Journal of Membrane Science 493 (2015) 340–348

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Physicochemical properties of alkaline doped polybenzimidazolemembranes for anion exchange membrane fuel cells

L. Zeng, T.S. Zhao n, L. An, G. Zhao, X.H. YanDepartment of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,Hong Kong SAR, China

a r t i c l e i n f o

Article history:Received 26 February 2015Received in revised form12 June 2015Accepted 13 June 2015Available online 2 July 2015

Keywords:PolybenzimidazoleFuel cellAnion exchange membraneIonic conductivityAlcohol permeability

x.doi.org/10.1016/j.memsci.2015.06.01388/& 2015 Elsevier B.V. All rights reserved.

esponding author.ail address: metzhao@ust.hk (T.S. Zhao).

a b s t r a c t

Alkaline doped PBI membranes are investigated for their physicochemical properties when employed inalkaline direct alcohol fuel cells. Alkali uptake, water uptake, alcohol permeability, ion conductivity,mechanical strength and evolution of the morphology for the alkaline doped PBI membranes are mea-sured. It is demonstrated that the alkaline doping process has a profound influence on the physico-chemical properties of PBI membranes. With a high alkaline doping level, the alkali uptake of alkalinedoped PBI membranes reaches 14%. The high alkaline uptake results in a separation of the polymerbackbones and partial cleavage of hydrogen bonds, which leads to a reduction in the mechanical strengthand an increase the alcohol permeability of alkaline doped PBI membranes. However, the high alkalinedoping level is beneficial to the ionic conductivity. The conductivity of alkaline doped PBI membranesincreases to 96.1 mS cm�1 at 363.15 K after PBI membranes are doped with 6 M KOH solution. Based onthese investigations, a mechanism is proposed to interpret the chemical interaction during the alkalinedoping process and its influence on physicochemical properties. The existence of a neutralization reac-tion and the re-establishment of hydrogen bonding networks should be attributed to the propertytransformation during the alkaline doping process.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

Alkaline direct alcohol fuel cells (ADAFCs) have been widelyinvestigated in the last few decades for their numerous merits andinteresting properties [1–3]. An alkaline medium can dramaticallyimprove the kinetics of both oxygen reduction reaction (ORR) andalcohol oxidation reaction (AOR) [1,4]. Therefore, non-preciousmetal catalysts based on abundant transition metals, such asnickel [5] and silver [6], can be employed as catalysts, withoutcompromising reaction rate and dramatically reducing systemcost. In addition, the conductive ions (typically OH�) in the anionexchange membranes (AEMs) transfer from the cathode to theanode, in an opposite direction to that of alcohol crossover. Thus,the electro-osmotic effect in ADAFCs can be mitigated, particularlyat high currents. Finally, alcohols are liquid fuels and exhibit highvolumetric energy densities [7]. They are also renewable fuels andcan be produced in large quantities from biomass or agriculturalproducts.

As a crucial component of ADAFCs, AEMs are particularly in-dispensable to anion conduction and fuel/oxidant separation.

Heterogeneous membranes containing ionic exchange materialsand inert compounds have recently been widely investigated[8–10]. In particular, much attention has been focused on alkalinedoped polybenzimidazole (PBI), due to its high hydroxide ionconductivity and excellent chemical stability in alkaline media.Although PBI is an electronic and ionic insulator, the pyridine-typenitrogen (–N¼) and pyrrole-type nitrogen (–NH–) in the benzi-midazole rings allow absorption and interaction with the free in-organic base (i.e., KOH), contributing to the ionic conductivity. Forinstance, Hou et al. [11] reported that KOH-doped PBI membranefor AEM direct methanol fuel cells (AEM DMFCs) exhibited highthermal stability, low methanol permeability and an ionic con-ductivity of 18.4 mS cm�1 recorded at room temperature; how-ever, the power density obtained at 363.15 K was as low as31 mW cm�2. Hou et al. [12] also demonstrated that a peak powerdensity of 60.9 mW cm�2 was achieved at 363.15 K when theanode was fed with 2 M ethanol and 2 M KOH solution. Modestovet al. [13] studied the MEAs fabricated with alkali-doped PBImembranes and non-platinum electrocatalysts; a power density of100 mW cm�2 was reached at 353.15 K using 2 M ethanol and 3 MKOH as the fuel for a single cell. The PBI-derivate membranes[14–17] were also synthesized and the ionic conductivity wastested; the PBI with pendant quaternary ammonium groups [16]exhibited a hydroxide conductivity of 56 mS cm�1 at 353.15 K.

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348 341

Recently, Aili et al. [18,19] measured the membrane compositionand ionic conductivity of three types of alkaline doped PBI mem-branes (linear, crosslinked and thermally cured PBI); it was de-monstrated that the PBI membrane compositions were not sig-nificantly changed while the conductivities of PBI membrane afterthe cross-linking process and thermal-curing process were es-sentially lower than that of linear PBI membrane. In addition to theionic conductivity, the alcohol crossover in alkaline doped PBImembranes [20,21], the assembly of PBI, supporting materials andmetal nanoparticles for fuel cell catalysts [22–24], and the dur-ability of alkaline-doped PBI [25] were also investigated. Our lit-erature review indicates that the alkaline doping process has aprofound influence on the performance of PBI membranes,whereas a systematic study of the influence of the alkaline dopingprocess on the physicochemical properties in terms of the ionicconductivity, thermal stability and mechanical properties, has notyet been addressed.

The objective of this work is to prepare solution-cast PBImembranes and systemically characterize the physicochemicalproperties of the alkaline doped PBI membranes. The surface/phase morphologies of PBI membrane before and after the alkalinedoping process were examined, and the chemical structure of thealkaline doped PBI was characterized. The alkali uptake, wateruptake, ionic conductivity, alcohol permeability and the mechan-ical properties of the PBI membrane doped by KOH solutions ofvarious concentrations were also tested. Finally, based on thesecharacterizations, a mechanism was proposed to elucidate theinteraction between the doped alkali and the PBI membrane.

2. Experimental

2.1 PBI membrane casting

PBI solution of concentration 26 wt% (intrinsic viscosity:0.73 dl g�1) was purchased from PBI Performance Products Inc.Based on the intrinsic viscosity, the molecular weight of PBI mem-brane calculated from Mark–Houwink expression is 123.798 Da [26].Concentrated PBI solution was diluted with dimethylacetamide(DMAC) to make a solution with a concentration of 4 mg mL�1. ThePBI solution was cast upon a clean glass plate using a micrometeradjustable film applicator. The plate was then introduced inside aforced convection oven. To completely evaporate the solvents, theoven temperature was dwelled at 363.15 K for 4 h, 393.15 K for 4 h,433.15 K for 6 h and 463.15 K for 2 h and subsequently cooled downroom temperature. PBI membranes with a thickness of 40–50 μmwere then peeled off from the glass plate.

2.2 Water uptake, alkali uptake and swelling ratio measurement

The PBI membrane samples were doped in KOH solutions ofvarious concentrations. During the doping process, a piece ofpristine PBI membrane was immersed in deionized (DI) water forthe purpose of comparison. The alkaline doping process lasted for10 days to ensure that the doping process was complete. Subse-quently, the wet weight (mwet), surface area (Awet) and thickness(Twet) of the membranes were then measured at room temperatureafter the excess alkaline solution on the PBI membrane surfacewas quickly wiped off with tissue paper. The PBI samples werethen dried at 383.15 K for 2 h in an oven and quickly weighed in aclosed vessel (mdry1) to prevent possible interference from CO2 inthe air [19]. The PBI samples were then treated in DI water at363.15 K for 2 h to remove the residual KOH and dried at 383.15 Kfor 2 h in an oven and quickly weighed in a closed vessel (mdry2).The surface area (Adry) and thickness (Tdry) of the dried PBI sampleswere thereafter measured. Accordingly, water uptake (WU), alkali

uptake (AU) and swelling ratio (SR) were determined by

WUm m

m100%

1wet dry

dry

1

2=

−×

( )

AUm m

m100%

2dry dry

dry

1 2

2=

−×

( )

SRA T A T

A T100%

3wet wet dry dry

dry dry=

−×

( )

2.3 Physical characterization

The surface morphology and the cross-section of the PBImembrane before and after the alkaline doping process were de-termined by scanning electron microscopy (JEOL-6300F). SEM-energy-dispersive X-ray spectroscopy (SEM-EDS) mapping wasoperated at 15 kV. FTIR spectra were collected between wave-numbers ranging from 4000 to 400 cm�1 by Fourier transforminfrared spectroscopy (FTS 600, Bio-Rad) with ATR accessory. X-rayphotoelectron spectroscopy (XPS) were performed on a PhysicalElectronics PHI5600 with X-ray source operated at 12 kV and350W. Survey spectra and high resolution spectra for each ele-ment were obtained at pass energy of 187.85 eV and 23.5 eV, re-spectively. All membranes were dried under a vacuum oven at373.15 K overnight before the surface analysis.

The phase image of atomic force microscopy was performed ina Nanoscope-MultiMode/Dimension (Digital Instruments). Theinstrument was equipped with a NanoWizard scanner. For tap-ping-mode AFM, a commercial Si cantilever of about 320 kHz re-sonant frequency was used. The set-point used for imaging variedbetween 1.0 and 1.5 V. All AFM phase images were shown as re-corded without any additional image processing.

2.4 Ionic conductivity measurement

The ionic conductivity of the PBI membrane was determinedwith a potentiostat (EG&G Princeton, model M2273) through atwo electrode conductivity clamp using an AC impedance method.The membranes were sandwiched by a pair of Au-coated stainlesssteel electrodes. The electrodes were then sandwiched betweentwo PTFE plates and placed in a home-made container filled withN2-saturated DI water to prevent possible interference from CO2. Afrequency range from 100 kHz to 1 Hz with a wave amplitude of10 mV was applied to the conductivity clamp to obtain the ACimpedance spectra. The membrane resistance (RΩ) can be ob-tained by calculating the intercept of the high frequency region.The ionic conductivity s can be obtained from:

LR A 4

σ =× ( )Ω

where L represents the thickness of the membrane and A re-presents the surface area of the membrane [27,28].

After the ion conductivity was measured at a given tempera-ture, the alkaline doped PBI membranes were taken out, quicklywiped off the surface water and submerged in the correspondingKOH solutions for 1 h and resembled to measure the ionic con-ductivity in the N2-saturated DI water. In such a short time in-terval, the conductivity of alkaline doped PBI membranes can beconsidered as stable and the influence of the alkali leaching outfrom the doped PBI matrix can be ignored.

2.5 Alcohol permeability measurement

The alcohol permeability through a membrane was measured

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348342

with a home-made apparatus. The membrane was tightly sand-wiched between two chambers with the same volume (40 ml).Chamber A was filled with 3 M methanolþ6 M KOH or 3 Methanolþ6 M KOH while Chamber B was filled with 6 M KOH. Thealcohol permeability of pristine PBI membrane was measured forthe purpose of comparison when Chamber A was filled with 3 Mmethanol or 3 M ethanol while Chamber B was filled with DIwater. The concentration of ethanol/methanol in Chamber B due topermeation was measured by gas chromatography (6890 GC In-strument, Agilent Technologies). Each chamber contained a mag-netic stirrer during the measurement. The alcohol concentration inChamber B can be determined by [26,29]

CAV

DKL

C t t 5B t A 0= ( − ) ( )( )

where C is the concentration of the alcohol, A and L are the areaand thickness of the membrane, respectively, D and K stand for thealcohol diffusivity and partition coefficient, V is the volume ofchamber, and a t0¼L2/D is the time lag. The alcohol permeabilityP¼DK is determined by the slope of the variation in alcohol con-centration with time.

Fig. 1. (a) surface morphology of pristine PBI; (b) surface morphology and (c) cross-sectoxygen and nitrogen elements.

2.6 Mechanical property measurement

The mechanical properties were measured with a micro-forcetesting machine (Alliance RT/5, MTS) with a crosshead speed of5 mm min�1 at room temperature. All the PBI membranes weredoped by KOH solutions of various concentrations for 10 days.Prior to the measurement, the alkaline doped PBI membraneswere taken out and wiped with tissue paper to remove the freealkaline solution on the surface. All samples were cut into a size of40 mm in length�5 mm in width, which were supported by a pairof fixture assemblies. The average values of modulus and the stressat break were determined from load–deformation curves of threetensile measurements. All the measurements were performed atroom temperature with 60% relative humidity.

3. Results and discussion

3.1 Morphology evolution of the alkaline doped PBI membrane

The morphologies of PBI membranes before and after the al-kaline doping process were determined by SEM-EDS, as presented

ion morphology of the alkaline doped PBI (6 M); (d)–(f) EDS mapping of potassium,

Fig. 2. AFM phase images of pristine PBI membrane (a) and alkaline doped PBI membranes: (b) 2.0 M KOH, (c) 4.0 M KOH and (d) 6.0 M KOH.

Fig. 3. FTIR of pristine PBI and alkaline doped PBI membrane (6 M KOH).

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348 343

in Fig. 1. It is seen that a uniform, dense and smooth membranewithout the existence of pores is formed both before and after thealkaline doping process (Fig. 1a and b). The results of elementmapping are presented in Fig. 1d–f. The signals of potassium andoxygen, derived from the doped alkali, are uniformly distributedon the entire membrane while the signal of nitrogen results fromthe original PBI membrane. This result indicates that the alkali iswell dispersed through the alkaline doped PBI membrane.

AFM was employed to further investigate the morphologyevolution during the alkaline doping process. The AFM phaseimages of pristine PBI and PBI doped with KOH solutions of variousconcentrations were recorded using a tapping mode under ambi-ent conditions, as presented in Fig. 2a–d. The dark areas and thebright areas in AFM phase images are ascribed to the hydrophilic

(ionic) domains and hydrophobic domains, respectively [30–33].The pristine PBI membrane exhibits a bright field, clarifying pris-tine PBI membranes possess hydrophobic property. In contrast, inalkaline doped PBI membranes, the hydrophilic domains appearand the boundaries between the hydrophilic and hydrophobicdomains become more apparent as the alkaline concentration isincreased. Meanwhile, the hydrophilic domain shows an inter-connected phase morphology, which is benefit to the hydroxideion transportation. It should be noted that AFM measurement wasperformed in the ambient environment with the relative humidityof about 60%. Very little amount of water molecule might be ab-sorbed on the surface of pristine PBI membrane. However, thedark fields in pristine PBI membrane are lighter than that in al-kaline doped PBI membranes, indicating that the water moleculeswere absorbed on the surface, instead of being doped in the PBImatrix.

3.2 Chemical structure of the alkaline doped PBI membrane

FTIR spectra of PBI membrane before and after alkaline dopingtreatment are presented in Fig. 3. The pristine PBI membrane ex-hibits an absorption peak at 1600 cm�1 attributing to C–Nstretching, 1544 cm�1 arising from –N–H stretching, and1284 cm�1 from the breathing mode of imidazole ring. Afterdoped by 6 M KOH, the PBI membrane exhibits absorption peakslocated at 1625 cm�1, 1510 cm�1 and 1120 cm�1, correspondingto –O–H stretching in the absorbed water, –N–K deformation andthe –N–K out of plane bending. It is apparent that the –N–Hstretching vibration (1284 cm�1) is replaced by –N–K deformation(1510 cm�1) and bending vibration (1120 cm�1) [34], implyingthat the hydrogen bonds are partially fractured while the cations(Kþ) react with benzimidazole segments in the PBI skeleton.

Fig. 4. (a) XPS survey spectra for PBI and alkaline doped PBI (6 M KOH); (b) thehigh resolution of N 1s XPS spectra; (c) deconvolution of N 1s spectra in the al-kaline doped PBI.

Fig. 5. Water uptake, alkali uptake (a) and contact angle measurement (b) of PBImembrane doped by KOH solutions of various concentrations at room temperature.Inset in panel (b): digital images of water drops on PBI membrane.

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348344

The chemical structure of PBI membrane before and after thealkaline doping process was further confirmed by XPS (Fig. 4a).After the alkaline doping process, the emergence of the K peaks(376.8 eV for K 2 s, 291.8 eV for K 2 p, 32.8 eV for K 3 s and 16.8 eVfor K 3 p) indicates that the alkali has successfully been doped onthe PBI membrane. The N 1s high-resolution spectra of PBImembrane before and after the doping process is shown in Fig. 4b,from which it is seen that N 1s spectra can be deconvoluted intotwo peaks (Fig. 4c) due to the double-bonded and single-bondednitrogen in the benzimidazole rings [35,36]. However, the bindingenergy for N 1s core level spectra in the alkaline doped PBImembrane is negatively shifted (0.25 eV) when compared withthat of in the pristine PBI membrane, indicating that the benzi-midazole segments are delocalized with some electrons and ne-gatively charged [37,38]. This result demonstrates that benzimi-dazole segments are deprotonated to form a polymeric salt.

3.3 Water uptake, alkali uptake and swelling ratio

The water uptake, alkali uptake were measured at room tem-perature. As shown in Fig. 5a, the WU of PBI membrane treated byDI water is limited to 10 wt%. At low KOH concentrations(r1.0 M), the WU and AU are also relatively low. Actually, thepristine PBI membrane can be neutralized with the doped alkalinesolution with the KOH concentration above 0.063 M assuming thatthe dissociation constant (pKa¼12.8 at 298.15 K) of benzimidazolein an aqueous solution [39] is appliedfor PBI membrane. But thealkali concentration in the alkaline doped PBI membranes is lowerthan that of in the bulk solution, hence the AU and WU is lowwhen the PBI membranes are doped with KOH solutions with alow concentration. It should be noted that the measured WU of PBImembrane treated by DI water and low KOH concentrations(r1.0 M) is lower than that of results reported in the literature[40,41], which is likely induced by the different molecular weight/structure and membrane thickness. However, the WU and AU al-most linearly increase when the KOH concentration is higher than1.0 M. The WU and AU respectively reach 45 wt% and 15 wt% whenPBI membranes are doped with 6 M KOH. With the similar

calculation adopted by Glipa et al. [42], the KOH concentration inthe PBI membrane doped by 6 M KOH is about 25 wt%, which isalmost the same content of the liquid phase alkaline solution.When being doped by 6 M KOH, the amount of doped alkali in thealkaline doped PBI membrane normalized to each PBI repeatingunit is 0.84, which is relatively lower than that of values reportedin the literature [18,19]. Similarly,the differentiae of AU might becaused by the different molecular weight/ structure of PBI mem-branes. When the concentration of KOH was further increased to8 M, PBI membranes crumbled or even dissolved in the alkalinesolution. The high WU will cause an increase in the water affinity ofthe alkaline doped PBI membranes. To determine the changes inhydrophilicity, the water contact angle was measured, as illustratedin Fig. 5b. It can be seen that the contact angle decreases with anincrease in the doped KOH concentration, indicating an increase inthe hydrophilicity of the alkaline doped PBI membranes. As showninset in Fig. 5b, the alkaline doped PBI membrane exhibits highwettability when PBI membrane is doped with 6 M KOH.

The swelling ratio of the PBI membrane doped by KOH solu-tions of various concentrations at room temperature were mea-sured, as shown in Fig. 6. The tendency of swelling ratio is similarwith that of AU, i.e., a low swelling ratio is achieved at a lowdoping level while the swelling ratio of alkaline doped PBI mem-branes almost linearly increases with an increase in KOH con-centration (Z1.0 M). Interestingly, the swelling ratio in thethickness direction is larger than that of in the length/width di-rection, which is an indicative of asymmetric swelling of alkalinedoped PBI membranes. Although the mechanism of asymmetricswelling is still unclear, the low swelling ratio in the length/widthdirection is beneficial to the dimensional stability during the fuelcell operation.

Fig. 6. Swelling ratio of PBI membrane doped by KOH solutions of various con-centrations at room temperature.

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348 345

3.4 Ionic conductivity

Ionic conductivity of alkaline doped PBI membranes is highlydependent on the AU (Fig. 7a). It is seen that the ionic conductivityof alkaline doped PBI membranes is extremely low when the PBImembrane is doped by a low alkaline concentration (i.e. 0.5 MKOH). An increase in the KOH concentration significantly enhancesthe ionic conductivity of the alkaline doped PBI membranes sincethe amount of associated KOH absorption in the molecular net-works is increased. After being doped in 6 M KOH, the alkalinedoped PBI membrane possesses an ionic conductivity of

Fig. 7. (a) Ionic conductivity of PBI membrane doped by KOH solutions of variousconcentrations; (b) Arrhenius plots of alkaline doped PBI membranes, the linesindicate the linear regression.

34.1 mS cm�1 at 303.15 K. Meanwhile, the ionic conductivity in-creases with elevated temperatures. The highest ionic conductivityof PBI doped by 6.0 M KOH reaches 96.1 mS cm�1 at 363.15 K,which is in the same range as previously reported for the KOHdoped PBI membrane [19,43]. This result can be explained as fol-lows. First, the intermolecular interaction reduces with the ele-vated temperature, resulting in an incompact structure and in-creased free volumes. Hence, the ionic conducting channels ex-pand, contributing to ion mobility. Meanwhile, the water uptakesof membranes increases with the elevated temperature, resultingin further promotion of ionic conductivity. It should be noted thatmassive water uptake would dilute the ionic strength of ionicconducting channels, thereby reducing the ionic conductivity ofthe membrane. However, this issue was not present in the alkalinedoped PBI membranes.

Fig. 7b presents the Arrhenius plots of PBI membranes dopedby KOH solutions of various concentrations. It is seen that ln s vs.1000/T fits linearly for all samples while the slopes for each of thesample differ. The activation energy (Ea) can be obtained from thefitted slope through the following equation: Ea¼�b�R. Here, R isthe gas constant (8.314 J K�1 mol�1) and b is the fitted slope.Based on the evaluation, the activation energy is 23.22 kJ mol�1,16.92 kJ mol�1, 16.00 kJ mol�1, 15.87 kJ mol�1, corresponding tothe PBI membranes doped by 0.5 M KOH, 2.0 M KOH, 4.0 M KOHand 6.0 M KOH, respectively. The decline of Ea with an increase inthe doped KOH concentration testifies that the membrane’s ionicchannels are well-established, thereby reducing the dependenceof ionic conductivity on temperature. The activation energy valuesare also close to the acid-doped PBI membranes [44,45], which isthe indicative of the similar transportation mechanism, i.e., theGrotthuss mechanism.

3.5 Alcohol permeability

The alcohol permeability in the pristine PBI and the alkalinedoped PBI membrane at different temperatures was measured andthe calculated permeability is summarized in Table 1. The pristinePBI exhibits a lowmethanol and ethanol permeability. Ethanol andmethanol are required to be hydrated and diffused through themembrane with the aid of absorbed water, which has a significantinfluence on the diffusivity [46,47]. The pristine PBI has a lowwater uptake, thereby resulting in a low alcohol permeability.After doped with 6 M KOH, the permeability of methanol andethanol significantly increases. For instance, the alkaline doped PBImembrane has an ethanol permeability of 21.57�10�7 cm2 s�1,which is much larger than that of the pristine PBI membrane(0.67�10�7 cm2 s�1) at 363.15 K. The increased permeability re-sults from the reduced intermolecular interaction as a result ofestablished ionic channels [48]. On the other hand, the perme-ability of methanol is higher than that of ethanol due to methanolhas a smaller hydrated molecular size, leading to a relatively lowresistance for methanol across the alkaline doped PBI membranes[49]. However, the methanol permeability of alkaline doped PBImembrane (27.05�10�7 cm2 s�1) at 363.15 K is in the same rangeas previously reported for the alkaline doped PBI membrane(28.7�10�7 cm2 s�1 at 333.15 K) [21] and lower than that of

Table 1Alcohol permeabilities (�10�7 cm2 s�1) of methanol and ethanol for pristine PBIand alkaline doped PBI membranes (6 M KOH) at various temperatures.

Pristine PBI Alkaline doped PBI

303.15 K 363.15 K 303.15 K 363.15 K3 M methanol 0.54 1.41 11.26 27.053 M ethanol 0.27 0.67 5.50 21.57

Fig. 8. The stress–strain curves of pristine PBI membrane and the alkaline dopedPBI membrane.

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348346

Nafion 117 membranes [50,51].

3.6 Mechanical property

Excellent mechanical properties are essential for AEMs tocounteract harsh processes in the course of fuel cell assemblingand operating conditions. It is generally considered that the me-chanical strength of polymer membranes is determined by theattractive forces among polymer molecules, such as dipole–dipoleinteraction (including hydrogen bond), induction forces and dis-persion forces between non-polar molecules. When a polymer isdoped with alkali, the ionic bonding and ion-dipole interactionsalso contribute to the mechanical strength [48]. The tensile–straincurves of pristine PBI and alkaline doped PBI membrane are shownin Fig. 8 and the calculated results are summarized in Table 2. Forpristine PBI membranes, the mechanical strength is pre-dominantly determined by the hydrogen bond between –N¼ and–NH– segments. In our experiments, the pristine PBI membranehas a tensile strength at break as high as 92.98 MPa with anelongation of 5.6%, indicating that the pristine PBI membranepossesses a superior mechanical strength. When the alkali is in-troduced, the originally hydrogen bonds are partially cleaved, re-sulting in the swelling ratio in the cross-section direction and in-creased separation of PBI backbones. These interactions decreasemolecular cohesion and deteriorate the PBI membrane’s me-chanical strength [48]. As seen in Fig. 8, the tensile strength atbreak dramatically decreases with the increased KOH concentra-tions. After the PBI membrane was doped by 6 M KOH, the tensilestrength at break and Young’s modulus reduce to 14.75 MPa and0.27 GPa, respectively. The percentage of reduction in the tensilestrength and Young’s modulus are approximately 84.1% and 87.1%,respectively, when compared with that of the pristine PBI mem-brane. However, the tensile strength demonstrated in this work isstill higher than that of AEMs reported in the literature [52–54]and satisfies the mechanical requirement in ADAFCs.

Table 2Mechanical properties of pristine PBI and alkaline doped PBI membranes.

Samples Tensile strengthat break(MPa)

Young’s Modulus(GPa)

Elongationat break (%)

PBI 92.98 2.09 5.62 M KOH-dopedPBI

65.16 1.32 36.5

4 M KOH-dopedPBI

25.12 0.45 72.1

6 M KOH-dopedPBI

14.75 0.27 81.5

Interestingly, the elongation at break of the alkaline doped PBImembrane increases with an increased KOH concentration. Theelongation at break for pristine PBI membrane is 5.6% when PBImembrane was saturated with DI water despite the low wateruptake, while the elongation at break is significantly increased to81.5% when PBI membrane was doped by 6 M KOH. This behaviorindicates that the alkaline doped PBI membranes exhibit an im-proved plasticity.

3.7 Mechanism

As discussed in the aforementioned sections, the alkalinedoping process has a profound influence on the physicochemicalproperties of PBI membranes. While He et al. [48] studied the ef-fects of phosphoric acid doping process on the mechanical prop-erties and proton conductivity, the current literature lacks a sys-tematic investigation on the properties of PBI membrane after thealkaline doping process.

Actually, the pristine PBI exhibits a weak alkalinity in aqueoussolutions, which can be proven by the dissociation constant(pKa¼12.8 at 298.15 K) of benzimidazole in an aqueous solution[39]. Due to the high concentration of alkaline solutions, however,PBI will fulfill the role of an acid by dissociating protons from thepyrrole-type nitrogen (–NH). Subsequently, the hydroxide ionsimpregnated in the polymer matrix react with the dissociatedprotons. This neutralization reaction is presented in Scheme 1a.The high KOH concentration promotes this reaction, resulting inmore –NH groups being substituted by the free alkali. This can bedemonstrated from the increased alkali uptake as shown in Sec-tion 3.3. Meanwhile, the strong hydrogen bonds are partiallycleaved due to the introduction of the polar bond (–N–K) and thepyrrole-type nitrogen (–NH) atoms are negatively charged. Thepotassium cations are then involved in the bridging negativelycharged nitrogen atoms and the doped alkali, as illustrated inScheme 1b. The combination between KOH solution and PBI willestablish hydrophilic ionic clusters to transport the hydroxide ions.The proposed hydroxide ion transport pathway is illustrated inFig. 9. With high KOH concentrations, the size of hydrophilic ionicclusters will increase and interconnect, as demonstrated by AFMmeasurement. Hence, the ionic conductivity of the alkaline dopedPBI membrane improves with the increased KOH concentration asshown in Section 3.4.

The increase in alcohol permeability in the alkaline doped PBImembrane can be also explained with the proposed mechanism.Since the water uptake increases accompanying with the increasedalkali uptake, the size of well-established ionic clusters will beenlarged. Therefore, the alcohols dissolved in water enable to befacilely transported through the alkaline doped PBI membranes.

In regard to the mechanical property, the high mechanicalstrength of pristine PBI results from strong hydrogen bonds be-tween the pyrrole-type nitrogen (–NH) and the pyridine-type ni-trogen (–N¼) (Fig. 9a). However, the strong hydrogen bond net-works are partially cleaved in the course of the alkaline dopingprocess, leading to the decrease in the mechanical strength. Al-though the hydrogen bonds among the bridging negativelycharged nitrogen atoms and the doped alkali will be formed, thisinteraction is much weaker than that of hydrogen bond betweenthe pyrrole-type nitrogen (–NH) and the pyridine-type nitrogen (–N¼). Therefore, the tensile strength at break and Young’s modulusof alkaline doped PBI membranes are significantly reduced withthe increased KOH concentration. Meanwhile, the elongation atbreak increases after the alkaline doping process, indicating thatthe membranes with high alkali content in the polymer matrixbecome more plastic. This plasticizing effect results from an in-creased flexibility of PBI backbones due to the reduction of mo-lecular cohesion.

Scheme 1. The possible reaction during the alkaline doping process: (a) neutralization reaction; (b) the hydrogen bonds before and after the alkaline doping process.

Fig. 9. Schematic of (a) the pristine PBI and (b) alkaline doped PBI membrane. Thepink ellipses and red lines are the hydrophilic ionic clusters and hydrogen bonds,respectively. The yellow arrows represent the possible hydroxide ions transportpathway. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

L. Zeng et al. / Journal of Membrane Science 493 (2015) 340–348 347

4. Conclusions

In summary, the influence of the alkaline doping process on thephysicochemical properties of PBI membrane was systemicallystudied and a mechanism was proposed. After being doped byKOH solutions of various concentrations, the morphology of thealkaline doped PBI membrane remained unchanged while a hy-drophobic/hydrophilic phase separation was established as a re-sult of the formation of hydrophilic ionic clusters in the PBI ske-leton. The modification of PBI membrane by KOH solution was alsoinvestigated by FTIR, demonstrating that the strong hydrogen-bonded networks were partially fractured while the cations (Kþ)reacted with benzimidazole segments (–NH) in the PBI skeleton.This modification has a profound influence on the ionic con-ductivity, alcohol permeability and mechanical properties. Weexperimentally testified that the ionic conductivity of the alkalinedoped PBI membrane reached 96.1 mS cm�1 at 363.15 K when PBImembrane was doped by 6 M KOH. Although the increase in thealcohol permeability and the deterioration of mechanical proper-ties were also induced by the decreasing the molecular cohesion,these properties of the alkaline doped PBI membranes still met therequirement for the application in ADAFCs. Finally, a mechanism

was proposed to interpret the interaction and their influence onphysicochemical properties. The existence of a neutralization re-action and the re-establishment of hydrogen bonding networksresulted in the property transformation during the alkaline dopingprocess.

Acknowledgements

The work described in this paper was fully supported by a grantfrom the Research Grants Council of the Hong Kong Special Ad-ministrative Region, China (Project no. 623313).

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