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Journal of Membrane Science 288 (2007) 51–60

Synthesis and characterization of the cross-linked PVA/TiO2

composite polymer membrane for alkaline DMFC

Chun-Chen Yang∗

Department of Chemical Engineering, Mingchi University of Technology, Taipei Hsien 243, Taiwan, ROC

Received 21 June 2006; received in revised form 26 October 2006; accepted 28 October 2006

Available online 11 November 2006

Abstract

A novel PVA/TiO2 composite polymer membrane was prepared by a solution casting method. Glutaraldehyde (GA) was used as a cross-linkerfor the composite polymer membrane in order to enhance the chemical, thermal and mechanical stabilities. The characteristic properties of the

cross-linked PVA/TiO2 composite polymer membranes were examined by thermal gravimetric analysis (TGA), X-ray diffraction (XRD), scanning

surface microscopy (SEM), and ac impedance method. The novel DMFC, consisting of an air cathode electrode with MnO2 carbon inks, an anode

electrode with PtRu black inks on carbon paper and the PVA/TiO2 composite polymer membrane, was assembled and examined. It was found that

the DMFC using this novel cheap PVA/TiO2 composite polymer membrane showed good electrochemical performance at ambient temperature

and pressure. The maximum peak power density of the alkaline DMFC is about 7.54 mW cm−2 at 60 ◦C and 1 atm.

© 2006 Elsevier B.V. All rights reserved.

Keywords: Direct methanol fuel cell (DMFC); PVA; Composite polymer membrane; TiO2; MnO2

1. Introduction

Direct methanol fuel cell (DMFC) [1–18] and solid poly-

mer electrolyte membranes fuel cell (PEMFC) [19–25] have

recently received a lot of attentions due to these power sources

presenting a high-energy efficiency and low emission of pollu-

tants. For PEMFC, hydrogen was used as fuel and delivered

the power density of 300–500 W cm−2 at 80–90 ◦C. While,

using hydrogen as a fuel may cause some problems, such

as production, storage and transportation of hydrogen. Due

to these reasons, the DMFC has attracted much attention

than PEMFC because of using liquid methanol fuel, which is

easy to deliver and store. More importantly, liquid fuel can

use at ambient temperature and pressure, which makes the

DMFC easily be applied on the portable 3C electronic devices

[13–18].

However, the development of acidic DMFC has faced several

serious problems: (i) slow methanol oxidation kinetics [1–3],

(ii) the poisoning of CO intermediate on the Pt surface [6], (iii)

the high methanol cross-over through the polymer membrane

∗ Tel.: +886 2 908 4309; fax: +886 2 2904 1914.

E-mail address: [email protected].

[5,7,9,10], and (iv) the high costs of the Nafion membrane and

Pt catalyst.Presently, the perfluorosulfonate ionomer membranes, such

as Nafion membrane (Dupont), are the primary polymer mem-

branes used on the DMFC. However, the commercial Nafion

polymer membranes showed serious methanol cross-over prob-

lem [5,9], which methanol permeates from the anode to the

cathode. The methanol permeation not only cause a loss of fuel

but also a mixed potential being formed at the cathode and lead-

ing to a lower electrochemical performance of the DMFC. Thus,

for the liquid methanol fuel cell, it is imperative that the most

important characteristic properties of a solid polymer membrane

on the DMFC must have a lower methanol permeation of liquid

fuel.

Alkaline polymer electrolytes based on PEO have been stud-

ied for application on Ni–Cd, Ni–Zn [26,27], and Ni–MH

secondary battery systems [28,29]. They reported the alkaline

PEO–KOHpolymer electrolyte exhibiting the ionicconductivity

around 10−3 S cm−1 at room temperature. Yang and Lin [30,31]

studied and prepared the alkaline polymer electrolyte mem-

brane based on PEO–PVA–KOH for use on secondary Ni–MH

and primary Zn-air batteries. Lewandowski et al. [32] synthe-

sized PEO–KOH polymer electrolyte for electric double layer

capacitors (EDLCs). Yang and Lin [33,34] also reported the

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.memsci.2006.10.048

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52 C.-C. Yang / Journal of Membrane Science 288 (2007) 51–60

PVA composite polymer electrolyte for use in Ni–MH and Zn-

air batteries. Agel et al. [35,36] prepared an alkaline anionic

exchange membrane for fuel cell by grafting quaternary amines

on the epichlorhydridepolymer withthe ionicconductivityabout

10−2 S cm−1 andwith anionic transportnumber (t −)greaterthan

0.9.

Recently, Cho et al. [37] prepared the semi-interpenetrating

polymer networks (IPNs) of Nafion and cross-linked

poly(AMPS) for DMFC. The IPNs membrane showed a

reduction of the methanol permeability and increased the

maximum power density as much as 22% (209 mW cm−2)

compared with Nafion (171 mWcm−2). Song et al. [38]

also prepared Nafion/montmorillonite (MMT) nanocompos-

ite membranes with lower methanol permeability (1.6×

10−7 cm3 cmcm−2 s−1) for DMFC. Those membranes demon-

strated that the Nafion/MMT nanocomposite membranes

delivered much higher maximum power density (about

100mWcm−2) at concentrated 10 M methanol feed at 70 ◦C.

Li et al. [39] prepared the sulfonated poly(ether ether ketone)

(SPEEK) membranes on the acidic DMFC. The methanolpermeability (1.3–1.4× 10−7 cm2 s−1) was about an order of

magnitude lower than that of Nafion 115 membrane (4.9 ×

10−6 cm2 s−1).

Sun et al. [40] synthesized characterized the sulfonated

poly(phthalazinone ether ketone) (SPPEK) membranes with

lower methanol permeability for DMFC. The maximum power

density of a single cell DMFC was 55 mW cm−2 at optimal

methanol concentration (3 M) at 70 ◦C. Shen et al. [41] reported

the performance of DMFC with radiation grafted polymer elec-

trolyte membranes. Three base polymer films (polyethylene

tetrafluoroethylene (ETFE), polyvinyl fluoride (PVDF), low-

density polyethylene (LDPE)) were grafted with polystyrenesulfonic acid (PSSA). The DMFC with PSSA grafted-PVDF

membrane showed the maximum peak power density about

58mWcm−2 at 80 ◦C.

For comparison, Yu and Scott [42–44] studied the elec-

trochemical performance of the alkaline DMFC with anion

exchange membranes. The DMFC performance with maxi-

mum power density of about 10 mW cm−2 was obtained in

a commercial quaternary-ammonium anion-exchange mem-

brane (Morgane-ADP, Solvay SA, Belgium). In addition, the

PVA/PWA based [45,46] membranes have been prepared and

applied on DMFC. Varcoe et al. [47–50] developed and char-

acterized the quaternary ammonium (as the counter ions bund

to the polymer backbone) radiation grafted ETFC [47], PVDFand FEP [50] alkaline anion exchange membrane (AAEM).

They [48] prepared the AAEM-MEAs that do not contain any

metal-cation Mn+ (i.e., K+, Na+) ions to avoid the carbonate

precipitation problem and to improve the long-term opera-

tion stability. It is a breakthrough for alkaline anion-exchange

membranes on fuel cell application. The peak power density

of 130 mWcm−2 for the H2 /O2 fuel cell with AAEM mem-

brane was obtained. While the maximum power density of

8.5mWcm−2 was obtained in a metal-cation-free methanol/O2

fuel cell with 2–2.5 bar back pressure at 80 ◦C. Recently, Yang

et al. [51] prepared the alkaline PVA/PAA IPNs membranes and

can be applied on the fuel cells.

The addition of ceramic filler into polymer matrix is allow to

reduce the glass transition temperature (T g) and the crystallinity

of the polymer, and also allow the increase the amorphous

phases of polymer matrix, then increase the ionic conductiv-

ity. There are various ceramic filler, such as Al2O3, TiO2 [5],

SiO2 [7], have been extensively studied. These experimental

results indicated improvements in the ionic conductivity, ther-

mal and mechanical properties as the different ceramic fillers

were added into the solid polymer electrolyte (SPE). The rea-

son for the increase of ionic conductivity of composite polymer

electrolyte was explained that the ceramic particle fillers in the

polymer matrix created some defects and free volume at inter-

face of between the ceramic particle and the polymer chain. In

this work, we attempted to disperse the nano-sized TiO2 par-

ticles into the PVA matrix act as a solid plasticizer capable of

enhancing chemical, thermal and mechanical stabilities of the

PVA-based composite polymer membrane.

TGA was used to analyze the thermal stability properties of

the cross-linked PVA/TiO2 compositepolymer membrane. XRD

was used to investigate the crystal structure of the PVA/TiO2

composite polymer membrane. SEM was used to examine

the surface morphology of the composite polymer film. The

ionic conductivity of alkaline PVA/TiO2 composite polymer

membranes was measured by ac impedance spectroscopy. The

characteristic properties of the cross-linked PVA/TiO2 poly-

mer membranes with different weight percents of TiO2 fillers

(1–20 wt.%) will be examined and discussed in detail.

In this work, the alkaline DMFC, composed of the air cathode

electrode loaded with MnO2 /BP2000 + CNT binary carbon inks,

the PtRu anode electrode (4.00 mg cm−2) and the cross-linked

PVA/TiO2 composite polymer membrane, was assembled and

examined. The PVA/TiO2 composite polymer membrane was atfirst prepared through directly blending PVA polymer with nano-

sized TiO2 (anatase, 7 nm) fillers under ultrasonic condition.

The obtained composite polymer membrane was then further

immersed in 5 wt.% GA solution for the cross-linking reaction.

For anodic methanol electro-oxidation reaction, cathodic oxy-

gen reduction reaction (ORR) and the overall reaction of the

DMFC in alkaline media can be described as follows:

Anodic reaction : CH3OH + 6OH− → CO2 + 5H2O + 6e−,

E0a = −0.810 V (versus SHE), (1)

Cathodic reaction :

3

2 O2 + 3H2O + 6e−

→ 6OH−

,

E0c = 0.402 V (versus SHE), (2)

Overall reaction : CH3OH +3

2O2 → CO2 + 2H2O,

E0cell = 1.21 V. (3)

Additionally, the electrochemical characteristics of the

DMFC with the cross-linked PVA/TiO2 composite polymer

membrane were investigated by the linear polarization and

potentiostatic methods; especially, for the peak power density

of the DMFC.

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C.-C. Yang / Journal of Membrane Science 288 (2007) 51–60 53

2. Experimental

2.1. Preparation of the cross-linked PVA/TiO2 composite

polymer membranes

PVA (Aldrich), nano-sized TiO2 fillers (7 nm, 338 m2 g−1,

Aldrich), and KOH (Merck) were used as received without

further purification. Degree of polymerization and saponifica-

tion of PVA were 1700 and 98–99%, respectively. The alkaline

PVA/TiO2 composite polymer membranes were prepared by

a solution casting method. The appropriate weight ratios of

PVA:TiO2 = 1:1–20 wt.% were dissolved in distilledwaterunder

stirring, respectively. The above resulting solution was stirred

continuously until the solution mixture became a homogeneous

viscous appearance at 85 ◦C for 2 h. The addition sequence of

powders and the time of blending in the vessel were well con-

trolled. The resulting mixture solution was poured out into a

Teflon container or Petri dish. The thickness of wet composite

polymer membrane is between 0.20 and 0.40 mm. The con-

tainer with viscous PVA/TiO2 composite polymer sample wasweighed again and then the excess water was allowed to evap-

orate slowly at 25 ◦C with a relative humidity of 30%. After

water solvent evaporation,the container withthe composite solid

polymer membrane was weighed again. The composition of the

PVA/TiO2 composite polymer membrane was determined from

the mass balance. Thethickness of the composite polymer mem-

brane was controlled at between 0.10 and 0.30 mm. The PVA

composite polymer membrane was cross-linked by immersing

in a solution of 5 wt.% glutaraldehyde (GA, 75% content in dis-

tilled water, Merck), 0.2–0.5vol.% HCl (used as a catalyst) and

acetone for the cross-linking reaction at 40 ◦C for different times

of 6–48 h. The preparation methods of the PVA-based polymerelectrolyte membranesby the solution casting method have been

reported in detailed [33,34,51].

2.2. Ionic conductivity, liquid uptake and ionic transport

property measurements

Conductivity measurements were made for alkaline

PVA/TiO2 composite polymer membrane by an ac impedance

method. The cross-linked PVA/TiO2 composite samples were

immersed in 32 wt.% KOH solutions for at least 24 h before test.

The alkaline PVA/TiO2 composite polymer membranes were

sandwiched between SS304 stainless steel, ion-blocking elec-

trodes, each of surface area 0.785 cm2, in a spring-loaded glassholder. A thermocouple was kept in close to the composite poly-

mer membrane for temperature measurement. Each sample was

equilibrated at the experimental temperature for at least 30 min

before measurement. The ac impedance measurements were car-

ried out using an Autolab PGSTAT-30 equipment (Eco Chemie

B.V., The Netherlands). The ac frequency range from 300 kHz

to 1 Hz at an excitation signal of 10 mV was recorded. The

impedance of the composite polymer membrane was recorded

at a temperature range between 30 and 70 ◦C. Experimental

temperatures were maintained within ±0.2 ◦C by a convection

oven. All alkaline PVA/TiO2 composite polymer membranes

were studied at least three times.

The pre-weighted, driedPVA/TiO2 composite polymer mem-

brane (W 0) was immersed in distilled (DI) water, 8 M KOH,

10M CH3OH aqueous solutions, respectively, and maintained

for24hat25 ◦C until the equilibrium was established. The com-

posite polymer membrane was taken out from the immersion

bath and the excess surface water was carefully removed. The

weight of the wet composite polymer membrane (W 1) was then

determined. The liquid uptake was calculatedfrom the following

equation:

Liquid uptake (%) =W 1 −W 0

W 0× 100% (4)

The ionic transport numbers of the cross-linked PVA/TiO2

composite polymer membranes were examined using a dynamic

Hittorf’s method [52] at 25 ◦C.

2.3. Crystal structure, morphology, and thermal analysis

The crystal structures of the PVA/TiO2 composite poly-

mer membranes were examined using a Philips X’Pert X-raydiffractometer (XRD) with Cu K radiation of wavelength

λ= 1.54056 A for 2θ angles between 10◦ and 80◦. The sur-

facemorphology and microstructure of the PVA/TiO2 composite

polymer membrane was examined by a Hitachi S-2600H scan-

ning electron microscope (SEM).

TGA thermal analysis was carried out using a Perkim-Elmer

Pyris 7 TGA system. Measurements were carried out by heating

from 25 to 500 ◦C, under N2 atmosphere at a heating rate of

10 ◦Cmin−1 with about 10 mg sample.

2.4. Preparation of the anode and cathode electrodes

The preparation of the catalyst slurry ink for the anode elec-

trode was prepared by mixing 70 wt.% PtRu black powders

(Alfa, HiSPEC 6000, PtRu black with Pt:Ru = 1:1 molar ratio),

30 wt.% PTFE binder solution (Dupont, 30 wt.% base solution),

and a suitable amount of distilled water and alcohol. The result-

ing PtRu black mixtures were ultrasonicated for 2 h. The PtRu

black inks were loaded onto the carbon paper (Sigracet GDL

10BC, Germany) by a paint-brush method to achieve a load-

ing of PtRu black of 4.0 mg cm−2. The as-prepared PtRu anode

electrode was dried in a vacuum oven at 100 ◦C for 2 h.

Thecarbon slurry for the gasdiffusion layer of the air cathode

was prepared with a mixture of 70 wt.% Shawinigan acety-

lene black (AB50) with specific surface area of 80 m2 g−1 and30 wt.% PTFE (Teflon-30 suspension) as a wet-proofing agent

andbinder. Thecarbon slurrywas coatedon theNi-foam as a cur-

rent collector and then pressed at a pressure of 120 kg cm−2.The

gas diffusion layer was then sintered at temperature of 360 ◦C,

30 min. The catalyst layer of the air electrode was then prepared

by spraying a mixture of a 15 wt.% of PTFE solution binder and

85 wt.% of mixed powders consisting of -MnO2 (electrolytic

manganese oxide, EMD) catalyst supported on binary carbons

(i.e., BP2000:CNT = 1:1). The Ni-foam current collector was

cut from 1 cm×1cm or 2cm× 4 cm. Both BP2000 and CNTs

carbons were used as the supporting materials for the cathode

electrode (so-called a binary carbon system). The preparation

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methods of the air cathode electrodes have been reported in

detailed [53,54].

2.5. Electrochemical measurements

The cross-linked PVA/TiO2 composite polymer membrane

was sandwiched between the sheets of the anode electrode

and the cathode electrode, and then hot-pressed at 60 ◦C for

120kgf cm−2 for 10 min to obtain a membrane electrode assem-

bly (MEA). The electrode area of the MEA was about 1 or

8 cm2.

The electrochemical measurements were carried out in a

two-electrode cell. The E –t curves of the alkaline DMFC with

the PVA/TiO2 composite polymer membrane were recorded at

a constant current density of 20 mA cm−2. All electrochemi-

cal measurements were performed on an Autolab PGSTAT-30

electrochemical system with GPES 4.8 package software (Eco

Chemie, The Netherlands). The electrochemical performances

of the alkaline DMFC with the cross-linked PVA/TiO2 com-

posite polymer membrane and the cathode electrode open tothe atmospheric air were studied in 2 M KOH + 2 M methanol

solutions at different temperatures.

3. Results and discussion

3.1. Crystalline structure and surface morphology

The X-ray diffraction measurement was performed to exam-

ine the crystallinity of the PVA/TiO2 composite polymer

membrane. Fig. 1 shows the diffraction pattern of the PVA/TiO2

compositepolymer membranesthat were cross-linked by 5 wt.%

GA solutions at 40◦

C for different times. It is well known thatthe PVA polymer exhibits a semi-crystalline structure with a

Fig. 1. XRD spectra of PVA/TiO2 (10 wt.%) composite polymer membranes.

large peak at a 2θ angle of 20◦ [33,34]. As can be seen clearly in

Fig.1, a large peak at2θ of20◦ for the PVA/TiO2 (10wt.%) com-

posite polymer membrane wasseen. But, it was also clearly seen

that the peak intensity of the PVA/TiO2 composite polymer film

greatly reduced when the membranes were further cross-linked

by GA for the time between 6 and 48 h. It is well known that the

hydroxyl groupsof PVA chemically react with aldehydesto form

acetal or semi-acteal linkages. Indeed, this chemically cross-

linking reaction on the PVA/TiO2 polymer membrane provides

greater chemical, thermal and mechanical stabilities for DMFC

applications. It was observed that the cross-linked PVA/TiO2

composite polymer membrane greatly augmented the domain of

amorphous region (i.e., the degree of crystallinity is decreased).

This indicates that the cross-linked PVA/TiO2 polymer mem-

brane becomes more amorphous. Notably, it was found that the

degree of amorphous of the composite membranes increases as

both the TiO2 ceramic fillers and the cross-linking reaction were

added into and treated on the PVA polymer, respectively.

SEM photographs for the PVA/TiO2 (20 wt.%) composite

polymer membrane at different magnifications, as shown inFig. 2(a) and (b), respectively. It was found that the surface

morphology of the PVA/TiO2 composite polymer sample shows

many different sizes of PVA–TiO2 aggregates or chunks that are

randomly distributed on the top surface. It was found that the

Fig. 2. SEM photographs for PVA/TiO2 (20 wt.%) composite polymer mem-

brane at: (a) 500×; (b) 5k ×.

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dimension of these TiO2 embedded in PVA chunks is about

1–20m. This indicates that the nano-TiO2 particles were not

dispersed well within the PVA polymer matrix, as shown in

Fig. 2(b).

As a whole, the compatibility of the PVA polymer and TiO2

particles is still uniform and homogenous when the weight per-

cent of nano-TiO2 fillers is less than 20 wt.%. As we know, the

higher the content of TiO2 (as a methanol permeation barrier) in

the PVA network matrix may help reduce the methanol cross-

over through the polymer membranes. However, it will become

another important issue how to obtain a uniform PVA/TiO2

composite membrane without nano-sized TiO2 chunks or aggre-

gates.

3.2. TGA thermal analysis

Fig. 3 shows TGA and differential gravimetric analysis

(DTG) thermographs of the PVA polymer membrane alone,

the PVA/TiO2 (15 wt.%) composite polymer membrane (with-

out GA) and the cross-linked PVA/TiO2 (15 wt.%) compositepolymer membrane (with GA) in this work, respectively. TGA

and DTG curves of the pure PVA polymer film reveal three

main weight loss regions, which appear as three peaks in the

DTG curves. The first region at a temperature of 80–100 ◦C

is due to the evaporation of physical weakly and chemically

strongly bound water; the weight loss of the membrane is about

7.39 wt.%. The second transition region at around 250–350 ◦C

is due to the degradation of PVA polymer membrane; the total

weightloss corresponds to this stage about 76.67 wt.%. Thepeak

of third stage at 425 ◦C is due to the cleavage backbone of PVA

polymer membrane (or so-called carbonation); the total weight

loss is 91.30 wt.% at 500◦

C, as listed in Table 1.Moreover, the TGA and DTG curves of the PVA/TiO2 com-

posite polymer film without cross-linked display three main

weight losses, which also appear as three peaks in the DTG

curves. The first stage at a temperature range of 80–100 ◦C is

Fig. 3. TGA thermograph of the cross-linked PVA/TiO2 (15 wt.%) composite

polymer membranes.

Table 1

The weight loss of the PVA polymer membrane, the PVA/TiO2 (15 wt.%) com-

posite polymer membrane without cross-linked and the PVA/TiO2 (15wt.%)

with GA cross-linked at different temperatures by TGA analysis

Types T (◦C)

100 250 350 500

PVA film (%) 7.39 13.94 76.67 91.30PVA/TiO2 (15 wt.%) SPE

(without cross-linked) (%)

6.03 9.50 69.47 84.54

PVA/TiO2 (15 wt.%) SPE

(cross-linked by GA) (%)

5.43 24.74 35.19 77.97

also due to the removal of bound water; the weight loss is about

6.03%. In fact, the second transition at around 250–350 ◦Cisdue

to thedegradation of PVA composite membrane; thetotal weight

loss corresponds to this stage about 69.47 wt.%. Obviously, the

second main weight loss is much less intense, compared with

the pure PVA polymer film. The third peak at 420 ◦C is due to

the breaking main chain of PVA polymer membrane; there is a

total weight loss of 84.54 wt.% at 500 ◦C.Furthermore, the TGA and DTG curves for the cross-linked

PVA/TiO2 composite polymer film exhibit three main weight

loss regions, which appear as three peaks in the DTG curves,

as shown in Fig. 3. The first stage at a range of 80–100 ◦C is

also due to the evaporation of bounding water; the weight loss is

about 5.43 wt.%. The second transition at around 120–300◦C is

due to the degradation of GA and PVA in the composite polymer

membrane; the total weight loss corresponds to this stage about

35.19 wt.% at 350 ◦C. Consequently, the second main weight

loss for the cross-linked PVA/TiO2 composite polymer mem-

brane greatly reduces, compared with that of pure PVA polymer

film. The third peak at 450◦

C is due to the degradation backboneof cross-linked PVA/TiO2 polymer membrane; however, there

is only a total weight loss of 77.97 wt.% at 500 ◦C.

Overall, the degradation peaks of the cross-linked PVA/TiO2

composite polymer samples are less intense and shift towards

higher temperatures. It can be concluded that the improved ther-

mal stability is probably due to the additive effect of the TiO2

filler and the chemical cross-linking reaction of between PVA

and GA.

3.3. Ionic conductivities and transport numbers

The typical ac impedance spectra for alkaline blend PVA/

TiO2 composite polymer membrane by directly blending PVA

polymer with TiO2 (2 wt.%) and KOH (without cross-linking

treatment) at different temperatures are shown in Fig. 4(a). The

ac spectra are typically non-vertical spikes for stainless steel

(SS) blocking electrodes, i.e., SS|PVA/TiO2 SPE|SS cell. Anal-

ysis of the spectra yields information about the properties of the

PVA/TiO2 polymer electrolyte, such as bulk resistance, Rb. Tak-

ing into account the thickness of the composite electrolyte films,

the Rb value was converted into the ionic conductivity value, σ ,

according to the following equation:

σ =L

RbA

(5)

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Fig. 4. Nyquiest plot of PVA/TiO2 SPE: (a) without cross-linked; (b) cross-

linked by GA.

where L is the thickness (cm) of the PVA/TiO2 polymer mem-

brane, A the area of the blocking electrode (cm2), and Rb is the

bulk resistance () of alkaline composite polymer membrane.

Typically, the Rb values for the blend PVA/TiO2 composite

polymer membranes are of the order of less than 1 and are

dependent on the contents of TiO2 fillers and KOH in the mem-

brane. Table 2 shows theionic conductivity values of thealkaline

blend PVA/TiO2 composite polymer membrane (without GA) at

different temperatures. As a result, the ionic conductivity value

is 0.0181 S cm−1 (σ in the order of 10−2 S cm−2) at 30 ◦C.

However, the ac impedance spectra for the cross-linked

PVA/TiO2 (2 wt.%) composite polymer membrane were also

Table 2

The conductivity values of PVA/TiO2 (2 wt.%) composite polymer membrane

(PVA direct blended with TiO2 and KOH) at different temperatures (without

cross-linked)

T (◦C) Parameters

L (cm) Rb () σ (Scm−1)

30 0.005 0.351 0.0181

40 0.005 0.286 0.0222

50 0.005 0.243 0.0262

60 0.005 0.218 0.0292

70 0.005 0.205 0.0310

Table 3

The conductivity values of cross-linked PVA/TiO2 (2 wt.%) (under 5wt.% GA

at 40 ◦C, 12 h) composites polymer membranes (SPE was dipped in 8 M KOH

for 24 h) at different temperatures

T (◦C) Parameters

L (cm) Rb () σ (Scm−1)

30 0.025 3.34 0.005640 0.025 2.72 0.0069

50 0.025 2.06 0.0091

60 0.025 1.62 0.0116

70 0.025 1.41 0.0134

obtained, as shown in Fig. 4(b). Notice that, the cross-linked

polymer membrane was immersed in 8 M KOH solutions for at

least 24 h before measurement. As a result, the Rb values for the

cross-linked PVA/TiO2 composite membranesare of theorder of

less than 3.50 andare dependent on the contents of TiO2 fillers

and the cross-linking conditions (σ in the order of10−3 S cm−2).

Table 3 shows the ionic conductivity values for the cross-linkedPVA/TiO2 (2 wt.%) composite polymer membrane at different

temperatures. Notably, the corresponding value of ionic con-

ductivity at 30 ◦C is about 0.0056 S cm−1. It was found that

the highest ionic conductivity value of alkaline cross-linked

PVA/TiO2 (20 wt.%) composite polymer membrane reaches

about 0.0120 S cm−1 at30 ◦C.Itcanbeseenclearlythattheionic

conductivity of the cross-linked PVA/TiO2 SPE decreases some

extent after the PVA/TiO2 polymer membrane is cross-linked by

gultaraldehyde.

As a matter fact, the cross-linked PVA/TiO2 composite mem-

brane may have less accessible free volume for the KOH

electrolyte; therefore, the ionic conductivity is in the order of

10−3 S cm−1 at ambient temperature. However, the cross-linkedPVA/TiO2 composite membranes show greater thermal prop-

erties (as indicated in TGA’s results) and chemical stability

properties (as indicated in liquid uptake’s results later). The tem-

perature dependence of the ionic conductivity is of theArrhenius

type:

σ = σ 0 exp

−Ea

RT

(6)

where σ 0 is a pre-exponential factor, E a the activation energy,

and T is the temperature in Kelvins. The log10(σ ) versus 1/ T

plots, as shown in Fig. 5, obtains the activation energy ( E a)ofthe

PVA composite SPE, which is highly dependent on the contentsof TiO2 fillers and the cross-linking conditions. The E a value for

alkaline cross-linked PVA/TiO2 composite polymer membrane

isin the order of11 kJmol−1 (normally cross-linked SPE shows

E a value over 20 kJ mol−1).

Fig. 6 shows the variation of the ionic conductivity values

versus the weight percents of TiO2 fillers for the cross-linked

PVA/TiO2 composite polymer membranes at 30 and 60 ◦C. The

improvement ionic conductivity for the cross-linked PVA/TiO2

composite polymer electrolyte exhibited when the content of

nano-TiO2 filler is over 10 wt.%. In contrast with, Yang and Lin

[33,34] reported the ionic conductivity values of the blend PVA

polymer electrolyte (free of TiO2 fillers) are about 0.0471 and

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Fig. 5. Arrehenius plot of PVA/TiO2 (2 wt.%) composite polymer membranes:

(a) PVA direct blend KOH without cross-linked; (b) the PVA/TiO 2 compositepolymer film cross-linked with GA.

0.0528 S cm−1,at20and60 ◦C, respectively. It can be concluded

that the optimum content of TiO2 ceramic filler in the PVA-

based composite polymer membrane must keep around 10 wt.%

in order to obtain a uniform composite SPE film.

Moreover, we found that the anionic transport number value

(t −) of alkaline cross-linked PVA/TiO2 (2 and15 wt.%) compos-

ite polymer membrane is about 0.80 and 0.70, respectively, by

Hittorf’s method. Comparatively, the anionic transport number

of alkaline blend PVA polymer electrolyte at ambient tempera-

ture is about 0.94–0.95 [52]. The lower value of anion transportnumber value for the cross-linked PVA/TiO2 SPE indicates that

the OH− ions (function as major charge carriers) are not easily

to transport in the matrix of cross-linked PVA/TiO2 SPE.

Fig. 6. The conductivities values vs. the contents of TiO2 fillers for the alkaline

cross-linked PVA/TiO2 composite polymer membranes: (a) 30◦

C; (b) 60◦

C.

Table 4

The liquid uptake (%) results for the PVA/TiO2 composite polymer membranes

at 25 ◦C

Types of membranes Solutions

DI water 8M KOH 97wt.% CH3OH

PVA + 2 wt.% TiO2 94.71 82.70 71.41

PVA + 5 wt.% TiO2 93.21 74.80 62.80PVA + 10 wt.% TiO2 95.50 65.42 54.35

PVA + 2 wt.% TiO2 + 5 wt.% GA 89.20 48.18 21.70

PVA + 5 wt.% TiO2 + 5 wt.% GA 83.50 53.65 15.20

PVA + 10 wt.% TiO2 + 5 wt.% GA 87.20 50.20 8.60

It is well known that the ionic transport property of poly-

mer electrolyte significantly influences the conductivity of

polymer electrolyte membrane. It was found that the con-

ductivity value of alkaline cross-linked PVA/TiO2 (20 wt.%)

composite polymer membrane (σ = 0.0120S cm−1) is much

lower than that of alkaline blend PVA–KOH polymer elec-

trolyte (σ = 0.0472S cm−1

) at ambient temperature. As a result,the ionic conductivity of alkaline cross-linked PVA/TiO2 com-

posite polymer membrane indeed decreases some extent when

the PVA/TiO2 polymer membrane was further cross-linked by

gultaraldehyde (GA). Nevertheless, the thermal and mechani-

cal properties of the cross-linked PVA/TiO2 composite polymer

membrane are greatly enhanced.

Besides, the liquid uptake (%) for DI water, 8 M KOH

and 97% CH3OH solutions shows in Table 4. As shown in

Table 4, the percent of DI uptake slightly decreased for both

the blend PVA/TiO2 and the cross-linked PVA/TiO2 composite

polymer membranes; the uptake results also indicate indepen-

dent of TiO2 content in PVA polymer membrane. However, the

percent of KOH uptake decreases about 20–30% for the cross-linked PVA/TiO2 composite polymer membrane. The chain

motion of cross-linked SPEs becomes less flexible (due to

the cross-linking). The free volume in the 3D PVA network

structure reduces, it may cause decrease the amount of KOH

solutions uptake [7]. Furthermore, the percent of CH3OH solu-

tion uptake significantly reduces when the PVA/TiO2 composite

polymer membrane is cross-linked by GA cross-linking treat-

ment.

Clearly, the cross-linkedPVA/TiO2 compositepolymer mem-

brane becomes rigid and less free volume; it is due to the duel

effects of TiO2 particle filler and GA. As it can be seen, the as-

prepared cross-linked composite polymer membrane becomesexcellent barrier for methanol cross-over. On the other hand,

it was found that the amount of 97 wt.% CH3OH solutions

uptake for the cross-linked PVA/TiO2 (10 wt.%) SPE is around

8.60 wt.%.

3.4. Electrochemical characterization of a single DMFC

Fig. 7 shows the E –t curves of the alkaline DMFC consist-

ing of the anode electrode with a loading of PtRu black of

4.0 mgcm−2, the cathode electrode with MnO2 carbon inks

of 3.63mgcm−2 and the cross-linked PVA/TiO2 (10 wt.%)

composite polymer membrane in 2 M KOH + 1–5M CH3OH

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Fig. 7. The E –t curves for the DMFC in 2 M KOH+ 1–5M CH3OH solutionsat 20 mAcm−2 at 60 ◦C at ambient pressure.

solutions at 20 mA cm−2 at 60 ◦C. In spite of a tendency to

fall at the beginning of the test, the cell potential are stabilized

and remain constant after 1 h, it shows the good electrochem-

ical stability of the DMFC. Table 5 shows the cell potentials

of the DMFC in different CH3OH concentrations at 60 ◦C at

1 atm. The results indicate that the highest cell working poten-

tial ( E cell = 0.320 V) of the DMFC is in 2 M KOH + 2 M CH3OH

solutions.

Fig. 8 shows the potential–current density curves and

power density–current density curves of the alkaline DMFC

( A = 8 c m2) at the temperature between 30 and 60◦C in

2 M K O H + 2 M C H3OH solution at ambient pressure. In

fact, the peak power density of 3.86 mW cm−2 was achieved

at E p,max = 0.238 V with a peak current density (ip,max) of

16.17 mA cm−2 at ambient temperature and pressure. On the

other hand, the maximum power density of 7.54 mW cm−2 was

obtained at E p,max = 0.258 V with a peak current density of

29.18 mA cm−2 at 60 ◦C. Moreover, Table 6 lists some electro-

chemical parameters, such as the open circuit potential ( E ocp),

the maximum peak power density (PDmax), the peak poten-

tial ( E p,max) and the peak current density (ip,max) at different

temperatures for the alkaline DMFC. It was found that the val-

ues of open circuit potential of the alkaline DMFC are about

Table 5

The chronopotentiostatic ( E –t ) curves of the alkaline DMFC in 2M

KOH + 1–5 M CH3OH solutions at i =20mAcm−2 at ambient temperature and

pressure

Concentration (M) Parameter

E cell (V)

1 0.308

2 0.320

3 0.286

4 0.283

5 0.283

Fig. 8. PD vs. current density curves for the alkaline DMFC (8 cm2) at different

temperatures in 2 M KOH+ 2 M CH3OH solutions at ambient pressure.

Table 6

The electrochemical parameters for the alkaline DMFC with the cross-linked

PVA/TiO2 (10wt.%) compositemembrane in2 M KOH + 2 M CH3OH solutions

at 1 atm at different temperatures

Parameters T (◦C)

25 30 40 50 60

E ocp (V) 0.779 0.810 0.776 0.788 0.845

E p,max (V) 0.238 0.226 0.226 0.240 0.258

ip,max (mAcm−2) 16.17 17.62 18.96 23.31 29.18

PDmax (mWcm−2) 3.86 4.00 4.29 5.59 7.54

0.77–0.84 V. As a result, the maximum peak powder density

of the alkaline DMFC increases as the operation temperature

increases. Finally, it is important to study the durability of

cross-linked PVA/SiO2 composite membrane on DMFC under

long-term operation. Fig. 9 demonstrates the result of long-term

Fig. 9. The long-term stability curves (for 50 h) of alkaline DMFC with the

PVA/TiO2 composite membrane at a constant load of 20 mA cm−2 in 2M

KOH+2M CH3OH solution under ambient condition operation (25 ◦C and

1 atm).

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stability test for the alkaline DMFC at 25 ◦C and 1 atm for 50h

operation. The measurement of cell potential versus time was

recorded at 20 mA cm−2 (the load current density is higher than

the ip,max = 16.17 mAcm−2 with a mean PD = 3.41 mW cm−2)

under ambient conditions for a total time of 50 h. During the

stability test, the measurement was carried out for a continu-

ous operation of 10 h plus 30 min off-period. It was found that

the cell working potential of 0.17 V for alkaline DMFC was flat

and stable at 20 mA cm−2; in contrast, the cell potential was

immediately from 0.17 V back to OCP (about 0.77 V) during

off-period.

Scrosati et al. [7] studied the cross-linked PVA/SiO2 com-

posite polymer membrane for a lab-type acidic DMFC using

2 M H2SO4 +2 M CH3OH solutions, the cell with a maximum

peak power density of only about 2.0 mW cm−2 at ambient tem-

perature and pressure. Furthermore, according to their results,

it was found that the stability of MEA with Nafion poly-

mer membrane seems not stable for their lab-type DMFC. By

contrast, the electrochemical performance of alkaline DMFC

system with the cross-linked PVA/TiO2 composite membrane(PD=3.86mWcm−2) shows better than that of acidic DMFC

cell with the cross-linked PVA/SiO2 composite membrane

(PD=2.00mWcm−2) at ambient conditions. It was demon-

strated that the alkaline DMFC system exhibits some advantages

over than that of the acidic DMFC system. In particular, the alka-

line DMFC with the air electrode is allowed to use non-precious

metal catalyst (i.e., with MnO2 catalyst inks instead of with

Pt/C inks) and also choose the cross-linked PVA/TiO2 com-

posite polymer membrane (i.e., not perfluorosulfonated cheap

polymer membrane instead of Nafion).

4. Conclusions

A novel cross-linked PVA/TiO2 composite polymer mem-

brane was prepared by a solution casting method. Alkaline

direct methanol fuel cell (DMFC) consisting of this novel cross-

linked PVA/TiO2 composite polymer membrane was assembled

and examined. The novel alkaline DMFC cell is comprised of

the air cathode electrode with MnO2 catalyst inks, the PtRu

anode electrode and the novel cross-linked PVA/TiO2 composite

polymer membrane. It was demonstrated that alkaline DMFCs

with these cross-linked PVA/TiO2 composite polymer mem-

branes show good electrochemical performances at ambient

temperatures and pressure. The maximum peak power den-

sity of the DMFC is about 7.54 mW cm−2 at 60 ◦C and 1 atm.From the economic and application point of view, the cross-

linked PVA/TiO2 composite polymer membranes are easily

prepared for the mass production and PVA is also a cheap poly-

mer material. These cross-linked PVA/TiO2 composite polymer

membranes show a highly potential candidate for the DMFC

applications.

Acknowledgement

Financial support from the National Science Council, Taiwan

(Project no: NSC-94-2214-131-002) is gratefully acknowl-

edged.

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