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Preparation and electrochemistry of graphene nanosheets–multiwalled carbon nanotubes hybrid nanomaterials as Pd electrocatalyst support for formic acid oxidation
Electrochimica Acta 62 (2012) 242–249
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemistry of graphene nanosheets–multiwalled carbon nanotubes hybrid nanomaterials as Pd electrocatalyst support for formic acid oxidation Sudong Yang a , Chengmin Shen b , Xiangjun Lu a , Hao Tong a , Jiajia Zhu a , Xiaogang Zhang a,∗ , Hong-jun Gao b,∗∗ a b
College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2011 Received in revised form 7 December 2011 Accepted 8 December 2011 Available online 16 December 2011 Keywords: Fuel cell Graphene nanosheets Palladium Catalysts Formic acid
a b s t r a c t Graphene nanosheets–MWCNTs (GNS–CNTs) composites were synthesized by in situ reduction method, and then palladium nanoparticles (NPs) were supported on the GNS–CNTs by a microwave-assisted polyol process. Microstructure measurements showed that the graphene nanosheets and the CNTs formed a uniform nanocomposite with CNTs absorbed on the graphene nanosheets surface and/or filled between the graphene nanosheets. Compared to Pd/Vulcan XC-72R carbon, Pd/GNS, or Pd/CNTs catalysts, the Pd/GNS–CNTs catalysts exhibit excellent electrocatalytic activity and stability for formic acid electrooxidation when the mass ratio of GO to CNTs is 5:1. The superior performance of Pd/GNS–CNTs catalysts may arise from large surface area utilization for NPs and enhanced electronic conductivity of the supports. Therefore, the GNS–CNTs composite should be a promising carbon material for application as electrocatalyst support in fuel cells. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The electrochemical oxidation of formic acid has attracted a lot of attention in the past few years due to the great potential of direct formic acid fuel cells (DFAFCs) as efficient energy suppliers for mobile and portable applications [1–3]. Recent progress in Pd-based catalysts has revealed that Pd nanoparticles (NPs) are a promising catalyst employed in DFAFCs due to their high catalytic activity [4–6]. The most commonly accepted mechanism of formic acid oxidation is the so-called ‘parallel or dual pathway mechanism’ and is described below [7]. The first mechanism, called “direct pathway,” involves direct oxidation of the acid to carbon dioxide: HCOOH + Me → CO2 + 2H+ + Me + 2e−
(“Me = Pt, Pd and other)
(1)
A second mechanism occurs when carbon monoxide adsorbs onto a “Me” surface, and two electrochemical steps follow: HCOOH + Me → Me–CO + H2 O
∗ Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626. ∗∗ Corresponding author. Tel.: +86 10 82648035; fax: +86 10 62556598. E-mail addresses: [email protected] (X. Zhang), [email protected] (H.-j. Gao). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.022
(2)
Me + H2 O → Me–OH + H+ + e−
(3)
Me–CO + Me–OH → 2Me + CO2 + H+ + e−
(4)
It is well established that Pd promotes the “direct pathway” mechanism of formic acid oxidation (Eq. (1)). A more recent investigation has shown that the slow adsorption of the “CO-like” intermediate might be the principal reason for the deactivation of Pd catalysts during formic acid oxidation [8]; such a conclusion is supported by a prior investigation using in situ infrared absorption spectroscopy [9]. On the other hand, a Pt electrode promotes the indirect mechanism (Eqs. (2)–(4)). In order to improve the catalytic character and lower the overall cost of fuel cells, Pd NPs are used to load on the conductive carbon materials [10–12], which not only maximize the availability of nanosized electrocatalyst surface area for electron transfer but also provide better mass transport of reactants to the electrocatalyst. Therefore, much effort has been devoted to developing novel catalyst supports. The recent emergence of graphene nanosheet has opened a new avenue for utilizing two-dimensional new carbon material as a support because of its unique properties [13–15]. Recently, Pd–graphene catalysts have become a hot topic of interest in fuel cells [16–19]. In particular, Chen et al. [16] showed Pd/graphene exhibited better catalytic performance and stability compared to the commercial Pd/C catalyst. Fu et al. [17] found that Pd/graphene shows better electrochemical activity for
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typical synthetic procedure for GNS–CNTs, 1 mL hydrazine hydrate is added into the resulting dispersion (100 mL) and the reaction mixture was kept at 100 ◦ C for 24 h under constant stirring. Finally, the solid was filtered, washed several times with distilled water and alcohol, and dried at 60 ◦ C for 12 h in a vacuum oven. The mass ratio of GO to CNTs for each of the composite supports was 1:1, 3:1, 5:1, and 7:1 denoted in this report as GNS–CNTs (1:1), GNS–CNTs (3:1), GNS–CNTs (5:1), and GNS–CNTs (7:1), respectively. The pure graphene nanosheets were prepared through the above-mentioned chemical process without the presence of CNTs.
Fig. 1. Schematic representation of the synthesis of Pd/GNS–CNTs catalysts.
electro-oxidation of formic acid than the Pd/C catalyst. Zhang et al. [18] reported about the deposition of Pd on graphene sheets and their electrochemical activity for ethanol oxidation was studied. Graphene sheets prepared through the exfoliation of graphite oxide leave behind some defects and vacancies, which can act as good anchoring sites for the deposition of metal NPs that can be used for fuel cell applications. However, as-reduced graphene sheets tend to form irreversible agglomerates through van der Waals interactions and even restack to form graphite, which sacrificed both large specific surface area and outstanding single-layer electric property of graphene [20]. In addition, graphene sheets prepared through the chemical reducing with hydrazine leave behind some defects and vacancies that will reduce the conductivity of the composite [21]. These problems severely restrict the further applications for the catalyst supports. The restacking can be prevented by using CNTs as spacers [22] which will result in the increase in the surface area and electronic conductivity and thereby the enhancement in performance. Thus, combination of graphene nanosheets and CNTs as novel catalyst support materials may increase catalytic activity of NPs. As a result, the CNTs increase the electronic conductivity of the nanosheets. And meanwhile, the CNTs ensure the high electrochemical utilization of graphene layers as well as the open nano-channels provided by three-dimensional GNS–CNTs hybrid material. In this paper, GNS–CNTs hybrid materials as catalyst support with different mass ratios were prepared by in situ chemical reduction method. Then the palladium was directly reduced onto the hybrid support (Fig. 1). Catalysts with different CNTs compositions were investigated. The results demonstrate that Pd/GNS–CNTs exhibits excellent catalytic activity and stability than Pd/GNS and Pd/CNTs catalyst for formic acid oxidation. 2. Experimental 2.1. Materials The majority of the chemicals including graphite power (SP grade), palladium chloride (PdCl2 ), hydrazine hydrate, and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd., China, with their purity in analytical grade. Multi-walled carbon nanotubes (CNTs, 20–40 nm in diameter) were purchased from Nanotech Port Co., Ltd. (Shenzhen, China) and purified by refluxing them in nitric acid (HNO3 ) for 6 h before use. Other chemical reagents were analytical grade and used as received without further purification. 2.2. Synthesis of the GNS–CNTs materials The graphite oxide (GO) was prepared according to modified Hummer’s method [23]. Exfoliation of GO and CNTs was achieved by ultrasonication of the dispersion using an ultrasonic bath. In a
2.3. Synthesis of the Pd/GNS–CNTs materials To obtain the Pd/GNS–CNTs (5:1) composites, in a typical procedure, 20 mg GNS–CNTs (5:1) powder was dispersed in 30 mL EG solvent by ultrasonic treatment for approximately 1 h and then 0.71 mL of 7 mg mL−1 PdCl2 solution was added under magnetic stirring. The PH value of this mixture was adjusted to 10.0 by adding 1 M NaOH aqueous solution. Subsequently, the solution was put into a microwave oven (1000 W, 2.45 GHz) and then was alternatively heated for 2 min and paused for 30 s for eight times at a maximum temperature of 140 ◦ C. The resulting slurry was centrifuged, washed with deionized water and then dried in a vacuum oven. For comparative purposes, a sample of Pd-loaded GNS–CNTs was also prepared under identical conditions. The same procedure was followed for the synthesis of Pd/GNS, Pd/CNTs, and Pd/Vulcan XC-72R carbon catalyst (denoted as Pd/C).
2.4. Preparation of electrode Glassy carbon (GC) electrode, 5 mm in diameter (electrode area 0.2 cm2 ), polished with 0.05 m alumina to a mirror-finish before each experiment, was used as substrates for supported catalysts. For the electrode preparation, typically, 3 mg catalyst was added into 0.5 mL of 0.05 wt.% Nafion solution, and then the mixture was treated for 1 h with ultrasonication for uniform dispersion. A measured volume (30 L) of this mixture was dropped by a microsyringe onto the top surface of the GC electrode. The asobtained catalyst modified GC electrode was employed as the working electrode in our experiments.
2.5. Instrument and measurement X-ray diffraction (XRD) analysis was carried out on Bruker D8-ADVANCE diffractometer with Cu K␣ radiation of wavelength = 0.15418 nm. The Raman spectroscopy was used to study the integrity and electronic structure of the samples on the Raman system YJ-HR800 with confocal microscopy. Transmission electron microscopy (TEM, JEOL JEM-2100) was applied to characterize the morphology. The analysis of the composition of the catalyst was obtained with a Thermo IRIS Intrepid II inductively coupled plasma atom emission spectrometry (ICP-AES) system. The electronic conductivities of the samples were measured by a four-point probe method (SDY-5 Four-Point probe meter). All electrochemical measurements were carried out with a CHI 660C electrochemical workstation system and a conventional three-electrode system. A Pt wire, the saturated calomel electrode (SCE) and the GC electrode were used as the counter electrode, the reference electrode and the working electrode, respectively. All electrolytes were deaerated by bubbling N2 for 20 min and protected with a nitrogen atmosphere during the entire experimental procedure. All experiments were performed at 25 ± 1 ◦ C.
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Fig. 2. XRD patterns of (a) Pd/CNTs, (b) Pd/GNS, (c) Pd/GNS–CNTs (1:1), (d) Pd/GNS–CNTs (3:1), (e) Pd/GNS–CNTs (5:1), and (f) Pd/GNS–CNTs (7:1).
3. Results and discussion The XRD patterns of Pd/CNTs, Pd/GNS, and Pd/GNS–CNTs are shown in Fig. 2. For Pd/GNS, the position of the (0 0 2) diffraction peak corresponds to the interlayer spacing 3.61 A˚ in graphene, which is a little larger than the d-spacing of well ordered graphite ˚ The small amount of functional groups and hydrogen (3.4 A). remaining might be the main reason for this difference. After the reduction by hydrazine, the graphene tends to form very dense agglomerates with layered structure in dry state, which results from the van der Waals interactions between the layers of graphene [24]. Compared with Pd/CNTs, with the lower amount of CNTs in GNS–CNTs composites, the characteristic diffraction peak of the C (0 0 2) with a broad peak is slightly shifted to a lower angle in the case of Pd/GNS–CNTs, which also can be attributed to the effect from the CNTs. The all as-prepared catalysts exhibit the strong diffraction peaks at 2 = 39.9◦ , 46.3◦ , 67.8◦ and 81.6◦ that can be assigned to the characteristic (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of Pd, respectively, which means palladium exists as face-centered-cubic (fcc) structure. Among them, the average particle sizes of the Pd/CNTs, Pd/GNS, and Pd/GNS–CNTs (5:1) catalysts, calculated from the (2 2 0) peak using the Debye–Scherrer equation, were 4.3, 4.1, and 4.0 nm, respectively. As shown in Fig. 3, the Raman spectrum of the graphene can be decomposed into two features: the well-documented D and G peaks, and the D band at 1351 cm−1 corresponding to defects or edge areas and G band at 1586 cm−1 related to the vibration of sp2 -hybridized carbon. Similarly, the Raman spectra of the
Fig. 3. Raman spectra of the GNS and GNS–CNTs (5:1).
GNS–CNTs (5:1) both display two prominent peaks at 1351 and 1586 cm−1 , corresponding to the D and G peaks, respectively. It is worth noting that, due to the incorporation of the CNTs, the intensity ratio of D/G in the GNS–CNTs (5:1) composite decreases in comparison with that in the graphene. The morphologies of Pd/CNTs, Pd/GNS, GNS–CNTs (5:1), and Pd/GNS–CNTs (5:1) were characterized by TEM. From the TEM images in Fig. 4a and b, the Pd NPs are slightly more uniformly dispersed on the graphene nanosheets than the CNTs due to the remained oxygen-containing functionalities on the surface of the graphene sheets. As shown in Fig. 4c, the graphene nanosheets and the CNTs formed a uniform nanocomposite with the CNTs absorbed on the graphene nanosheet surface and/or filled between the graphene nanosheets (the arrows indicate the presence of CNTs in the graphene sheets). It is clearly seen that the GNS–CNTs supports are decorated by the nanosized Pd particles with very few aggregations. Further high-resolution TEM (HRTEM) image (the insert in Fig. 4d) analysis indicated lattice fringes with an interfringe ˚ which is close to the interplane disdistance of approximately 2.2 A, tance of the [1 1 1] planes in the fcc structured Pd. From TEM images, the mean size of the Pd NPs decorated on the GNS–CNTs (5:1) was about 4.0 nm. Clearly, in this study, the microwave-assisted polyol process plays a key role in keeping a similar Pd particle size on the three supports, while the different Pd particle dispersion mainly depends on the carbon support. The slightly lower Pd dispersion on CNTs is mainly due to the inert graphite layers and the higher surface tension on the CNTs support. As can be seen in Fig. 4d, the graphene sheets in the GNS–CNTs exist distributed CNTs between or on the graphene sheets. Based on these, we speculate that CNTs in the hybrid materials of GNS–CNTs set up a fairly conductive network, which may facile charge-transfer and mass-transfer processes. The practical composition of Pd in different samples was evaluated by ICP-AES analysis. The obtained ICP-AES composition of the Pd/C, Pd/GNS, Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1), Pd/GNS–CNTs (5:1), and Pd/GNS–CNTs (7:1) catalysts for metal loading was 18.6, 20.1, 18.5, 18.8, 19.0, 19.2, and 19.6 wt.%, respectively. The electrochemically active surface area (ECSA) provides important information regarding the number of available active sites [25]. Fig. 5a shows the cyclic voltammograms of different electrocatalyst composites by scanning the potential from −0.3 to 0.8 V vs SCE at a scan rate of 20 mV s−1 in 0.25 M H2 SO4 . From Fig. 5a, it can be seen that the area of hydrogen adsorption and desorption peak for Pd/GNS–CNTs (5:1) electrocatalysts is bigger than the other electrocatalysts. The difference of current density can be also ascribed mainly to the difference in surface areas and only to an insignificant extent to the presence. The ECSA was estimated by integrating the voltammogram corresponding to hydrogen desorption (QH ) by adapting the assumption of 212 C cm−2 from the electrode surface. The ECSA for Pd/C, Pd/GNS, Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1), Pd/GNS–CNTs (5:1), and Pd/GNS–CNTs (7:1) was estimated to be 69.3, 75.2, 72.1, 80.3, 83.6, 87.8, and 78.1 m2 g−1 , respectively. The results show that the Pd/GNS–CNTs (5:1) electrode has the highest ECSA and Pd utilization, while the Pd/C has the lowest. The lowest ECSA and Pd utilization for the Pd/CNTs are most likely due to the inert graphite layers and the higher surface tension of the CNTs support. Compared with Pd/GNS–CNTs (5:1), the higher ECSA of Pd/GNS–CNTs (5:1) is likely due to the fact that the electrode possesses a three dimensional structure and better conductive paths. In addition, the double layer capacitance was also obtained from the cyclic voltammetry data. The electrical capacitance is a measure of the surface area, both of Pd and support composites that can be accessed approached by electrons as well as protons [10]. The increased double-layer thickness of the Pd/GNS–CNTs (5:1) based electrodes reflects the higher specific surface area of the support composite.
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Fig. 4. TEM images of (a) Pd/CNTs, (b) Pd/GNS, (c) GNS–CNTs (5:1), and (d) Pd/GNS–CNTs (5:1). The insert is HRTEM image of a single Pd NP.
To optimize the electrode activity, different ratios of GO to CNTs were examined by cyclic voltammetry and chronoamperometry. Fig. 5b shows the cyclic voltammogram in 0.25 M H2 SO4 containing 0.25 M formic acid. As shown in Fig. 5b, the cyclic voltammogram features are in good agreement with the literature [10]. The two formic acid oxidation peaks during positive potential scanning, probably correspond to the two reaction steps (Eq. (5)) and (Eq. (6)) of reaction: HCOOH → reactive intermediates
(5)
reactive intermediates → CO2 + 2H+ + 2e−
(6)
The positive scan oxidation peak current density and peak potential of the catalysts are summarized in Table 1. It is clear that the electrocatalyst with the mass ratio of GO to CNTs equal to 5:1 gave the best performance. This value is substantially higher than those for pristine graphene nanosheets, Vulcan XC-72R carbon, or CNTs. The highest peak currents were observed on Pd/GNS–CNTs (5:1), indicating the highest catalytic activity for HCOOH oxidation, nearly twice than that on Pd/GNS electrode catalysts. And their corresponding peak potentials are located at 0.09 V, 30 mV more negative than that for the Pd/GNS catalysts. As shown in Table 1, the content of CNTs in the Pd/GNS–CNTs catalysts affects the catalytic activity for formic acid oxidation. With the CNTs content
Fig. 5. (a) Cyclic voltammograms of the as-prepared catalysts in 0.25 M H2 SO4 solution at a scan rate of 20 mV s−1 ; (b) cyclic voltammograms of formic acid oxidation on the as-prepared catalysts in 0.25 M H2 SO4 solution containing 0.25 M HCOOH at a scan rate of 50 mV s−1 .
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Table 1 Comparison of electrochemical characterization of Pd-based catalysts. Sample
Ep (V)
ip (mA cm−2 )
i600 (mA cm−2 )
Pd/C Pd/GNS Pd/CNTs Pd/GNS–CNTs (1:1) Pd/GNS–CNTs (3:1) Pd/GNS–CNTs (5:1) Pd/GNS–CNTs (7:1)
0.12 0.12 0.12 0.10 0.11 0.09 0.10
15.79 17.15 24.34 24.03 26.79 33.61 20.17
0.82 0.54 1.18 1.47 2.89 3.50 1.20
Ep , the peak potential of the positive scan; ip , the peak current density of the positive scan; i600 , the current density after 600 s.
increasing, the current density increases at first and then decreases with the excess amount of CNTs in the catalysts. It can be seen that the best performance of the Pd/GNS–CNTs catalysts was obtained when the mass ratio of GO to CNTs is 5:1; too high and too low CNTs contents both cause a decrease in the activity of catalyst. The significant enhancement for the catalytic activity of HCOOH oxidation on the Pd/GNS–CNTs electrode was related to the three-dimensional GNS–CNTs electrode structure. Furthermore, these nanosheets possess large surface areas, and particles can be deposited on both sides of these sheets [26]. Additionally, the CNTs attached onto the graphene surface can prevent the reduced GO from aggregation and restacking, and possess large surface areas because both the faces of graphene are accessible in their applications. Given single or few layered graphene with less agglomeration may help to facilitate the transmission of the electrolyte through the surface of the catalyst. Furthermore, the relationship between the specific activity and CNTs contents in the Pd/GNS–CNTs catalysts is displayed in Fig. 6. It was found that the content of CNTs in the Pd/GNS–CNTs catalysts affected the catalytic activity for formic acid oxidation. With the CNTs content increasing, the current density increases at first and then decreases with the excess amount of CNTs in the catalysts. These results proved that CNTs as one of the composite supporting materials loaded Pd NPs played a key role on the oxidation of formic acid. The stability of Pd-based electrocatalyts is extremely important for their real applications in DFAFCs. Chronoamperometric experiments were widely applied to explore the catalytic stability and reaction mechanism [27]. The long-term activity and durability of the Pd-based catalysts were further assessed by chronoamperometry test (i–t curve) with the potential fixed at 0.1 V for 600 s as shown in Fig. 7. A gradual decrease can be seen in the similar model for all catalysts in the oxidation current density with time, indicating thereby the poisoning of electrocatalysts. However, the initial current densities and limiting current densities for formic acid oxidation on Pd/GNS–CNTs (5:1) catalyst are found to be higher than the other catalysts in the whole process. The current
Fig. 6. Relationship between the specific activity and CNTs contents, the specific activity was obtained from the current density of forward peaks for formic acid electrooxidation on the Pd/GNS–CNTs catalysts.
Fig. 7. Chronoamperometry curves for glass carbon electrodes modified with different catalysts in 0.25 M H2 SO4 solution containing 0.25 M HCOOH at 0.1 V.
densities at the end of each test are listed in Table 1. Notably, Pd/GNS–CNTs showed superior activity and durability as compared to their Pd/GNS counterparts. The electro-oxidation current on Pd/GNS–CNTs (5:1) electrode at 600 s is 1.2–6.5 times as high as that on the others. The much better long-term electrocatalytic activity of Pd/GNS–CNTs (5:1) catalysts is due to the electrode structure, which is advantageous for efficient diffusion and transport of intermediates or by-product. The results further demonstrate that the Pd/GNS–CNTs (5:1) catalyst exhibits the best performance. This result is in agreement with its behavior in the cyclic voltammogram.
Fig. 8. (a) Cyclic voltammograms of formic acid oxidation on the as-prepared catalysts at a scan rate of 50 mV s−1 ; (b) chronoamperometry curves for different catalysts at 0.1 V in 0.25 M H2 SO4 solution containing 0.25 M HCOOH.
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Fig. 9. (a and b) The linear sweep voltammetry curves for formic acid oxidation on Pd/GNS and Pd/GNS–CNTs (5:1) electrode at different scan rates; (c) relationship of peak current density with the square root of scan rates; (d) electrochemical impedance spectroscopy from 100 kHz to 10 mHz for different catalyst materials in 0.25 M H2 SO4 solution containing 0.25 M HCOOH.
For comparison, Pd/GNS–CNTs (5:1)-2 was synthesized by ultrasonication of graphene and CNTs suspensions with the same CNTs content. Graphene was prepared by the chemical process without the presence of CNTs. As shown in Fig. 8, Pd/GNS–CNTs (5:1) catalyst showed superior activity and durability as compared with Pd/GNS–CNTs (5:1)-2 catalyst for formic acid electro-oxidation. The enhanced performance could attribute to the well-dispersed Pd NPs on the uniform morphology of GNS–CNTs (5:1) composite. It suggests that CNTs is a better “separator” material to separate and stabilize the graphene in the GNS–CNTs (5:1) material. The GNS–CNTs (5:1) material has a larger surface contact or coverage area as compared to GNS–CNTs (5:1)-2. Hence, the chance of graphene agglomerated to form graphite platelets is less in the GNS–CNTs (5:1) composite. In addition, the composite maintains the larger surface to facilitate the transport of the electrolyte and formic acid through the surface of the catalyst. Thus more active material will participate in the electrochemical reaction to give a higher activity. The linear sweep voltammetry (LSV) curves of the Pd/GNS–CNTs (5:1) and Pd/GNS electrode at various rates in 0.25 M H2 SO4 solution containing 0.25 M HCOOH are shown in Fig. 9a and b. It can be seen that oxidation potential and peak current density for formic acid oxidation become more prominent with the scan rates increasing. It is indicated that the oxidation of formic acid is an irreversible electrode process. The peak current density of Pd/GNS–CNTs (5:1) is greater than that of Pd/GNS, anticipating higher electrochemical activity. Fig. 9c shows the relation of peak current (ip ) to the square root of scan rates (v1/2 ) for Pd/GNS–CNTs (5:1) composite electrodes as a comparison with Pd/G. It can be seen that ip depends on v1/2 linearly, confirming that a diffusion-controlled process takes place. According to the Eq. (7) [28], provided that both electrodes have same number of n, A, and C0 *, where n, A, C0 * stand for electron transfer numbers, electrode area, and initial concentration,
respectively, which is almost the case, diffusion coefficients (DGNS–CNTs and DGNS ) for Pd/GNS and Pd/GNS–CNTs (5:1) composite electrodes are compared. From Eq. (8), it is evident that CNTs improve the diffusion coefficient of the electrode. Additionally, the improved electron-transfer kinetics at the Pd/GNS–CNTs catalyst can limit the amount of intermediates. Furthermore, the threedimensional electrode structure of Pd/GNS–CNTs is expected to be advantageous for efficient diffusion and transport of by-product [29,30]. Therefore, the GNS–CNTs as a support of Pd-based electrocatalyst for formic acid oxidation have a better performance. ip = (2.69 × 105 )n3/2 AD0 C0 ∗ v1/2 1/2
DGNS–CNTs = DGNS
(ip /v1/2 )GNS–CNTs (ip /v1/2 )GNS
(7)
2 = 2.6
(8)
The electronic conductivity of Pd/GNS–CNTs (5:1) hybrid materials (ca. 5.3 S m−1 ) is about four times that of the Pd/GNS (ca. 1.2 S m−1 ), demonstrating the improved electron transport due to the existence of the CNTs. The fact that CNTs increase the conductivity of the electrode can also be derived from the electrochemical impedance spectroscopy (EIS). EIS spectra of formic acid oxidation on electrodes Pd/GNS and Pd/GNS–CNTs (5:1) are shown in Fig. 9d. Both electrodes exhibit a semicircle at higher frequency region and a straight line at lower frequency region (Fig. 9d). At high frequencies, the diameter of the semicircle has been considered as the charge transfer resistance representing the rate of charge exchange between ions in aqueous and composite at electrochemical interface [31]. It can be seen that the diameter of the semicircle of Pd/GNS–CNTs (5:1) is smaller than that of Pd/GNS. This suggests that formic acid oxidation proceeds much easier on electrode Pd/GNS–CNTs (5:1) than on electrode Pd/GNS. The straight line at low frequency in the EIS suggests the presence of Warburg diffusion resistance [32]. The results confirm that CNTs
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4. Conclusions
Fig. 10. Schematic diagram explaining the conversion of adsorbed COads species to CO2 on Pd/GNS–CNTs hybrids.
play a key role in increasing the conductivity of the composite electrode. A more recent investigation demonstrated that the slow adsorption of the “CO-like” intermediate might be the principal reason for the deactivation of Pd catalysts during formic acid oxidation [8]. As we know, the graphene nanosheets prepared by the chemical reduction with hydrazine leave behind some residual oxygen groups on the graphene. The remarkably strong antipoisoning activity of the Pd/GNS electrocatalysts has been found to be associated with the type and surface density of covalently bound oxygen containing groups remained on the GNS support. The presence of residual oxygen groups on the graphene support can promote the oxidation of CO adsorbed, COads , on the active Pd sites via the mechanism [33]. The proposed mechanism is described in the following equations and in the schematic of Fig. 10: GNS + H2 O → GNS–(OH)ads + H+ + e−
(forward scan)
(9)
Pd–COads + GNS–(OH)ads → CO2 + Pd + GNS + H+ + e −
(reverse scan)
(10)
Dissociative adsorption of water molecules on the GNS support creates GNS–(OH)ads surface groups adjacent to Pd NPs (Eq. (9)), which readily oxidize COads groups on the peripheral Pd atoms (Eq. (10)). The hydrophilic nature of GNS promotes water activation and is the major driver in this mechanism. Based on the above results, these favorable properties may be attributed to three aspects. First, the CNTs are a better “separator” material as regards the separation and stabilization of graphene in the GNS–CNTs (5:1) material. The GNS–CNTs (5:1) material has a larger surface contact or coverage area which can provide a support for anchoring well-dispersed Pd NPs. Additionally, the CNTs work as a highly conductive matrix for quickly providing electrons which might be favorable for electrochemical reaction. In addition, the special frameworks and properties of Pd/GNS–CNTs hybrids were helpful to facilitate the transport of the electrolyte onto the surface of the catalysts and thus reduced the liquid sealing effect, which would enhance the active surface area for electrochemical reactions. Finally, a possible functional effect of support may also occur during electrode reactions. The presence of residual oxygen groups on GNS–CNTs (5:1) plays a role on the removal of carbonaceous species from the adjacent Pd sites, which can promote the oxidation of formic acid [33].
In summary, graphene nanosheets–MWCNTs hybrid materials with different mass ratios were prepared by the in situ reduction methods. The GNS–CNTs were used as the support of the Pd NPs (Pd/GNS–CNTs) for formic acid electrooxidation and their electrochemical properties have been investigated. The improved performance of Pd/GNS–CNTs compared to Pd/GNS and Pd/CNTs has been attributed to inhibit the agglomeration of graphene sheets and increased electrical conductivity brought about by CNTs. It is believed that the stability and electrical conductivity of GNS–CNTs composite are increased by CNTs, on the other hand, the aggregation or restacking of graphene to form graphite platelets is effectively prevented by CNTs. Thus, GNS–CNTs can be used as a more suitable and promising electrode material for formic acid fuel cells. Acknowledgements The work was supported by National Natural Science Foundation of China (No. 20873064), Natural Science Foundation of Jiangsu Province (No. BK2011030). S.D. Yang also gratefully acknowledged the support by Graduate Student Innovation Foundation of Jiangsu Province (CX09B 075Z) and NUAA Research Funding (No. NS2010165). References [1] B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.N. Xia, Science 324 (2009) 1302. [2] Y.M. Zhu, S.Y. Ha, R.I. Masel, J. Power Sources 130 (2004) 8. [3] S.J. Kang, J. Lee, J.K. Lee, S.Y. Chung, Y. Tak, J. Phys. Chem. B 110 (2006) 7270. [4] W.P. Zhou, A. Lewera, R. Larsen, R.I. Masel, P.S. Bagus, A. Wieckowski, J. Phys. Chem. B 110 (2006) 13393. [5] Y. Zhu, Y.Y. Kang, Z.Q. Zou, Q. Zhou, J.W. Zheng, B.J. Xia, H. Yang, Electrochem. Commun. 10 (2008) 802. [6] J.T. Zhang, C.C. Qiu, H.Y. Ma, X.Y. Liu, J. Phys. Chem. C 112 (2008) 13970. [7] J.M. Feliu, E. Herrero, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cells, vol. 2, Wiley, New York, 2003, p. 679. [8] X.W. Yu, P.G. Pickup, Electrochem. Commun. 11 (2009) 2012. [9] H. Miyake, T. Okada, G. Samjeské, M. Osawa, Phys. Chem. Chem. Phys. 10 (2008) 3662. [10] S.D. Yang, C.M. Shen, Y.Y. Liang, H. Tong, W. He, X.Z. Shi, X.G. Zhang, H.J. Gao, Nanoscale 3 (2011) 3277. [11] J.S. Zheng, X.S. Zhang, P. Li, J. Zhu, X.G. Zhou, W.K. Yuan, Electrochem. Commun. 9 (2007) 895. [12] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [13] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282. [14] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [15] N. Behabtu, J.R. Lomeda, M.J. Green, A.L. Higginbotham, A. Sinitskii, D.V. Kosynkin, D. Tsentalovich, A.N.G. Parra-Vasquez, J. Schmidt, E. Kesselman, Y. Cohen, Y. Talmon, J.M. Tour, M. Pasquali, Nat. Nanotechnol. 5 (2010) 406. [16] X.M. Chen, G.H. Wu, J.M. Chen, X. Chen, Z.X. Xie, X.R. Wang, J. Am. Chem. Soc. 133 (2011) 3693. [17] J. Yang, C.G. Tian, L. Wang, H.G. Fu, J. Mater. Chem. 21 (2011) 3384. [18] Z.L. Wen, S.D. Yang, Q.J. Song, L. Hao, X.G. Zhang, Acta Phys.-Chim. Sin. 26 (2010) 1570. [19] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, Electrochem. Commun. 13 (2011) 182. [20] S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558. [21] C. Gomez-Navarro, J.C. Meyers, R.S. Sundaram, A. Chuvilin, S. Kurash, M. Burghard, K. Kern, U. Kaizer, Nano Lett. 10 (2010) 1144. [22] R.I. Jafri, T. Arockiados, N. Rajalakshmi, S. Ramaprabhu, J. Electrochem. Soc. 157 (2010) B874. [23] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [24] J. Yan, T. Wei, B. Shao, F.Q. Ma, Z.J. Fan, M.L. Zhang, C. Zheng, Y.C. Shang, W.Z. Qian, F. Wei, Carbon 48 (2010) 1731. [25] B. Seger, P.V. Kamat, J. Phys. Chem. C 113 (2009) 7990. [26] R.S. Sundaram, C. Gomez-Navarro, K. Balasubramanian, M. Burghard, K. Kern, Adv. Mater. 20 (2008) 3050. [27] M.C. Zhao, C. Rice, R.I. Masel, P. Waszczuk, A. Wieckowskib, J. Electrochem. Soc. 151 (2004) A131.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemistry of graphene nanosheets–multiwalled carbon nanotubes hybrid nanomaterials as Pd electrocatalyst support for formic acid oxidation Sudong Yang a , Chengmin Shen b , Xiangjun Lu a , Hao Tong a , Jiajia Zhu a , Xiaogang Zhang a,∗ , Hong-jun Gao b,∗∗ a b
College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2011 Received in revised form 7 December 2011 Accepted 8 December 2011 Available online 16 December 2011 Keywords: Fuel cell Graphene nanosheets Palladium Catalysts Formic acid
a b s t r a c t Graphene nanosheets–MWCNTs (GNS–CNTs) composites were synthesized by in situ reduction method, and then palladium nanoparticles (NPs) were supported on the GNS–CNTs by a microwave-assisted polyol process. Microstructure measurements showed that the graphene nanosheets and the CNTs formed a uniform nanocomposite with CNTs absorbed on the graphene nanosheets surface and/or filled between the graphene nanosheets. Compared to Pd/Vulcan XC-72R carbon, Pd/GNS, or Pd/CNTs catalysts, the Pd/GNS–CNTs catalysts exhibit excellent electrocatalytic activity and stability for formic acid electrooxidation when the mass ratio of GO to CNTs is 5:1. The superior performance of Pd/GNS–CNTs catalysts may arise from large surface area utilization for NPs and enhanced electronic conductivity of the supports. Therefore, the GNS–CNTs composite should be a promising carbon material for application as electrocatalyst support in fuel cells. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction The electrochemical oxidation of formic acid has attracted a lot of attention in the past few years due to the great potential of direct formic acid fuel cells (DFAFCs) as efficient energy suppliers for mobile and portable applications [1–3]. Recent progress in Pd-based catalysts has revealed that Pd nanoparticles (NPs) are a promising catalyst employed in DFAFCs due to their high catalytic activity [4–6]. The most commonly accepted mechanism of formic acid oxidation is the so-called ‘parallel or dual pathway mechanism’ and is described below [7]. The first mechanism, called “direct pathway,” involves direct oxidation of the acid to carbon dioxide: HCOOH + Me → CO2 + 2H+ + Me + 2e−
(“Me = Pt, Pd and other)
(1)
A second mechanism occurs when carbon monoxide adsorbs onto a “Me” surface, and two electrochemical steps follow: HCOOH + Me → Me–CO + H2 O
∗ Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112626. ∗∗ Corresponding author. Tel.: +86 10 82648035; fax: +86 10 62556598. E-mail addresses: [email protected] (X. Zhang), [email protected] (H.-j. Gao). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.12.022
(2)
Me + H2 O → Me–OH + H+ + e−
(3)
Me–CO + Me–OH → 2Me + CO2 + H+ + e−
(4)
It is well established that Pd promotes the “direct pathway” mechanism of formic acid oxidation (Eq. (1)). A more recent investigation has shown that the slow adsorption of the “CO-like” intermediate might be the principal reason for the deactivation of Pd catalysts during formic acid oxidation [8]; such a conclusion is supported by a prior investigation using in situ infrared absorption spectroscopy [9]. On the other hand, a Pt electrode promotes the indirect mechanism (Eqs. (2)–(4)). In order to improve the catalytic character and lower the overall cost of fuel cells, Pd NPs are used to load on the conductive carbon materials [10–12], which not only maximize the availability of nanosized electrocatalyst surface area for electron transfer but also provide better mass transport of reactants to the electrocatalyst. Therefore, much effort has been devoted to developing novel catalyst supports. The recent emergence of graphene nanosheet has opened a new avenue for utilizing two-dimensional new carbon material as a support because of its unique properties [13–15]. Recently, Pd–graphene catalysts have become a hot topic of interest in fuel cells [16–19]. In particular, Chen et al. [16] showed Pd/graphene exhibited better catalytic performance and stability compared to the commercial Pd/C catalyst. Fu et al. [17] found that Pd/graphene shows better electrochemical activity for
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typical synthetic procedure for GNS–CNTs, 1 mL hydrazine hydrate is added into the resulting dispersion (100 mL) and the reaction mixture was kept at 100 ◦ C for 24 h under constant stirring. Finally, the solid was filtered, washed several times with distilled water and alcohol, and dried at 60 ◦ C for 12 h in a vacuum oven. The mass ratio of GO to CNTs for each of the composite supports was 1:1, 3:1, 5:1, and 7:1 denoted in this report as GNS–CNTs (1:1), GNS–CNTs (3:1), GNS–CNTs (5:1), and GNS–CNTs (7:1), respectively. The pure graphene nanosheets were prepared through the above-mentioned chemical process without the presence of CNTs.
Fig. 1. Schematic representation of the synthesis of Pd/GNS–CNTs catalysts.
electro-oxidation of formic acid than the Pd/C catalyst. Zhang et al. [18] reported about the deposition of Pd on graphene sheets and their electrochemical activity for ethanol oxidation was studied. Graphene sheets prepared through the exfoliation of graphite oxide leave behind some defects and vacancies, which can act as good anchoring sites for the deposition of metal NPs that can be used for fuel cell applications. However, as-reduced graphene sheets tend to form irreversible agglomerates through van der Waals interactions and even restack to form graphite, which sacrificed both large specific surface area and outstanding single-layer electric property of graphene [20]. In addition, graphene sheets prepared through the chemical reducing with hydrazine leave behind some defects and vacancies that will reduce the conductivity of the composite [21]. These problems severely restrict the further applications for the catalyst supports. The restacking can be prevented by using CNTs as spacers [22] which will result in the increase in the surface area and electronic conductivity and thereby the enhancement in performance. Thus, combination of graphene nanosheets and CNTs as novel catalyst support materials may increase catalytic activity of NPs. As a result, the CNTs increase the electronic conductivity of the nanosheets. And meanwhile, the CNTs ensure the high electrochemical utilization of graphene layers as well as the open nano-channels provided by three-dimensional GNS–CNTs hybrid material. In this paper, GNS–CNTs hybrid materials as catalyst support with different mass ratios were prepared by in situ chemical reduction method. Then the palladium was directly reduced onto the hybrid support (Fig. 1). Catalysts with different CNTs compositions were investigated. The results demonstrate that Pd/GNS–CNTs exhibits excellent catalytic activity and stability than Pd/GNS and Pd/CNTs catalyst for formic acid oxidation. 2. Experimental 2.1. Materials The majority of the chemicals including graphite power (SP grade), palladium chloride (PdCl2 ), hydrazine hydrate, and ethylene glycol (EG) were purchased from Sinopharm Chemical Reagent Co., Ltd., China, with their purity in analytical grade. Multi-walled carbon nanotubes (CNTs, 20–40 nm in diameter) were purchased from Nanotech Port Co., Ltd. (Shenzhen, China) and purified by refluxing them in nitric acid (HNO3 ) for 6 h before use. Other chemical reagents were analytical grade and used as received without further purification. 2.2. Synthesis of the GNS–CNTs materials The graphite oxide (GO) was prepared according to modified Hummer’s method [23]. Exfoliation of GO and CNTs was achieved by ultrasonication of the dispersion using an ultrasonic bath. In a
2.3. Synthesis of the Pd/GNS–CNTs materials To obtain the Pd/GNS–CNTs (5:1) composites, in a typical procedure, 20 mg GNS–CNTs (5:1) powder was dispersed in 30 mL EG solvent by ultrasonic treatment for approximately 1 h and then 0.71 mL of 7 mg mL−1 PdCl2 solution was added under magnetic stirring. The PH value of this mixture was adjusted to 10.0 by adding 1 M NaOH aqueous solution. Subsequently, the solution was put into a microwave oven (1000 W, 2.45 GHz) and then was alternatively heated for 2 min and paused for 30 s for eight times at a maximum temperature of 140 ◦ C. The resulting slurry was centrifuged, washed with deionized water and then dried in a vacuum oven. For comparative purposes, a sample of Pd-loaded GNS–CNTs was also prepared under identical conditions. The same procedure was followed for the synthesis of Pd/GNS, Pd/CNTs, and Pd/Vulcan XC-72R carbon catalyst (denoted as Pd/C).
2.4. Preparation of electrode Glassy carbon (GC) electrode, 5 mm in diameter (electrode area 0.2 cm2 ), polished with 0.05 m alumina to a mirror-finish before each experiment, was used as substrates for supported catalysts. For the electrode preparation, typically, 3 mg catalyst was added into 0.5 mL of 0.05 wt.% Nafion solution, and then the mixture was treated for 1 h with ultrasonication for uniform dispersion. A measured volume (30 L) of this mixture was dropped by a microsyringe onto the top surface of the GC electrode. The asobtained catalyst modified GC electrode was employed as the working electrode in our experiments.
2.5. Instrument and measurement X-ray diffraction (XRD) analysis was carried out on Bruker D8-ADVANCE diffractometer with Cu K␣ radiation of wavelength = 0.15418 nm. The Raman spectroscopy was used to study the integrity and electronic structure of the samples on the Raman system YJ-HR800 with confocal microscopy. Transmission electron microscopy (TEM, JEOL JEM-2100) was applied to characterize the morphology. The analysis of the composition of the catalyst was obtained with a Thermo IRIS Intrepid II inductively coupled plasma atom emission spectrometry (ICP-AES) system. The electronic conductivities of the samples were measured by a four-point probe method (SDY-5 Four-Point probe meter). All electrochemical measurements were carried out with a CHI 660C electrochemical workstation system and a conventional three-electrode system. A Pt wire, the saturated calomel electrode (SCE) and the GC electrode were used as the counter electrode, the reference electrode and the working electrode, respectively. All electrolytes were deaerated by bubbling N2 for 20 min and protected with a nitrogen atmosphere during the entire experimental procedure. All experiments were performed at 25 ± 1 ◦ C.
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Fig. 2. XRD patterns of (a) Pd/CNTs, (b) Pd/GNS, (c) Pd/GNS–CNTs (1:1), (d) Pd/GNS–CNTs (3:1), (e) Pd/GNS–CNTs (5:1), and (f) Pd/GNS–CNTs (7:1).
3. Results and discussion The XRD patterns of Pd/CNTs, Pd/GNS, and Pd/GNS–CNTs are shown in Fig. 2. For Pd/GNS, the position of the (0 0 2) diffraction peak corresponds to the interlayer spacing 3.61 A˚ in graphene, which is a little larger than the d-spacing of well ordered graphite ˚ The small amount of functional groups and hydrogen (3.4 A). remaining might be the main reason for this difference. After the reduction by hydrazine, the graphene tends to form very dense agglomerates with layered structure in dry state, which results from the van der Waals interactions between the layers of graphene [24]. Compared with Pd/CNTs, with the lower amount of CNTs in GNS–CNTs composites, the characteristic diffraction peak of the C (0 0 2) with a broad peak is slightly shifted to a lower angle in the case of Pd/GNS–CNTs, which also can be attributed to the effect from the CNTs. The all as-prepared catalysts exhibit the strong diffraction peaks at 2 = 39.9◦ , 46.3◦ , 67.8◦ and 81.6◦ that can be assigned to the characteristic (1 1 1), (2 0 0), (2 2 0), and (3 1 1) crystalline planes of Pd, respectively, which means palladium exists as face-centered-cubic (fcc) structure. Among them, the average particle sizes of the Pd/CNTs, Pd/GNS, and Pd/GNS–CNTs (5:1) catalysts, calculated from the (2 2 0) peak using the Debye–Scherrer equation, were 4.3, 4.1, and 4.0 nm, respectively. As shown in Fig. 3, the Raman spectrum of the graphene can be decomposed into two features: the well-documented D and G peaks, and the D band at 1351 cm−1 corresponding to defects or edge areas and G band at 1586 cm−1 related to the vibration of sp2 -hybridized carbon. Similarly, the Raman spectra of the
Fig. 3. Raman spectra of the GNS and GNS–CNTs (5:1).
GNS–CNTs (5:1) both display two prominent peaks at 1351 and 1586 cm−1 , corresponding to the D and G peaks, respectively. It is worth noting that, due to the incorporation of the CNTs, the intensity ratio of D/G in the GNS–CNTs (5:1) composite decreases in comparison with that in the graphene. The morphologies of Pd/CNTs, Pd/GNS, GNS–CNTs (5:1), and Pd/GNS–CNTs (5:1) were characterized by TEM. From the TEM images in Fig. 4a and b, the Pd NPs are slightly more uniformly dispersed on the graphene nanosheets than the CNTs due to the remained oxygen-containing functionalities on the surface of the graphene sheets. As shown in Fig. 4c, the graphene nanosheets and the CNTs formed a uniform nanocomposite with the CNTs absorbed on the graphene nanosheet surface and/or filled between the graphene nanosheets (the arrows indicate the presence of CNTs in the graphene sheets). It is clearly seen that the GNS–CNTs supports are decorated by the nanosized Pd particles with very few aggregations. Further high-resolution TEM (HRTEM) image (the insert in Fig. 4d) analysis indicated lattice fringes with an interfringe ˚ which is close to the interplane disdistance of approximately 2.2 A, tance of the [1 1 1] planes in the fcc structured Pd. From TEM images, the mean size of the Pd NPs decorated on the GNS–CNTs (5:1) was about 4.0 nm. Clearly, in this study, the microwave-assisted polyol process plays a key role in keeping a similar Pd particle size on the three supports, while the different Pd particle dispersion mainly depends on the carbon support. The slightly lower Pd dispersion on CNTs is mainly due to the inert graphite layers and the higher surface tension on the CNTs support. As can be seen in Fig. 4d, the graphene sheets in the GNS–CNTs exist distributed CNTs between or on the graphene sheets. Based on these, we speculate that CNTs in the hybrid materials of GNS–CNTs set up a fairly conductive network, which may facile charge-transfer and mass-transfer processes. The practical composition of Pd in different samples was evaluated by ICP-AES analysis. The obtained ICP-AES composition of the Pd/C, Pd/GNS, Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1), Pd/GNS–CNTs (5:1), and Pd/GNS–CNTs (7:1) catalysts for metal loading was 18.6, 20.1, 18.5, 18.8, 19.0, 19.2, and 19.6 wt.%, respectively. The electrochemically active surface area (ECSA) provides important information regarding the number of available active sites [25]. Fig. 5a shows the cyclic voltammograms of different electrocatalyst composites by scanning the potential from −0.3 to 0.8 V vs SCE at a scan rate of 20 mV s−1 in 0.25 M H2 SO4 . From Fig. 5a, it can be seen that the area of hydrogen adsorption and desorption peak for Pd/GNS–CNTs (5:1) electrocatalysts is bigger than the other electrocatalysts. The difference of current density can be also ascribed mainly to the difference in surface areas and only to an insignificant extent to the presence. The ECSA was estimated by integrating the voltammogram corresponding to hydrogen desorption (QH ) by adapting the assumption of 212 C cm−2 from the electrode surface. The ECSA for Pd/C, Pd/GNS, Pd/CNTs, Pd/GNS–CNTs (1:1), Pd/GNS–CNTs (3:1), Pd/GNS–CNTs (5:1), and Pd/GNS–CNTs (7:1) was estimated to be 69.3, 75.2, 72.1, 80.3, 83.6, 87.8, and 78.1 m2 g−1 , respectively. The results show that the Pd/GNS–CNTs (5:1) electrode has the highest ECSA and Pd utilization, while the Pd/C has the lowest. The lowest ECSA and Pd utilization for the Pd/CNTs are most likely due to the inert graphite layers and the higher surface tension of the CNTs support. Compared with Pd/GNS–CNTs (5:1), the higher ECSA of Pd/GNS–CNTs (5:1) is likely due to the fact that the electrode possesses a three dimensional structure and better conductive paths. In addition, the double layer capacitance was also obtained from the cyclic voltammetry data. The electrical capacitance is a measure of the surface area, both of Pd and support composites that can be accessed approached by electrons as well as protons [10]. The increased double-layer thickness of the Pd/GNS–CNTs (5:1) based electrodes reflects the higher specific surface area of the support composite.
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Fig. 4. TEM images of (a) Pd/CNTs, (b) Pd/GNS, (c) GNS–CNTs (5:1), and (d) Pd/GNS–CNTs (5:1). The insert is HRTEM image of a single Pd NP.
To optimize the electrode activity, different ratios of GO to CNTs were examined by cyclic voltammetry and chronoamperometry. Fig. 5b shows the cyclic voltammogram in 0.25 M H2 SO4 containing 0.25 M formic acid. As shown in Fig. 5b, the cyclic voltammogram features are in good agreement with the literature [10]. The two formic acid oxidation peaks during positive potential scanning, probably correspond to the two reaction steps (Eq. (5)) and (Eq. (6)) of reaction: HCOOH → reactive intermediates
(5)
reactive intermediates → CO2 + 2H+ + 2e−
(6)
The positive scan oxidation peak current density and peak potential of the catalysts are summarized in Table 1. It is clear that the electrocatalyst with the mass ratio of GO to CNTs equal to 5:1 gave the best performance. This value is substantially higher than those for pristine graphene nanosheets, Vulcan XC-72R carbon, or CNTs. The highest peak currents were observed on Pd/GNS–CNTs (5:1), indicating the highest catalytic activity for HCOOH oxidation, nearly twice than that on Pd/GNS electrode catalysts. And their corresponding peak potentials are located at 0.09 V, 30 mV more negative than that for the Pd/GNS catalysts. As shown in Table 1, the content of CNTs in the Pd/GNS–CNTs catalysts affects the catalytic activity for formic acid oxidation. With the CNTs content
Fig. 5. (a) Cyclic voltammograms of the as-prepared catalysts in 0.25 M H2 SO4 solution at a scan rate of 20 mV s−1 ; (b) cyclic voltammograms of formic acid oxidation on the as-prepared catalysts in 0.25 M H2 SO4 solution containing 0.25 M HCOOH at a scan rate of 50 mV s−1 .
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Table 1 Comparison of electrochemical characterization of Pd-based catalysts. Sample
Ep (V)
ip (mA cm−2 )
i600 (mA cm−2 )
Pd/C Pd/GNS Pd/CNTs Pd/GNS–CNTs (1:1) Pd/GNS–CNTs (3:1) Pd/GNS–CNTs (5:1) Pd/GNS–CNTs (7:1)
0.12 0.12 0.12 0.10 0.11 0.09 0.10
15.79 17.15 24.34 24.03 26.79 33.61 20.17
0.82 0.54 1.18 1.47 2.89 3.50 1.20
Ep , the peak potential of the positive scan; ip , the peak current density of the positive scan; i600 , the current density after 600 s.
increasing, the current density increases at first and then decreases with the excess amount of CNTs in the catalysts. It can be seen that the best performance of the Pd/GNS–CNTs catalysts was obtained when the mass ratio of GO to CNTs is 5:1; too high and too low CNTs contents both cause a decrease in the activity of catalyst. The significant enhancement for the catalytic activity of HCOOH oxidation on the Pd/GNS–CNTs electrode was related to the three-dimensional GNS–CNTs electrode structure. Furthermore, these nanosheets possess large surface areas, and particles can be deposited on both sides of these sheets [26]. Additionally, the CNTs attached onto the graphene surface can prevent the reduced GO from aggregation and restacking, and possess large surface areas because both the faces of graphene are accessible in their applications. Given single or few layered graphene with less agglomeration may help to facilitate the transmission of the electrolyte through the surface of the catalyst. Furthermore, the relationship between the specific activity and CNTs contents in the Pd/GNS–CNTs catalysts is displayed in Fig. 6. It was found that the content of CNTs in the Pd/GNS–CNTs catalysts affected the catalytic activity for formic acid oxidation. With the CNTs content increasing, the current density increases at first and then decreases with the excess amount of CNTs in the catalysts. These results proved that CNTs as one of the composite supporting materials loaded Pd NPs played a key role on the oxidation of formic acid. The stability of Pd-based electrocatalyts is extremely important for their real applications in DFAFCs. Chronoamperometric experiments were widely applied to explore the catalytic stability and reaction mechanism [27]. The long-term activity and durability of the Pd-based catalysts were further assessed by chronoamperometry test (i–t curve) with the potential fixed at 0.1 V for 600 s as shown in Fig. 7. A gradual decrease can be seen in the similar model for all catalysts in the oxidation current density with time, indicating thereby the poisoning of electrocatalysts. However, the initial current densities and limiting current densities for formic acid oxidation on Pd/GNS–CNTs (5:1) catalyst are found to be higher than the other catalysts in the whole process. The current
Fig. 6. Relationship between the specific activity and CNTs contents, the specific activity was obtained from the current density of forward peaks for formic acid electrooxidation on the Pd/GNS–CNTs catalysts.
Fig. 7. Chronoamperometry curves for glass carbon electrodes modified with different catalysts in 0.25 M H2 SO4 solution containing 0.25 M HCOOH at 0.1 V.
densities at the end of each test are listed in Table 1. Notably, Pd/GNS–CNTs showed superior activity and durability as compared to their Pd/GNS counterparts. The electro-oxidation current on Pd/GNS–CNTs (5:1) electrode at 600 s is 1.2–6.5 times as high as that on the others. The much better long-term electrocatalytic activity of Pd/GNS–CNTs (5:1) catalysts is due to the electrode structure, which is advantageous for efficient diffusion and transport of intermediates or by-product. The results further demonstrate that the Pd/GNS–CNTs (5:1) catalyst exhibits the best performance. This result is in agreement with its behavior in the cyclic voltammogram.
Fig. 8. (a) Cyclic voltammograms of formic acid oxidation on the as-prepared catalysts at a scan rate of 50 mV s−1 ; (b) chronoamperometry curves for different catalysts at 0.1 V in 0.25 M H2 SO4 solution containing 0.25 M HCOOH.
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Fig. 9. (a and b) The linear sweep voltammetry curves for formic acid oxidation on Pd/GNS and Pd/GNS–CNTs (5:1) electrode at different scan rates; (c) relationship of peak current density with the square root of scan rates; (d) electrochemical impedance spectroscopy from 100 kHz to 10 mHz for different catalyst materials in 0.25 M H2 SO4 solution containing 0.25 M HCOOH.
For comparison, Pd/GNS–CNTs (5:1)-2 was synthesized by ultrasonication of graphene and CNTs suspensions with the same CNTs content. Graphene was prepared by the chemical process without the presence of CNTs. As shown in Fig. 8, Pd/GNS–CNTs (5:1) catalyst showed superior activity and durability as compared with Pd/GNS–CNTs (5:1)-2 catalyst for formic acid electro-oxidation. The enhanced performance could attribute to the well-dispersed Pd NPs on the uniform morphology of GNS–CNTs (5:1) composite. It suggests that CNTs is a better “separator” material to separate and stabilize the graphene in the GNS–CNTs (5:1) material. The GNS–CNTs (5:1) material has a larger surface contact or coverage area as compared to GNS–CNTs (5:1)-2. Hence, the chance of graphene agglomerated to form graphite platelets is less in the GNS–CNTs (5:1) composite. In addition, the composite maintains the larger surface to facilitate the transport of the electrolyte and formic acid through the surface of the catalyst. Thus more active material will participate in the electrochemical reaction to give a higher activity. The linear sweep voltammetry (LSV) curves of the Pd/GNS–CNTs (5:1) and Pd/GNS electrode at various rates in 0.25 M H2 SO4 solution containing 0.25 M HCOOH are shown in Fig. 9a and b. It can be seen that oxidation potential and peak current density for formic acid oxidation become more prominent with the scan rates increasing. It is indicated that the oxidation of formic acid is an irreversible electrode process. The peak current density of Pd/GNS–CNTs (5:1) is greater than that of Pd/GNS, anticipating higher electrochemical activity. Fig. 9c shows the relation of peak current (ip ) to the square root of scan rates (v1/2 ) for Pd/GNS–CNTs (5:1) composite electrodes as a comparison with Pd/G. It can be seen that ip depends on v1/2 linearly, confirming that a diffusion-controlled process takes place. According to the Eq. (7) [28], provided that both electrodes have same number of n, A, and C0 *, where n, A, C0 * stand for electron transfer numbers, electrode area, and initial concentration,
respectively, which is almost the case, diffusion coefficients (DGNS–CNTs and DGNS ) for Pd/GNS and Pd/GNS–CNTs (5:1) composite electrodes are compared. From Eq. (8), it is evident that CNTs improve the diffusion coefficient of the electrode. Additionally, the improved electron-transfer kinetics at the Pd/GNS–CNTs catalyst can limit the amount of intermediates. Furthermore, the threedimensional electrode structure of Pd/GNS–CNTs is expected to be advantageous for efficient diffusion and transport of by-product [29,30]. Therefore, the GNS–CNTs as a support of Pd-based electrocatalyst for formic acid oxidation have a better performance. ip = (2.69 × 105 )n3/2 AD0 C0 ∗ v1/2 1/2
DGNS–CNTs = DGNS
(ip /v1/2 )GNS–CNTs (ip /v1/2 )GNS
(7)
2 = 2.6
(8)
The electronic conductivity of Pd/GNS–CNTs (5:1) hybrid materials (ca. 5.3 S m−1 ) is about four times that of the Pd/GNS (ca. 1.2 S m−1 ), demonstrating the improved electron transport due to the existence of the CNTs. The fact that CNTs increase the conductivity of the electrode can also be derived from the electrochemical impedance spectroscopy (EIS). EIS spectra of formic acid oxidation on electrodes Pd/GNS and Pd/GNS–CNTs (5:1) are shown in Fig. 9d. Both electrodes exhibit a semicircle at higher frequency region and a straight line at lower frequency region (Fig. 9d). At high frequencies, the diameter of the semicircle has been considered as the charge transfer resistance representing the rate of charge exchange between ions in aqueous and composite at electrochemical interface [31]. It can be seen that the diameter of the semicircle of Pd/GNS–CNTs (5:1) is smaller than that of Pd/GNS. This suggests that formic acid oxidation proceeds much easier on electrode Pd/GNS–CNTs (5:1) than on electrode Pd/GNS. The straight line at low frequency in the EIS suggests the presence of Warburg diffusion resistance [32]. The results confirm that CNTs
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4. Conclusions
Fig. 10. Schematic diagram explaining the conversion of adsorbed COads species to CO2 on Pd/GNS–CNTs hybrids.
play a key role in increasing the conductivity of the composite electrode. A more recent investigation demonstrated that the slow adsorption of the “CO-like” intermediate might be the principal reason for the deactivation of Pd catalysts during formic acid oxidation [8]. As we know, the graphene nanosheets prepared by the chemical reduction with hydrazine leave behind some residual oxygen groups on the graphene. The remarkably strong antipoisoning activity of the Pd/GNS electrocatalysts has been found to be associated with the type and surface density of covalently bound oxygen containing groups remained on the GNS support. The presence of residual oxygen groups on the graphene support can promote the oxidation of CO adsorbed, COads , on the active Pd sites via the mechanism [33]. The proposed mechanism is described in the following equations and in the schematic of Fig. 10: GNS + H2 O → GNS–(OH)ads + H+ + e−
(forward scan)
(9)
Pd–COads + GNS–(OH)ads → CO2 + Pd + GNS + H+ + e −
(reverse scan)
(10)
Dissociative adsorption of water molecules on the GNS support creates GNS–(OH)ads surface groups adjacent to Pd NPs (Eq. (9)), which readily oxidize COads groups on the peripheral Pd atoms (Eq. (10)). The hydrophilic nature of GNS promotes water activation and is the major driver in this mechanism. Based on the above results, these favorable properties may be attributed to three aspects. First, the CNTs are a better “separator” material as regards the separation and stabilization of graphene in the GNS–CNTs (5:1) material. The GNS–CNTs (5:1) material has a larger surface contact or coverage area which can provide a support for anchoring well-dispersed Pd NPs. Additionally, the CNTs work as a highly conductive matrix for quickly providing electrons which might be favorable for electrochemical reaction. In addition, the special frameworks and properties of Pd/GNS–CNTs hybrids were helpful to facilitate the transport of the electrolyte onto the surface of the catalysts and thus reduced the liquid sealing effect, which would enhance the active surface area for electrochemical reactions. Finally, a possible functional effect of support may also occur during electrode reactions. The presence of residual oxygen groups on GNS–CNTs (5:1) plays a role on the removal of carbonaceous species from the adjacent Pd sites, which can promote the oxidation of formic acid [33].
In summary, graphene nanosheets–MWCNTs hybrid materials with different mass ratios were prepared by the in situ reduction methods. The GNS–CNTs were used as the support of the Pd NPs (Pd/GNS–CNTs) for formic acid electrooxidation and their electrochemical properties have been investigated. The improved performance of Pd/GNS–CNTs compared to Pd/GNS and Pd/CNTs has been attributed to inhibit the agglomeration of graphene sheets and increased electrical conductivity brought about by CNTs. It is believed that the stability and electrical conductivity of GNS–CNTs composite are increased by CNTs, on the other hand, the aggregation or restacking of graphene to form graphite platelets is effectively prevented by CNTs. Thus, GNS–CNTs can be used as a more suitable and promising electrode material for formic acid fuel cells. Acknowledgements The work was supported by National Natural Science Foundation of China (No. 20873064), Natural Science Foundation of Jiangsu Province (No. BK2011030). S.D. Yang also gratefully acknowledged the support by Graduate Student Innovation Foundation of Jiangsu Province (CX09B 075Z) and NUAA Research Funding (No. NS2010165). References [1] B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.N. Xia, Science 324 (2009) 1302. [2] Y.M. Zhu, S.Y. Ha, R.I. Masel, J. Power Sources 130 (2004) 8. [3] S.J. Kang, J. Lee, J.K. Lee, S.Y. Chung, Y. Tak, J. Phys. Chem. B 110 (2006) 7270. [4] W.P. Zhou, A. Lewera, R. Larsen, R.I. Masel, P.S. Bagus, A. Wieckowski, J. Phys. Chem. B 110 (2006) 13393. [5] Y. Zhu, Y.Y. Kang, Z.Q. Zou, Q. Zhou, J.W. Zheng, B.J. Xia, H. Yang, Electrochem. Commun. 10 (2008) 802. [6] J.T. Zhang, C.C. Qiu, H.Y. Ma, X.Y. Liu, J. Phys. Chem. C 112 (2008) 13970. [7] J.M. Feliu, E. Herrero, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cells, vol. 2, Wiley, New York, 2003, p. 679. [8] X.W. Yu, P.G. Pickup, Electrochem. Commun. 11 (2009) 2012. [9] H. Miyake, T. Okada, G. Samjeské, M. Osawa, Phys. Chem. Chem. Phys. 10 (2008) 3662. [10] S.D. Yang, C.M. Shen, Y.Y. Liang, H. Tong, W. He, X.Z. Shi, X.G. Zhang, H.J. Gao, Nanoscale 3 (2011) 3277. [11] J.S. Zheng, X.S. Zhang, P. Li, J. Zhu, X.G. Zhou, W.K. Yuan, Electrochem. Commun. 9 (2007) 895. [12] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [13] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282. [14] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183. [15] N. Behabtu, J.R. Lomeda, M.J. Green, A.L. Higginbotham, A. Sinitskii, D.V. Kosynkin, D. Tsentalovich, A.N.G. Parra-Vasquez, J. Schmidt, E. Kesselman, Y. Cohen, Y. Talmon, J.M. Tour, M. Pasquali, Nat. Nanotechnol. 5 (2010) 406. [16] X.M. Chen, G.H. Wu, J.M. Chen, X. Chen, Z.X. Xie, X.R. Wang, J. Am. Chem. Soc. 133 (2011) 3693. [17] J. Yang, C.G. Tian, L. Wang, H.G. Fu, J. Mater. Chem. 21 (2011) 3384. [18] Z.L. Wen, S.D. Yang, Q.J. Song, L. Hao, X.G. Zhang, Acta Phys.-Chim. Sin. 26 (2010) 1570. [19] M.H. Seo, S.M. Choi, H.J. Kim, W.B. Kim, Electrochem. Commun. 13 (2011) 182. [20] S. Stankovich, D. Dikin, R. Piner, K. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558. [21] C. Gomez-Navarro, J.C. Meyers, R.S. Sundaram, A. Chuvilin, S. Kurash, M. Burghard, K. Kern, U. Kaizer, Nano Lett. 10 (2010) 1144. [22] R.I. Jafri, T. Arockiados, N. Rajalakshmi, S. Ramaprabhu, J. Electrochem. Soc. 157 (2010) B874. [23] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339. [24] J. Yan, T. Wei, B. Shao, F.Q. Ma, Z.J. Fan, M.L. Zhang, C. Zheng, Y.C. Shang, W.Z. Qian, F. Wei, Carbon 48 (2010) 1731. [25] B. Seger, P.V. Kamat, J. Phys. Chem. C 113 (2009) 7990. [26] R.S. Sundaram, C. Gomez-Navarro, K. Balasubramanian, M. Burghard, K. Kern, Adv. Mater. 20 (2008) 3050. [27] M.C. Zhao, C. Rice, R.I. Masel, P. Waszczuk, A. Wieckowskib, J. Electrochem. Soc. 151 (2004) A131.
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