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A novel phosphoric acid loaded quaternary 1,4-diazabicyclo-[2.2.2]-octane polysulfone membrane for intermediatetemperature fuel cells

Xu Wang a,*, Chenxi Xu a, Bernard T. Golding b, Masih Sadeghi b, Yuancheng Cao a,Keith Scott a

aSchool of Chemical Engineering and Advanced Materials, Merz Court, Newcastle upon Tyne NE1 7RU, UKb School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK

a r t i c l e i n f o

Article history:

Received 23 February 2011

Received in revised form

20 March 2011

Accepted 26 March 2011

Available online 29 April 2011

Keywords:

Phosphoric acid loaded membrane

DABCO

Polysulfone

Chloromethylation

Quaternization

Fuel cell

* Corresponding author.E-mail address: xu.wang@ncl.ac.uk (X. W

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.03.143

a b s t r a c t

Phosphoric acid loaded quaternary 1,4-diazabicyclo-[2.2.2]-octane (DABCO) polysulfone

was synthesised with different degrees of substitution (DS) and the membranes were

characterized. The polymer structure was investigated using NMR and FT-IR spectra. The

effect of DS on ionic conductivity and fuel cell performance are described. Conductivities of

0.12 and 0.064 S cm�1 were achieved for the high degree of substitution membrane (DS106)

and low degree of substitution membrane (DS58), respectively. Fuel cell tests gave a high

power output of 400 mW cm�2 using H2 and O2 at 150 �C and atmospheric pressure.

Therefore, PA/QDPSU membrane has potential applications for intermediate temperature

fuel cells (ITFCs).

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction at elevated temperature (>120 �C) offers a number of potential

Fuel cells are a key enabling technology for the future

hydrogen economy. Nafion based proton exchangemembrane

fuel cells (PEMFCs) play an important role for fuel cell devel-

opment. However, the high cost of Nafion membrane and Pt

group catalysts hinders the commercialization and further

development of PEMFC. In addition, it is sensitive to carbon

monoxide which can lead to irreversible adsorption on the

catalyst and the complex PEMFC system experiences reduced

efficiency due to water management requirements. To over-

come the shortcomings of low temperature PEMFC, operation

ang).2011, Hydrogen Energy P

advantages: a) high CO tolerance, b) faster kinetics, c) avoid-

ance of cathode flooding, d) increased system efficiency by

removing water management units [1].

Compared to aqueous phosphoric acid fuel cells (PAFCs)

operateathigher temperatures (150e200 �C) thanPEMFCs, solid

state electrolytes provide advantages such as a less corrosive

environment for catalysts and lower internal resistance losses.

Thephosphoric acid loadedpolybenzimidazole (PBI) is thebest-

known example which has produced reasonably successful

membranes for fuel cells with excellent thermal, chemical

stability and good conductivity [1]. A number of insoluble

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Synthesis scheme of quaternary DABCO

polysulfone.

Fig. 3 e FT-IR spectra of QDPSU and PA/QDPSU.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 8 5 5 0e8 5 5 6 8551

heteropolyacids (e.g.Cs2.5H0.5PMo12O40andCs-HPAs)wereused

for PBI compositemembrane to improve the ionic conductivity

of PBImembrane for high and lowphosphoric acid doping level

[2,3]. Recently, a TiO2/PBI composite membrane was prepared

by Lobato and co-workers, they found the water retention and

acid loading level were improved compared to pristine PBI

which leading toahighconductivityof (0.18Scm�1 at125 �C) [4].

Fig. 2 e 1H NMR spectra of d

Such composite membrane gave a peak power density of

800 mW cm�1 at 150 �C. Sulfonated poly (ether ether ketone)

(SPEEK) was characterized and the increase in degree of sulfo-

nation resulted in increased glassy transition temperature (Tg)

and enhanced membrane hydrophilicity which lead to

a improved proton conductivity [5]. Xing et al. synthesised

a sulfonated poly (biphenyl ether sulfone) membranes exhibi-

ted a good conductivity (0.18 S cm�1 at 25 �C) and high thermal

stability (up to 280 �C) [6]. Lately, they reported a montmoril-

lonite/sulfonated poly (phenylether sulfone)/PTFE composite

membrane with low swelling ratio and good stability at high

temperature and high strength conditions [7].

As an alternative method of preparing PEM for ITFCs, Li

et al. reported quaternized polymers as matrices for H3PO4

and they added that the quaternary ammonium group is

stable in the operating condition of ITFCs and it has a good

ifferent DS of CM-PSU.

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bonding with phosphoric acid [8,9]. Our previous work on

quaternary ammonium polysulfone (QAPSU) loaded with

H3PO4 exhibited a promising fuel cell performance (700

mW cm�1 at 150 �C and 1 bar pressure) [9,10]. There are still

few reports on quaternary ammonium polymers for ITFCs

compared to the reports on PBI based composite membranes

and sulfonated polymer membranes. In this work, a quater-

nary 1,4-diazabicyclo-[2.2.2]-octane (DABCO) polysulfone

(QDPSU) was synthesised with different degrees of substitu-

tion (DS). Compared to the synthetic method for QAPSU, the

carcinogenic chloromethyl methyl ether was replaced by

a safer chloromethylation procedure (in-situ generation of

chloromethyl methyl ether).

Fig. 4 e SEM images and element maps of PA/QDPSU-DS 58, a)

e) EDX analysis.

2. Experimental

2.1. Synthesis of quaternary DABCO polysulfone

The synthesis of QDPSU was achieved by two steps as shown

in Fig. 1. First, the method for synthesis of choloromethylated

polysulfone (CM-PSU) by a FriedeleCrafts like reaction was

used, as developed by Avram and co-workers [11]. 10 g

(2.26 mmol) polysulfone (P-1700, Udel) was dissolved in

400 cm3 (mL) of 1,2-dichloroethane. 6.78 g (22.6 mmol) para-

formaldehyde and 24.6 g (22.6 mmol) trimethylchlorosilane

were added to the solution with magnetic stirring. Stannic

and b) cross-section view, c) phosphorous, d) sulphur and

Fig. 5 e Ionic conductivity of a) different degree of

substitution PA/QDPSU membrane and b) Arrhenius plot

obtained from ionic conductivity data.

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chloride 1.178 (0.452 mmol) was added dropwise and the

reaction proceeded for 12e72 h at 50 �C, to produce different

degrees of substitution of CM-PSU. The white CM-PSU was

precipitated in high purity ethanol (99.9%) to remove the

excess of reactants. The product was purified by dissolving in

dimethylacetamide (DMAc), filtering and precipitating with

distilledwater several times. After that treatment, the product

was dried in high vacuum to remove any residual solvent and

kept in a vacuum before further use.

In order to obtain non-crosslinked QDPSU, the quaterni-

zation at a different degree of substitution (DS) was completed

by adding CM-PSU and DABCO in DMAc in a molar ratio of

1(eCH2Cl):5(DABCO). Themixture was stirred at 80 �C for 14 h,

whereupon the product was precipitated using diethyl ether

and dried in a vacuum.

To preparemembranes, QDPSUwas dissolved in DMAc and

the solutionwas cast onto an optical glass at 60 �C overnight to

evaporate the solvent. QDPSU membrane was immersed in

11.0 M phosphoric acid at room temperature for three days to

obtain the PA/QDPSUmembrane. All the chemicals were used

as received.

2.2. 1H NMR analysis

1H NMR (400 MHz) was performed on JEOL-ECS 400 using

deuterochloroform as the solvent and tetramethylsilane

(TMS) as the internal reference.

2.3. SEM and EDX analysis

A low vacuum environmental scanning electron microscopy

(ESEM, JSM-5300LV, Japan) incorporated with energy disper-

sive X-ray (EDX) spectroscopy was used to investigate the

morphology andmicrostructure of PA/QDPSUmembrane. The

membranewas immersed in liquid N2 for 10min andwas then

broken off to investigate the morphology of membrane cross-

section.

2.4. FT-IR analysis

Fourier transform infrared spectroscopy (FT-IR) of samples

was measured on a Varian 800 FT-IR spectrometer system

between 4000 and 400 cm�1.

2.5. Ionic conductivity measurement

A four-point probe technique was used for membrane

conductivity measurement. The membrane was contacted

with platinum foils and connected to a frequency response

analyser (FRA, Voltech TF2000, UK). All samples were cut into

1 cm wide, 5 cm long strips and placed on the platinum foil

with 0.5 cm gap between them. AC impedancemeasurements

were carried out at frequencies between 1 and 20 kHz. The

polymer membranes were held at the desired conditions of

temperature for 30 min to reach steady state.

The final results were calculated using Equation (1):

s ¼ id=vA (1)

Where s is the membrane conductivity (S cm�1), i is the

response current, v is the response voltage, d is the distance

between platinum foil and A is the membrane area of cross-

section.

2.6. MEA preparation and fuel cell test

Catalyst inks were prepared by blending carbon-supported

catalysts (50 wt.% Pt/C, Alfa Aesar) and polytetrafluoro-

ethylene (PTFE) solution (60 wt.% Aldrich) in a watereethanol

mixture under ultrasonic vibration for 10 min [2,12]. Gas

diffusion electrodes (carbon paper) incorporated with wet

proofed micro-porous layer (H2315 T10AC1), obtained from

Freudenburg (FFCCT, Germany), were used as substrates to

deposit the catalyst layer for both anode and cathode. The

catalyst inks were sprayed onto the micro-porous gas diffu-

sion layer, at 100 �C, and the electrodes were held at

a temperature of 150 �C for 2 h. The Pt loading was

0.7 mg cm�2. The required amount of PA acid, which was

diluted with a water/ethanol mixture, was added to the

surface of the electrodes by means of a micropipette and

electrodes were kept in an oven, at 80 �C, for 1 h to dry

the residue of water/ethanolmixture [12]. TheMEAwas finally

obtained by hot pressing the electrodes on 4.5 mol% phos-

phoric acid loaded membranes at 150 �C for 10 min at a pres-

sure of 40 kg cm�2.

Table 1 e Membrane properties of QDPSU.

Sample Degree ofsubstitution/%

Doping level, % Conductivity/S cm�1,@150 �C

Ea/kJ mol�1 Thickness/mm

PA/QDPSU�58 58 102b 0.06 13.2 55

PA/QDPSU�106 106 193c 0.12 12.8 65

QDPSU�180a 180 e e e e

a Brittle in the presence of water.

b Equivalent to 4.6 per repeating unit (PRU).

c Equivalent to 8.6 per repeating unit (PRU).

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The catalyst ink was prepared either by sonicating Pt/C

(50 wt.%, Alfa Aesar) and PTFE dispersion (60% wt., Aldrich) in

a watereethanol mixture, for PTFE based MEAs. The MEA was

fixed between two high-density graphite blocks (impregnated

withphenolic resin)with parallel gas flowchannels. The active

electrode area was 1 cm2. Electric cartridge heaters were

mounted at the rear of the graphite blocks to maintain the

desired temperature, which was monitored by imbedded

thermocouples and controlled with a temperature controller.

Gold-plated steel bolts were screwed into the blocks to allow

electrical contact. H2 and O2were fed to the cell at flow rates of

0.4 and 0.7 dm3 min�1, respectively. These flow rates were

some 4 times more than stoichiometric requirements corre-

sponding to themaximumcurrent obtained from the cell tests.

Fig. 6 e Fuel cell performance using different DS of

PA/QDPSU, a) DS58, b) DS106, at 150 �C, 0.7 mgPt cmL2

(50 wt.% Pt/C) for both anode and cathode, under

atmospheric pressure, dry gases.

3. Results and discussion

3.1. 1H NMR spectra

The NMR spectra for pristine PSU and CM-PSU (DS 58 and 106)

are shown in Fig. 2. In all spectra, the characteristics of poly-

sulfone backbone were found. The multi-peaks of protons (a,

b, c, d, f, g and h position) on aromatic ring are in the region

between 6.8 and 7.8 ppm. The proton peak at e position on CH3

is at 1.7 ppm. The peaks at 7.3 ppm and 1.6 ppm are attributed

by CDCl3 and H2O respectively. In the spectra of CM-PSU, the

appearance of new peak (i) at 4.6 ppm indicates the formation

of chloromethylated polysulfone (CH2Cl), based on the calcu-

lation of integrated area of the peak, the degree of substitution

(DS) was obtained using Equation (2) [13]:

DS ¼ 2AHi=AHg � 100% (2)

Where AHiand AHg are integral area of Hi (i position, proton in

�CH2Cl) and Hg (g position, proton adjacent to�SO2� group in

the aromatic ring) in the spectra of CM-PSU.

For the chloromethylation reaction time of 12 and 32 h, the

different DS of PSU were 58 and 106% (DS58 and DS106),

respectively. The 1H NMR spectra suggest the successful

synthesis of chloromethylated polysulfone with different DS.

3.2. FT-IR

Fig. 3 shows the infrared spectra of QDPSU and PA/QDPSU. For

both membranes, the O and S]O vibra-

tion peaks are found at 1241 and 1375 cm�1. The small peaks

at 2924 cm�1, 1617 cm�1 and the sharp peak at 1487 cm�1 are

attributed by quaternary ammonium group stretching vibra-

tion [14]. For PA/QDPSU membrane, a significant vibration

peakwas found at 966 cm�1 is attributed by the P]O vibration.

The data suggest the successful preparation of QDPSU and

PA/QDPSU membranes.

3.3. SEM and EDX

The morphology of PA/QDPSU membrane was investigated

using scanning electron microscopy (SEM) and energy

dispersive X-ray (EDX). The SEM images (Fig. 4) of the

membrane cross-section reveal a dense and non-porous

structure as shown in Fig. 4a and b. The element mapping of

P and S as shown in Fig. 4c and d, further suggest that the

homogenous structure of PA/QDPSU membrane was formed.

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In Fig. 4e, the EDX analysis of PA/QDPSU shows C, S

element at 0.25 and 2.4 keV, respectively, which are contrib-

uted by polysulfone. The Al element peak was contributed by

the sample holder. The significant P element peak at 2.0 keV

suggests a complete substitution of Cl� to H2PO4� after 3 days

doping at room temperature. Thus, the degree of chlor-

omethylation is the same as the degree of substitution (DS)

after quaternization with DABCO.

3.4. Ionic conductivity

Fig.5a shows the variation of ionic conductivity with temper-

ature for the membranes prepared in this work. The conduc-

tivities of the DS106 membrane were approximately twice

those of the DS58 membrane and increased with temperature

in the range of 80e150 �C under non-humidified conditions.

Conductivities of 0.12 and 0.064 S cm�1 were achieved for

DS106 and DS58 at 150 �C, respectively. The data suggest that

the ionic conductivity of QDPSU depended upon the DS of PSU.

Higher DS of QDPSU was synthesised, however, DS180

membrane is too brittle in the presence of water and thus is

not suitable for fuel cell applications.

Fig. 5b shows Arrhenius plots of the conductivity data and

the calculated activation energies of ion transport were 13.2

and 12.8 kJ mol�1, respectively for DS58 and DS106

Fig. 7 e a) IR corrected polarization curves of DS58 and

DS106, b) Tafel plots obtained from polarization curves in

Fig. 7a.

membranes. The data indicate that the ionic conductivity of

PA/QDPSU was related to the number of function groups

(degree of substitution) andwas independent of the activation

energy for ion transport. The properties of themembranes are

listed in Table 1.

3.5. Fuel cell performance

Fig. 6 shows the polarization and power density curves of fuel

cells using hydrogen and oxygen at different temperatures.

Both MEAs with different DS of QDPSU showed increased fuel

cell performancewith increasing temperature. The open circuit

potentials at temperatures of 150 �Cwere 0.97 and 0.90 V for the

DS58 and DS106 membranes, respectively. The high DS of PA/

QDPSU led to a superior fuel cell performance (400 mW cm�2)

than the low DS PA/QDPSU (323 mW cm�2). The fuel cell

performance was similar to the result published using greater

catalyst loading (0.81 mgPt cm�2) and a thinner membrane

(20 mm) [9].

The internal cell conductivities estimated from the linear

region of the polarization curves obtained at 150 �C were

0.015Scm�1 and0.023Scm�1 for theDS58andDS106membrane

basedMEAs respectively. These values aremuch lower than the

membrane conductivities indicating significant voltage losses

due to electrode polarization and electrode layer resistances.

Fig. 7 shows IR corrected polarization curves of the fuel cells

operated at 150 �C. From the plot of the IR correct voltage vs ln

( j ), the slope (Tafel type) of the lines were 120 and 103 mV per

decade. Ignoring the contribution associatedwith thehydrogen

oxidation reaction, these values are close to literature reported

values for phosphoric acid loaded PBI fuel cells [15].

4. Conclusions

A novel PA/QDPSU was synthesised via a FriedeleCrafts like

reaction followed by quaternization with DABCO, which is safe

and easy to handle. Different degrees of substitution (DS) can be

achieved by this method. The ionic conductivity and fuel cell

performance are related to the DS. The higher DS of QDPSU

results in significantly better ionic conductivity and fuel cell

power density. However, too high of the DS (such as DS180) will

compromise themembranemechanicalpropertywhich leads to

the unfavourable results for fuel cell applications. A high power

density output of 400mW cm�2 can be achieved using DS106 of

PA/QDPSUmembrane at 150 �C and atmospheric pressure.

Acknowledgement

The authors thank the UK EPSRC for financial support.

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