Three-phase Hybrid Facilitated Transport Hollow Fiber Membranes for Enhanced CO2 Separation

ContentslistsavailableatScienceDirect

Applied Materials Today

journalhomepage:www.elsevier.com/locate/apmt

Three-phase hybrid facilitated transport hollow fiber membranes for enhanced CO ₂ separation

Saravanan Janakiram

, Juan Luis Martín Espejo

, Karen Karolina Høisæter

, Arne Lindbråthen

, Luca Ansaloni

, Liyuan Deng

aDepartment of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Trondheim, NO-7491, Norway

bDepartment of Sustainable Energy Technology, SINTEF Industry, 0373 Oslo, Norway

a rt i c l e i n f o

Article history:

Received 2 July 2020 Revised 13 August 2020 Accepted 17 August 2020

Keywords:

Facilitated transport CO 2separation Hollow fiber membrane Graphene oxide Mobile carriers

a b s t r a c t

Theconfigurationofthinfilmcomposite(TFC)intheformofhollowfiberisdesiredforgasseparation membranestoachieve bettergaspermeationand higher packingdensity.Inthiswork, wedeveloped andtestedTFChollowfibermembraneswithadefect-free,ultrathin(200nm)hybridfacilitatedtrans- portselectivelayerconsistingofthreephases,i.e.,ahostpolymericmatrixwithfixed-sitecarriers,a2D inorganicfiller,and,aCO2-philicmobilecarrier.Theeffectoflateralsizeofgrapheneoxide(GO)-based fillersonCO₂permeationwerestudiedindetail,andthemodifiedsize-optimizedporousGO(pGO)fillers werefoundto enhanceCO₂ permeationataverylow loading of0.2wt%. The optimizedhybrid ma- terialswerethencombined withselectedmobile carriers,whichinteract withCO2 reversiblytoform carbonate/carbene-CO₂adducttofurtherenhancetheCO₂permeationperformance.Theresultinghybrid facilitatedtransportmembraneswithmobilecarriersshowcaseaCO2 permeanceofupto825GPUwith aCO2/N2separationfactorof31andaCO2/CH4of20.Thesemembranesalsoexhibitincreasedresistance tocarriersaturationphenomenatypicaloffacilitated transportmembranes, showingpotential forCO₂ separationapplicationsalsoatelevatedpressures.

© 2020 The Authors. Published by Elsevier Ltd.

ThisisanopenaccessarticleundertheCCBYlicense.(http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Membranesofferapotentialsolutionforapplicationsrelatedto CO₂ separationduetotheir highmodularity,lower footprint,eas- ier up-scaling, and lower environmental impact when compared toconventionalamine-basedabsorptionsystems[1–4].Inorderto be competitive and industrially attractive for gas separation ap- plications, membranes should be characterized withhighperme- ability and selectivity. Traditional polymeric membrane materials sufferfromaninherent permeability-selectivity“trade-off”,which limitstheseparationperformance,reducingthecommercialization potential of membranetechnology despite thelow-cost and easy scalabilitybenefits[5,6].

Research efforts on developing high performance membrane materials to overcome the trade-off are mainly focused on in- creasing permeability andselectivity through various approaches [7–11],suchasengineeringhybridmaterialstocombinetheadvan- tagesofinorganicnanofillersandincorporatingCO₂ reactivecarri- ersinpolymermatricesforfacilitatedtransport ofCO₂.Nonethe-

∗ Corresponding author

E-mail address: [email protected] (L. Deng).

less, these developed materials have usually been reported as self-standingthick films, typically in the orderof microns. How- ever,thesuccessfuldevelopmentofmembranematerialsisbench- marked with improvements in transmembrane CO₂ flux (perme- ance), which reduces the effective membrane area required to achievetargetedseparation[12–14].Suchmembranesare,ingen- eral,fabricated in the form ofthin film composite (TFC) as flat- sheetorhollowfibermembraneswithastable,selectivelayertyp- icallywithathicknessofafewhundrednanometers[15].

Typical challenges accompanying the fabrication of ultrathin membranes include retention of the permeation properties of the material that is previously developed and evaluated as self- standingthickfilms.Additionally,whentheselectivelayeriscom- posedofahybridmaterial,thepresenceoftwodistinctphasesin- creases the complexity ofachieving defect-free coating, while in hollowfiberconfiguration,thecoatingprocedureneedsfurtherop- timizationduetotheircurvedtopology[16].

Limitedstudieshavebeenreportedonfabricatinghollowfiber thincomposite membranes witha hybrid selective layer forCO₂ separation applications. Dai et al. [17] have embedded ZIF-8 nanofillersof200nmsizeinUltem® 1000(polyetherimide)poly- mer, resulting in an asymmetric hollow fiber membrane with a https://doi.org/10.1016/j.apmt.2020.100801

2352-9407/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license. ( http://creativecommons.org/licenses/by/4.0/ )

CO₂permeanceof30GPUwithCO₂/N₂separationfactorof25un- der mixedgas conditions. Smaller sized ZIFs (less than 100 nm) were dispersed in Pebaxpolymer by Sutrisana et al. [18]in the formofhollowfibers toincrease theplasticizationresistance ob- taining stable CO₂ permeance of 350 GPU with the correspond- ingCO₂/N₂ selectivityof32.Zhangetal.[19]reportedaCO₂ per- meance of 415 GPU with CO₂/N₂ separation factor of 43 using grapheneoxide(GO)inPebaxcoatedonPVDFhollowfibersalbeit withcharacterizationonlybysinglegastests.

There exist a variety of nanofillers that enhance CO₂ perme- ation properties in conventional polymeric membranes. Both 1D nanofillers like nanocellulose [20,21] and 2D materials like GO [22]havebeensuccessfullyusedinpolymericmatricesintheform ofhybridmembranesforCO₂ separation,exploitingtheir highas- pect ratio geometries [1,8]. Nevertheless, polymeric membranes containingsuchfillersareseldomstudiedashybridmembranesin theformofthincompositehollowfiberswithultra-thinselective layers.Hollowfibermembranesareparticularlyinterestingforgas separationduetotheir maximumachievable packingdensity.The highsurface-to-volumeratioarisingfromthe2Dstructuretriggers nanoscopicpropertychangeswhen dispersedinpolymeric matri- ces.Effectivedispersionoftheseplatelets,evenundersmallquan- tities,affects reorientingof the polymer chain packing, inducing changesin crystallinity,fractional free volumeand CO₂ solubility [23]. Theuse ofa very smallloading ofthese 2D fillers helpsin coatingdefect-freehybridselectivelayersforTFCmembranes,but their highaspectratio can still posea considerablechallenge for hollowfiberconfiguration.

Another approach to overcome the permeability-selectivity

“trade-off” is by introducing CO₂-reactive carriers into the poly- meric selective layer. The so-called “facilitated transport” mem- branes transfer CO₂ through an additional reactive pathway compared with conventional polymers that follow the solution- diffusionmechanismonly [24]. The reactive carriersare typically amine groups fixed to the backbone of the polymeric chain. Fa- cilitatedtransportisfoundbeneficial inlow-pressureapplications likepost-combustioncapture[9],asthereactivesitesbecomesat- uratedathigherCO₂ partialpressures inthefeed[25].Thisphe- nomenon,known as“carriersaturation”, leadsto an eventual re- ductionin permeationperformance of facilitatedtransport mem- branesforapplicationsthatinvolveelevatedfeedpressures.Small CO₂-philicmoleculesthatreversibly reactwithCO₂ canbe added to the polymeric host matrices as “mobile carriers” to increase CO₂transport[26].Thelowmolecularweightofthesecompounds leadstoan increase inthe densityofCO₂-reactive groupsin the membranematrix and can help in combating the carriersatura- tionphenomena. Nonetheless, thereactions withcarriers require watertoaidCO₂ transportacrossthemembrane.Conveniently,GO nanosheetsbeinghydrophilic,inducethepolymerchaindisruption whenaddedtofacilitatedtransportmatrices,leadingtodistributed water-richchannelsandincreasedCO₂ solubility.Thesesynergistic effectsofGOandmobilecarriersonthegasseparationproperties ofsuchhybridmembranesrelyhighlyonthedistributionandsur- facechemistryoftheaddednanosheetsaswellastheirlateraldi- mensionsinadditiontothepropertiesofthemobilecarrier.

In this work, we report the first three-phase hybrid facili- tated transport membranes (HFTMs) in TFC hollow fiber config- uration with outstanding separation performance. Sterically hin- dered polyallylamine (SHPAA) was used as the facilitated trans- portpolymermatrixandGO-based2Dnanoplateletswereembed- dedasnanofillers,whichwerebothsize-optimizedandphysically modifiedtofavourgastransportpropertiesupondispersioninthe membrane matrix. Water-soluble mobile carrier phases compris- ing of CO₂-reactive smallmolecules were optimized andstudied for their interaction with CO₂. The optimal content of the mo- bilecarriersinthepolymericmatrixtobenefitCO₂transportwere

Scheme 1. Steric hindrance of polyallylamine.

determined. Ultrathin selective layers less than 200 nm were coated as hollow fiber membranes. The fabricated membranes were tested for CO₂/N₂ separation at 1.7 bar and CO₂/CH₄ sep- aration at elevated pressures (up to 20 bar) to simulateflue gas andbiogasconditions,respectively.Theeffectoffeedpressureon CO₂/CH₄separationpropertieswasalsostudied.

2. Materialsandmethods 2.1. Materials

Poly(allylamine hydrochloride) (Mw = 120,000–200,000) was bought from Thermo Fisher Scientific, Sweden. 2-bromobutane (≥98%, Mw = 137.02) purchased from Sigma-Aldrich, Norway wasused forthe synthesis ofSHPAA. GrapheneOxide dispersion (2.5wt%inwater)wassuppliedbyGraphene-XT,Italyandusedas diluteddispersions. Hydrogenperoxide (H₂O₂,30%inwater)used in the synthesis of porous Graphene Oxide (pGO) was supplied by Sigma-Aldrich, Norway. L-proline Reagentplus® (≥99 wt%), 1- (2-Aminoethyl)piperazine (99wt%), 1-Ethyl-3-methylimidazolium acetate (97 wt%) and sarcosine (N-Methylglycine) (98 wt%) were purchasedfromSigma-Aldrich,Norway.Potassiumhydroxide(pel- lets, 99.9%), polyvinyl alcohol (Mw = 89,000–98,000, 89% hy- drolyzed), Deuterium oxide (99.9%), 3-(Trimethylsilyl) propionic- 2,2,3,3-d₄ acid sodium salt (TMSP, 98%) were used as received from Sigma-Aldrich, Norway. Poly(p-phenylene oxide) (PPO) hol- low fibers used as coating supports (inside diameter of 350 μm andoutsidediameter of540μm) were obtained fromParker A/S Norway. CO₂/N₂ mixture (10 vol.% CO₂ in N₂), CO₂/CH₄ mixture (40 vol.%CO₂ inCH₄), andCH₄ (99.95%),that wereused forper- meationtestsweresuppliedbyAGA,Norway.

2.2. Methods

2.2.1. Synthesisofstericallyhinderedpolyallylamine

Polyallylamine hydrochloride was purified by reacting with equivalentamounts ofKOH inMeOH, precipitatingKClin a one- step process. Subsequently, purifiedPAA wasmodified intopoly- N-isobutyl allyl amine (SHPAA) by reaction with an equivalent amountof2-bromobutaneandKOHinMeOHat50°C(Scheme1).

Theresultingpolymer waspurifiedbyseparatingprecipitatedKCl crystalsfollowedbydryinginaN₂ atmosphereat60°C.

2.2.2. Synthesisofnanofillers

The GO dispersion as received wasfirst diluted to 1 mg g⁻¹ solution followed by pH adjustment to 10 using 1M NaOH. The diluted solution wassonicated ina bath sonicator for30 minat 25 °C. The dispersion is then subject to ultrasonic disintegration (Vibra-Cell^TM UltrasonicLiquid Processor) atanamplitudeof 60%

inanicebathwitha3-spulsefollowedbya2-sbreak.Thispro- cedurewascarriedouttosimultaneouslyexfoliateandcontrolthe size ofGO flakesby varying thetime ofoperation.In thisstudy, the sonicationwas carriedout for3,6 or 9h, andthe resulting GO flakes were referred to as GO3, GO6, and GO9, respectively.

Furthermore, thesesize-controlled GO dispersionswere then hy- drothermally treated to introduce random non-selective pores as describedbyLeeetal.andXuetal.[27,28].TheGOdispersionwas

mixedwith3wt%H₂O₂solution,andthemixturewasstirredvig- orouslyfor10minfollowedbybathsonicationfor10min.There- upon,themixtureistreatedinaTeflonautoclavefor6hat180°C.

The resulting pGO dispersions derived from GO3, GO6 and GO9 sampleswerenamedaspGO3,pGO6andpGO9,respectively.

2.2.3. Synthesisofmobilecarriers

Mobile carriers considered in this work are two amino acid saltsandanionicliquid.Equivalentamountsof

ι

were dissolved in DI water to form a solution of 10 wt% total solids.Thesolutionwasthenstirredathighspeed (~800rpm)at roomtemperaturefor12htoformpotassiumL-prolinate(ProK).

Similarly, equivalent amounts of 1-(2-Aminoethyl) piperazine andsarcosine were stirredin calculatedquantities ofDIwater at room temperature to obtain 37.7 wt% of 2-(1-piperazinyl) ethy- laminesarcosinate(PZEA-SARC).

1-Ethyl-3-methylimidazolium acetate ([Emim][OAc]) was dis- solvedinDIwatertoforma10wt%solutionandstirredovernight atroomtemperature.

2.2.4. Coatingofhollowfibermembranes

Purified anddried SHPAA after modification was dissolved in DIwatertoobtaina6wt%solution,andthepolymersolutionwas stirredfor atleast 2days atroom temperatureto obtain a clear polymer solution. Inthe case ofPVA, a 4wt% solution waspre- paredby dissolving PVApelletsin DIwater at80°C for4h un- der reflux conditions.Apolymer blend ofSHPAA andPVAinthe weightratioof9:1wasusedinallthemembranes.PVAwasadded duetoitsexcellentfilm-formingabilities.

Forcasting solution preparation, calculatedquantities ofpoly- mer solutions were added to DI water and diluted to a casting solution concentration of0.15 wt%polymer. The amounts ofmo- bile carriers were reportedasthe composition ofmobile carriers inthetotalorganiccontent(polymer+mobile carrier),whilethe amounts ofnanofillers, which are considered asadditives to the organic matrix,were reportedas their loadings in terms oftotal organicsolidcontent(includingthepolymerandmobilecarriers), asdescribedinEqs.(1)and(2),respectively.

cmc=100 ×

w_pol+wmc

(1)

c_{n f} = 100×

(2)

where w_mc is the weight of mobile carrier (g), w_pol is the total weight ofthedrypolymer(g),andw_nfis theweightofnanofiller (g).cmcisthecomposition ofmobilecarrierstotheorganicphase (wt%),andc_nfistheloadingofthenanofiller(wt%).

For facilitatedtransport membranes withmobile carriers, cal- culatedquantitiesofthemobilecarriersolutionswerefirstdiluted with waterfollowed by drop-wise addition ofpolymer solutions, so that the overall solid content (polymer + mobile carrier) re- mainedat0.15wt%.ForHFTMs,bothGOandpGOnanofillerswere dispersed inSHPAA/PVAsolutionattwofillerloadings of0.2wt%

and0.5wt%. ForHFTMswithmobile carriers,leandispersionsof nanofillerswere initiallyprepared,followedbydrop-wiseaddition of both mobile carriersolutions andsubsequently polymer solu- tionsincalculatedquantitates.

PPOhollowfibermembraneswere usedassubstratestofabri- catehollowfibermembranes.ThechosenPPOsupportshaveathin unselective skinlayerthat helpsin coatingthe ultrathinselective layerwithlowmolecularweightadditives.Theskinlayeralsopre- ventsporepenetration andactsasadditionalmechanicalsupport to theselective layer,especially athigh pressurefeedconditions.

The PPO supports were hung vertically withthe ends sealed us- ing paperclips, which alsocreates tensionand avoidsslackening

offibers. DI waterwas used towash the fibers twice toremove possibledust particlessticking tothe surface,followed by drying inairatroomtemperature. Thethinselectivelayerwasachieved bydipcoating thefibersusingthecastingsolutioninbothdirec- tions ata constantlow speed (in the rangeof6–8 cms⁻¹) with atime intervalof30 minbetweensuccessivecoatingprocedures.

Coating in opposite directions ensures defect-free selectivelayer.

Additionally,theleanviscosityofcastingsolutionowingtothelow solidcontentleadstouniformityofselectivelayerthicknessinde- pendentofcoating speedandfiller loading.Thefibers werethen driedat room temperaturefirst,followed by dryingat60 °Cun- der vacuumfor 2h to remove residual solvent components.The resultingfibersexhibitashinyappearanceduetothepresenceof ultrathinselectivelayercoating.

Inorderto assemblethe coatedhollowfiber(HF)membranes into a module, a few fibers (in the range of 2–5) were inserted carefullyintoa pre-assembledstainless-steelHFmoduledesigned using ¼ inch ⅜ -inch Swagelok^TM fittings. The ends were then sealed using epoxy adhesive. The bore side of the fibers was openedbybreakingoff thecuredadhesiveonanextensionmould.

2.3.Materialandmembranecharacterization

2.3.1. Nanofillercharacterization

Fourier-transforminfrared(FTIR)spectroscopywasusedtode- termine changes in the chemical structure of the GO nanofillers duringtheultrasoundtreatment.ThiswasdonebyusingaThermo NicoletNexusspectrometerequippedwithasmartendurance re- flectioncellwithadiamondcrystalinattenuatedtotalreflectance mode. The spectra were built averaging 16 scans with a resolu- tionof 4cm⁻¹ betweenthe rangeof 4000 cm⁻¹ and 800cm⁻¹. ThesameprocedurewasusedtostudytheinteractionofCO₂with PZEA-Sarc,asthe loaded solutionformed solid precipitates.A 30 wt% solution of PZEA-Sarc was used for the studies. For obtain- ingspectraofnanofillers,5– 10mLof1mgg⁻¹ dispersionsofthe nanofillersweredriedonamicroscopeglassslideintheventilated ovenat60°C.Acleanslidewasusedforobtainingthebackground spectra,followedbyanalysiswiththeslides containingdeposited nanofillers.

Renishaw InVia Reflex Spectrometer systemwas used for ob- tainingRamanSpectraofGOnanofillersusing532nmwavelength incidentlaserintherangeof3200cm⁻¹ and100cm⁻¹.Laserin- tensitywasvaried between5–20% based onthe response by the samples to signal. The samples were prepared by drying a few drops ofGO/pGO dispersionson a microscopic slidefollowed by dryingintheventilatedovenat60°C.

The GO and pGO nanosheets were imaged using a Scanning TransmissionElectronMicroscope(S(T)EM,HitachiS-5500,Hitachi High Technologies America, Inc., USA). The bright field detector wasusedfortransmissionmeasurements. The sampleswere pre- paredbydispersinga dropofdilutedispersionson 300meshCu grids(ElectronMicroscopySciences,FCF300-Cu).

Topographicanalysis ofnanofillers wasdone using an Atomic Force Microscope (AFM)(AFM DimensionIcon, Bruker, USA).The sampleswere preparedby dryinga dropofdispersion onfreshly cleavedMicasheetsfollowedbydryingatroomtemperature.The imagingwasdoneinScanAsystQNMtappingmodeusingasilicon nitridetip.

2.3.2. Characterizationofmobilecarriers

Forthe thermal stabilityevaluation of mobile carriers loaded inthepolymermatrices,filmswithvariousmobilecarriercontent were madeand testedin a thermogravimetric analyser (TGA, TG 209F1Libra,Netzsch, Germany).About 10–15mgofthesamples wereloadedinaceramiccrucibleandheatedataconstantrateof 20Kmin⁻¹ underN₂ atmosphere(purge rate60mlmin⁻¹)from

roomtemperatureto 800 °C. The correspondingchanges in mass wererecordedasafunctionoftemperature.

ToidentifytheinteractionsofCO₂ withmobilecarriers,liquid- state NMR was used. The NMR experiments were performedon a Bruker 600 MHz Avance III HD spectrometer equippedwith a 5 mm cryogenic CP-TCI z-gradient probe. The obtained spectra wereanalysed in thesoftware BrukerTopSpin 4.0.7. The samples forNMR were preparedby bubblingCO₂ atroom temperaturein mobilecarriersolutions (65%[Emim][OAc]inD₂O/H₂O;10%ProK inH₂O)foraperiodof24h–48h.Deuterated waterwasusedas the “lock” solvent, and TMSP was used as an internal reference standard. The loaded ProK solution was placed in an NMR tube, andthelock solventwas placedin an inserted coaxialinsert. To beabletolock[Emim][OAc]containingsamples,thisconfiguration wasreversed.The loaded solutionwasplaced ina coaxialinsert, whichwasthen placedinsidean NMRtube filledwiththe“lock”

solvent.

2.3.3. Membranecharacterization

FieldEmissionSEMAPREO (FEI,ThermoFisherScientific,USA) equippedwith an in-lens detector under both standard and im- mersionmodewasusedforstructuralandmorphologicalanalysis ofthemembranes.Samplesforcross-sectionalanalysisofHFwere obtainedbyfreeze-fracturinginliquidN₂.Allsampleswerecoated withafewnanometersofPd/Ptalloypriortomeasurementstoin- creasethesampleconductivity.

2.3.4. Gaspermeationperformance

The fabricated HF membranes were evaluated for gas perme- ation performance using humid mixed gas permeation test rigs, asreportedinourpreviousstudies [20,21].Thefeedcomposedof 90/10v/vCO₂/N₂ mixtureor40/60v/vCO₂/CH₄mixture.Theflow rateofthefeedwas300mlmin⁻¹fortheCO₂/N₂ testsand400–

600ml min⁻¹ forthe CO₂/CH₄ tests. The differencein feedflow rateswasmainlytorecoupdifferencesinmembraneareasandtar- getingaverylowstagecutofbelow5%.ThesweepgasforCO₂/N₂ testswas CH₄, while forthe CO₂/CH₄ tests, N₂ was used as the sweepgas.Inbothcases,feedandthesweepgasstreamswerehu- midifiedinabubbletankbeforethemembranemodule.Theshell sideofthemembraneswasusedforthefeedgasandtheboreside ofthefibers wasused asthepermeate/sweep side. Thepressure onthefeedsidewasmaintainedconstantat1.7barfortheCO₂/N₂ testsandvariedbetween2to20barfortheCO₂/CH₄ tests.Sweep sidepressure washeld at1.02 bar. The temperatureofoperation wasmaintained at 35°C for all tests. The exit gas compositions inbothfeedandsweepsideweremonitoredcontinuouslyusinga calibratedgas chromatograph(490 Micro GC, Agilent, forCO₂/N₂ testsandMG5, SRI InstrumentsInc., forCO₂/CH₄ tests).The per- meanceofcomponent ‘i’wasobtained usingthe followingequa- tion

Pi=

(

)

<p_i,f,pi,r>−pi,p

where the total permeate flow Vp is in ml s⁻¹ measured at the exit using a bubble flow meter at steady-state conditions.

y_H2O and y_i denote the molar fraction of the water and per- meating species in the permeate flow, respectively. Partial pres- sures p_i,f, p_i,r and p_i,p of the species ‘i’ in the feed, retentate, and permeate, respectively, are in cm Hg⁻¹. p_i,f, p_i,r is the av- erage of feed andretentate partial pressure. Permeance of com- ponents are represented in GPU, where 1 GPU = 10⁻⁶ cm³(STP) cm⁻² s⁻¹ cmHg⁻¹= 3.35 × 10⁻¹⁰ mol m⁻² s⁻¹ Pa⁻¹. The sepa- rationfactoriscalculatedusingconcentrationsofeachcomponent accordingtotheequation

α

y_j/x_j (4)

Fig. 1. S(T)EM imaging of (A) GO3 and (B) pGO3.

wherey andx identifythegascontentinthe permeateandfeed side, respectively. The experiments have all been performedat a lowstagecut(<5%)topreventconcentrationpolarizationphenom- ena,whichreducestheinfluenceofthemodule performanceand allowsretrievalofthematerialperformance.

3. Resultsanddiscussion

3.1. CharacterizationofGO-basednanoplatelets

3.1.1. S(T)EM

Theformationofnon-selectiveporesinGOnanosheetsthrough thehydrothermaltreatmentisconfirmedbyrepresentativeS(T)EM imagingofGO3andpGO3asseeninFig1.The2DGOnanosheets seem stacked into a few layers similar to the previous reports intheliterature [29,30] Themorphology ofrandomnon-selective pores shown in Fig. 1B is also confirmed by previous studies [27,28].

3.1.2. AFM

Thesonication-assisted exfoliationprocedurewasemployed to obtain monolayers of GOin water dispersion. Inorder to ensure the reproducibility of the method, the concentration of GO dis- persion was kept constant at 2 mg mL⁻¹, and the sample vol- umewasmaintained at300 mLfor all procedures.In thisstudy, three sonicationdurations were chosen to simultaneouslyreduce thelateral dimensionsduringthe exfoliationprocess. Thesonica- tion process impartsrandom fragmentationof2D nanosheets in- duced by mechanical failure of defective sp³ regions [31]. These random lacerations are followed by propagation of cracks lead- ing to reduced flake sizes. Several studies have reported the ef- fectofsonicationtimeonthelateraldimensionsofGOnanosheets [23,32–34]. Typical studies involve AFM and TEM to analyse the distribution of flake dimensions and aspect ratios. In this study, AFManalysisrevealedthepresenceoflargeflakeswithlateraldi- mensionsof morethan 1μm forGO3.Subsequent sonicationre- sulted in smaller flakes in the range of 400–800 nm and down to less than 500 nm for GO6 and GO9, respectively (Fig. 2A–C).

Allsampleswerethensubjecttohydrothermaltreatmenttointro- ducerandompores.WhilesizeestimationofpGOflakeswithAFM waschallenging dueto their morphology,representative imaging of pGO flakes (pGO6, Fig. 2D) show a further reduction in flake sizeafterthehydrothermaltreatment.Thissizereductionwascon- firmedwithrelative increase inpresence ofcarbonyl groups(ob- served fromFTIR) that wereexposed along theedges ofthepGO whencomparedtoGO.

3.1.3. FTIR

Chemical changes in the GO nanoplatelets during the hy- drothermaltreatmentprocesswasstudiedusingFTIRspectroscopy.

The sonication procedures barely affected the chemical structure oftheGOandpGOnanoplateletsrenderingsimilarspectraforthe

Fig. 2. AFM imaging of representative GO nanosheets used for HF membranes of (A) GO3 (B) GO6 (C) GO9 and (D) pGO6.

Fig. 3. FTIR spectra of pre-dried GO6 and pGO6.

family of GO andpGO individually (Figure S1). Previous studies report similar resultson the chemicalcomposition of 2D materi- alsunalteredwiththeultrasonicfragmentationprocess[32].How- ever,discerniblepeakchangesappearedbetweentheGOandpGO as seenin Fig. 3.Characteristic GO peaks include-C-O-C-alkoxy stretching at 1050 cm⁻¹ andthe corresponding epoxy stretching vibrationat1250cm⁻¹.Thepeakat1412cm⁻¹ isassignedtothe deformation of surface –OH groups [35]. Since the modification processinvolvesoxidationusinghydrogenperoxide,C=Ocarbonyl

Fig. 4. Raman Spectra at room temperature of GO6 and pGO6 on a glass slide.

stretchwasobserved at1723 cm⁻¹,whiletheotheroxygen func- tionalgroupswerepreserved[36].Interestingly,C=Cstretchingvi- brations were confirmed at1680 cm⁻¹. These transpolyacetylene segments indicate that the pGO modificationprocedure also cre- atedstabilizedendgroupsatgrainboundaries.

3.1.4. Ramanspectroscopy

Raman spectra of thin films of GO and pGO deposited on a glass slidewere obtained to characterize the structure andqual- ityofvariousGOandpGOnanofillers.Fig.4comparestheRaman

Fig. 5. SEM imaging of neat SHPAA/PVA membrane; the freeze-fractured hollow fiber (A) and cross-section (B).

spectra of representative GO6 and pGO6. The spectrum of GO6 clearlymarks4 activebands: -D band at~1340 cm⁻¹ indicating disordercausedbygraphiteedges;Gband at~1570cm⁻¹ indicat- ingin-phasevibrationofsp² hybridizedgraphitelattice [37];two other weak bands(2D and2D’) associated to2D, whichis a dis- persiveovertoneofD band that also signifies thelayering of the GOnanosheets.Thesplittingoftheoriginal2Dbandintooverlap- ping2Dand2D’peaksdenotethepresenceoffew-layergraphene planes, confirming successful exfoliation of platelets both in GO andpGO[38].

Two interesting signature peaks around ~1130 cm⁻¹ and

~1720cm⁻¹appearedforallpGOnanofillers.Thelatterisassigned as D’ band, which is a result of double resonance Raman pro- cess indicative of defective structure [39,40]. The intensity of D’

bandrelativetootherbandsishigherinallpGOsamplesthanGO nanofillers(FigureS2),representingincreasedbasal planedefects inlinewiththeexpectations.Thebandat1130cm⁻¹ hasbeensel- domreportedingraphene.Itcanbeassignedtotranspolyacetylene speciesatgrain boundariesandsurfaces [41]andalsoassociated to the presence of holes created in the GO flakes [42] after the hydrothermaltreatment. The presence oftranspolyacetylene seg- ments in pGO is alsoconfirmed by the FTIR peak at 1680 cm⁻¹ assignedto C=C stretching frequency(Fig.3). Another important characterizationofGOqualityintermsofdisorderlinessattributed tosp³inpredominantlysp²hybridizationisstudiedusingtheI_D/I_G ratio.The ratioremained similar at~1 for all GO andpGO sam- ples,denoting no obviousreduction during thesonication orthe hydrothermaltreatmentprocess(FigureS2)[43].

3.2.Morphologyofhybridhollowfibermembranes

Figs. 5 and6 show the SEM imagingofthe cross-section and the outer surface of the hollow fibers coated with the different castingsolutions. Allmobilecarriersformedsmooth topologiesin theconcentrationsstudied inlinewithobservationsmadein our previousstudy[26].The totalsolid contentinthecasting solution wasmaintainedlow(0.15wt%),whichresultedinanultrathinse- lectivelayer thicknessof ~200 nm on the skinlayer ofPPO hol- lowfibers,asseeninFig.5B.Thefaciledip-coatingprocedurealso ensuresthein-planealignmentofGOduetoshearalignment[19]. Theneatpolymer membraneanda representativepolymermem- brane containing 20% Pro-K showcased a rather smooth topog- raphy as seen in Figs. 5A and 6A, B. Alternatively, the surfaces offabricatedmembranesalsoexhibiteddiscerniblecircumferential spotsinbothGOandpGOcontainingmembranesarisingfromthe boundariesoftheunderlyingplatelets(Figs.6Cand7D).

3.3. Effectofmobilecarriersinfacilitatedtransportmembranes

In facilitated transport membranes, CO₂ permeates through a reactive pathwayin additionto the solutionanddiffusion mech- anism [20,44].Theamine groupsattachedtothe backboneofthe polymericmatrixreversiblyreactwithCO₂ inthepresenceofwa- tertotransportCO₂acrossthemembrane.Althoughpolymerslike polyvinylamineandpolyallylaminecontainahighdensityofamine groupsrelativetothehydrocarboncontentintherepeatingunitof thepolymer,theircontributiontoincreasedCO₂transportrelieson theaccesstotheaminegroupsforCO₂ andtheproximitytoform continuous channels for CO₂ reaction andtransfer. In this work, stericallyhinderedpolyallylaminepolymerwaschosen toincrease the accessibility of amine groups to CO₂ [45]. Nevertheless, the amine groups are still locked in themain chain, restrictingtheir mobilityin the water-swollenmatrix. Hence, toincrease thedif- fusivityofthesereactivespecies,CO₂-philiccompoundsareadded inthematrixasmobilecarriers[26].Theadditionofthesemobile carriers not only increases the density of CO₂-philic moieties in thepolymermatrixbutalsoenhancesthemobilityofCO₂-reacted species,thusincreasing CO₂ diffusivityinthehostmatrix.Impor- tantcharacteristicsofsuchmobilecarriersforbeneficialuseinfa- cilitated transport membranes include (1) low molecular weight (highermobility),(2)highCO₂ uptakecapacity,and(3)capability offormationofweak bondwithCO₂ that enhancesthe transport ofCO₂ throughwater-swollenmembranematrices(reversibleCO₂ association/dissociation)andfacilitate its releaseat thepermeate side.

ThreedifferentCO₂-philiccompoundswereselectedtobeused inthisstudytoresultinfacilitatedtransportmembranesasmobile carriers.Thehostpolymermatrixwasloadedwithmisciblemobile carriers-oneionicliquid([Emim][OAc])andtwoaminoacidsalts (ProK andPZEA-SARC)withvaryingamine chemistry.Allthemo- bile carriers are characterized withlow vapor pressure and they havesufficientthermalstabilityasadditivesformembranesinthe consideredCO₂separationapplications,asstudiedusingTGAanal- ysisshowninFigureS3.

[Emim][OAc],aroom-temperatureionicliquid,reactswithCO₂ via carbene route and forms a carbene-CO₂ adduct. The forma- tion of the adduct is confirmed by NMR spectra of the CO₂ bubbled [Emim][OAc] aqueous solution in addition to bicarbon- ate/carbonatespecies(FigureS4). Oneofthe mainadvantagesof using [Emim][OAc] as mobile carrier compared to the other two amino acidsis that its carbene-routed interaction withCO₂ does notinfluencetheviscosityofthesolution,whichmaybebeneficial inreducingthemasstransferresistanceintheselectivelayerupon the sorption of CO₂ [46]. The carbene-CO₂ adduct (as shown in

Fig. 6. Surface SEM images of membranes with (A) neat SHPAA/PVA (B) SHPAA/PVA with 20% ProK (C) SHPAA/PVA with 0.2 wt% GO6 (D) SHPAA/PVA with 0.2 wt% pGO6.

Scheme2A)hasalsobeenreportedtohavefasterdiffusiondespite thebulkinessoftheimidazoliniumringduetoitsplanarmolecular structure [47].Additionally,thepresenceofwaterinhibitsthein- teractionoftheacetategroupwithCO₂ toformaceticacid,which mightresultinpHchangesinthesystem[48].However,abovean optimumcontent of10 wt%,theionicliquidwasfound toreduce theCO₂ transport.Thiseffectcanbeduetotheincreasedconcen- trationofacetategroupsthatleadtopHchangesuponinteraction with CO₂, forming acetic acid, which decreased the efficiencyof amine groups present in the host polymer matrix. Nevertheless, atanoptimalcontentof10wt%,theCO₂ permeance increasedto 716GPUwithaCO2/N2separationfactorof32(Fig.7).

ProK (potassium L-prolinate), a secondary amino acid, reacts with CO₂ to form carbamate and bicarbonate/carbonate species [49].The presenceofthesespeciesformed uponinteraction with CO₂withthemobilecarrier(representedinScheme2B)wasiden- tified by NMR studies (FigureS5). Studies indicate that the for- mation ofcarbonate/bicarbonatespeciesismorefavouredinProK than the carbamate intermediate dueto the steric hindrance ef- fect [49,50].Increasing the content ofProK mobile carrierled to increasing CO₂ permeance from 407GPU to 730 GPU at20 wt%

withnosignificantchangeinCO₂/N₂ separationfactor,asseenin Fig.7.FurtheradditionledtomarginalchangesinCO₂permeance, which canbe attributedto smallchanges inthe thicknessofthe selectivelayerwithincreasingsolidsincoatingsolutions[26].

PZEA-Sarcisanaminoacidsaltcontainingoneprimaryamine, two secondary amines (one from sarcosine) and one tertiary amine.CO₂interactswiththismobilecarriertoformprimaryand

secondary mono-carbamates (Scheme 2C). Unlike both ProK and [Emim][OAc]whereliquidstateNMRwasusedforanalysis, PZEA- Sarc crystallized to form semi-solid precipitates upon saturation withCO₂ at 30wt% concentration inwater. Lowerreaction time led to undetectable CO₂-loaded compounds in NMR. Hence, FTIR studies were carried out to study the interaction instead of ¹H and¹³C NMR with solid precipitates. The presence of carbamate speciesinaqueous solutionsis confirmedby thespectra. (Figure S6).TheadditionofPZEA-SARCincreasedtheCO₂permeanceonly atahighcontentof30wt% to635GPUwhencomparedto ProK and[Emim][OAc]whichcanbeattributedtomorestableandbulky carbamatesformedwiththeinteraction ofCO2 andtheincreased viscosityofCO₂-loadedmobilecarriers.

3.4.Effectofnanoplateletsinfacilitatedtransportmembranes

HomogeneousdispersionsofGO andpGOwithvarying lateral dimensionsenabledsuccessfulcoatingofHFTMsandtheresulting membranes were evaluated for CO₂/N₂ separation performances.

Theresultinggasseparationperformancesofhybridmembranesat two differentloadings (0.2wt% and 0.5wt%) are summarised in Fig.8.

Underoptimized conditions,both GO andpGO plateletsserve in creating fast transport channels for CO₂ due to increased sorption and water channel redistribution due to their large as- pect ratio. GO3 is characterized with the highest average lat- eraldimensions(largerthana micron), andhencethebarrier ef- fect contributed to decreased CO₂ permeance at both loadings

Scheme 2. Interaction of the three mobile carriers with CO 2(A. [Emim][OAc]; B. ProK; C. PZEA-Sarc).

Fig. 7. CO 2/N 2mixed gas permeation performance of various facilitated transport membranes as a function of mobile carrier content measured at 35 °C, 1.7 bar.

(Filled shapes denote CO 2 permeance and empty shapes denote corresponding CO 2/N 2separation factors).

of nanosheets (Fig. 8A). The corresponding pGO3 at 0.2 wt%, however, increased the CO₂ permeance marginally to 470 GPU due to a drop in the resistance to gas transport due to in- plane pores (Fig. 8B). The further addition of fillers leads to a drop in CO₂ permeance, which can be attributed to the in- creased barrier effect of GO due to the larger nanosheet size.

GO6 and pGO6 hybrids exhibited a sharp increase in CO₂ permeance at 0.2 wt% to 530 GPU and 780 GPU, respectively (Fig. 8A and B). This phenomenon can be attributed to the op- timallateral dimensions of the GO platelets studied.Pristine GO nanosheetsarerenownedfortheirbarrierefficacyastheirhighas- pectratioinflictresistance togastransport. Hence inhybridma- trices,the average platelet size plays a pivotalrole. For instance, large-sizeGOeffectivelyimpartschangesinpolymer-orpolymer- GOinterfaceinfavourofgastransport,butalsosimultaneouslyin- creasesbarriereffectforgas transportby increasingthe diffusion pathways.On theother hand,a similareffectcan be expectedin thecorresponding pGOnanofiller, which,however,offers reduced resistancetoCO₂ transport throughthenon-selectiveporeswhile thepreserved2Dstructurestill impartspropertychangesinpoly-

mer chains inthe surrounding environment.Additionally,the in- creasedexposureofedgegroupscanalsocontributetohigherCO₂ affinity.Theamountoftheseedgegroupsisinverselyproportional totheaveragelateraldimensions[51].Consequently,theappropri- ateaverage lateraldimensionsof GO6andpGO6 presenttheop- timaleffect,leadingtohigherCO₂ permeancewhencompared to theotherfillersatthesameloading.

Thus, the composition of 0.2 wt% nanofiller marked optimal nanosheet dimensionsand loading where the expositionof edge groups and disruption of chain packing (which is dependent on flake size and dispersion) reach the maximum, simultaneously overcoming the barrier resistance to gas permeation induced by theGO/pGO basalplanes.Undertheseconditions,the nanosheets form continuous CO₂ permeation pathways along the fairly long CO₂-philic GO/pGO surface with reoriented water channels sur- roundingthe2D structure.Furtherloadingofboththesefillersre- sultedin adrop inCO₂ permeance asthevolume fractionofGO phase increases. Both GO9 and pGO9 resulted in increased CO₂ permeance at all loadings; this phenomenon may be due to the smallest average flake size and better dispersion in the polymer matrix. However, the smaller flake sizes inhibitthe formation of continuouspermeationpathwayssimilartoGO6/pGO6nanosheets.

HenceCO₂transportinthesehybridsisreliantmainlyonthepoly- merpropertychangesasdiscussedearlier.Interestingly,at0.2wt%

loading, all hybrids (both GO andpGO) had little influence over CO₂/N₂ selectivity.Thiscanbe attributedto thelowvolumefrac- tionofnanosheetsinthehybridstoeffectivelyhinderpermeation ofN₂withlongertransportpathwayscircumventingthelaminates.

Under high loading of 0.5 wt%, all GO-based hybrids showcased a marginally increased CO₂/N₂ selectivity (from 32to 38) owing tohighereffectiveresistancetoN₂ permeationwhencomparedto CO₂(Fig.8A).ThecorrespondingpGOhybridshadasimilareffect onCO₂/N₂ selectivity,althoughtoalesserextent,duetothepres- enceofnon-selectivepores.

3.5. Three-phasehybridfacilitatedtransportmembranesforCO₂ separation

In an attemptto leverage the advantage ofmobile carriers in HFTMs, 0.2wt% ofpGO wasdispersed in a polymer matrixcon- taining 10wt% [Emim][OAc] and20 wt% ProK.The compositions werechosenaccordingtotheoptimalcompositiondetectedinthe

Fig. 8. CO 2/N 2mixed gas permeation performance of various hybrid membranes containing (A) 0.2 wt% GO and 0.5 wt% GO (B) 0.2 wt% pGO and 0.5 wt% pGO measured at 35 °C.

Fig. 9. Mixed gas permeation performance of various HFTMs measured at 35 °C for (A) CO 2/N 2gas pair at 1.7 bar and (B) CO 2/CH 4gas pair at 2 bar.

experiments andreported in theprevious sections.The resulting three-phaseHTFMshadasmallincreasedperformanceforCO₂/N₂ separationonlyinthecaseofProKcontainingmembranes,increas- ing the CO₂ permeance up to 810 GPU as seen in Fig. 9A. This phenomenon canbea resultofincreasedreactivespeciesmaking useofdistributedwaterchannelsmadeavailablebythepGOfillers inthepolymer matrix.Suchbehaviour isbetterpronounced with CO₂/CH₄ separation performances, as seen in Fig. 9B. For these tests, the feedgas consisting of 40/60 v/v CO₂/CH₄ mixture was

used, mimicking typical biogas composition [52,53]. Bothmobile carrierscontainingmembranes werecharacterizedwithamarked increaseinCO₂permeancewhiletheCO₂/CH₄separationfactorre- mainedconstant around 20.The HFTMcontaining 0.2wt% pGO6 with 20% ProK peaked at a CO₂ permeance of 825 GPU with a CO₂/CH₄ separation factor of 20, while the neat polymer had a CO₂ permeance of 497GPU with a CO₂/CH₄ separation factor of 21at afeedpressure of2bar. The correspondingHFTM contain- ing0.2wt%pGO6withoutmobilecarrierswaslimitedto727GPU

Fig. 10. CO 2/CH 4mixed gas permeation performance of various 0.2 wt% HFTMs measured at 35 °C.

withCO₂/CH₄ feedgasmixture.AsimilarincreaseinCO₂ perme- anceof10%[Emim][OAc]containingmembraneupto782GPUwas observed.The mobilecarrierscontainingmembranesexhibitedan increasedCO₂ permeance atthetotalupstream pressureof2bar withtheCO₂/CH₄ feedmixturesduetotheincreasedpartialpres- sureCO₂ andlowerstagecut(higherfeedflow).

3.6.Influenceoffeedpressureinhybridfacilitatedtransport membranes

Investigatingtheeffectoffeedpressureonthepermeationper- formance of a facilitated transport membrane is a common ap- proachto examine the carrier saturation phenomenon, the char- acteristicfeaturecommonlyused toconfirm thefacilitated trans- portmechanism. Italsoallows toexplore themembranesforap- plicationsatelevatedpressures,likebiogasupgrading,naturalgas sweeteningandpre-combustion CO₂ capture, whichaids in opti- mizingprocessdesignofmembrane-basedseparationunits[52].

Herein, the effectof feedpressure onthe CO₂/CH₄ separation performanceofthe fabricatedthree-phaseHFTMs wasstudiedby varyingfeedpressures.Theupstreampressurewasincreasedfrom 2bartoamaximumof20bar.Increasingfeedpressureledtofur- therdistinguishableseparationperformance basedonthecompo- sitionofHFTMs.Facilitatedtransportmembranesarecharacterized withcarriersaturationphenomenon at ahighpartial pressure of CO₂ inthefeed.SincetheavailabilityoffixedCO₂ carriers(amine groups) in the polymer matrix is limited, increasing CO₂ partial pressureinthefeedgasleadstocarriersaturation,hencedecreases theCO₂ permeance.Consequently, inallthesystemsdiscussed in thiswork, increasing pressure in thefeed side reflects a drop in permeanceofCO₂,asseeninFig.10,whichisobservedfortypical facilitatedtransportmembranes[25].Interestingly,themembranes withloading 2D fillers, both GO6 andpGO6, exhibited increased resistancetocarriersaturationphenomenon,especiallyatthepres- suresof5barand10bar,whichcanbeattributedtoincreasedwa- terretentionduetoreinforcementofmatrixbythehighaspectra- tionanosheets.Thus,thecorrespondingCO₂permeancesremained at340 GPUand450 GPU whencompared with neat polymer at 300GPU at5 bar, which istypical operating pressure forbiogas upgrading[54].However,thisresistanceseemedtobelesspromi- nentatthehigherpressuresof15barand20barwhereneatpoly- merrecordedhigherpermeances.Thisbehaviourcanbeattributed toCO₂-inducedswellingintheneatpolymer,whichispossiblyde- layedinallnanofiller-containingHFTMsduetodistributedpolymer

chain packing.Regardless, the three-phase HFTMs that contained mobile carriers showcased further resistance to the carrier satu- ration phenomenondue tothe increase ofavailable CO₂ carriers, asexpected. Theeffectremainsevidentacrosstheentirepressure rangeof testingfor both 10wt% [Emim][OAc] and 20wt% Pro-K loaded membranes. These membranes showcased a CO₂ perme- ance of463GPUand468GPUandCO₂/CH₄ separationfactorsof 24and25,respectively,at5bar.

4. Conclusions

Three-phasehybridfacilitatedtransportmembranescontaining GO-based2DfillersandCO₂-philicmobilecarriersinultrathinse- lective layers were successfully fabricated and tested. GO-based fillers were found to benefit HFTMs to increase the CO₂ separa- tionpropertiesdependingontheirlateraldimensionsandloading.

pGO fillersderived fromsize-optimized GO nanosheetsenhanced CO₂ permeationeffectively ataloading of0.2wt%. TheseHFTMs were characterized witha highCO2 permeance of 780 GPUand a corresponding CO₂/N₂ separationfactorof 30.Facilitatedtrans- port membranes with mobile carriers that reversibly react with CO₂ were also fabricated asTFC hollow fibers.It was found that the mobile carriers ProK and [Emim][OAc] were able to improve the separation performance ofthe SHPAA/PVA membranedue to theirhighmobilityandreversibleinteractionwithCO₂toformbi- carbonate/carbonatespeciesandcarbene-CO₂adducts,respectively.

As a newconcept, three-phase hybrid facilitatedtransport mem- branes were fabricated andthe resulting membranes exhibited a CO₂permeanceof825GPU.Thesemembraneswereevaluatedfor bothCO₂/N₂andCO₂/CH₄gaspairsandtheseparationfactorwas found to be 31 for CO₂/N₂ and 20 forCO₂/CH₄. Due to the rel- ativeincrease in the content ofCO2-philicspeciesand reinforce- ment with the addition of pGO, these three-phase HFTMs were stableforfeedpressures ofup to20 barandexhibitedincreased resistancetocarriersaturationphenomena.Thishighstabilityand gas separation performance, when combinedwith easily scalable hollowfiberconfiguration,establishes the commercialviabilityof thefabricatedmembranesforCO₂separationapplications.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingfinan- cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.

CRediTauthorshipcontributionstatement

Saravanan Janakiram: Conceptualization, Methodology, Writ- ing -original draft, Writing -review & editing.JuanLuis Martín Espejo: Validation, Investigation. KarenKarolina Høisæter: Re- sources,Investigation.ArneLindbråthen:Resources,Writing-re- view&editing.LucaAnsaloni:Writing-review&editing.Liyuan Deng: Writing- review& editing,Project administration,Funding acquisition.

Acknowledgements

This workis a partofthe NANOMEMC2 projectsupported by theEuropean Union’sHorizon 2020ResearchandInnovation pro- gram underGrant Agreement n°727734 andthe FaT H2project supported by the Norwegian Research Council (No.294533). The authorsthankDr.RanyMirantifordiscussionsontheRamanSpec- troscopy. RicardoWanderley andProf. Hanna Knuutila are grate- fully acknowledged for the discussions on mobile carriers. We thankProf. HoBum Parkandhis groupforthe knowledgetrans- fer in GO modification. The Research Council of Norway is ac- knowledged for the support tothe Norwegian Micro- andNano- FabricationFacility,NorFab,projectnumber245963/F50.

Supplementarymaterials

Supplementary material associated with this article can be found,intheonlineversion,atdoi:10.1016/j.apmt.2020.100801. References

[1] S. Janakiram, M. Ahmadi, Z. Dai, L. Ansaloni, L. Deng, Performance of nanocom- posite membranes containing 0D to 2D nanofillers for CO 2 separation: a re- view, Membranes (Basel) (2018) 8, doi: 10.3390/membranes8020024 . [2] W.M. McDanel , M.G. Cowan , N.O. Chisholm , D.L. Gin , R.D. Noble , Fixed-site–

carrier facilitated transport of carbon dioxide through ionic-liquid-based epoxy-amine ion gel membranes, J. Memb. Sci. 492 (2015) 303–311 . [3] Y. Zhang, J. Sunarso, S. Liu, R. Wang, Current status and development of mem-

branes for CO 2/CH 4separation: a review, Int. J. Greenh. Gas Control. 12 (2013) 84–107, doi: 10.1016/j.ijggc.2012.10.009 .

[4] D.M. D’Alessandro, B. Smit, J.R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chemie - Int. Ed. 49 (2010) 6058–6082, doi: 10.1002/

anie.2010 0 0431 .

[5] L.M. Robeson, The upper bound revisited, J. Memb. Sci. 320 (2008) 390–400, doi: 10.1016/j.memsci.2008.04.030 .

[6] B.D. Freeman, Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes, Macromolecules 32 (1999) 375–380, doi: 10.1021/

ma9814548 .

[7] H.B. Park, J. Kamcev, L.M. Robeson, M. Elimelech, B.D. Freeman, Maximizing the right stuff: the trade-off between membrane permeability and selectivity, Science (80) 356 (2017) eaab0530, doi: 10.1126/science.aab0530 .

[8] M. Ahmadi, S. Janakiram, Z. Dai, L. Ansaloni, L. Deng, Performance of mixed matrix membranes containing porous two-dimensional (2D) and three- dimensional (3D) fillers for CO 2 separation: a review, Membranes (Basel) 8 (2018) 50, doi: 10.3390/membranes8030050 .

[9] Y. Han, W.S.W. Ho, Recent advances in polymeric membranes for CO 2capture, Chinese J. Chem. Eng. 26 (2018) 2238–2254, doi: 10.1016/j.cjche.2018.07.010 . [10] J. Wu, S. Japip, T.S. Chung, Infiltrating molecular gatekeepers with coexist-

ing molecular solubility and 3D-intrinsic porosity into a microporous polymer scaffold f or gas separation, J. Mater. Chem. A. 8 (2020) 6196–6209, doi: 10.1039/

c9ta12028a .

[11] J. Wu, J. Liu, T.-S. Chung, Structural tuning of polymers of intrinsic microporos- ity via the copolymerization with macrocyclic 4- tert -butylcalix[4]arene for enhanced gas separation performance, Adv. Sustain. Syst. 2 (2018) 180 0 044, doi: 10.10 02/adsu.20180 0 044 .

[12] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng.

Chem. Res. 41 (2002) 1393–1411, doi: 10.1021/ie0108088 .

[13] G. Dong, H. Li, V. Chen, Challenges and opportunities for mixed-matrix mem- branes for gas separation, J. Mater. Chem. A. 1 (2013) 4610–4630, doi: 10.1039/

C3TA00927K .

[14] Z. Dai, J. Deng, Q. Yu, R.M.L. Helberg, S. Janakiram, L. Ansaloni, L. Deng, Fabrica- tion and evaluation of bio-based nanocomposite TFC hollow fiber membranes for enhanced CO 2capture, ACS Appl. Mater. Interfaces. 11 (2019) 10874–10882, doi: 10.1021/acsami.8b19651 .

[15] C.Z. Liang, W.F. Yong, T.S. Chung, High-performance composite hollow fiber membrane for flue gas and air separations, J. Memb. Sci. 541 (2017) 367–377, doi: 10.1016/j.memsci.2017.07.014 .

[16] L. Ansaloni, L. Deng, Advances in polymer-inorganic hybrids as membrane ma- terials, 2016. 10.1016/B978-0-08-10 0408-1.0 0 0 07-8.

[17] Y. Dai, J.R. Johnson, O. Karvan, D.S. Sholl, W.J. Koros, Ultem ®/ZIF-8 mixed ma- trix hollow fiber membranes for CO 2/N 2 separations, J. Memb. Sci. 401–402 (2012) 76–82, doi: 10.1016/j.memsci.2012.01.044 .

[18] P.D. Sutrisna, J. Hou, H. Li, Y. Zhang, V. Chen, Improved operational stability of Pebax-based gas separation membranes with ZIF-8: a comparative study of flat sheet and composite hollow fibre membranes, J. Memb. Sci. 524 (2017) 266–279, doi: 10.1016/j.memsci.2016.11.048 .

[19] Y. Zhang, Q. Shen, J. Hou, P.D. Sutrisna, V. Chen, Shear-aligned graphene ox- ide laminate/Pebax ultrathin composite hollow fiber membranes using a facile dip-coating approach, J. Mater. Chem. A. 5 (2017) 7732–7737, doi: 10.1039/

c6ta10395b .

[20] S. Janakiram, X. Yu, L. Ansaloni, Z. Dai, L. Deng, Manipulation of fibril surfaces in nanocellulose-based facilitated transport membranes for enhanced CO 2cap- ture, ACS Appl. Mater. Interfaces. 11 (2019) 33302–33313, doi: 10.1021/acsami.

9b09920 .

[21] S. Janakiram, L. Ansaloni, S.-A. Jin, X. Yu, Z. Dai, R.J. Spontak, L. Deng, Humidity-responsive molecular gate-opening mechanism for gas separation in ultraselective nanocellulose/il hybrid membranes, Green Chem. 22 (2020) 3546–3557, doi: 10.1039/D0GC00544D .

[22] S. Janakiram, J. Luis Martín Espejo, X. Yu, L. Ansaloni, L. Deng, Facilitated trans- port membranes containing graphene oxide-based nanoplatelets for CO 2sep- aration: effect of 2D filler properties, J. Memb. Sci. (2020) 118626 In press, doi: 10.1016/j.memsci.2020.118626 .

[23] J. Shen, M. Zhang, G. Liu, K. Guan, W. Jin, Size effects of graphene oxide on mixed matrix membranes for CO 2separation, AIChE J. 62 (2016) 2843–2852, doi: 10.1002/aic.15260 .

[24] L. Deng, M.B. Hagg, Carbon nanotube reinforced PVAm/PVA blend FSC nanocomposite membrane for CO 2/CH 4separation, Int. J. Greenh. Gas Control.

26 (2014) 127–134, doi: 10.1016/j.ijggc.2014.04.018 .

[25] Y. Zhao, B.T. Jung, L. Ansaloni, W.S.W. Ho, Multiwalled carbon nanotube mixed matrix membranes containing amines for high pressure CO 2/H 2separation, J.

Memb. Sci. 459 (2014) 233–243, doi: 10.1016/j.memsci.2014.02.022 .

[26] Z. Dai, J. Deng, L. Ansaloni, S. Janakiram, L. Deng, Thin-film-composite hollow fiber membranes containing amino acid salts as mobile carriers for CO 2sepa- ration, J. Memb. Sci. (2019), doi: 10.1016/j.memsci.2019.02.023 .

[27] H. Lee, S.C. Park, J.S. Roh, G.H. Moon, J.E. Shin, Y.S. Kang, H.B. Park, Metal- organic frameworks grown on porous planar template with exceptionally high surface area: promising nanofiller platforms for CO 2separation, J. Mater. Chem.

A. 5 (2017) 22500–22505, doi: 10.1039/C7TA06049A .

[28] Y. Xu, Z. Lin, X. Zhong, X. Huang, N.O. Weiss, Y. Huang, X. Duan, Holey graphene frameworks for highly efficient capacitive energy storage, Nat. Com- mun. 5 (2014) 1–8, doi: 10.1038/ncomms5554 .

[29] J. Wang, Y. Wang, Y. Zhang, A. Uliana, J. Zhu, J. Liu, B. Van Der Bruggen, Zeolitic imidazolate framework/graphene oxide hybrid nanosheets functional- ized thin film nanocomposite membrane for enhanced antimicrobial perfor- mance, ACS Appl. Mater. Interfaces 8 (2016) 25508–25519, doi: 10.1021/acsami.

6b06992 .

[30] P.T. Lan Huong, N. Tu, H. Lan, L.H. Thang, N. Van Quy, P.A. Tuan, N.X. Dinh, V.N. Phan, A.T. Le, Functional manganese ferrite/graphene oxide nanocompos- ites: Effects of graphene oxide on the adsorption mechanisms of organic MB dye and inorganic As(v) ions from aqueous solution, RSC Adv. 8 (2018) 12376–

12389, doi: 10.1039/c8ra00270c .

[31] K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett, A. Zettl, Determination of the local chemical structure of graphene oxide and reduced graphene oxide, Adv.

Mater. 2 (2010) 4 467–4 472, doi: 10.10 02/adma.2010 0 0732 .

[32] A. Liscio, K. Kouroupis-Agalou, X.D. Betriu, A. Kovtun, E. Treossi, N.M. Pugno, G. De Luca, L. Giorgini, V. Palermo, Evolution of the size and shape of 2D nanosheets during ultrasonic fragmentation, 2D Mater 4 (2017) 025017, doi: 10.

1088/2053-1583/aa57ff.

[33] Z.Y. Xia, S. Pezzini, E. Treossi, G. Giambastiani, F. Corticelli, V. Morandi, A. Zanelli, V. Bellani, V. Palermo, The exfoliation of graphene in liquids by elec- trochemical, chemical, and sonication-assisted techniques: a nanoscale study, Adv. Funct. Mater. 23 (2013) 4684–4693, doi: 10.1002/adfm.201203686 . [34] U. Khan, A. O’Neill, M. Lotya, S. De, J.N. Coleman, High-concentration solvent

exfoliation of graphene, Small 6 (2010) 864–871, doi: 10.10 02/smll.20 0902066 . [35] B.D. Ossonon, D. Bélanger, Synthesis and characterization of sulfophenyl- functionalized reduced graphene oxide sheets, RSC Adv. 7 (2017) 27224–27234, doi: 10.1039/c6ra28311j .

[36] N. Sharma, V. Sharma, Y. Jain, M. Kumari, R. Gupta, S.K. Sharma, K. Sachdev, Synthesis and characterization of graphene Oxide (GO) and reduced graphene oxide (rGO) for gas sensing application, Macromol. Symp. 372 (2017) 170 0 0 06, doi: 10.10 02/masy.20170 0 0 06 .

[37] F. Tuinstra, J.L. Koenig, Raman spectrum of graphite, J. Chem. Phys. (1970) 53, doi: 10.1063/1.1674108 .

[38] L.M. Malard, M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, Raman spec- troscopy in graphene, Phys. Rep. 473 (2009) 51–87, doi: 10.1016/j.physrep.2009.

02.003 .

[39] R.J. Nemanich, S.A. Solin, First- and second-order Raman scattering from finite- size crystals of graphite, Phys. Rev. B 20 (1979) 392, doi: 10.1103/PhysRevB.20.

392 .

[40] C.H. Lui, L.M. Malard, S. Kim, G. Lantz, F.E. Laverge, R. Saito, T.F. Heinz, Ob- servation of layer-breathing mode vibrations in few-layer graphene through combination raman scattering, Nano Lett. 12 (2012) 5539–5544, doi: 10.1021/

nl302450s .