New DMFC Anode Structure Consisting of Platinum Nanowires

May 12, 2007 - The DMFC performance test further showed that the integrated electrode yielded a higher performance than did the conventional E-TEK ele...
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J. Phys. Chem. C 2007, 111, 8128-8134

New DMFC Anode Structure Consisting of Platinum Nanowires Deposited into a Nafion Membrane Z. X. Liang and T. S. Zhao* Department of Mechanical Engineering, The Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China ReceiVed: February 11, 2007; In Final Form: April 4, 2007

We propose a new DMFC anode structure consisting of platinum nanowires that are electrochemically deposited into the partial layer of a Nafion membrane. The formed platinum-nanowire network in such a Pt-Nafion integrated electrode not only provides the electron conduction paths but also functions as the catalyst for the methanol oxidation reaction, while the remaining part of the membrane with no Pt keeps on the function as the electrolyte. Transmission electron microscopy (TEM) images showed that Pt nanowires were uniformly distributed into the partial layer of the membrane, with the diameter ranging from 2 to 3 nm. Cyclic voltammetry tests showed that the Pt-Nafion integrated electrode possessed a larger electrochemically active surface area than did the conventional E-TEK electrode. The DMFC with this new anode structure demonstrated a lower rate of methanol crossover as the result of the incorporation of Pt nanowires into the hydrophilic pores of the membrane. The DMFC performance test further showed that the integrated electrode yielded a higher performance than did the conventional E-TEK electrode.

Introduction The direct methanol fuel cell (DMFC) has recently attracted much interest as it has been identified as a promising candidate to compete with conventional batteries for powering portable electronic devices.1-2 The membrane electrode assembly (MEA) is the key component of the DMFC, in which the electrochemical reactions take place and the electrical energy is produced. The MEA consists of three components: the anodic elecrode, the cathodic electrode, and the polymer electrolyte membrane (PEM). The electrode used in the DMFC is generally prepared by two methods: (1) to brush the as-prepared catalyst ink directly onto the gas diffusion layer; (2) to spray the catalyst ink onto a PTFE blank sheet, and then to transfer the dried catalyst layer onto the Nafion membrane.3-4 In these two methods, the first and key step is to prepare the catalyst ink, which is a mixture of a calculated amount of catalyst and Nafion resin in suitable solvents. It has been reported that the Nafion has an optimal content in the ink, which ranges generally from 10% to 40% for the anode.5-7 A higher Nafion content will lead to a higher proton conductivity and increased porosity, while an excess of Nafion may form a thin film on the catalyst aggregates that may lead to a poor electron conductivity.7 In other words, in preparing electrodes by the conventional method, an increase in the proton conductivity by increasing the Nafion content may cause a decrease in the electron conductivity. As a result, the conventional electrode usually shows a lower catalyst utilization, about 50% in an optimized conventional electrode.1 Additionally, the study of the Nafion morphology and distribution in the catalyst layer showed that it was difficult for Nafion aggregates to penetrate into the mesopores of the catalyst layer in conventional electrodes.5,8 Without the contact with Nafion resin, the catalyst sites in the mesopores contribute little to the overall electrode activity, which leads to the lower catalyst utilization in conventional electrodes. To achieve a * Corresponding author. Tel.: (852) 2358 8647. E-mail: [email protected].

higher catalyst utilization, it is desirable to explore alternative designs of the electrode for fuel cells. New Design of the Anode Structure As discussed above, the balance of the proton and electron conductivities greatly limits the catalyst utilization in conventional electrodes. It can be inferred that distinguishing the conduction paths for electrons and protons may provide a solution to this problem. In line with this idea, in this work we propose a new design of the anode, as sketched in Figure 1, in which the nanowire-like platinum catalyst resides in a partial layer of a Nafion membrane. In this new electrode design, the platinum-nanowire network to be formed not only provides the electron conduction paths but also functions as the catalyst for the methanol oxidation reaction, while the remaining part of the membrane with no Pt keeps on the function as the electrolyte. As such, the electron and proton conduction paths can be separated, which is expected to be beneficial to acquire a high catalyst utilization of the electrode. This new electrode design is termed as the Pt-Nafion integrated electrode, as opposed to the conventional anode that is hot-pressed onto the outer surface of the Nafion membrane. The new integrated electrode was prepared by the electrochemical deposition method. Electrochemical deposition has been widely used to prepare metal nanowires. Recently, it has been reported that the electrochemical deposition method can offer a novel way to deposit catalysts selectively at the desired locations in the electrode with both ionic and electronic accessibility.9-14 Missiroli et al.13 studied the properties of PtRu catalysts prepared by the electrochemical deposition on the C-Nafion supports, and found that the optimized PtRu/CNafion catalysts displayed a higher catalytic activity than that of a commercial system. Shen et al.14 prepared the electrode by depositing platinum directly at the interface of the gas diffusion layer and the solid polymer electrolyte. However, the catalyst activity to the MOR was reported to be much lower

10.1021/jp0711747 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/12/2007

New DMFC Anode Structure

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Figure 3. Schematic of the homemade setup for the platinum electrodeposition (-3.5 mA cm-2 was applied for the electrodeposition). Figure 1. Schematic of the Pt nanowire-Nafion integrated electrode. Black zone: platinum; white color: hydrophilic pore in Nafion membrane.

Figure 2. Schematic of the homemade setup for the copper plating.

TABLE 1: Composition of Copper Plating Solution CuSO4‚5H2O concentration

10 g/L

Rochelle salts NaOH NiCl2‚6H2O HCHO 45 g/L 10 g/L

0.7 g/L

10 mL/L

than the conventional electrode, which might result from the different catalyst particles size of the prepared electrode (200 nm) and the commercial one (5 nm). Experimental Preparation of the Pt-Nafion Integrated Electrode. To electrodeposit platinum into the pores of the Nafion membrane, a copper layer of several micrometers was first plated onto the membrane surface to act as the working electrode. Cu-Nafion Bilayer Membrane Preparation. The commercial Nafion 115 membranes (Electrochem. Inc.) were treated according to the membrane cleaning procedure detailed elsewhere.15 Briefly, the Nafion 115 membranes were boiled at 100 °C for 1 h in 3% H2O2 solution (Riedel-deHae¨n), deionized (D.I.) water, 0.5 M H2SO4 solution (AnalaR), and again in D.I. water. The Nafion membrane was assembled into the homemade setup (shown in Figure 2) with only one face immersed in the plating solution. The plating solution composition is listed in Table 1.16 Before plating, the Nafion membrane has to be sensitized and activated. First, the Nafion membrane was treated with SnCl2 solution (20 g/L SnCl2‚2H2O (nacalai tesque), 40 mL/L HCl (37%, TEDIA)) for 2 min, followed by rinsing with D.I. water for ten times. Then, the sensitized Nafion membrane was activated with AgNO3 solution (4.9 g/L AgNO3 (AnalaR), 23 mL/L NH3‚H2O (28%, Wako)) for 7 min, followed by another rinsing with D.I. water for ten times. The plating of copper was carried out at room temperature with oxygen bubbling for 30 min. The Cu-Nafion 115 membrane was then removed from the plating setup and assembled into the electrodeposition setup (shown in Figure 3) to carry out the platinum electrodeposition.

Pt Electrodeposition. We used a gold-coated stainless steel foil as the current collecting contact, which was in contact with the Cu layer on the membrane. The electrodeposition experiment was carried out under the galvanostatic mode on the Autolab PG30 potentiostat. The electrodeposition solution was composed of 60 mM tetraammineplatinum dichloride (Aldrich) and 1.0 M potassium chloride (Riedel-deHae¨n). The purpose of the addition of KCl is to decrease the voltage loss due to the large solution resistance during the electrodeposition. The deposition of platinum was carried out in two steps. First, platinum was deposited onto the surface of the Nafion membrane to act as the current collector for the further application as an electrode in the DMFC. This was achieved by the chemical displacement with the previously plated copper for 100 s. The reaction is as follows:

Pt(NH3)42+ + Cu ) Pt + Cu2+ + 4NH3 Second, the electrodeposition of platinum was carried out at a current density of -3.5 mA cm-2 for 95 min under nitrogen bubbling. A Pt foil (1 × 1 cm2) was used as the counter electrode. After the platinum deposition, the copper layer was removed in the 0.05 M CuCl2 solution (containing 3% HCl) in an ultrasonic bath for 2 h.17 The membrane was then boiled at 100 °C in 3% H2O2 and D.I. water for 1 h, respectively. The membrane was boiled in 2 M sulfuric acid for 12 h to remove the tin hydroxide and exchange other cations, followed by rinsing and boiling in D.I. water to remove excess sulfuric acid in the membrane. The Nafion membrane was dried in the vacuum oven at 60 °C for 2 h before and after the platinum deposition. Then the platinum loading was measured to be 4 mg cm-2 by weighing the dried membrane before and after the deposition. Characterization of the Pt-Nafion Integrated Electrode. The X-ray diffraction (XRD) measurements for the Nafion 115 and the Pt-Nafion electrode were carried out by using a Phillips PW 1830 diffractometer with a Cu KR radiation source operated at 40 keV and at a scan rate of 0.05° s-1. Scanning electron microscopy (SEM) (JEOL-6700F) was used to examine the surface morphology of the prepared samples. A transmission electron microscope (TEM) (Philips CM-20) was used to examine the Pt distribution of the prepared Pt-Nafion 115 electrode. The sample was prepared by cutting 1.0 mm wide strips. A silicone embedding mold was slit to open when bent and the strip was positioned in the slit to allow the cross sectioning. Then, the epoxy was added to the samples until it became hardened. A Leica ultra-microtome was used to cut the membrane to obtain a membrane slice with a thickness of about 70 nm for the TEM characterization.

8130 J. Phys. Chem. C, Vol. 111, No. 22, 2007 Electrochemical Characterization of the Pt-Nafion Integrated Electrode. To characterize the activity of the PtNafion integrated electrode as the anode, the commercial E-TEK electrode was employed as the cathode (Pt black with 4 mg cm-2). The MEA with an active area of 1.0 cm2 was fabricated by sandwiching the Nafion 115 membrane between the anode and cathode; which were hot pressed at 135 °C under the pressure of 4 MPa for 3 min. The anode gas diffusion layer was prepared with waterproofed Toray carbon paper TGPH090 with the PTFE loading of 12%, which was then coated with Vulcan carbon powder with the loading of 2 mg cm-2. For comparison, we also fabricated a MEA with a Nafion 115 membrane and the commercial E-TEK electrodes (Pt black with 4 mg cm-2) as the anode and cathode. The electrochemically active surface area (ECSA) was measured by using the cyclic voltammetry (CV) technique in the fuel cell configuration. The experiments were conducted by connecting, respectively, the anode of the cell to the working electrode and the cathode to the reference and counter electrodes of the potentiostat. During the course of the experiment, the anode was fed with D.I. water (N2 was purged to D.I. water for about 30 min before it was fed to the cathode) at 1.0 mL min-1, and the cathode was fed with hydrogen gas at 20.0 mL min-1. The CV curves for the anode were recorded by applying the potential between -0.05 and 1.0 V at a scan rate of 20 mV s-1. The anode polarization experiment was conducted by connecting, respectively, the anode of the cell to the working electrode and the cathode to the reference and counter electrodes of the potentiostat. The cathode was fed with H2 at 20 mL min-1 and the anode was fed with 1.0 M methanol solution at 1.0 mL min-1. The anode polarization curve was recorded by applying the potential between 0 and 0.72 V at a scan rate of 3 mV s-1. The rate of methanol crossover was evaluated by the voltammetric method using the DMFC configuration. The experiments were performed by connecting, respectively, the cathode of the cell to the working electrode and the anode to the reference and counter electrodes of the potentiostat. During the experiment, the anode of the cell was fed with 1.0 M methanol solution at a flow rate of 1 mL min-1 and the cathode of the cell was fed with D.I. water (nitrogen-bubbled) at a flow rate of 1 mL min-1. The methanol electro-oxidation curve was recorded by applying a potential between 0.1 and 1.0 V at a scan rate of 1 mV s-1. The limiting current density was regarded as the value for methanol crossover rate. The DMFC performance test was conducted at 75.0 ( 0.2 °C by feeding 1.0 M methanol at a flow rate of 1 mL min-1 using a high-pressure piston pump (Model Series III, Scientific Systems Inc.) and by feeding dry oxygen gas into the cathode at a flow rate of 20 mL min-1 at ambient pressure by fixing the load current, which was controlled with an electric load system (BT2000, Arbin Instrument Inc.). The cell resistance was measured by the current-interruption method. Results and Discussion Figure 4 shows the SEM images of the surface morphology of the Nafion membrane at the different stages of treatment. Figure 4, parts a and b, shows the surface morphology of the silver-activated Nafion 115 membrane in different magnifications. From Figure 4a, it can be observed that the silver particles are uniformly distributed on the membrane, from which it could be inferred that the sensitization of the membrane surface is uniform in the first step. The uniform distribution of silver particles on the membrane ensured the even plating of copper

Liang and Zhao onto the membrane in the next step. Figure 4b shows that the size of silver particles is in the range of 2-3 µm. Figure 4c shows the copper morphology on the Cu-plated membrane surface. As expected, the copper layer formed on the membrane was complete and uniform, which made it possible to obtain a uniform platinum electrodeposition in the next step. Additionally, the lighter dots on the surface may be attributed to silver particles introduced in the activation step. Figure 4d shows the surface morphology of the synthesized Pt-Nafion electrode. The platinum is observed to form a thin “film” on the surface of the membrane, which serves as the electrical conductor in the DMFC anode, as discussed in the following section. The wrinkles in the Pt thin film might be caused by the dehydration of the membrane in the vacuum chamber of the SEM instrument. Figure 4e shows the energy-dispersive X-ray spectrometer (EDS) result of the prepared Pt-Nafion electrode, from which the elements and the content are calculated and listed in Table 2. The elements copper and tin were not found, indicating that they had been removed completely in the preparation process. The presence of elements C, F, and O is due to the base Nafion membrane, although the surface mainly consists of platinum. These elements are detected because X-ray could penetrate into several micrometers in depth, and hence the EDS results are the combined effect of the surface and the bulk elemental information. Additionally, a small amount of silver was detected, which was introduced in the activation process of the membrane. Figure 5 shows the TEM images of the cross-sectional morphology of the Pt-Nafion integrated electrode. The overall morphology presented in Figure 5a shows that the platinum is present both on the surface of the membrane and in the bulk membrane. The platinum on the surface formed a thin layer, which acts as the current collect when the Pt-Nafion integrated electrode is used as the anode in the DMFC. The thickness of the platinum layer is found to be not uniform, which is in the range of 0.6-1.4 µm. The platinum thickness nonuniformity could result in a large contact resistance when the Pt-Nafion integrated electrode is used as the electrode in the fuel cell, as discussed later. The nonuniformity in thickness can be explained as follows. There exist many “defects” at the interface between the metal layer and membrane in forming the Cu-Nafion bilayer membrane. And these defects might prevent the platinum ions from being reduced at these positions in the electrodeposition. First, hydrogen bubbles are produced and reside on the membrane surface in the copper plating process, which may result in defects in the formed copper layer. Second, defects may also be formed during the displacement plating of platinum before electrodeposition. The Pt(II) ion transports through the Nafion membrane and is reduced to the metal state by the copper, whereas this process may be carried out nonuniformly due to the presence of silver particles on the Cu-Nafion membrane surface. Figure 5b shows the detailed morphology of platinum particles on the surface of the membrane (at the left side of Figure 5a.). The platinum particles on the membrane surface exhibit a narrow size distribution, mainly in the range of 5-6 nm. The platinum distribution and morphology in the bulk membrane presented in Figure 5, parts c and d, indicate that the platinum deposited in the membrane is in the nanowire state. The Pt nanowires exhibit a uniform and narrow diameter distribution of 2-3 nm, and these nanowires are found to form networks in the membrane. The metal wire diameter is much smaller than that of the results reported elsewhere,18-19 in which the silver wire diameter was in the range of several hundred nanometers. This can be explained by the different Nafion membrane properties and the electrodeposition conditions. Chou

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Figure 4. SEM images of the surface morphologies of the Nafion membrane at different stages of treament: (a) surface of the silver-activated Nafion 115 membrane at the magnification of 200×, (b) surface of the silver-activated Nafion 115 membrane at the magnification of 500×, (c) copper morphology on the Cu-plated Nafion 115 membrane surface, (d) surface of the synthesized Pt-Nafion 115 membrane, and (e) energydispersive X-ray spectrometer (EDS) result of the prepared Pt-Nafion 115 membrane.

TABLE 2: EDS Results of the Elements and Content in the Pt-Nafion 115 Membrane element

wt %

atom %

C O F Ag Pt

14 3 27 4 51

37 6 47 1 8

et al.18-19 employed a recast Nafion membrane made at room temperature as the template for the synthesis of silver nanowires. It is understood that there are many large ”holes” of several hundred nanometers in the recast Nafion membrane that are caused by the evaporation of solvents in the preparation

process.20-21 Therefore, the silver nanowires deposited in the recast Nafion membrane exhibit a diameter of several hundred nanometers. In this work, we employed a commercial Nafion 115 membrane. It is well understood that the extruded Nafion 115 membrane does not contain such holes due to the different preparation routes. Hence, the formation of the nanowire of platinum directly proves the existence of hydrophilic channels in the commercial Nafion membrane. This result could be regarded as a proof to support the Gierke’s model,22 which proposed that there exist hydrophilic channels formed by the side chain and crystalline region formed by the fluorocarbon backbone in the Nafion membrane. It should be noted that the platinum-nanowire network is electronically interconnected, as

8132 J. Phys. Chem. C, Vol. 111, No. 22, 2007

Liang and Zhao

Figure 5. TEM images of the cross-sectional morphology of the Pt-Nafion 115 membrane: (a) the overall morphology of the Pt-Nafion 115 membrane (detailed information at positions (b) and (c) is given in Figure 4, parts b-d), (b) the platinum particles morphology on the left side of the Pt-Nafion 115 membrane, (c) and (d) the platinum nanowires morphology in the bulk Nafion membrane.

it is formed by the electrochemical deposition method. Thus, the proton paths (hydrophilic channels in the membrane) and the electron paths (Pt nanowires) are separated and do not interfere with each other in this Pt-Nafion integrated electrode. Figure 6 shows the cyclic voltammetry results of the PtNafion integrated electrode and the commercial E-TEK Pt electrode. The hydrogen desorption peaks formed in the lower potential region are used to calculate the electrochemically active surface area (ECSA). It is found that the Pt-Nafion integrated electrode has a higher ECSA (11.5 m2/g Pt) than does the E-TEK electrode (7.3 m2/g Pt). The conventional electrode was formed directly by mixing the Nafion resin (proton conductor) and the catalyst (electron conductor). An excess content of one component generally leads to a poor conductance of the other component. For example, a higher Nafion content in the electrode leads to a higher proton conductivity, whereas a lower

electron conductivity, which limits the utilization of the catalyst in the conventional electrode.5-7,23 Additionally, it has been found that the lack of Nafion resin in the mesopores of the catalyst layer leads to a lower catalyst utilization in conventional electrodes.5,8 However, these problems can be avoided in the new electrode. As stated above, the catalyst nanowire network was formed in the hydrophilic pores of the Nafion membrane by the electrochemical deposition. In this way, the electron conduction paths (Pt nanowires) and the proton conduction paths (Nafion resin) do not interfere with each other, and therefore a higher electrochemically active surface area can be achieved for the new electrode. Figure 7 shows the XRD patterns of the Nafion 115 membrane and the Pt-Nafion integrated electrode. The membrane shows a peak around 2θ ) 18-20°, which can be attributed to the overlap of the two peaks from amorphous and

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Figure 6. Cyclic voltammetry comparison of the integrated Pt-Nafion electrode and the commercial E-TEK Pt electrode

Figure 8. The direct methanol fuel cells performance comparison of the Pt-Nafion integrated electrode and the E-TEK electrode. Cell temperature: 75 °C; methanol concentration and flow rate: 1.0 M and 1.00 mL min-1; oxygen: 20 sccm and ambient pressure.

TABLE 3: DMFC Parameters of the MEA with E-TEK Anode and the MEA with the Pt-Nafion 115 Integrated Anode

E-TEK MEA Pt-Nafion integrated MEA

OCVa (V)

Ib (mA cm-2)

Rc (ohm cm2)

0.42 0.52

140 97

0.47 0.61

a OCV: open-circuit voltage. b i: limiting current density, representing the methanol crossover rate. c R: cell specific resistance.

Figure 7. XRD patterns of the Nafion 115 membrane and the PtNafion 115 integrated electrode.

the crystalline scattering of the hydrophobic polyfluorocarbon backbone.24 Except for this characteristic Nafion peak, the PtNafion integrated electrode shows the characteristic platinum peaks at 2θ ) 39.85, 46.35, 67.3, and 81.35°, which correspond to Pt (111), (200), (220), and (311), respectively. The mean Pt particle size can be calculated by the Scherrer equation:

L)

0.9λKR1 B(2θ) cos θ

(1)

It turned out to be 6.4 nm from the (220) diffraction peak, which is consistent with the TEM results. Additionally, the measurement of the full width at half-maximum (fwhm) of the Nafion peak shows that the Pt-Nafion membrane has a smaller value (2.92°) than does the Nafion membrane (3.67°), which indicates that the incorporation of the Pt nanowire increased the crystalline degree of the Nafion membrane. This characteristic is beneficial for the suppression of methanol crossover, as discussed later. Figure 8 shows the DMFC performance comparison between the Pt-Nafion integrated electrode and the E-TEK electrode. The integrated electrode showed a higher open-circuit voltage (also listed in Table 3) and better performance at low current densities (