Performance Investigation of Membrane Electrode Assemblies for High Temperature Proton Exchange Membrane Fuel Cell

doi:10.4236/jpee.2013.15016

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Journal of Power and Energy Engineering, 2013, 1, 95-100

http://dx.doi.org/10.4236/jpee.2013.15016 Published Online October 2013 (http://www.scirp.org/journal/jpee)

Performance Investi gation of Membrane Elec trode

Assemblies for High Tem perature Proton Exchange

Membrane Fu el C el l

Huaneng Su, Sivakumar Pasupathi, Bernard Bladergroen, Vladimir Linkov, Bruno G. Pollet

HySA Systems Competence Centre, South African Institute for Advanced Materials Chemistry, University of the Western Cape,

Private Bag X17, Bellville 7535, South Africa.

Email: [email protected]

Received August 2013

ABSTRACT

Different types of ABPBI (poly(2,5-benzimidazole)) membranes and polymer binders were evaluated to investigate the

performance of MEAs for high temperature proton exchange membrane fuel cell (HT-PEMFC). The properties of the

prepared MEAs were evaluated and analyzed by polarization curve, electrochemistry impedance spectroscopy (EIS),

cyclic voltammetry (CV) and durability test. Th e results showed that MEA with modified ABPBI membrane (AM) has

satisfactory performance and durability for fuel cell application. Compare to conventional PBI or Nafion binders, poly-

tetrafluoroethylene (PTFE) and polyvinylidene difluoride (PVDF) are more attractive as binders in the catalyst layer

(CL) of gas diffusion electrode (GDE) for HT-PEMFC.

Keywords: High Temperature Proton Exchange Membrane Fuel Cell; ABPBI (Poly(2,5-Benzimidazole)); Polymer

Binders; Gas Diffusion Electrode; Membrane Electrode Assembly

1. Introduction

Polybenzimidazole (PBI) based high temperature proton

exchange membrane fuel cells (HT-PEMFCs) have at-

tracted more and more attention in these years due to

their advantages over low temperature PEMFCs based on

perfluorosulphonic acid polymer electrolytes ( e.g. Nafion)

[1]. However, the sluggish kinetics of the oxygen reduc-

tion reaction (ORR) [2] and the transport limitations of

protons and reactants in cathode, especially in the pres-

ence of phosphoric acid (PA), limit the cell performance

of the high temperature PEMFC. Therefore, enhancing

the cell performance is one of the most important issues

for high temperature PEMFC being more widely consi-

dered as an alternative to the low temperature PEMFC

systems.

Membrane electrode assembly (MEA) is the most im-

portant component in high temperature PEMFC system

and it plays a major role in determining cell performance.

It consists of a proton exchange membrane sandwiched

between two gas diffusion electrodes (GDEs), which pos-

sess a porous structure that allows easy transport of reac-

tant gases and water to and from the catalytically active

zone. Therefore, the components of MEA, i.e. electrolyte

membrane and GDEs, have significant influence on the

performance of high temperature PEMFC.

Several types of electrolyte membrane, such as poly

[2,2’-(m-phenylene)-5,5’-bibenzimidazole (PBI or mPBI),

poly(2,5-benzimidazole) (ABPBI) and their derivatives

[3]; several types of polymer ionomers, such as PBI [4-6],

polytetrafluoroethylene (PTFE) [7,8], polyvinylidene dif-

luoride (PVDF) [9,10] and Nafion [11,12], can be used in

the MEA as proton exchange membranes and catalyst

layer (CL) binders for high temperature PEMFC. The phy-

sicochemical properties of these membranes and binders

are different from each other; consequently the resultant

MEAs have their own advantages and shortcomings. In

this work, the MEAs with four types of ABPBI mem-

branes and four types of binders were investigated to

evaluate their fuel cell performances. The properties of

these MEA were characterized by single cell polarization,

electrochemical analysis and dur ability test.

2. Experimental

2.1. Preparation of GDEs

Hispec 4000 Pt/C catalyst (40 wt% Pt, Johnson Matthey)

was used in this study. All GDEs were prepared by our

newly developed spraying method [13]. The catalyst powd-

ers were deposited onto the microporous layer of com-

mercially available GDL (Freudenberg, Germany). The

Performance Investigation of Membrane Electrode Assemblies for High Temperature Proton Exchange Membrane Fuel Cell

catalyst loadings were calculated by weighing the GDEs

before applying the catalyst inks, and then after applica-

tion and oven drying for overnight. The platinum load-

ings of all GDEs (both anode and cathode) used for this

study are 0.5 mg∙cm−2, unless otherwise stated.

2.2. Preparation of MEAs

The membranes used in this study are ABPBI (denoted

as AM, AM 55, AMcl and APcl), which were supplied

by FuMA-Tech. The properties (thickness, composition,

process method etc.) of these membranes vary from each

other. For doping with PA, the membranes were im-

mersed in 85% acid solution for certain time at 100˚C

until their acid doping level of about 3.7 molecules of

H3PO4 per polymer repeating unit (PRU) were obtained.

Before being used, the membrane was taken from the PA

bath, and the superficial acid onto the membrane was

thoroughly wiped off with lab tissue. To ge the r w ith gaskets

made of fluorinated polymer, the MEA was assembled

by sandwiching the doped membrane between two GDEs

impregnated with PA in a single cell fixture (BalticFuel-

Cells GmbH, Germany) without a preceding hot-pressing

step. The active areas of all MEAs are 5 cm2, unless oth-

erwise stated.

2.3. Single Cell Test and Electrochemical

Characteriz ation

The cells were operated at 150˚C (unless otherwise stated)

and 2 N∙mm−2 piston pressure in a FuelCon Evaluator C

test station (FuelCon, Germany). Pure hydrogen was fed

to the anode and air to the cathode respectively, with

flow rates (unless otherwise stated) of 100 ml∙min−1 (hy-

drogen) and 250 ml∙min−1 (air), at ambient pressure. Both

hydrogen and air were used as dry gases, directly from

the compressed bottles without external humidification.

Electrochemical impedance spectroscopy (EIS) and cyc-

lic voltammetry (CV) were performed using an Autolab

PGSTAT 30 Potentiostat/Galvanostat (Metrohm). EIS

measurements were carried out at a cell voltage of 0.6 V

with amplitude of 5 mV, and in the frequency range of

100 mHz to 20 kHz. The impedance data were obtained

by calculation and simulation with Autolab Nova soft-

ware. Voltammetric measurements, undertaken to study

the electrochemical active surface area (EASA), were con-

ducted using dry N2 at the cathode (working electrode)

and dry H2 at the anode (counter electrode and reference

electrode) at cell working temperature. Cyclic voltam-

mograms were recorded from 1.2 V to 0.05 V at a scan

rate of 0.05 V∙s−1.

3. Results and Discussion

3.1. Effect of Electrolyte Membrane

Figure 1 shows the FT-IR spectra of the four ABPBI

membranes. The characteristic peaks of PBI are shown in

all membranes; hydrogen-bonded N-H stretching at 3184

cm−1, the free non-hydrogen-bonded N-H stretching at

3415 cm−1, and C=C and C=N stretching bands for ben-

zimidazole group at 1612, 1590, and 1443 cm−1 can be

observed from all the membrane samples [14], indicating

that no significant structural changes took place in the

bulk polymer of the above ABPBI membranes. However,

it should be noted that sharp new peaks in the zone

ranged between 800 and 1300 cm−1 are observed for the

three modified ABPBI membranes (AM, AM 55 and

AMcl), which are indicatives of interactions between the

additives and ABPBI groups [11]. Interactions between

the additives and ABPBI groups should increase the sta-

bility of the modified ABPBI membranes, as demon-

strated in the durability test later in current study (see

Section 3.3).

The single cell performance comparison of the MEA

with AM, AM 55, AMcl and APcl are shown in Figure 2.

It should be noted that the polymer binders used in these

MEAs are PVDF (the binder content in CL has been pre-

optimized for this comparison), which is the preferred

4000 3000 2000 10000

Intensity / a.u.

W avenum b er / cm

− 1

AM5 5

AMc l

APcl

Figure 1. FT-IR spectra of various ABPBI membranes.

0.0 0.4 0.8 1.2 1.6

0.2

0.4

0.6

0.8

1.0

AM 55

AMcl

APcl

Current density

/ A cm

− 2

Cell voltage / V

0.0

0.1

0.2

0.3

0.4

ower density / W cm

− 2

Figure 2. Polarization curves and power density curves of

HTPEMFC with MEAs using different ABPBI membranes,

operated at atmosphere pressure and 150˚C.

Performance Investigation of Membrane Electrode Assemblies for High Temperature Proton Exchange Membrane Fuel Cell

binder pre-optimized in Section 3.2. It is clear that the

MEA prepared by AM membrane yields much better per-

formance than the MEA prepared with other membranes.

At a working voltage of 0.6 V, the current density of the

MEA with AM membrane reaches 0.208 A∙cm−2, the

maximum power density of the MEA with GDE-1 can

reach 0.363 W∙cm−2 at 0.313 V.

To understand the performance difference of the MEAs

prepared with these membranes, overall analysis on the

polarization curves and electrochemical measurements

on both GDEs are performed. Generally, the po larization

curve of a PEMFC could be divided into three segments

(corresponding to different electrochemical processes) ac-

cording to its different voltage drop rates. The initial

drop of the curve at a very low current density is due to

the sluggish kinetics of oxygen reduction at the cathode,

determined by the nature of the electrodes. It can be seen

in Figure 2 that all MEAs show similar voltage drop in

this region (0 - 0.1 A cm–2), which is reasonable consi-

dering the GDEs used in all MEAs were the same. To

prove this point, CV measurements are performed to

study the EASAs of the all MEAs, as shown in Figure 3

(only H2 desorption peaks were showed for clarity).

The corresponding EASAs were calculated from the

H2 desorption peak of each voltammogram and the re-

sults are summarized in Table 1. It can be seen that the

EASA values of all MEAs are between 17.4 and 22.3

m2∙g−1, only small differences were detected among these

MEAs. The EASA results are certainly consistent with

their performances (activation polarization zone) pre-

sented in Figure 2.

Figure 3. Cyclic voltammograms of the MEAs with differ-

ent ABPBI membranes.

Table 1. Electrochemical characteristics of the MEAs with

different ABPBI membranes.

MEA type RΩ (Ω) Rct (Ω) EASA (m2∙g−1)

AM 0.05 0.07 22.3

AM 55 0.12 0.13 17.4

AMcl 0.13 0.11 18.7

APcl 0.12 0.13 19.1

The subsequent drop in the polarization curve is as-

cribed to ohmic loss, which originates from ionic flow

through the electrolyte membrane, and from electron

flow through the electrode layers, current collectors and

flow field plates. As shown in Figure 2, the four MEAs

present much different decreasing slopes in the linear

region, implying that they had different ohmic resis-

tances. To verify the resistances of the single cells with

these MEAs, in situ impedance measurements are per-

formed at the cell voltage of 0.6 V, as shown in Figure 4.

Only one semicircular loop can be observed in the Ny-

quist plot as the electrode process is dominated by ORR

[15]. Through simulation with a simple RC equivalent

circuit, their corresponding cell resistances (RΩ) and charge

transfer resistances (Rct) can be calculated, which are also

presented in Table 1. It can be seen that there is no sig-

nificant difference in cell ohmic resistance for the MEAs

with the last three membranes (i.e. AM 55, AMcl and

APcl), which is consistent with the similar decreasing

slopes in the linear regions of their polarization curves

presented in Figure 2. However the ohmic resistance of

the MEA with AM membrane is only 0.05 Ω, less than

half of the other membranes. It suggests that AM mem-

brane has higher proton conductivity than other mem-

branes because all other parts in the test fixture during

testing were identical. Moreover, the charge transfer re-

sistance of the AM MEA is much smaller than those of

the other three, which suggests that AM MEA yielded a

more efficient electrochemical active layer, which means

that the interactions between the GDE and the AM mem-

brane are more efficient.

The last voltage drop at high current density is due to

mass transport limitations o ccurring in the electrodes and

the membrane. However, from Figure 2 it can be seen

that, for all MEAs, the vo ltage drop rates in low cell vo l-

tage region (<0.4 V) of their polarization curves are al-

most the same with that in their linear regions, which

means that no obvious mass transfer limitations in these

Figure 4. In situ impedance curves of the MEAs with dif-

ferent membranes, at a cell voltage of 0.6 V.

0.05 0.10 0.15 0.20 0.25

-0.02

0.00

0.02

0.04

0.06

AM 55

AMcl

APcl

−

Z' ' / Ω

Z' / Ω

Performance Investigation of Membrane Electrode Assemblies for High Temperature Proton Exchange Membrane Fuel Cell

MEAs even at the high current densities. It is unders-

tandable when considering the high operating tempera-

ture (150˚C, only water vapor existed in the GDEs) and

the high stoichiometries reactants (ca. 7/7.5 for H2/A ir at

0.4 A∙cm–2) due to the small active area (~5 cm2) even at

low gases flow rates [16].

From all these analyses and electrochemical results, it

can be concluded that the good performance of the MEA

with AM membrane is primarily attributable to the high-

er proton conductivity and good interactivity of the elec-

trolyte membrane and the GDEs, which makes a more

efficient electrochemical active layer, accordingly the

minor ohmic resistance and charge transfer resistance.

For these reasons, the AM membrane, which performed

best at usual working voltage of 0.6 V and maximum

power density, is chose for all subsequent studies.

3.2. Effect of Polymer Binders

Figure 5 shows the performance comparison of the MEA

with the four different polymer binders. It is clear that the

PTFE and PVDF GDEs yield much better performance

than the GDEs prepared with PBI and Nafion binders in

all regions of the polarization curve. At a working vol-

tage of 0.6 V, the current density of the MEA with PVDF

GDEs reaches 0.53 A∙cm−2, 121% higher than that (0.24

A∙cm−2) of the MEA with PBI GDEs. The maximum

power density of the MEA with PTFE GDEs can reach

0.61 W∙cm−2 at 0.35 V. These values are almost the best

results yet reported for similar PA-doped PBI fuel cell

and operated using air, which are comparable to the per-

formances of the commercial MEAs with high Pt load-

ings [17]. This is mainly attribut able to the properties of

PTFE and PVDF binders that they exist in the CLs as a

fiber phase, which makes catalyst particles less likely to

be encapsulated in the binder, then making more Pt sur-

face available in the CLs. On the contrary, PBI and Na-

fion polymer ionomers are easily covered on the surface

of the catalyst particles, which could impose mass trans-

port limitation in CLs due to the low gas permeability of

these films formed on the catalyst sites [15].

3.3. Durability

From above physical characterizations and electrochem-

ical analysis results, it can be concluded that good MEA

performance (at usual working voltage of 0.6 V) can be

delivered by using PVDF as CL binder and AM mem-

brane as electrolyte membrane. However, the stability or

durability of this MEA is also a major concern for the

real application and commercialization of HT-PEMFC.

The remarkable long term stability of PA-doped PBI

MEA is achieved in some research groups’ works [9,

10,18,19]. To verify the stabilities of the MEA with AM

membrane and PVDF binder, a short term durability test

was performed, as shown in Figure 6.

0.0 0.5 1.0 1.5 2.0 2.5

0. 2

0. 4

0. 6

0. 8

1. 0

PVDF

PTFE

PBI

Nafion

Cell voltage / V

Current density / A cm

− 2

Figure 5. Polarization curves of PA-doped ABPBI (AM

membrane) fuel cell using GDEs prepared with different

polymer binders, operated with flow rates of 200 ml∙min−1

(hydrogen) and 1000 ml∙min−1 (air), at ambient pressure

and 160˚C.

Figure 6. The durability test of the MEA with AM mem-

brane and PVDF binder.

It can be seen that the cell voltage of the MEA remains

at ~0.56 V without obvious drop after almost 1000 h op-

eration. The degradation rate calculated by linear fitting

of cell voltage data points after the MEA activation is

about 56.8 μV∙h–1, which are acceptable for most appli-

cations [12,18-20].

4. Conclusion

Four types of ABPBI membranes and four types of po-

lymer binders were evaluated to investigate the perfor-

mance of MEAs for high temperature proton exchange

membrane fuel cell (HT-PEMFC). The results showed

that MEA with modified ABPBI membrane (AM) as

electrolyte and PVDF as CL binder has satisfactory per-

formance and durability for fu e l cell ap plication. At usual

working voltage of 0.6 V and cell temperature of 150˚C,

the peak power density reached 0.363 W∙cm−2, and the

current density at usual working voltage 0.6 V was up to

Performance Investigation of Membrane Electrode Assemblies for High Temperature Proton Exchange Membrane Fuel Cell

0.208 A∙cm−2, which are comparable to the results yet

reported for similar MEAs with Pt loading of ~0.5

mg∙cm−2. The MEA showed good durability in a short

term operation: the cell voltage remained at ~0.56 V

without obvious drop after almost 1000 h operation.

5. Acknowledgements

This work is supported by Hydrogen and Fuel Cell

Technologies RDI Programme (HySA), funded by the

Dep artment of Science and Technology in South Africa

(project KP 1-S01).

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