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Influence of the bolt torque on PEFC performance withdifferent gasket materials

I. Gatto*, F. Urbani, G. Giacoppo, O. Barbera, E. Passalacqua

CNR-ITAE, Institute for Advanced Energy Technologies “N. Giordano” Via Salita S. Lucia sopra Contesse, 598126 Messina, Italy

a r t i c l e i n f o

Article history:

Received 19 April 2011

Received in revised form

14 July 2011

Accepted 16 July 2011

Available online 9 August 2011

Keywords:

PEFCs

GDL compression

Torque moment

Gasket

Cell deformation

a b s t r a c t

In this work, different gasket materials (NBR, expanded PTFE and PTFE) with different

thicknesses were investigated by evaluating the electrochemical performance as function

of a torque moment applies to fasten the cell. Because the materials composing the gasket

are subjected to a different deformation, depending on their mechanical properties,

a different compression was obtained on the GDL as a function of the clamping force.

These effects influence the cell performance, above all the diffusive region of the polar-

isation curves, where the problems related to the mass transport are more important.

These problems are minimised when the cell is fed with gas pressurized at 3 barabs, in fact

at higher pressure the gas concentration is higher and the diffusion is favoured despite the

lowering of GDL porosity due to the compression.

When the gas pressure is 1 barabs, the cell performance is more evidently affected by the

GDL compression and contact resistance increase.

In any case, an optimal clamping force was found to be as a function of the mechanical

properties of materials composing the gasket. The NBR and Expanded PTFE reached the

best performance with a torque moment of 11Nm while the PTFE reached similar perfor-

mance at 9Nm. It was found that thinner PTFE is more stable than others during the time,

with an average power density of 250 mWcm�2 and the lowest standard deviation. The

expected over-compression of the GDL is prevented by distortion of the clamping plates.

This distortion results in unexpectedly good cell performance.

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

reserved.

1. Introduction

In polymer electrolyte fuel cells, all components are generally

assembled between clamping plates by applying a torque

moment on the tightening bolts; as a consequence the

clamping force plays an important role for stack realisation

[1e7]. Previous studies have shown that the clamping load

(pressure) in the assembly of a single cell plays an important

role in optimizing the performance of the fuel cell. The

clamping load of a large fuel cell stack affects the lifetime and

performance of the stack system in several ways. Too large

a clamping load may cause some components in the stack to

produce a stress high enough to give rise to plastic deforma-

tion even cracks [8], while an unreasonable small clamping

load may produce a high contact electrical resistance at the

interface of the gas diffusion layer (GDL) and the bipolar plate

(BPP), and also may cause leakage of water or fuel in the seal

interfaces [1].

An excessive compression of the components, in particular

the GDL, increases the mass transport problems with

* Corresponding author. Tel.: þ39 090 624 240; fax: þ39 090 624 247.E-mail address: gatto@itae.cnr.it (I. Gatto).

Avai lab le a t www.sc iencedi rec t .com

journa l homepage : www.e lsev ier . com/ loca te /he

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0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2011.07.066

Author's personal copy

a consequent reduction of cell performance at high current

density [9e13].

Contrarily, a non optimal tightening of the cell does not

warrant a perfect gas seal and, moreover, it could cause

a higher contact resistance between the MEA and graphite

plates causing a reduction of the cell performance. This

matter has been focused on by many papers, dealing with the

effect of GDL compression on performance loss at high

current density. In the assembly process of a fuel cell stack,

a sufficient contact pressure at the interface between neigh-

boring components should be provided, since the ECR will be

dramatically reduced with the increasing interface contact

pressure. However, too large a clamping force will result in an

excessive resistance to the transport of reactants in the GDL

and even in the flow channels due to the deformation of the

GDL. Therefore, for a given clamping force, how to obtain the

minimum ECR is a very important target in fuel cell stack

design [14]. Some authors considered also variation of the

electrical contact resistance between gas diffusion layer (GDL)

and catalyst layer (CL) by changing the compression. They

found that the contact resistance between the GDL and CLwas

relatively large than the contact resistance between the GDL

and graphite current collector, in addition its significant

variation with compression suggest that uneven compression

pressure on the active area [15]. Moreover, it was found that

the compression produces a reduction of pore volume in the

GDL and consequently a reduction of gas permeability,

a better contact resistance and an improved contact between

GDL and other components [16e21]. Other authors reported

that the cell performance decreases when increasing the

compression and often the fuel cell works under over-

compression, even if an optimal compression ratio exists

[22,23]. A study in which the compression effect on gas

permeability, conductivity and bulk density of different GDL

has also been carried out [24].

In addition, an inhomogeneous GDL compression could

cause a temperature gradient in the cell that enhances the

degradation phenomena with consequent possible lifetime

problems [25]. In fact, the inhomogeneous compression cau-

ses partial deformation of the GDLs with a consequent inho-

mogeneous current density distribution. This variation in the

local current density accelerates the membrane degradation

and influences the cell durability [26].

Another key components in the cell assembly are the

gaskets. It was found that there is an optimal difference in

thickness between gaskets and GDL, in order to prevent

problems related to an excessive GDL compression [27].

Moreover, because of the gaskets are typically placed between

the graphite plates and the MEA to guarantee a good sealing,

the chemical and mechanical characteristics and stability of

the gasket materials were investigated. In fact these proper-

ties are critical for both sealing and the electrochemical

performance of the cell [28,29]. Ge et al. used a specific cell

fixture to evaluate the effect of the GDL compression on PEFC

performance and by considering two different GDL materials

[22], they found that fuel cell performance first increases with

the increase of compression, then decreases with the increase

of compression after passing a certain point.

In this work, the relation between clamping force, GDL

compression and mechanical properties of the gaskets used,

is illustrated by using a conventional cell hardware in which

bolts are used as a tightening system. It is described in terms

of a difference in performance by operating at a different

operative pressure of the reactant gases. Also discussed is

how the clamping force influences the cell performance as

a function of the material used as a gasket and an optimal

torque moment is established for each material. Moreover, an

evaluation of the thickness influence, by using the same

material, elucidated that there is an improvement of perfor-

mance both in terms of average power density and stability.

The cell deformation due to the clamping force increase,

generates results that partially agree with the results reported

by Ge et al. [22], in which cell deformation is avoided by using

a specific test fixture. This aspect was numerically investi-

gated [30e38], in which a finite element modelling and pres-

sure distribution measurements were carried out on

a conventional cell.

2. Experimental

2.1. Electrodes and MEAs preparation

The electrodes were prepared by an application of catalyst

and micro porous layers on carbon cloth (Textron) substrate

Table 1 e Used gasket materials.

Gasket Material Supplier Thickness Youngmodulus(MPa)

NBR Reinforced

nitrile rubber

ATAG 0.20 mm >50e100

PTFE-020 PTFE DIFLON 0.20 mm 410e550

PTFE-015 PTFE DIFLON 0.15 mm 410e550

Exp-PTFE PTFE GORETEX 0.50 mm 10e100

Fig. 1 e Set up for contact pressure measurement.

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through a spray technique [39]. A diffusion layer (GDL) of

Shanwiningan Acetylene Black (Chevron) and 50% wt/wt of

PTFEwas sprayed onto carbon cloth and heat treated at 350 �C.The catalytic layer was obtained by using a 30%wt/wt Pt/

Vulcan (E-TEK) catalyst mixed with a Nafion solution (Aldrich,

5%wt/wt), ammonium carbonate (Carlo Erba), water and

sprayed onto GDL [40]. The thickness of the GDL was

measured with a thickness gauge (Mitutoyo absolute ID-C112

PB), and a value of about 350 mmwas obtained. The electrodes

were dried at 150 �C. The Pt loading was maintained as

a constant at 0.1 mg cm�2 for all the prepared electrodes.

MEAs were obtained by hot pressing the electrodes onto

commercial Nafion 115 membrane at 130 �C for 3 min. The

membranes were previously purified in a 5%vol. H2O2 solution

(Carlo Erba) and in a 1M H2SO4 solution (Carlo Erba).

2.2. Gasket material selection

In a fuel cell, gaskets are normally used to generate the

insulation of anodic and cathodic compartments and to avoid

gas cross over. Generally, they form a frame around MEA in

the un-active zone of the flow field. Because the cell plates are

subject to a compression, the gasket thickness and material

can influence the cell performance. Different materials were

selected to be used as gaskets in the cell hardware. As repor-

ted in Table 1, the first gasket was composed of nitrile rubber

reinforcedwith cotton yarn having a thickness of 0.2 mm. The

other gaskets are based on PTFE polymer and differ among

themselves in thickness (PTFE-020 and PTFE-015) and fabri-

cation process (Expanded PTFE).

2.3. Electrochemical characterisation

The electrochemical tests were carried out in a 25 cm2

commercial single cell (GlobeTech) connected to a Fuel Cell

Test Station. The cell hardware consists of two flow field

graphite plates assembled with two copper clamping plates,

tightened by four stainless steel bolts. Different torque

moments were used to tighten the cell hardware in a range

between 7 and 13 Nm and each kind of gasket was tested in

these conditions. The measurements were performed at 80 �Cin H2/air. Gas pressures of 1 and 3 barabs were used with

a constant gas flux of 1.5 and 2 times the stoichiometric value

at 1 A cm�2 for hydrogen and air, respectively. A relative

humidity (RH, %) of 100% was fixed for both reactant gases.

The polarisation curves, were recorded by means of a test

station equipped with software for the automatic data

acquisitions and the cell resistance was measured with an

Agilent milliohmmeter by a static method at a frequency of

1 KHz. An electronic load (Agilent mod. 6050A) able to appre-

ciate a voltagewith an accuracy of�0.1%was used to carry out

the polarisation curves and time-test measurement.

00.10.20.30.40.50.60.70.80.9

1

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1

0 200 400 600 800 1000 1200

Cel

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ell P

oten

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Current Density, mA/cm2

Current Density, mA/cm2

7 Nm9 Nm11 Nm13 Nm

NBR 3bar

NBR 1bar

0 200 400 600 800 1000 1200

7 Nm9 Nm11 Nm13 Nm

Fig. 2 e Polarisation curves as a function of torque moment

at two different pressures for NBR gasket.

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0 200 400 600 800 1000 1200Current Density, mA/cm2

Cel

lPot

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7 Nm9 Nm11 Nm13 Nm

NBR

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0 200 400 600 800 1000 1200

Current Density, mA/cm2

Cel

lPot

entia

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7 Nm9 Nm11 Nm

PTFE-020

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0 200 400 600 800 1000 1200Current Density, mA/cm2

Cel

lPot

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7 Nm9 Nm11 Nm13 Nm

Exp-PTFE

Fig. 3 e Polarisation curves as a function of torque moment

for different gaskets.

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The time-test on all MEAs was carried out at a constant

value of cell potential (0.4e0.5 V) recording the current density

variation as a function of time with intervals of about 300 s for

about 10 h with a shut down and start up after about 4 h (one

day of tests).

2.4. Contact pressure measurement

Contact pressuremeasurements were carried out by using the

TEKSCAN system composed of a polymeric support that

allocates a wide number of pressure piezoresistive sensors

that permit a pressure numerical map and a coloured repre-

sentation through a digital interface. A Iscan #5076 sensorwas

used to perform the experimental tests. This sensor has

a square matrix of 83.8 � 83.8 mm consisting of 1936 sensing

elements, a spatial density of 27.6 sensel/cm2 (corresponding

to a spatial resolution of about 1.90� 1.90mm), and a pressure

saturation rating (Psat) of 2.4 MPa. Pressure resolution is about

10 kPa, with a maximum sampling rate of about 100 Hz.

Measurements were conducted by placing the sensitive

sensor between the electrode and tightening the cell hardware

at a different clamping force, as shown in Fig. 1aeb.

3. Results and discussion

Because many efforts are directed towards the reduction of

the operating gas pressure from 3 barabs towards atmospheric

pressure, an evaluation of the influence of torque moment on

cell performance was carried out at 3 and 1 barabs. This

evaluation was conducted by using NBR as a gasket, consid-

ered as a starting point for these kinds of measurements

(Fig. 2).

As shown, the cell potential increases by increasing the TM,

and the maximum of performance was reached at 11Nm for

both 1 and 3 barabs. When a TM of 13Nm was applied,

a decrease of the cell potential was observed at each current

density step. This is mainly due to an excessive GDL

compression that limits the mass transfer through the GDL to

the catalyst sites. According to the Nerst equation, the

performance obtained at 3 barabs is always higher than that

recorded at 1 barabs, however the influence of TM increase is

less evident at the higher operative pressure. Moreover the

higher gas pressure minimizes the problems of diffusion due

to the reduced porosity for GDL compression. As above

mentioned the influence of TM increment on the fuel cell

performance is more evident at 1barabs, then the further

analyses were conducted at this pressure only.

At the beginning the test using three different materials

maintaining the same thickness was carried out (NBR, PTFE-

020 and Exp-PTFE). Nevertheless, Exp-PTFE has an initial

thickness of 0.5 mm, this material exhibits a high deform-

ability, in fact after already tightening at 7Nm, a thickness

0.20 mm was measured with a thickness gauge (Mitutoyo

absolute ID-C112 PB).

Fig. 3 shows the polarisation curves obtainedwith different

gaskets as a function of the torque moment (TM).

It is possible to notice that all of the tested materials react

in the same way to the bolt torque. In fact by increasing the

clamping force the cell performance improves until

a maximum and then decreases with the increase of

compression. NBR shows that the influence of the clamping

force acts on all of the IeV curve (ohmic and diffusive region),

in the othermaterials performances aremostly affected in the

diffusive region. For this reason the voltage corresponding to

a current density of 800 mA/cm2 was chosen as the most

representative for all the materials studied.

Fig. 4 reports the cell potential at 800 mA/cm2 and cell

resistance variation as a function of the TM relative to all

samples. As first the cell resistance of PTFE-020 > NBR > Exp-

PTFE in the overall TM range.

As highlighted in the polarisation curves above,

a maximum of performance by increasing TM, was shown for

each gasket. This maximum value seems to be related to the

gasket material properties, in particular the stiffness; the cell

performance decreases after this maximum value, indicating

an excessive compression of the GDL that compromises the

mass transfer and water management. The increase of cell

performance for all materials between TM ¼ 7Nm and

Fig. 4 e Cell potential @800 mA cmL2 and cell resistance as

a function of torque moment for different gaskets.

Fig. 5 e Mechanical similitude of gasket-MEA-clamping plate system.

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TM ¼ 9Nm, is directly related to a decrease of cell resistance, in

fact passing from 7 to 9Nm the contact pressure between the

electrodes and graphite plates improves.

In the TM range 9e11Nman increase of the cell resistance is

observable while the cell potential has a different behaviour

as a function of the considered gaskets. In particular, NBR and

Exp-PTFE show an opposite trend compared to that expected

where an increase of cell resistance would suggest a decrease

in the performance. On the contrary, the PTFE-020 follows the

expected trend.

To explain this evidence it is necessary to consider two

different aspects: the first involving the cell deformation with

the clamping force increase, and the second concerning the

stiffness of the gasket materials. The combination of these

two phenomena causes the particular behaviour for the

different examined gaskets.

With regard to the first aspect, the increase of bolt torque

leads to a deformation of clamping plates, generating an

uneven pressure contact between GDL and gasket area. In this

way, the compression load of the GDL in the central active

area decreases: thus at most its average porosity could

increase as the cell resistance, and the two effect play an

important role for the overall cell performance. The preva-

lence of one aspect compared to the other could be justified

considering the gasket stiffness. In general, the increase of cell

resistance leads to worst cell voltages, otherwise an increase

of GDL porosity enhance the mass transfer and then the cell

performance [4,5,22]. To explain how this two effects combine

themselves, an equivalent mechanical model of the cell is

proposed (Fig. 5a). Two springs in parallel having a different

spring constant (Kg and KGDL in Fig. 5b) represent the GDL and

gasket stiffness. Them can be considered as an equivalent

spring (Fig. 5c) with a constant Keq ¼ Kg þ KGDL. In this case the

total clamping force (FTot) is equilibrated by FGDL þ Fg and

therefore it depends on the spring constant (Kg and KGDL)

strictly related to the stiffness of the material. Then the effect

of uneven pressure contact is more pronounced as the

difference in stiffness between the gasket (sg) and GDL (sGDL)

becomes more elevated. Considering that the GDL stiffness is

always lower than the gasket stiffness then two different

situations are possible: for sg � sGDL (case of NBR and Exp-

PTFE), the performance improves because the effect due to

the reduction of the GDL compression is predominant

compared to the increase of the cell resistance. For sg » sGDL

(case of PTFE-020) the trend is the opposite, because the

increase of the cell resistance has a predominant effect

compared to the reduced GDL compression.

The proposed spring model can explain that, as rigid is the

gasket, as lower is the force that is distributed upon the GDL.

This justifies the highest cell resistance obtained using PTFE-

020 (that has the highest stiffness among the three adopted

materials) and the unexpected behaviour of the cell voltage

increase with the cell resistance increase (case of NBR at 9-

11Nm).

By increasing the TM from 11 to 13 Nm a decrease of both

cell resistance and performance is evident for NBR and Exp-

PTFE samples; in this case an excessive contact pressure

enhances the electrical contact but reduces the electrode

porosity by emphasizing the mass transfer problems and

water management.

A comparison of IeV curves at 1 barabs for each used gasket,

tightened with the optimal torque moment, is reported in

Fig. 6.

Both the NBR and Exp-PTFE reached the best performance

at 11Nm, the highest current density was reached by using

Exp-PTFE. PTFE-020 shows the best results at 9Nm, but in any

case, is lower than the other two gaskets. This behaviour

could be attributable to the highest cell resistance due to the

Fig. 7 e Cell potential @800 mA cmL2 and cell resistance as

a function of torque moment for PTFE gaskets with

different thicknesses.

Fig. 8 e Stability test in terms of power density for different

used gaskets.

0

0.1

0.2

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0.6

0.7

0.8

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Current Density,mA/cm2

NBR 11 Nm

PTFE-0209 Nm

Exp-PTFE11 Nm

P=1 bar

Fig. 6 e Comparison of IeV curves for different used

gaskets obtained at each optimal TM found.

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cell deformation, that predominates if compared to the effect

of the reduced GDL compression.

To evaluate the influence of the gasket thickness,

a 0.15 mm PTFE (PTFE-015) was used and compared to PTFE-

020, as reported in Fig. 7.

As expected, the trend of cell performance and resistance

is the same, except that the cell resistance with PTFE-015 is

always lower. This is due to the higher initial GDL compres-

sion induced by the difference in thickness between the

gasket and the diffusion layer for the PTFE-015. In this case the

optimal torque moment is also 9Nm, even if the variation, in

the range 7e11Nm, is less pronounced than PTFE-020. The

PTFE-015 seems to be a good compromise between the cell

performance and mass transport phenomena.

Fig. 8 compares the short stability tests recorded for each

gasket with the optimal torque moment.

Unstable behaviour is evident except for PTFE-015, which

gives the lowest standard deviation percentage compared to

the average power density, as reported in Fig. 9.

The better stability of PTFE-015 can be explained by

considering the cell hardware deformation during the tight-

ening and cell operation. This effect was pointed out through

contact pressure measurements carried out using the system

described in the experimental section. Fig. 10 reports the

pressure contact contours at different TM (<3, 7, 9, 11 Nm)

referred to the PTFE-020 system.

As can be seen the average contact pressure in the gasket

region remains almost unaltered, except for the corner of the

gasket, where the contact pressure increases until it reaches

the saturation value of sensor (over 10 MPa). The centre of the

electrode tends to unload during the loading phase, resulting

in a low compressed zone below the minimum instrument

sensitivity (<0.7 MPa). This unloaded area increases by

increasing the clamping torque. This effect confirms the

hypothesis of plate deformation during the tightening. When

the thinner gasket 0.15 mm was used, the higher initial GDL

compression was advantageous in terms of lower contact

resistance and probably of higher GDL compression unifor-

mity. This implied more uniform distribution of the gas

diffusion layer porosity and consequently, a better current

density distribution on the electrode surface. Moreover using

Fig. 10 e Contour plot of contact pressure measured at <3, 7, 9 and 11Nm for PTFE-020 system.

Fig. 9 e Average power density and standard deviation for

the different gaskets obtained at each optimal TM found.

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a thinner gasket (PTFE-015) allow the GDL to retain the initial

compression also when the deformation of the clamping plate

begins. Taking into account these effects an improvement of

the overall cell performance in terms of stability and the

average power density was recorded.

In any case, plate deformation is a non negligible

phenomenon in a test cell that use bolts for tightening.

4. Conclusions

This paper illustrated the torquemoment influence on the cell

performance, particularly at low pressure. Experimental

results on different gasket materials show that gasket thick-

ness and its physical properties greatly influence the perfor-

mance of the cell. A maximum of performance was reached

with different torque moments, depending on the material

properties.

By increasing the clamping force the electrical contact

between electrodes and plates is enhanced, but a lower cell

resistance does not necessarily imply a better cell

performance.

Deformation of the plates play an important role and

greatly affect the cell performance.

Nevertheless, the measured local value of contact pressure

is related to the specific hardware adopted in this work;

therefore, the obtained results could be interesting for anyone

running a cell compressed by bolts, where clamping plates

deformation is present. Moreover, this effect linearly prevents

correlating the torque moment and the contact pressure. In

addition, the use of different gasket materials changes the

contact pressure distribution on the GDL, in the same torque

moment, affecting the fuel cell performance.

Acknowledgements

The authors are grateful to Prof. R. Montanini for providing the

TEKSCAN experimental apparatus.

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