pristine 6FDA-durene and 6FDA-durene-GO membranes as a function of temperature in a nitrogen atmosphere are shown in Fig. 3(c ). To remove the moisture trapped in the membranes, all samples were dried at 100 °C for 8 h in a vacuum oven before the TGA test. From the TGA curves, it can be clearly seen that both 6FDA-durene and 6FDA-durene-GO membranes could withstand high temperatures, and there were no apparent weight loss until approximately 450 °C. The 6FDAdurene-GO membranes exhibit a slightly higher degradation temperature than the 6FDA-durene membranes. The char yield also increased with increasing the GO loading content from 0. 1% to 1% percent. The improved thermal stability was attributed to the incorporation of GO, which built strong carbon bridges between polyimide chains and strengthen the structural stability of the polymer matrix. The mechanical properties were evaluated by an Instron model 5542 at the speed of 10 mm min -1 with at least ve lm specimens (30 mm long, 5 mm wide, and about 0. 05 mm thick), the results were averaged scan. As shown in Fig. 3(d), the stress-strain curves show that that PI-GO membranes were stronger than PI membranes. The mechanical properties of PI and PI-GO membranes are summarized in Table 1. The PI-GO composite lms about 50 mm thick are exible and exhibit good mechanical properties. The tensile strengths and Young's modulus of PI-GO membranes are a little higher than the original PI, which is strengthen by GO bridge. As shown in Fig. 3 (e), the Raman spectrum of GO exhibited bands at 1350 cm -1 and 1590 cm -1 due to the D and G bands, respectively. The functional groups on GO surface made some defects which led to D-band (disorders of sp 2 network) appearance while G-band represented sp 2bonded carbon vibration. For PI-GO-1. 0%-500 °C carbon membranes, the ratio of peak intensities (I D /I G ) is 1. 8, which is the typical degree of crystallinity (ordered carbon) for CMS membrane derived from precursor polymers. As shown in Fig. 3 (f), the BET specic surface area of the PI-GO-500 °C membrane was 58 m 2 g -1. The pore size distributions that were calculated from the corresponding adsorption isotherms by the NLDFT method, the PI-GO-500 °C membrane exhibited a peak centered at 0. 36 nm. 

As shown in Fig. 4, the photograph, cross section and surface morphology of the PI-GO CMS membranes were summarized and compared. The internal microstructures of PI-GO CMS membranes were revealed by the cross-section SEM micrographs, which changed from smooth to rough with increasing the GO nanosheet contents. There was no apparent aggregation of GO, indicating its good compatibility with the polymer matrix. The top surface SEM images also indicated that the in situ polymerization of GO improved the interface compatibility between the GO nanosheets and polymer matrix.
section_name: Characterization of polymers

section_content: The pure gas permeability and ideal selectivity of 6FDA-durene and 6FDA-durene-GO membranes are summarized in Table 2. When the loading content of GO increased, the permeability of H 2 increased slightly from 756 to 958. The permeability of N 2 increases from 65 to 69, with an increase of only 6%. To clearly elucidate the GO incorporation effect into the polymer matrix, Fig. 5 a Residual weight (%) at 800 °C in nitrogen. b T 5% = 5% weight loss temperature (obtained by TGA at a heating rate of 10 °C min -1 in nitrogen). 

Fig. 4 Photograph, cross section and surface morphology of the PI-GO CMS membranes. 

in Fig. 5, the permeability and selectivity of H 2 shi near to the upper bound lines with a small loading of graphene oxide. 

While the separation performance of 6FDA-durene-GO membranes for almost all of the pure gases could not surpass the 2008 Robeson upper bound line, even though, the permeability of all gases increased with increasing the GO loading content, while the selectivity of H 2 /CO 2, H 2 /N 2, and H 2 /CH 4 just increased a little with the continuous loading of GO. It is because the high-aspect ratio GO nanosheets in polyimide matrix increase the length of the tortuous path of gas diffusion and generate a rigidied interface between the polymer matrix and llers, which reduce the mobility of polymer chains and restrict the diffusion of larger molecules but favor the diffusion of small molecules H 2 with less resistance, thus improving gas diffusivity selectivity. In order to take full advantages of the GO nanosheets, the 6FDA-durene-GO membranes were treated at high temperature to improve the free volume and gas separation performance. The pure gas permeabilities and ideal selectivities of PI-GO membranes carbonized at different temperatures for H 2, CO 2, O 2, N 2 and CH 4 were summarized in Table 2. The gas permeability followed the order of P (H 2 ) > P (CO 2 ) > P (O 2 ) > P (N 2 ) > P (CH 4 ), which was in reverse order of gas molecular size, indicating that the gas permeation of PI-GO CMS membranes was dominated by the molecular sieving mechanism. As the partial carbonization or methyl (-CH 3 ) group degradation occurred, the micropores (6. 55 Å) and ultrane micropores (2. 97 and 3. 91 Å) were generated aer thermal treatment, which were conrmed from the XRD curves. Therefore, the gas permeability improved substantially aer carbonization at high temperature. The H 2 permeability of PI-GO-0. 1%-500 °C was almost 30 times higher than that of the original PI-GO-0. 1% membrane. As the pyrolysis temperature increased, the gas permeabilities of all gases were gradually reduced. When the carbonization temperature was raised from 500 to 600 °C, however, the average reduction rate of gas permeability increased with increasing of gas molecular size, such as H 2 < CO 2 < O 2 < N 2 < CH 4, which resulted in a gradual increase in the ideal selectivities for H 2 /N 2, H 2 /CH 4, and H 2 /CO 2 gas pairs. This result could be attributed to the shrinkage of the pore structure that greatly restricted the diffusion of larger gas molecules. In particular, the PI-GO-1. 0%-600 °C membrane exhibited extraordinarily high H 2 /N 2 (157) and H 2 /CH 4 (243) selectivity, which was related to its interlayer spacing of GO nanosheet being much smaller than the molecular size of N 2 and CH 4, as conrmed by the above XRD results. 

Aer high-temperature treatment over 500 °C, the partial carbonization or methyl (-CH 3 ) group degradation occurred, which was conrmed from the TGA degradation curves. As a result, a lightly cross-linked and rigid polymer networks were generated under the guidance of 2D GO nanosheets. In the high-temperature treatment process, the GO nanosheets would regulate the packing of polymer chain, and manipulate the free volume of 6FDA-durene-GO membranes. This process would generate more micropores (6. 55 Å) and ultrane micropores (2. 97 and 3. 91 Å), which improved the gas permeability and selectivity. The dimensions of the ultrane micropores (2. 97 and 3. 91 Å) could discern the gas kinetic diameters (2. 89-3. 8 Å). As a result, the gas selectivity was mainly determined by ultra-ne micropores, while the gas permeability was largely related to the micropores (6. 55 Å). As presented in Fig. 5, both the gas permeability and selectivity were improved substantially aer thermal treatment, and the separation performance of 6FDAdurene-GO-500 °C for almost all gas pairs could surpass the 2008 Robeson upper bound line. Aer thermal treatment at 500 and 550 °C for 1 h, the selectivity of H 2 /CO 2, H 2 /N 2, and H 2 /CH 4 increased by one order of magnitude. As the carbonization temperature increased, ultrahigh selectivities for H 2 /CO 2 (17), H 2 /N 2 (157), and H 2 /CH 4 (243) gas pairs were achieved while maintaining moderate H 2 gas permeabilities. The excellent gas separation performance of the prepared CMS membranes may be attributed to the guidance of 2D GO nanosheets and the structural evolution during pyrolysis. The major transport pathway for gas molecules might be the structural pores within GO akes, and the gas selectivity was probably due to the interlayer spacing between GO nanosheets. It's sure that the PI-GO based CMS membranes for H 2 /N 2, H 2 /CH 4, and H 2 /CO 2 gas pairs can produce high purity hydrogen, and the high H 2 permeability will achieve competitive separation costs. Therefore, the novel CMS membranes derived from PI-GO polymeric precursor offer great application potential in hydrogen purication.
section_name: Pure gas permeability and selectivity

section_content: To further evaluate the feasibility of PI-GO CMS membranes, the gas separation performance for binary gas mixtures of H 2 /N 2 (50%/50%) was also tested. As presented in Fig. 6, both pure gas and mixed gas separation performance could overcome the 2008 Robeson upper bound line. The H 2 permeability and corresponding selectivity in mixed-gas experiments were lower than those obtained in pure-gas experiments under the same conditions. This phenomenon can be ascribed to the competitive sorption effect of mixed gases on the xed Langmuir sorption sites, and the affinity of CO 2, N 2 and CH 4 for the sorption sites overwhelms H 2. As demonstrated in Fig. 6, the H 2 / CH 4 selectivity under mixed-gas conditions was slightly lower than the ideal selectivity under pure-gas conditions at the feed pressure of 3. 5 atm. This phenomenon was due to a combination of competitive sorption (reduction in H 2 permeability) and plasticization (increase in CH 4 permeability). Similar behavior was also observed in other reported membranes. 47 Surprisingly, the PI-GO-1. 0%-600 °C membrane displayed ultrahigh H 2 /N 2 (157) and H 2 /CH 4 (243) selectivity, which were better than those of state-of-the-art PEI, PPO, PIM-700 °C, TB-800 °C, glucose, PIM-6FDA-OH-630 °C, PIM-6FDA-OH-800 °C, and some polyimide membranes. 

The gas permeability and selectivity are two important parameters used to evaluate membrane separation performance. The gas permeability followed the order of P (H 2 ) > P (CO 2 ) > P (O 2 ) > P (N 2 ) > P (CH 4 ), which was in reverse order of the gas kinetic diameter, indicating that the gas permeation of PI-GO CMS membranes was dominated by the molecularsieving mechanism. The ultrahigh H 2 /N 2 (117) gas selectivity was probably due to the interlayer spacing between GO nanosheets and selective structural pores within PI-GO membrane aer thermal treatment. As presented in Fig. 7, there are more straight and upright pathways aer thermal treatment, compared with the tortuous pathways through parallelstacked GO nanosheets. Before thermal treatment, the H 2 permeability and H 2 /N 2 selectivity of PI-GO-1. 0% were only 958 Barrer and 13. 88, respectively. While, aer thermal treatment, the H 2 permeability of PI-GO-1. 0%-500 °C membrane increased up to 19 368 Barrer, which was approximately 20 times higher than that of the pristine PI-GO-1. 0% membrane. At the same time, the H 2 /N 2 selectivity of PI-GO-1. 0%-500 °C increased to 50. 16, which was 3. 6 times higher than that of the original membrane. The great increase of both permeability and selectivity were due to the pore generation in GO nanosheets and polymer matrix aer thermal treatment. Therefore, the major transport pathways for these gas molecules might be the free volume from the partial degradation of polyimide and selective structural pores within GO akes aer thermal treatment.
section_name: Mixed gas permeability and selectivity

section_content: The gas sorption behaviors of 6FDA-durene and 6FDA-durene-GO membranes were analyzed via the dual-volume pressure decay method using the XEMIS microbalance system. The gas sorption isotherms of the PI-GO and PI-GO-500 °C membranes are presented in Fig. 8. It's clear to see that the gas concentrations to the pressure axis are convex curves, the nonlinear sorption behaviors agree well with the typical trend of polymeric membranes that can be described by the so-called dual sorption model. Henry's law mode believes that the gas absorbed in membrane is similar to gas molecules dissolved in a rubbery polymer where the sorption concentration linearly increases with increasing the pressure. While the Langmuir mode considers that the gas sorption in membrane has a capacity limit because it behaves like a hole-lling process. 48, 49 herefore, the gas sorption behaviors of PI and PI-GO membranes follow the dual sorption mode (Henry's law mode and Langmuir mode). So as can be clearly seen from Fig. 8, the gas concentrations increase nonlinearly with increasing the gas feeding pressure up to 10 bar. For the same gas, the gas concentration in different membranes follows the order of PI-GO-1. 0% > PI-GO-0. 5% > PI-GO-0. 1% > PI, since membranes with higher content of GO, which can effectively restructure the architecture, have more micropores to capture gas molecules. Aer thermal treatment, the gas concentration increases considerably and follows the order of PI-GO-1. 0%-500 °C > PI-GO-0. 5%-500 °C > PI-GO-0. 1%-500 °C > PI, the degradation of sensitive groups on GO nanosheets could create more micropores to hold gas molecules. The gas concentration increases with increasing temperature and follows the order of PI-GO-1. 0%-600 °C > PI-GO-0. 5%-550 °C > PI-GO-0. 1%-500 °C, more micropores would be created at higher temperature. While for the same membrane, the H 2 has the highest sorption capability, the concentrations of different gases follow the order of H 2 > CO 2 > CH 4 > O 2 > N 2, that's probably because the graphite structure from the decomposition of polymer and GO have a strong affinity with H 2 gas molecules. 

The solubility coefficient could be calculated from this equation S = c/p, and is in the form of a secant slope from the sorption isotherm at a certain pressure. As observed in Fig. 8, the solubility coefficients increase with increasing the GO content. As the temperature increases, the solubility coefficients increase especially for H 2 gas, which is approximately 5 times higher than the N 2, CO 2, and CH 4. The corresponding solubility coefficients and diffusion coefficients at 3. 5 atm are listed in Table 3. Compared with the pristine PI membrane, the incorporation of GO into PI polymer matrix improves the solubility coefficients, and aer thermal treatment, the PI-GO membranes have improved solubility coefficients and diffusion coefficients simultaneously. Thus, the gas permeability of pristine PI-GO membrane increases with increasing the GO content, and the gas selectivity of the thermally treated PI-GO membrane increases with increasing the temperature from 500 °C to 600 °C. As the carbonization temperature increased, the gas sorption capability of CMS membranes increased rapidly and followed the order of PI-GO-1. 0%-600 °C > PI-GO- 1. 0%-550 °C > PI-GO-1. 0%-500 °C. The H 2 molecules had better interactions with the ordered graphite structure than with the turbostratic polymeric structure.
section_name: Gas sorption

section_content: Graphene oxide tuned polyimide hydrogen purication membranes were prepared from in situ polymerization. The ultraselective CMS membranes for H 2 /N 2, H 2 /CH 4 and H 2 /CO 2 binary gas were prepared from the PI-GO precursor at carbonization temperatures of 500-600 °C. Compared with the original PI-GO membranes, both the H 2 /N 2, H 2 /CH 4, H 2 /CO 2 selectivity and H 2 permeability greatly increased aer thermal treatment at high temperature. The gas permeability followed the order of P (H 2 ) > P (CO 2 ) > P (O 2 ) > P (N 2 ) > P (CH 4 ), which was in reverse order of gas molecular size, indicating that the gas permeation of PI-GO CMS membranes was dominated by the molecular sieving mechanism. In the thermal carbonization process, the turbostratic polymeric structure gradually became denser and more ordered carbon structure under the guidance of 2D GO nanosheets. More ultrane-micropores (2. 97 Å) were created for improved H 2 selectivity, while the micropores (6. 55 Å) and ultramicropores (3. 91 Å) dominated the gas permeability. As the carbonization temperature increased, the gas sorption capability of CMS membranes increased and followed the order of PI-GO-1. 0%-600 °C > PI-GO-1. 0%-550 °C > PI-GO-1. 0%-500 °C, and the H 2 molecules had better interaction with the ordered graphite structure than the turbostratic polymeric structure. Therefore, the H 2 /N 2, H 2 /CH 4, and H 2 /CO 2 selectivity increased owing to the substantial enhancement of diffusion and solubility selectivity. The PI-GO-1. 0%-550 °C membrane exhibited an ultrahigh H 2 /N 2 selectivity of 117 and H 2 permeability of 7462 Barrer. The CMS membranes with ultrahigh gas selectivity and permeability have bright application prospects in hydrogen purication.
section_name: Conclusions

section_name: Acknowledgements

section_content: The authors would like to thank the nancial support from the National Key Research and Development Program of China ( 2022YFB3805200, 2019YFA0210000 ), the National Natural Science Foundation of China ( 12275270, 11975225, 12175232 ), the Young Talent Program ( KY2400000020 ), the Double First-Class Initiative University of Science and Technology of China ( YD2400002003, KY2400000004 ) and the Fundamental Research Funds for the Central Universities ( WK2400000004, KY2400000008 ). Special thanks are given to Professor Hanqing Yu, Zhouqing Xie, Guoping Sheng, Yang Mu, Xianwei Liu, Wenwei Li, and Jiejie Chen for their valuable assistance and fruitful discussions in the research process.
section_name: Acknowledgements

section_name: Conflicts of interest

section_content: There are no conicts to declare.
section_name: Conflicts of interest