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Demonstration of a 20 W class high-temperature polymerelectrolyte fuel cell stack with novel fabrication ofa membrane electrode assembly

Hye-Jin Lee a,1, Byoung Gak Kim b,1, Dong Hoon Lee b,1, Se Jin Park b, Yongmin Kim b,Jeung Woo Lee c, Dirk Henkensmeier b, Suk Woo Namb, Hyoung-Juhn Kim b,*,Hwayong Kim a,**, Ju-Yong Kim d,***a School of Chemical and Biological Engineering, San 56-1, Sillinm-dong Gwanak-Gu, Seoul 151-744, South Koreab Fuel Cell Center, Korea Institute of Science and Technology, 39-1 Hawolgok, Seoul 136-791, South Koreac Fuel Cell R&D Center, POSCO Power, 13-9, Jukcheonri Hunghae-eup Buk-gu, Pohang City, Gyeonbuk, South KoreadCorporate R&D Center, Samsung SDI, 428-5 Gongse, Giheung, Yongin, Gyeonggido 446-577, South Korea

a r t i c l e i n f o

Article history:

Received 12 December 2010

Received in revised form

2 February 2011

Accepted 3 February 2011

Available online 5 March 2011

Keywords:

High-temperature polymer electro-

lyte fuel cell

Phosphoric acid-doped

polybenzimidazole

Membrane electrolyte assembly

Fuel cell stack

* Corresponding author. Tel.: þ82 2 958 5299** Corresponding author. Tel.: þ82 2 880 7406*** Corresponding author. Tel.: þ82 31 288 460

E-mail addresses: hjkim25@kist.re.kr (H.-1 Equally contributed to as a first author.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.02.014

a b s t r a c t

Acid-doped polybenzimidazole (PBI) membrane and polytetrafluoroethylene (PTFE)-based

electrodes are used for the membrane electrode assembly (MEA) in high-temperature

polymer electrolyte fuel cells (HTPEFCs). To find the optimum PTFE content for the catalyst

layer, the PTFE ratio in the electrodes is varied from 25 to 50 wt%. To improve the

performance of the electrodes, PBI is added to the catalyst layer. With a weight ratio of

PTFE to Pt/C of 45:55 (45 wt% PTFE in the catalyst layer), the fuel cell shows good perfor-

mance at 150 �C under non-humidified conditions. When 5 wt% PBI is added to the elec-

trodes, performance is further improved (250 mA cm�2 at 0.6 V). Our 20 W class HTPEFC

stack is fabricated with a novel MEA. This MEA consists of 8 layers (1 phosphoric acid-

doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-diffusion media, 2 gas-sealing

gaskets). The sub-gasket mitigates the destruction of a highly acid-doped PBI membrane

and provides long-term durability to the fuel cell stack. The stack operates for 1200 h

without noticeable cell degradation.

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

reserved.

1. Introduction However, low-temperature PEFCs (LTPEFCs) should be oper-

Fuel cells offer several advantages for solving problems such

as environmental contamination and fossil fuel diminish-

ment. Among various kinds of fuel cells, polymer electrolyte

fuel cells (PEFCs) have been in the spotlight because of their

fast start-up, low operation temperature and high efficiency.

; fax: þ82 2 958 5199.; fax: þ82 2 889 6695.8; fax: þ82 31 288 4646.J. Kim), hwayongk@snu.a

2011, Hydrogen Energy P

ated below 100 �C and water management has to be carefully

controlled to achieve high performance of this type of cell. In

addition, LTPEFCs demand CO (carbon monoxide)-free

hydrogen as a fuel. For these reasons, high-temperature PEFCs

(HTPEFCs) (operating at >120 �C) have been studied to over-

come the drawbacks of LTPEFCs [1,2].

c.kr (H. Kim), Juyong.j.kim@samsung.com (J.-Y. Kim).

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

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Most HTPEFCs employ acid-doped polybenzimidazole (PBI)

derivatives in the fabrication of a fuel cell membrane [2e4].

Although PBI itself does not have proton conductivity, it

develops proton conductivity after being doped with a strong

inorganic acid, such as phosphoric acid. Acid-doped PBI can be

prepared by two different methods: post acid-doping with PBI

membrane and in situ acid-doping by directmembrane casting

from a polymerization mixture in polyphosphoric acid (PPA)

[5]. The former method produces a membrane which has

strong mechanical strength, but a limited doping level. The

latter results in a high doping level, but low mechanical

strength. In order to obtain an acid-doped PBI membrane for

effective HTPEMFC operation, the two methods are used by

carefully controlling the acid-doping level; however, there has

been no solid strategy for fabricating a PBI membrane which

has a high doping level, uses an easy membrane-casting

method and exhibits strong mechanical strength.

Previously we reported several results which addressed the

synthesis, membrane fabrication, and fabrication of the

membrane electrode assembly (MEA) for PBI membranes. For

high-temperature fuel cell operation, in situ fuel cellmembrane

was fabricated from a polymerization mixture in PPA. In this

case, high proton conductivity could be obtained, but low

mechanical strength remained a problem for MEA fabrication

and long-term stability. In this report, we use a novel MEA

fabrication method that produces a very durable HTPEFC. The

MEA has an 8-layered structure consisting of 1 phosphoric

acid-doped PBI membrane, 2 electrodes, 1 sub-gasket, 2 gas-

diffusion media, and 2 gas-sealing gaskets (Fig. 1). Most

components of this MEA are similar to those of other types for

Fig. 1 e Schematic drawing of th

low-and high-temperature PEFCs. The unique component of

this MEA is a sub-gasket. The sub-gasket ensures that the

membrane structure maintains proton conductivity and long-

term durability. Commercialized MEAs of low-temperature

PEFCs are equipped with a sub-gasket which has at least the

same thickness as that of an electrolytemembrane [6,7]. In our

case, however, the thickness of the sub-gasket is thinner than

that of the phosphoric acid-doped PBI membrane. During

single cell fabrication, the phosphoric acid-doped PBI

membrane is squeezed only as far as the thickness of the sub-

gasket.Asa result, the structuredoesnotdegrade for single cell

and stack operation, resulting in excellent durability.

In order to achieve good performance of the MEA of our

HTPEFC, the electrode was also optimized. Recently, binders

such as polytetrafluoroethylene (PTFE) and PBI for the Pt/C

catalysthavebeenstudiedtofabricate theelectrodes [8,9]. Inthis

work,weusedamixtureof PTFEandPBI as thecatalyst binder to

provide goodproton conductionandmaintainapathway for air.

Using this type of MEA, a 20 W class HTPEFC stack was fabri-

cated. In the sections which follow, we present new findings

associated with our novel MEA and 20 W class HTPEFC stack

which demonstrate their capacity for long-term durability.

2. Experimental

2.1. Materials

3,3-Diaminobenzidine (99%, Aldrich), terephthalic acid (>99%,

TCI), isophthalic acid (>99%, TCI), phosphoric acid (85%,

e 8-layered MEA structure.

Fig. 2 e Comparison of MEA shapes after MEA fabrication

without (a) and with (b) a sub-gasket. Membrane: p-PBI.

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Aldrich) and polyphosphoric acid (PPA) (115% phosphoric acid

equivalent, Aldrich) were used as received. A 46.1 wt% Pt/C

catalyst (Tanaka), gas-diffusion cloth (withmicroporous layer,

HT1410-W from E-Tek), and 60 wt% PTFE dispersion in water

(Aldrich) were used for the MEA fabrication.

2.2. Synthesis of p-PBI (poly[2,20-(p-phenylene)-5,50-bibenzimidazole]) and membrane fabrication

3,3-Diaminobenzidine (3.9 g, 0.018 mol) and terephthalic acid

(3.0 g, 0.018 mol) were polymerized with polyphosphoric acid.

The reaction temperature was kept at 250 �C for 24 h and the

viscous polymer solution was poured onto a glass plate and

flattened by a doctor blade to obtain a uniform thickness

(300 mm). The membrane was maintained at an ambient

condition (25 �C, 45% relative humidity) and the polyphosphoric

acid was hydrolyzed to phosphoric acid. The p-PBI membrane

was used without a further phosphoric acid-doping process.

Fig. 3 e The polarization curves of MEAs with different wt%

in the electrode. A number preceded by P refers to the wt%

of PTFE in the electrode under dry state. Test conditions:

150 �C; non-humidified; ambient pressure; flow rate of H2

100 sccm and air 300 sccm.

2.3. Preparation of m-PBI (poly[2,20-(m-phenylene)-5,50-bibenzimidazole]) solution

The m-PBI was synthesized using a similar method as

described in Section 2.2. Instead of terephthalic acid, iso-

phthalic acid was used for the synthesis of m-PBI. After the

polymerizationwas finished, the polymermixturewas poured

into water. The fibrous polymer was washed with a 10 wt%

K2CO3 solution andwater several times to obtain the de-doped

m-PBI. Then it was dried under reduced pressure for 24 h at

60 �C. The dried m-PBI (0.15 g) was dissolved in 10 g of

dimethyl acetamide (DMAc).

2.4. Preparation of electrode

Typical procedure for the electrode with 45% PTFE under dry

state: Pt/C catalyst (1 g), PTFE dispersion (2 g), iso-propanol

(100 mL), water (25 mL) and with (or without) m-PBI solution

(4.1 g) prepared as the method in the section 2.3, were mixed

by a homogenizer for 1 h. The catalyst slurry was sprayed on

carbon cloth and dried in a vacuum oven to remove residual

solvents. The catalyst-loaded electrode was sintered at 350 �Cfor 30 min under a nitrogen atmosphere.

2.5. Single cell performance test

The catalyst-loaded electrodes and phosphoric acid-doped

p-PBI membrane were assembled to make MEA using a PTFE

sub-gasket (thickness: 250 mm) with (or without) hot-pressing

processing at 140 �C under 400 kPa for 4min. The active area of

the single cell was 10 cm2. The Pt loadings of both cathode and

anode were 1 mg cm�2, respectively. Dry hydrogen and air

were used and the cell temperature was kept at 150 �C.Polarization curves and impedance spectra were obtained by

the method described by Kim et al. [5].

2.6. Fabrication of the MEA and 20 W class PEFC stack

A 20W class high-temperature PEFC was fabricated by using

the MEAs as prepared in Section 2.4. The active area of the

Fig. 4 e Polarization curves of MEAs prepared with and

without a hot-pressing method. PTFE content: 45wt% in

the electrode under dry state. Test conditions: 150 �C; non-humidified; ambient pressure; flow rate of H2 100 sccm and

air 300 sccm.

Fig. 6 e Polarization curves of MEAs which were hot-

pressed with 5 wt% or 10 wt% m-PBI and without m-PBI in

electrodes. Numbers followed by P and PBI are the wt% of

PTFE and m-PBI, respectively, in the electrode under dry

state. The MEA was hot-pressed. Test conditions: 150 �C;non-humidified; ambient pressure; flow rate of H2

100 sccm and air 300 sccm.

Fig. 5 e Impedance spectra of MEAs fabricated with and

without a hot-pressing method at 0.85 V.

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MEA was 25 cm2. The stack was composed of 7 cells. Each cell

had an 8-layered structure (1 phosphoric acid-doped PBI

membrane, 2 electrodes, 1 sub-gaskets, 2 gas-diffusionmedia,

2 gas-sealing gaskets) (Fig. 1). We used graphite bipolar plates

which has serpentine channels for anode and cathode.

3. Results and discussion

3.1. MEA fabrication and cell performance

Highly acid-doped p-PBI membrane has a relatively low

mechanical strength.Themembranebreakseasilyduringsingle

cell preparation or fuel cell operation. The boundary of the

Fig. 7 e 20 W class HTFC. Active ar

activeareaisparticularlysusceptibletodamage,probablydueto

the pressure difference between the electrolyte membrane

under gas-diffusion media and the gas-sealing gaskets. Fig. 2

shows the MEA with and without a sub-gasket after single cell

fabrication. Without the sub-gasket, the membrane was

destroyed by the higher pressure under the gas-sealing gaskets.

In the case of single cell fabricationwith a sub-gasket, however,

destruction of the membrane was not observed.

The 8-layered MEA, which has a sub-gasket, was used for

the fuel cell operation at 150 �C under non-humidified

ea of MEA: 25 cm2; 7 cell stack.

Fig. 8 e Cell performance of 20 W class HTPEFC at 150 �C under non-humidified and ambient pressure. The MEA were

fabricated using hot-pressing method with 5 wt% m-PBI in electrodes. Active area of MEA: 25 cm2; 7 cell stack. (a) Constant

current operation at 5 A with 750 sccm of H2 and 2500 sccm of air; (b) IV curves initial, 600, 1200 h operation.

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 ) 5 5 2 1e5 5 2 6 5525

conditions and ambient pressure. To determine the ideal PTFE

amount in the electrodes, we carried out experiments by

varying the weight ratio of PTFE from 25 to 50% in the elec-

trodes under dry state. As shown in Fig. 3, cell performance

improved when PTFE content increased, although there was

a limit after which cell performance was compromised. At

a PTFE weight percentage of 50, performance declined

precipitously. These results may occur because as the amount

of PTFE is enhanced, binding ability may improve, resulting in

Fig. 9 e Cell performance of 7 cells of a 20 W class HTPEFC at 1

a function of time at 5 Awith 750 sccm of H2 and 2500 sccm of air

difference between the best and the worst cell of the stack at 0

high cell performance, but with too much PTFE in the elec-

trodes, hydrogen and oxygen may be blocked and the resis-

tance of the electrode layer may increase.

In preparing the 8-layered MEA, we fabricated it both with

and without a hot-pressing method. To mitigate the resis-

tance between the membrane, the electrodes and the gas-

diffusion layers, we pressed theMEA at 140 �Cunder 40 KPa for

4 min. A performance comparison of cells containing MEAs

which were fabricated with and without a hot-pressing

50 �C under non-humidified and ambient pressure as

. (a) Initial; (b) 600 operation; (c) 1200 h operation (d) Current

.6 V in initial, 600 and 1200 h operations.

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method is shown in Fig. 4. The hot-pressed MEA resulted in

higher cell performance than with a pristine MEA. This was

confirmed by examining the impedance spectra (Fig. 5) which

showed that the Ohmic resistance of the hot-pressedMEAwas

smaller than that of the pristine one.

Catalyst binders, such as PBI and PTFE, were used for MEA

fabrication. PBI binder is excellent for three-phase boundary

formation. Previously we clarified the role of PBI and phos-

phoric acid in an electrode [10,11]. J. Lobato et al. [12] have also

reported on the importance of the ratio of PBI and phosphoric

acid in electrodes. PTFE binder, which has generally been used

in phosphoric acid fuel cells (PAFCs) [13], has the advantage of

promoting good transfer of fuel and air. In this reported work,

we used amixture of PTFE andm-PBI for the catalyst binder to

provide a good pathway for gases as well as proper proton

conduction. We fixed PTFE content at 45 wt% and varied the

content ofm-PBI to 5 and 10wt%. Fig. 6 shows the dependence

on cell performance of m-PBI content. When 5 wt%m-PBI was

added in the electrode, the cell performance of the MEA was

higher than that of pristine MEA without m-PBI in the elec-

trode. However, cell performance decreased when the 10 wt%

m-PBI was added in the electrode. Cell performance was very

dependent on them-PBI content in the electrode. Theremight

be an optimum combination for m-PBI and PTFE, so this shall

be the subject of our next experiment in an effort to improve

cell performance.

3.2. 20 W class high-temperature PEFC stack operation

Seven 8-layered MEAs, which were hot-pressed with 5 wt%

m-PBI in the electrodes, were used to fabricate a 20 W class

HTPEFC stack (Fig. 7). Fig. 8 presents the cell performance of

the stack as a function of time. Constant current operation at 5

A did not cause deterioration of the stack in 1200 h. IV curves

also indicated that after 600 and 1200 h of operation, perfor-

mance of the cells in the stack was not compromised.

During the stack operation, we tested the cell performance

of 7 cells of the stack (Fig. 9). Initially the 7 cells showed

different cell performance. The performance difference

between the best cell and the worst cell was 63 mA cm�2.

Surprisingly, as operation time increased, the difference

decreased. In addition, stack performancewas not affected, as

shown in Fig. 8. It is not clear why this phenomenon occurred.

One possible answer is that the phosphoric acid distribution

evened out in each cell because each MEA was equipped with

a sub-gasket whichmade the thickness of eachmembrane the

same. This could account for the similar performance of each

cell as operation time increased.

4. Conclusions

In order to improve HTPEFC performance, PTFE and m-PBI

were combined to make catalyst binder in the electrode. In

addition, a hot-pressing method was shown to be very effec-

tive for the mitigation of Ohmic resistance, resulting in high

cell performance. A novel MEA fabrication method was

developed for the acid-doped PBI membrane. It involved the

insertion of a sub-gasket to avoid deterioration of the PBI

membrane during cell fabrication and operation. We

fabricated a 20 W class HTPEFC stack using 7 MEAs. The stack

was operated for 1200 h without noticeable performance

degradation. We also found out that the 7 cells composing the

stack demonstrated similar performance, an outcome which

is not fully understood at this time. Electrochemical and

physical analysis of our MEA will be carried out to understand

the driving force behind these performance results.

Acknowledgements

This work was supported by the New and Renewable Energy

R&D Program (2008-N-FC12-J-01-2-100), and a grant

(M2009010025) from the Fundamental R&D Program for Core

Technology of Materials funded by the Ministry of Knowledge

Economy (MKE), Republic of Korea.

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