Hydroxide Conduction Enhancement of Chitosan Membranes by Functionalized Mxene
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materials Article Hydroxide Conduction Enhancement of Chitosan Membranes by Functionalized MXene Lina Wang 1,* and Benbing Shi 2,* 1 College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, China 2 School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China * Correspondence: [email protected] (L.W.); [email protected] (B.S.) Received: 7 September 2018; Accepted: 17 November 2018; Published: 21 November 2018 Abstract: In this study, imidazolium brushes tethered by –NH2-containing ligands were grafted onto the surface of a 2D material, MXene, using precipitation polymerization followed by quaternization. Functionalized MXene was embedded into chitosan matrix to prepare a hybrid alkaline anion exchange membrane. Due to high interfacial compatibility, functionalized MXene was homogeneously dispersed in chitosan matrix, generating continuous ion conduction channels and then greatly enhancing OH− conduction property (up to 172%). The ability and mechanism of OH− conduction in the membrane were elaborated based on systematic tests. The mechanical-thermal stability and swelling resistance of the membrane were evidently augmented. Therefore, it is a promising anion exchange membrane for alkaline fuel cell application. Keywords: functionalized MXene; chitosan; anion exchange membrane; hydroxide conductivity; alkaline fuel cell 1. Introduction As an efficient, clean, and zero-emission power generation technology, fuel cells can effectively solve energy crises and environmental pollution issues as the focus of global governments. Compared with other types of fuel cells, the ones with alkaline anion exchange membranes have attracted wide attention because of: (1) Low working temperature (60–90 ◦C); (2) high rate of oxygen reduction reaction and catalytic stability, which can decrease the Pt amount or use non-noble metal catalysts such as Ag and Ni instead, thus significantly reducing the cost; (3) low fuel permeability (permeability of methanol: <10−6 m−2 s); and (4) facile water and heat management [1–3]. Alkaline anion exchange membranes are one of the essential components of alkaline fuel cells and are capable of blocking fuel and conducting OH−. Its performance, especially OH− conductivity, directly determines the open circuit voltage and energy output of fuel cells. However, currently available alkaline membranes all have low OH− conductivities owing to a low diffusion coefficient of OH− [4] (movement rate of OH− is 56% of that of H+) and poor channel connectivity inside. Thus, the commercialization of fuel cells has been seriously limited. Alkaline anion exchange membranes are mainly prepared by functionalized polymers, including polyphenyl ether [5], polybenzimidazole [6], and polyetheretherketone [7]. OH− is mainly conducted through conductible cation aggregation zones inside membranes, so the continuity of these zones plays a decisive role. Since traditional polymers have similar functionalized and non-functionalized areas, continuous ion conduction channels cannot form through self-assembly [8,9]. Recently, block polymers have been prepared to significantly increase the OH− conductivity of membranes through self-assembly-generated interconnected ion cluster channels due to entropy-driven microphase separation of hydrophilic and hydrophobic chain segments. Nevertheless, the preparation processes Materials 2018, 11, 2335; doi:10.3390/ma11112335 www.mdpi.com/journal/materials Materials 2018, 11, 2335 2 of 11 Materials 2018, 11, x FOR PEER REVIEW 2 of 11 forpreparation block polymers processes are for complicated block polymers and the are distribution complicated of and groups the distribution cannot be accurately of groups controlled, cannot be soaccurately it is difficult controlled, and non-universal so it is difficult to generate and non- interconnecteduniversal to generate conduction interconnected channels depending conduction on functionalizedchannels depending polymers on func [10].tionalized polymers [10]. Comparatively,Comparatively, organic–inorganic organic–inorganic hybridization hybridization is is a a convenientconvenient andand effectiveeffective strategystrategy toto elevateelevate thethe ionion conductionconduction performanceperformance ofof membranes.membranes. OnOn oneone hand,hand, thethe surfacesurface ofof functionalizedfunctionalized inorganicinorganic nano-fillernano-filler allowsallows formationformation ofof continuouscontinuous ionion conductionconduction channels.channels. OnOn thethe otherother hand,hand, addingadding functionalizedfunctionalized nano-fille nano-fillerr can can connect connect non-conduction non-conduction dead dead zone zones,s, thereby thereby significantly significantly raising raising the theconductivity conductivity [11,12]. [11, 12In ].addition, In addition, addition addition of inor ofganic inorganic nano-filler nano-filler can markedly can markedly augment augment the thermal the thermalstability stabilityand mechanical and mechanical performance performance of membranes. of membranes. At present, At inorganic present, nano-fillers inorganic nano-fillers are mainly arestructurally mainly structurallyclassified into: classified (1) Zero-dimensional into: (1) Zero-dimensional nanoparticles nanoparticles (SiO2 [13], TiO (SiO2 [14],2 [13 etc.),], TiO (2)2 [one-14], etc.),dimensional (2) one-dimensional nanotubes (halloysit nanotubese (halloysitenanotubes nanotubes[15], carbon [15 ],nanotubes carbon nanotubes [16], etc.), [16 ],and etc.), (3) and two- (3) two-dimensionaldimensional nanolamellar nanolamellar materials materials (graphene (graphene oxide oxide [17], [17 ],hydrotalcite hydrotalcite [18], [18], montmorillonite montmorillonite [[19],19], MoSMoS22 [20],[20], etc.). etc.). Two-dimensional Two-dimensional nano nanolamellarlamellar materials, materials, which which have have high high specific specific surface surface areas areas that thatbenefit benefit the theconstruction construction of long-r of long-rangeange and and wide wide continuous continuous channels, channels, are are thus thus superior superior to otherother materialsmaterials [[21].21]. Nevertheless,Nevertheless, routineroutine two-dimensionaltwo-dimensional nanolamellarnanolamellar materialsmaterials havehave limitedlimited functionsfunctions owingowing toto low contents and uneven uneven distribution distribution of of su surfacerface functional functional groups. groups. Thus, Thus, it itis isimperative imperative to todevelop develop novel novel functionalized functionalized two-dimensional two-dimensional nanolamellar nanolamellar materials materials for for preparing hybrid anionanion exchangeexchange membranes.membranes. InIn thisthis study, we selected a a new new lamellar lamellar material material rich rich in in oxygen-containing oxygen-containing groups, groups, MXene, MXene, to tograft graft imidazolium imidazolium brushes brushes through through precipitation precipitation polymerization polymerization followed followed by quaternization.quaternization. Afterwards,Afterwards, wewe fabricatedfabricated aa hybridhybrid membranemembrane byby usingusing functionalizedfunctionalized MXeneMXene andand chitosanchitosan matrixmatrix withwith excellentexcellent film-forming film-forming ability. ability. Moreover, Moreover, we systematically we systematically studied studied the construction the construction of interfacial of conductioninterfacial channelsconduction and channels the mechanism and the for mechanism ion conduction for byion characterizing conduction theby microstructurecharacterizing andthe testingmicrostructure related properties. and testing related properties. (a) (b) (c) 2.2. ExperimentalExperimental SectionSection 2.1.2.1. MaterialsMaterials TitaniumTitanium hydridehydride (TiH(TiH22,, 99%,99%, 325-mesh),325-mesh), titaniumtitanium carbidecarbide (TiC,(TiC, 99%,99%, 325-mesh),325-mesh), aluminumaluminum powderspowders (Al, (Al, 99%, 99%, 325 325 mesh), mesh), and and 1-vinylimidazole 1-vinylimidazo werele were purchased purchased from Alfafrom Aesar Alfa (Shanghai,Aesar (Shanghai, China). HydrofluoricChina). Hydrofluoric acid (HF, acid 49%) (HF, was 49%) bought was from bought Aladdin from Reagent Aladdin Inc. Reagent (Shanghai, Inc. (Shanghai, China). Dimethyl China). sulfoxideDimethyl (DMSO,sulfoxide analytic (DMSO, grade), analytic glacial grade), acetic glacial acid acetic (analytic acid grade), (analytic and grade), potassium and hydroxidepotassium (analytichydroxide grade) (analytic were grade) obtained were from obtained Tianjin from Kermel Tianjin Chemical Kermel Chemical Reagent Co.,Reagent Ltd. Co., (Tianjin, Ltd. (Tianjin, China). CSChina). (degree CS of(degree deacetylation: of deacetylation: 95%) was 95%) provided was pr byovided Zhejiang by Golden-ShellZhejiang Golden-Shell Pharmaceutical Pharmaceutical Co., Ltd. (Yuhuan,Co., Ltd. Zhejiang,(Yuhuan, China).Zhejiang, China). 2.2.2.2. ApparatusApparatus AA TEM-100CXTEM-100CX transmissiontransmission electronelectron microscopemicroscope was purchased from JEOL ( (Boston,Boston, MA, USA). AnAn FTIRFTIR ThermoThermo NicoletNicolet IR200IR200 FourierFourier transformtransform infraredinfrared spectroscopespectroscope waswas boughtbought fromfrom ThermoThermo ScientificScientific (Waltham, MA, MA, USA). USA). A AD8 D8 Advance Advance ECO ECO X-ray X-ray diffractometer diffractometer was obtained was obtained from Bruker from Bruker(Karlsruhe, (Karlsruhe, Germany). Germany). A TGA-50 A TGA-50 thermal thermal gravimetric gravimetric analyzer analyzer was provided