synthesis and characterization of composite membrane with three-dimensionally ordered macroporous...

doi: 10.1149/1.28309492008, Volume 155, Issue 3, Pages B303-B308.J. Electrochem. Soc.

Dai Yamamoto, Hirokazu Munakata and Kiyoshi Kanamura DMFCThree-Dimensionally Ordered Macroporous Polyimide Matrix for Synthesis and Characterization of Composite Membrane with

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Journal of The Electrochemical Society, 155 �3� B303-B308 �2008� B303

Synthesis and Characterization of Composite Membranewith Three-Dimensionally Ordered Macroporous PolyimideMatrix for DMFCDai Yamamoto,a,b Hirokazu Munakata,a,b,* and Kiyoshi Kanamuraa,b,*,z

aDepartment of Applied Chemistry, Graduate School of Urban Environmental Science,Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, JapanbCore Research for Evolutional Science and Technology, Japan Science and Technology Agency,Kawaguchi, Saitama 332-0012, Japan

A composite membrane consisting of three-dimensionally ordered macroporous �3DOM� polyimide matrix and a proton-conducting gel polymer was prepared for direct methanol fuel cells �DMFCs�. The matrix effect on the physico-electrochemicalproperties of the composite membrane was investigated in order to compare 3DOM silica composite membrane containing thesame proton-conducting polymer. From the comparison of 3DOM polyimide matrix with the 3DOM silica matrix reported in ourprevious paper, it was confirmed that the polyimide matrix has some structural advantages. In particular, the high uniformity of3DOM structure provides fairly continuous proton-conducting pathways in the composite membrane, resulting in considerablyhigh proton conductivity of 1.7 � 10−1 S cm−1 at 60°C under 90% relative humidity, which is about three times greater than thatobtained with the 3DOM silica composite membrane. Furthermore, high mechanical strength of 3DOM polyimide suppressedpolymer swelling in the composite membrane, which resulted in low methanol permeability of 9.4 � 10−7 cm2 s−1 in 20 mol dm−3

methanol solution. Consequently, 1.3 � 105 S cm−3 s was obtained as transfer selectivity of the composite membrane, which wasabout ten times larger than that of Nafion membrane.© 2008 The Electrochemical Society. �DOI: 10.1149/1.2830949� All rights reserved.

Manuscript submitted August 22, 2007; revised manuscript received December 10, 2007.Available electronically January 22, 2008.

0013-4651/2008/155�3�/B303/6/$23.00 © The Electrochemical Society

Polymer electrolyte fuel cells �PEFCs� are interesting electro-chemical devices and environmentally friendly power sources be-cause of their high energy-conversion efficiency and power densitywith no or low emissions.1,2 The direct methanol fuel cell �DMFC�is a kind of PEFC and it can feed methanol directly to the anodewithout a reformer, so that it has high energy density as a powersource and utilizes liquid fuel. It is one of the promising powersources for portable applications.3,4 However, the DMFC has someproblems to be solved for practical use. One is methanol penetrationthrough a membrane from anode to cathode, so-called crossover.5-9

This behavior is mainly induced by expansion of an electrolytemembrane by water or methanol absorption and results in a chemi-cal short-circuit reaction at the cathode, which induces lowering ofthe cell voltage and fuel utilization. Feeding of highly concentratedmethanol solution is desirable to fabricate compact DMFCs �highenergy density of fuel cell�; however, it enhances the crossover.

In PEFCs, Nafion, which is a kind of perfluorosulfonic acid poly-mer, is usually used as a membrane electrolyte due to its high protonconductivity and chemical stability. However, Nafion membrane iseasily expanded by methanol absorption when it is used for DMFCs.Therefore, alternative electrolyte membranes with high resistance tomethanol absorption are required.10 So far, various sulfonated poly-mers based on polysulfones,11,12 polyetheretherketones,13,14

polyimides,15,16 and so on have been synthesized and used inDMFCs. However, they must be sulfonated at a certain level torealize adequate proton conductivity for DMFC use. Consequently,these polymers have a similar nature to Nafion membrane, so thatthe suppression of crossover is practically difficult. Another ap-proach is the addition of inorganic fillers such as silica and zirconiaparticles into electrolyte membranes.17-19 The chemical and physicalproperties of membranes can be controlled by the kind and contentof fillers. On increasing the content of fillers, the mechanical stabil-ity of membrane increases; however, it also disturbs proton conduc-tivity. Thus, high proton conductivity and low methanol permeabil-ity are incompatible.

In our previous work, we reported a composite membrane com-posed of a three-dimensionally ordered macroporous �3DOM� silicamembrane and a proton-conducting polymer electrolyte.20 The silica

* Electrochemical Society Active Member.z E-mail: [email protected]

address. Redistribu128.59.62.83Downloaded on 2012-09-12 to IP

matrix have mechanical strength high enough to suppress polymerexpansion in the pores. Therefore, lower methanol crossover thanthat of Nafion membrane was successfully obtained in the compositemembrane. The hard nature of silica matrix also makes it possible toprepare self-standing composite membranes, even though the fillingpolymer is soft. This feature is favorable for constructing a smalland lightweight DMFC system without supporting components suchas separators. However, it is not suitable for some applications, par-ticularly where membrane flexibility is required. It is well knownthat engineering plastics have high mechanical strength, chemicalstability, thermal stability, as well as flexibility. Thus, we employedthe concept of developing composite polymer electrolyte membranewith 3DOM silica matrix.21 The present 3DOM proton-conducting

Figure 1. Schematic illustrations of 3DOM polyimide and composite elec-trolyte membrane.

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B304 Journal of The Electrochemical Society, 155 �3� B303-B308 �2008�B304

polyimide membrane has a more uniform structure than 3DOMsilica as schematically shown in Fig. 1, so that it is expected to be abetter matrix for DMFC composite membranes.

In this paper, the properties of 3DOM polyimide and its compos-ite membranes, such as thermal stability, swelling ratio, water up-take, as well as proton conductivity and methanol permeability, wereprecisely evaluated. The DMFC using the composite membrane wasalso examined.

Experimental

Silica particles �550 nm� were provided as 20.5 wt % aqueoussuspension from Nippon Shokubai Co., Ltd. A mixed cellulosemembrane filter with pore size of 0.1 �m �A010A090C� was pur-chased from Advantec Toyo Kaisha, Ltd. Figure 2 shows chemicalstructures of polyamic acid and polyimide. We used polyamic acidas a precursor of polyimide, and its 10 wt % dimethylacetoamidesolution was supplied by JFE Chemical Corporation. Nafion solu-tion �5 wt %� was purchased from Sigma-Aldrich Corporation. Pt/C�37.9 wt %� and Pt–Ru/C �53 wt %� used as catalysts, respectively,for cathode and anode were purchased from Tanaka Kikinzoku Ko-gyo K. K. All other chemicals were purchased from Wako PureChemical Industries, Ltd., and used without further purification.

3DOM polyimide membrane was prepared by the colloidal crys-tal templating method.22,23 A dilute suspension containing 550 nmsilica particles, prepared by mixing 2 mL of the silica suspensionand 30 mL of water, was filtrated through a membrane filter under avacuum of 1 � 104 Pa. The deposited silica layer was peeled offfrom the membrane filter and calcined at 1100°C for 2 h to obtain asilica template. A polyamic acid solution was filled into the vacantspace of silica template and thermally cured at 300°C for 1 h. Theseimpregnation and heat processes were repeated three times to fillpolyimide completely into the silica template. The silica templatewas then removed by etching with 10 wt % aqueous hydrofluoricacid solution to obtain 3DOM polyimide membrane.

Figure 2. Chemical structures of polyamic acid and polyimide.


A proton-conducting polymer electrolyte was prepared by poly-merization of 2-acrylamido-2-methylpropanesulfonic acid �AMPS�.N,N�-methylenebisacrylamide �MBA� and ammonium persulfate�APS� were used as a bridging agent and an initiator of polymeriza-tion, respectively. The aqueous solution containing AMPS, MBA,and APS with concentrations of 4.82, 6.49 � 10−2, and 4.38� 10−2 mol kg−1, respectively, was filled into the 3DOM polyimideby vacuum impregnation and then polymerized at 60°C for 1 h toobtain the composite membrane. The AMPS gel composition usedwas an optimized one as reported in our previous paper.20

Conversion of polyamic acid to polyimide was confirmed byFourier transform infrared analysis �FTIR 670 plus, JASCO�. Apolyamic acid solution was cast on a glass plate and dried at roomtemperature to obtain its thin film. The prepared films heated atdifferent temperatures were used for FTIR measurement in order toevaluate the conversion of polyamic acid to polyimide. Thermalstability of 3DOM polyimide was measured by thermogravimetricdifferential thermal analysis �DTG-60, Shimadzu Corporation�, andthe thermogravimetric curve was obtained in nitrogen atmospherefrom room temperature to 800°C at a heating rate of 5°C min−1.The structures of 3DOM polyimide and its composite membraneswere observed with a scanning electron microscope �JSM-5310,JEOL�. The surface area of the membrane was determined by theBrunauer–Emmett–Teller �BET� method �Belsorp II, BEL JAPANINC.�.

The electrolyte membrane was soaked in water at room tempera-ture for 24 h. The equilibrated membrane was then picked up fromwater, and excess water on the membrane surface was quicklywiped. Then, the weight �Wwet� and area �Swet� of the membranewere measured. Subsequently, the membrane was dried in vacuo for24 h, and then the weight �Wdry� and area �Sdry� in the dry state weremeasured. The water uptake and swelling ratio of the membranewere estimated by Eq. 1 and 2, respectively

Water uptake =Wwet − Wdry

Wdry� 100% �1�

Swelling ratio =Swet − Sdry

Sdry� 100% �2�

The uptake and swelling ratio in methanol solution were also esti-mated by the same procedure.

The methanol permeability of electrolyte membranes was mea-sured using a two-compartment glass cell, in which the membranewas set at the middle of the cell. One compartment �VA = 15 mL�was filled with methanol solution, and the other �VB = 15 mL� wasfilled with ultrapure water. Both compartments were kept stirringduring the experiment. According to the methanol concentration gra-dient across the membrane, methanol permeation to the water com-partment is induced. The methanol permeability �PM� of the mem-brane can be given by Eq. 324

Methanol permeability PM =CB�t�VL

ACA�3�

where CA and CB are methanol concentrations in the methanol-solution compartment and water compartment, respectively. L and Aare the membrane thickness and area. The methanol concentration inthe water compartment as a function of time �CB�t�� was determinedby gas chromatography �GC-14B, Shimadzu Corporation�.

The proton conductivity of the electrolyte membrane was mea-sured with an impedance analyzer �4192A, Yokogawa-Hewlett-Packard, Ltd.� in the frequency range from 100 Hz to 1 MHz. Amembrane was clamped between two gold electrodes using a home-made Teflon cell and kept under controlled temperature and humid-ity. The diameter of the Au electrode was adjusted to 5 mm. Fromthe Cole–Cole plot, the resistance of the membrane was estimated,and then the conductivity �� was calculated from Eq. 4


B305Journal of The Electrochemical Society, 155 �3� B303-B308 �2008� B305

Proton conductivity � =l

A � R�4�

where A is the electrode area and l and R are the membrane thick-ness and ohmic resistance, respectively.

A catalyst ink prepared from Pt/C or Pt–Ru/C, 5 wt % Nafionsolution, water, and glycerol was painted on a carbon paper �EC-TP1-060, Toray�, and it was dried at 120°C. The Pt/C and Pt–Ru/Cwere used, respectively, for cathode and anode, and those loadingswere adjusted to 3.0 mg cm−2. The Nafion loading for both elec-trodes was fixed to 1.5 mg cm−2. A membrane electrode assembly�MEA� was prepared by attaching the carbon papers with catalystlayers on both sides of the composite membrane. The cell test wasconducted at 80°C. Methanol solution �2 mol dm−3� and dry oxy-gen gas were fed to anode and cathode at flow rates of 5 and135 mL min−1, respectively.

Results and Discussion

3DOM polyimide membrane was obtained using a flat silica tem-plate with 200 �m thickness composed of 550 nm silica particles.The thickness of the prepared 3DOM polyimide membrane de-pended on the number of impregnations of polyamic acid into thesilica template and adjusted to 150 �m. The prepared polyimidemembrane had adequate mechanical strength and flexibility for amatrix of composite membrane. Figure 3 shows surface scanningelectron microscopy �SEM� images of the silica template and the3DOM polyimide membrane. It was found that the silica templateconsisted of regularly ordered particles, and the 3DOM polyimidemembrane had its inverse structure. The porosities of silica colloidaltemplate and 3DOM polyimide were estimated to be 23 and 80%,respectively, which were close to theoretical values expected fromhexagonal arrays of silica particles �26%� or three-dimensionallyordered pores �74%�. Smaller pores were also observed inmacropores of the 3DOM polyimide membrane. These are connect-

Figure 3. Typical surface SEM images of �a� silica template and �b� 3DOMpolyimide.


ing windows among the macropores. The average sizes of themacropores and connecting windows were estimated to be 400 and100 nm, respectively, from SEM observation. The uniform connect-ing windows suggests that the 3DOM polyimide membrane had ahighly ordered structure.

Table I shows BET surface areas of the silica template and3DOM polyimide membrane. The silica template had a surface areaclose to an ideal one estimated by the closed packed structure of550 nm silica spheres. However, the 3DOM polyimide membraneshowed a value about two times larger than the theoretical value,suggesting that the surface of polyimide polymer was slightly rough.This feature of 3DOM polyimide membrane is different from the3DOM silica membrane previously prepared by us. The ordering ofmacropores of 3DOM silica membrane is not so high compared withthat of 3DOM polyimide membrane. In other words, 3DOM silicamembrane has no ideal connecting pores, but 3DOM polyimidemembrane has an ideal one. This difference is important for theproton-conducting pathway. In a sense of structural point, 3DOMpolyimide may be more suitable material than 3DOM silica.

Figure 4 shows FTIR spectra of polyamic acid films treated atdifferent temperatures. The film without heat-treatment had absorp-tion peaks at 1230, 1500, and 1600 cm−1 corresponding to C–O–Cstretching vibration between two aromatic rings, breathing mode ofthe aromatic ring, and –CONH– coupled deformation of amide,respectively.15,16,25 With increasing heat-treatment temperature, thepeak at 1600 cm−1 gradually disappeared and new peaks appeared at1370 and 1730 cm−1. The latter peaks are attributed to CvO andC–N–C stretching vibrations and reflect the formation of imidesrings, i.e., polyimide. From the spectra, it was confirmed that thecomplete conversion of polyamic acid to polyimide occurred at300°C. Figure 5 shows the thermogravimetry of polyimide film and3DOM polyimide membrane. The 3DOM polyimide membrane ex-hibited an almost similar thermoprofile to polyimide film. Theweight loss observed at 500°C was attributed to decomposition of

Table I. Experimental and theoretical BET surface areas of silicatemplate and 3DOM polyimide membrane.

SampleBET surface area

�m2 g−1�Theoretical value

�m2 g−1�

Silica template 6.04 5.203DOM polyimide 68.5 20.7

Figure 4. FTIR spectra of polyamic acid films treated at different tempera-tures.



the polymer main chain.15,16 This result suggests that the thermalstability of polyimide was not changed by formation of the 3DOMstructure.

A composite membrane was prepared by injection of AMPSpolymer electrolyte into a 3DOM polyimide membrane. As shownin Fig. 6, the pores of 3DOM polyimide were completely occupiedby AMPS polymer. The effect of 3DOM polyimide matrix on thesuppression of polymer expansion was evaluated from water ormethanol uptake and the membrane swelling ratio. Figure 7 showsthe uptake and swelling ratio of membranes measured in water andmethanol solution. AMPS gel polymer absorbed a large amount ofwater and methanol and swelled drastically. The uptake and swellingratio in water were estimated as 1200 and 460%, respectively, andthose values gradually decreased to 990 and 330% with increasingmethanol concentration, respectively. This behavior was opposite tothat observed in Nafion membrane and was due to different affinityof AMPS polymer to methanol.26 The composite membrane hardlyswelled in water and methanol solution, in which the uptake andswelling ratio were reduced to less than 3 and 6%, respectively,regardless of methanol concentration. This result shows that the3DOM polyimide matrix had a reasonable mechanical strength andworked effectively to suppress AMPS polymer expansion. However,the mechanical strength of 3DOM polyimide is lower than that of3DOM silica matrix, and suppression for expansion of polymer in-

Figure 5. Thermogravimetric curves of 3DOM �solid line� and film �dashedline� polyimide membranes in nitrogen atmosphere.

Figure 6. Typical cross-sectional SEM image of 3DOM composite mem-brane.


jected in the 3DOM polyimide matrix is slightly weaker than that in3DOM silica matrix. This is due to expansion of 3DOM polyimideitself.

Figure 8 shows methanol permeability of electrolyte membranesas a function of methanol concentration. The methanol permeabilityin AMPS membrane decreased with methanol concentration, whichwas well-correlated with the results of uptake and swelling tests. Bythe way, methanol is not absorbed by polyimide matrix, so thatmethanol permeation in the composite membrane occurs in theAMPS electrolyte phase. Consequently, the composite membraneshowed a similar concentration profile of methanol permeability tothat observed in AMPS polymer. However, the AMPS polymer in-jected in the pores of 3DOM polyimide matrix cannot expand due tomechanical suppression by the matrix as shown in Fig. 7. So, themethanol permeability was reduced to about one-fifth in the com-posite membrane, and better performance than Nafion membranewas attained. Especially, the composite membrane has an advantagewhen using concentrated methanol solutions above 10 mol dm−3.However, from the comparison of 3DOM polyimide composite elec-trolyte with 3DOM silica composite membrane, it can be seen thatthe reduction of methanol permeability by 3DOM polyimide matrixis smaller that that by 3DOM silica matrix. When a matrix withhigher mechanical strength is used for preparing composite mem-brane, more reduction of methanol permeability is attained, and si-multaneously the membrane become more fragile. 3DOM polyimide

Figure 7. Uptakes �closed symbol� and swelling ratios �open symbol� of3DOM composite ��, ��, AMPS ��, ��, and Nafion ��, �� membranesmeasured in water and methanol solutions at 30°C.

Figure 8. Methanol permeability of 3DOM composite ��, AMPS ��, andNafion �� membranes as a function of methanol concentration at 30°C.


B307Journal of The Electrochemical Society, 155 �3� B303-B308 �2008� B307

membrane may have reasonable suppression of methanol permeabil-ity and mechanical properties as the matrix for composite electrolytemembrane. A selection of the matrix may depend on the kind ofapplication. 3DOM polyimide composite membrane is near toNafion membrane, compared with hard 3DOM silica compositemembrane.

Figure 9 shows Arrhenius plots for proton conductivity of elec-trolyte membranes. The composite membrane showed higher protonconductivity than Nafion membrane at all temperatures, and 1.7� 10−1 S cm−1 was obtained at 60°C under 90% relative humidity.This value is about three times greater than that previously obtainedin the 3DOM silica composite membrane including AMPS gelpolymer.20 Proton transfer in 3DOM composite membrane occurs inthe polymer electrolyte phase. Therefore, the polymer in the3DMOM pores must be connected to each other for proton transferacross the membrane. The 3DOM matrix fabricated here had highuniformity as shown in Fig. 3, which makes it possible to form a lotof connecting windows that are expected to work as proton transferchannels between the 3DOM pores. This is one reason why protonconductivity of 3DOM polyimide composite membrane is higherthan that of 3DOM silica composite membrane. Table II shows theactivation energy of proton conduction estimated from Fig. 9. Simi-lar values were obtained for AMPS and its 3DOM composite mem-brane, suggesting that the proton conduction mechanism of AMPSpolymer was maintained in the composite membrane.

In order to compare the performance of membranes for DMFCs,the transfer selectivity �� between protons and methanol moleculeswas estimated as a ratio of proton conductivity against methanolpermeability. As listed in Table III, the composite membrane exhib-ited a higher � value than AMPS and Nafion membranes in allmethanol concentrations, and its � value increased with methanolconcentration. When 20 mol dm−3 methanol solution was fed, thehighest value of 1.3 � 105 S cm−3 s, which was about 10 timeslarger than that of Nafion membrane, was successfully attained.

Figure 9. Arrhenius plots for proton conductivity of 3DOM composite ��,AMPS ��, and Nafion �� membranes under 90% relative humidity.

Table II. Activation energy of proton conduction in 3DOM com-posite, AMPS, and Nafion membranes under 90% relativehumidity.

SampleConductivity at 60°C

�S cm−1�Activation energy

�eV�

Composite 0.17 0.13Nafion 0.10 0.23AMPS 0.55 0.16


Transfer selectivity has been obtained for 3DOM silica compositemembrane27 and is larger than that of the 3DOM polyimide com-posite membrane. However, the proton conductivity of 3DOM silicacomposite membrane is lower than that of 3DOM polyimide mem-brane. The high transfer selectivity for 3DOM silica compositemembrane is caused by extreme suppression of membrane expan-sion. At this moment, two kinds of composite membranes have beenprepared with different transfer features for proton and methanol.For DMFC, the selection of composite membrane is important. Forexample, 3DOM polyimide membrane may be suitable for higherpower application, but 3DOM silica membrane may be preferred forsmall portable devices with higher energy density.

Figure 10 shows the polarization curve of DMFC using the com-posite membrane. Even though the MEA was fabricated by simplecontact of carbon papers with catalyst layers to the composite mem-brane, the maximum current density of 250 mA cm−2 and powerdensity of 25 mW cm−2 were obtained at 0.2 V. The open-circuitvoltage of 0.53 V was a little smaller than that reported for Nafionmembrane in a similar test condition ��0.6 V�.28 This result is cor-related with the comparison of methanol permeability shown in Fig.8. Considering the concentration dependence of methanol perme-ability in the composite membrane, the feeding of highly concen-trated methanol solutions seems to be preferable. However, it hasbeen confirmed that our cell has relatively high contact resistancesbetween the catalyst layer and membrane, particularly in highmethanol concentration. This is due to the mechanically hard natureof 3DOM polyimide composite membrane, namely the difference inswelling properties between the composite membrane and Nafion. Itis expected that the cell performance is enhanced by improving thecontact between the composite membrane and catalyst electrodes. Inour previous paper, we reported a new fabrication method of MEAonto Nafion membrane using an electrophoretic deposition �EPD�process.29 This EPD process has been applied to the formation ofvarious kinds of materials such as filters, solid oxide electrolyte

Table III. Transfer selectivity „�… of 3DOM composite, AMPS,and Nafion membranes measured in different methanol concen-trations at 30°C.

Sample

Selectivity �� 104 S cm−3 s�

2 mol dm−3 4 mol dm−3 10 mol dm−3 20 mol dm−3

Composite 4.4 7.2 9.0 13Nafion 2.5 2.2 1.5 1.4AMPS 3.5 4.7 5.4 7.3

Figure 10. Polarization curve �� and power density �� of DMFC using3DOM composite membrane by feeding 2 mol dm−3 methanol solution at5 mL min−1 flow rate and dry oxygen gas at 135 mL min−1 at 80°C.



layers, and dielectric layers. Therefore, EPD may be one of thepreferred methods of fabricating catalyst layers on composite mem-branes.

Conclusion

3DOM polyimide matrix prepared by the colloidal crystal tem-plating method for alternative composite membranes of Nafionexhibited many excellent properties, such as highly orderedstructure, 80% volume capacity, thermal stability up to 500°C, highmechanical strength, and methanol-absorption stability. The lattertwo properties provided low methanol permeability �e.g., 9.4� 10−7 cm2 s−1 in 20 mol dm−3 methanol solution� in the compos-ite membrane prepared by injection of AMPS gel polymer into the3DOM pores, even though AMPS gel easily permeated methanol.This behavior was due to suppression of AMPS expansion by the3DOM matrix and also observed in the uptake and swelling tests. Incomparison with the 3DOM silica matrix reported previously, the3DOM polyimide had a highly uniform structure. This uniformitywas reflected in the proton conductivity of composite membrane andabout twice larger proton conductivity than Nafion membrane wassuccessfully obtained. Consequently, the transfer selectivity of 1.3� 105 S cm−3 s, which was about ten times larger than that ofNafion membrane, was attained in composite membrane. We alsoexamined the cell performance of MEA using the composite mem-brane and obtained a maximum power density of 25 mW cm−2 at0.2 V and 80°C by feeding 2 mol dm−3 methanol, although theMEA fabrication was conducted by simple contact of the compositemembrane to catalyst layers. Therefore, it is expected that the cellperformance is enhanced by improving the contact between thecomposite membrane and catalyst electrodes.

Tokyo Metropolitan University assisted in meeting the publication costs
of this article.

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