performance of a poly(2,5-benzimidazole)-based polymer electrolyte membrane fuel cell

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Performance of a poly(2,5-benzimidazole)-based polymerelectrolyte membrane fuel cell

Jose J. Linares, Cassandra Sanches, Valdecir A. Paganin, Ernesto R. Gonzalez*

Instituto de Quımica de Sao Carlos, University of Sao Paulo, Av. Trabalhador Sao-Carlense, 400 CP 780, CEP 13560-970 Sao Carlos, SP, Brazil

a r t i c l e i n f o

Article history:

Received 8 September 2011

Received in revised form

15 November 2011

Accepted 5 December 2011

Available online 30 December 2011

Keywords:

High temperature PEMFC

Cell performance

ABPBI

CO tolerance

Humidity

Temperature

* Corresponding author. Tel.: þ55 1633739899E-mail address: [email protected] (E.R.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.12.030

a b s t r a c t

The performance of an ABPBI-based High Temperature H2/O2 PEMFC system was studied

under different experimental conditions. Increasing the temperature from 130 to 170 �C

improved the cell performance, even though further increase was not beneficial for the

system. Humidification of the H2 stream ameliorated this behaviour, even though oper-

ating above 170 �C is not advisable in terms of cell performance. A significant electrolyte

dehydration seems to negatively affect the fuel cell performance, especially in the case of

the anode. In the presence of 2% vol. CO in the H2 stream, the temperature exerted

a positive effect on the cell performance, reducing the strong adsorption of this poison on

the platinum sites. Moreover, humidification of the H2 þ CO stream increased the

maximum power densities of the cell, further alleviating the CO poisoning effects. Actual

COeO2 fuel cell results confirmed the significant beneficial effect of the relative humidity

on the kinetics of the CO oxidation process.

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

reserved.

1. Introduction high temperatures [6,7]. Also, an enhancement of the oxygen

HighTemperature (120e200 �C) Polymer ElectrolyteMembrane

Fuel Cells (HT-PEMFC) are a promising alternative to low

temperature (up to 90 �C) Nafion� based PEMFC [1e4]. The idea

for developing this system arose from the limitations of the

traditional system, primarily from the point of view of the

tolerance to fuel impurities, such as the carbon monoxide

coming from a hydrocarbon reforming process [5]. In this

process, typical carbon monoxide contents of thousands of

ppm are obtained, making strictly mandatory the imple-

mentation of advanced CO cleaning processes (Partial Oxida-

tion, Water Gas Shift, Membrane Technology), which rises the

complexity and the cost of the system. One alternative to

overtake this limitation is to take advantage of the behaviour

of the CO adsorption process on platinum. This process

exhibits a negative value of the entropy, being disfavoured at

; fax: þ55 1633739903.Gonzalez).2011, Hydrogen Energy P

reduction reaction could be expected under those conditions.

Increasing the operational temperature requires materials

with a high thermal, mechanical and chemical stability, low

fuel and comburent permeability, and high proton conduc-

tivity [8]. Among the different existing options, poly-

benzimidazoles (PBIs) emerged as one of the most interesting

ones. Polybenzimidazoles are a family of highly aromatic

thermoplastic polymers with a high glass transition temper-

ature (Tg > 400), and a basic nature due to a pair of unpaired

electrons of two nitrogen atoms in the imidazole ring. Pure

PBIs are electronic and ionic insulators, but properly impreg-

nated with an acid, they become good proton conductors. One

of the most suitable acids is phosphoric acid. This acid is not

aggressive to the polymer structure, and has got a low vapour

pressure and a high chemical stability, allowing its use at high

temperatures, such as in the case of Phosphoric Acid Fuel

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

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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 7 ( 2 0 1 2 ) 7 2 1 2e7 2 2 0 7213

Cells (PAFC). Therefore, from the combination of PBIs with

H3PO4 arises a very interesting candidate for HT-PEMFC.

The most extensively used PBI is the Poly[2,2-(m-phenyl-

ene)-5,5-bibenzimidazole] (m-PBI). As Li et al. [1] recently

showed in a review, over the last 16 years, since the first

proposal of use of H3PO4-doped m-PBI for fuel cell application

by Prof. Savinell’s research group [9], there has been an expo-

nential growth in the interest for this material. Another poly-

mer of the family of the polybenzimidazole is the poly(2,5-

benzimidazole) (ABPBI). ABPBI can be easily synthesized from

a single monomer, 3,4-diaminobenzoic acid, obtaining more

easily high molecular weights [10]. This polymer was firstly

proposed for fuel cell applications by Litt et al. [11] in 1997, and

significantly developed by Prof. Gomez-Romero’s research

group [12]. However, less attention has been paid to this

system due to intrinsic handling difficulties, especially during

the preparation of the membrane (use of strong acids for the

casting procedure), aswell as, somehow, a lower performance.

Nonetheless, important straightforward steps have been

recently taken improving significantly the performance of this

system, as demonstrated by a series of recent papers [13e18].

In this context, this paper aims to study the behaviour of

a high temperature ABPBI-based PEMFC under different

experimental conditions. In the first part, it is analyzed the

influence of two typical operating variable, such as the

temperature and the humidification of the anode stream on

the cell performance, when operating the cell on neat

hydrogen. In the second part, the same study and protocol is

applied, but feeding the system with hydrogen contaminated

with carbon monoxide. Finally, pure CO was fed into the cell

operating on CO/O2 in order to assess the real capacity of the

system for oxidizing CO.

2. Materials and method

2.1. ABPBI polymer synthesis and membrane casting

ABPBI polymer was prepared by a polycondensation process

described elsewhere [19,20]. Briefly, it consisted of the auto-

polycondensation of 3,4-diaminobenzoic acid (97%, Aldrich)

in polyphosphoric acid (115% Aldrich) under inert atmosphere

at 150e180 �C for 5 h. Afterwards, the polymer solution was

poured into water, neutralized with NaOH, washed several

times with Milli-Q water, and dried at 80 �C overnight in

a vacuum oven. Once isolated the polymer powder, this was

dissolved in methanesulfonic acid (99.5%, Aldrich) at room

temperature, leaving the system until complete solution for

24 h. The polymer solution was then spread onto an optically

plane tempered glass plate, evaporating the solvent at 100 �C.Membranes were peeled off the support by immersion in

water, neutralized in a 10% wt. NH4OH solution, washed

several times in boiling Milli-Q water, and stored in 80%

H3PO4, at least for one week.

2.2. Preparation of the electrodes and the membrane-electrode-assembly

Electrodes were prepared as follows. A diffusion layer was

made with carbon powder (Vulcan XC-72R) and 15% w/w.

Teflon TE-3893 (Dupont) was applied homogeneously over

a carbon cloth (PWB-3, Stackpole) by vacuum filtration. On top

of this layer a catalyst ink was applied by brushing. The ink

was composed of a Pt/C catalyst (20% Pt on Vulcan XC-72R, E-

TEK Inc.), PBI (from Aldrich at 5% in N,N0-dimethylacetamide,

DMAc), and DMAc as solvent. The process took place on a hot

aluminium plate (60 �C). The platinum loading for both elec-

trodes was 0.5 mg/cm2, with a PBI loading normalized with

respect to the carbon catalyst loading (C/PBI ratio) of 20 [21].

Afterwards, the electrodes were cured in an oven at 190 �C for

2 h, and then impregnatedwith a 10%H3PO4 (loading of 20mg/

cm2), leaving them overnight in order to ensure the complete

soaking of the electrodes.

In order to prepare the membrane-electrode-assembly

(MEA), a piece of membrane was taken out from the H3PO4

doping bath, and placed between the electrodes (active area of

4.62 cm2). Hot pressing was carried out with the aid of a press,

applying 1 tonne and 150 �C for 15 min. MEAs prepared were

stored in sealed plastic bags for future use in the fuel cell.

2.3. Electrochemical measurements

The cell hardware consisted of two graphite monopolar

plates, into which a 4-channel serpentine geometry was

machined.Within the graphite plates, heating rodswere fitted

in order to heat the cell up. Temperature was controlled with

the aid of a temperature controller (Flyever). Temperatures

used in this study were 130e150e170e190 �C. During the

measurements, pure H2 or 2% vol. CO in H2 (White Martins)

were fed into the cell at a flow rate of 100 ml min�1. The

comburent, pure oxygen (White Martins) was introduced at

a flow rate of 75 ml min�1. In the case of the measurements in

which hydrogen was also introduced in the cathodic

compartment, the flow was readjusted to 100 ml min�1, and

humidified by bubbling inwater at room temperature. Relative

humidity of the fuel stream was controlled by applying the

fuel un-humidified or after bubbling it through a water

chamber at 25 �C or 60 �C. In order to prevent condensation of

the water vapour present in the hydrogen stream, the length

of the connecting pipe between the humidifier and the fuel

cell was shortened and electric heat traced. Assuming

complete saturation of the hydrogen stream at the corre-

sponding temperature, water vapour partial pressures were

approximately 2,500 and 20,000 Pa, respectively. Higher

temperatures of the water chamber did not lead to enhance-

ment in the cell performance, most likely due to an excessive

dilution of the fuel streams as Lobato et al. [22] demonstrated.

Previous to the polarization curves measurements at the

different temperatures and water vapour partial pressures,

the system was polarized at 0.6 V until stabilization of the

current (normally 2e3 h). Next, polarization curves were

carried out under potentiostatic conditions, waiting for

a stable value of the current. They were recorded with the aid

of a potentiostat/galvanostat AUTOLAB PGSTAT 302 (Ecoche-

mie) equipped with a current booster of 20 A from the lowest

to the highest temperature. Measurements were performed

from open circuit voltage (OCV) down to 100 mV.

In the case of the operation with H2 also in the cathode,

polarization curves were carried out under galvanostatic

conditions, from 0 to 90% of the maximum current density



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Fig. 1 e (a) Polarization curves (solid symbols) and power

density (hollow symbols), (b) iR-corrected polarization

curves, (c) Ohmic resistance vs. current density, and (d)

Polarization curves when operating the cell on H2/H2 at

different temperatures (-: 130 �C; C: 150 �C; :: 170 �C;

;: 190 �C). Cell operated on dry neat hydrogen at

atmospheric pressure. When operating on H2/H2 the


achieved during the normal operation of the cell. Eventually,

in the case of the CO/O2 tests, the system was polarized

potentiostatically from OCV down to 0 V. The ohmic (iR)

contribution was assessed by AC impedance, taking the

uncompensated resistance obtained at high frequencies for

each point of the polarization curves.

3. Results and discussion

3.1. Performance on neat hydrogen fuel

3.1.1. Influence of the temperature on the cell performanceThe influence of the temperature on the cell behaviour of an

ABPBI-based PEMFC is displayed in Fig. 1. Firstly, Fig. 1(a)

shows the influence of the temperature on the actual cell

performance, and on the power density drawn from the cell.

In Fig. 1(b) the iR-corrected polarization curves are depicted.

Fig. 1(c) collects the corresponding values of the ohmic resis-

tance of the polarization curves of Fig. 1(a). Fig. 1(d) shows the

results corresponding to the H2/H2 operation of the cell. The

cell was operated on dry hydrogen and oxygen.

Influence of the temperature on the cell performance

shows an increment of the performance up to 150 �C, with no

further increases above this temperature, and even a decrease

when operating at 190 �C. The maximum power peak is ob-

tained at 150 �C with a value of 343 mW cm�2, decreasing at

190 �C to 257mWcm�2. This behaviour is not easily explained.

Indeed, in the literature, it is generally accepted that

increasing the temperature from 120 to 200 �C comes

accompanied of a monotonous enhancement of the cell

performance [22e25]. Lobato et al. [26] reported a similar

influence of the cell temperature on the analogous m-PBI-

based PEMFC system, showing that when the cell was left to

stabilize for 24 h at different temperatures, the subsequent

polarization curves showed an optimum performance at

150 �C. They explained their results mainly in terms of the

electrolyte dehydration, which increases the ohmic resis-

tance, and as a consequence of this, diminishes the electrode

performance, since the electrolyte within it is a key element

allowing the transportation of protons generated/consumed

in the electrochemical reactions. However, looking at Fig. 1(b)

and (c), it can be seen that actual larger ohmic resistance are

only observed when operating the cell at low current densi-

ties, region in which a detailed observation of the iR-corrected

polarization curves shows the expected positive intrinsic

effect of the temperature. Contrarily, at high current densities,

the ohmic resistance decreases with the temperature,

whereas the cell performance does not improve above 150 �C[if the ohmic resistance in the catalyst layer diminishes, an

enhancement in the electrode performance collected in

Fig. 1(b) may be expected]. The behaviour of the ohmic resis-

tance can be simply explained in terms of the larger hydration

of the electrolyte when operating the cell at high current

density, due to the large amount of water vapour produced in

the cathode. On the other hand, the explanation for the

cathode/pseudo-reference electrode was humidified at

room temperature.



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curves, (c) Ohmic resistance vs. current density, and (d)

Polarization curves when operating the cell on H2/H2 at

different water vapour pressures (-: dry; C: 2,500 Pa; ::

20,000 Pa). Cell operated at 190 �C with neat hydrogen at

atmospheric pressure.


behaviour of the electrode is more complex. The cell perfor-

mance comes from the summation of the anodic and cathodic

polarizations. It is generally, and most of the time correct to

assume, that most of it comes from the oxygen reduction

reaction [27]. Nonetheless, for this H3PO4-doped system, the

anode possesses a non-negligible contribution, as Seland et al.

[28] demonstrated. Indeed, having a look at the results when

operating the cell on H2/H2 mode [Fig. 1(d)], they are similar to

those of the overall polarization curves, with an enhancement

when the temperature was increased from 130 �C to 150 �C,and significantly worsening when operating at 190 �C.Therefore, a significant contribution of the performance

decays at high temperatures may come from the anode. The

electrolytewithin it is operating under strict dry conditions, so

that it can undergo a severe dehydration at the highest

temperature (despite the fact that in global terms the

ohmic resistance decreases with the temperature above

50 mA cm�2). This leads to a decrease in the performance of

the anode for the hydrogen oxidation reaction, and hence, in

the global cell performance.

3.1.2. Influence of the relative humidity on the cellperformanceIn view of the previous results, it seems interesting to study

the influence of introducing a pre-humidified hydrogen

stream on the anode. The presence of water vapour has been

reported to significantly enhance the ABPBI membrane

conductivity [19]. Therefore, it may be expected to observe

a beneficial effect of the relative humidity on the cell perfor-

mance. In this sense, Fig. 2 displays the cell performance at

190 �C after humidifying the fuel 25 �C and 60 �C. The

temperature of 190 �Cwas intentionally chosen because of the

significant drop in the cell performance ascribed to the

dehydration in the anode, as seen in Fig. 1(d). Fig. 2 displays

the cell performance at 190 �C as a function of fuel humidity in

a sequence similar to Fig. 1.

The pre-humidification of the hydrogen stream shows

a noticeable influence on the cell performancewhen operating

at 190 �C. The higher thewater vapour pressure, the higher the

maximum power of the cell, from 257 mW cm�2 for dry

hydrogen, to 358 mW cm�2 for a water vapour pressure of

20,000 Pa, which corresponds to an increase of 39%. Lobato

et al. [22] observed a similar behaviour, even though not as

notorious as in these results. As it canbe seen fromFig. 2(c), the

pre-humidification of the hydrogen stream causes a notable

decrease in the ohmic resistance of the system, attributed to

the increase in the electrolyte conductivity, althoughhumidity

is only applied in the anode. Large conductivity assists in

increasing the electrode performance with the relative

humidity. The importance of the relative humidity on the

performance of the anode is shown in Fig. 1(d). As can be

observed, the potential of the H2/H2 system diminishes the

higher the relative humidity in the anode streams, so the

hydrogen oxidation reaction is significantly enhanced when

operating with humidified fuel. Moreover, Liu et al. [29] re-

ported that the relative humidity increased the oxygen solu-

bility through the acid doped PBI. A similar behaviour should

not be disregarded with the hydrogen solubility in acid doped

ABPBI. Hydrogen solubility in doped polybenzimidazoles

system is significantly lowered compared to Nafion� [30].




3.1.3. Combined influence of the temperature and the relativehumidity on the cell performanceOnce studied individually the influence of each operating

variable, it can be interesting to analyze the combined effect

of both variables on the cell performance. The maximum

power density of the cell was elected as variable for compar-

ison purposes. Fig. 3 shows its values at the different

temperatures and water vapour pressures used in this work.

As it can be seen in Fig. 3, independent of the pre-

humidification of the hydrogen stream, the use of tempera-

tures above 150 �C does not result beneficial in terms of cell

performance. The pre-humidification of the anode stream

improves the cell performance, due to the expected increase

in the electrolyte conductivity and, consequently, in the anode

performance. However, this still undergoes a severe dehy-

dration phenomenon at 170 and 190 �C. Indeed, calculation of

the relative humidity under the cell operating temperature

leads to values ranging from 0.2% for the humidified hydrogen

stream at 25 �C, with the cell operating at 190 �C, to 7.5% for

the humidified hydrogen streams at 60 �C, with the cell

operating at 130 �C, which corresponds, at the temperatures of

130 and 190 �C, and assuming equilibrium between the

phosphoric acid and the surrounding water vapour, to phos-

phoric acid concentrations above 100% [31], i.e., extremely

dehydrated acid already experiencing auto-polymerization

reactions [32]. Therefore, although the ohmic resistance of

the system decreases with the increase in the water vapour

pressure at all the temperatures [tendencies of the variation of

the ohmic resistance with the relative humidity for the four

temperatures is the same than that of Fig. 2(c)], the anode still

operates under severe anhydrous conditions at 170 and 190 �C.This leads to a decrease of its performance when operated the

cell at such high temperatures. In fact, the anodic curves at

different temperatures, for each level of water vapour pres-

sure, display the same tendency than that of Fig. 1(d).

In summary, it is recommended to operate the cell at an

intermediate temperature in the range of 150e170 �C, and

under moderately humidified hydrogen streams if high power

density is desired. As a matter of fact, in agreement with

Lobato et al. [22], when this system was operated with fuel

130 150 170 190200

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500

Ma

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p

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Fig. 3 eMaximum power density drawn from the cell at the

different temperatures and water vapour partial pressures

(-: dry; C: 2500 Pa; :: 20,000 Pa). Cell operated with neat

hydrogen at atmospheric pressure.

humidification at 90 �C, no enhancement in the cell perfor-

mance was observed (results not shown), so humidification at

60 �C should be enough.

3.2. Performance on CO contaminated hydrogen fuel

One of the great advantages of this high temperature PEMFC

system is its versatility for operation with CO containing

hydrogen coming, for example, from a hydrocarbon reformate

system. Therefore, it is interesting to extend the study of the

operating variables to a system using this type of “more

realistic” fuels.

3.2.1. Influence of the temperature on the cell performanceFig. 4 shows the results regarding the influence of the

temperature on the cell performance when operating the cell

on dry H2 þ CO fuel at 130e150e170e190 �C.CO is known to be one of the most stringent poisons for

PEMFC, since it strongly adsorb onto platinum sites, inacti-

vating them, and leading to a decrease in the anode perfor-

mance. As it can be seen in Fig. 4(a), operating at high

temperature for CO containing hydrogen streams alleviates

the CO detrimental effects. Indeed, for a 2% in volume CO, the

maximum power continuously increases from 34.4 mW cm�2

at 130 �C, to 206.9 mW cm�2 at 190 �C. This behaviour can be

explained in terms of the negative value of the entropy for the

CO adsorption on platinum [6,7], reducing its coverage the

higher the temperature, in agreement with those reported in

the literature for similar PBI [33e37] and ABPBI [38] systems.

The iR-corrected polarization curves also show that the

performance of the electrodes significantly improves for

higher operating temperatures. Fig. 4(c) helps to understand

the characteristics of the polarization curves. It can be

observed a significant decrease of the cell potential in the

H2 þ CO/H2 system the higher the operating temperatures,

despite the expected higher dehydration of the acid doped

electrolyte. In fact, a careful analysis of the curves reveals that

this decrease diminishes the higher the temperature. At those

high temperatures (170e190 �C) the CO poisoning effects are

so depressed that the electrolyte dehydration effects start to

govern the cell performance.

3.2.2. Influence of the relative humidity on the cellperformanceAs it was reported in Section 3.1.2, the relative humidity

possesses a significant influence on the cell performance,

improving it by reducing the undesirable dehydration process

within the anode. In the presence of CO, aside from this effect,

its presence may be beneficial for several reasons: (i) the

presence of larger amounts of water could result in a higher

level of H2O and OH coverage, as a result of an increase in its

partial pressure, depressing that of CO [40], and (ii) according

to the bifunctional mechanism for surface CO removal [41],

eOH species must be present on the platinum surface, in

order to oxidize PteCO to CO2 and free catalytic sites.

With these premises, it is interesting to study the influence

of humidifying the hydrogen stream on the performance with

a H2 þ CO fuel. Fig. 5 displays the results operating the cell at

the lowest temperature, 130 �C, where the poisoning effects of

CO are more noticeable.



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curves, and (c) Polarization curves when operating the cell

on H2 D CO/H2, at different temperatures (-: 130 �C; C:

150 �C; :: 170 �C; ;: 190 �C). Cell operated on dry H2 D 2%

CO at atmospheric pressure.

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curves, and (c) Polarization curves when operating the cell

on H2 D CO/H2 at different water vapour pressures (-: dry;

C: 2,500 Pa; :: 20,000 Pa). Cell operated at 130 �C on dry

H2 D 2% CO at atmospheric pressure.


As shown in Section 3.1.2, the pre-humidification of the

fuel stream enhances the cell performance even in the

presence of CO. Thus, the maximum power density varies

from 34.4 mW cm�2 on dry hydrogen, up to 50.6 mW cm�2

when the fuel stream was humidified at 60 �C. These results

are in good agreement with those displayed by Modestov

et al. [35] and Jiang et al. [42]. Humidification of the fuel

stream in the presence of CO is beneficial for the fuel cell

performance. Fig. 5(b) depicts the corresponding iR-corrected

polarization curves. As it can be seen, electrodes perform

better when water vapour is introduced into the anode

stream, with most of this enhancement happens at the

anode [see Fig. 5(c)].

Nevertheless, in view of these results, it becomes complex

to discern the intrinsic positive effect of the water vapour on

the CO tolerance from the enhancement of the electrode

performance or just by the reduction of the ohmic resistance.

For this purpose, it will be better to consider the results dis-

cussed in Section 3.3, in which the CO tolerance will be

assessed in terms of comparison of the maximum power

densities operating the cell on hydrogen contaminated with

CO and on neat hydrogen.



130 150 170 1900

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150

200

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300M

ax

im

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Fig. 6 eMaximum power density drawn from the cell at the

different temperatures and water vapour partial pressures

(-: dry; C: 2,500 Pa; :: 20,000 Pa). Cell operated on dry

H2 D 2% CO at atmospheric pressure.

130 150 170 1900.0

0.2

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PH

+C

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H

Temperature / °C

Fig. 7 e Evaluation of the CO tolerance of the cell at the

different temperature and water vapour partial pressures

(-: dry; C: 2,500 Pa; :: 20,000 Pa). PH2 : maximum power

density when operating with pure H2; PH2DCO: maximum

power density when operating the cell with H2 D 2% CO.


3.2.3. Combined influence of the temperature and the relativehumidity on the cell performanceFinally, it is interesting to study the combined influence of

both operating variables on the cell performance. As in Fig. 3,

maximum power densities were used for comparison. Fig. 6

displays the corresponding results.

As it can be seen in Fig. 6, the temperature shows a constant

positive effect on the cell performance, independent of the

water vapour pressure in the anode stream, due to the above-

mentioned relieving effect on the CO adsorption on Pt.

However, according to Fig. 6, the effect seems to reinforcewhen

the cell was operated with the most humidified fuel. Under

these conditions, apart from the beneficial effects of the water

on theCO tolerance, the decrease in the ohmic resistance in the

anode also helps in strengthening the best performance at high

temperature. Likewise, the relative humidity also shows

a positive effect on the cell performance in the presence of

carbon monoxide. Notwithstanding, this effect becomes more

visible at the highest temperature, which can be explained

taking into account that at 190 �C the effects of the dehydration

becomes more dominant, so that it will be more beneficial to

operate with a humid H2 þ CO stream. Furthermore, as previ-

ously noted, the presence of the water also proves to be bene-

ficial due to the reduction of the CO partial pressure, directly

related to the degree of CO coverage [39].

In summary, when operating the cell on H2 þ CO fuel, with

significant amounts of carbonmonoxide, like in this study (2%

in vol.), it is desirable to operate the cell at high temperatures

(above 170 �C) and with a humid fuel, in order to minimize the

CO adsorption on the platinum sites and the negative effect of

the electrolyte dehydration within the anode.

3.3. CO tolerance

As it was done in previous sections, for analyzing the

combined effect of the temperature and the relative humidity

in the anode stream, the maximum power density will be

used, and in this particular case, Fig. 7 displays the ratio

between maximum power peaks when operating the cell on

CO contaminated H2 and pure H2.

Fig. 7 shows that the CO tolerance of the system signifi-

cantly increases with the temperature, and to a lower extent,

with the relative humidity. In the case of the temperatures,

this fashion clearly relies on the lower level of CO poisoning

when operating at high temperatures. In the case of the

relative humidity, cell performance increases when oper-

ating on humid fuel, particularly at low temperatures, where

the CO effects will be more stringent. Modestov et al. [35] and

Jiang et al. [42] also showed in their works, the presence of

water vapour in the H2 þ CO stream is beneficial, increasing

the performance of the cell. In agreement with them, the

presence of water vapour improves the CO tolerance in

terms of a lower degree of CO coverage, stemming from the

decrease in the CO partial pressure, and the larger coverage

of platinum sites by eOH species. Furthermore, water is an

active element required for performing the electrochemical

oxidation of CO on the platinum sites, according to the well-

known bifunctional mechanism. In order to confirm this,

Fig. 8 displays the cell performance when the system was

operated on CO/O2 at 130 �C and different water vapour

partial pressures.

Polarization curves show an improvement in the cell

performance when the cell is operated with humid CO. The

increase in the water vapour partial pressure leads to a larger

level of eOH coverage on the catalyst surface, and conse-

quently, a lower level of CO coverage [42]. A surface with this

distribution of species is expected to be more favourable for

oxidizing the CO according to the bifunctional mechanism.

Also, the OCV of the cell increases with the relative humidity,

probably due to the above-mentioned larger eOH coverage on

the platinum surface, leading to an earlier onset potential for

the CO electroxidation process, as Modestov et al. [40] re-

ported. Hence, the pre-humidification leads to a faster CO

electroxidation process, which helps to “clean” more



0 1 2 3 4 5 6

0

15

30

45

60

75

90C

ell v

oltag

e / m

V


Fig. 8 e Cell performance for a CO/O2 configuration at

130 �C for different water vapour partial pressures (-: dry;

C: 2,500 Pa; :: 20,000 Pa).


effectively the Pt surface. Notwithstanding, currents are

apparently rather poor, and might lead to think that this

electroxidation hardly participates in the increased CO toler-

ance of the system at high temperature. However, any

reduction in the CO coverage on the electrode surface will

have a positive impact on the cell performancewhen operated

with H2 þ CO, due to the significant amount of actives sites

inactivated by the carbon monoxide [39]. Hence, the use of

a humid fuel stream increases the CO tolerance in terms of

a faster CO electroxidation process.

4. Conclusions

The following conclusions can be extracted from this work:

� When operating on pure hydrogen, it is advisable to use

intermediate temperature (150e170 �C) and pre-humidify

the hydrogen stream in order to obtain the best perfor-

mance for an ABPBI-based PEMFC. Under these conditions,

the best trade-off between kinetics and ohmic resistance is

obtained. Higher temperatures lead to undesirable

phenomena of electrolyte dehydration in the anode, harm-

ing the global cell performance, even when the hydrogen

stream is humidified.

� When operating onH2þCO, high temperatures (170e190 �C)and a pre-humidified hydrogen stream are desirables for

maximizing the cell performance. The application of these

conditions seems to reduce the CO coverage on the plat-

inum surfaces, as well as accelerates the CO electroxidation

reaction. This, in combination with a less detrimental

dehydration process in the anode, leads to a larger cell

performance and enhanced CO tolerance.

� Optimum conditions for a flexible operation of the cell,

independent of the presence of moderates CO percentages

could be: (i) cell temperature of 170 �C, and (ii) pre-

humidification of the fuel stream at a water vapour partial

pressure of 20,000 Pa.

Acknowledgements

Authorswant to thank to the Fundacao de Apoio a Pesquisa do

Estado de Sao Paulo (FAPESP), to the Conselho Nacional de

Desenvolvimento Cientıfico e Tecnologico (CNPq), and to the

Coordenacao de Aperfeicoamento de Pessoal de Nıvel Supe-

rior (CAPES) for the financial support. In this sense, Jose

J. Linares thanks FAPESP for a post-doctoral fellowship awar-

ded under the reference number 2010/07108-3.

r e f e r e n c e s

[1] Li Q, Jensen JO, Savinell RF, Bjerrum NJ. High temperatureproton exchange membranes based on polybenzimidazolefor fuel cells. Prog Polym Sci 2009;34:449e77.

[2] Bose S, Kuila T, Hien Nguyen TX, Kim NH, Lau K-T, Lee JH.Polymer membranes for high temperature proton exchangemembrane fuel cell: recent advances and challenges. ProgPolym Sci 2011;36:813e43.

[3] Li Q, He R, Jensen JO, Bjerrum. PBI-based polymermembranes for high temperature fuel cells e preparation,characterization and fuel cell demonstration. Fuel Cells 2004;4:147e59.

[4] Li Q, He R, Jensen JO, Bjerrum NJ. Approaches and recentdevelopment of polymer electrolyte membranes for fuel cellsoperating above 100 �C. Chem Mater 2003;15:4896e915.

[5] Dayton DC. Fuel cell integration. A Study of the impactsof gas quality and impurities; June 2001. Milestonecompletion report for the Chemistry of BioEnergy SystemsDivision of the National BioEnergy Center. Referencenumber: NREL/MP-510e30298.

[6] Dhar HP, Christner LG, Kush AK, Maru HC. Performancestudy of a fuel cell Pt-on-C anode in presence of CO and CO2,and calculation of adsorption parameters for CO poisoning.J Electrochem Soc 1986;133:1574e82.

[7] Dhar HP, Christner LG, Kush AK. Nature of CO adsorptionduring H2 oxidation in relation to modeling for CO poisoningof a fuel cell anode. J Electrochem Soc 1987;134:3021e6.

[8] Savadogo O. Emerging membranes for electrochemicalsystems: part II. High temperature composite membranes forpolymer electrolyte fuel cell (PEFC) applications. J PowerSources 2004;127:135e61.

[9] Wainright JS, Wang J-T, Weng D, Savinell RF, Litt M. Acid-doped polybenzimidazoles: a new polymer electrolyte.J Electrochem Soc 1995;142:L121e3.

[10] Asensio JA, Gomez-Romero P. Recent developments onproton conducting poly(2,5-benzimidazole) (ABPBI)membranes for high temperature polymer electrolytemembrane fuel cells. Fuel Cells 2005;5:336e43.

[11] Litt M, Ameri R, Wang Y, Savinell R, Wainright J.Polybenzimidazoles/phosphoric acid solid polymerelectrolytes: mechanical and electrical properties. Mat ResSoc Symp Proc 1999;458:313e23.

[12] Asensio JA, Sanchez EM, Gomez-Romero P. Proton-conducting membranes based on benzimidazolepolymersfor high-temperature PEMfuelcells. A chemical quest. ChemSoc Rev 2010;39:3210e39.

[13] Kim S-K, Kim T-H, Ko T, Lee J-C. Cross-linked poly(2,5-benzimidazole) consisting of wholly aromatic groups forhigh-temperature PEM fuel cell applications. J Membr Sci2011;373:80e8.

[14] Ong A-L, Jung G-B, Wu C-C, Yan W-M. Single-step fabricationof ABPBI-based GDE and study of its MEA characteristics for




high-temperature PEM fuel cells. Int J Hydrogen Energ 2010;35:7866e73.

[15] Wippermann K, Wannek C, Oetjen H-F, Mergel J, Lehnert W.Cell resistances of poly(2,5-benzimidazole)-based hightemperature polymer membrane fuel cell membraneelectrode assemblies: time dependence and influence ofoperating parameters. J Power Sources 2010;195:2806e9.

[16] Zheng H, Petrik L, Mathe M. Preparation and characterisationof porous poly(2,5benzimidazole) (ABPBI) membranes usingsurfactants as templates for polymer electrolyte membranefuel cells. Int J Hydrogen Energy 2010;35:3745e50.

[17] Wannek C, Lehnert W, Mergel J. Membrane electrodeassemblies for high-temperature polymer electrolyte fuelcells based on poly(2,5-benzimidazole) membranes withphosphoric acid impregnation via the catalyst layers. J PowerSources 2009;192:258e66.

[18] Wannek C, Kohnen B, Oetjen H-F, Lippert H, Mergel J.Durability of ABPBI-based MEAs for high temperaturePEMFCs at different operating conditions. Fuel Cells 2008;8:87e95.

[19] Linares JJ, Sanches C, Paganin VA, Gonzalez ER. Poly(2,5-Benzimidazole) membranes: physico-chemicalcharacterization and high temperature PEMFC. Appl ECSTrans 2011;41:1579e93.

[20] Ashino NM, Camargo APM, Morgado DL, Frollini E,Gonzalez ER. Development of electrolyte membranes for fuelcells operating at intermediate temperatures (130e200�C).ECS Trans 2009;19:51e62.

[21] Lobato J, Canizares P, Rodrigo MA, Linares JJ, Pinar FJ. Studyof the influence of the amount of PBI-H3PO4 in the catalyticlayer of a high temperature PEMFC. Int J Hydrogen Energ2010;35:1347e55.

[22] Lobato J, Canizares P, Rodrigo MA, Linares JJ, Ubeda D,Pinar FJ. Study of the catalytic layer in polybenzimidazole-based high temperature PEMFC: effect of platinum contenton the carbon support. Fuel Cells 2010;10:312e9.

[23] Jespersen JL, Schaltz E, Kær SK. Electrochemicalcharacterization of a polybenzimidazole-based hightemperature proton exchange membrane unit cell. J PowerSources 2009;191:289e96.

[24] Zhang J, Tang Y, Song C, Zhang J. Polybenzimidazole-membrane-based PEM fuel cell in the temperature range of120e200 �C. J Power Sources 2007;172:163e71.

[25] Mamlouk M, Scott K. The effect of electrode parameters onperformance of a phosphoric acid-doped PBI membrane fuelcell. Int J Hydrogen Energy 2010;35:784e93.

[26] Lobato J, Canizares P, Rodrigo MA, Linares JJ. PBI-basedpolymer electrolyte membranes fuel cells. Temperatureeffects on cell performance and catalyst stability.Electrochim Acta 2007;52:3910e20.

[27] Jalani NH, Ramani M, Ohlsson K, Buelte S, Pacifico G,Pollard R, et al. Performance analysis and impedancespectral signatures of high temperature PBI-phosphoric acidgel membrane fuel cells. J Power Sources 2006;160:1096e113.

[28] Seland F, Berning T, Børresen B, Tunold R. Improving theperformance of high-temperature PEM fuel cells based on PBIelectrolyte. J Power Sources 2006;160:27e36.

[29] Liu Z, Wainright JS, Litt MH, Savinell RF. Study of the oxygenreduction reaction (ORR) at Pt interfaced with phosphoricacid doped polybenzimidazole at elevated temperature andlow relative humidity. Electrochim Acta 2006;51:3914e23.

[30] Mamlouk M, Scott K. Phosphoric acid-doped electrodes fora PBI polymer membrane fuel cell. Int J Energ Res 2011;35:507e19.

[31] MacDonald DI, Boyack JR. Density, electrical conductivity,and vapor pressure of concentrated phosphoric acid. J ChemEng 1969;14:380e4.

[32] Ma Y-L, Wainright JS, Litt MH, Savinell RF. Conductivity ofPBI membranes for high-temperature polymer electrolytefuel cells. J Electrochem Soc 2004;151:A8e16.

[33] Li Q, He R, Gao J-A, Jensen JO, Bjerrum NJ. The CO poisoningeffect in PEMFCs operational at temperatures up to 200�C.J Electrochem Soc 2003;150:A1599e605.

[34] Andreasen SJ, Vang JR, Kær SK. High temperature PEM fuelcell performance characterisation with CO and CO2 usingelectrochemical impedance spectroscopy. Int J HydrogenEnerg 2011;36:9815e30.

[35] Modestov AD, Tarasevich MR, Filimonov VY, Davydova ES.CO tolerance and CO oxidation at Pt and PteRu anodecatalysts in fuel cell with polybenzimidazole-H3PO4

membrane. Electrochim Acta 2010;55:6073e80.[36] Moser H, Perchthaler M, Hacker V, Siebenhofer M.

Experimental study of CO and temperature impact on HighTemperature Proton Exchange Membrane Fuel Cell (HT-PEMFC) performance and current distribution. AIChE annualmeeting, conference proceedings; 2009.

[37] Wang C-P, Chu H-S, Yan Y-Y, Hsueh K- L. Transientevolution of carbon monoxide poisoning effect of PBImembrane fuel cells. J Power Sources 2007;170:235e41.

[38] Krishnan P, Park J-S, Kim C-S. Performance of a poly(2,5-benzimidazole) membrane based high temperature PEM fuelcell in the presence of carbon monoxide. J Power Sources2006;159:817e23.

[39] Kohlmayr G, Stonehart P. Adsorption kinetics for carbonmonoxide on platinum in hot phosphoric acid. ElectrochimActa 1973;18:211e23.

[40] Modestov AD, Tarasevich MR, Leykin AY. COelectrooxidation study on Pt and PteRu in H3PO4 using MEAwith PBI-H3PO4 membrane. J Power Sources 2011;196:2994e3002.

[41] Lopes PP, Freitas KS, Ticianelli EA. CO tolerance of PEMFCanodes: mechanisms and electrode designs. Electrocatal2010;1:200e12.

[42] Jiang R, Russell Kunz H, Fenton JM. Influence of temperatureand relative humidity on performance and CO tolerance ofPEM fuel cells with Nafion�-Teflon�-Zr(HPO4)2 highertemperature composite membranes. Electrochim Acta 2006;51:5596e605.



performance of a poly(2,5-benzimidazole)-based polymer electrolyte membrane fuel cell

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