Composite Proton Exchange Membranes Based on Chitosan and Phosphotungstic Acid Immobilized One-Dimensional Attapulgite for Direct Methanol Fuel Cells

nanomaterials

Article Composite Proton Exchange Membranes Based on Chitosan and Phosphotungstic Acid Immobilized One-Dimensional Attapulgite for Direct Methanol Fuel Cells

Wen-Chin Tsen

Department of Fashion and Design, Lee-Ming Institute of Technology, New Taipei City 243, Taiwan; [email protected]; Tel.: +886-02-29097811

Received: 30 June 2020; Accepted: 19 August 2020; Published: 21 August 2020

Abstract: In order to obtain biopolymer chitosan-based proton exchange membranes with excellent mechanical properties as well as high ionic conductivity at the same time, natural attapulgite (AT) with one-dimensional (1D) structure was loaded with a strong heteropolyacid and also a super proton conductor, phosphotungstic acid (PWA), using a facial method. The obtained PWA anchored attapulgite (WQAT) was then doped into the chitosan matrix to prepare a series of Chitosan (CS)/WQAT composite membranes. The PWA coating could improve the dispersion and interfacial bonding between the nano-additive and polymer matrix, thus increasing the mechanical strength. Moreover, the ultra-strong proton conduction ability of PWA together with the interaction between positively charged CS chains and negatively charged PWA can construct eﬀective proton transport channels with the help of 1D AT. The proton conductivity of the composite membrane (4 wt.% WQAT 1 loading) reached 35.3 mS cm− at 80 ◦C, which was 31.8% higher than that of the pure CS membrane. Moreover, due to the decreased methanol permeability and increased conductivity, the composite 2 membrane with 4% WQAT content exhibited a peak power density of 70.26 mW cm− fed at 2 M 2 methanol, whereas the pure CS membrane displayed only 40.08 mW cm− .

Keywords: chitosan; attapulgite; phosphotungstic acid; proton exchange membranes; fuel cells

1. Introduction Direct methanol fuel cell (DMFC) is a highly efficient and eco-friendly energy conversion device that can directly convert chemical energy into electrical energy by using liquid methanol as the anode fuel. Moreover, when compared with proton exchange membrane fuel cells (PEMFCs) using gaseous hydrogen as the fuel, DMFCs have a wider application prospect especially in the fields of portable devices and vehicles because they can fully utilize the existing infrastructure for gasoline [1–3]. As a key component in a DMFC, the proton exchange membrane (PEM) plays an important role of a separator of anode and cathode as well as a deliverer of protons [4,5]. PEMs must possess high proton conductivity, excellent methanol barrier ability, and good thermal and mechanical properties. Up to now, perfluorosulfonic acid (PFSA) membranes (represented by Dupont’s Nafion) are known as currently commercial PEMs in PEMFCs for their relatively high proton conductivity and good chemical and electrochemical stabilities [6]. However, the high methanol permeability of PFSAs is a big obstacle when utilizing PFSAs as PEMs in DMFCs. In addition, the very high cost and limited operation temperature of PFSAs also bring adverse effects on their widespread application in DMFCs [7]. Therefore, numerous researchers have turned to seek cost-effective and high-performance alternatives to PFSAs.

Nanomaterials 2020, 10, 1641; doi:10.3390/nano10091641 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1641 2 of 15

In recent years, chitosan (CS), a natural polymer, has attracted wide attention as a PEM material for its abundance in nature, environmental friendliness and low cost [8,9]. As the only degradable polycationic bio-polysaccharide in nature, CS is a deacetylated product of chitin and has been applied to various fields such as food chemistry, biomedicine, cosmetics, textile, papermaking and so on. CS has a large number of amino and hydroxyl groups, and good ability of alcohol–water separation, which has preferential permeability to water under the condition of alcohol resistance. So, CS is also a good candidate of PEMs to solve the problem of methanol permeation [8]. In the process of membrane preparation, the hydrophilic groups of CS generate strong intra molecular and intermolecular hydrogen-bonding interactions, which drive CS chains to form numerous crystalline regions and cross-linked networks. On the one hand, the high crystallinity packs the CS chains and thus decreases the free volume cavities (only 0.56 nm), which can effectively prevent methanol penetration through the CS membrane [10], but on the other hand highly crystallized CS has very low proton conductivity because the ion conduction in a PEM mainly occurs in the amorphous phase rather than crystalline phase [8,9]. Additionally, the poor mechanical properties of CS are also a concern when using CS-based PEMs operation in fuel cells. To improve the mechanical properties and proton conductivity, many efforts have been taken to modify CS. For example, chemical modification [11,12] (e.g., sulfonation, phosphorylation, chemical cross-linking) and physical modification [13–16] (e.g., blending with other polymers, organic-inorganic hybrids). Among these methods, preparation of organic-inorganic composites is considered as a facile and efficient approach to solve the problems facing CS because this method can combine both their merits and sometimes generate the synergistic effect. Many inorganic nano-additives such as 0-dimensional (0D) nanoparticles [17–19] (e.g., silica, titania, zirconia, zeolites, silicon-aluminum oxides), one-dimensional (1D) nanotubes or nanorods [14–16,20–22] (e.g., carbon nanotubes, halloysite tubes) and two-dimensional (2D) nanoplates [23,24] (e.g., montmorillonite, grapheme oxide) have been extensively applied to modify polymers to improve their thermal and mechanical stabilities while endowing them with some new properties due to the synergistic effect. Among those inorganic nanomaterials, 1D nanotubes or nanorods stand out for their unique anisotropic 1D shape because such nanomaterials can contribute to construct better channel-like ion transport pathways in the composites for proton conduction [25–27]. Generally, nanomaterials including 1D nano-additives need to be surface modified to improve the compatibility between nanomaterials and polymer matrix, and thus, can fully play their functions. Wen and co-workers designed and prepared functionalized carbon nanotubes (CNTs) with different surface coating substances (such as chitosan, sulfate zirconia, SiO2, TiO2, organic long-chain ions) to modify CS to fabricate a series of composite membranes [14–16,26,28]. These surface coating materials can not only promote the dispersion of CNTs, and thus, fully play the reinforcement role of CNTs, but also improve the proton conductivity due to the newly formed proton conducting network along the surface of functionalized CNTs. Apart from 1D CNTs, 1D clay nanotubes or nanorods have also been used as an effective additive in the field of PEMs. Wang et al. [21] synthesized halloysite nanotubes bearing sulfonated polyelectrolyte brushes (SHNTs) via distillation-precipitation polymerization and then added into CS matrix to prepare nanohybrid membranes. The SHNTs is helpful to create continuous pathways along the nanotube, thus generating acid-base pairs at SHNT-CS interface, which can work as low-barrier proton-hoping sites. Moreover, the wider pathways could form with the aid of the long brushes on SHNTs in the CS matrix. Recently, phosphotungstic acid (PWA), recognized as a strong heteropolyacid and a super proton conductor, was immobilized onto the surface of poly (vinylidene fluoride) (PVDF) electrospun nanofibers to obtain a new three-dimensional proton conducting network [29]. After impregnating with CS, which can tightly anchor PWA to avoid its leaching out from PEMs in water or methanol, the resulting composite membrane demonstrated ultrahigh proton conductivity and satisfactory fuel cell performance. Inspired by that, herein, we utilized PWA as a surface modification material to coat 1D natural clay attapulgite (AT) with a theoretical formula of Si Mg O (OH) (H O) 4H O[30]. 8 5 20 2 2 4· 2 It should be mentioned that AT is cheap and abundant in nature when compared with other carbon Nanomaterials 2020, 10, 1641 3 of 15 nanomaterials [23,25]. Then, the obtained PWA-decorated AT was added into the CS matrix to fabricate novel organic-inorganic composite membranes. Expectedly, the PWA coating could improve the dispersion and interfacial bonding between the nano-additive and polymer matrix, thus increasing the mechanical strength. Moreover, the ultra-strong proton conduction ability of PWA together with the interaction between positively charged CS chains and negatively charged PWA can construct effective proton transport channels with the help of 1D AT. The effects of PWA-coated AT on the structure and properties of CS were evaluated. The single direct methanol fuel cell performance was also tested.

2. Materials and Methods

2.1. Materials Attapulgite clay was obtained from the Huaiyuan Mining Industry Co. (Huai’an, Jiangsu Province, China). Chitosan (Mw = 500 kDa) with a degree of 90% deacetylation was purchased from Zhejiang Aoxing Biotechnology CO., Ltd. (Yuhuan, Zhejiang Province, China), DC5700 + [(CH3O)3Si(CH2)3N (CH3)2(C18H37)Cl−] was bought from Green Chem. International Co., Ltd. (Shanghai, China). BaCl2 was purchased from Annaiji Chemical Agent CO., Ltd. (Beijing, China). Ethanol (A.R.), acetic acid (A.R.), PWA (A.R.), and ammonia (A.R.) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sulfuric acid (98%) was supplied by Kaifeng Shenma Group Co., Ltd. (Kaifeng, China).

2.2. Synthesis of Phosphotungstic Acid Immobilized Attapulgite Phosphotungstic acid immobilized on attapulgite (WQAT) was prepared using the following main three steps: Firstly, AT was ultrasonically dispersed in a H2SO4 solution (2 M) at 60 ◦C for 2 h. 2 The suspension was ﬁltered, followed by rinsing with deionized water until SO4 − was undetectable by a BaCl2 solution. The obtained sample was dried and gained acid-activated AT. Secondly, 2 g of acid-activated AT was dispersed into the mixture of ethanol and deionized water (1:1, volume ratio), and ultrasonically oscillated for 30 min. After adjusting the pH to 10–12 using NaOH solution, 15 mL of DC5700 was added to the mixture under ultrasonic dispersing at 70 ◦C for 120 min, and then slowly stirred for 12 h. The mixture was ﬁltered, repeatedly washed by water and dried at 80 ◦C to obtain quaternized AT (QAT). Finally, WQAT was obtained by immersing QAT in a PWA solution 1 (5 mg mL− ) for 24 h and then washed by deionized water to remove the residual PWA, and dried in a vacuum oven at 60 ◦C overnight.

2.3. Preparation of CS-Based Composite Membranes An amount of 0.7 g of CS was dissolved in a 20 mL of acetic acid aqueous solution (2% v/v) and stirred at room temperature. Simultaneously, a certain amount of nanoparticle was dispersed into a 10 mL of acetic acid solution with ultrasonic treatment for 30 min. Then, these two solutions were mixed together and stirred vigorously for another 24 h. The resultant homogenous mixture was cast onto a clear glass plate and dried at 40 ◦C for 24 h to obtain composite membranes. To easily uncover the membranes and remove the remained acetic acid, the membrane was immersed in a NaOH solution for 2 h and then rinsed with deionized water until the pH value was 7. Subsequently, the membrane was ionic cross-linked by immersion in a H2SO4 solution (2 M) for 24 h and then extensively washed with water to remove the residual H2SO4. Finally, the membrane was dried under vacuum at 30 ◦C for 24 h. The obtained composite membranes are named as CS/WQAT-x, where x is the mass percent of the WQAT in the CS matrix. The average thickness of the dry membranes falls in the range of 50–55 µm.

2.4. Characterization Nicolet 380 Fourier transform infrared spectrometer (Thermo Electron Co., Waltham, MA, USA) and a X-ray photoelectron spectroscopy (XPS, ThermoFisher Scitiﬁc Co., Waltham, MA, USA) were used to character the chemical structure of AT and modiﬁed AT samples. The thermal properties of Nanomaterials 2020, 10, 1641 4 of 15

WQAT and composite membranes were conducted by thermo gravimetric analysis (TGA). TGA was carried out on a STA 499F instrument (NETZSCH Co., Germany) from 30 to 800 ◦C with a heating 1 rate of 10 ◦C min− under nitrogen atmosphere after being kept for 5 min at 130 ◦C to remove the absorbed water. The thermal transition behavior of the membranes was tested on a diﬀerential scanning calorimeter (DSC, NETZSCH Co., Selb, Germany). The samples were ﬁrst preheated from room 1 temperature to 130 ◦C with 10 ◦C min− under nitrogen atmosphere, then cooled to 90 ◦C, and reheated to 260 ◦C. In order to verify the dispersion of nanorods in the composite membranes, Scanning Electron Microscope (JSM-6510, Electronics Co., Fukuoka, Japan) was used to observe the surface and cross-sections of the membranes at 5 kV. The membrane sample was freeze-fractured in liquid nitrogen previously and then sputtered with gold. The mechanical properties of the hybrid membranes (1.0 4.0 cm) were investigated by a universal tensile testing machine (AG-IC 5KN, Shimadzu Co., × 1 Kyoto, Japan) using an elongation rate of 2 mm min− at room temperature. Water uptake and swelling ratio of the membranes were carried out through measuring the changes in the weight and area of the samples in dry and wet conditions. After weighing the weight (Wdry) and recording the size (Adry), the dry membrane was soaked in deionized water for 24 h at 80 ◦C and then recorded the weight (Wwet) and size (Awet) of the wet sample. Water uptake and swelling ratio of the membrane are calculated according to the following equations:

Wwet Wdry Water uptake (%) = − 100% (1) Wdry ×

Awet Adry Area swelling(%) = − 100% (2) Adry × Proton conductivity was obtained by electrochemical impedance spectroscopy (EIS) on an electrochemical station (Autolab PGSTAT 302N, Netherland) with voltage amplitude of 5 mV in the frequency range of 1 Hz–1 MHz. Initially, all the membranes were fully hydrated by immersing in deionized water for 24 h, and then put onto a home-made two-electrode mould with platinum wires as electrodes. After that, the measurements were carried out in the temperature range of 20–80 ◦C. 1 The calculation formula of proton conductivity (σ, S cm− ) is as follows:

L σ = (3) A R × where L (2.16 cm) is the distance between the two electrodes, and R and A are the resistance (Ω) and cross-sectional area (cm2) of the membrane sample, respectively.

2.5. DMFC Performance and Methanol Crossover Test

2 The DMFC single cell performance was evaluated at 70 ◦C with an active area of 4 cm (2 cm 2 cm). 2 × 2 The anode and cathode catalysts were Pt-Ru with a loading of 4.0 mg cm− and Pt of 2.0 mg cm− , respectively. The membrane was sandwiched between the anode and cathode, and then hot pressed 1 for 3 min at 125 ◦C and 2 MPa to obtain a membrane electrode assembly. Methanol solution (1 mol L− 1 H2SO4 solution in different ratio) was fed as a fuel into the anode at a flow rate of 1 mL min− by a 1 peristaltic pump, and oxygen was fed into the cathode at a flow rate of 90 mL min− . The current density (I) and potential (V) of the fuel cells were recorded by an electrochemical workstation (Autolab 1 PGSTAT 302N, Herisau, Switzerland) at a scan rate of 5 mA s− . Methanol crossover was tested using a voltammetric method. A methanol solution (1, 2 or 5 M) 1 was fed at a flow rate of 1.0 mL min− into the anode side of the MEA while the cathode side was 1 kept in an inert N2 atmosphere at a flow rate of 90 mL min− . The whole test setup was kept at a temperature of 70 ◦C. By applying a positive potential at the cathode side, the flux rate of permeating methanol was determined by measuring the steady-state limiting current density resulting from the complete electro-oxidation at the membrane/Pt catalyst interface at the cathode side. Nanomaterials 2020, 10, 1641 5 of 15 Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 15

3.3. Results Results and and Discussion Discussion

3.1.3.1. Synthesis Synthesis and and Characterization Characterization of of WQAT WQAT TheThe synthesis synthesis processprocess of WQAT WQAT could could be be divided divided into into three three steps steps as asillustrated illustrated in Scheme in Scheme 1: The1: Thefirst ﬁrst step step is a conventional is a conventional acid acidtreatment, treatment, which which can introduce can introduce some somehydroxyl hydroxyl groups groups on the onsurface the surfaceof AT. With of AT. the With aid theof these aid of introduced these introduced ‐OH groups, -OH DC5700, groups, DC5700,a long‐chain a long-chain silane coupling silane agent coupling with agentquaternary with quaternary ammonium ammonium groups, grafted groups, onto grafted the ontosurface the of surface AT to of obtain AT to QAT obtain through QAT through typical typicalhydrolysis hydrolysis and condensation and condensation reactions. reactions. In the following In the following step, with step, the with AT thenanorods AT nanorods and the and surface the surfacegrafted grafted quaternary quaternary ammonium ammonium groups groupsacting as acting a template as a template and anchoring and anchoring sites respectively, sites respectively, PWA, a PWA,heteropolyacid a heteropolyacid with strong with acidity strong and acidity have and excellent have proton excellent transport proton ability, transport which ability, can whichimmobilize can immobilizeon AT through on AT a throughstrong acid a strong‐base interaction acid-base interactionbetween positively between positivelycharged QAT charged and QATnegatively and negativelycharged PWA. charged PWA.

Scheme 1. Synthesis process of WQAT and CS/WQAT-x composite membranes. Scheme 1. Synthesis process of WQAT and CS/WQAT‐x composite membranes. FTIR, TG, and XPS were used to characterize the synthesized WQAT powders. The chemical compositionFTIR, TG, of pristine and XPS AT were and modifiedused to characterize AT was confirmed the synthesized by FTIR as WQAT shown inpowders. Figure1 a.The As chemical for the FTIRcomposition spectra of of AT, pristine QAT, AT and and WAQT, modified the three AT was samples confirmed showed by the FTIR characteristic as shown in absorption Figure 1a. bands As for 1 ofthe attapulgite FTIR spectra at 3400–3600 of AT, QAT, cm− and, which WAQT, correspond the three to samples the four showed hydroxyl the structures characteristic of the absorption hydroxyl stretchingbands of vibrationattapulgite of at bonds 3400–3600 to aluminum cm−1, which and/or correspond magnesium, to thethe hydroxyl four hydroxyl stretching structures vibration of the of adsorbedhydroxyl water, stretching and the vibration bending of vibration bonds to of aluminum zeolite water and/or in the magnesium, channels of ATthe [ 31hydroxyl]. The absorption stretching 1 peaksvibration at 1030 of adsorbed and 1018 water, cm− andwere the characteristic bending vibration peaks of zeolite symmetric water and in the asymmetric channels stretchingof AT [31]. vibrationThe absorption of Si-O peaks of Si-O-Si at 1030 and and Si-O-Al, 1018 cm respectively.−1 were characteristic Besides, the peaks FTIR of spectrum symmetric of and QAT asymmetric appeared 1 twostretching new absorption vibration of peaks Si‐O at of 2924.1 Si‐O‐Si and and 2853.0 Si‐O‐Al, cm respectively.− , which corresponded Besides, the FTIR to the spectrum C-H stretching of QAT vibrationappeared of two CH 3newand CHabsorption2 in the alkylpeaks chain at 2924.1 of DC5700, and 2853.0 indicating cm−1 that, which AT wascorresponded successfully to modified the C‐H by DC5700. The characteristic IR bands of HPW in WQAT were somewhat different from those of stretching vibration of CH3 and CH2 in the alkyl chain of DC5700, indicating that AT was successfully 1 1 bulkmodified HPW. by Among DC5700. the The four characteristic characteristic IR bands bands of of HPW, HPW those in WQAT at 1080 were cm− somewhat(P-Oa) and different 981.4 cm from− (W-O ) were overlapped by the strong and broad Si-O-Si band and Si-O band of AT, while those at thosed of bulk HPW. Among the four characteristic bands of HPW, those at 1080 cm−1 (P‐Oa) and 981.4 890.3 cm 1 (W-O -W, corner-sharing) and 798.8 cm 1 (W-O -W, edge-sharing) shifted to 897.1 cm 1 cm−1 (W−‐Od) wereb overlapped by the strong and broad− Si‐Oc‐Si band and Si‐O band of AT, while those− and 817.0 cm 1, respectively. These shifts are probably assigned to the formation of the secondary at 890.3 cm−1− (W‐Ob‐W, corner‐sharing) and 798.8 cm−1 (W‐Oc ‐W, edge‐sharing) shifted to 897.1 cm−1 structureand 817.0 of cm phosphotungstic−1, respectively. acid These with shifts a strong are probably chemical assigned interaction to betweenthe formation oxygen of terminatedthe secondary of + heteropolyacidstructure of phosphotungstic polyanion and acid H5O with2 of a QAT,strong which chemical illustrated interaction HPW between has a good oxygen stability terminated on the of surface of QAT [32]. heteropolyacid polyanion and H5O2+ of QAT, which illustrated HPW has a good stability on the surface of QAT [32].

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Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 15

FigureFigure 1. 1.( a()a) FTIR FTIR and and ( (bb)) XPSXPS full-scanfull‐scan spectra spectra of of AT AT and and functionalized functionalized AT. AT.

Figure 1. (a) FTIR and (b) XPS full‐scan spectra of AT and functionalized AT. XPSXPS was was used used to to verify verify the the elements elements (such (such asas AlAl 2p, Si 2p, 2p, Si Si 2s, 2s, C C 1s, 1s, O O 1s, 1s, and and others) others) presented presented onon the the surface surface of of AT. AT. From From the the XPS XPS spectrum spectrum of of QATQAT (Figure(Figure1 1b),b), it can be found found that that a a new new N1s N1s peak peak XPS was used to verify the elements (such as Al 2p, Si 2p, Si 2s, C 1s, O 1s, and others) presented appearedappeared at at 401.86 401.86 eV, eV, which which was was assigned assigned to to thethe quaternaryquaternary amine amine groups groups from from the the grafted grafted DC5700 DC5700 on the surface of AT. From the XPS spectrum of QAT (Figure+ 1b), it can be found that a new N1s peak chains,chains, indicating indicating that that DC5700 DC5700 long long chains chains containing containing -NR ‐NR3 3groups+ groups were were successfully successfully grafted grafted onto onto the appeared at 401.86 eV, which was assigned to the quaternary amine groups from the grafted DC5700 surfacethe surface of AT of through AT through the reaction the reaction between between the the ethoxy ethoxy group group of of DC5700 DC5700 and and the the hydroxyl hydroxyl groups groups of chains, indicating that DC5700 long chains containing ‐NR3+ groups were successfully grafted onto AT.of For AT. WQAT, For WQAT, except except for the for peaks the peaks similar similar to QAT, to aQAT, new a peak new of peak W4f of at W4f a binding at a binding energy energy of 35.34 of eV the surface of AT through the reaction between the ethoxy group of DC5700 and the hydroxyl groups 35.34 eV became remarkably visible. The result also proved that HPW was anchored onto the surface becameof AT. remarkably For WQAT, visible. except Thefor the result peaks also similar proved to that QAT, HPW a new was peak anchored of W4f onto at a the binding surface energy of DC5700 of of DC5700 modified AT through the strong electrostatic interaction between+ ‐NR3+ cations of grafted modified35.34 eV AT became through remarkably the strong visible. electrostatic The result interaction also proved between that HPW -NR3 wascations anchored of grafted onto the DC5700 surface and DC57003 and PW12O403− anions of PWA. PWof12 DC5700O40 − anions modified of PWA.AT through the strong electrostatic interaction between ‐NR3+ cations of grafted To determine the coating content of PWA in WQA, TGA was carried out in a nitrogen flow from DC5700To determine and PW12 theO403 coating− anions content of PWA. of PWA in WQA, TGA was carried out in a nitrogen flow from 30 to 800 °C. The obtained TGA and derivative thermo‐gravimetric (DTG, dW/dT) curves are shown 30 to 800To ◦determineC. The obtained the coating TGA content and derivative of PWA in thermo-gravimetric WQA, TGA was carried (DTG, out dW in a/dT) nitrogen curves flow are from shown in Figure 2a,b. As for the pristine AT, it showed a three weight loss thermal degradation behavior: in30 Figure to 8002 a,b.°C. The As forobtained the pristine TGA and AT, derivative it showed thermo a three‐gravimetric weight loss (DTG, thermal dW/dT) degradation curves are behavior: shown The first loss is due to the adsorbed water and zeolite water in the attapulgite; the second loss comes Thein firstFigure loss 2a,b. is due As tofor the the adsorbed pristine AT, water it showed and zeolite a three water weight in the loss attapulgite; thermal degradation the second behavior: loss comes from the removal of structural water; the third loss is because of the destruction of the internal fromThe the first removal loss is due of structural to the adsorbed water; water the third and loss zeolite is because water in of the the attapulgite; destruction the of thesecond internal loss comes hydroxyl hydroxyl groups of AT. As for QAT, the first weight loss (about 4.06%) was lower than that of AT groupsfrom ofthe AT. removal As for of QAT, structural the first water; weight the loss third (about loss 4.06%) is because was lowerof the than destruction that of ATof (9.86%),the internal which (9.86%), which was mainly attributed to the hydrophobic organic long chains covering the surface of washydroxyl mainly groups attributed of AT. to theAs hydrophobicfor QAT, the first organic weight long loss chains (about covering 4.06%) thewas surface lower than of AT. that Apart of AT from AT. Apart from this, QAT had a weight loss of 12.24% in the range of 150–270 °C and 29.09% in the this,(9.86%), QAT hadwhich a weight was mainly loss of attributed 12.24% in to the the range hydrophobic of 150–270 organicC and long 29.09% chains in covering the range the of surface 280–600 of C, range of 280–600 °C, which should be due to the introduction◦ of a new bond of Si‐O‐Si between◦ AT. Apart from this, QAT had a weight loss of 12.24% in the range of 150–270 °C and 29.09% in the whichorgano should‐silane be coupling due to the agent introduction and AT and of the a new decomposition bond of Si-O-Si of DC5700 between organic organo-silane molecular couplingchains. range of 280–600 °C, which should be due to the introduction of a new bond of Si‐O‐Si between agentAfter and PWA AT andcoating, the decompositionWQAT exhibited of a DC5700 similar organicdecomposition molecular procedure chains. Afterto that PWA of QAT. coating, From WQAT the organo‐silane coupling agent and AT and the decomposition of DC5700 organic molecular chains. exhibiteddifference a similar of the final decomposition residual content procedure between to thatQAT of and QAT. WQAT, From the the loading difference content of the of PWA final residualcould After PWA coating, WQAT exhibited a similar decomposition procedure to that of QAT. From the contentbe calculated between to QATbe 11.48%, and WQAT,because thePWA loading can keep content thermally of PWA stable could until 700 be calculated°C [33]. to be 11.48%, difference of the final residual content between QAT and WQAT, the loading content of PWA could because PWA can keep thermally stable until 700 ◦C[33]. be calculated to be 11.48%, because PWA can keep thermally stable until 700 °C [33].

Figure 2. TGA (a) and DTG (b) curves of AT, QAT and WQAT. Figure 2. TGA (a) and DTG (b) curves of AT, QAT and WQAT. Figure 2. TGA (a) and DTG (b) curves of AT, QAT and WQAT.

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3.2. Characterization of Pure CS and CS/WQAT Composite Membranes 3.2. Characterization of Pure CS and CS/WQAT Composite Membranes 3.2.1. Structure and Morphology 3.2.1. Structure and Morphology The dispersion of nanofillers in an organic‐inorganic composite system can affect the load The dispersion of nanofillers in an organic-inorganic composite system can affect the load transfer transfer from the polymer matrix to stiff inorganic nanofiller. Besides, the good distribution of 1D from the polymer matrix to stiff inorganic nanofiller. Besides, the good distribution of 1D WQAT in the WQAT in the composite membranes could also contribute to form new channel‐like pathways for composite membranes could also contribute to form new channel-like pathways for proton transport. proton transport. The cross‐sectional morphology of the composite membranes was probed using The cross-sectional morphology of the composite membranes was probed using Scanning Electron Scanning Electron Microscope (SEM) as shown in Figure 3a–f. The pure CS membrane (Figure 3a) Microscope (SEM) as shown in Figure3a–f. The pure CS membrane (Figure3a) showed a homogeneous showed a homogeneous and impact morphology without obvious cracks or micro‐voids. With the and impact morphology without obvious cracks or micro-voids. With the introduction of WQAT, which introduction of WQAT, which are nanorods with a diameter of ~10 nm and length of several microns are nanorods with a diameter of ~10 nm and length of several microns (as shown in Figure3g), obvious (as shown in Figure 3g), obvious white particle‐like substances can be observed in the composites (as white particle-like substances can be observed in the composites (as shown in Figure3b–f). These white shown in Figure 3b–3f). These white particles are WQAT nanorods distributed in the CS matrix after particles are WQAT nanorods distributed in the CS matrix after liquid nitrogen quenching during the liquid nitrogen quenching during the preparation of SEM cross‐sectional samples. Clearly, these preparation of SEM cross-sectional samples. Clearly, these WQAT nanorods were uniformly dispersed WQAT nanorods were uniformly dispersed in the polymer matrix as for the sample CS/WQAT‐2 and in the polymer matrix as for the sample CS/WQAT-2 and CS/WQAT-4, which can be verified by the CS/WQAT‐4, which can be verified by the Energy Dispersive X‐Ray Spectroscopy (EDX) mapping Energy Dispersive X-Ray Spectroscopy (EDX) mapping images of Si (Figure3h) and W (Figure3i). images of Si (Figure 3h) and W (Figure 3i). However, some WQAT agglomerations (marked with However, some WQAT agglomerations (marked with yellow circles in Figure3d–f) can be seen in the yellow circles in Figure 3d–f) can be seen in the cross‐sections of the composite membranes when the cross-sections of the composite membranes when the content of WQAT was more than 4%. content of WQAT was more than 4%.

Figure 3. CrossCross-sectional‐sectional SEM SEM images images of of the the membranes: membranes: (a) (CS,a) CS, (b) (CS/WQATb) CS/WQAT-2,‐2, (c) CS/WQAT (c) CS/WQAT-4,‐4, (d) (CS/WQATd) CS/WQAT-8,‐8, (e) ( CS/WQATe) CS/WQAT-10,‐10, (f) ( fCS/WQAT) CS/WQAT-15;‐15; (g (g) )SEM SEM image image of of WQAT; WQAT; EDX EDX mapping mapping images of Si (h) and W (ii)) distribution inin thethe CSCS/WQAT/WQAT-4‐4 membrane. membrane.

Nanomaterials 2020, 10, 1641 8 of 15

Nanomaterials 2019, 9, x FOR PEER REVIEW 8 of 15 Generally, the proton conduction in PEMs mainly occurs in amorphous phase rather than crystallineGenerally, phase the [8, 9proton]. Due toconduction the existence in ofPEMs intramolecular mainly occurs hydrogen in amorphous bonds between phase oxygen rather atoms than andcrystalline intermolecular phase [8,9]. hydrogen Due to bonds the existence between of amino intramolecular and hydroxyl hydrogen groups bonds in CS between molecule, oxygen CS regularly atoms accumulatesand intermolecular into a semi-crystalline hydrogen bonds structure between during amino the processand hydroxyl of film formation.groups in TheCS molecule, XRD patterns CS ofregularly the as-prepared accumulates membranes into a semi are‐crystalline depicted structure in Figure during4. The the pure process CS membrane of film formation. exhibited The typical XRD patterns of the as‐prepared membranes are depicted in Figure 4. The pure CS membrane exhibited three characteristic diffraction bands at 2θ = 12.2◦, 30.8◦ and 40.5◦. Obviously, as for the composite typical three characteristic diffraction bands at 2θ = 12.2°, 30.8° and 40.5°. Obviously, as for the membranes, the intensity of the strong band of CS matrix at 12.21◦ decreased first and then increased withcomposite increasing membranes, WQAT contents.the intensity Among of the all strong the membranes, band of CS CSmatrix/WQAT-4 at 12.21° haddecreased the weakest first di andffraction then intensity.increased with The reasonincreasing for thisWQAT trend contents. of intensity Among change all the is membranes, maybe because CS/WQAT the ordered‐4 had arrangement the weakest ofdiffraction CS molecular intensity. chains The is partlyreason destroyed for this trend by the of added intensity WQAT, change and is thus, maybe reduced because their the crystalline ordered domains.arrangement However, of CS molecular when the contentchains is of partly WAQT destroyed was high by (e.g., the aboveadded 4%), WQAT, the agglomeratedand thus, reduced inorganic their particlescrystalline (as domains. illustrated However, in SEM images when in the Figure content3) had noof obviousWAQT damagewas high ability (e.g., to theabove crystal 4%), areas. the agglomerated inorganic particles (as illustrated in SEM images in Figure 3) had no obvious damage This effect of WQAT on the crystallization ability of CS matrix may also bring some influences on the ability to the crystal areas. This effect of WQAT on the crystallization ability of CS matrix may also proton conductivity, which will be discussed in Section 3.2.4. bring some influences on the proton conductivity, which will be discussed in Section 3.2.4.

Figure 4. Comparison of XRD patterns of CS and CS/WQAT composite membranes. Figure 4. Comparison of XRD patterns of CS and CS/WQAT composite membranes. 3.2.2. Thermal and Mechanical Properties of the Composite Membranes 3.2.2. Thermal and Mechanical Properties of the Composite Membranes As a key component in PEMFCs or DMFCs, PEMs must have excellent thermal and mechanical stabilitiesAs a key for component long-time operation. in PEMFCs Theor DMFCs, thermal PEMs properties must have of the excellent as-prepared thermal membranes and mechanical were investigatedstabilities for by long TGA-DTG‐time operation. and DSC The as shown thermal in Figureproperties5. From of the Figure as‐5prepareda, the pure membranes CS membrane were exhibitedinvestigated two by thermal TGA‐ weightDTG and loss DSC stages: as shown (i) The firstin Figure stage may5. From be related Figure to 5a, the the degradation pure CS membrane of chitosan sideexhibited groups two around thermal 200–250 weight◦C; loss (ii) stages: the second (i) The stage first in stage the temperature may be related range to of the 250–350 degradation◦C is the of decompositionchitosan side groups of polymer around backbone 200–250 of °C; CS, (ii) which the second is similar stage to thein the result temperature in the literature range [of14 250–350]. After the °C incorporationis the decomposition of WQAT, of polymer the CS/ WQATbackbone composite of CS, which membranes is similar demonstrated to the result in similar the literature degradation [14]. behaviorAfter the to incorporation that of the pure of CS WQAT, membrane. the CS/WQAT By comparing composite the initial membranes decomposition demonstrated temperature similar of the firstdegradation stage, the behavior CS/WQAT to composite that of the membranes pure CS showedmembrane. a slightly By comparing higher degradation the initial temperature decomposition than thattemperature of pure CS of from the thefirst DTG stage, curves. the ThisCS/WQAT may be duecomposite to the interaction membranes between showed phosphotungstic a slightly higher acid anddegradation -NH2 of chitosan,temperature which than partly that inhibits of pure the CS movement from the of polymerDTG curves. segments This and may thus be increases due to the thermalinteraction stability between of the phosphotungstic composites. To acid further and confirm ‐NH2 of the chitosan, thermal which behavior, partly DSC inhibits was alsothe movement conducted fromof polymer 90 to 260segments◦C, and and the thus obtained increases curves the thermal are shown stability in Figure of the5 b.composites. Clearly, the To decompositionfurther confirm temperaturesthe thermal behavior, (Td) increased DSC was from also 217.4 conducted◦C (pure from CS membrane) 90 to 260 °C, to and 232.2 the◦C obtained (CS/WQAT-15 curves membrane) are shown within Figure the increase 5b. Clearly, in WQAT the content, decomposition verifying temperatures the enough thermal (Td) increased stability of from our composite 217.4 °C system(pure forCS themembrane) application to 232.2 in PEMFCs °C (CS/WQAT and DMFCs.‐15 membrane) with the increase in WQAT content, verifying the enough thermal stability of our composite system for the application in PEMFCs and DMFCs.

Nanomaterials 2020, 10, 1641 9 of 15 Nanomaterials 2019, 9, x FOR PEER REVIEW 9 of 15

Figure 5. ((a)) TGA TGA-DTG‐DTG and ( b) DSC curves of CS/WQAT CS/WQAT compositecomposite membranes.membranes. PEMs should possess enough mechanical strength and adequate flexibility to meet the requirements PEMs should possess enough mechanical strength and adequate flexibility to meet the of fuel cell assembly and operation. The tensile strength, elongation and modulus of the pure CS requirements of fuel cell assembly and operation. The tensile strength, elongation and modulus of and CS/WQAT composite membranes are shown in Table1. The pure CS membrane possessed a the pure CS and CS/WQAT composite membranes are shown in Table 1. The pure CS membrane tensile strength of 41.31 MPa, elongation of 17.01% and Young’s modulus of 618.9 MPa. As for the possessed a tensile strength of 41.31 MPa, elongation of 17.01% and Young’s modulus of 618.9 MPa. composite membranes, the tensile strength, elongation and Young’s modulus values increased with As for the composite membranes, the tensile strength, elongation and Young’s modulus values the increase in the content of WQAT up to 4 wt.%. The improvement of tensile strength and Young’s increased with the increase in the content of WQAT up to 4 wt.%. The improvement of tensile strength modulus might be attributed to the good dispersion of WQAT, which contributes to the formation and Young’s modulus might be attributed to the good dispersion of WQAT, which contributes to the of more acid-base pairs, and thus, can partly prevent the slippage of CS chains when the composites formation of more acid‐base pairs, and thus, can partly prevent the slippage of CS chains when the are subjected to stress. The increase of elongation, which is unlike other typical inorganic nanofiller composites are subjected to stress. The increase of elongation, which is unlike other typical inorganic reinforced composite systems, may be ascribed to the flexible DC5700 long chains grafted on AT. nanofiller reinforced composite systems, may be ascribed to the flexible DC5700 long chains grafted Among all the composite membranes, the CS/WQAT-4 membrane exhibited the highest tensile strength, on AT. Among all the composite membranes, the CS/WQAT‐4 membrane exhibited the highest elongation and Young’s modulus of 58.65 MPa, 23.21% and 1152.69 MPa, which improved by 42.0%, tensile strength, elongation and Young’s modulus of 58.65 MPa, 23.21% and 1152.69 MPa, which 36% and 86.25% respectively when compared to those of the pure CS membrane. However, with improved by 42.0%, 36% and 86.25% respectively when compared to those of the pure CS membrane. the further increase in WQAT content, the overall decrease of mechanical properties is due to the However, with the further increase in WQAT content, the overall decrease of mechanical properties agglomeration of WQAT, which weakens its ability of strengthening and toughening. Fortunately, is due to the agglomeration of WQAT, which weakens its ability of strengthening and toughening. the mechanical properties of the composite membranes were still better than those of the pure CS Fortunately, the mechanical properties of the composite membranes were still better than those of membrane. In summary, the above result showed that the incorporation of WQAT can enhance the the pure CS membrane. In summary, the above result showed that the incorporation of WQAT can thermal and mechanical stabilities of CS matrix, making the composite membranes more suitable for enhance the thermal and mechanical stabilities of CS matrix, making the composite membranes more DMFCs application. suitable for DMFCs application.

Table 1. Mechanical properties of the pure CS and CS/WQAT composite membranes. Table 1. Mechanical properties of the pure CS and CS/WQAT composite membranes.

SampleSample Tensile Tensile Strength Strength (MPa) (MPa) Elongation Elongation (%) (%) Young’s Young’s Modulus Modulus (MPa) (MPa) CS 41.31 2.0 17.07 4.8 618.90 10 CS 41.31 ± 2.0± 17.07 ± 4.8± 618.90 ± 10± CS/WQAT-2 50.14 2.7 20.89 3.1 990.48 11 CS/WQAT‐2 50.14 ± 2.7± 20.89 ± 3.1± 990.48 ± 11± CS/WQAT-4 58.65 2.5 23.21 1.8 1152.69 14 ± ± ± CS/WQATCS/WQAT-8‐4 58.65 52.37 ± 2.52.2 23.21 19.96 ± 1.82.5 1152.69 980.78 ± 1416 ± ± ± CS/WQATCS/WQAT-10‐8 52.37 48.67 ± 2.21.8 19.96 18.36 ± 2.54.5 980.78 890.54 ± 1612 ± ± ± CS/WQATCS/WQAT-15‐10 48.67 42.8 ± 1.83.8 18.36 16.36 ± 4.52.4 890.54 720.54 ± 1214 ± ± ± CS/WQAT‐15 42.8 ± 3.8 16.36 ± 2.4 720.54 ± 14 3.2.3. Water Uptake and Area Swelling of the Membranes 3.2.3. Water Uptake and Area Swelling of the Membranes Water molecules play an important role for proton conduction in proton exchange membranes becauseWater H+ moleculesions transport play generally an important along role the for hydrogen-bonded proton conduction network in proton formed exchange by water membranes (diﬀusion mechanism)because H+ ions or through transport the generally ion exchange along groups the hydrogen dissociated‐bonded by water network (hopping formed mechanism). by water (diffusion However, toomechanism) high water or uptake through generally the ion results exchange in excessive groups swellingdissociated of PEMsby water and thus(hopping sharply mechanism). decreases theirHowever, mechanical too high stability. water uptake In addition, generally it should results be in mentioned excessive thatswelling too big of swellingPEMs and ratio thus of sharply a PEM alsodecreases brings their a severe mechanical methanol stability. crossover In addition, from the it should anode be to mentioned cathode through that too electro-osmosis, big swelling ratio thus of a PEM also brings a severe methanol crossover from the anode to cathode through electro‐osmosis, thus producing a mixed potential to decrease the open circuit voltage. Therefore, the water

Nanomaterials 2020, 10, 1641 10 of 15 producing a mixed potential to decrease the open circuit voltage. Therefore, the water absorption and swelling of PEMs are important indicators for judging the quality of proton exchange membranes. The water uptake values of the CS/WQAT composite membranes with various weight fractions of WQAT at different temperatures are shown in Table2. Compared with the pure membrane, the water absorption values of the composite membranes increased by 8.35% to 41.16% at 80 ◦C with different content WQAT. This increase in water uptake may be because AT itself has many hydroxyls on its structure, which is helpful to absorbing water. Besides, the existence of heteropolyacid anions on the surface of AT could also contain water molecules through hydrogen bonds. As for the effect of temperature on moisture uptake, all the membranes showed an increasing trend of water content with the increase in temperature. Generally, the increased temperature can accelerate the movement of water molecules and polymer chains of chitosan, which makes H2O easily diffuse into the membranes.

Table 2. Water uptake (%) of the pure CS and CS/WQAT composite membranes.

Sample 20 ◦C 40 ◦C 60 ◦C 80 ◦C CS 68.15 5.24 72.03 6.52 88.05 7.38 114.11 7.7 ± ± ± ± CS/WQAT-2 71.21 3.37 74.00 5.61 90.11 4.28 121.17 8.5 ± ± ± ± CS/WQAT-4 72.09 3.58 74.76 4.53 92.69 3.52 161.08 7.3 ± ± ± ± CS/WQAT-8 70.05 4.12 74.86 5.72 86.21 6.92 150.93 5.7 ± ± ± ± CS/WQAT-10 68.44 4.87 75.86 3.57 89.27 5.47 137.78 8.3 ± ± ± ± CS/WQAT-15 65.49 3.73 75.98 4.38 85.87 3.39 123.64 9.5 ± ± ± ±

As for the area swelling of the composite membranes (as shown in Table3), all the membranes display increased area swelling values with the testing temperatures, which is similar to the result of water uptake. However, unlike the change trend of water uptake with the content of WQAT, the area swelling values follow a slight decrease trend, indicating the increased dimensional stability of the composites. The result is probably due to the strong interfacial interaction between CS and WQAT. In summary, the above results indicated that the composite membranes possessed higher water uptake ability and dimensional stability than those of the pure CS membrane.

Table 3. Area swelling ratio (%) of the composite membranes.

Sample 20 ◦C 40 ◦C 60 ◦C 80 ◦C CS 5.17 4.57 9.50 4.36 17.81 2.52 38.02 5.33 ± ± ± ± CS/WQAT-2 5.16 3.97 9.32 4.38 15.64 5.31 35.56 6.30 ± ± ± ± CS/WQAT-4 4.20 5.37 8.16 5.35 14.92 3.30 30.02 4.31 ± ± ± ± CS/WQAT-8 4.52 2.97 9.79 5.07 17.0 3.36 33.92 3.67 ± ± ± ± CS/WQAT-10 4.88 5.34 9.86 4.27 16.60 6.01 33.65 5.47 ± ± ± ± CS/WQAT-15 5.68 4.85 9.90 4.31 16.07 4.77 39.22 4.36 ± ± ± ±

3.2.4. Proton Conductivity, Methanol Permeability and Single Cell Performance The proton conductivities of the as-prepared membranes were tested at 100% R.H. in the temperature range from 20 to 80 ◦C using a two-probe electrochemical impedance spectroscopy method, and the results are shown in Figure6. According to the curves, the conductivity values of the composite membranes increased firstly and then decreased with the addition of WQAT. Among all the 1 membranes, the CS/WQAT-4 membrane showed a highest conductivity of 35.3 mS cm− at 80 ◦C that 1 was about 37% higher than that of the pure CS membrane (25.8 mS cm− ). To better understand the effect of WQAT on the proton conductivity, we also prepared the CS/AT composite membrane with 4% AT loading and tested its proton conductivity under the same condition. The result showed the proton 1 conductivity of the CS/AT (4% AT content) composite membrane was 28.8 mS cm− at 80 ◦C, which was much lower than that of the CS/WQAT composite membrane with the same inorganic nanofiller content, indicating the positive influence of the PWA surface medication on AT. This significantly Nanomaterials 2020, 10, 1641 11 of 15 improvedNanomaterials proton 2019, 9, conductivity x FOR PEER REVIEW and may be explained as follows: (1) the reduced degree of crystallinity11 of 15 of chitosan matrix (as proven in Figure5) is conducive to proton transport; (2) the super-acidic PWA of crystallinity of chitosan matrix (as proven in Figure 5) is conducive to proton transport; (2) the coated on the surface of AT can act as new H+ transfer sites; (3) with the assistance of 1D structure super‐acidic PWA coated on the surface of AT can act as new H+ transfer sites; (3) with the assistance of AT, the homogeneously dispersed WQAT (as shown in Figure3c) could construct continuous of 1D structure of AT, the homogeneously dispersed WQAT (as shown in Figure 3c) could construct proton transport nanopathways along the interface between CS and WQAT because of the acid-base continuous proton transport nanopathways along the interface between CS and WQAT because of interaction between the two components [29]; (4) the increased water uptake ability can also contribute the acid‐base interaction between the two components [29]; (4) the increased water uptake ability can to the fast transport of H+. However, it can also be noted that further increasing WQAT did not also contribute to the fast transport of H+. However, it can also be noted that further increasing WQAT produce higher proton conductivity. For example, the CS/WQAT-15 composite membrane exhibited did not produce higher proton conductivity. For example, the CS/WQAT‐15 composite membrane a conductivity of 25.8 mS cm 1 (80 C) that was 73% of that of the CS/WQAT-4 membrane. Such a exhibited a conductivity of 25.8− mS ◦cm−1 (80 °C) that was 73% of that of the CS/WQAT‐4 membrane. trend is unsurprising because the aggregated inorganic nanofiller may partly block the fast proton Such a trend is unsurprising because the aggregated inorganic nanofiller may partly block the fast transportation in the composite to some extent. proton transportation in the composite to some extent.

Figure 6. Proton conductivity of the composite membranes at different temperatures. Figure 6. Proton conductivity of the composite membranes at different temperatures. In addition to proton conductivity, the penetration of methanol is another serious problem deservingIn addition to be discussed to proton because conductivity, methanol the crossover penetration can not of only methanol partly wasteis another the anode serious fuel, problem but also adeservingffect the output to be discussed voltage and because fuel cellmethanol performance crossover [34 ].can Considering not only partly the optimalwaste the properties anode fuel, of thebut CSalso/WQAT-4 affect the composite output voltage membrane and fuel among cell all performance the composites, [34]. Considering this sample was the selectedoptimal properties to be further of evaluatedthe CS/WQAT for the‐4 methanolcomposite permeability membrane among and single-cell all the composites, performance. this The sample methanol was crossoverselected to was be testedfurther using evaluated a voltammetric for the methanol method permeability [35], in which and the single limited‐cell current performance. density that The comes methanol from crossover complete electro-oxidationwas tested using a of voltammetric methanol permeation method [35], at the in interfacewhich the of limited membrane current/catalyst density is usedthat comes to evaluate from thecomplete methanol electro permeability.‐oxidation of Generally, methanol the permeation higher the at methanol the interface crossover of membrane/catalyst current density, is the used more to seriousevaluate the the methanol methanol permeability. permeability. Figure Generally,7a compares the higher the methanolthe methanol crossover crossover current current density density, for the more serious the methanol permeability. Figure 7a compares the methanol crossover current the hybrid membrane (CS/WQAT-4) at 70 ◦C in different methanol solutions (1 M, 2 M and 5 M); itdensity can be for seen the that hybrid the crossover membrane current (CS/WQAT density‐4) gradually at 70 °C increasedin different with methanol the methanol solutions concentration. (1 M, 2 M Thisand is5 M); because it can methanol be seen that permeability the crossover is more current serious density when gradually using a methanol increased solution with the with methanol higher concentration. This is because methanol permeability is more serious when using a methanol solution CH3OH concentration as the anode fuel, due to the bigger concentration gradient diffusion. At the samewith time,higher we CH also3OH compared concentration the methanol as the crossoveranode fuel, current due to densities the bigger of the concentration pure CS, CS/ WQAT-4gradient anddiffusion. commercial At the Nafion same time, 212 membraneswe also compared in a 2 M the methanol methanol solution. crossover As current shown densities in Figure 7ofb, the Nafion pure CS, CS/WQAT‐4 and commercial Nafion 212 membranes in a 2 M methanol2 solution. As shown in 212 membrane exhibited the highest current density of 535.9 mA cm− among the three membrane −2 samples,Figure 7b, indicating Nafion 212 its poormembrane methanol exhibited barrier the ability highest due current to its highly density hydrophobic of 535.9 mA/hydrophilic cm among phase the separatedthree membrane morphology samples, [35]. Asindicating for the pure its CSpoor membrane, methanol it alsobarrier showed ability a relatively due to high its currenthighly hydrophobic/hydrophilic2 phase separated morphology [35]. As for the pure CS membrane, it also density of 445 mA cm− , while this value for the CS/WQAT-4 hybrid membrane was only 325 mA showed2 a relatively high current density of 445 mA cm−2, while this value for the CS/WQAT‐4 hybrid cm− . This result revealed that the hybrid membrane had excellent methanol barrier ability, which −2 ismembrane attributed was to the only formation 325 mA of cm tortuous. This pathwaysresult revealed against that penetrating the hybrid the membrane methanol had molecules excellent in themethanol composites. barrier ability, which is attributed to the formation of tortuous pathways against penetrating the methanol molecules in the composites.

Nanomaterials 2019, 9, x FOR PEER REVIEW 12 of 15 Nanomaterials 20192020,, 910, x, 1641FOR PEER REVIEW 1212 of of 15 15

FigureFigure 7.7. Comparison of the methanol crossover current densities of (a) CSCS/WQAT/WQAT-4‐4 atat 7070 ◦°CC inin Figuremethanolmethanol 7. Comparison solutionssolutions with of different dithefferent methanol methanol methanol crossover concentrations; concentrations; current densities (b (b) )CS, CS, CS/WQAT of CS (/WQAT-4a) CS/WQAT‐4 and and Nafion‐ Nafion4 at 70 212 212°C at in at70 methanol solutions with different methanol concentrations; (b) CS, CS/WQAT‐4 and Nafion 212 at 70 70°C◦ inC ina 2 a M 2 Mmethanol methanol solution. solution. °C in a 2 M methanol solution. InIn order order to to further further evaluate evaluate the possibility the possibility of the practicalof the practical application application of our composite of our membranes, composite singlemembranes,In DMFC order single wasto further equipped DMFC evaluate was and testedequipped the at possibility 70 and◦C usingtested of methanol at the 70 °Cpractical using solution methanolapplication as the anode solution of fuelour as andcomposite the oxygen anode membranes,asfuel the and cathode oxygen single gas. as FigureDMFC the cathode8 awas depicts equipped gas. the Figure potential-current and 8atested depicts at 70 densitythe °C potentialusing (I-V) methanol and‐current the power solutiondensity density-current as(I‐ V)the and anode the fueldensitypower and density curves oxygen of‐current theas the CS density/cathodeWQAT-4 curves gas. composite Figure of the membrane CS/WQAT8a depicts at ‐the4 1composite M, potential 2 M and membrane‐current 5 M methanol density at 1 M, concentrations. (I 2‐V) M andand the5 M powerFrommethanol Figure density concentrations.8a,‐current the open density circuit From curves Figure voltages of 8a, the (OCVs) the CS/WQAT open were circuit‐ 0.744 composite voltages V, 0.67 V membrane(OCVs) and 0.64 were V at respectively 10.74 M, V,2 M 0.67 and when V 5 and M methanolthe0.64 methanol V respectively concentrations. concentrations when theFrom methanol were Figure 1 M, concentrations8a, 2 the M and open 5 M,circuit were which voltages 1 M, showed 2 M (OCVs)and a 5 similar M, were which trend 0.74 showed V, to 0.67 that a similarV of and the 2 0.64methanoltrend V respectivelyto that crossover. of the when methanol Simultaneously, the methanol crossover. the concentrations Simultaneously, peak power were densities the 1 M, peak 2 were M power and 45.23, 5 M, densities 70.26 which and showedwere 59.1 45.23, mW a similar cm70.26− trendrespectivelyand 59.1to that mW atof 1cmthe M,− 2methanol 2respectively M and 3crossover. M at methanol 1 M, Simultaneously, 2 solutions.M and 3 M It the ismethanol obvious peak power solutions. that the densities CS It/ WQAT-4is wereobvious 45.23, composite that 70.26 the andmembraneCS/WQAT 59.1 mW exhibited‐4 composite cm−2 respectively a highest membrane power at exhibited1 densityM, 2 M at a andhighest 2 M 3 methanol. M power methanol density So we solutions. further at 2 M comparedmethanol. It is obvious the So we single-cell that further the CS/WQATperformancecompared‐ 4the ofcomposite thesingle CS/‐WQAT-4cell membrane performance composite exhibited of membrane, thea highest CS/WQAT pure power CS‐4 density and composite commercial at 2 Mmembrane, methanol. Nafion 212 Sopure membranes we CSfurther and comparedusingcommercial 2 M methanolthe Nafion single 212 as‐cell the membranes performance anode fuel using at 70of 2◦the C.M FiguremethanolCS/WQAT8b shows as‐ 4the composite thatanode the fuel maximum membrane, at 70 °C. power Figure pure density 8bCS shows and of 2 commercialNafionthat the 212 maximum membrane Nafion 212power was membranes 79.87 density mW usingof cm Nafion− , 2 which M methanol212 was membrane only as slightly the wasanode higher 79.87 fuel thanmW at 70 thatcm °C.−2 of , Figurewhich the CS 8b/wasWQAT-4 shows only 2 −2 thatcompositeslightly the maximumhigher membrane than power that (70.26 of density the mW CS/WQAT cmof −Nafion) under‐4 composite212 the membrane same membrane test was condition. 79.87 (70.26 mW mW As cm a cm contrast,,− 2)which under the was the DMFC onlysame −2 2 slightlyequippedtest condition. higher with Asthan the a pure contrast,that CSof the membrane the CS/WQAT DMFC outputequipped‐4 composite the with peak the membrane power pure densityCS membrane(70.26 of mW as low outputcm as) 40.08under the peak mW the power cmsame− , testwhichdensity condition. was of onlyas lowAs 57% a as contrast, of40.08 that mW ofthe our DMFCcm composite−2, which equipped was membrane. withonly the57% The pure of improved that CS membrane of our proton composite output conductivity themembrane. peak together power The densitywithimproved decreased of asproton low methanol asconductivity 40.08 crossover mW cmtogether− may2, which be with responsible was decreased only 57% for methanol the of satisfactorythat of crossover our fuelcomposite cellmay performance be membrane. responsible of The our for improveddesignedthe satisfactory composite proton fuel conductivity membranes.cell performance together of withour designed decreased composite methanol membranes. crossover may be responsible for the satisfactory fuel cell performance of our designed composite membranes.

Figure 8. Polarization and power density curves of DMFC tests of (a) the CS/WQAT-4 composite Figure 8. Polarization and power density curves of DMFC tests of (a) the CS/WQAT‐4 composite membrane at diﬀerent methanol concentrations, and (b) diﬀerent PEMs at 70 C with a 2 M methanol Figuremembrane 8. Polarization at different and methanol power concentrations, density curves and of DMFC(b) different tests PEMsof (a) atthe 70 CS/WQAT◦ °C with a 2‐4 M composite methanol solution as the anode fuel. membranesolution as at the different anode fuel.methanol concentrations, and (b) different PEMs at 70 °C with a 2 M methanol solution as the anode fuel.

Nanomaterials 2020, 10, 1641 13 of 15

4. Conclusions In summary, super-strong proton conductor, PWA, anchored AT was prepared and directly used as a novel nanoﬁller employed in the chitosan matrix to fabricate composite proton exchange membranes. The mechanical strength of the composite membranes increased owing to the incorporation of uniformly-dispersed 1D WQAT, which can serve as physical cross-linking points to prohibit the rupture of polymer chains. Moreover, the ultra-strong proton conduction ability of PWA together with the interaction between positively charged CS chains and negatively charged PWA could construct eﬀective proton transport channels with the help of 1D AT. As a result, the proton conductivity of the CS/WQAT-4% composite membrane increased by 31.8% when compared with that of the pure CS membrane at 80 ◦C. Besides, the methanol permeability of the composite membrane can also be remarkably decreased. The increased ionic conductivity and decreased methanol permeability lead to the increase of maximum power density for the composite electrolyte, which exhibited the value of 2 70.26 mW cm− at 70 ◦C (2 M methanol as the anode fuel), whereas the pristine membrane displayed 2 only 40.08 mW cm− .

Funding: This research received no external funding. Conﬂicts of Interest: The author declares no conﬂict of interest.

References

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