ORIGINAL PAPER

Strengthen the performance of sulfonated poly(ether etherketone) as proton exchange membranes with phosphonic acidfunctionalized carbon nanotubes

Wen Zhang1 &Hui Zheng1 &Chengyi Zhang1 & Baochen Li1 & Feifei Fang1 &YuxinWang1

Received: 5 December 2016 /Revised: 2 February 2017 /Accepted: 13 February 2017 /Published online: 23 February 2017# Springer-Verlag Berlin Heidelberg 2017

Abstract Nanocomposite polymers based on phosphonic ac-id functionalized carbon nanotubes (CNT-POH) and sulfonat-ed poly(ether ether ketone) (SPEEK) have been fabricated andemployed as highly efficient proton exchange membranes.CNT-POH were synthesized through the grafting of carbonnanotubes (CNT) with diethylphosphatoethyl triethoxysilaneand subsequent acidification of phosphate to phosphonic acidligands. Incorporating CNT-POH into SPEEK matrix im-proves the proton conductivity at different temperatures andrelative humidity, which can be attributed to the homogeneousdispersion of highly hydrophilic phosphonic acid groups andthe formation of proton transport channels in the membrane.The methanol permeability of the composite membranes isalso decreased, owing to the increased tortuosity of the meth-anol transport channel. The CNT in SPEEK matrix also en-hance the dimensional stability and mechanical property re-markably. Consequently, this phosphonic acid functionalizedCNT/SPEEK composite membrane (SPEEK-POH) is a poten-tial candidate for application in direct methanol fuel cells(DMFC).

Keywords Phosphonic acid . SPEEK . Carbon nanotubes .

Proton exchangemembrane

Introduction

Direct methanol fuel cells (DMFC), with the advantages ofliquid fuel and high power and energy density, are believed tobe a very promising portable power source [1, 2]. The protonexchangemembrane (PEM) is a critical component in DMFC,which is employed to transport protons and impede methanolcrossover from the anode to the cathode.

The most commonly used PEM materials for DMFC aresulfonated polymers. Perfluorosulfonated ionomers, such asNafion, possess superior chemical durability and high protonconductivity. But their high methanol permeability and highcost limit their practical application in commercial DMFC.Sulfonated poly(ether ether ketone) (SPEEK) membrane isone of the most promising membranes because of its lowcosting, high mechanical strength, and fine chemical durabil-ity [3, 4]. A SPEEK membrane can be produced with a highdegree of sulfonation (DS) [5] so as to achieve high protonconductivity. However, with increasing DS, the SPEEKmem-brane would demonstrate an increase in methanol permeabil-ity and decrease in mechanical property. To solve the problem,embedding particulate fillers in the SPEEK matrix of mem-brane was attempted by many researchers [6–9].

Recently, strengthening the performance of proton ex-change membranes with carbon nanotubes (CNT) as fillerswas reported [10–13]. Own to huge length-diameter ratio,carbon nanotubes (CNT) have mechanical properties that arefar in excess of conventional materials used in engineeringpolymer composites. Effective reinforcement of polymersusing carbon nanotubes is feasible if CNT is dispersed uni-formly in polymers. There would be no short circuit if keepingthe content of CNT lower than the percolation threshold [14].Incorporating the polymer matrices with CNT can also con-siderably reduce the methanol permeability [15, 16].However, a decline in the proton conductivity of the CNT

Electronic supplementary material The online version of this article(doi:10.1007/s11581-017-2030-0) contains supplementary material,which is available to authorized users.

* Wen Zhangzhang_wen@tju.edu.cn

1 School of Chemical Engineering and Technology, State KeyLaboratory of Chemical Engineering, Tianjin Key Laboratory ofMembrane Science and Desalination Technology, Tianjin University,Tianjin 300072, People’s Republic of China

Ionics (2017) 23:2103–2112DOI 10.1007/s11581-017-2030-0

embeddedmembrane was observed, because of the changes inthe orientation and size of proton-channels caused by thefillers [17, 18]. To circumvent this weakness and thus to im-prove the performance of PEM, the incorporation of CNTmodified with strong protonic acid could be a better choice.Suzana et al. [19] embedded sulfonated multi-walled carbonnanotubes into SPEEK matrix, showing a better DMFC per-formance than Nafion membranes. Yeong et al. [12] incorpo-rated sulfonated poly(arylene sulfone) (SPAS) membranewithsulfonated CNT, and the resulted composite membranes ex-hibit higher ion conductivity, enhanced mechanical strength,and lower methanol permeability compared with the unfilledSPAS membrane. However, almost all the reported CNTfillers in Nafion or SPEEK were modified by sulfonic acid,and there is no report on phosphonic acid functionalized CNTfor PEM use, to the best of our knowledge.

Compared with sulfonic acid, phosphonic acid demon-strates significant potential advantages for usage in PEM interms of charge carrier density, thermal stability, and oxidationresistance [20, 21]. Owning to the dynamic hydrogen-bondnetworks and amphoteric peculiarity, the neat phosphonic acidhas a relatively high conductivity. Because of the strong hy-drogen bonding, the proton transport in phosphoric acid isdominated by structural diffusion instead of vehicular diffu-sion [22, 23]. Even under the conditions of intermediate tem-perature and low humidity, phosphonic acid shows rather highproton conductivity [24]. Zhang et al. [23] prepared a phos-phoric acid doped quaternized poly(ether ether ketone) mem-brane, which has a high proton conductivity and tensilestrengths at elevated temperature. Jiang et al. [25] preparedpoly(vinylphosphonic acid-co-divinylbenzene)/silica nano-tubes and showed enhanced proton conductivity and stability.

However, the chemical bonding of carbon nanotubes withphosphonic acid groups has not been reported so far, to ourbest knowledge.

Here, we report the synthesis and characterization of a nov-el phosphonic acid-grafted CNT/SPEEK composite mem-brane endowing the membrane with high conductivity at dif-ferent humidity. The doping effect of CNT on methanol per-meability, proton conductivity, mechanical properties, thermalstability, ion-exchange capacity, swelling ratio, and water up-take was presented and discussed.

Experimental

Materials

Multi-walled carbon nanotubes (CNT, diameter 10–20 nm,specific surface area 40–300 m2/g, purity >95%) was pur-chased from Shenzhen Nanometer Gang Co., Ltd. Poly(etherether ketone) (PEEK) pellets (450P, Mw¼ 38.3 K) were re-ceived from Victrex. Diethylphosphatoethyl triethoxysilane(PETES, 98%) was purchased from Gelest. Anhydrous di-methyl formamide (DMF) was obtained via the distillationof a DMF-benzophenone solution containing metal Na. Theother chemicals were analytical reagent grade and used with-out further purification.

Functionalization of CNT

The synthesis of phosphonic acid functionalized CNT con-tains four steps, as illustrated in Fig. 1. The oxidation ofCNT was realized by a modified Hummers method [26].

Fig. 1 Synthesis of phosphonicacid functionalized CNT

2104 Ionics (2017) 23:2103–2112

Typically, 20 mL of concentrated sulfuric acid, 0.5 g of CNT,and 0.5 g of NaNO3 were sequentially added to a 250 mLround-bottom flask immersed in an ice bath at 0–5 °C andstirred for 15 min. Then, 2.8 g of KMnO4 was added slowlyover 20 min and stirred for 2 h. The flask was then transferredto a water bath at 35 °C. About 35 mL of deionized water wasadded to the flask very carefully followed by additional stir-ring for 1 h at 90 °C. The mixture and 3 mL 30% H2O2 wasadded to 100 mL deionized water and stirred for 1 h. Finally,the mixture was filtrated and the oxidized CNT precipitatewas dried overnight. This oxidized CNT is labeled as CNT-COH. Then, CNT-COH was grafted with phosphonate esterby reacting 0.3 g CNT-COH with 0.01 mol PETES in 50 mLanhydrous DMF at 120 °C under N2 atmosphere for 24 h.After the reaction with PETES, the phosphonate ester of thefunctionalized CNT-COH was cleaved to phosphonic acidgroups by refluxing in 48 wt% H2SO4 solution; ca. 1 g ofthe product was suspended in 300 mL H2SO4 solution andrefluxed, with stirring, for 24 h at 105 °C, then filtrated. Theresulting powder was washed repeatedly with water till thefiltrate was neutral, and then dried overnight. This phosphonicacid functionalized CNT powder is labeled as CNT-POH.

Incorporating SPEEK with CNT

SPEEK (63% sulfonated degree) was prepared using the samemethod previously reported by our laboratory [27, 28].Membranes were prepared by solution casting method. In atypical procedure, a solution comprising 0.008 g CNT-COHor CNT-POH, 0.4 g SPEEK, and 3.6 g dimethylacetamide(DMAc) were stirred and then sonicated to acquire a homo-geneous solution. The blend membranes consisting of 2 wt%functional CNT were prepared via the solution casting meth-od. The resulting membranes were dried at 80 °C for 12 h,then 120 °C for 4 h. After that, the membranes were soaked in1 mol/L H2SO4 at room temperature for 24 h and then washedwith deionized water, repeated three times. The thickness ofthe above membranes was controlled to be in 90–100 um. Forcontrastive research, the unmodified SPEEK membrane wascasted in the above method without functional CNT. The in-corporating SPEEKwith CNT-COH and CNT-POH is labeledas SPEEK-COH and SPEEK-POH, respectively.

Characterization

13C and 31P cross-polarization magic angle spinning solid-state nuclear magnetic resonance (MAS NMR) data weremeasured on a Bruker AV300 NMR spectrometer, referencedto tetramethylsilane and phosphoric acid, respectively. Themorphology and structure of the CNT were observed by atransmission electron microscopy (TEM, The Netherlands,FEI TECNAI F-20). The accelerating voltage was 200 kV.X-ray photoelectron spectroscopy (XPS) was implemented

on a PHI Quantera SXM spectrometer with a monochromaticAl X-ray at 24.9 W. Microstructural and composition analysiswere conducted via a field-emission scanning electron micro-scope (SEM, JXA-8530F, JEOL) with the electron probe mi-croscopic analyzer (EPMA) and the energy dispersive X-rayspectroscopy (EDX). Prior to the SEM measurements, themembrane samples were submerged in 2 mol/L sodium hy-droxide solution for 12 h, washed thoroughly with deionizedwater, and fractured in liquid nitrogen to expose the cross-section area.

Thermal gravimetric analysis (TGA) was carried out underconstant N2 flow at a heating rate of 10 °C/min using a thermalanalyzer (Netzsch STA 449 F1).

Water uptake, swelling ratio, ion-exchange capacity,and proton conductivity

The membranes were dried at 100 °C in a vacuum oven for24 h. And the loss on drying can be expressed by the subtrac-tionmethod in the percentage of weight. The membranes werecutted to the size of 20 mm × 20 mm, and then immersed indeionized water at different temperatures between 30 and60 °C. After 24 h, the outside surface of the wet membraneswas lightly wiped with filter paper. The water uptake can becalculated using the following equation:

Water uptake %ð Þ ¼ Wwet−Wdry

� �

Wdry� 100

The swelling experiments were carried out in 1 mol/Lmethanol solution at different temperatures in the aboveway. And the size of dried membranes (before swelling) andwetted membranes (after swelling) were measured. The swell-ing ratio can be calculated using the following equation:

Swelling ratio %ð Þ ¼ Swet−Sdry� �

Sdry� 100

The number of available surface sites was analyzed viaacid-base titration

Before titration, the dried membranes were immersed in asaturated NaCl solution for 24 h to displace the H+ ions withNa+ ions. The value of ion-exchange capacity (IEC) was cal-culated as

IEC mmol=gð Þ ¼ NNaOH � VNaOH

Massdry� 1000

After full hydration by soaked in deionized water, themembranes were clamped vertically between two compart-ments, which were filled with 1 mol/L methanol solution(compartment A) and deionized water (compartment B),

Ionics (2017) 23:2103–2112 2105

respectively. The magnetic agitators were used to homogenizethe solution contained in the two compartments. The concen-tration ofmethanol was monitored in real time by a differentialrefractometer (LCD201, WINOPAL) and a peristaltic pump.The methanol permeability can be determined using the fol-lowing equation:

P cm2=s� � ¼ S � VA � L

A� CB0

where S is the slope of the least squares fitting line according tothe methanol concentration versus time in compartment A, andVA (mL) is the volume of the compartment A. L (cm) is thethickness of membranes and A (cm2) is the diffusion area. CB0

(mol/L) is the initial methanol concentration in compartment B.Two point probe alternating current impedance spectrosco-

py (PARSTAT 2273, 90 USA) were used to determine thethrough-plane proton conductivity of the membranes. The fre-quency range was from 0.1 kHz to 1 MHz. Before the protonconductivity testing, all the samples were stuck in deionizedwater for 48 h at room temperature. The measurements atdifferent temperatures and relative humidity were carried bythe membrane test system (Scribner Associates Inc., MTS740), and the membrane was maintained at constant tempera-ture and relative humidity for 20 min prior to each measure-ment. The more experimental details for the measurement ofproton conductivity can be founded in ElectronicSupplementary Information. All the above samples in this partwere tested with three specimens.

Results and discussion

Characterization of functional CNT

The CNT-COH synthesized by the Hummers method haveinherited the one-dimensional structure of carbon nanotubes,as described by their TEM (Fig. 2a) and SEM (Fig. 2c) im-ages. After functionalized with phosphonic acid groups, theTEM (Fig. 2b) and SEM (Fig. 2d) images of CNT-POH haveno noticeable change and the structure of CNT-COH is wellpreserved. The C, O, P, and Si element signals in the wide scanXPS spectra of the CNT-POH (Fig. S2) support the successfulincorporation of phosphonic acid groups to the surface ofCNT-COH. The C1s (Fig. 2e) and O1s (Fig. 2f) narrow scanXPS spectra of the CNT-POH further characterize the chem-ical structure of the organic portion. The characteristic peaksof C–O–Si at 285.3 eV in Fig. 2e and 533.6 eV in Fig. 2fcorrespond to the chemical linkage between CNT and phos-phonic acid groups [29]. The covalent attachment and integ-rity of the organic functionalities are also confirmed by 13Cand 31P MAS NMR spectroscopies (Fig. 2h, i). In Fig. 2h, theintense peaks appear between 90 and 140 ppm are assigned tothe unsaturated carbon atoms in CNT and their oxygen-containing groups. The existence of the phosphonic acidgroups on the CNT can be evidenced by the intense peaks at0 and 21 ppm, which are attributed to the carbon atoms linkeddirectly to silicon and phosphorus (−Si–CH2–CH2–P). InFig. 2i, the intense peak at 33 ppm is indicative of the freephosphonic acid groups (−CH2PO (OH)2).

150 100 50 0 -50

2

13C ppm

31P ppm

1

C in CNT

(h)

-50 0 50 100

(i)

33 ppm

290 288 286 284 282

C-C/C-H

C-OH/C-O-Si

Inte

nsity

Inte

nsity

Inte

nsity

Inte

nsity

B.E.(eV)

C 1s

C=O

(e)

536 534 532 530

C=O

C=O/P(O)OH

Si-O

B.E.(eV)

O 1s

C-OH/C-O-Si

(f)

(c) (d)(a) (b)

Fig. 2 TEM images of aCNT-COH and bCNT-POH. SEM images of cCNT-COH and dCNT-POH, eC1s and fO1s narrow scanXPS spectra of CNT-POH, h 13C MAS NMR spectrum and i 31P MAS NMR spectrum of CNT-POH

2106 Ionics (2017) 23:2103–2112

The FTIR spectra of CNT-COH and CNT-POH are shownin Fig. 3. In the spectrum of CNT-COH, a broad and intensiveband at 3430 cm−1 is due to the bending and stretching vibra-tions of adsorbed molecular water. The bands at 1720 and1577 cm−1 are assigned with C=O and C–O stretching vibra-tion bands of –COOH groups. The peak at about 1184 cm−1 isassociated with the C–C–O stretching vibration. All these ev-idences confirm that CNT-COH are –OH and –COOH func-tionalized carbon nanotubes. These peaks are consistent withthe literature [30]. In the spectrum of CNT-POH, the broadpeaks at 1640 and 1573 cm−1 are attributed to the –P (O) OHgroups. The peak at 1257 cm−1 is due to the P=O stretchingvibrations. The asymmetric stretching of C–O–Si groups is at1087 cm−1, indicating that covalent bonds are formed betweenCNT and phosphonic acid groups [31, 32]. The strong peaksat 801 and 1018 cm−1 are assigned to the stretching of the P–Cand P–O groups, respectively [32]. As a consequence, it canbe confirmed by FTIR that the phosphonic acid group hasbeen successfully grafted onto CNT-COH.

The element contents of CNT-COH and CNT-POH weremeasured by EDS. Their results were listed in Table 1. XPSelemental analysis was also carried out for CNT-POH, andXPS results (listed in the parentheses in Table 1) are similarto the results determined by EDS. The amount of phosphonicacid groups anchored to the CNT-COH was calculated by

phosphorus content from XPS elemental analysis. And theresults (2.58 mmol/g) are about one tenth of the addingamount in the synthesis process. The functionalization of theCNT-COH is realized by the cross-linking reactions betweenthe silanol groups on PETES and the surface functionalgroups on CNT-COH (such as hydroxyl and epoxy groups).

The thermogravimetric analysis curves of CNT-COH andCNT-POH are illustrated in Fig. 4 (red dashed curves). Theweight losses of CNT-COH and CNT-POH before 170 °C areattributed to the evaporation of the physical adsorbed water. Itis noteworthy that the dehydration speed of CNT-POH ismuch slower than that of CNT-COH, which indicates thatthe phosphonic acid groups in CNT-POH have a higher affin-ity to water molecules than oxygen-containing functionalgroups in CNT-COH. After 170 °C, the weight loss of CNT-POH is associated with the decomposition of oxygen-containing functional groups and phosphonic acid groups.For CNT-COH, the obvious weight loss between 200 and240 °C is associated with the decomposition of carboxylicacid groups. From the residual weight at 800 °C of the twosamples, the amount of phosphonic acid groups attached toCNT-COH can be calculated and the result is listed in Table 1,and this result is similar to that determined by XPS.

Characterization of SPEEK incorporated with functionalCNT

Figure 5a–c, characterized by SEM, exhibits the surface mor-phologies of the SPEEK, SPEEK-COH, and SPEEK-POH.The surface of all samples shows smooth and dense surfaces,which indicated a uniform dispersion of CNT-COH and CNT-POH in the SPEEK matrix. Figure 5d–f presents the cross-sectional SEM images of the three membranes. There is nodistinct micro phase separation, pores or mesh structure in allthe membranes. And the overall morphology of the membraneis uniform, with no obvious cracks or other structural defectsthat can be observed on their surfaces and cross sections. For amore detailed analysis on the presence of the elements in themembranes, the EMPA spectra was performed to determinethe elements and their distributions in the cross sections ofsamples, as shown in Fig. 5k–n. And the analysis areas ofEPMA are labeled with the white rectangles in Fig. 5d–f.The EPMA mappings of SPEEK (Fig. 5k) and SPEEK-COH (Fig. 5m) show the uniform presence of carbon, oxygen,

Fig. 3 FTIR spectra of CNT-COH and CNT-POH

Table 1 The element contents ofCNT-COH and CNT-POH Element contents (%) EDS (XPS) (wt%) Phosphonic acid (mmol/g)

C O P Si XPS TG

CNT-COH 78.2 21.8 – – 2.58 3.51CNT-POH 64.9

(61.3)

19.6

(22.8)

7.6

(7.2)

7.8

(8.6)

Ionics (2017) 23:2103–2112 2107

and sulfur elements. In the EPMA mappings of SPEEK-POH(Fig. 5n), there are well-distributed dispersions for both P and

Si elements in the cross-sectional of SPEEK-POH, indicatingthat CNT-POH are homogeneously embedded in SPEEK ma-trix. This might profit by the high dispersibility of the CNT-POH in the DMAc solutions and SPEEK molecular chains.Because of the strong interfacial interaction between CNT-POH and SPEEK, the fabricated membranes have a homoge-neous microstructure after the solvent DMAc volatilization.

Thermogravimetric analysis (TGA) is used to examine thethermal property of the SPEEK, SPEEK-COH and SPEEK-POH membranes. After being heated at 120 °C for 4 h, themembranes were placed in atmosphere at 25 °C for 24 h be-fore measuring. Fig. 4 shows the TGA curves of the threemembranes. It can be seen from the SPEEK and SPEEK-COH curves that there is a loss of 8% weight due to theevaporation of the adsorbed water before 170 °C. And forSPEEK-POH, there is a slight loss of 3% weight at this stage.The speed of adsorbedwater loss for SPEEK-POHwas slowerthan the SPEEK and SPEEK-COH, which indicates that thereis a strongest intermolecular force between phosphonic acidgroups and adsorbed water molecules. Further increase of thetemperature brings about a two-stage weight loss. The firststage from 280 to 360 °C is related to the loss of sulfonic acid

Fig. 4 Thermogravimetric analysis of CNT-COH, CNT-POH, SPEEK,SPEEK-COH, and SPEEK-POH

Fig. 5 Surface SEM images of a SPEEK, b SPEEK-COH, and c SPEEK-POH. Cross-sectional SEM images of d SPEEK, e SPEEK-COH, and fSPEEK-POH. Cross-sectional EPMA mappings of k SPEEK, m SPEEK-COH, and n SPEEK-POH

2108 Ionics (2017) 23:2103–2112

groups. The second stage from 500 to 600 °C is originatedfrom the decomposition of the main chain and residual organiccomponents. As shown in Table 2, it is worth noting that theweight loss for SPEEK-POH in this two stages are much lessthan the other two, which implies that the phosphonic acidgroups loaded in the matrix of the SPEEK have an enhancedthermal stability.

The typical stress-strain curves of the three membranes aremeasured and the parameters of mechanical properties arelisted in Table 2. The addition of the CNT-COH and CNT-POH to the SPEEK matrix results in enhanced mechanicalproperties. The improvements in strength and stiffness aredirectly attributed to the inclusion of the CNT reinforcement.Due to the one-dimensional nanostructure, CNT with a highdispersibility can serve as the bridge between the polymerchains and develop weak physical cross-linking networkstructure. They form an inter-percolating network throughthe membranes, hence reinforcing the mechanical properties.Compared with SPEEK-COH, the mechanical properties ofSPEEK-POH are more enhanced. The strong acidity of phos-phonic acid groups in SPEEK-POH makes them more dis-persed in DMAc solvents than the CNT-COH. Higher tensilestrength will allow the SPEEK-POH membrane to resist theformation of pinholes and tears during operation in a PEMFC.

In XRD patterns performed between 10° and 50°, as shownin Fig. 6, there are broad peaks overlays on the 2θ range of15°–25° for the SPEEK and the composite membranes. It iscan be inferred that all the three membranes has a low crys-tallinity. However, the peaks shown in SPEEK-COH andSPEEK-POH are more flatter than the pristine SPEEK mem-brane. Because of the in situ incorporating CNT-COH orCNT-POH, the structure of pristine SPEEK membrane is dis-ordered, lead to the lower crystallinity of SPEEK-COH andSPEEK-POH [33]

During the actual operation of DMFC, the dimensionalstability of PEM is crucial for maintaining the stability ofmembrane electrode assembly. Figure 7 demonstrates the di-mensional swelling of SPEEK, SPEEK-COH, and SPEEK-POH after being soaked in 1 mol/L methanol solutions atdifferent temperatures for 24 h. It is found that the dimensionalchange of SPEEK-COH and SPEEK-POH composite mem-branes is less than the pristine SPEEK membrane. Because of

the good compatibility between CNT and SPEEK matrix, thetensile strength of the composite membranes is increased by75%.

The water uptakes of the SPEEK, SPEEK-COH, andSPEEK-POH versus temperatures are shown in Fig. 8. Asthe temperatures increases from 30 to 60 °C, the water uptakesof the three membrane samples are all increased. This varia-tion can be ascribed to the swelling of bulk membrane matrix,causing the facile penetration of water molecule [34]. SPEEK-COH and SPEEK-POH composite membranes show higherwater uptake than the pristine SPEEK membrane when thetemperature changes from 30 to 60 °C. This can be explainedby the formation of more hydrophilic channels and inherentmicro/mesopores within the CNT/SPEEK composite mem-branes than within the pristine SPEEK membrane.Compared with oxygen-containing functional groups onCNT-COH, the phosphonic acid groups functionalized onCNT-POH has a higher water retention capacity.

The methanol permeability of the three membranes,SPEEK, SPEEK-COH, and SPEEK-POH, was measured in1 mol/L methanol solutions at different temperatures to inves-tigate their performance for DMFC application. And the re-sults are illustrated in Fig. 9. The methanol permeability of thethree membranes increased monotonically from 30 to 60 °C.And the inclusion of CNT-COH or CNT-POH has reduced the

Table 2 Weight loss, mechanical properties, ion-exchange capacity, and activation energy of SPEEK, SPEEK-COH, and SPEEK-POH

TGAweight loss (%) Mechanical properties Ion-exchangecapacity (mmol/g)

Activation energy of protonconductivity (kJ/mol)

0–170 °C 280–360 °C 500–600 °C Young’smodulus(Mpa)

Tensilestrength(Mpa)

Elongation atbreak (%)

SPEEK 7.92 11.50 16.74 88 20 153 1.98 27.2

SPEEK-COH 6.83 9.67 14.62 156 32 122 1.99 24.9

SPEEK-POH 3.36 4.98 7.56 198 35 120 2.07 22.9

10 20 30 40 50

SPEEK-POH

SPEEK-COHInte

nsity

2 (o

)

SPEEK

θFig. 6 XRD spectra of SPEEK, SPEEK-COH, and SPEEK-POH

Ionics (2017) 23:2103–2112 2109

methanol permeability of the SPEEK matrix, compared withthe SPEEK membrane at the same temperatures. The dis-persed CNT-COH and CNT-POH nanoparticles hinder themethanol crossover through the membranes. This decreasingin methanol permeability can be ascribed to the embedding ofCNT into the hydrophilic channel of the SPEEK matrix. As aresult, the tortuosity of the methanol transport channel is in-creased, and the methanol diffusion pathway in the compositemembranes become flexuose, leading to the increased transferresistance for methanol fuel.

The ionic exchange capacity (IEC) is correspond to thequantity of protonic acid sites per unit mass of membrane.Table 2 shows the IEC of SPEEK, SPEEK-COH, andSPEEK-POH membranes. Because of the high density phos-phonic acid groups on grafted on CNT, the IEC values ofSPEEK-POH are larger than the SPEEK and SPEEK-COHmembranes. While the IEC of membranes SPEEK-COH isbarely affected by embedding of CNT-COH. That is becausethere is no strong protonic acid site on CNT-COH.

Figure 10 shows the proton conductivities of the threemembranes, SPEEK, SPEEK-COH, and SPEEK-POH, at

different temperatures with 100% relative humidity. For theSPEEK-COH, with the CNT-COH incorporated into theSPEEK, the proton conductivity is slightly increased. Due tothe hydrophilic oxygen-containing functional groups on CNT-COH nanoparticles, the water molecules absorbed on CNT-COH can play the part of vehicles for proton diffusion.Therefore, the proton conductivity of membrane SPEEK-COH increases at each temperature [35, 36]. On the otherhand, the proton conductivity of SPEEK-POH shows a sub-stantially increasing at each temperature. CNT-POH aredressed with phosphonic acid groups, which can providecharge sites for proton transport. Because of the ion clustersformed by strong protonic acid-phosphonic acid groups, theproton transport channels are well-connected, which leads tothe increasing of the proton conductivity for the membraneSPEEK-POH. The Arrhenius plots of conductivity for thethree membranes are also depicted in Fig. 10. The protonconductivities of all the three membranes increase when thetemperatures elevates from 30 to 60 °C. The activation energyof proton conductivity (Ea) is listed in Table 2. The decline of

30 35 40 45 50 55 60

0

1

2

3

4

5

6

7

Me

tha

no

l p

erm

ea

bility (

cm

2/s

)

Temperature (o

C)

SPEEK

SPEEK-COH

SPEEK-POH

×10-7

Fig. 9 Methanol permeability of SPEEK, SPEEK-COH and SPEEK-POH

30 35 40 45 50 55 60

0

50

100

400

500

600

Dim

en

sio

na

l sw

ellin

g (

%)

Temperature (o

C)

SPEEK

SPEEK-COH

SPEEK-POH

Fig. 7 Dimensional swelling of SPEEK, SPEEK-COH, and SPEEK-POH

30 35 40 45 50 55 60

40

80

120

160

Wate

r u

pta

ke (

%)

Temperature (o

C)

SPEEK

SPEEK-COH

SPEEK-POH

Fig. 8 Water uptake of SPEEK, SPEEK-COH and SPEEK-POH

3.0 3.1 3.2 3.3

0.05

0.14 SPEEK

SPEEK-COH

SPEEK-POH

Pro

ton

Co

nd

uctivity (

S c

m-1)

1000/T (K-1

)

Fig. 10 Proton conductivity of SPEEK, SPEEK-COH and SPEEK-POHat different temperatures

2110 Ionics (2017) 23:2103–2112

the activation energy is in the sequence of SPEEK < SPEEK-COH < SPEEK-POH. This sequence is consistent with thewater uptake at different temperatures. This consistency canbe explained by the embedded CNT nanoparticles, which af-ford more channel for fast proton diffusion at the interfacebetween CNT and the macromolecular chains of SPEEK.

Figure 11 shows the proton conductivity of the three mem-branes at 60 °C with different relative humidity (RH) (60–100%). All the three samples show the dependence of theconductivity on hydration, and their conductivity increasesobviously as RH increases from 60 to 100% RH. When RHis below 60%, the conductivity is too small to measure pre-cisely. On varying the RH, the proton conductivities ofSPEEK-POH ranged from 0.006 to 0.16 S cm−1. As comparedto SPEEK-COH and SPEEK, SPEEK-POH exhibits muchhigher conductivity at each humidity. On the basis of the ioniccluster theory [37], due to the hydrophilic phosphonic acid onCNT-POH, the numerous absorbed water can swell theSPEEK molecular chain and enhance the interconnection ofthe ionic clusters in membrane matrix. Therefore, the mem-brane CNT-POH has the highest proton conductivity at lowRH.

Conclusions

A novel proton exchange membrane was successfully devel-oped via incorporation of hydrophilic phosphonic acid groupsfunctionalized CNT into SPEEK matrix. The introduction ofphosphonic acid can improve the proton conductivity of theSPEEK matrix by one time because of the formation of well-connected proton transport channels. The tensile strength ofthe composite membrane is also strengthened by 75% becauseof the good mechanical properties of CNT nano-materials andtheir homogeneous dispersion in SPEEK. The phosphonicacid functionalized CNT particles can positively affect thehindering properties against methanol molecules.

Accordingly, this phosphonic acid functionalized SPEEKcomposite membrane exhibits a potential as PEM in directmethanol fuel cells.

Acknowledgements We acknowledge the financial support from theScience and Technology Program of Tianjin (15ZCZDSF00040,16JCQNIC06000) and the Program of Introducing Talents of Disciplineto Universities (B06006).

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