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Available online at www.sciencedirect.com

Journal of Membrane Science 310 (2008) 577585

Preparation and characterization of CPPO/BPPO blend membranesfor potential application in alkaline direct methanol fuel cell

Liang Wu, Tongwen Xu, Dan Wu, Xin Zheng

Functional Membrane Laboratory, School of Chemistry and Material Science, University of Science and Technology

of China (USTC), Hefei, Anhui 230026, PR China

Received 11 September 2007; received in revised form 25 November 2007; accepted 27 November 2007

Available online 4 December 2007

Abstract

A series of hydroxyl-conducting anion-exchange membranes were prepared by blending chloroacetylated poly(2,6-dimethyl-1,4-phenylene

oxide) (CPPO) with bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO), and their fuel cell-related performances were evaluated.

The resulting membranes exhibited high hydroxyl conductivities (0.0220.032 S cm1 at 25 C) and low methanol permeability (1.35 107 to

1.46 107 cm2 s1). All the blend membranes proved to be miscible or partially miscible under the investigations of scanning electron microscopy

(SEM) and differential scanning calorimeters (DSC). By condition optimization, the blendmembranes with 3040 wt% CPPO are recommended for

application in direct methanolalkalinefuel cells because theyshowedlow methanolpermeability, excellent mechanical propertiesand comparatively

high hydroxyl conductivity.

2007 Elsevier B.V. All rights reserved.

Keywords: Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO); Blend membrane; Hydroxyl ion conductivity; Methanol permeability; Alkaline fuel cell

1. Introduction

The direct methanol fuel cell (DMFC) has attracted con-

siderable attention as an alternative energy source due to its

high power density, slight pollution, high efficiency, and instant

refueling. DMFC, which produces a power of about 500 W

and runs at the temperature ranging from 20 to 50 C, is

one of the ideal substitutes for batteries in portable devices,

such as laptops, cellular phones, digital cameras, and human

portable power packs [1,2]. Generally speaking, there are two

kinds of DMFC: (i) proton-conducting DMFC, which uses a

cation-exchange membrane with sulfuric acid groups, such as

Nafion and sulfonated polymeric membranes and (ii) hydroxyl-

conducting DMFC, which uses an anion-exchange membrane

instead.

In recent years, proton-conducting membrane-based DMFC

has attracted much attention. This research treats of mem-

brane development [3,4], the oxidative mechanism inside the

electrodes and in the interface between electrode and mem-

Corresponding author. Tel.: +86 55 1360 1587.

E-mail address: [email protected] (T. Xu).

brane, and fuel cell performances [58]; however, there are stillsome obstacles to proton-conducting membrane-based DMFCs

application: (i)the relatively slow methanol oxidation kinetics

on the anode catalyst; (ii) CO poisoning of electrode catalyst at

low temperature; (iii) high costs of the membrane, catalyst, and

separator; (iv) parasitic methanol crossover [913].

Direct methanol alkaline fuel cell (DMAFC) also deserves

equal attention since it has a number of potential advantages

as follows [1418]: (i) Methanol oxidation in alkaline media is

kinetically faster than that in acidic media; (ii) Non-precious

metal catalysts, such as Ni and Ag, can be used due to the

methanol oxidation catalysts are less sensitive in alkaline media

than in acidic media; (iii) Methanol crossover from the anode

to the cathode can be reduced by the alkaline anion-exchange

membrane since the electro-osmotic ion transport occurs in the

opposite direction. If crossover can be eliminated, it is possible

to keep methanol from being diluted and employ thin mem-

branes with low electrical resistance, which will lead to the

maximization of energy density.

Therefore,in recent years, more researchers turned theirinter-

est to DMAFC [1418,5,6], and they focused on preparation

of anion exchange membranes with both lower methanol per-

meability [19] and manufacturing cost than those of Nafion.

0376-7388/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.memsci.2007.11.039
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.memsci.2007.11.039http://localhost/var/www/apps/conversion/tmp/scratch_3/dx.doi.org/10.1016/j.memsci.2007.11.039mailto:[email protected]

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578 L. Wu et al. / Journal of Membrane Science 310 (2008) 577585

A series of membrane materials have been thus reported for

application in DMAFC, such as polysiloxane containing qua-

ternary ammonium groups [20]; aminated poly(oxyethylene)

methacrylates [21]; quaternized polyethersulfone cardo [22],

radiation-grafted poly(vinylidene fluoride) (PVDF) and poly

(tetrafluoroethene-co-hexafluoropropylene) (FEP) [16,23], and

quaternized poly(phthalazinon ether sulfone ketone), etc. [24].

The corresponding methods used for material preparing include

chemical modification and pore grafting. In spite of their diver-

sity, these membrane materials are not suitable for further

application in DMAFC due to some practical reasons. For

examples, non-fluorinated polymer membranes have poor fuel

cell-related performances, and fluorinated polymer such as FEP

are high in cost.

Among engineering plastics, poly(2,6-dimethyl-1,4-

phenylene oxide) (PPO) is a unique material in view of

its strong hydrophobicity, high glass transition temperature

(Tg =212C) and hydrolytic stability [25]. Despite the simplic-

ity in structure when compared with other aromatic polymers,

PPO can easily conduct many polymer-analogous reactionsin both aryl- and benzyl-positions, such as electrophilic

substitution on the benzene ring of PPO, radical substitution of

the hydrogen from the methyl groups of PPO, and nucleophilic

substitution of the bromomethylated PPO [26]. It can be

expected that these substituted materials have good miscibility

since they all have PPO backbones. Therefore, to explore

the miscible blend membranes for DMAFC, in this paper,

new composite anion-exchange membranes for fuel cells will

be prepared from the blends of chloroacetylated poly(2,6-

dimethyl-1,4-phenylene oxide) (CPPO) and bromomethylated

PPO (BPPO), and their structures and fundamental properties

will be discussed upon their relative contents.

2. Experimental

2.1. Materials

Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) of intrin-

sic viscosity 0.57 dl g1 in chloroform at 25 C was obtained

from Institute of Chemical Engineering of Beijing (PR China).

CP-grade trimethylamine (TMA) aqueous solution (33.3%),

AR-grade ethanol, chloroform chlorobenzene and bromine were

purchased from Shanghai Sinopham Chemical Reagent Co. Ltd.

(China). Anhydrous aluminum chloride was purchased from

Shanghai Meixing Chemical and Engineering Co. Ltd. and pul-verized and dried (120 C) before use. Deionized water was used

in experiments throughout.

2.2. Bromination and chloroacetylation

PPO was brominated according to the method reported in

detail in our previous paper [27]. In brief, 12 g of PPO was

dissolved in chlorobenzene to form an 8 wt% solution, and

this solution was taken to bromination at the boiling point

(130132 C) after addition of 16 g of bromine (diluted with

cholorobenzene). After precipitation (in methanol), washing,

and drying (80

C for 1 day), brominated PPO (BPPO) was

obtained. The benzyl substitution was about 100% according

to 1H NMR (Unity plus 400) measurements.

Chloroacetylation of PPO was conducted by FriedelCrafts

reaction [28] and theparticular procedures areas follows:(i) 12 g

of PPO was dissolved in 100 ml chloroform to form an 8 wt%

solution in a 250 ml three-neck flask which was equipped with

a tube filled with CaCl2 and a stirrer; (ii) 13.5 g of anhydrous

aluminum chloride was then added into the solution with stren-

uous stirring so as to achieve a complete dissolution; (iii) 8 ml

1,3-bis-chloroacetyl was added dropwise into the solution and

the mixture was stirred at 40 C for 4 h. The final chloroacety-

lated PPO (CPPO) was obtained by depositing the solution in

ethanol and then drying it at 60 C for 48h. The 1H NMR mea-

surement (Unity plus 400) showed that CPPO used here was

the one with a phenylic substitution degree of about 50.3%

[29].

2.3. Membrane preparation

The blend membranes were prepared by casting thechlorobenze solution of CPPO and BPPO mixture on a cleaning

glass plate and evaporating the solvent at 30 C in a con-

vection oven over 24 h. The base membranes, which were

detached without mechanical stress from the glass plate, were

then washed with 0.01 mol dm3 sodium hydroxide solution,

0.01 mol dm3 hydrochloric acid solution, and water in turn.

Then they were quaternary-aminated in a trimethylamine aque-

ous solution (0.91 mol dm3) for 48 h. Finally, the membranes

were rinsed with 0.5 mol dm3 HCl solution and changed into

Cl form in 1moldm3 NaCl solution. The blend ratio is

calculated as the relative content of CPPO in the total poly-

mers regardless of the solvent. The final membranes weredesignated using the weight ratio of CPPO in the base blend

membrane (i.e., before quaternary-amination), e.g., CPPO-x

represents the weight ratio of CPPO is x% in the base blend

membrane.

2.4. Ion exchange capacity and water uptake

The ion exchange capacity (IEC) of the membrane was deter-

mined by using the Mohr method, in which the membrane in

Cl form was converted into SO42 form after immersing in

a Na2SO4 aqueous solution (0.5 mol dm3) for 8 h. The chlo-

ride ions released from the membrane were titrated with an

0.1moldm3 AgNO3 aqueous solution. The IEC values were

calculated from the released chloride ions and expressed as

mequiv. g1 of dry membrane (in Cl form).

For water uptake measurement, the dried and pre-weighed

CPPO/BPPO membranes (W0) were immersed in deionized

water at 25 C for 24h and thentaken out of the bath. The excess

liquid was carefully removed from the membrane surface by fil-

ter papers, and the wet weight (W1) was then determined. The

water uptake (WU) was calculated according to the following

equation:

WU% = 100(W1 W0)

W0

(1)

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L. Wu et al. / Journal of Membrane Science 310 (2 008) 57758 5 579

2.5. Hydroxyl conductivity

The hydroxyl conductivity measurement is similar to the

proton conductivity measurement using the normal four-point

probe technique [30] and the procedures are as follows: (1)

Specify the frequency region in the Bode Plot where the phase

angle is very low (near zero) and (2) Draw the Nyquist Plot

for this frequency region and read the membrane resistance

from the plot. In our manuscript, the entire testing frequency

region is from 1 MHz to 50 Hz, and the determined frequency

region (where phase angle is near zero) from Bode Plot is about

104 to 500 Hz. The ionic conductivity () was calculated from

the membrane resistance according to the following equation

[30,31]:

=L

RWd(2)

where R is the obtained membrane resistance, L the dis-

tance between potential-sensing electrodes (here 1 cm) and W

and d are the width (here 1 cm) and thickness of the mem-brane, respectively. During the measurement, the cell was

immersed in purified water, so the ionic conductivity obtained

was corresponding to the fully hydrated membrane at room

temperature.

2.6. Methanol permeability

The methanol permeability was measured using a home-

made permeation-measuring cell with two compartments and

magnetic stirring. Compartment A was filled with 150 cm3 of

20% (v/v) methanol solution, compartment B was filled with

150 cm3 of deionized water. The membrane was positioned

between these two compartments, and the diameter of the diffu-

sion area was 1.0 cm. During experiment, the diffusion cell kept

stirring slowly, and the methanol concentration of compartment

A was kept unchanged by feeding continuously from a container

filled with 20% (v/v) methanol solution. The concentration of

methanol diffused from compartment A to B across the mem-

brane was monitored with time using refractive index detector

(RI750F, Younglin Instrument Co., Korea), which was driven by

a Masterflex pump through a 1 mm diameter silicon pipe at a con-

stant speed of 1.0 cm3 min1. This detector was also connected

to compartment B and calibrated with various methanol concen-

trations. The output signal was converted to the digital signal by

data module (Autochro, Younglin Instrument Co., Korea) andtransferred for a programmed computer.

After measuringthe change in methanol concentration during

permeation and calculating the slope of the resulting curves,

the methanol permeability (P) was obtained using the following

equation [31]:

CB(t) =A

VB

P

LCA(t t0) (3)

where CA is the initial methanol concentration of the methanol

solution, CB(t) the methanol concentration in the deionized

water-filled compartment at time t, VB the liquid volume in the

deionized water-filled compartment,L the thicknessof themem-

brane, A the effective permeation area and t0 is the time lag, i.e.,

the selected starting point for linear regressions.

2.7. Thermal analysis: DSC and TGA

The DSC spectra of the un-aminated base membranes were

obtained on DSC model 60, while the TGA spectra of the

aminated membranes were obtained on the DTG model 60H

fromShimadzu Company, respectively. Measurements wereper-

formed in nitrogen in the temperature range of 10600 C at a

heating rate of 10 Cmin1.

2.8. Morphological observations

The surface morphologies of prepared membranes were

investigated using scanning electron microscopy (XT30 ESEM-

TMP PHILIP). Those membranes were dried and then coated

with gold before observation.

2.9. Mechanical analysis

The tensile strength of dry membrane was measured using

a Microtest 5000 tensile stage from Gatan, Inc. The cross-

sectional area of the sample was calculated according to its

known width and thickness. The films were then placed between

the flat-faced grips of the testing machine. The speed of testing

was set at the rate of 100 N min1.

3. Results and discussion

3.1. IEC

IEC canprovide information on the density of ionizable func-

tional groups in the membrane which are responsible for the

charge nature of the membrane and thus for membrane con-

ductivity. Fig. 1 plots the IEC data of membranes prepared

from the blends of BPPO and CPPO. It is demonstrated that

IEC decreases as CPPO content increases. This trend can be

ascribed to the membrane composition and amination degree of

each component. As mentioned in the experimental section, the

employed BPPO has 100% benzyl substitution, and CPPO has

Fig. 1. IEC and water uptake of blend membranes.

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a phenyl substitution of about 50%. After treating with tertiary

amine (trimethyl amines (TMA) aqueous solution in this paper),

both bromomethyl groups and chloroacetyl groups can form the

quaternary ammonium groups and contribute to the IEC. How-

ever, bromomethyl groups are more reactive than its polystyrene

counterpart [32] while chloroacetyl groups are hard to react with

TMA due to the steric hindrance. This may be responsible for

the trend of change in IEC.

Fig. 1 also lists the theoretical IEC values, which are cal-

culated from the bromination and chloroacetylation degrees by

assuming the complete reaction with TMA. It can be observed

that, in comparison with the experimental IEC, the theoretical

IEC is relatively large. Moreover, this difference increases as

CPPO content increases. For example, in the case of CPPO-10,

the difference between experimental and theoretical IEC value

is 1 mequiv. g1 dry, but in the case of CPPO-50, the difference

increases to 1.9 mequiv. g1. This can be ascribed to the fact that

the amination cannot be conducted completely in experiments,

and that there exists a competition between bromomethyl groups

and chloroacetyl groups while aminating, which was discussedabove.

As for a further explanation on the difference mentioned

above, one may take into consideration that bromomethyl

groups and chloroacetyl groups have different competences for

electrophilic reaction during base membrane treatment. In par-

ticular, because of the electron withdrawing effect of carbonyl

groups, it is more likely for chloroacetyl groups, in com-

parison with bromomethyl groups, to undergo FriedelCrafts

reaction with the hydrogen of phenyl at a low temperature and

even without any catalyst [28]. The corresponding chemical

equation is shown in Scheme 1. This leads to the difficulty

when chloroacetyl groups are being aminated, and furtherresults in the difference between theoretical and experimental

IECs.

3.2. Water uptake

As expected, water uptake shows the same trend as IEC. i.e.,

it decreases as CPPO content increases (Fig. 1). In particular,

the water uptakes for the membranes with CPPO contents of 10,

20, 30, 40, and 50 wt% are 280.3, 179.2, 137.4, 79.7, and 44.3%,

respectively. Notably, the decrease is very sharp. In our previous

studies, it was found that anion exchange membrane prepared

from BPPO had an extremely high water uptake and swelling

degree due to the formation of hydrophilic quaternary ammo-

nium groups during the reaction of bromomethyl groups with

TMA [27]. In contrast, the membrane prepared by quaternary-

aminating CPPO with TMA had an extremely low water uptake

dueto thehydrophobiceffect of chloroacetyl groups [29]. There-

fore, although it is usually accepted that a high IEC leads to a

high water uptake and both of quaternary-aminated bromethyl

groups (in BPPO) and chloroacetyl groups (in CPPO) can con-

tribute to IEC, a desired anion-exchange membrane with high

IEC and low water uptake can be achieved if one controls the

hydrophobicity by adjusting the CPPO content in BPPO/CPPOblends.

3.3. Thermal behavior

The thermal behaviorsof studied blends were examinedusing

DSC, TGA. Our preliminary results proved that the trends are

similar between membranes with different blend ratios, so Fig. 2

shows the DSC data for three CPPO/BPPO blend base mem-

branes CPPO-10, CPPO-30, CPPO-40 as well as BPPO and

CPPO pure polymeric membranes. It is observed that these

blend membranes exhibit only one broad Tg peak, in the region

between the twopure polymers. And those broad Tg peaks mightbe considered as two overlapping Tg peaks of the respective

polymer. This kind of characteristic indicates the miscibility or

Scheme 1. FriedelCrafts reaction of chloroacetyl groups during heat treatment.

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Fig. 2. DSC curves of CPPO/BPPO blend membranes.

partial miscibility between blend components at these mixing

ratios [33].

The thermal stability of the final membrane, as a function

of weight loss ratio, was investigated via TGA at a heating

rate of 10 Cmin1 in nitrogen, and the corresponding results

are shown in Fig. 3. Obviously, the thermal degradation course

Fig. 3. TGA curves of blend membranes.

can be divided into four stages for those membranes. The ini-

tial weight loss of those blend membranes, which starts below

100 C and continues to 160 C, is attributed to the bond water.

But aminated CPPO almost have no weight loss in this tem-

perature region. According to the discussion in Section 3.1, the

chloroacetyle substitution degree of CPPO is only 50wt%, and

Fig. 4. SEM micrographs of CPPO/BPPO blend membranes with different CPPO contents: (a) CPPO-20; (b) CPPO-30; (c) CPPO-40; (d) CPPO-50.

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those chloroacetyl groups are difficult to be quaternized. Hence,

it is reasonable that aminated CPPO has little weight loss in

this temperature region for little water absorbed in it. The sec-

ond degradation stage of blend membranes appeared at around

300 C,while that of aminated CPPO startsat about 200 C. This

kind of degradation is ascribed to the splitting-off of quaternized

bromomethyl or chloroacetyl groups in the membranes. More-

over, the beginning temperature of this decomposition in the

TGA curves shifts from 290 to 310 C when the BPPO content

increasesfrom 60 to 90%. Therefore, allof those indicate that the

thermal stability of the blend is enhanced by increasing BPPO

content. The third thermal degradation region is at about 340 C

and can be ascribed to the decomposition of chemical bonds that

are formed according to the chemical equation in Section 3.1.

The last weight loss region at about 420 C is attributed to the

decomposition of the main chain of the polymers. These results

indicate that all those blend membranes are thermally stable at

about 300 C, which is good enough for usage in DMAFC. On

the other hand, the blends with higher BPPO contents have a

higher amount of char residue (Fig. 3), indicative of a higherthermal stability. All of those results suggest that BPPO is more

heat stable than CPPO, and thus the thermal stability of blends

can be enhanced by increasing BPPO content.

3.4. Miscibility and mechanical properties

The morphology of the membranes fracture surface was

observed using field emission scanning electron microscopy

(XT30ESEM-TMP PHILIP), and the micrographs are presented

in Fig. 4ad for the samples CPPO-20, CPPO-30, CPPO-40, and

CPPO-50, respectively. Obviously, the dense fracture surfaces

can be observed, suggesting that the two blended componentsare miscible or partially miscible. The result is consistent with

the earlier DSC observations, where a single broad peak was

observed for the blend membranes with different weight ratios.

All of those micrographs show rough fracture surfaces,which

provides some information regarding membranes mechanical

properties since the roughness may be associated with co-

continuous blend morphologies. The patterns, formed in the

process of fracture energy dissipating during tensile failure, can

be ascribed to plastic deformation of the CPPO domains when

CPPO presents as the dispersed phase in this blend system.

The rumpled surface accompanying cavitations (Fig. 4ac), are

indicative of ductile fracture. In contrast, the smooth, semicircu-

lar zones andnext ripplezonesin Fig.4d indicate brickle fracture[34], which results in a low tensile strength. These qualitative

judgments are confirmed by the tensile strength data which are

plotted in Fig. 5. Obviously, CPPO-20, CPPO-30, and CPPO-

40 membranes have rougher fractures than CPPO-50 membrane

does. Additionally, the blend does not become remarkably

rougher until CPPO content increases over 30 wt%. One reason-

able explanation is that the morphology of the blend undergoes

a remarkable change from a discrete CPPO/continuous BPPO

pattern at low CPPO contents to a co-continuous blend morphol-

ogyat higherCPPO contents such as 40 wt%CPPO content [35].

Thus, as the tensile strength increase with an increase in CPPO

content. For example, CPPO-40 shows a relatively high tensile

Fig. 5. Tensile strength of blend membranes with different CPPO contents.

strength (c.f. Fig. 5). Nevertheless, as discussed in Section 3.1, it

is easier for chloroacetyl groups to undergo FriedelCrafts reac-tion with hydrogen of phenyl than bromomethyl groups due to

the electron withdrawing effect of carbonyl groups. Therefore,

the membranes with higher CPPO content such as CPPO-50

has more chances to form a partially-crosslinking structure than

other membranes, which results in a brickle fracture as shown

in Fig. 4d, and further a decrease in tensile strength as shown in

Fig. 5. The round dark regions shown in Fig. 4d are pores formed

by gas bubbles during membrane preparation. As described in

Section 2.3, solution casting method was adopted for membrane

preparation here, and those bubbles in the casting solution can-

notbe removed completely duringbase membrane treatmentdue

to the formation of partially-crosslinking structures. Notably,

there are few round dark regions in the micrographs of mem-braneCPPO-20,membrane CPPO-30, andmembrane CPPO-40,

which have less dense structures than membrane CPPO-50.

3.5. Hydroxyl conductivity

Four-probe longitudinal measurement was adopted here to

determine hydroxyl conductivity in the plane of the membrane.

As shown in the experimental section, the quaternized mem-

branes were first soaked in 2 mol dm3 NaOH solution for

8 h to turn into OH form, and then they were taken out and

washed several times with deionized water to remove absorbed

ions. Then 1 cm 4 cm membranes were mounted for hydroxylconductivity determination in the measuring cell, which was

immersed in purified water.

Fig. 6 shows the ionic conductivity of blending membrane

as a function of CPPO content. The blend membranes which

contain 20, 30, and 40 wt% of CPPO show the conductivities

of 0.0325, 0.0276, and 0.0221 S cm1, respectively. Those high

values show consistency to the high IEC or water uptake as

discussed above. Surprisingly, the hydroxyl conductivities of

these membranes are much higher than those of reported anion-

exchange membranes. For example, the initial studies of fuel

cell conducted by Yu and Scott showed that the conductivity

of a commercial anion exchange membrane, Morgane

-ADP

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Fig. 6. Hydroxyl ion conductivity of blend membranes.

(Solvay, S.A.), was 600% lowerthan those of Nafion membranes

[14]. As well known, the proton conductivity for Nafion 117 isabout 0.08 S cm1. It suggests that the hydroxyl ion conduc-

tivity of the present membranes can attain 3040% of proton

conductivity in Nafion 117. In general, from the viewpoint of

conductivity, CPPO/BPPO blend membranes reported in this

paper are qualified for application in DMAFC.

More interestingly, when CPPO content increases to 50%,

the conductivity has a sharp decrease, e.g., the conduc-

tivity decreases from 0.0221 for membrane CPPO-40 to

0.000558 S cm1 for membrane CPPO-50 (c.f. Fig. 6). The

results can be explained by conductive ion cluster formation

mechanism. The clustering theory of ions in organic polymers

holds that ions in organic media exist probably as pairs orhigher multiplets [36], which could aggregate to form a clus-

ter at relatively low temperatures. For CPPO/BPPO blending

membranes, the hydrophilic quaternary ammonium groups may

aggregate into hydrophilic ionic clusters in the hydrophobic

polymer matrix. Furthermore, the clustering theory also sug-

gests that there is a critical concentration below which cluster

formation is energetically unfavorable. Hence, the ionic clus-

ters are isolated in the continuous hydrophobic domain if the

density of quaternary ammonium groups is low. However, when

BPPO content is high, these ionic clusters aggregate energet-

ically and form a random distribution of ion channels with

good connectivity, where the hydroxyl ions can transport eas-

ily. The introduction of hydrophobic CPPO component reducesthe density of hydrophilic quaternary ammonium groups, so it

is reasonable that the decreasing tendency in ionic conductiv-

ity was mainly influenced by the introduction of hydrophobic

CPPO into the hydrophilic aminated BPPO matrix. As for the

CPPO-50 membrane, the quaternary bromomethylated ammo-

nium groups are not enough to form ion clusters and thus the

conduction of hydroxyl ions therein is restricted. Furthermore,

the conductivity of the CPPO-50 membrane decreased sharply

when compared to other blending membranes. There may be

two reasons: the first one is related to the clustering theory men-

tioned above, and the other is related to the formation of dense

microstructures. There are more chances for the CPPO-50 mem-

Fig. 7. Methanol permeability of blend membranes.

brane to undergo FriedelCraft reaction than other membranes

since it has a high CPPO content and thus denser microstructure

as compared with others. Therefore, the mobility and size of theion clusters in the blend membranes might be restricted. Both

of these two reasons resulted in so lower hydroxyl conductivity

for CPPO-50.

3.6. Methanol permeability

Apart from high hydroxyl conductivity, low methanol per-

meability is another requirement for practical application

in fuel cells. Fig. 7 shows the methanol permeability of

blend membranes. Obviously, all the membranes show low

methanol permeability for the opposite transfer direction of

hydroxyl ions and methanol. For example, CPPO-20, CPPO-30, CPPO-40, and CPPO-50 membranes have the methanol

permeability of 1.46 107, 1.44 107, 1.35 107, and

1.30 107 cm2 s1, respectively. It is difficult to compare the

methanol permeability with a standard anion-exchange mem-

brane for alkaline fuel cells since the reported experimental

conditions aredifferent in cases by cases. However, Nafion 117

is thestandardmembranefor protonconductive fuel cells,whose

methanol permeability was measured as 21.4 107 cm2 s1

under the identical conditions. In comparison, all the blend

membranes prepared in this work have much less methanol

permeability than this Nafion membrane.

It is possible that there exists barriers to methanol in the blend

membranes and the mechanism may be related to the hydropho-bicity as well as the poor methanol affinity of the CPPO matrix

as compared with perfluorinated hydrocarbon. The connective

channels formed by the aggregation of hydrophilic clusters is

favorable for methanol transport in membrane, but the interac-

tion between CPPO and BPPO, mentioned above, restricts the

formation of large hydrophilic clusters due to the hydrophobicity

of CPPO, and correspondingly leads to a much lower methanol

permeability than Nafion 117.

In summary, considering the requirements for practical appli-

cation in fuel cells, the blend membranes with CPPO content

ranging from 3040 wt% are good choices since they have high

conductivity (i.e., high IEC and water uptake), dimensional

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stability (low swelling degree), and low methanol permeabil-

ity.

4. Conclusions

New hydroxyl-conducting membranes were prepared by

blending CPPO with BPPO. The similar structures and inter-

action of the two components endued the resulting membranes

with good miscibility and mechanical properties as well as high

thermal stability.

All the membranes were characterized by conventional prop-

erties, such as water uptake and IEC, and the fuel cell-related

performances, such as conductivity and methanol permeabil-

ity. The results showed that the properties, such as water

uptake, IEC, hydroxyl conductivity, and methanol perme-

ability, decrease as CPPO content increases. By properly

balancing these properties, blend membranes with 3040 wt%

CPPO are strongly recommended for application in DMAFC.

Such membranes showed high hydroxyl ion conductivity

of 0.0210.027 S cm1 and low methanol permeability of1.351.44 107 cm2 s1.

Acknowledgements

This research was supported in part by the National

Science Foundation of China (No. 20636050) and the NSFC-

KOSEF Scientific Cooperation Program (No. 20611140649).

Special thanks will be given to Prof. Moon (the Laboratory

for the Environment-Oriented Electrochemical Engineering of

Gwangju Institute of Science and Technology, Korea) for his

kindly providing the facilities for measuring hydroxyl ion con-

ductivity and methanol permeability.

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