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Electrochimica Acta 56 (2011) 4439–4444

Contents lists available at ScienceDirect

Electrochimica Acta

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mproved gas diffusion electrodes for hybrid polymer electrolyte fuel cells

urat Ünlü, Junfeng Zhou, Irene Anestis-Richard, Hyea Kim, Paul A. Kohl ∗

chool of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100, USA

r t i c l e i n f o

rticle history:eceived 24 November 2010eceived in revised form 31 January 2011ccepted 3 February 2011vailable online 1 March 2011

eywords:nion exchange ionomerybrid fuel cellatalyst utilization

a b s t r a c t

In this study, the performance of the anionic electrodes for hybrid polymer electrolyte fuel cells wasimproved. The anion exchange membrane (AEM) electrodes were initially characterized as the cathodeon a proton exchange membrane (PEM) anode/membrane half-assembly (i.e. hybrid polymer electrolytefuel cell). The electrode performance was improved by tailoring the ionomer distribution within theelectrode structure so as to better balance the electronic, ionic, and reactant transport within the catalystlayer. An ionomer impregnation method was used to achieve a non-uniform ionomer distribution andhigher performance. Traditional electrode fabrication methods (i.e. thin-film method) lead to a uniformionomer distribution. The peak power density at 70 ◦C for a H2/O2 hybrid fuel cell was 44 mW cm−2 usingthe thin-film electrode, and 120 mW cm−2 using the ionomer impregnated electrode. A hydrophobic

lkaline electrodehree-phase boundary

additive used in the catalyst layer further improved the electrode performance, giving a peak powerdensity of 315 mW cm−2 for H2/O2 at 70 ◦C. Electrochemical impedance spectroscopy was used as anin situ diagnostic tool to help understand the origin of the electrode improvements. The increase inperformance was attributed to improved catalyst utilization due to the creation of facile gas transportdomains in the AEM electrode structure. Similarly, the AEM anode prepared by ionomer impregnation

lenem me

with polytetrafluoroethyones made by the thin-fil

. Introduction

Anion exchange membrane (AEM) fuel cells have attracted sig-ificant interest because they have the potential to overcome thehallenges of durability and cost while achieving efficient energyonversion. [1–3]. The alkaline environment in AEM fuel cells offersast electrokinetics and reduced corrosion mitigating the needor platinum-based catalysts. AEMs can also lower the degree ofuel crossover in-part due to the opposite direction of electro-smotic drag in an AEM cell compared to a PEM cell. Additionally,ovel AEM/PEM hybrid designs simplify the water managementnd enable fuel cell operation without external humidification [4].owever, the cell performance obtained for AEM and AEM/PEMybrid fuel cells is currently modest compared to PEM fuel cells.ecent studies have shown that the moderate performance of AEM

uel cells is largely due to the poor performance of the electrodes5,6]. PEM fuel cells have undergone extensive development over

period of decades. While AEM fuel cells take advantage of the

dvances learned from PEM technology, numerous incrementaldvances are necessary. This study addresses one critical issue inEM electrode assemblies.

∗ Corresponding author.E-mail addresses: kohl@gatech.edu, paul.kohl@chbe.gatech.edu (P.A. Kohl).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.02.017

resulted in a three-fold increase in the peak power density compared tothod, which has no polytetrafluoroethylene.

© 2011 Elsevier Ltd. All rights reserved.

The fabrication method for the membrane electrode assemblyplays an important role in fuel cell performance [7–10]. Modifica-tion of the electrode fabrication techniques can lead to structures,which have a better combination of ionic and electronic conduc-tance, along with pathways for transport of gaseous reactants andproducts to and from the catalytic sites. In addition, intimate bond-ing of the ionomer to the catalyst surface is crucial for high currentcells. Thin-film electrodes are used in the fabrication of PEM fuelcell because they are simple and lead to high catalyst utilizationand power density [9–10]. Previous AEM electrodes were producedby the thin-film method, which involved painting or spraying thecatalyst ionomer ink on either the gas diffusion layer or membrane[5,6,11,12]. However, the cell performances were very modest com-pared to PEM fuel cells.

Recently, the AEM electrodes were successfully characterizedas the cathode on a PEM membrane electrode half-assembly [13].This novel design enabled in situ characterization of the AEM elec-trodes by electrochemical impedance spectroscopy, which gaveinsights into the limiting processes. The analysis of AEM electrodesmade by the thin-film method showed that low catalyst utiliza-tion was the primary reason for the low performance. In particular,

poor mass transport of the reactant to the catalyst sites and highwater uptake in the AEM ionomer inhibited access of reactant gasto the catalyst, which led to poor use of the catalyst area. The lossof active catalytic sites is more pronounced at high current den-sity. The rate of water generation increases with higher current

4 ica Acta 56 (2011) 4439–4444

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ensity causing flooding and additional blockage of the catalyticites.

In this study, we have modified the fabrication methods forEM electrodes so as to improve the electrochemically active sur-

ace area in the catalyst layer. An ionomer impregnation methodas used to change the ionomer distribution in the catalyst layer

o achieve greater free volume to enhance gas diffusion and massransport. The AEM electrodes were characterized as the cathoden an AEM/PEM hybrid fuel cell. The effect of fabrication method onhe AEM anode performance was also investigated. Hydrophobicdditives were used in the catalyst layer to further enhance the gasransport properties.

. Experimental

The AEM electrodes were characterized on a semi-hybrid mem-rane electrode assembly (MEA), as described in [13]. Hybrid MEAs

nclude one conventional PEM-based electrode on a Nafion coreembrane and a second AEM electrode. All the PEM-based elec-

rodes in the hybrid MEAs used in this study were identical. Theatalyst was 20% Pt on carbon (E-TEK), Pt/C, for both types oflectrodes. Toray carbon paper (TGPH-090) was used as the gasiffusion layer.

The PEM electrodes were made by a conventional thin-filmethod, which involves painting catalyst ink onto carbon paper.

he ink was prepared by mixing a Nafion solution, Pt/C, isopropyllcohol, and water. The weight ratio of isopropanol to water is 0.8.he ink was sonicated for 15 min and then cast onto Toray carbonaper (TGPH-090).

The ionomer used for the AEM electrode was poly (arylenether sulfone) functionalized with quaternary ammonium groupsynthesized for this study, as described previously [14]. TheEM ionomer has an ion exchange capacity of 1.14 mmol/g and

onic conductivity of 23 mS cm−1 at 25 ◦C. The AEM ionomer wasissolved as a 1 wt% solution in dimethylformamide. The AEM elec-rodes were prepared by the thin-film and ionomer impregnation

ethods.

.1. Thin-film method

This method involves painting the catalyst and ionomer from aolvent-based mixture onto carbon paper. The catalyst ink was pre-ared by mixing the Pt/C catalyst, ionomer, and a solvent mixtureomposed of methanol and dimethyl formamide (1:1 weight ratio).he catalyst inks were sonicated for 15 min and then cast onto Torayarbon paper (TGPH-090). The ionomer loading was 0.6 mg cm−2.

.2. Ionomer impregnation method

The catalyst and ionomer were sequentially coated onto theas diffusion layer. The catalyst, with or without, polytetrafluo-oethylene (PTFE) was suspended in a water and isopropyl alcoholixture. The mixture was sprayed onto the carbon paper. After the

lectrode was allowed to dry at room temperature, the electrodeas sintered at 250 ◦C for 1 h. An ionomer and solvent mixture

DMF:methanol in 1:1 weight ratio) was sprayed on the catalysturface. The ionomer loading was 0.6 mg cm−2. If PTFE was used,he percent PTFE in the dry electrode was 10% by weight.

The AEI electrodes were allowed to dry at room temperatureollowed by an immersion in an aqueous 0.1 M KOH solution toxchange OH− for Cl−. The electrodes were rinsed thoroughly and

ried at room temperature. The catalyst loadings were 0.5 mg cm−2

t for all the electrodes. A mixture consisting of 100 �L of Nafion®

5% suspension) and isopropanol mixture (1:2 by volume) wasprayed onto the AEM and PEM electrodes before assembling thelectrodes onto the membrane. The hybrid MEAs were assembled

Fig. 1. The effect of fabrication method on the AEM electrode performance as thecathode at 70 ◦C and 100% relative humidity for H2/O2. The AEM electrodes werefabricated by thin-film and ionomer impregnation methods. The solid symbols cor-respond to the power density curves.

in two steps. In the first step, the PEM electrode was pressed ontoNafion® 212 at 2 MPa gauge pressure and 135 ◦C temperature for3 min. In the second step, the AEM electrode was pressed onto thePEM half-cell assembly at 2 MPa and 50 ◦C for 3 min.

The fuel cell test procedure was performed as previouslyreported [15]. The fuel cell hardware assembly (Fuel Cell Tech-nologies) consisted of a pair of Poco graphite blocks with asingle-serpentine flow pattern. All MEAs were preconditioned byoperation at 600 mV to achieve steady state conditions for 24 hbefore performing the current–voltage (I–V) polarization exper-iments. Electrochemical measurements were performed using aArbin BT2000 potentiostat. All fuel cell tests were conducted usingH2/O2 at ambient pressure. EIS and electrode polarization experi-ments were used to monitor the cell performance. EIS experimentswere performed at different cell voltages using a PAR 2273 poten-tiostat/galvanostat. The EIS experiments used frequencies from10 mHz–10 kHz. The amplitude of the ac voltage was 10 mV. Theunit of impedance data was reported as � cm2 considering thegeometric, superficial surface area of the electrode.

3. Results and discussion

A first characterization of the AEM electrodes was performed ina hybrid fuel cell condition as the cathode. Currently, PEM mem-branes and electrode assembly methods are superior to AEM ones.Using an AEM cathode on a PEM membrane with a PEM anodehalf assembly is a useful way to isolate the AEM cathode effectsand characterize the AEM electrode [13]. The thermodynamics ofthis hybrid configuration have been evaluated recently [15]. Whenoperated at steady state with no net consumption or productionof protons (i.e. overall reaction is 2H2 + O2 → 2H2O), the Nerns-tian change in potential at the electrodes is off-set by the junctionpotentials, yielding a 1.2 V cell voltage. Two hybrid MEAs with iden-tical PEM anodes and membranes with different AEM cathodes,prepared by the thin-film and impregnation method, were pre-pared. The MEA with the thin-film AEM cathode was designatedas MEA-T. The MEA with the ionomer impregnated AEM cathodewas designated as MEA-I.

Fig. 1 shows the voltage–current polarization and power curves◦

for two hybrid fuel cells with AEM cathodes using H2/O2 at 70 C.

The performances of the two MEAs are similar at low currentdensity, i.e. <50 mA cm−2. At higher current density, MEA-I hadsignificantly higher current than MEA-T. The maximum power den-sities are 44 mW cm−2 for MEA-T, and 125 mW cm−2 for MEA-I.

M. Ünlü et al. / Electrochimica Acta 56 (2011) 4439–4444 4441

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more catalytic reaction sites and the water product is more easily

ig. 2. Schematic diagrams of the electrode structures made by (a) thin-film and (b)onomer impregnation methods.

ince both MEAs have identical PEM anodes and membranes, theifference between the two MEAs is attributed to the change in theEM catalyst structure as made by the two methods, as depicted

n the schematic of the electrodes shown in Fig. 2. The distributionf ionomer in the catalyst layer plays a key role in obtaining bothonic and electronic conductance, along with pathways for the massransport of reactant and products.

In the thin-film method, the ionomer is well distributed withinhe catalyst layer, as shown Fig. 2a. This catalyst–ionomer matrixrovides for good ionic transport but leads to a less free volume foras transport. The previous analysis of the AEM electrodes showedhat the gas transport limitation was the primary reason for theow performances in thin-film AEM electrodes [6]. This limitation

ainly arises from the high water uptake, which causes swellingnd poor mass transport within the AEM ionomer. The excessiveater uptake blocks the catalytic sites, decreasing the active sur-

ace area. This loss of active catalytic sites is more pronounced atigher current density when the rate of water generation is great-st. Since the water generated cannot be removed fast enough fromhe catalyst layer, access to the catalytic sites is decreased. Thus,

Fig. 4. Impedance spectra of hybrid MEAs collected at 600 mV. T

Fig. 3. The performance of hybrid fuel cell comprising the AEM cathode made byionomer impregnation with PTFE additive. The PTFE content is 9% relative to thecatalyst layer. The fuel cell was tested at 70 ◦C and 100% relative humidity for H2/O2.

the performance drops in the MEA-T electrodes becomes capped athigh current density.

In the impregnation method, the ionomer solution is sprayedon the surface of the catalyst-on-GDL layer. Only a fraction of theionomer penetrates into the catalyst layer. Also, the distribution ofionomer within the catalyst layer is not uniform. A large fractionof ionomer remains close to the membrane, as shown in Fig. 2b.The catalyst layer facing to the GDL holds a lower ionomer con-tent, resulting in greater free volume. These voids are close to theGDL layer and provide a pathway for transport of gaseous reac-tants to the catalyst sites and water created during the reactionaway from the catalyst. Thus, the reactant gases gain access to

removed.It is important to note that the impregnation method results

in less interfacial contact area between the catalyst and ionomer,decreasing the ionic conductance. However, this does not affect the

he spectra was normalized by the geometric surface area.

4 ica Acta 56 (2011) 4439–4444

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ell performance because gas transport, not ionic conductance, ishe rate-limited feature in the reaction sequence.

Hydrophobic PTFE was added to the catalyst layer in the ionomermpregnation AEM electrode to further decrease the loss of activeurface area due to water blockage. The PTFE-catalyst mixture wasast onto the gas diffusion layer and sintered at 250 ◦C to removehe surfactants and solvents. Then, the AEM ionomer was impreg-ated into the PTFE-bonded catalyst layer. The resulting electrode,EA-I with PTFE, was assembled on the PEM half-cell and tested

s the cathode in the hybrid configuration. The cell performances shown in Fig. 3. The peak power density is 315 mW cm−2 for

2/O2 at 70 ◦C, which is significantly higher than the correspond-ng electrode without PTFE in Fig. 1, 127 mW cm−2. This advancen electrode performance is due to the improved mass transport inhe catalyst layer from the use of PTFE. The PTFE provides passagesor the transport of gas species and water products. It is also likelyhat PTFE can introduce some porosity to the catalyst layer, furtherncreasing the accessibility of the catalyst surface.

Electrochemical impedance spectroscopy was used to helpnderstand the nature of the improved catalyst utilization. Fig. 4hows the impedance spectra collected at 600 mV after 24 h ofteady state performance for each of the three hybrid MEAs (MEA-T,EA-I, and MEA-I with PTFE). All MEAs had similar high fre-

uency response, the x-intercept on the left side of the semi-circleoop, ca. 220 m� cm2. Typically, high frequency resistance reflectsotal ohmic resistance with no parallel capacitive component.t is dominated by the membrane resistance and contact resis-ances between the membrane and electrode. This observationuggests that the three electrode structures each create a goodhmic contact between the PEM membrane and the AEM cath-de. All spectra have a single, medium frequency loop, whichhanges with the potential. This is a typical response for kineticallyontrolled behavior. The AEM cathode dominates the complex (par-llel resistance–capacitance) electrode impedance measured in theybrid MEA with the AEM cathode [6]. Since all MEAs have theame PEM anode, any change in the electrode impedance betweenhe three hybrid configurations is attributed to the cathode perfor-

ance. In Fig. 4, there is a substantial variation in the diameter ofoops which corresponds to the charge transfer resistance in theseells, RCT. MEA-I with PTFE has the smallest RCT, consistent withts superior performance (Fig. 4). MEA-T has the highest RCT dueo its method of construction and balance between the electronic,onic, and gas transport attributes. Understanding this interactionetween electronic, ionic, and transport attributes is important

nformation which can help introduce further improvements.The charge transfer resistance, RCT, is determined by fundamen-

al interfacial reaction kinetic parameter, R0CT , and the true surface

rea of the electrode, A0, Eq. (1) [6].

CT = R0CT

A0(1)

CT is the measured value in ohms from impedance spectroscopy,hereas R0

CT is the resistance per active area, � cm2. Even thoughCT includes an intrinsic area term, the true surface area factor isommonly omitted in the literature in favor of the superficial area.t is more difficult to determine the true surface area in a particularperating fuel cell than the superficial area. Thus, in this report, thenit of RCT is also reported as � cm2 considering geometric, super-cial surface area of the electrode, and not the catalytic surfacerea.

Since all electrodes use the same ionomer (i.e. reaction envi-

onment), R0

CT is presumably the same in all MEAs, although theeaction environment (e.g. pH) can change with current densityue to consumption of reactants and production of products. Thus,he differences in RCT for the three types of AEM electrodes arise

ostly due to changes in active surface area between the AEM elec-

Fig. 5. The peak power densities of three hybrid MEAs with three different AEMcathodes as a function of relative humidity at 70 ◦C. Hydrogen and oxygen gases weresupplied at ambient pressure with the stoichiometric ratio of 2 and 1.5, respectively.

trodes. Although the same amount of catalyst (and catalyst area)is present in each electrode, A0 is determined by that area wherea three-phase boundary exists between the gas diffusion, ionic,and electronic conducting domains. The lack of any one of thesedomains will render the catalyst area ineffective and result in alower A0. As explained above, the thin film method yields insuf-ficient contact between the catalysts and gas transport domains,which limits the active surface area, particularly at high currentdensity where swelling chokes off the supply or reactant. Simi-larly, the AEM cathode in MEA-I with PTFE presumably has bothlowest water content and more effective mass transport domains,increasing A0. This greater A0 results in the lower RCT, as shown inFig. 4.

A unique aspect of the hybrid cell configuration with the AEMcathode on the PEM half-cell is the creation of water near the AEMcathode, where it is consumed: self-humidification [4]. The waterproduced is efficiently transported and used by the cathode becauseit is generated between the AEM electrode and PEM membrane. Thewater generated within the MEA enables self-hydration of the MEAwhen the cell was operates at low relative humidity conditions.Fig. 5 shows the peak power density at different relative humid-ity conditions for the feed gases for each of the MEAs with differentAEM cathodes. Each power density point was collected after 24 h ofoperation at 600 mV. Each MEA had higher performance at lowerrelative humidity due to faster water removal from the catalystlayer. This lowered the water swelling and increased the access ofthe reactant gases to the catalytic reaction sites. The peak powerdensity increased by 231%, 89%, and 17% for the MEA-T, MEA-I,and MEA-I with PTFE electrodes, respectively, when the relativehumidity decreased from 100% to 0%. The performance enhance-ment at lower relative humidity was more pronounced for thethin-film electrode and ionomer impregnated electrode withoutPTFE, which collaborates the idea that the loss of performance withthose electrodes was indeed due to the high water content in theelectrode structures. However, the ionomer impregnated electrodewith PTFE does not suffer from the flooding to the same extentdue to its hydrophobic nature, and thus had a smaller relativeimprovement.

Fig. 6 shows the current–voltage behavior of the hybrid MEAsoperating at 70 ◦C with dry gas feeds. MEA-I with PTFE has the

highest peak power density, 369 mW cm−2. This performance ishigher than the ones reported recently; 258 mW cm−2 by Guet al. [16] and 230 mW cm−2 by Poynton et al. [17]. Even though,the power density of 400 mW cm−2 was reported by Piana et al.[18] the cell performance decayed quickly at steady state opera-

M. Ünlü et al. / Electrochimica A

Fig. 6. The performance of three hybrid MEAs with AEM electrode as the cathodeat 70 ◦C and 0% relative humidity for H2/O2.

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ig. 7. The performance of the hybrid MEAs with the AEM electrode as the anodet 70 ◦C and 100% relative humidity for H2/O2.

ion. The peak power density for MEA-T and MEA-I are 144 and91 mW cm−2, respectively. Interestingly, MEA-T has a slightlyigher performance than MEA-I at the current densities lower than50 mA cm−2. This is likely to be due to insufficient hydration inEA-I compared to MEA-T, which leads to lower ionic conduc-

ion. However, the performance of MEA-I sharply decreases above50 mA cm−2. The rate of water generation at higher current den-ities floods the voids in the catalyst layer and blocks the catalysturfaces even though there is no external humidification. Thisbservation shows that the mass transport limitation in the cata-yst layer for the thin-film AEM electrodes is substantial. In contrast,

EA-I has fast mass transport in the catalyst layer and the water isore easily removed. The loss of catalytic sites due to excess water

t high current densities is minimal.The AEM/PEM hybrid cells can have many configurations and

se of an AEM anode is also of interest [11]. The effect of fabricationethod on the AEM electrode as the anode was also investigated.

ig. 7 shows the polarization curves for the hybrid MEAs with AEMlectrodes as anodes on the PEM membrane/cathode assembly. TheEM electrodes are made of thin-film and ionomer impregnationith PTFE methods. The cathodes were identical PEM electrodes.

imilar to the improved AEM cathode performances, the AEMnode made by ionomer impregnation with PTFE results in highererformance relative to the one made by thin-film method. Theerformance increases from 95 mW cm−2 with thin-film anode to

cta 56 (2011) 4439–4444 4443

375 mW cm−2 with impregnated PTFE AEM anode. This improve-ment is also attributed to higher catalyst utilization in the AEManode made by ionomer impregnation method with PTFE additive.

It is important to note that the thin-film method was shown tohave higher performance than the ionomer impregnation methodfor PEM fuel cells because of better contact between the cata-lyst particles and polymer electrolyte, i.e. high ionic conductivity[9–10]. However, in order to be an effective electrode, goodelectrical conductance and mass transport properties must alsobe provided. The electrode fabrication method must provide astructure, which balances these requirements. For example, thethin-film method favors ionic conductivity over the gas transportand electronic conductance. In contrast, the ionomer impregnationmethod provides good electrical contact between the catalyst par-ticles and provided greater porosity promoting the access of thegaseous reactants to more effective catalytic sites. Thus, depend-ing on the morphology and gas solubility properties, each polymerelectrolyte and ionomer is likely to require a different composi-tion and electrode structure to obtain the optimum three-phaseboundary.

In particular, AEM and PEM materials have different proper-ties. PEMs have good ionic conductivity, mass transport, and O2solubility. The thin-film method yields good performance withPEM electrodes. However, the thin-film AEM electrodes show infe-rior performance to the ionomer impregnation method electrodes.These results show that AEM materials have lower mass transportproperties, likely through lower gas solubilities and diffusion, com-pared to PEMs. The optimum electrode architectures can vary fordifferent polymer electrolytes depending on their physical proper-ties.

4. Conclusions

The effect of thin-film and ionomer impregnation methods forAEM electrodes were evaluated. The AEM electrodes were charac-terized as the electrodes on AEM/PEM hybrid fuel cells. The ionomerimpregnation method led to superior fuel cell performance com-pared with the thin-film method. The improvement was attributedto the non-uniform distribution of the AEM ionomer in the catalystlayer, which created free volumes close to the gas diffusion layer.These domains increase the catalyst surface area in contact with gasreactants. The use of a hydrophobic additive in the catalyst layeralso improved the catalyst utilization by decreasing the loss of cat-alytic sites due to the excessive water uptake. The improvementsin the electrode performance were examined with electrochem-ical impedance spectroscopy. This study shows that the physicalproperties of polymer electrolytes (e.g., water uptake, ionic con-ductivity) require different electrode architectures and fabricationprotocols to obtain high performances.

Acknowledgement

The authors gratefully acknowledge the financial support theArmy Research Laboratory, contract LCHS22067.

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