(2007) 865–870www.elsevier.com/locate/ssi

Solid State Ionics 178

Palladium composite membranes using supercritical CO2 impregnationmethod for direct methanol fuel cells

Donghyun Kim a, Junho Sauk b, Jungyeon Byun a, Kab Soo Lee c, Hwayong Kim a,⁎

a School of Chemical and Biological Engineering and Institute of Chemical Process, Seoul National University,Shinlim-dong, Gwanak-Gu, Seoul 151-744, South Korea

b CTO Energy 1 Team, Samsung SDI Co., Ltd., 575, Shin-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do, 443-731, South Koreac Environmental System Engineering, Kimpo College, San 14-1, Ponae-ri, Wolgot-myun, Gyounggi-do, 415-761, South Korea

Received 20 September 2006; received in revised form 12 December 2006; accepted 17 February 2007

Abstract

Palladium composite membranes were synthesized using the supercritical impregnation method in order to reduce their methanol permeationproperties in direct methanol fuel cells. Pd(II)(acetylacetonate)2 was dissolved in supercritical CO2 and impregnated into the Nafion membranes.The impregnated Nafion membranes were converted to the palladium deposited Nafion membranes by treating them with various concentrationsof a reducing agent, sodium borohydride (NaBH4), at 50 °C for 2 h.

The morphology of the surface and palladium distributions of the Pd/Nafion composite membranes were investigated by transmission electronmicroscope (TEM), electron dispersive spectrometry (EDS) and electron probe micro analysis (EPMA). To evaluate the palladium loading, themembranes were analyzed by ICP-AES. The prepared palladium composite membranes were characterized by measuring their methanolpermeabilities and ion conductivities. Then, the characterized properties were compared with those of Nafion. Finally, the performance of a cellemploying the palladium composite membrane was evaluated using a DMFC single cell.© 2007 Elsevier B.V. All rights reserved.

Keywords: Palladium; Supercritical CO2; Impregnation; DMFC

1. Introduction

The Direct Methanol Fuel Cell has received considerableattention because of its many advantages, namely its highenergy density, easy storage, supply of fuel and lack of need forfuel reforming [1–3]. Nafion is commonly used as the solidelectrolyte in DMFCs, due to its good chemical and physicalproperties and ion conductivity. However, Nafion is quitepermeable to methanol, and this leads to the so-called mixedpotential phenomena and decreases the efficiency of the fuelcell. In order to improve the performance of DMFCs, it isnecessary to reduce their methanol permeability [4,5]. Severalapproaches have been taken to solve this problem. On suchapproach involves modified Nafion membranes such as theradiation-modified Nafion membrane [6].

⁎ Corresponding author. Tel.: +82 2 880 7406; fax: +82 2 888 6695.E-mail address: hwayongk@snu.ac.kr (H. Kim).

0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2007.02.034

The concept of the palladium-Nafion membrane wasproposed in order to reduce the methanol permeability, becausepalladium allows the selective transport of protons and watermolecules, while restricting the passage of methanol molecules.Sun et al. [7] suggested that high concentrations of methanolcould be used as a fuel at low temperature to obtain betterperformance Nafion membranes with a Pd layer were depositedby electroless plating. Hejzea et al. [8] prepared Pd-coatedNafion 117 by electroless deposition from commercial platingbaths and found that it had reduced methanol permeabilitycompared to the bare Nafion 117 membrane. However, severecracking problems occur on the Pd layer when it is depositedonto the membranes by the electroless plating method. We triedto enhance the performance to keep the thickness of the Pd layerin the membrane as thin as possible.

Recently, impregnation techniques using supercritical fluids,particularly supercritical CO2 (scCO2), have attracted a greatdeal of attention. Using scCO2 as a swelling agent, it is possible

866 D. Kim et al. / Solid State Ionics 178 (2007) 865–870

to obtain polymer modifications by avoiding thermal stressesand plasticizing effect on polymers during impregnation [9].Supercritical fluids exhibit both gas-like and liquid-likeproperties. They can dissolve organic solid compounds likeorganic solvents, and possess low viscosity and high diffusivitylike gases. Supercritical carbon oxide (scCO2) has severaladvantages as an impregnation solvent for polymer membranes.High penetration of impregnation substances is expected, due tothe low viscosity and high diffusivity of scCO2, while thesolubility of the solutes is easily controlled by adjusting thepressure. Moreover, the removal of the scCO2 from the productis easy [10]. Yoda et al. [11] reported that a metal/polyamidenanocomposite film could be prepared by impregnation, usingscCO2 as a solvent for the metal precursor. Saquing et al. [12]synthesized Platinum/carbon aerogel nanocomposites using asupercritical deposition method.

In this study, we synthesized palladium-impregnated Nafioncomposite membranes by supercritical-impregnation and chem-ical reduction. The morphology and structure of the Pd-impregnated Nafion composite membranes were investigatedby TEM, EDS, and EPMA. To confirm the loading ofpalladium, the membranes were analyzed by ICP-AES. Themethanol permeability and ion conductivity were also measuredin order to characterize the composite membranes and comparethem with a Nafion 117 membrane. The performance of thecomposite membrane was examined in a DMFC unit cell.

2. Experimental

2.1. Material

The Nafion117 (Dupont) membrane was converted into theproton form by treating it for 1 h in 10 wt.% H2SO4 (98%,Aldrich) at 80 °C. Then, the membrane was soaked in deionizedwater for 1 h at 80 °C in order to remove the excess sulfuric acid[13].

The palladium precursor, Pd(acac)2 (Palladium(II)acetylace-tonate, 97%) was purchased from Aldrich and the reducingagent, NaBH4 (sodium borohydride), was purchased fromJUNSEI. Carbon dioxide with a purity of 99.99% waspurchased from Korea Industrial Gas and used as received.

Fig. 1. Schematic diagram of the impregnation and polymerization apparatu

2.2. Preparation of Pd-impregnated Nafion composite membrane

A batch type high-pressure vessel that was fitted with awindow to observe the phase changes was used for theimpregnation process. The pretreated Nafion 117 membranewas cut into strips (5 cm×5 cm) and then placed inside thevessel with the palladium precursor, Pd(acac)2 (10 mg). Carbondioxide was supplied using a gas booster pump (MaxproTechnologies Inc., Model DLE 75-1). CO2 was charged into thevessel until the desired pressure was reached. The impregnationstep was started by raising the temperature to 80 °C and wasallowed to continue for 4 h at a pressure of 20 MPa. A diagramof the supercritical impregnation apparatus is given in Fig. 1.After the impregnation process, the reactor was cooled undercritical temperature, and the pressure decreased by releasing theCO2.

The precursor impregnated Nafion 117 was immersed insodium borohydride in order to prepare the Pd layer bychemical reduction. We used various concentrations of NaBH4

solution (0.5, 2, 10, 100 mM).

2.3. Characterization of composite membrane

To observe the size of Pd nanoparticle impregnated in theNafion membrane, Transmission electron microscope (TEM,JEOL 2010) analysis was carried out and the Pd distribution ofthe composite membrane was confirmed by electron dispersivespectrometry (EDS, Oxford INCA Energy). The surface wascoated with platinum to prevent it from becoming charged [14].Electron probe micro analysis (EPMA, JXA-8900R) was used toanalyze the cross-section of the membranes, and they werecoated with carbon to prevent charging.

In order to determine how much palladium was impregnated,the synthesized membranes were analyzed by Inductivelycoupled plasma-atomic emission spectroscopy (ICP-AES,OPTIMA 4300DV). Samples were decomposed in acids bymicrowave digestion.

The methanol permeability of the composite membrane wasmeasured using an in-house diffusion cell at room temperature.A 2 M methanol solution (20 ml) and pure water were fedthrough opposite sides of the membrane. Both parts of the cell

s (P=pressure gauge; T= temperature gauge; PR=pressure regulator).

Fig. 2. Transmission electron microscope of Pd/Nafion membranes reduced with (a) 0.5 mM, (b) 2 mM, (c) 10 mM and (d) 100 mM.

867D. Kim et al. / Solid State Ionics 178 (2007) 865–870

were stirred during the permeation experiments. After a fixedperiod of time, the amount of methanol that diffused through themembrane into the other side of the vessel was determined bygas chromatography (GC-2010, Shimadzu), using a gaschromatograph equipped with a packed column (Propark Q)and a flame ionization detector (FID). The methanol perme-ability was determined from the equation developed by Tricoli[15].

The ion conductivity of the hydrated composite membranewas investigated by means of electrochemical impedancespectroscopy (EG&G Model 273A potentiostat/galvanostat)over the frequency range of 1 Hz–100 kHz with an ACperturbation of 10 mV. The four-probe method was used todetermine the resistance of the composite membrane.

2.4. MEA fabrication

Before preparing the MEA, the Pd-impregnated Nafioncomposite membrane was boiled in 10 wt.% H2SO4 to convert itfrom the Na-form to the H-form. The anode and cathode werePtRu/C 60 wt.% and Pt/C 60 wt.% electrodes (from E-tek, Inc.),and the average catalyst loading of each electrode was 4 mgcm−2. The membrane electrode assembly (MEA) was fabricat-ed by hot pressing with a pressure of 13.8 MPa at 125 °C for2 min.

Fig. 3. Mapping of palladium element at surface of Pd/Na

2.5. Single cell test

The composite membrane was tested under DMFC condi-tions. The DMFC single cell tests for the Pd-impregnatedNafion membrane and pure Nafion membranes were performedat 80 °C.

A 2 M or 5 M methanol solution was pumped into theDMFC anode at a flow rate of 2 ml min−1. The cathode wasfed with air of 300 ml min−1 and the pressure of betweenanode and cathode was 200 kPa. The pressure of air wascontrolled by a back-pressure regulator. The current–voltageand power density curves were investigated using anelectronic load.

3. Results and discussion

Fig. 2 shows the TEM image of the Pd/Nafion membrane. InFig. 2(a), the size of Pd particles was 4–5 nm (scale bar: 20 nm).However in Fig. 2(b)–(d) the size of Pd particles was 20–70 nm(scale bar: 50 nm). As the concentration of NaBH4 solutionincreased, the Pd particle size was increased and aggregated.Especially, Pd particles in Pd/Nafion membranes reduced with100 mMNaBH4 solution were 10 times larger than that of Fig. 2(a). Because of lower concentration of reducing agent solution,the number of seeds to reduce the Pd precursors was deceased,

fion membrane reduced with 2 mM by EDS analysis.

Fig. 4. Mapping of palladium element at cross-section of Pd/Nafion membrane reduced with (a) 0.5 mM, (b) 2 mM, (c) 10 mM and (d) 100 mM by EMPA analysis.

Fig. 5. Palladium contents of Pd/Nafion membranes.

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and reduction rate was slow, therefore the size of Pd particle wasdecreased.

We were able to confirm the existence of Pd in themembranes, by using EDS analysis to map out the dis-tribution of palladium element. The EDS profile is shownFig. 3. The surface of the membrane was covered uniformlywith Pd.

We also attempted to confirm the presence of palladiumusing SEM and EDS analysis at the cross-section, but since themembrane was not prepared by electroless plating onto theNafion membrane, the palladium layer was situated in thematrix. Therefore, the palladium layer was not easy to observe.Thus, we tried a more detailed analysis by EPMA. Fig. 4 showsthe EPMA cross-section image of the Pd/Nafion membranes.As shown in Fig. 4, the Pd layer was very thin. In the membranereduced by 0.5 mM NaBH4 solution shown in Fig. 4(a), Pdparticles were distributed over a substantial depth, but the levelof Pd was quite low. On the other hand, the membranes shownin Fig. 4(b)–(d) had a thin layer of Pd and, as the concentrationof the reducing agent increased, the layer of palladium becamethicker and the level of Pd became higher. This was because,when the concentration of the reducing agent solution washigher, the reduction reaction proceeded more quickly at the

surface, and the Pd layer that was formed at the surface hinderedthe dissolution of the impregnated Pd precursor into thereducing agent solution.

Fig. 5 shows the ICP-AES results for the Pd/Nafionmembranes. The Pd contents of membranes were 0.304,0.493, 0.544 and 0.561 wt.% for the composite membranesreduced by the 0.5, 2, 10 and 100 mM NaBH4 solutions,

Table 1Comparison of the ion conductivity and methanol permeability of Pd/Nafionmembranes at room temperature

Ion conductivity(S cm−1)

Methanol permeability(cm2 s−1)

Nafion 117 0.0740 1.20×10−6

0.5 mM 0.0833 6.63×10−7

2 mM 0.0765 4.38×10−7

10 mM 0.0734 3.05×10−7

100 mM 0.0726 1.60×10−7

Fig. 7. Cell performance of Pd/Nafion membrane reduced 2 mM NaBH4

solution at 80 °C DMFC system with 5 M methanol.

869D. Kim et al. / Solid State Ionics 178 (2007) 865–870

respectively. This coincided with the results of the EPMAanalysis, in that the amount of Pd was proportional to theconcentration of NaBH4.

The ion conductivity of the Pd/Nafion membranes is shownin Table 1. The highest ion conductivity was observed for thePd/Nafion membrane reduced by the 0.5 mM NaBH4 solution.However, the differences in the proton conductivities for the Pd/Nafion samples were not significant, due to the pseudo-protonconducting of the Pd particles. Especially, the ion conductivitiesof the samples reduced by the 10 and 100 mM NaBH4 solutionswere lower than those of Nafion. When the Pd particles weresmall and dispersive in the case of the Pd/Nafion membranereduced by 0.5 mM NaBH4, the ion conductivity was increased,because Pd particles act as proton conductors. However, as theconcentration of the reducing agent increased, the palladiumparticles that were formed in the membrane became larger, thuscausing the activity of Pd as a proton conducting carrier to bediminished.

The methanol permeability of these membranes is shown inTable 1 as a function of the concentration of the reducingagent. As the NaBH4 concentration increased, the permeabilitywas decreased. The permeability of Nafion 117 was1.20×10−6 cm2 s−1, as calculated by the Tricoli equation,while that of the Pd/Nafion membrane reduced by 100 mMNaBH4 solution was 1.60×10−7 cm2 s−1. The methanolpermeability of the Pd/Nafion membrane reduced by 100 mMNaBH4 solution was 13% less than that of Nafion 117. Sincelayer with Pd nanoparticles can act as a blocking layer whichblocks the pathway of methanol diffusion. As the NaBH4

Fig. 6. Cell performances of Pd/Nafion membranes according to changing con

concentration increased, the Pd contents of membranes wereincreased, then the permeability of methanol was decreased.

Fig. 6 shows the cell performances of the Pd/Nafionmembranes prepared with different concentrations of reducingagent. We tested the membranes in a DMFC single cell with2 M methanol at 80 °C with air in the cathode. All of the Pd/Nafion membranes exhibited better cell performance thanNafion 117 and the best cell performance was observed forthe Pd/Nafion membrane reduced by 2 mM NaBH4 solution.At 0.35 V, its current density was 260 mA cm−2, while thatof Nafion 117 was 180 mA cm−2. The performance of thePd/Nafion membrane reduced by 100 mM NaBH4 solutionwas lower than that of the membrane reduced by 0.5 mMNaBH4 solution. Although the former membrane had a lowermethanol permeability, its ion conductivity was lower and itsinterfacial resistance was higher than that of the lattermembrane, due to the layer of Pd being composed of bulkyparticles at the surface.

Fig. 7 shows the cell performances using 5 M methanolsolution as the fuel under the same conditions. The currentdensity of the Pd/Nafion membrane reduced with 2 mM NaBH4

solution was twice that of Nafion 117. This means that Pd/

centration of reducing agent at 80 °C DMFC system with 2 M methanol.

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Nafion membrane is more suitable for DMFCs with a highconcentration of methanol.

4. Conclusion

Pd/Nafion membranes were prepared by impregnatingpalladium acetylacetonate into Nafion membranes in thepresence of supercritical carbon dioxide (scCO2). Since theimpregnated membranes were reduced, they could be used asproton exchange membranes. The size and distribution of Pdparticles in these membranes were observed with TEM. Wewere able to confirm the presence of Pd in the membranes byusing EDS analysis to map out the distribution of palladiumelement. The cross-sections of the Pd/Nafion membranes wereinspected by EPMA, in order to monitor the distribution of thePd particles. Through the ICP-AES analysis, the Pd loadingswere found to range from 0.304 to 0.561 wt.% of the Nafionmembranes. The ion conductivities of some of the Pd/Nafionmembranes were higher than that of Nafion 117. The methanolpermeability of these membranes was studied as a function ofthe concentration of reducing agent. As the reducing agentconcentration increased, the crossover phenomenon deceased.The cell performances of the Pd/Nafion membranes were asmuch as 44% higher than that of Nafion 117 when using 2 Mmethanol solution at 80 °C and 0.35 V. The Pd/Nafionmembranes prevented the methanol crossover problem effi-

ciently, and are suitable for use as the electrolyte of DMFCswith a high concentration of methanol.

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