Water Transport through a Proton-Exchange Membrane (PEM)Fuel Cell Operating near Ambient Conditions: Experimental and

Modeling Studies

D. S. Falcao,† C. M. Rangel,‡ C. Pinho,† and A. M. F. R. Pinto*,†

Departamento de Engenharia Quımica, Centro de Estudos de Fenomenos de Transporte, Faculdade deEngenharia da UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal, and Unidade deElectroquımica de Materiais, Instituto Nacional de Engenharia, Tecnologia e InoVacao (INETI), Paco 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-exchangemembrane (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]. Thecatalytic layers have a load of 70% Pt/C and 0.5 mg of Pt/cm2 on both sides, and the membrane is made ofNafion 112. A multiserpentine configuration for the anode and cathode flow channels is used. Experimentswere carried out under different anode and cathode relative humidities (RHs) and flow rates. Predictions froma previously developed one-dimensional model, coupling mass- and heat-transfer effects, are compared toexperimental polarization curves. The influence of the anode and cathode relative humidification level on thecell performance is explained under the light of the predicted water content across the membrane. Under theoperating conditions studied, the net water flux of water is toward the anode. Accordingly, the influence ofthe anode humidification is not significant, and the influence of the cathode humidification has a high impactin fuel cell performance. Results show that fuel cell performance is better for experiments where higher watercontent values were obtained. In comparison to the anode feed flow rate influence, the influence of the cathodefeed flow rate has a major impact in fuel cell performance.

1. Introduction

Fuel cells are an innovative alternative to current powersources, with potential to achieve higher efficiencies withrenewable fuels with minimal environmental impact. In par-ticular, the proton-exchange membrane (PEM) fuel cells (FCs)are today in the focus of interest as one of the most promisingdevelopments in power generation, with a wide range ofapplications in transportation and portable electronics. Althoughprototypes of fuel cell vehicles and residential fuel cell systemshave already been introduced, their cost must be reduced andtheir efficiencies enhanced.

Several coupled fluid flow, heat and mass transport processesoccur in a fuel cell in conjunction with the electrochemicalreactions. Generally, PEMFCs operate bellow 80 °C. Anodicoxidation of hydrogen produces protons that are transportedthrough the membrane to the cathode where the reduction ofoxygen generates water. One of the most important operationalissues of PEMFCs is the water management in the cell.1,2

To achieve optimal fuel cell performance, it is critical to havean adequate water balance to ensure that the membrane remains

hydrated for sufficient proton conductivity, while cathodeflooding and anode dehydration are avoided.3-5

The water content of the membrane is determined by thebalance between water production and three water-transportprocesses: electro-osmotic drag of water (EOD), associated withproton migration through the membrane, back diffusion fromthe cathode, and diffusion of water to/from the oxidant/fuel gasstreams. Understanding the water transport in the PEM is a guidefor materials optimization and development of new membrane/electrodes assemblies (MEAs).

Recent studies6 reported the influence of various operatingconditions on fuel cell performance, such as temperatures,pressures, and humidity of reactant gases. On the basis of theseinvestigations, the optimum conditions are operation at higherpressure and elevated temperature with the humidified reactantgases. Yan et al.7 also studied the influence of various operatingconditions, including the cathode flow rate, cathode inlethumidification temperature, and cell temperature on the per-

* To whom correspondence should be addressed. Telephone:+351225081675. E-mail: apinto@fe.up.pt.

† Faculdade de Engenharia da Universidade do Porto.‡ Instituto Nacional de Engenharia, Tecnologia e Inovacao (INETI).(1) Eikerling, M.; Kharkats, Yu. I.; Kornyshev., A. A.; Volfkovrch, Yu.

M. Phenomenological theory of electro-osmotic effect and water manage-ment in polymer electrolyte proton-conducting membranes. J. Electrochem.Soc. 1998, 145, 2684–2699.

(2) Eikerling, M.; Kornyshev, A. A.; Kucerhak, A. R. Water in polymerelectrolyte fuel cells: Friend or foe? Phys. Today 2006, 59, 38.

(3) Baschuk, J. J.; Li, X. Modeling of polymer electrolyte membranefuel 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. Materialsand processes for small fuel cells. Solid State Ionics 2002, 8312.

(6) Amirinejad, M.; Rowshanzamir, S.; Eikani, M. H. Effects ofoperating parameters on performance of a proton exchange membrane fuelcell. J. Power Sources 2006, 161 (2), 872–875.

(7) Yan, W. M.; Chen, C. Y.; Mei, S. C.; Soong, C. Y.; Chen, F. Effectsof operating conditions on cell performance of PEM fuel cells withconventional or interdigitated flow field. J. Power Sources 2006, 162 (2),1157–1164.

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10.1021/ef8004948 CCC: $40.75 2009 American Chemical SocietyPublished on Web 12/09/2008

formance of a PEMFC. Experimental results showed that cellperformance is enhanced with increases in cathode inlet gasflowrate,cathodehumidificationtemperature,andcell temperature.

There are few studies on PEMFCs with a multiserpentineflow channel configuration. Li et al.8 indicated that this designensures adequate water removal by the gas flow through thechannel and no stagnant area formation at the cathode surfaceas a result of water accumulation. Watkins et al.9 reported that,under the same experimental conditions, the output power fromthe cell could be increased by almost 50% with this type offlow-field plate.

In this work, the effect of cathode and anode flow rates andrelative humidity on the performance and power of a PEMFCwith multiserpentine channels is studied and some results areexplained and compared to the predictions of a recentlydeveloped 1D model.10

2. Experimental Section

2.1. Apparatus. A schematic drawing of the experimentalapparatus used in this work is shown in Figure 1. Pure hydrogen(humidified or dry) as fuel and air (humidified or dry) as an oxidantare used. The pressure of the gases is controlled by pressureregulators (air, Norgreen 11400; H2, Europneumaq mod. 44-2262-241) and flow rates controlled by flow meters (KDG, Mobrey).The reactants humidity and temperatures are monitored by adequatehumidity and temperature probes (air, Testo; H2, Vaisala).

The humidification of air and hydrogen gases is conducted inErlenmeyer flasks by a simple bubbling process. To control thehumidification temperature, each Erlenmeyer flask is thermallyisolated and surrounded with an electrical resistance (50 W/m)activated by a Osaka OK 31 digital temperature controller. Thesame procedure is applied along the connecting pipes from thehumidification point up to the entrance of the fuel cells to guaranteethe temperature stabilization of each reacting gas flow as well asto 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 fromTTI is used. This device could work with five different operatingmodes: (1) constant current, two possibilities were available, 0-8A (with 1 mA resolution) and 0-80 A (10 mA resolution), with aprecision of (0.2% + 20 mA; (2) constant voltage, two possibilitieswere available, Vmin up to 8 V (1 mA resolution) and Vmin up to 80

(8) Li, X.; Sabir, I. Review of bipolar plates in PEM fuel cells: Flow-field 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) Falcao, D. S.; Oliveira, V. B.; Rangel, C. M.; Pinho, C.; Pinto,A. M. F. R. Water transport through a PEM fuel cell: A one-dimensionalmodel with heat-transfer effects. Chem. Eng. Sci., manuscript submitted.

Figure 1. Schematic representation of the experimental setup.

Figure 2. Flow channel configuration and dimensions.

398 Energy & Fuels, Vol. 23, 2009 Falcão et al.

V [10 mA resolution (where Vmin is 10 mV for a low-power situationand 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, operatingrange from 0.01 up to 1 A/V (1 A/V resolution) and from 0.2 up

to 40 A/V (resolution of 0.01 A/V), with a precision of 0.5% + 2digits; and (5) constant resistance, operating range from 0.04 up to10 Ω (0.01 Ω resolution) and from 2 to 40 Ω (with 0.1 Ωresolution), with a precision of 0.5% + 2 digits.

This load was connected to a data acquisition system composedby 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 componentsof the PEMFC were “in house”-designed, with exception of theMEA. A Paxitech seven-layer MEA (Nafion 112) with 25 cm2

active surface area is used. The channel configuration used for theanode and cathode flow channels is represented in Figure 2.The channel depth is 0.6 mm for the hydrogen flow and 1.5 mmfor the air flow.

2.3. Experimental Conditions. In this work, a set of conditionswas used as the base condition. Using this set of conditions andchanging one variable, it is possible to evaluate the influence ofthis parameter on the cell temperature. The studied operatingconditions were the cell temperature, anode humidification, cathodehumidification, anode flow rate, and cathode flow rate. The baseconditions are summarized in Table 1.

Two experiments were performed at two different cell temper-atures, 298 and 313 K. To study the influence of the anode/cathodehumidification, dry hydrogen/air was introduced (to achieve lowerhumidity levels) and hydrogen/air was introduced at room temper-ature (to achieve intermediate humidity levels). In another set ofexperiments, the anode and cathode flow rates were set to doublethe base values. For each one of the studied conditions, the inletwater concentration at both sides of the cell was accuratelydetermined using the relative humidity and inlet temperature values.

3. Results and Discussion

In a previous work, Falcao et al.10 developed a semi-analyticalone-dimensional model considering the effects of coupled heatand mass transfer, along with the electrochemical reactionsoccurring in a PEMFC. The model can be used to predict the

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) 298anode flow temperature (K) 313anode relative humidity (%) 70cathode flow temperature (K) 313cathode relative humidity (%) 70anode pressure (atm) 1.2cathode pressure (atm) 2anode flow rate (slpm) 0.15cathode flow rate (slpm) 0.7

Table 2. Inlet Water Concentrations

experienceanode inlet water

concentration (mol/cm3)cathode inlet water

concentration (mol/cm3)

1 (base condition) 2.0 × 10-6

2.0 ×10-62 (anode T ) 298 K;

RH ) 76%)9.8 × 10-7

3 (anode T ) 298 K;RH ) 7%)

9.0 × 10-8

Table 3. Inlet Water Concentrations

experiencecathode inlet water

concentration (mol/cm3)

anode inlet waterconcentration

(mol/cm3)

1 (base condition) 2.0 × 10-6

2.0 ×10-62 (cathode T ) 298 K;

RH ) 94%)1.2 × 10-6

3 (cathode T ) 298 K;RH ) 1%)

1.3 × 10-8

Figure 4. (a) Voltage versus current density and (b) power density versus current density for two different cell temperatures.

Figure 5. Membrane temperature versus current density for differentcell temperatures, model predictions.

Water Transport through a PEM Fuel Cell Energy & Fuels, Vol. 23, 2009 399

hydrogen, oxygen, and water concentration profiles in the anode,cathode, and membrane as well as to estimate membrane watercontents and the temperature profile across the cell.

In this work, the developed model is used to predict thepolarization curve for the base-operating conditions (Table 1).The model predictions and experimental results are comparedin Figure 3. For low current densities, the model predicts verywell the experimental results. For higher densities, the modelpredictions are higher than experimental results. This discrep-ancy is a common feature of single-phase models because theeffect of reduced oxygen transport because of water floodingat the cathode at high current density is not accounted for.

Model predictions are also useful to better understandexperimental results. The membrane water content is a goodindicator of membrane humidification and is easily calculatedusing this simple one-dimensional model. In this work, model

predictions of the membrane water content and temperaturesare used to explain some experimental results.

3.1. Fuel Cell Temperature. In Figure 4, the polarizationand power curves obtained in two experiments with differentcell temperatures (298 and 313 K) are presented.

In this range of low cell temperature operation, the influenceon fuel cell performance is minimal. This range of temperatureswas selected, bearing in mind the portable applications (exclud-ing the use of heating equipment).

In Figure 5, the predicted variation of membrane temperaturewith current density is presented for both fuel cell temperatures.

As expected, the cell temperature increases with currentdensity because of the cathode exothermic reaction (highercurrents correspond to higher amounts of heat released).Although the cell temperature is different for the two experi-ments (15 K variation), the temperature profile through themembrane is quite similar (differences of 3 K). According to

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 valuesof anode humidification (current density of 0.1 A/cm2), modelpredictions.

Figure 8. (a) Voltage versus current density and (b) power density versus current density for different values of cathode humidification.

Figure 9. Water content (λ) along the membrane for different valuesof cathode humidification (current density of 0.1 A/cm2), modelpredictions.

400 Energy & Fuels, Vol. 23, 2009 Falcão et al.

these results, it is predictable that the two conditions lead tosimilar fuel cell performances, as shown in Figure 4.

3.2. Anode Humidification. Experiments with differentanode relative humidification levels were performed (Table 2).The corresponding polarization and power curves are plottedin Figure 6.

For the used MEA, the manufacturer indicates that there isno need to humidify the anode stream. As is evident from theplots of Figure 6, the influence of the anode humidification onthe performance of the cell is not significant. These results arein agreement with the MEA manufacturer specifications. Suchcell behavior could be useful for portable applications becausethe use of a humidifier for the anode stream could be avoided.

The model predictions of the water content in the membranetrough parameter λ (the ratio of the number of water moleculesto the number of charged SO3

-H+ sites) are presented in Figure7 for the same three experiences.

For the conditions studied, the net flow of water is towardthe anode. For these conditions, the amount of water is higherfor the cathode side because of the importance of water transportby electro-osmotic drag and water generation by the reaction.As can also be seen from the plots, the water content near theanode catalyst layer is lower for the three curves, in particularfor the less humidified anode. These results are in accordancewith experiments. The water content is similar, and conse-quently, the cell performance is similar too.

3.3. Cathode Humidification. To analyze the cathode hu-midification influence, two experiments were performed andcompared to the results obtained with the base condition (Figure8). The different values of the inlet water concentrationsdetermined are presented in Table 3.

In contrast to the case analyzed previously, the cathodehumidification level has a significant impact on the fuel cellperformance. As indicated above, the water management is acritical issue. Water acts like a proton shuttle in the membraneand catalyst layers because excessive water amounts filling the

pores inhibit the access to active sites and block the transportof gaseous reactants and products. On the contrary, dehydrationof anodic regions because of electro-osmotic drag can cause abreakdown of proton conductivity and even a structural deg-radation of the PEM. The membrane must therefore have anideal humidification level to achieve optimal performances. Theplot of the predicted values of the water content across themembrane for the same three experiences (corresponding to acurrent density of 0.1 A/cm2) is shown in Figure 9 andcontributes to a better explanation of the results shown inFigure 8.

These values are in agreement with experiments since theintermediate water content value (experiment 2) leads to thebest performance (Figure 8). For this condition, the mean watercontent through the cell is higher corresponding to an enhancedproton conductivity and consequently a better performance. Nopredicted anode dehydration occurs for all of the studiedconditions.

3.4. Anode Feed Flow Rate Influence. For the base opera-tion condition, the hydrogen flow rate used corresponds to astoichiometric ratio of 1, at 1 A/cm2 (a hydrogen flow ratesufficient even for current densities up to 25 A). An experimentwith a stoichiometric ratio of 2 was also performed. The resultsfor both conditions are represented in Figure 10.

As shown in Figure 10, the hydrogen flow rate increase hasno significant influence on the fuel cell performance. Theseresults are expected since, for both conditions, the hydrogenflow rate is largely in excess, even for high values of the currentdensity.

3.4. Cathode Feed Flow Rate Influence. For the basecondition, the air flow rate corresponds to a stoichiometric ratioof 3, at 1 A/cm2. An experiment with a cathode stoichiometricratio of 6 was performed to check the influence of increasingthe air flow rate on the cell performance. The results arepresented in Figure 11.

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.

Water Transport through a PEM Fuel Cell Energy & Fuels, Vol. 23, 2009 401

As expected, the air flow rate increase improves fuel cellperformance probably to an enhanced water removal. Theimprovement in the fuel cell performance is more significantfor higher current densities because of the more pronouncedformation of water at these conditions. Consequently, it isadvantageous to work with higher air flow rates when usinghumidified cathode feeds, namely, for high current densities.

4. Conclusions

In the present study, an experimental study on the perfor-mance of an “in-house”-developed PEM fuel cell with 25 cm2

of active membrane area is described. A multiserpentineconfiguration for the anode and cathode flow channels was used.Experiments were carried out under different anode and cathodeRHs and flow rates. The influence of the anode and cathoderelative humidification level on the cell performance is explainedunder the light of the predictions of water content across themembrane from a recently developed model. Under the operat-ing conditions studied, the net water flux of water is toward

the anode and, accordingly, the influence of the anode humidi-fication is not significant. These results are in accordance withthe specifications of the manufacturer. The cathode humidifi-cation has a more important impact on the cell performanceprobably because of a more significant effect on the protonconductivity. An enhanced performance was obtained for thecondition where a higher water content was obtained probablybecause of a better proton conductivity.

The influence of the anode and cathode feed flow rates wasalso studied.

This work is the starting point for a more detailed study,aiming at the setup of optimized and tailored MEAs adequatefor different applications (namely, low-humidity operation).

Acknowledgment. The partial support of “Fundacao para aCiencia e TecnologiasPortugal” through project POCI/EME/55497/2004 is gratefully acknowledged. POCTI (FEDER) also supportedthis work via CEFT.

EF8004948

402 Energy & Fuels, Vol. 23, 2009 Falcão et al.