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MnO2-reduced graphene oxide nanocomposite for methanol electro-oxidation in alkaline media
Electrochemistry Communications 26 (2013) 63–66
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Preparation of Pd/MnO2-reduced graphene oxide nanocomposite for methanol electro-oxidation in alkaline media Rui Liu a, b, Haihui Zhou a, b,⁎, Jia Liu a, b, Yuan Yao c, Zhongyuan Huang a, b, Chaopeng Fu a, b, Yafei Kuang a, b,⁎ a b c
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China School of Marine Science and Technology, Harbin Institute of Technology, Weihai, 264209, PR China
a r t i c l e
i n f o
Article history: Received 11 September 2012 Received in revised form 9 October 2012 Accepted 10 October 2012 Available online 17 October 2012 Keywords: Graphene MnO2 Pd nanoparticles Methanol oxidation
a b s t r a c t Manganese dioxide modified reduced graphene oxide supported Pd nanoparticles (Pd/MnO2-RGO) were prepared facilely by a chemical approach. The as-prepared materials were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The electrocatalytic behavior of the Pd/MnO2-RGO for methanol oxidation was studied using cyclic voltammetry and chronoamperometry. The results indicate that the Pd/MnO2-RGO nanocomposites exhibit higher catalytic activity and better stability than Pd/RGO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) as power sources have attracted enormous attention in recent years. As one of the most effective catalysts of DMFCs, Pt has been widely used for many years [1–4]. However, the high cost and limited reserve of Pt hinder its application in DMFCs. Pd is very suitable as a Pt-alternative material due to its relatively abundant resource and desirable catalytic activity [5–7]. So far, many Pd-based catalysts, such as Pd-oxides, have been investigated as electrode materials for methanol oxidation [8,9]. Among all the metal oxides, MnO2 is attractive for its low cost and excellent molecular adsorption ability [10]. Pd/β-MnO2 was prepared and proved to display excellent electrochemical performances for methanol oxidation in alkaline media [11]. Hence, it will be interesting to see if the incorporation of MnO2 into Pd particles is a good way to fabricate a new effective catalyst for DMFCs. Graphene oxide (GO), one of the most important derivatives of graphene, is characterized by a layered structure with oxygen functional groups bearing on the basal planes and edges. It is well known that reduced graphene oxide (RGO) could be facilely obtained by chemical reduction of GO in aqueous solutions [12]. The RGO not only has high surface area, but also could reduce aggregation of nanoparticles due to its crinkly structure and the retained oxygen atoms in the film of RGO compared to multi-walled carbon nanotubes (MWCNTs) and Vulcan XC-72. Hence, RGO is expected to be an ideal catalyst carrier [13,14]. ⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China. Tel.: +86 731 88821603; fax: +86 731 88713642. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.10.019
To the best of our knowledge, there is not any report on the application of MnO2-RGO as the catalyst support in DMFCs. In this work, MnO2 modified reduced graphene oxide supported Pd nanoparticles (Pd/MnO2-RGO) were fabricated for the first time by co-reduction of MnO2 modified GO (MnO2-GO) and (NH4)2PdCl6. The electrocatalytic performance of the synthesized Pd/MnO2-RGO was evaluated by methanol oxidation in alkaline media. 2. Experimental 2.1. Preparation of catalysts All reagents with analytical grade were used without further purification, deionized water was used throughout this study. GO was prepared from nature graphite flakes (325 mesh, 99.8%) by a modified Hummers method [15]. MnO2-GO was prepared as follows: 21 mg of GO and 5.5 mg of KMnO4 dispersed uniformly in deionized water. Then 15 mL of ethylene glycol was added dropwise under vigorous stirring at 40 °C for 1 h. Finally, the product was separated by centrifugation, washed repeatedly with deionized water, and dried in vacuum oven. To prepare Pd/MnO2-RGO, 11.46 mg of MnO2-GO was added to 20 mL ethanol–water (1:1, v/v ratio) solution with 2 h ultrasonic treatment. Then, 4.5 mL of 0.006 M (NH4)2PdCl6 was added. Subsequently, excess freshly prepared NaBH4 was delivered by drops into the above suspension under continuous stirring. After reacting 12 h at room temperature, Pd/MnO2-RGO can be obtained by centrifugation, washing and drying. For comparison, Pd/RGO was prepared using the similar method. The palladium loading of every catalyst was 20 wt.%.
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Fig. 1. The formation mechanism of Pd/MnO2-RGO.
2.2. Characterization of catalysts Morphologies of the catalysts were characterized by transmission electron microscopy (TEM, JEOL-3010) and scanning electron microscopy (SEM, Hitachi S-4800). Structure characterization of the catalysts was carried out by X-ray diffraction (XRD, Rigaku D/Max Ultima II diffractometer). Electrochemical properties of the catalysts were measured on a CHI 440A electrochemical workstation (Shanghai Chenhua Instrument Factory, China) using a conventional three-electrode system with a catalyst modified glassy carbon (GC, S = 0.071 cm2) working electrode, a platinum wire counter electrode and a saturated calomel reference electrode (SCE). Before the surface coating, the GC was polished with
0.3 μm alumina powder. Then 1 mg of catalyst was dispersed ultrasonically in 1 mL of H2O; 10 μL of the well-dispersed catalyst suspension was dropped on the surface of the GC electrode and dried at room temperature. After that, 5 μL of Nafion (0.5 wt.%) was placed on the surface of the catalyst modified GC electrode and dried before electrochemical experiments. 3. Results and discussion 3.1. Mechanism of Pd/MnO2-RGO formation Fig. 1 shows the formation mechanism of the Pd/MnO2-RGO. In the first step, the modified Hummers method was employed to prepare GO.
Fig. 2. SEM image of MnO2-GO (a); SEM (b) and TEM (c) images of Pd/MnO2-RGO; XRD patterns (d) of GO(1), MnO2-GO(2) and Pd/MnO2-RGO(3).
R. Liu et al. / Electrochemistry Communications 26 (2013) 63–66
65
Then MnO2 were trapped by the oxygenated functional groups on the surface of GO using KMnO4 as MnO2 precursor and ethylene glycol as reducing agent, and then we got a brown uniform suspension (MnO2-GO). The incorporation of MnO2 could offer more active sites for Pd loading. And the oxygenous functional groups of GO could also serve as anchor sites bonding with metal ions. In the last step, PdCl62 − and GO were co-reduced by NaBH4 with continuous magnetic stirring, and finally well dispersed Pd nanoparticles supported on MnO2-RGO were obtained. 3.2. Structural and compositional analysis SEM and TEM images of MnO2-GO and Pd/MnO2-RGO are shown in Fig. 2. As shown in Fig. 2a, most of MnO2 particles on the surface of GO are agglomeratic and non-uniform, leading a rougher surface. However, in Fig. 2b, Pd/MnO2 nanoparticles are well-dispersed on the surface of RGO, without reaggregation, indicating that during the process of loading Pd, MnO2 can not only promote the homogenous dispersion of Pd nanoparticles, but also change its own existence form. And what's more, the retained oxygen atoms in the film of RGO could serve as binding sites to prevent nanoparticles aggregating together and keep Pd nanoparticles well adhered. Fig. 2c shows that most Pd particles have an average size of 5 nm. The small size and good dispersion of Pd are very conducive to methanol electro-oxidation. XRD patterns of GO, MnO2-GO and Pd/MnO2-RGO are shown in Fig. 2d. The typical diffraction peak of GO at 2θ = 11.6° shifts to a larger angle (2θ = 24.6°) due to the reduction of GO by NaBH4. As seen from Fig. 2d (2), a few broad peaks with less intensity at around 2θ = 36.5° and 66.7° are attributed to MnO2 (110) and (020) respectively [16,17], which can still be observed in Fig. 2d (3). It means that the structure of MnO2 in both MnO2-GO and Pd/MnO2-RGO is similar. The diffraction peaks at 2θ = 39.2°, 45.1° and 68.4° are associated with the (111), (200) and (220) facets of Pd crystal, indicating that Pd/MnO2-RGO is composed of Pd and MnO2. 3.3. Electrochemical properties of Pd/MnO2-RGO Fig. 3a shows the cyclic voltammograms (CVs) of Pd/RGO and Pd/MnO2-RGO electrodes in 0.5 M KOH + 1 M CH3OH solution. The onset potential of faradaic current (Eonset), forward anodic peak current density (If), backward anodic peak current density (Ib) and forward peak potential (Ep) for methanol oxidation on Pd/MnO2-RGO and Pd/RGO electrodes are shown in Table 1. The values of Eonset and Ep on Pd/MnO2-RGO (−0.60 V, −0.20 V) are more negative than those on Pd/RGO (−0.53 V, −0.19 V) and Pd/β-MnO2 nanotubes (−0.55 V, 0.04 V) [11]. The If of Pd/MnO2-RGO (20.4 mA cm−2) is much higher than that of Pd/RGO (6.0 mA cm−2). This indicates that the electrocatalytic activity of Pd/MnO2-RGO is obviously higher than that of Pd/RGO. It is well known that the ratio of If to Ib (If/Ib) can be used to evaluate the catalyst tolerance to the intermediate carbonaceous species accumulated on electrode surface [18]. The higher If/Ib value indicates higher tolerance to the intermediate carbonaceous species. The If/Ib on Pd/MnO2-RGO is 4 times as large as that on Pd/RGO, which suggests that Pd/MnO2-RGO has less carbonaceous accumulation and hence is much more tolerant toward CO poisoning. Fig. 3b shows the CVs of different catalysts in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s − 1. The ratio of MnO2 to RGO was adjusted by changing MnO2 and RGO contents at a fixed Pd loading of 0.028 mg·cm−2. Different weight ratios of MnO2 to RGO, 1:15, 1:7, 3:13 and 1:3, were studied. It can be seen that all catalysts with MnO2 display large If and almost the same Ib. Among these catalysts, Pd/MnO2-RGO exhibits the highest catalytic activity when the weight ratio of MnO2 to RGO is 1:7. Hence, the incorporation of MnO2 can greatly enhance the catalyst activity. Fig. 3c shows chronoamperometric curves of Pd/RGO and Pd/MnO2RGO (1:7) in 0.5 M KOH + 1 M CH3OH at −0.25 V. The rapid current
Fig. 3. (a) CVs of Pd/RGO and Pd/MnO2-RGO in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s−1. (b) CVs of different catalysts in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s−1. All the catalysts have a fixed Pd loading of 0.028 mg cm−2. (c) Chronoamperometric curves of Pd/RGO and Pd/MnO2-RGO (1:7) in 0.5 M KOH+1 M CH3OH at −0.25 V.
decay shows the poisoning of catalysts. It is noticeable that the current decay on Pd/MnO2-RGO is slower than that on Pd/RGO, indicating that the incorporation of MnO2 into Pd particles can enhance catalytic Table 1 The Eonset, Ep, If and Ib for methanol electro-oxidation on different catalysts. Catalysts
Eonset (V vs. SCE)
Ep (V vs. SCE)
If (mA cm−2)
Ib (mA cm−2)
If/Ib
Pd/RGO Pd/MnO2-RGO (1:7)
−0.53 −0.60
−0.19 −0.20
6.0 20.4
5.6 4.7
1.1 4.3
66
R. Liu et al. / Electrochemistry Communications 26 (2013) 63–66
stability toward methanol oxidation. It can be explained as follows: Firstly, MnO2 can offer more active sites and improve the dispersion of Pd particles, which results in the larger active surface of Pd/MnO2-RGO; Secondly, the surface adsorbed hydroxyl of MnO2 may remove the adsorbed carbonyl on the surface of Pd, and then dissociation–adsorption of methanol proceeds quickly. The reaction [19] can be written as Eqs. (1)–(3). −
MnO2 þ OH →MnO2 −OHads þ e
−
−
ð1Þ
−
Pd−ðCH3 OHads Þ þ 4OH →Pd−COads þ 4H2 O þ 4e
−
ð2Þ
−
Pd−COads þ MnO2 −OHads þ OH →Pd þ MnO2 þ CO2 þ H2 O þ e : ð3Þ MnO2 would increase the concentration of OHads species on the catalyst surface, and these OHads can react with CO-like intermediate species to produce CO2 or watersoluble products, releasing the active sites on Pd for further electrochemical reaction. 4. Conclusions In summary, Pd/MnO2-RGO nanocomposites have been successfully prepared. The SEM and TEM images reveal that Pd particles are welldispersed on the surface of MnO2-RGO with an average size of 5 nm. CVs showed that the electrocatalytic activity of the Pd/MnO2-RGO for methanol oxidation is much higher than that of the Pd/RGO, and the chronoamperograms indicate that the Pd/MnO2-RGO has much higher stability than the Pd/RGO. All the results imply that the Pd/MnO2-RGO has potential application in DMFC field.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No.51071067, 21271069, J1210040) and Science Technology Project of Hunan Province, China (Grant No. 2010GK3208, 2011GK3136). References [1] Y. Li, W. Gao, L. Ci, C. Wang, P.M. Ajiayan, Carbon 48 (2010) 1124. [2] L. Wang, Y. Nemoto, Y. Yamauchi, Journal of the American Chemical Society 133 (2011) 9674. [3] H. Wang, S. Ishihara, K. Ariga, Y. Yamauchi, Journal of the American Chemical Society 134 (2012) 10819. [4] D. Zhao, X. Guo, Y. Gao, F. Gao, ACS Applied Materials & Interfaces 4 (2012) 2865. [5] Z.P. Sun, X.G. Zhang, Y.Y. Liang, H.L. Li, Electrochemistry Communications 11 (2009) 557. [6] C. Xu, L. Cheng, P. Shen, Y. Liu, Electrochemistry Communications 9 (2007) 997. [7] Y. Zhao, X. Yang, J. Tian, F. Wang, L. Zhan, International Journal of Hydrogen Energy 35 (2010) 3249. [8] C. Xu, P. Shen, Y. Liu, Journal of Power Sources 164 (2007) 527. [9] C. Xu, Z. Tian, P. Shen, S.P. Jiang, Electrochimica Acta 53 (2008) 2610. [10] S.B. Ma, K.W. Nam, W.S. Yoon, X.Q. Yang, K.Y. Ahn, K.H. Oh, K.B. Kim, Journal of Power Sources 178 (2008) 483. [11] M.W. Xu, G.Y. Gao, W.J. Zhou, K.F. Zhang, H.L. Li, Journal of Power Sources 175 (2008) 217. [12] Z. Huang, H. Zhou, C. Li, F. Zeng, C. Fu, Y. Kuang, Journal of Materials Chemistry 22 (2012) 1781. [13] C.N.R. Rao, A.K. Sood, R. Voggu, K.S. Subrahmanyam, Journal of Physical Chemistry Letters 1 (2010) 572. [14] D. Li, M.B. Muller, S. Gil Je, R.B. Kaner, G.G. Wallace, Nature Nanotechnology 3 (2008) 101. [15] W.S. Hummers, R.E. Offeman, Journal of the American Chemical Society 80 (1958) 1339. [16] J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang, F. Wei, Carbon 48 (2010) 3825. [17] S.B. Ma, K.Y. Ahn, E.S. Lee, K.H. Oh, K.B. Kim, Carbon 45 (2007) 375. [18] A. Pozio, M. De Francesco, A. Cemmi, F. Cardellini, L. Giorgi, Journal of Power Sources 105 (2002) 13. [19] C. Zhou, F. Peng, H. Wang, H. Yu, J. Yang, X. Fu, Fuel Cells 11 (2011) 301.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Preparation of Pd/MnO2-reduced graphene oxide nanocomposite for methanol electro-oxidation in alkaline media Rui Liu a, b, Haihui Zhou a, b,⁎, Jia Liu a, b, Yuan Yao c, Zhongyuan Huang a, b, Chaopeng Fu a, b, Yafei Kuang a, b,⁎ a b c
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China School of Marine Science and Technology, Harbin Institute of Technology, Weihai, 264209, PR China
a r t i c l e
i n f o
Article history: Received 11 September 2012 Received in revised form 9 October 2012 Accepted 10 October 2012 Available online 17 October 2012 Keywords: Graphene MnO2 Pd nanoparticles Methanol oxidation
a b s t r a c t Manganese dioxide modified reduced graphene oxide supported Pd nanoparticles (Pd/MnO2-RGO) were prepared facilely by a chemical approach. The as-prepared materials were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The electrocatalytic behavior of the Pd/MnO2-RGO for methanol oxidation was studied using cyclic voltammetry and chronoamperometry. The results indicate that the Pd/MnO2-RGO nanocomposites exhibit higher catalytic activity and better stability than Pd/RGO. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) as power sources have attracted enormous attention in recent years. As one of the most effective catalysts of DMFCs, Pt has been widely used for many years [1–4]. However, the high cost and limited reserve of Pt hinder its application in DMFCs. Pd is very suitable as a Pt-alternative material due to its relatively abundant resource and desirable catalytic activity [5–7]. So far, many Pd-based catalysts, such as Pd-oxides, have been investigated as electrode materials for methanol oxidation [8,9]. Among all the metal oxides, MnO2 is attractive for its low cost and excellent molecular adsorption ability [10]. Pd/β-MnO2 was prepared and proved to display excellent electrochemical performances for methanol oxidation in alkaline media [11]. Hence, it will be interesting to see if the incorporation of MnO2 into Pd particles is a good way to fabricate a new effective catalyst for DMFCs. Graphene oxide (GO), one of the most important derivatives of graphene, is characterized by a layered structure with oxygen functional groups bearing on the basal planes and edges. It is well known that reduced graphene oxide (RGO) could be facilely obtained by chemical reduction of GO in aqueous solutions [12]. The RGO not only has high surface area, but also could reduce aggregation of nanoparticles due to its crinkly structure and the retained oxygen atoms in the film of RGO compared to multi-walled carbon nanotubes (MWCNTs) and Vulcan XC-72. Hence, RGO is expected to be an ideal catalyst carrier [13,14]. ⁎ Corresponding authors at: College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, PR China. Tel.: +86 731 88821603; fax: +86 731 88713642. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.10.019
To the best of our knowledge, there is not any report on the application of MnO2-RGO as the catalyst support in DMFCs. In this work, MnO2 modified reduced graphene oxide supported Pd nanoparticles (Pd/MnO2-RGO) were fabricated for the first time by co-reduction of MnO2 modified GO (MnO2-GO) and (NH4)2PdCl6. The electrocatalytic performance of the synthesized Pd/MnO2-RGO was evaluated by methanol oxidation in alkaline media. 2. Experimental 2.1. Preparation of catalysts All reagents with analytical grade were used without further purification, deionized water was used throughout this study. GO was prepared from nature graphite flakes (325 mesh, 99.8%) by a modified Hummers method [15]. MnO2-GO was prepared as follows: 21 mg of GO and 5.5 mg of KMnO4 dispersed uniformly in deionized water. Then 15 mL of ethylene glycol was added dropwise under vigorous stirring at 40 °C for 1 h. Finally, the product was separated by centrifugation, washed repeatedly with deionized water, and dried in vacuum oven. To prepare Pd/MnO2-RGO, 11.46 mg of MnO2-GO was added to 20 mL ethanol–water (1:1, v/v ratio) solution with 2 h ultrasonic treatment. Then, 4.5 mL of 0.006 M (NH4)2PdCl6 was added. Subsequently, excess freshly prepared NaBH4 was delivered by drops into the above suspension under continuous stirring. After reacting 12 h at room temperature, Pd/MnO2-RGO can be obtained by centrifugation, washing and drying. For comparison, Pd/RGO was prepared using the similar method. The palladium loading of every catalyst was 20 wt.%.
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Fig. 1. The formation mechanism of Pd/MnO2-RGO.
2.2. Characterization of catalysts Morphologies of the catalysts were characterized by transmission electron microscopy (TEM, JEOL-3010) and scanning electron microscopy (SEM, Hitachi S-4800). Structure characterization of the catalysts was carried out by X-ray diffraction (XRD, Rigaku D/Max Ultima II diffractometer). Electrochemical properties of the catalysts were measured on a CHI 440A electrochemical workstation (Shanghai Chenhua Instrument Factory, China) using a conventional three-electrode system with a catalyst modified glassy carbon (GC, S = 0.071 cm2) working electrode, a platinum wire counter electrode and a saturated calomel reference electrode (SCE). Before the surface coating, the GC was polished with
0.3 μm alumina powder. Then 1 mg of catalyst was dispersed ultrasonically in 1 mL of H2O; 10 μL of the well-dispersed catalyst suspension was dropped on the surface of the GC electrode and dried at room temperature. After that, 5 μL of Nafion (0.5 wt.%) was placed on the surface of the catalyst modified GC electrode and dried before electrochemical experiments. 3. Results and discussion 3.1. Mechanism of Pd/MnO2-RGO formation Fig. 1 shows the formation mechanism of the Pd/MnO2-RGO. In the first step, the modified Hummers method was employed to prepare GO.
Fig. 2. SEM image of MnO2-GO (a); SEM (b) and TEM (c) images of Pd/MnO2-RGO; XRD patterns (d) of GO(1), MnO2-GO(2) and Pd/MnO2-RGO(3).
R. Liu et al. / Electrochemistry Communications 26 (2013) 63–66
65
Then MnO2 were trapped by the oxygenated functional groups on the surface of GO using KMnO4 as MnO2 precursor and ethylene glycol as reducing agent, and then we got a brown uniform suspension (MnO2-GO). The incorporation of MnO2 could offer more active sites for Pd loading. And the oxygenous functional groups of GO could also serve as anchor sites bonding with metal ions. In the last step, PdCl62 − and GO were co-reduced by NaBH4 with continuous magnetic stirring, and finally well dispersed Pd nanoparticles supported on MnO2-RGO were obtained. 3.2. Structural and compositional analysis SEM and TEM images of MnO2-GO and Pd/MnO2-RGO are shown in Fig. 2. As shown in Fig. 2a, most of MnO2 particles on the surface of GO are agglomeratic and non-uniform, leading a rougher surface. However, in Fig. 2b, Pd/MnO2 nanoparticles are well-dispersed on the surface of RGO, without reaggregation, indicating that during the process of loading Pd, MnO2 can not only promote the homogenous dispersion of Pd nanoparticles, but also change its own existence form. And what's more, the retained oxygen atoms in the film of RGO could serve as binding sites to prevent nanoparticles aggregating together and keep Pd nanoparticles well adhered. Fig. 2c shows that most Pd particles have an average size of 5 nm. The small size and good dispersion of Pd are very conducive to methanol electro-oxidation. XRD patterns of GO, MnO2-GO and Pd/MnO2-RGO are shown in Fig. 2d. The typical diffraction peak of GO at 2θ = 11.6° shifts to a larger angle (2θ = 24.6°) due to the reduction of GO by NaBH4. As seen from Fig. 2d (2), a few broad peaks with less intensity at around 2θ = 36.5° and 66.7° are attributed to MnO2 (110) and (020) respectively [16,17], which can still be observed in Fig. 2d (3). It means that the structure of MnO2 in both MnO2-GO and Pd/MnO2-RGO is similar. The diffraction peaks at 2θ = 39.2°, 45.1° and 68.4° are associated with the (111), (200) and (220) facets of Pd crystal, indicating that Pd/MnO2-RGO is composed of Pd and MnO2. 3.3. Electrochemical properties of Pd/MnO2-RGO Fig. 3a shows the cyclic voltammograms (CVs) of Pd/RGO and Pd/MnO2-RGO electrodes in 0.5 M KOH + 1 M CH3OH solution. The onset potential of faradaic current (Eonset), forward anodic peak current density (If), backward anodic peak current density (Ib) and forward peak potential (Ep) for methanol oxidation on Pd/MnO2-RGO and Pd/RGO electrodes are shown in Table 1. The values of Eonset and Ep on Pd/MnO2-RGO (−0.60 V, −0.20 V) are more negative than those on Pd/RGO (−0.53 V, −0.19 V) and Pd/β-MnO2 nanotubes (−0.55 V, 0.04 V) [11]. The If of Pd/MnO2-RGO (20.4 mA cm−2) is much higher than that of Pd/RGO (6.0 mA cm−2). This indicates that the electrocatalytic activity of Pd/MnO2-RGO is obviously higher than that of Pd/RGO. It is well known that the ratio of If to Ib (If/Ib) can be used to evaluate the catalyst tolerance to the intermediate carbonaceous species accumulated on electrode surface [18]. The higher If/Ib value indicates higher tolerance to the intermediate carbonaceous species. The If/Ib on Pd/MnO2-RGO is 4 times as large as that on Pd/RGO, which suggests that Pd/MnO2-RGO has less carbonaceous accumulation and hence is much more tolerant toward CO poisoning. Fig. 3b shows the CVs of different catalysts in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s − 1. The ratio of MnO2 to RGO was adjusted by changing MnO2 and RGO contents at a fixed Pd loading of 0.028 mg·cm−2. Different weight ratios of MnO2 to RGO, 1:15, 1:7, 3:13 and 1:3, were studied. It can be seen that all catalysts with MnO2 display large If and almost the same Ib. Among these catalysts, Pd/MnO2-RGO exhibits the highest catalytic activity when the weight ratio of MnO2 to RGO is 1:7. Hence, the incorporation of MnO2 can greatly enhance the catalyst activity. Fig. 3c shows chronoamperometric curves of Pd/RGO and Pd/MnO2RGO (1:7) in 0.5 M KOH + 1 M CH3OH at −0.25 V. The rapid current
Fig. 3. (a) CVs of Pd/RGO and Pd/MnO2-RGO in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s−1. (b) CVs of different catalysts in 0.5 M KOH + 1 M CH3OH at a scan rate of 50 mV s−1. All the catalysts have a fixed Pd loading of 0.028 mg cm−2. (c) Chronoamperometric curves of Pd/RGO and Pd/MnO2-RGO (1:7) in 0.5 M KOH+1 M CH3OH at −0.25 V.
decay shows the poisoning of catalysts. It is noticeable that the current decay on Pd/MnO2-RGO is slower than that on Pd/RGO, indicating that the incorporation of MnO2 into Pd particles can enhance catalytic Table 1 The Eonset, Ep, If and Ib for methanol electro-oxidation on different catalysts. Catalysts
Eonset (V vs. SCE)
Ep (V vs. SCE)
If (mA cm−2)
Ib (mA cm−2)
If/Ib
Pd/RGO Pd/MnO2-RGO (1:7)
−0.53 −0.60
−0.19 −0.20
6.0 20.4
5.6 4.7
1.1 4.3
66
R. Liu et al. / Electrochemistry Communications 26 (2013) 63–66
stability toward methanol oxidation. It can be explained as follows: Firstly, MnO2 can offer more active sites and improve the dispersion of Pd particles, which results in the larger active surface of Pd/MnO2-RGO; Secondly, the surface adsorbed hydroxyl of MnO2 may remove the adsorbed carbonyl on the surface of Pd, and then dissociation–adsorption of methanol proceeds quickly. The reaction [19] can be written as Eqs. (1)–(3). −
MnO2 þ OH →MnO2 −OHads þ e
−
−
ð1Þ
−
Pd−ðCH3 OHads Þ þ 4OH →Pd−COads þ 4H2 O þ 4e
−
ð2Þ
−
Pd−COads þ MnO2 −OHads þ OH →Pd þ MnO2 þ CO2 þ H2 O þ e : ð3Þ MnO2 would increase the concentration of OHads species on the catalyst surface, and these OHads can react with CO-like intermediate species to produce CO2 or watersoluble products, releasing the active sites on Pd for further electrochemical reaction. 4. Conclusions In summary, Pd/MnO2-RGO nanocomposites have been successfully prepared. The SEM and TEM images reveal that Pd particles are welldispersed on the surface of MnO2-RGO with an average size of 5 nm. CVs showed that the electrocatalytic activity of the Pd/MnO2-RGO for methanol oxidation is much higher than that of the Pd/RGO, and the chronoamperograms indicate that the Pd/MnO2-RGO has much higher stability than the Pd/RGO. All the results imply that the Pd/MnO2-RGO has potential application in DMFC field.
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