Pd electroless plated Nafion® membrane for high concentration DMFCs

Pd electroless plated Nafion® membrane for high concentration DMFCs

Journal of Membrane Science 259 (2005) 27–33 Pd electroless plated Nafion® membrane for high concentration DMFCs Hai Sun a,d , Gongquan Sun a,∗ , Sul...

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Journal of Membrane Science 259 (2005) 27–33

Pd electroless plated Nafion® membrane for high concentration DMFCs Hai Sun a,d , Gongquan Sun a,∗ , Suli Wang a,d , Jianguo Liu d , Xinsheng Zhao a,d , Guoxiong Wang a,d , Hengyong Xu c , Shoufu Hou c , Qin Xin b a

Direct Alcohol Fuel Cell Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China b State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China c 801 Group, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China d Graduate School of the Chinese Academy Sciences, Beijing 100039, China Received 14 December 2004; received in revised form 5 February 2005; accepted 8 February 2005 Available online 6 June 2005

Abstract A Pd layer was deposited on Nafion® membrane by electroless plating in order to reduce methanol crossover. The composite membrane was characterized by using the following techniques of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDXS) and transmission electron microscopy (TEM). Methanol crossover was determined by the electrochemical method. The experimental results indicated that Pd film was successfully deposited onto the Nafion® membrane. The methanol permeability was effectively suppressed by the Pd layer, with methanol-related current decreasing from 64 mA/cm2 (Jlim ) to 57 mA/cm2 for 1 M methanol and from 267 mA/cm2 to 170 mA/cm2 for 5 M methanol, respectively. Furthermore, by employing the novel Pd-Nafion® composite electrolyte, the performance of single fuel cell was obviously improved with a high concentration of methanol such as 5 M, indicating that depositing Pd layer onto the Nafion® membrane is an effective method to suppress the methanol crossover. © 2005 Elsevier B.V. All rights reserved. Keywords: DMFC; Methanol crossover; Pd; Electroless plating

1. Introduction There is a growing interest in the research of direct methanol fuel cells (DMFCs) as portable power sources and electric devices due to their high energy density and simplicity in the system structure such as easy storage and supply of fuel and needing no fuel reforming or humidification [1–3]. It is known that Nafion® perfluorosulfonic acid polymers are commonly used as the electrolyte due to their good chemical and thermal resistance and ionic conductivity in DMFCs [4,5]. Unfortunately, the present perfluorosulfonate membranes are known to be quite permeable to methanol, leading to methanol crossover to the oxygen cathode in the DMFCs, causing depolarization losses at the cathode and fuel losses and consequently poor performance. In order ∗

Corresponding author. Tel.: +86 411 84379063; fax: +86 411 84379063. E-mail address: [email protected] (G. Sun).

0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.02.017

to improve the performance of DMFCs, it is necessary to eliminate or at least to reduce methanol crossover to some extent. Recently, considerable effort has been focused on this problem. Additionally, the unique properties of palladium membrane which is permeable to protons, yet completely resistant to the transport of methanol are very promising and accommodate to the requirement of a solid electrolyte, which can be utilized in DMFCs. Pu et al. [6] investigated the performance of composite membranes that were formed by sandwiching a dense Pd foil (25 ␮m thick) between two Nafion® 115 membranes. Though the Pd-sandwiched membrane was active for proton transport and methanol crossover was eliminated, the performance of a PEMFC with the membrane was still very poor. Choi et al. [7] reported a study on the DMFCs performance with Pd-layered Nafion® membrane. The thin palladium films were deposited onto the surface of Nafion® 117 membrane using a sputtering technique. The

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membrane with a Pd layer of 20 nm-thick showed improved performance and a higher open circuit voltage (OCV) in a DMFC compared to the unmodified Nafion® 117. Yoon et al. [8] found that the DMFC’s performances employing Nafion® membranes modified by the sputter-deposited thin films were almost independent of the Pd layer thickness although both the methanol permeability and the protonic conductivity through the modified membranes decreased with increasing thickness of the Pd layer. It is a fact that modifying Nafion® membrane by sputtering was an effective way to suppress methanol crossover, but on the other hand, the Pd films were unstable and easily delaminated from the membrane surface. Furthermore, there were many cracks in the film, from which methanol permeation was still very severe. In order to reduce the possibility of embrittling due to α–β phase transition, Ma et al. [9] sputtered thin Pt/Pd–Ag/Pt layer on the surface of Nafion® membrane and found that it was effective in reducing methanol crossover. The cell with the composite membrane gave a higher OCV and better cell performance than that with a single Nafion® membrane. Generally speaking, it was an effective method to deposit Pd or Pd-based films by sputtering, but it was hard to sandwich the film between two Nafion® membranes by hot pressing. In this case, the film was easy to peel off from the Nafion® membrane. Additionally cracks were inclined to form in the membrane. Palladium membrane is widely used for hydrogen separation and purification [10,11], hydrogen recovery from process streams [12,13] and membrane reactors for H2 -related reactions (e.g. hydrogenation [14–16], dehydrogenation [17,18], methane reforming [19], ammonia decomposition [20], etc.). Several recognized film deposition techniques have been successfully used in fabricating thin supported palladium membranes, among which the technique of electroless plating is a popular method of preparing palladium membranes [21–23]. A substantial amount of works have been published on Pd membrane synthesis via electroless plating [24–27]. However, little work has been done to deposit Palladium on Nafion® membrane by electroless plating which should be a promising way to modifying Nafion® membrane to suppress methanol crossover. When palladium is deposited on Nafion® membrane, not only palladium is formed on the Nafion® membrane but also palladium particles can be formed in the Nafion® membrane by impregnating the pores. Methanol crossover can be prevented or reduced by the Pd nanophases in a Nafion® polymer membrane electrolyte [28]. Kim et al. investigated the palladinized Nafion® for DMFC application and found that thin tortuous Pd film on the Nafion® membrane improved the cell performance [29]. In the present investigation, a different method of electroless plating from Kim [29] was used to prepare Pd-layered Nafion® membrane which was characterized with XRD, SEM, EDXS and TEM techniques. The single DMFC performance employing Pd-layered Nafion® membranes and unaltered Nafion® membranes as the electrolyte were also evaluated and compared here.

Fig. 1. Preparation protocol of Pd layer modified Nafion® membrane.

2. Experimental 2.1. Preparation of Pd-layered membranes Before the preparation of Pd-layered membrane, the membranes (Nafion® 115) were treated by soaking them successively at 80 ◦ C for more than 1 h in the following solutions: 3% H2 O2 , deionized water, 0.5 M H2 SO4 , and finally deionized water. Pd membrane was deposited on Nafion® membrane by modified electroless plating shown in Fig. 1: Nafion® (H+ ) membrane was immersed in 2000 ppm Pd(II)SO4 solution at room temperature; then immersed in N2 H4 solution (50% received, N2 H4 :H2 O = 1:9 in volume) to reduce Pd(II) at room temperature. This procedure was repeated for five times to obtain a uniformly activated membrane. Then the seeded membranes were immerged in plating solution (Pd: 2000 ppm) to prepare Pd membranes at 30 ◦ C by the autocatalysis of Pd seeds. During the process of electroless plating one side of the Nafion® membrane was protected by a Teflon block in order to deposit Pd membrane on only one side of the Nafion® membrane. The composition of plating solution is listed in Table 1. The reagents were from Union Reagent Factory, Shenyang. 2.2. Characterization of Pd/Nafion® membrane Methanol crossover was determined electrochemically by measuring oxidation current of the permeating methanol [30,31] with a DMFC except that inert, well-humidified N2 instead of O2 was fed into the cathode. During the test, the flow rate of methanol solution (1 mol/L, 5 mol/L) at the anode side was kept at 1 mL/min while the nitrogen flow rate at the cathode was 40 mL/min. The current that was generated by methanol oxidation at the cathode of the DMFC was recorded as the voltage applied by the potentiostat (EG&G, M273A, PARC) was swept at 2 mV/s from 0.1 V to 0.9 V. As the voltage increased, the methanol oxidation current increased until a current density was reached where the oxidation reaction was “limited” by the transport of methanol through the membrane to the cathode side, referred to as the limiting current. Table 1 Composition of plating solution Palladium precursor Complexing agent Reducing agent

1.0 g PdCl2 15.0 g NH4 Cl 75.0 mL NH4 OH (16 M) 6.0 g Na2 PO2

H. Sun et al. / Journal of Membrane Science 259 (2005) 27–33

The temperatures of the cell and the humidifier were maintained at 30 ◦ C and 80 ◦ C, respectively. The conductivity of the membranes in the direction as shown in Fig. 3b was measured at 30 ◦ C and 0.2 MPa with two electrode method using an impedance measurement (EG&G, M273A, PARC and Model 5210 Lock In Amplifier, Perkin-Elmer Instruments), in the frequency range of 100 mHz to 100 kHz and oscillating voltage of 5 mV. XRD patterns of all samples were obtained with a Rigaku X-3000 X-ray powder diffractometer using Cu K␣ radiation with a Ni filter. The scan range was from 5◦ to 85◦ , and the scan rate was 4◦ /min. The morphologies of the surface and cross-sections of the Pd-layered Nafion® membrane were investigated with a JEOL JSM-5600LV SEM, which was integrated with an EDXS to analyze the chemical composition of the samples. Ultramicrotomy is a specimen preparation technique widely used in the biological specimens for TEM and has the ability to produce fairly large, uniform thin areas. Blom et al. [32] have prepared cross-sections samples of proton exchange membrane fuel cells by ultramicrotomy for TEM. In order to preserve the spatial relationship among the various components of the Pd-layered Nafion® membranes, a small piece of the composite membrane was embedded in an epoxy resin (Epon 812) before sectioning with a knife on an ultramicrotome. Resins were polymerized at 60 ◦ C for 24 h. Samples at a thickness of about 50 nm were microtomed at room temperature using a LKB-V ultramicrotome. The thin sections were then examined in JEOL JEM-2000EX TEM at 100 kV. 2.3. Membrane electrode assembly (MEA) fabrication Before preparing MEA Pd-layered Nafion® 115 membrane was boiled in 0.5 M NaOH, DI water to obtain Na+ -type membrane (for Nafion® 115). The catalyst was unsupported Pt–Ru black and Pt black (both from Johnson Matthey Corp.) for the anode and cathode, respectively. The Nafion® content was 15 wt% and 10 wt% in the anode and cathode ink composition, respectively. To prepare the CCM (catalyst coated membrane), a piece of Pd-layered Nafion® 115 membrane was placed on a vacuum table, then polymer electrolyte such as Nafion® (5 wt% solution, Aldrich Chemical Company Inc.) was applied onto the surface of Pd film and dried for 1 h to form layer I (0.5 mg/cm2 ) in order to improve the contact between Pd films and catalyst layer. Then the anode catalyst ink was applied onto layer I to form a catalyst layer. The cathode catalyst inks were directly sprayed onto the other side of Nafion® membrane. The loading of Pt–Ru black was ca. 3.7 mg/cm2 , and the loading of Pt black was ca. 2.3 mg/cm2 . For comparison, an MEA with pure Nafion® membrane was fabricated by the same method. The procedures were carefully controlled so that their fabrication conditions were kept identical. After hot pressing at 190 ◦ C and 1.6 MPa for 2 min to reinforce the contact between the catalyst layer and the composite membrane, the MEAs were immersed into 1 mol/L

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Fig. 2. XRD patterns of Pd thin film formed on Nafion® 115 membranes.

H2 SO4 solution to be transformed into H+ form and finally rinsed in DI water. 2.4. Single DMFC test The MEA was sandwiched between two stainless steel plates, in which crossed channels were adopted for methanol or oxygen flow. Steel meshes and silicon gaskets were used for collecting current and sealing, respectively. Electrical heaters and thermocouples were embedded in the plates to control operating temperature. A pump was employed to supply aqueous methanol solution from a reservoir without back pressure. Oxygen was fed from a cylinder, and the pressure was controlled by a pressure regulator. In all experiments, 1 M or 5 M methanol solution was pumped through the DMFC anode flow field at a flow rate of 1 mL/min for 4 h to activate the MEA before collecting data. The cathode was fed with oxygen under pressure of 0.2 MPa and flow rate of 4.3 mL/s. Current–voltage and power density curves were obtained stepwise using an Arbin electronic load and without IR corrected.

3. Results and discussion 3.1. Characterization of Pd-layered Nafion® membranes XRD patterns of the Pd layer are shown in Fig. 2. The diffraction peaks at 40◦ , 46◦ , 68◦ , 82◦ are attributed to Pd (1 1 1), (2 0 0), (2 2 0), (3 1 1) crystalline facet, respectively [8], which can give an indicator that in the Pd layer, Pd presents a highly crystallized and fcc crystalline structure. The amorphous band at ca. 18◦ could be ascribed to the crystalline peak of Nafion® [33]. Fig. 3 presents the SEM topography of Pd-layered Nafion® 115 membrane. From Fig. 3a we can see clearly that palladium can be deposited on the Nafion® membrane and the local crack-free areas of the palladium film are very

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Fig. 3. SEM images and EDX spectra of a Pd-layered Nafion® membrane.

compact, which can block methanol. Unfortunately, some cracks were found on the surface of the electroless plated Pd membrane. It was believed that some of the cracks might be caused by the residual stress between the plated Pd layer and Nafion® polymer membrane. When the composite membrane was taken out from water and dried, the Nafion® membrane shrunk to a degree, but the Pd film cannot shrink as much as the membrane, so some cracks were developed eventually in the Pd films. Some of the cracks might be produced when SEM samples were prepared. Some steps were also found on the surface. These might be formed by the unevenly coating during the process of Pd particles growing into the film or by the residual stress between the plated Pd layer and Nafion® polymer membrane as described above. Fig. 3b shows a scanning electron micrograph of a cross-section of the Pd-layered Nafion® 115. The thickness of the Pd membrane is ca. 5 ␮m. Pd membrane appears to be well attached to the Nafion® membrane and no gaps can be found between them. Fig. 3c shows EDXS spectra of Pd distribution through the cross-section as shown in Fig. 3b. There was obviously a Pd region which corresponds to the bright layer in Fig. 3b, which indicated that most Pd was successfully deposited onto the surface of Nafion® membrane to form compact Pd film. During the electroless plating, the Nafion® 115 membranes were immersed in diluted Pd(II) solution. Driven by the difference of Pd(II) concentration in the bulk region of solution and in the membrane, and by the electrostatic force between cations (Pd(II) and anions (–SO3 − ), the cations

would diffuse into the Nafion® membrane. Subsequently, the Pd(II) cation in the membrane would be reduced to Pd particles by hydrazine and Pd particles would remain in the membrane. There will be more particles near the interface of Pd film/Nafion® membrane and less particles in the interior of the membrane. The TEM image of the cross-section of Pdlayered Nafion® membrane is shown in Fig. 4. Apparently there existed many particles in the Nafion® membrane. A gradient distribution of the Pd particles is observed with less Pd particles as you move away from the Pd layer. The integration of Pd in the Nafion® membrane and the firmly attached Pd layer reduce the chance of delamination between Pd layer and Nafion® membrane. Furthermore, the gap that might be produced between Pd layer and Nafion® membrane if they were simply hot pressed together is avoided, and continuous paths are created for the proton transport through the Pd-layered Nafion® membrane. As shown in Table 2 conductivities of both membranes are in the same order and the conductivity of

Table 2 Limiting current from methanol permeation through a membrane/electrode assembly and proton conductivity of membranes at 30 ◦ C Jlim for methanol permeation (mA/cm2 ) Nafion®

115 Pd/Nafion® 115

1M

5M

64 57

267 170

Conductivity (S/cm)

1.7 × 10−2 2.0 × 10−2

H. Sun et al. / Journal of Membrane Science 259 (2005) 27–33

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Fig. 5. Current–voltage (I–V) curves of DMFC with 1 M methanol feed at 30 ◦ C.

Fig. 4. TEM image of a Pd-layered Nafion® membrane sheet.

Pd-layered Nafion® 115 is slightly higher than that of unaltered Nafion® 115. Thus the internal resistance of the MEA can be reduced and the performance can be improved. 3.2. Methanol permeability The limiting current (Jlim ) of methanol permeation through the MEA is listed in Table 2. It is clear that the methanol crossover was effectively suppressed by Pd layer. When 1 mol/L methanol was fed, Jlim decreased from 64 mA/cm2 for Nafion® 115 to 57 mA/cm2 for Pd-layered Nafion® 115; when the concentration of methanol increased to 5 mol/L, Jlim was reduced remarkably from 267 mA/cm2 to 170 mA/cm2 . The mechanism of proton transport in the Pdlayered Nafion® membrane is different from that in Nafion® membranes. In the latter case, the proton is associated with water and methanol during diffusion [34]. In the former case, however, only the proton, without associated water or methanol, can diffuse through the Pd alloy film. According to Pu [6], a hydrogen loaded palladium foil can be viewed as a proton permeable membrane: reductive adsorption of protons occurs on the surface facing the fuel anode, hydrogen diffuses through the palladium, and hydrogen atoms on the surface facing the Nafion® membrane are oxidatively desorbed as protons. When a thin Pd foil of 25 ␮m thick was sandwiched between two sheets of Nafion® 117, Shim et al. [35] demonstrated that the hydrogen transfer rate through the barrier was greatly enhanced if a negative bias potential with reference to the cell anode was applied on the barrier.

115 membrane using different methanol concentrations (1 mol/L and 5 mol/L) at 30 ◦ C. It can be clearly seen from Figs. 5 and 6 that, in both methanol concentrations, the DMFC performance using Pd-layered Nafion® 115 is better than those using pure Nafion® 115. It can also be seen that, at lower methanol concentration (1 mol/L MeOH) feed, DMFC using Pd-layered Nafion® 115 showed a slightly higher performance than that of a Nafion® 115 membrane., increasing from a maximum 36 mW/cm2 for the pure Nafion® 115 to 45 mW/cm2 with Pd-layered Nafion® 115 membrane. When 5 mol/L methanol was used, the maximum power density of the Pd-layered Nafion® 115 was 72 mW/cm2 , while the performance of MEA with pure Nafion® 115 membrane was only 32 mW/cm2 . When 5 mol/L methanol was used, the maximum power density was increased by 60% for Pdlayered Nafion® 115 compared to that using 1 M methanol but decreased 11% for pure Nafion® 115, respectively. This result was similar slightly to that of Kim et al. [28] using Pdimpregnated nanocomposite Nafion® membrane. As far as open circuit voltage (OCV) was concerned, there were some

3.3. Cell performance Figs. 5 and 6 show the respective performance of DMFCs with pure Nafion® 115 membrane and Pd-layered Nafion®

Fig. 6. Current–voltage (I–V) curves of DMFC with 5 M methanol feed at 30 ◦ C.

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differences under two conditions. When 1 mol/mL methanol was used, the OCVs were almost same for both kinds of membranes. However, when 5 mol/L methanol was used, the OCVs were lower than that of 1 mol/L for both membranes, and the OCV of Pd layered Nafion® membrane was higher than that of pure Nafion® membrane. The increase in the performance of DMFCs using Pd-layered Nafion® 115 membrane should be ascribed to the fact that the permeation of methanol from the anode to the cathode was reduced by the Pd membrane even though there were some cracks on its surface. The results indicated that a high concentration of methanol as a fuel could be applicable in DMFC at low temperature by using Pd-layered Nafion® membrane by electroless plating.

4. Conclusions A Pd layer modified Nafion® membrane was obtained by electroless plating. The SEM results indicated that Pd layer can be attached to Nafion® membrane closely and firmly, and TEM results showed that the Pd layer was integrated with the Nafion® membrane by Pd particles near the interface. The methanol crossover through Nafion® membrane was significantly reduced by the Pd layer. In the case of Pd-Nafion® composite membrane, when higher concentrations of methanol (5 mol/L) were used the DMFC delivered much better performance than that using 1 mol/L MeOH, and their corresponding peak power density increased from 45 mW/cm2 to 72 mW/cm2 . However, with pure Nafion® , the performance of DMFC decreased slightly from 36 mW/cm2 to 32 mW/cm2 when methanol concentration changed from 1.0 mol/L to 5.0 mol/L. This suggested that high concentrations of methanol could be used as fuel at low temperature to obtain better performance with Pd layer modified Nafion® membrane by electroless plating.

Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 20173060), Hi-Tech Research and Development Program of China (2003AA517040) and Innovation Foundation of Chinese Academy of Sciences (K2003D2).

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