Energy & Fuels 2009, 23, 397–402
397
Water Transport through a Proton-Exchange Membrane (PEM)
Fuel Cell Operating near Ambient Conditions: Experimental and
Modeling Studies
D. S. Falcão,† C. M. Rangel,‡ C. Pinho,† and A. M. F. R. Pinto*,†
Departamento de Engenharia Quı́mica, Centro de Estudos de Fenómenos de Transporte, Faculdade de
Engenharia da UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal, and Unidade de
Electroquı́mica de Materiais, Instituto Nacional de Engenharia, Tecnologia e InoVação (INETI), Paço do
Lumiar, 22 1649-038 Lisboa, Portugal
ReceiVed June 22, 2008. ReVised Manuscript ReceiVed October 7, 2008
In the present work, an experimental study on the performance of an “in-house”-developed proton-exchange
membrane (PEM) fuel cell with 25 cm2 of active membrane area is described. The membrane/electrodes assembly
(MEA), from Paxitech, has seven layers [membrane/catalyst layers/gas diffusion layers (GDLs)/gaskets]. The
catalytic layers have a load of 70% Pt/C and 0.5 mg of Pt/cm2 on both sides, and the membrane is made of
Nafion 112. A multiserpentine configuration for the anode and cathode flow channels is used. Experiments
were carried out under different anode and cathode relative humidities (RHs) and flow rates. Predictions from
a previously developed one-dimensional model, coupling mass- and heat-transfer effects, are compared to
experimental polarization curves. The influence of the anode and cathode relative humidification level on the
cell performance is explained under the light of the predicted water content across the membrane. Under the
operating conditions studied, the net water flux of water is toward the anode. Accordingly, the influence of
the anode humidification is not significant, and the influence of the cathode humidification has a high impact
in fuel cell performance. Results show that fuel cell performance is better for experiments where higher water
content values were obtained. In comparison to the anode feed flow rate influence, the influence of the cathode
feed flow rate has a major impact in fuel cell performance.
1. Introduction
Fuel cells are an innovative alternative to current power
sources, with potential to achieve higher efficiencies with
renewable fuels with minimal environmental impact. In particular, the proton-exchange membrane (PEM) fuel cells (FCs)
are today in the focus of interest as one of the most promising
developments in power generation, with a wide range of
applications in transportation and portable electronics. Although
prototypes of fuel cell vehicles and residential fuel cell systems
have already been introduced, their cost must be reduced and
their efficiencies enhanced.
Several coupled fluid flow, heat and mass transport processes
occur in a fuel cell in conjunction with the electrochemical
reactions. Generally, PEMFCs operate bellow 80 °C. Anodic
oxidation of hydrogen produces protons that are transported
through the membrane to the cathode where the reduction of
oxygen generates water. One of the most important operational
issues of PEMFCs is the water management in the cell.1,2
To achieve optimal fuel cell performance, it is critical to have
an adequate water balance to ensure that the membrane remains
* To whom correspondence should be addressed. Telephone:
+351225081675. E-mail: [email protected].
† Faculdade de Engenharia da Universidade do Porto.
‡ Instituto Nacional de Engenharia, Tecnologia e Inovação (INETI).
(1) Eikerling, M.; Kharkats, Yu. I.; Kornyshev., A. A.; Volfkovrch, Yu.
M. Phenomenological theory of electro-osmotic effect and water management in polymer electrolyte proton-conducting membranes. J. Electrochem.
Soc. 1998, 145, 2684–2699.
(2) Eikerling, M.; Kornyshev, A. A.; Kucerhak, A. R. Water in polymer
electrolyte fuel cells: Friend or foe? Phys. Today 2006, 59, 38.
hydrated for sufficient proton conductivity, while cathode
flooding and anode dehydration are avoided.3-5
The water content of the membrane is determined by the
balance between water production and three water-transport
processes: electro-osmotic drag of water (EOD), associated with
proton migration through the membrane, back diffusion from
the cathode, and diffusion of water to/from the oxidant/fuel gas
streams. Understanding the water transport in the PEM is a guide
for materials optimization and development of new membrane/
electrodes assemblies (MEAs).
Recent studies6 reported the influence of various operating
conditions on fuel cell performance, such as temperatures,
pressures, and humidity of reactant gases. On the basis of these
investigations, the optimum conditions are operation at higher
pressure and elevated temperature with the humidified reactant
gases. Yan et al.7 also studied the influence of various operating
conditions, including the cathode flow rate, cathode inlet
humidification temperature, and cell temperature on the per(3) Baschuk, J. J.; Li, X. Modeling of polymer electrolyte membrane
fuel cells with variable degrees of water flooding. J. Power Sources 2000,
86, 181–195.
(4) Biyikoglu, A. Review of proton exchange fuel cell models. Int. J.
Hydrogen Energy 2005, 30, 1185–1212.
(5) Chang, H.; Kim, J. R.; Cho, S. Y.; Kim, H. K.; Choi, K. H. Materials
and processes for small fuel cells. Solid State Ionics 2002, 8312.
(6) Amirinejad, M.; Rowshanzamir, S.; Eikani, M. H. Effects of
operating parameters on performance of a proton exchange membrane fuel
cell. J. Power Sources 2006, 161 (2), 872–875.
(7) Yan, W. M.; Chen, C. Y.; Mei, S. C.; Soong, C. Y.; Chen, F. Effects
of operating conditions on cell performance of PEM fuel cells with
conventional or interdigitated flow field. J. Power Sources 2006, 162 (2),
1157–1164.
10.1021/ef8004948 CCC: $40.75  2009 American Chemical Society
Published on Web 12/09/2008
398 Energy & Fuels, Vol. 23, 2009
Falcão et al.
Figure 1. Schematic representation of the experimental setup.
formance of a PEMFC. Experimental results showed that cell
performance is enhanced with increases in cathode inlet gas
flow rate, cathode humidification temperature, and cell temperature.
There are few studies on PEMFCs with a multiserpentine
flow channel configuration. Li et al.8 indicated that this design
ensures adequate water removal by the gas flow through the
channel and no stagnant area formation at the cathode surface
as a result of water accumulation. Watkins et al.9 reported that,
under the same experimental conditions, the output power from
the cell could be increased by almost 50% with this type of
flow-field plate.
In this work, the effect of cathode and anode flow rates and
relative humidity on the performance and power of a PEMFC
with multiserpentine channels is studied and some results are
explained and compared to the predictions of a recently
developed 1D model.10
2. Experimental Section
2.1. Apparatus. A schematic drawing of the experimental
apparatus used in this work is shown in Figure 1. Pure hydrogen
(humidified or dry) as fuel and air (humidified or dry) as an oxidant
are used. The pressure of the gases is controlled by pressure
regulators (air, Norgreen 11400; H2, Europneumaq mod. 44-2262241) and flow rates controlled by flow meters (KDG, Mobrey).
The reactants humidity and temperatures are monitored by adequate
humidity and temperature probes (air, Testo; H2, Vaisala).
(8) Li, X.; Sabir, I. Review of bipolar plates in PEM fuel cells: Flowfield designs. Int. J. Hydrogen Energy 2005, 30, 359–371.
(9) Watkins, D. S.; Dircks, K. W.; Epp, D. G. U.S. Patent 5,108,849,
1992.
(10) Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.; Pinho, C.; Pinto,
A. M. F. R. Water transport through a PEM fuel cell: A one-dimensional
model with heat-transfer effects. Chem. Eng. Sci., manuscript submitted.
Figure 2. Flow channel configuration and dimensions.
The humidification of air and hydrogen gases is conducted in
Erlenmeyer flasks by a simple bubbling process. To control the
humidification temperature, each Erlenmeyer flask is thermally
isolated and surrounded with an electrical resistance (50 W/m)
activated by a Osaka OK 31 digital temperature controller. The
same procedure is applied along the connecting pipes from the
humidification point up to the entrance of the fuel cells to guarantee
the temperature stabilization of each reacting gas flow as well as
to control the operating temperature of the fuel cell.
For the measurement and control of the cell electrical output,
an electric load reference LD300 300W DC electronic load from
TTI is used. This device could work with five different operating
modes: (1) constant current, two possibilities were available, 0-8
A (with 1 mA resolution) and 0-80 A (10 mA resolution), with a
precision of (0.2% + 20 mA; (2) constant voltage, two possibilities
were available, Vmin up to 8 V (1 mA resolution) and Vmin up to 80
Water Transport through a PEM Fuel Cell
Energy & Fuels, Vol. 23, 2009 399
Figure 5. Membrane temperature versus current density for different
cell temperatures, model predictions.
Figure 3. Voltage versus current density for the base condition,
experimental results and model predictions.
Table 1. Set of Conditions Used in This Work
cell temperature (K)
anode flow temperature (K)
anode relative humidity (%)
cathode flow temperature (K)
cathode relative humidity (%)
anode pressure (atm)
cathode pressure (atm)
anode flow rate (slpm)
cathode flow rate (slpm)
298
313
70
313
70
1.2
2
0.15
0.7
Table 2. Inlet Water Concentrations
experience
cathode inlet water
anode inlet water
concentration (mol/cm3) concentration (mol/cm3)
1 (base condition)
2 (anode T ) 298 K;
RH ) 76%)
3 (anode T ) 298 K;
RH ) 7%)
2.0 × 10-6
9.8 × 10-7
9.0 × 10-8
2.0 ×10-6
Table 3. Inlet Water Concentrations
experience
1 (base condition)
2 (cathode T ) 298 K;
RH ) 94%)
3 (cathode T ) 298 K;
RH ) 1%)
cathode inlet water
concentration (mol/cm3)
2.0 × 10-6
1.2 × 10-6
1.3 × 10-8
anode inlet water
concentration
(mol/cm3)
2.0 ×10-6
to 40 A/V (resolution of 0.01 A/V), with a precision of 0.5% + 2
digits; and (5) constant resistance, operating range from 0.04 up to
10 Ω (0.01 Ω resolution) and from 2 to 40 Ω (with 0.1 Ω
resolution), with a precision of 0.5% + 2 digits.
This load was connected to a data acquisition system composed
by Measurement Computing boards installed in a desktop computer.
The used data acquisition software was DASYLab.
2.2. Fuel Cell Design. In the present work, all of the components
of the PEMFC were “in house”-designed, with exception of the
MEA. A Paxitech seven-layer MEA (Nafion 112) with 25 cm2
active surface area is used. The channel configuration used for the
anode and cathode flow channels is represented in Figure 2.
The channel depth is 0.6 mm for the hydrogen flow and 1.5 mm
for the air flow.
2.3. Experimental Conditions. In this work, a set of conditions
was used as the base condition. Using this set of conditions and
changing one variable, it is possible to evaluate the influence of
this parameter on the cell temperature. The studied operating
conditions were the cell temperature, anode humidification, cathode
humidification, anode flow rate, and cathode flow rate. The base
conditions are summarized in Table 1.
Two experiments were performed at two different cell temperatures, 298 and 313 K. To study the influence of the anode/cathode
humidification, dry hydrogen/air was introduced (to achieve lower
humidity levels) and hydrogen/air was introduced at room temperature (to achieve intermediate humidity levels). In another set of
experiments, the anode and cathode flow rates were set to double
the base values. For each one of the studied conditions, the inlet
water concentration at both sides of the cell was accurately
determined using the relative humidity and inlet temperature values.
3. Results and Discussion
V [10 mA resolution (where Vmin is 10 mV for a low-power situation
and 2 V for 80 A), with a precision of (0.2% + 2 digits; (3)
constant power, the available power range goes from 0 to 320 W,
with a precision of 0.5% + 2 W; (4) constant conductance, operating
range from 0.01 up to 1 A/V (1 A/V resolution) and from 0.2 up
In a previous work, Falcão et al.10 developed a semi-analytical
one-dimensional model considering the effects of coupled heat
and mass transfer, along with the electrochemical reactions
occurring in a PEMFC. The model can be used to predict the
Figure 4. (a) Voltage versus current density and (b) power density versus current density for two different cell temperatures.
400 Energy & Fuels, Vol. 23, 2009
Falcão et al.
Figure 6. (a) Voltage versus current density and (b) power density versus current density for different anode humidifications.
Figure 7. Water content (λ) along the membrane for different values
of anode humidification (current density of 0.1 A/cm2), model
predictions.
Figure 9. Water content (λ) along the membrane for different values
of cathode humidification (current density of 0.1 A/cm2), model
predictions.
hydrogen, oxygen, and water concentration profiles in the anode,
cathode, and membrane as well as to estimate membrane water
contents and the temperature profile across the cell.
In this work, the developed model is used to predict the
polarization curve for the base-operating conditions (Table 1).
The model predictions and experimental results are compared
in Figure 3. For low current densities, the model predicts very
well the experimental results. For higher densities, the model
predictions are higher than experimental results. This discrepancy is a common feature of single-phase models because the
effect of reduced oxygen transport because of water flooding
at the cathode at high current density is not accounted for.
Model predictions are also useful to better understand
experimental results. The membrane water content is a good
indicator of membrane humidification and is easily calculated
using this simple one-dimensional model. In this work, model
predictions of the membrane water content and temperatures
are used to explain some experimental results.
3.1. Fuel Cell Temperature. In Figure 4, the polarization
and power curves obtained in two experiments with different
cell temperatures (298 and 313 K) are presented.
In this range of low cell temperature operation, the influence
on fuel cell performance is minimal. This range of temperatures
was selected, bearing in mind the portable applications (excluding the use of heating equipment).
In Figure 5, the predicted variation of membrane temperature
with current density is presented for both fuel cell temperatures.
As expected, the cell temperature increases with current
density because of the cathode exothermic reaction (higher
currents correspond to higher amounts of heat released).
Although the cell temperature is different for the two experiments (15 K variation), the temperature profile through the
membrane is quite similar (differences of 3 K). According to
Figure 8. (a) Voltage versus current density and (b) power density versus current density for different values of cathode humidification.
Water Transport through a PEM Fuel Cell
Energy & Fuels, Vol. 23, 2009 401
Figure 10. (a) Voltage versus current density and (b) power density versus current density for different anode feed flow rates.
Figure 11. (a) Voltage versus current density and (b) power density versus current density for different cathode feed flow rates.
these results, it is predictable that the two conditions lead to
similar fuel cell performances, as shown in Figure 4.
3.2. Anode Humidification. Experiments with different
anode relative humidification levels were performed (Table 2).
The corresponding polarization and power curves are plotted
in Figure 6.
For the used MEA, the manufacturer indicates that there is
no need to humidify the anode stream. As is evident from the
plots of Figure 6, the influence of the anode humidification on
the performance of the cell is not significant. These results are
in agreement with the MEA manufacturer specifications. Such
cell behavior could be useful for portable applications because
the use of a humidifier for the anode stream could be avoided.
The model predictions of the water content in the membrane
trough parameter λ (the ratio of the number of water molecules
to the number of charged SO3-H+ sites) are presented in Figure
7 for the same three experiences.
For the conditions studied, the net flow of water is toward
the anode. For these conditions, the amount of water is higher
for the cathode side because of the importance of water transport
by electro-osmotic drag and water generation by the reaction.
As can also be seen from the plots, the water content near the
anode catalyst layer is lower for the three curves, in particular
for the less humidified anode. These results are in accordance
with experiments. The water content is similar, and consequently, the cell performance is similar too.
3.3. Cathode Humidification. To analyze the cathode humidification influence, two experiments were performed and
compared to the results obtained with the base condition (Figure
8). The different values of the inlet water concentrations
determined are presented in Table 3.
In contrast to the case analyzed previously, the cathode
humidification level has a significant impact on the fuel cell
performance. As indicated above, the water management is a
critical issue. Water acts like a proton shuttle in the membrane
and catalyst layers because excessive water amounts filling the
pores inhibit the access to active sites and block the transport
of gaseous reactants and products. On the contrary, dehydration
of anodic regions because of electro-osmotic drag can cause a
breakdown of proton conductivity and even a structural degradation of the PEM. The membrane must therefore have an
ideal humidification level to achieve optimal performances. The
plot of the predicted values of the water content across the
membrane for the same three experiences (corresponding to a
current density of 0.1 A/cm2) is shown in Figure 9 and
contributes to a better explanation of the results shown in
Figure 8.
These values are in agreement with experiments since the
intermediate water content value (experiment 2) leads to the
best performance (Figure 8). For this condition, the mean water
content through the cell is higher corresponding to an enhanced
proton conductivity and consequently a better performance. No
predicted anode dehydration occurs for all of the studied
conditions.
3.4. Anode Feed Flow Rate Influence. For the base operation condition, the hydrogen flow rate used corresponds to a
stoichiometric ratio of 1, at 1 A/cm2 (a hydrogen flow rate
sufficient even for current densities up to 25 A). An experiment
with a stoichiometric ratio of 2 was also performed. The results
for both conditions are represented in Figure 10.
As shown in Figure 10, the hydrogen flow rate increase has
no significant influence on the fuel cell performance. These
results are expected since, for both conditions, the hydrogen
flow rate is largely in excess, even for high values of the current
density.
3.4. Cathode Feed Flow Rate Influence. For the base
condition, the air flow rate corresponds to a stoichiometric ratio
of 3, at 1 A/cm2. An experiment with a cathode stoichiometric
ratio of 6 was performed to check the influence of increasing
the air flow rate on the cell performance. The results are
presented in Figure 11.
402 Energy & Fuels, Vol. 23, 2009
As expected, the air flow rate increase improves fuel cell
performance probably to an enhanced water removal. The
improvement in the fuel cell performance is more significant
for higher current densities because of the more pronounced
formation of water at these conditions. Consequently, it is
advantageous to work with higher air flow rates when using
humidified cathode feeds, namely, for high current densities.
4. Conclusions
In the present study, an experimental study on the performance of an “in-house”-developed PEM fuel cell with 25 cm2
of active membrane area is described. A multiserpentine
configuration for the anode and cathode flow channels was used.
Experiments were carried out under different anode and cathode
RHs and flow rates. The influence of the anode and cathode
relative humidification level on the cell performance is explained
under the light of the predictions of water content across the
membrane from a recently developed model. Under the operating conditions studied, the net water flux of water is toward
Falcão et al.
the anode and, accordingly, the influence of the anode humidification is not significant. These results are in accordance with
the specifications of the manufacturer. The cathode humidification has a more important impact on the cell performance
probably because of a more significant effect on the proton
conductivity. An enhanced performance was obtained for the
condition where a higher water content was obtained probably
because of a better proton conductivity.
The influence of the anode and cathode feed flow rates was
also studied.
This work is the starting point for a more detailed study,
aiming at the setup of optimized and tailored MEAs adequate
for different applications (namely, low-humidity operation).
Acknowledgment. The partial support of “Fundação para a
Ciência e TecnologiasPortugal” through project POCI/EME/55497/
2004 is gratefully acknowledged. POCTI (FEDER) also supported
this work via CEFT.
EF8004948