Electrochemistry Communications 12 (2010) 607–610

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Electrochemistry Communications

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High performance direct methanol fuel cells basedon acid–base blend membranes containing benzotriazole

Wen Li a, Arumugam Manthiram a,*, Michael D. Guiver b, Baijun Liu c

a Electrochemical Energy Laboratory & Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712, USAb Department of Energy Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Koreac Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 18 January 2010Received in revised form 8 February 2010Accepted 10 February 2010Available online 14 February 2010

Keywords:Direct methanol fuel cellProton exchange membraneAromatic polymerMethanol crossoverAcid–base interactionsBlend membranes

1388-2481/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.elecom.2010.02.011

* Corresponding author. Fax: +1 512 471 7681.E-mail address: rmanth@mail.utexas.edu (A. Mant

Novel acid–base blend membranes consisting of acidic sulfophenylated poly(ether ether ketone ketone)(Ph-SPEEKK) and various amounts of basic polysulfone tethered with 5-amino-benzotriazole (PSf-BTraz)have been prepared and characterized. The blend membranes show higher proton conductivity and lowerliquid uptake and dimensional swelling compared to plain Ph-SPEEKK and sulfonated poly(ether etherketone) (SPEEK) membranes. The Ph-SPEEKK/PSf-BTraz blend membranes with optimized basic polymercontents exhibit lower methanol crossover and higher performance with improved stability in directmethanol fuel cells (DMFC) at various methanol concentrations (1–10 M) than plain Ph-SPEEK andNafion-115 membranes.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

The proton exchange membrane (PEM) plays a critical role inthe overall performance of direct methanol fuel cells (DMFC).Much attention has been directed towards the development ofalternative membranes as the currently used Nafion membranesuffers from high methanol permeability. Among them, sulfonatedaromatic polymers such as sulfonated poly(ether ether ketone)(SPEEK) and sulfonated polysulfone (SPSf) are currently consideredgood candidates due to their low cost, good thermal and mechan-ical stabilities, and low methanol permeability [1,2]. However,in many aromatic PEMs, the sulfonic acid groups are attacheddirectly to the main-chain, and the operating temperature andmethanol concentration are limited due to the undesired highdimensional swelling at elevated temperatures and high methanolconcentrations.

Recently, polymers with sulfonic acid groups attached to thependant side groups have been shown to exhibit better hydrolytic,oxidative, and swelling stabilities compared to those with the sul-fonic acid groups attached directly to the polymer main-chain [3–5]. For example, sulfophenylated poly(ether ether ketone ketone)(Ph-SPEEKK), which has high proton conductivity and a high ionexchange capacity (IEC) of 1.80 meq./g, exhibit excellent mechani-

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hiram).

cal properties with good thermal, oxidative, and dimensional sta-bility in hot water [3,4]. Also, blend membranes consisting of anaromatic acidic polymer like SPEEK and an N-heterocycle tetheredbasic polymer like benzimidazole tethered PSf (PSf-BIm) have beenshown in recent years to exhibit better performance in DMFC thanthe plain acidic polymers [6–8]. However, not much attention hasbeen focused on blend membranes in which the sulfonic acidgroups are attached to the pendant side groups. Also, almost nodata are available on the dimensional and performance stabilitiesof the Ph-SPEEKK membrane at high methanol concentrations(>5 M).

We present here a novel blend membrane based on acid–baseinteractions between the acidic polymer Ph-SPEEKK and the basicpolymer polysulfone tethered with 5-amino-benzotriazole (PSf-BTraz) (Fig. 1). The triazole containing heterocycles could facilitateproton transfer through the Grotthuss mechanism easily comparedto other N-heterocycles due to the multiple nitrogen atoms inBTraz [9,10]. A comparison of the properties and performances inDMFC of the Ph-SPEEKK/PSf-BTraz blend membranes with thoseof Ph-SPEEKK, SPEEK, and Nafion-115 membranes is presented.

2. Experimental

The PSf-BTraz (1.58 BTraz per repeating unit) [10], Ph-SPEEKK(100% degree of sulfonation) [4], and SPEEK (48% degree of sulfona-tion) [1] samples were synthesized as reported elsewhere. The

Fig. 1. Chemical structures of sulfophenylated poly(ether ether ketone ketone) (Ph-SPEEKK) and polysulfone tethered with 5-amino-benzotriazole (PSf-BTraz).

608 W. Li et al. / Electrochemistry Communications 12 (2010) 607–610

membranes consisting of the plain SPEEK or Ph-SPEEKK polymersor the blend polymers Ph-SPEEKK/PSf-BTraz were prepared bythe solution casting method with DMAc solutions (�10% w/w)[6]. The thickness of the membranes was kept constant at

Table 1Comparison of IEC, proton conductivity, liquid uptake, and swelling of Nafion 115, plain SPpolymer contents.

Nafion 115 SPEEK Ph-SPEE

IEC (meq./g) 0.89 1.46 1.80Proton conductivity (mS/cm) 25 �C 91 39 46

65 �C 142 72 82[SO3H]/[BTraz] ratio – – –

Liquid uptake (%)25 �C Water 31 38 32

1 M 35 42 372 M 39 47 455 M 43 64 5410 M 56 159 97

65 �C Water 37 46 371 M 39 56 422 M 48 59 525 M 52 212 8810 M 88 632 597

Swelling (%)25 �C Water 11 14 12

1 M 13 17 152 M 16 22 195 M 25 33 2610 M 44 76 65

65 �C Water 13 17 141 M 16 21 192 M 19 28 245 M 32 40 3410 M 62 143 132

60 ± 5 lm by controlling the polymer content in the DMAcsolution.

The ion exchange capacity (IEC) values of the membranes weredetermined by acid–base titration using phenolphthalein as an

EEK, plain Ph-SPEEKK, and Ph-SPEEKK/PSf-BTraz blend membranes with various basic

KK Ph-SPEEKK + 2 wt.%PSf-BTraz

Ph-SPEEKK + 5 wt.%PSf-BTraz

Ph-SPEEKK + 8 wt.%PSf-BTraz

1.74 1.66 1.5251 64 5992 114 10234.1 13.2 8.0

29 24 2233 29 2536 33 2944 37 3478 65 5833 30 2437 34 2843 37 3276 61 50

455 265 164

11 9 813 11 1017 14 1122 18 1451 48 4213 12 915 12 1120 17 1329 24 20

118 103 85

200m2 ) Methanol Crossover

(a) Nafion - 115

W. Li et al. / Electrochemistry Communications 12 (2010) 607–610 609

indicator [8]. Proton conductivities of the membranes were mea-sured in the longitudinal direction with a HP 4192 ALF ImpedanceAnalyzer in the frequency range of 5 Hz–100 kHz with an appliedvoltage of 10 mV.

The equilibrium liquid uptake (Wuptake) and dimensional swell-ing (lswelling) of the membranes were calculated as

Wuptake ¼ ðWwet �WdryÞ=Wdry � 100% ð1Þ

lswelling ¼ ðlwet � ldryÞ=ldry � 100% ð2Þ

The dry mass (Wdry) and dimension (ldry) of the membraneswere measured after drying the sample at 110 �C under vacuumfor 24 h. The wet mass (Wwet) and dimension (lwet) were measuredafter equilibrating the sample with de-ionized water or methanolsolution at desired temperatures for 24 h and blotting carefullywith a filter paper to remove the surface water.

Membrane–electrode assemblies (MEAs) were fabricated byuniaxially hot-pressing the anode and cathode onto the membraneat 120 �C for 3 min [8]. Commercial 60 wt.% Pt–Ru (1:1) (E-TEK) onVulcan carbon and 60 wt.% Pt (Johnson Matthey) on Vulcan carbonsprayed onto the gas-diffusion layer (E-TEK) were used as, respec-tively, the anode and cathode. The catalyst loading was 2.5 mg/cm2

on each side. Electrochemical performances of the MEAs were eval-uated with a single cell hardware (5 cm2 active area) and feedingmethanol solution and humidified oxygen to the anode and cath-ode, respectively, at a flow rate of 2.5 mL/min and 200 mL/minwithout back pressurization. Methanol crossover through themembranes was evaluated by a voltammetric method [9] withthe same setup by changing the cathode feed to an inert humidi-fied N2.

0 100 200 300 400

0.2

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Current Density (mA/cm2)

Cel

l Vo

ltag

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Polarization Curve Nafion - 115 Plain Ph-SPEEKK 2 wt. % PSf-BTraz 5 wt. % PSf-BTraz 8 wt. % PSf-BTraz

(b)

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(b) Plain Ph-SPEEKK(c) 2 wt. % PSf-BTraz(d) 5 wt. % PSf-BTraz(e) 8 wt. % PSf-BTraz (a)

(a)

Fig. 2. Comparison of the (a) methanol crossover current densities and (b)performances in DMFC of the Ph-SPEEKK/PSf-BTraz blend membranes havingvarious basic polymer contents with those of Nafion-115 and plain Ph-SPEEKK.Methanol concentration: 1 M, cell temperature: 65 �C.

3. Result and discussion

3.1. IEC and proton conductivity

Table 1 compares the IEC and proton conductivity values of Naf-ion, plain SPEEK and Ph-SPEEKK, and blend membranes consistingof Ph-SPEEKK and PSf-BTraz. Ph-SPEEKK with about 100% of degreeof sulfonation has a higher IEC than plain SPEEK, resulting in higherproton conductivity due to greater number of dissociable protons.

All the blend membranes exhibit lower IEC than plain Ph-SPEEKK membrane due to the acid–base interaction between thesulfonic acid and benzotriazole groups. Interestingly, the protonconductivity increases with increasing PSf-BTraz contents up to5 wt.% as the acid–base interaction facilitates proton transferthrough both the vehicle and Grotthuss mechanisms. The in-creased proton conductivity of the blend membranes could alsobe due to the wider hydrophilic channels caused by an insertionof the heterocycle groups into the ionic channel compared to thatin the plain membrane [6,8]. However, since there are a greaternumber of acid groups than basic groups in the membranes (Table1), the predominant vehicle mechanism could be perturbed whentoo much of BTraz is introduced into the ionic channel. As the re-sult, the proton conductivity decreases slightly on increasing thePSf-BTraz content to 8 wt.%.

3.2. Liquid uptake and swelling

Liquid uptake and swelling are critical issues, especially foroperation at high methanol concentrations in DMFCs. As seen inTable 1, although the liquid uptake and swelling of all the mem-branes increase with increasing methanol concentration and tem-perature, the Ph-SPEEKK based membranes show much lowerliquid uptake and swelling than SPEEK membrane despite higherIEC values. More importantly, at a given temperature and methanol

concentration, all the blend membranes show much reduced liquiduptake and swelling than their plain counter-part, and the valuesdecrease with increasing PSf-BTraz content in the membrane.The data reveal that the lower hydrophilicity of PSf-BTraz andthe acid–base interaction in the blend membranes could effectivelyenhance the swelling stability of the membranes. The difference inthe swelling between the blend and plain membranes becomeseven larger at high methanol concentration (5–10 M), and theblend membranes (5 and 8 wt.%) show lower or similar values toNafion-115 membrane with no evidence of instability in methanolsolutions, indicating that the Ph-SPEEKK blend membranes havethe potential to be used in DMFCs operating with high methanolconcentrations.

3.3. Methanol crossover and fuel cell performance

Fig. 2(a) compares the methanol crossover, and the Ph-SPEEKKmembrane (�60 lm) with half the thickness of the Nafion-115membrane (�125 lm) shows lower methanol crossover due tothe smaller ionic channels usually found with the sulfonated aro-matic polymers [8]. More importantly, the Ph-SPEEKK/PSf-BTrazmembranes show much lower methanol crossover than the plainacid polymers due to the acid–base interaction and hydrophobicityof PSf-BTraz, which is consistent with the lower liquid uptake andswelling. The decrease in methanol crossover with increasing basicpolymer content further supports this assertion; the methanol

0 200 400 600

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W/c

m2 )

Cel

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Current Density (mA/cm 2)

Tcell

= 80 oC, 1 M Methanol Solution

Nafion-115 Plain Ph-SPEEKK Ph-SPEEKK + 5 wt. % PSf-BTraz

(a)0

80

160

240

0 4 8

100

150

200

Tcell

= 65 oC Methanol Concentration: 1 - 10 M

Nafion-115 Ph-SPEEKK + 5 wt. % PSf-BTraz

Max

imu

m P

ow

er D

ensi

ty (

mW

/cm

2 )

Methanol Concentration (M)

(b)

Fig. 3. Comparison of the (a) DMFC performances and power densities at 80 �C with1 M methanol solution and (b) maximum power density as a function of methanolconcentration at 65 �C of the Ph-SPEEKK/PSf-BTraz blend membranes having 5 wt.%PSf-BTraz with those of Nafion-115 and plain Ph-SPEEKK membranes.

610 W. Li et al. / Electrochemistry Communications 12 (2010) 607–610

crossover of the blend membrane with 8 wt.% PSf-BTraz is about60% of that of the plain Ph-SPEEKK membrane. The lower methanolcrossover is further confirmed by the higher open-circuit voltage(OCV) of the blend membrane (0.78 V) compared to that of Naf-ion-115 membrane (0.65 V) due to the smaller voltage loss oncathode side.

Fig. 2(b) compares the performances in DMFC. Although theplain Ph-SPEEKK membrane shows performance similar to that ofNafion-115 membrane at 65 �C with 1 M methanol solution, thePh-SPEEKK/PSf-BTraz blend membranes exhibit much higher fuelcell performance compared to Nafion-115 due to the increasedproton conductivity and much suppressed methanol crossover.The Ph-SPEEKK/PSf-BTraz blend membrane with optimized basicpolymer content (5 wt.% PSf-BTraz) having the highest proton con-ductivity exhibits higher power density (110 mW/cm2) at 0.4 Vthan Nafion-115 (63 mW/cm2).

Fig. 3(a) also compares the polarization curves and power den-sities of Nafion-115, plain Ph-SPEEKK, and Ph-SPEEKK/PSf-BTraz(with 5 wt.% PSf-BTraz) membranes at 80 �C with 1 M methanolconcentration. The power density values are increased at elevatedtemperatures due to the better reaction kinetics in the electrodesand increased proton conductivity. The power density at 0.4 V ofthe blend membrane (176 mW/cm2) is much higher than thoseof both the plain Ph-SPEEKK (126 mW) and Nafion-115(128 mW/cm2) membranes.

Fig. 3(b) compares the maximum power densities of the blendmembrane with optimized PSf-BTraz content and Nafion-115

membrane as a function of methanol concentration. The blendmembrane out-performs Nafion-115 membrane at all methanolconcentrations due to much suppressed methanol permeabilitycombined with comparable proton conductivity. As the methanolconcentration increases, the difference in the maximum powerdensity values between the blend membranes and Nafion-115 be-comes larger. The blend membrane shows the highest power den-sity of 154 mW/cm2 with 5 M methanol, which is 1.4 times higherthan that of Nafion-115 membrane. Although the power densitydecreases with both the membranes on increasing the methanolconcentration to 10 M due to increased swelling, the blend mem-brane still shows higher power density than Nafion-115. It ismainly due to the lower methanol crossover and increased protonconductivity of the blend membrane compared to its counter-partplain membrane. The lower methanol crossover is also reflected ina higher OCV with the blend membranes compared to that withthe Nafion-115 membrane at higher methanol concentrations.The study demonstrates that the blend membranes offer the po-tential to operate at higher methanol concentrations compared toNafion-115, increasing significantly the energy density and possi-bly lowering the cathode catalyst loading.

4. Conclusion

Acid–base blend membranes consisting of Ph-SPEEKK and PSf-BTraz exhibit better performance in DMFC than plain Ph-SPEEKKand Nafion membranes due to increased proton conductivity andsuppressed methanol crossover. Moreover, the blend membranesexhibit lower liquid uptake and better swelling stability withincreasing methanol concentration from 1 to 10 M compared tothe plain Ph-SPEEKK and Nafion membranes due to the acid–baseinteractions and hydrophobicity of PSf-BTraz, which results insuperior DMFC performance stability at high methanol concentra-tions. To the best of our knowledge, the power density values ob-tained here are the highest values reported for non-fluorinatedmembranes at high methanol concentrations.

Acknowledgements

Financial support by the Office of Naval Research MURI GrantNumber N00014-07-1-0758 is gratefully acknowledged. MG andBL acknowledge support for the PSf-BTraz and Ph-SPEEKK synthe-sis work, respectively, by the WCU program through the NationalResearch Foundation of Korea Ministry of Education, Science andTechnology (No. R31-2008-000-10092-0) and the National NaturalScience Foundation of China (No. 50973040).

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