Ultraporous Palladium Supported on ... - Wiley Online Library

DOI: 10.1002/fuce.201200159

M. Sawangphruk1,2*, A. Krittayavathananon1, N. Chinwipas1, P. Srimuk1, T. Vatanatham1, S. Limtrakul1, J. S. Foord3 1

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Center of Excellence on Petrochemical and Materials Technology, Department of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand Chemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Rd, Oxford, OX1 3TA, UK

Received September 24, 2012; accepted April 04, 2013; published online June 10, 2013

Abstract Ultraporous Pd nanocrystals for electrocatalysis applications were fabricated using a direct electrodeposition method on three differing carbon supports: flexible carbon fiber paper (CFP), and CFP modified with either graphene oxide nanosheets or their chemically reduced forms using a simple spray coating technique. The electrocatalytic activity of these electrodes was investigated for the direct electro-oxidation reaction of methanol in alkaline media. Pd deposited on the CFP modified with reduced graphene oxide (rGO) has excellent poisoning tolerance to carbonaceous species and a significantly better catalytic activity toward methanol oxidation than the other two catalyst support materials. Pd/rGO/CFP

1 Introduction Direct methanol fuel cells (DMFCs) are one of the most promising alternative power sources for powering portable devices [1]. DMFCs offer a number of advantages such as high density of liquefied methanol, high-energy conversion efficiency (∼40%), high electron density (6 electrons a methanol molecule), and low operating temperature (50–200 °C) [1]. However, there are a number of drawbacks inhibiting practical uses of DMFCs. The first one is that direct oxidation of methanol within a fuel cell stack requires large quantities of precious catalysts, e.g. Pt. The second problem is the selfpoisoning of Pt by CO, one of intermediates in methanol oxidation, leading to relatively slow reaction kinetics [2]. A key challenge in the development of fuel cell technology is there-

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in 2.0 M CH3OH in 2.0 M NaOH yields a specific current density of 241 mAmg–1 cm–2 determined at the anodic oxidation peak. It is believed that the collaborative effects due to the three-dimensional ultraporous Pd nanocrystals and fast electron transfer owing to high conductivity of rGO nanosheets play an important role in enhancing the catalytic performance of Pd/rGO/CFP toward methanol oxidation in alkali media. Keywords: Carbon Fiber, Direct Methanol Fuel Cells, Graphene, Oxidation of Methanol, Palladium, Reduced Graphene Oxide

fore the development of stable, high activity, and lower cost catalytic materials. To this end, reducing Pt dependence with metallic alloys, e.g. RuPt and developing Pt-free electrocatalysts have been investigated extensively over the past decade [1, 3]. In this context, the use of Pd and Pd-based bimetallic materials is of interest [4]. Pd shows useful catalytic activity and has better activity than Pt-based catalysts for alcohol oxidation in alkali media [5–7]. For example, the state-of-the-art anion-exchange membrane direct ethanol fuel cells (DEFCs) in alkali media using Pd-based catalysts have recently been demonstrated to yield ultrahigh power density (185 mW cm–2 at 60 °C) [8]

– [*] Corresponding author, [email protected]

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ORIGINAL RESEARCH PAPER

Ultraporous Palladium Supported on Graphene-Coated Carbon Fiber Paper as a Highly Active Catalyst Electrode for the Oxidation of Methanol


Sawangphruk et al.: Ultraporous Palladium Supported on Graphene-Coated Carbon Fiber Paper while the state-of-the-art peak power density achieved by the conventioanl proton exchange membrane (PEM) DEFCs using Pt-based catalyst was only 79.5 mW cm–2 at 90 °C [9]. PEM-direct alcohol fuel cells have rather low power density due to the poor kinetic reaction of ethanol oxidation on the surface of catalyst [10]. In addition, Pd is 2–3 times cheaper than Pt and at least fifty times more abundant [4]. The possibilities of using alternative catalysts have now been boosted by advances in alkaline anion exchange membranes, which are enabling the development of alkaline analogues of the conventional acid-based direct fuel cell [5, 6]. Improved alcohol oxidation kinetics can be realized using basic media, so possibly allowing the use of less expensive catalysts. The catalytic activity of Pd nanostructures, e.g. nanoparticles [11], nanoplate arrays [12], nanotrees [13], nanowire arrays [14], nanoflowers [15], nanoclusters [16], and nanoporous films [4, 17] depends on their morphologies and structures. Among such nanostructures, nanoporous Pd with large specific surface areas and more active sites providing the specific current of 149 A g–1 toward the methanol oxidation in alkali media has a great potential to be utilized in fuel cells [4]. Previously, nanoporous Pd was produced using electrochemically dealloying Pd–Al alloy [4] or Pd30Ni50P20 metallic glass [17]. In the present work, we employ a one-step electrodeposition approach, which is capable of producing ultraporous and highly active “naked” Pd catalyst [18], displaying high catalytic activity. This technique is well recognized as a simple, scalable, and cheap technique for fabricating catalyst electrodes [15]. In addition, to obtain high surface area of Pd catalysts in a fuel cell, Pd nanostructures are normally dispersed or coated on some form of conductive carbon support, which also significantly influences system performance. Therefore great efforts have been made to develop optimized supporting materials. There are a number of carbon supporting materials in use including, e.g. carbon nanodots [18], carbon black (Vulcan XC-72) [11] carbon cloth [19], carbon nanotubes [20], helical carbon nanofibers [21], and graphene oxide (GO) nanosheets [16]. Graphene is the newest carbon support material to achieve attention [22]. It has several outstanding properties necessary for developing good fuel cell electrodes, e.g. low electrical resistivity, high surface area, high flexibility, cost effectiveness, simple handling, and high electrochemical stability [22]. Previous work explored the use of GO platelets as a support modifier for Pd electrocatalysts in alkaline DMFC applications and reported promising results [16]. However, GO has limited electrical activity, about 3 orders of magnitude lower than reduced GO (rGO) itself [23]. The present work therefore explores the use of rGO nanosheets, prepared by reduction of GO with hydrazine hydrate [24], in the same fuel cell applications to see if significant further improvement can be obtained by the reduction step. The rGO nanosheets were coated on carbon fiber paper (CFP) using a simple spray-coating technique. The CFP widely used in the membrane fuel cells is an electrically graphitic sheet of the randomly arranged short carbon fibers and has several out-

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standing properties of a good electrode for the development of DMFCs, e.g. low electrical resistivity, high flexibility, cost effectiveness, simple handling, and high electrochemical stability [25]. An ultraporous Pd nanostructure was electrodeposited on rGO-coated CFP. The as-fabricated exhibited excellent poisoning tolerance to carbonaceous species for the electro-oxidation of methanol when compared to other previous work.

2 Experimental 2.1 Chemicals and Materials Reagents used in this study were of analytical grade and used as received without further purification, encompassing graphite powder (20–40 lm, Sigma–Aldrich), sulfuric acid (98%, QRec), hydrogen peroxide (30%, Merck), potassium permanganate (99%, Ajax Finechem), sodium nitrate (99.5%, QRec), hydrazine hydrate (80%, Merck), acetone (99.5%, QRec), methanol (Merck, ≥99.9%), sodium hydroxide (Carlo Erba, 97%), and palladium (II) nitrate dihydrate (40% Pd basis, Sigma–Aldrich). Graphitized CFP with the trade name of SIGRACET® GDL 10 BA (thickness = 400 lm, electrical resistance 18 MX cm, Millipore). 2.2 Preparation of rGO Nanosheets The chemical oxidation of graphite to produce GO is an appropriate method for the large scale production of graphene materials [26, 27]. GO was therefore synthesized in this way, using specifically a Hummers method [28] with our modification. This preparation process was recently reported elsewhere [24]. Briefly, 3.0 g graphite powder and 1.5 g NaNO3 were added to concentrated 150 ml H2SO4 while stirring (100 rpm) in an ice bath for 1 h. 8.0 g KMnO4 was slowly added to the mixture at 25 °C for 2 h without stirring. 90 ml Milli-Q water was added to the suspension with continuous stirring (100 rpm) at 95 °C for 12 h. Then, 30 ml H2O2 (30%v/v) was slowly added to the diluted suspension. For the purification, the mixture was filtered through polyester fiber (Carpenter Co., USA). The filtrate was centrifuged at 6000 rpm for 15 min and the remaining solid material (GO) was then washed in succession with 200 ml of water, 100 ml HCl (30% v/v), and 100 ml ethanol. This process was repeated 3 times. The final product GO was collected by filtration and vacuum dried. For reduction of GO to produce rGO, 100 mg GO powder was sonicated in 30 ml Milli-Q water for 30 min. 3 ml hydrazine hydrate was added to the as-dispersed suspension. The mixture was then refluxed at 98 °C for 24 h. The final product, rGO was eventually harvested using the same purification method of GO. Note, a brief schematic diagram presenting the synthesis processes of GO and rGO nanosheets is shown in Figure 1.

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Sawangphruk et al.: Ultraporous Palladium Supported on Graphene-Coated Carbon Fiber Paper

3 Results and Discussion Fig. 1 A schematic diagram showing the synthesis processes of GO and rGO nanosheets as well as the fabrication of Pd/rGO/CFP electrode.

2.3 Spray Coating of rGO Nanosheets on CFP and Catalyst Preparation rGO powder was sonicated in acetone for dispersion (1 mg ml–1). The dispersion was coated on a CFP substrate using a spray-coating technique (Paasche Airbrush Company, USA). The airbrush head used a coating pressure and temperature of 20 psi and 25 °C, respectively. The specific loading of rGO on the CFP was around 4 mg cm–2. Pd nanocrystals were electrodeposited onto the rGO-coated CFP electrode using cyclic voltammetry, cycling the potential from –0.75 V to 0 V versus Ag/AgCl in 1 mM palladium (II) nitrate in 0.5 M H2SO4 at a scan rate of 10 mV s–1 for 10 cycles (see Figure 1). This provided a mass loading of Pd catalyst of ca. 0.4 mg cm–2 as determined by a thermogravimetric analysis (TGA 2960 in a flow of air at a heating rate of 10 °C min–1, TA instruments).

3.1 Proposed Electrodeposition Mechanism of Ultraporous Pd Catalyst A Pd nanocatalyst was electrodeposited onto the surface of the rGO/CFP electrode using cyclic voltammetry. Cyclic voltammograms (CVs) in Figure 2a were obtained by cycling potential from 0 to –0.75 V versus Ag/AgCl to the rGO/CFP electrode in 1 mM Pd(NO3)2 in 0.5 M H2SO4 at the scan rate of 10 mV s–1 for 10 cycles, at which range of potentials, Pd is deposited on the electrode. Features develop in the voltammograms as the number of cycles increase which must be associated with the accumulation of Pd at the electrode surface. During the cathodic scan, the cathodic current density increases at potential more negative than –0.19 V versus Ag/AgCl (see a magnified image in Figure 2b) associated with a prominent reduction peak at –0.25 V versus Ag/AgCl (see peak ii of Figure 2) which arises from the reduction of H+ to form adsorbed and absorbed hydrogen in/on the Pd nanoparticles which develop [29–31]. On the other hand, during the anodic scan an oxidation peak at –0.15 V versus Ag/AgCl (see peak i of Figure 2) corresponding to the electrochemical oxidation of these species [31]. The thermodynamic potential

2.4 Structural Characterization and Electrochemistry Fourier transform infrared spectroscopy (FTIR) spectra were acquired on a Perkin–Elmer System 2000 spectrometer. The specimens were prepared by grinding the dried powder mixed with potassium bromide (KBr) to a fine powder and then compressing it under high pressure into thin pellets. Raman spectra were recorded on a Senterra Dispersive Raman spectroscope (Bruker Optics, Germany) with a laser excitation wavelength of 514 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out using JSM-

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Fig. 2 (a) Electrodeposition process of ultraporous Pd nanocrystals by cycling potential from 0 to –0.75 V versus Ag/AgCl to rGO-coated CFP electrode in 1 mM Pd(NO3)2 in 0.5 M H2SO4 at the scan rate of 10 mV s–1 for 10 cycles and (b) magnified CVs of (a) from the applied potentials of –0.3 to 0 V versus Ag/AgCl.

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35CF operated at 20 keV and JEM 1230 with an acceleration voltage of 200 kV (JEOL Ltd., Japan), respectively. Energydispersive X-ray spectroscopy (EDX) was used to do elemental analysis of the as-prepared electrodes. X-ray diffraction (XRD) with the X’Pert Pro Mrd system (Philips) was employed to characterize the crystalline structure of as-prepared Pd catalysts. Electrochemical measurements were carried out in alkali media (NaOH) at room temperature (25 °C) using a computer-controlled m-AUTOLAB II potentiostat (Eco-Chemie, Utrecht, The Netherlands) equipped with a FRA2 frequency response analyzer module running GPES/ FRA software under three-electrode cell including a working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode saturated in KCl.


Sawangphruk et al.: Ultraporous Palladium Supported on Graphene-Coated Carbon Fiber Paper for gaseous hydrogen production is about –0.24 V versus Ag/AgCl at the pH used so it is likely this contributes to the increasing background current observed at more negative potentials. This becomes more significant as the amount of Pd deposited increases due to the electrocatalytic properties of the deposited Pd. A small negative baseline shift occurs as the amount of Pd deposited increases which is not seen for the final deposited structure in the absence of the Pd salt. It therefore suggests that this shift arises because the reduction of the Pd salt becomes more efficient as the amount of Pd on the surface increases, or some other Pd-driven reduction process such as the reduction of oxygen is being enhanced.

Fig. 4 (a) EDX and (b) XRD spectra of as-electrodeposited Pd on rGO-coated CFP electrode.

3.2 Physical Characterization A range of physical methods was employed to confirm the chemical integrity of the samples produced. The GO and rGO materials prepared as described above were dispersed in acetone using ultrasonication at a concentration of 0.5 mg ml–1. The photographs of GO and rGO dispersions in acetone as well as other physical properties characterized were previously reported by our group elsewhere [24]. The TEM image in Figure 3a resolves individual small graphene nanosheets of different sizes. The SEM image in Figure 3b shows the rGO material sprayed onto flexible CFP, so-called rGO/CFP. A single carbon fiber having a diameter of about 8 lm coated by rGO nanosheets can be resolved in Figure 3b. The average particle (lateral) size of rGO nanosheets is