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Mesoporous active carbon dispersed with ultra-fine platinum nanoparticles and their electrochemical properties
Diamond & Related Materials 18 (2009) 303–306
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Mesoporous active carbon dispersed with ultra-fine platinum nanoparticles and their electrochemical properties J.Y. Hwang a, A. Chatterjee b, C.H. Shen b, J.H. Wang c, C.L. Sun d, Oliver Chyan e, C.W. Chen a, K.H. Chen b,d, L.C. Chen b,⁎ a
Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan Department of Chemistry, National Taiwan University, Taipei, Taiwan d Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan e Department of Chemistry, University of North Texas, Texas, USA b c
a r t i c l e
i n f o
Available online 31 October 2008 Keywords: Mesoporous carbon Cyclic voltammetry (CV) Platinum carbon catalyst Ion exchange
a b s t r a c t Active carbons were prepared by polymer blend with various chemical concentrations and carbonization conditions. The resultant porosity and electrochemical properties were studied by nitrogen adsorption– desorption isotherm and cyclic voltammetry techniques, and compared with those of commercial carbon black, Vulcan XC-72R. The polymer-derived active carbons exhibit a mesoporous structure, large surface area, low capacitance, and high electrochemical activity, thus are suitable for catalyst support. By ion-exchange method with appropriate functionalization treatments (three different ways were conducted in this paper) on these carbon supports, we have achieved a uniform dispersion of ultra-fine Pt nanoparticles (~2 nm) with a total loading amount up to 10 wt.% and an exchange efficiency of 60%, much higher than the values of Vulcan XC-72R. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Highly dispersed platinum (Pt) on a conductive material is commonly used as catalyst for electrochemical reactions, such as those occurred in fuel cell systems. In these applications, Pt nanoparticles (NPs) with a diameter of 2–3 nm are desirable for optimal catalytic performance and cost effectiveness, while active carbons (ACs) with low density and large surface area are widely used as catalyst supports [1,2]. Conventional ACs are used as gas adsorbents or filters due to their extremely large surface area (N1000 m2/g) and microporous structure (b2 nm). However, in the case of electrochemical catalyst support, mesoporous structure (2 to 50 nm) is preferred since micropores are too small to be efficient active sites for catalyst particles. Meanwhile, the ability of electron transfer, which can be characterized by conductivity measurement, is also important. The cyclic voltammetry (CV) technique has been used for characterization and estimation of material properties, including capacitance effect and electrochemical activity, for electrochemical applications. Capacitance effect relates to the tendency of charge accumulation and chemical adsorption on the surface of material, whereas electrochemical activity suggests the ability of transporting electrons not only within the electrode material, but also between the electrode and electrolyte. As a catalyst support, the capacitance should be low while the electrochemical activity should be high in order to have fast response [3]. ⁎ Corresponding author. Tel.: +886 2 33665249; fax: +886 2 23655404. E-mail address: [email protected] (L.C. Chen). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.014
For catalyst (Pt) loading, ion-exchange method was reported as an effective technique to obtain highly dispersed platinum carbon catalyst (Pt/C) [4]. In contrast, the conventional impregnation method often results in significant coagulation of Pt on the surface [5]. It is suggested that the functional groups on the surface of carbon support could enhance Pt dispersion [6], and also affect the loading amount of Pt. In this paper, mesoporous active carbons (MPAC) have been successfully synthesized with a polymer blend method [7]. It is found that the resultant properties of ACs depend on the composition of blended polymers and carbonization conditions. Vulcan XC-72R, a commercial carbon black popularly used in fuel cell systems, is taken as a reference sample. Three different functionalization treatments, namely, ozone treatment, nitric acid immersion and partial oxidation followed by sodium hypochlorite immersion, were conducted on our MPAC and Vulcan XC-72R, followed by ion-exchange reaction for Pt loading. Morphology and microstructure studies using SEM and TEM suggested that highly dispersed Pt NPs (b5 nm) can be achieved on properly functionalization-treated MPAC. A loading amount up to 10 wt.% and a reaction efficiency approaching 60% were obtained. 2. Experimental 2.1. Preparation of active carbon Three organic polymers [7,8]: Furfuryl alcohol (FA, 98% ACROS OGANICS), Pyrrole (99% ACROS OGANICS), and Poly ethylene glycol (PEG) methylether (average molecular weight of 350, ACROS
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OGANICS) were used as the source materials for polymer blend. After mixing the source materials, nitric acid (as polymerization catalyst) was added gradually during 30 min while stirring and cooling continued until solidification. Various chemical concentrations and carbonization conditions were experimented (Table 1). After 30 min of argon purging, the solidified mixtures were heated in argon flow (50– 300 sccm) at a rate of 10 °C per min to a designated temperature (550– 850 °C) and held for 1 h. Finally, the carbonized sample was ball milled to powder. 2.2. Functionalization of carbon supports Three ways to functionalize the surface of carbon supports were used: (1) ozone treatment, under flowing O3 ambient at 40 mg/h for 5 days [9]; (2) nitric acid immersion, using concentrated HNO3 (65%) at 60 °C for 4 h [5,6,10]; (3) two-step functionalization, i.e., partial oxidation using 10% O2 in Ar at 350 °C for 5 h followed by immersing in sodium hypochlorite (NaClO, 13% ACROS OGANICS) at room temperature for 30 min [4,8]. These functionalization processes were experimented using MPAC, a subset of carbon supports prepared with specific conditions shown in Table 1. For comparison, identical functionalization experiments were also performed on Vulcan XC72R. 2.3. Ion-exchange process Pt(NH3)Cl2 (Aldrich, 98%) was used as precursor. The ratio of Pt to carbon supports was controlled at 18.7 wt.%, typically, by immersing 0.5 g carbon supports into 20 ml distilled water with pre-dissolved 0.2 g Pt(NH3)Cl2. The reactions were conducted at room temperature for 1 h, and the Pt/C catalysts were centrifuged, washed with water, and dried at 60 °C for 24 h. Lastly, the dried Pt/C samples were reduced in 10% H2 in Ar flow at 180 °C for 4 h. 2.4. Analyses of surface area and pore distribution Nitrogen adsorption–desorption isotherms were measured at −196 °C with Micromeritics ASAP 2010 instrument. The surface areas were calculated according to the BET method (P/P0 = 0.05–0.30) derived by Brunauer, Emmett, and Teller, and the total pore volume was calculated at P/P0 = 0.91, where P/P0 means the relative pressure, and P0 is atmosphere pressure. 2.5. Demonstration of electrochemical properties A specific electrode was prepared by using a gold wire with a 1 mm diameter as a current collector. The Au wire was polished and inserted Table 1 Chemical composition, carbonization temperature and gas flow rate of preparing active carbon
AC-01 AC-03 AC-07 AC-08-50 AC-08-100 AC-08-200 AC-08-300 AC-09 MPAC AC-11-550C AC-11-650C AC-11-750C AC-11-850C Vulcan XC-72R a
FA (ml)
PEG (ml)
Pyrrole (ml)
Carbonization temp. (°C)
Gas flow rate (sccm)
Surface area (m2/g)
6 6 6 6
0 2 4 4
0 0 0 1
4 2 6
4 4 4
1 1 2
650 650 650 650 650 650 650 650 650 550 650 750 850
100 100 100 50 100 200 300 100 100 100 100 100 100
109 264 231 250 291 278 224 352 441 267 314 207 103 225 (257a)
Data from Cabot.
Fig. 1. Adsorption–desorption isotherm and pore distribution of (a) MPAC and (b) Vulcan XC-72R.
into a Teflon plate and sealed by silicone [11]. Carbons were filled in the cavity with 1 mm diameter and 0.5 mm depth. Cyclic voltammetric curves were measured with μAutolab Potentialstat/Golvanostat Type III instrument in two kinds of electrolytes: 1 M H2SO4, to assess the capacitance effect and 1 M H2SO4 with 10 mM K3[Fe(CN)6], to measure the electrochemical activity. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V versus standard hydrogen electrode (SHE) at 25 °C) was used as reference electrode. 2.6. Characterization of Pt/C The morphology was characterized by high-resolution fieldemission scanning electron microscope (FESEM, JEOL-FESEM6700) and transmission electron microscope (TEM, Philips Tecnai 20). For measuring Pt loading amount, carbon support was combusted and then the residual was dissolved in aqua regia. The Pt concentration in diluted aqua regia solution was analyzed by inductively-coupledplasma atomic-emission spectrometry (ICP-AES). 3. Results and discussion 3.1. Surface area and pore structure Table 1 shows the results of surface areas of Vulcan XC-72R and ACs prepared by polymer blend method. Within the range of process parameters, both the carbonization temperature and the polymer
J.Y. Hwang et al. / Diamond & Related Materials 18 (2009) 303–306
blend composition showed more pronounced effects than the gas flow rate on the surface area of ACs. The largest surface area was obtained at a gas flow rate of 100 sccm and a carbonization temperature of 650 °C. Meanwhile, Raman analyses showed that the ratio of the intensity of G-band and D-band increased further with the carbonization temperature, indicating better structural order and possible enhancement of conductivity. However, the surface area decreased dramatically above a carbonization temperature of 650 °C, at which hydrogen atoms were gradually eliminated from the network of carbon polymeric chains and the electrical conductivity increased rapidly as the polymeric chains became an interconnected conducting network. Above that temperature, the porous properties became poor though the conductivity improved. Therefore, unless indicated otherwise, a gas flow rate of 100 sccm and a carbonization temperature of 650 °C were applied. The composition of blended polymers also showed a strong effect on the resultant surface area. When the composition of FA:PEG:Pyrrole was kept at 2:4:1, the resultant carbon exhibited a mesoporous structure and a surface area of 441 m2/g, nearly twice the corresponding value of Vulcan XC-72R (225 m2/g). For convenience, the term mesoporous active carbon (MPAC) refers to the carbon material prepared under this particular condition hereafter. Fig. 1 is the nitrogen adsorption–desorption isotherms and pore size distribution (inset) of MPAC and Vulcan XC-72R. The isotherm
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Fig. 3. Voltammetric curves measured at (□) 50 mV/s, ( ) 30 mV/s and (—) 10 mV/s in electrolyte of 1 M H2SO4 plus 10 mM K3(Fe(CN)6). Upper inset graph shows the relation of oxidation peak current and square root of scan rate. Lower inset graph presents the definition of peak current.
shape of MPAC can be categorized as type 4, which possesses a hysteresis loop [12], usually associated with an adsorption process of mesoporous solids wherein capillary condensation has occurred. In contrast, the isotherm shape of Vulcan XC-72R displays type 2, characteristic of physical adsorption of gases by non-porous solids. 3.2. Electrochemical properties The CV curves of MPAC and Vulcan XC-72R are shown in Fig. 2 (a) and (b). In only sulfuric solution (solid lines), the larger area enclosed indicates the larger capacitance. In other words, ions are more strongly adsorbed on the surface of electrode and this might interfere with the occurrence of specific reactions. With the presence of solvated K3[Fe(CN)6] (dash lines), the redox couple appears as electrons transfer at the interface of electrode and electrolyte. For an ideal electrode, the potential difference between the oxidation peak and reduction peak is 59 mV [13]. The measured potential difference is usually larger than that. Also, the higher the peak current is, the more the transferred electrons, implying the better electrochemical activity. In Fig. 2 (a), MPAC shows extremely low current performance in sulfuric solution compared to that in solution with solvated K3[Fe (CN)6], and the potential difference is close to 100 mV. There are no other reaction peaks present in both electrolytes. The results shown in Fig. 2 suggest that MPAC is a good electrode material for electrochemical applications. In contrast, for Vulcan XC-72R (Fig. 2 (b)), though the total current performance is higher than that of MPAC, its capacitance effect is significant. Besides, the potential difference is smaller than 59 mV, due to the adsorption effect [13], i.e., the current contribution of redox
Table 2 Pt content measured by ICP-AES
Designated ratio MPAC MPAC-O2 MPAC-O3 MPAC-HNO3 MPAC-fun XC-O3 XC-HNO3 XC-fun Fig. 2. Cyclic voltammetry analyses of (a) MPAC and (b) Vulcan XC-72R. Measurements were conducted in electrolytes of 1 M H2SO4 (solid line) and 1 M H2SO4 plus 10 mM K3(Fe(CN)6) (dash line) with scan rate of 10 mV/s.
a
Treatment
Pt wt.%
Efficiency (%)
None 10% O2 at 350 °C for 5 h Ozone oxidationa HNO3 at 80 °C for 4 h Activation and functionalizationb Ozone oxidationa HNO3 at 80 °C for 4 h Activation and functionalizationb
18.7 0.03 0.74 ± 0.04 0.82 ± 0.04 2.54 ± 0.05 10.82 ± 0.2 1.35 ± 0.02 2.11 ± 0.04 2.84 ± 0.05
0.16 3.96 4.38 13.58 57.86 7.22 11.28 15.19
Flowing O3 of 40 mg/h in ambience for 5 days. Heat treatment at 350 °C for 5 h in 10% O2 in Ar flow, followed by immersion in NaClO. b
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an acid-treated carbon black. In this study, several funtionalization processes were performed on MPAC and Vulcan XC-72R. The Pt loading amount on these treated carbon blacks is shown in Table 2. For Vulcan XC-72R, the Pt loading amounts are about 2 wt.%, similar to those reported by Yasuda et al. [6]. On the other hand, the Pt loading amount in MPAC after functionalization is over 10 wt.%, a basic loading amount reported for use in fuel cells [17]. Though acid treatments are conventionally used to form a large number of carboxylic acid groups, the surface area slightly decreased according to the BET measurements. The oxygen activation, however, could only form weak acid functional groups, but the surface area increased in contrast. This pre-treatment makes the MPAC more hydrophilic and therefore the following NaClO functionalization dramatically increased the number of functional groups, even stronger than nitric acid treatment [4]. The combination of oxygen activation and NaClO functionalization hence substantially increased the Pt loading amount. Subpanels (a) and (b) of Fig. 4 are SEM and TEM, respectively, images of Pt-loaded MPAC with 10 wt.% Pt. It is quite clear that PtNPs (averaged at 2 nm) are uniformly distributed. This demonstrates that the functional groups on the surface of carbon blacks effectively enhance the dispersion and also suppress the coagulation of PtNPs. 4. Conclusion
Fig. 4. Electron microscope images of Pt/MPAC. (a) SEM. (b) TEM.
couple comes from reactants adsorbed on the electrode surface rather than fresh reactants continuously diffusing from electrolyte. This property is unwanted for an electrode material. Furthermore, in both electrolytes, the rising part at high potential represents the formation of oxygen, and the arrow-marked peak symbolizes the reduction of formed oxygen [14]. In K3[Fe(CN)6] electrolyte, there is another small redox couple observed at ~ 0.1 V. These phenomena might be caused by the functional groups or impurities in Vulcan XC-72R. Fig. 3 is the CV curves of MPAC in sulfuric solution with solvated K3 [Fe(CN)6] measured at three different scan rates, and lower inset shows how the peak current was determined. The peak current and square root of scan rate (upper inset), show a linear relation with a correlation coefficient of 0.9944, suggesting that the electrochemical reaction is diffusion-controlled [15,16]. In other words, if the uniformity of electrolyte can be improved, i.e., the lower the concentration gradient during reaction, the performance could even be enhanced further. 3.3. Ion-exchange Yasuda et al. [6] reported that the Pt loading using the ionexchange method is dependent on the amount of carboxylic acid on the surface of carbon blacks and 37.4 wt.% Pt loading was obtained on
With polymer blend method, the properties of carbon blacks were optimized by controlling the concentration of blended chemicals and carbonization parameters. As corroborated by BET and CV measurements, a mesoporous active carbon with larger surface area (441 m2/g), low capacitance effect, and electrochemical activity has been prepared successfully. In contrast, though commercial carbon black, Vulcan XC-72R, shows electrochemically active, the large capacitance and adsorption make it an inferior electrode material. Highly dispersed PtNPs on carbon supports were prepared by ionexchange method, but the loading amount strongly depended on the functionalization treatments. After oxygen oxidation at 350 °C followed by NaClO functionalization, a Pt loading amount of 10 wt.% and exchange efficiency of 59% have been obtained in MPAC. References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
J.R. Anderson, Structure of Metallic Catalysts, Academic Press, London, 1975. C.N. Satterfield, Heterogeneous Catalysts in Practice, McGraw-Hill, New York, 1980. C.A. Frysz, D.D.L. Chung, Carbon vol. 35 (1997) 1111. D. Richard, P. Gallezot, in: B. Delmon, P. Grange, P.A. Jacobs, G. Poncelet (Eds.), Preparation of Catalysts 4, vol. 31, Stud. Surf. Sci. Catal, 1987, pp. 71–79, (Preparation of Highly Dispersed Carbon Supported Platinum Catalysts). K. Amine, K. Yasuda, H. Takenaka, Ann. Chim. Sci. Mat. vol. 23 (1998) 331. K. Yasuda, Y. Nishimura, Mater. Chem. Phys. vol. 82 (2003) 921. J. Ozaki, N. Endo, W. Ohizumi, K. Igarashi, M. Nakahara, A. Oya, Carbon vol. 35 (1997) 1031. T. Vergunst, F. Kapteijn, J.A. Moulijn, Appl. Catal. A vol. 213 (2001) 179. D.B. Mawhinney, J.T. Yates Jr., Carbon vol. 39 (2001) 1167. T. Ftrlink, W. Visscher, J.A.R. van Veen, J. electroanalytical Chem. vol. 382 (1995) 65. M. Umeda, M. Kokubo, M. Mohamedi, I. Uchida, Electrochimica vol. 48 (2003) 4867. J.W. Patrick, Porosity in Carbons: Characterization and Applications, 1995. P.H. Rieger, Electrochemistry, Chapman and Hall, New York, 1994, pp. 183–194. C.H. Hamann, A. Hamnett, W. Vielstich, Electrochemistry, Wiley-VCH, Weinheim, 1998. H. Luo, Z. Shi, N. Li, Z. Gu, Q. Zhuang, Anal. Chem. vol. 73 (2001) 915. C.G. Hu, W.L. Wang, S.X. Wang, W. Zhu, Y. Li, Diamond and Related Material vol. 12 (2003) 1295. E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A vol. 173 (1998) 259.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Mesoporous active carbon dispersed with ultra-fine platinum nanoparticles and their electrochemical properties J.Y. Hwang a, A. Chatterjee b, C.H. Shen b, J.H. Wang c, C.L. Sun d, Oliver Chyan e, C.W. Chen a, K.H. Chen b,d, L.C. Chen b,⁎ a
Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan Department of Chemistry, National Taiwan University, Taipei, Taiwan d Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan e Department of Chemistry, University of North Texas, Texas, USA b c
a r t i c l e
i n f o
Available online 31 October 2008 Keywords: Mesoporous carbon Cyclic voltammetry (CV) Platinum carbon catalyst Ion exchange
a b s t r a c t Active carbons were prepared by polymer blend with various chemical concentrations and carbonization conditions. The resultant porosity and electrochemical properties were studied by nitrogen adsorption– desorption isotherm and cyclic voltammetry techniques, and compared with those of commercial carbon black, Vulcan XC-72R. The polymer-derived active carbons exhibit a mesoporous structure, large surface area, low capacitance, and high electrochemical activity, thus are suitable for catalyst support. By ion-exchange method with appropriate functionalization treatments (three different ways were conducted in this paper) on these carbon supports, we have achieved a uniform dispersion of ultra-fine Pt nanoparticles (~2 nm) with a total loading amount up to 10 wt.% and an exchange efficiency of 60%, much higher than the values of Vulcan XC-72R. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Highly dispersed platinum (Pt) on a conductive material is commonly used as catalyst for electrochemical reactions, such as those occurred in fuel cell systems. In these applications, Pt nanoparticles (NPs) with a diameter of 2–3 nm are desirable for optimal catalytic performance and cost effectiveness, while active carbons (ACs) with low density and large surface area are widely used as catalyst supports [1,2]. Conventional ACs are used as gas adsorbents or filters due to their extremely large surface area (N1000 m2/g) and microporous structure (b2 nm). However, in the case of electrochemical catalyst support, mesoporous structure (2 to 50 nm) is preferred since micropores are too small to be efficient active sites for catalyst particles. Meanwhile, the ability of electron transfer, which can be characterized by conductivity measurement, is also important. The cyclic voltammetry (CV) technique has been used for characterization and estimation of material properties, including capacitance effect and electrochemical activity, for electrochemical applications. Capacitance effect relates to the tendency of charge accumulation and chemical adsorption on the surface of material, whereas electrochemical activity suggests the ability of transporting electrons not only within the electrode material, but also between the electrode and electrolyte. As a catalyst support, the capacitance should be low while the electrochemical activity should be high in order to have fast response [3]. ⁎ Corresponding author. Tel.: +886 2 33665249; fax: +886 2 23655404. E-mail address: [email protected] (L.C. Chen). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.014
For catalyst (Pt) loading, ion-exchange method was reported as an effective technique to obtain highly dispersed platinum carbon catalyst (Pt/C) [4]. In contrast, the conventional impregnation method often results in significant coagulation of Pt on the surface [5]. It is suggested that the functional groups on the surface of carbon support could enhance Pt dispersion [6], and also affect the loading amount of Pt. In this paper, mesoporous active carbons (MPAC) have been successfully synthesized with a polymer blend method [7]. It is found that the resultant properties of ACs depend on the composition of blended polymers and carbonization conditions. Vulcan XC-72R, a commercial carbon black popularly used in fuel cell systems, is taken as a reference sample. Three different functionalization treatments, namely, ozone treatment, nitric acid immersion and partial oxidation followed by sodium hypochlorite immersion, were conducted on our MPAC and Vulcan XC-72R, followed by ion-exchange reaction for Pt loading. Morphology and microstructure studies using SEM and TEM suggested that highly dispersed Pt NPs (b5 nm) can be achieved on properly functionalization-treated MPAC. A loading amount up to 10 wt.% and a reaction efficiency approaching 60% were obtained. 2. Experimental 2.1. Preparation of active carbon Three organic polymers [7,8]: Furfuryl alcohol (FA, 98% ACROS OGANICS), Pyrrole (99% ACROS OGANICS), and Poly ethylene glycol (PEG) methylether (average molecular weight of 350, ACROS
304
J.Y. Hwang et al. / Diamond & Related Materials 18 (2009) 303–306
OGANICS) were used as the source materials for polymer blend. After mixing the source materials, nitric acid (as polymerization catalyst) was added gradually during 30 min while stirring and cooling continued until solidification. Various chemical concentrations and carbonization conditions were experimented (Table 1). After 30 min of argon purging, the solidified mixtures were heated in argon flow (50– 300 sccm) at a rate of 10 °C per min to a designated temperature (550– 850 °C) and held for 1 h. Finally, the carbonized sample was ball milled to powder. 2.2. Functionalization of carbon supports Three ways to functionalize the surface of carbon supports were used: (1) ozone treatment, under flowing O3 ambient at 40 mg/h for 5 days [9]; (2) nitric acid immersion, using concentrated HNO3 (65%) at 60 °C for 4 h [5,6,10]; (3) two-step functionalization, i.e., partial oxidation using 10% O2 in Ar at 350 °C for 5 h followed by immersing in sodium hypochlorite (NaClO, 13% ACROS OGANICS) at room temperature for 30 min [4,8]. These functionalization processes were experimented using MPAC, a subset of carbon supports prepared with specific conditions shown in Table 1. For comparison, identical functionalization experiments were also performed on Vulcan XC72R. 2.3. Ion-exchange process Pt(NH3)Cl2 (Aldrich, 98%) was used as precursor. The ratio of Pt to carbon supports was controlled at 18.7 wt.%, typically, by immersing 0.5 g carbon supports into 20 ml distilled water with pre-dissolved 0.2 g Pt(NH3)Cl2. The reactions were conducted at room temperature for 1 h, and the Pt/C catalysts were centrifuged, washed with water, and dried at 60 °C for 24 h. Lastly, the dried Pt/C samples were reduced in 10% H2 in Ar flow at 180 °C for 4 h. 2.4. Analyses of surface area and pore distribution Nitrogen adsorption–desorption isotherms were measured at −196 °C with Micromeritics ASAP 2010 instrument. The surface areas were calculated according to the BET method (P/P0 = 0.05–0.30) derived by Brunauer, Emmett, and Teller, and the total pore volume was calculated at P/P0 = 0.91, where P/P0 means the relative pressure, and P0 is atmosphere pressure. 2.5. Demonstration of electrochemical properties A specific electrode was prepared by using a gold wire with a 1 mm diameter as a current collector. The Au wire was polished and inserted Table 1 Chemical composition, carbonization temperature and gas flow rate of preparing active carbon
AC-01 AC-03 AC-07 AC-08-50 AC-08-100 AC-08-200 AC-08-300 AC-09 MPAC AC-11-550C AC-11-650C AC-11-750C AC-11-850C Vulcan XC-72R a
FA (ml)
PEG (ml)
Pyrrole (ml)
Carbonization temp. (°C)
Gas flow rate (sccm)
Surface area (m2/g)
6 6 6 6
0 2 4 4
0 0 0 1
4 2 6
4 4 4
1 1 2
650 650 650 650 650 650 650 650 650 550 650 750 850
100 100 100 50 100 200 300 100 100 100 100 100 100
109 264 231 250 291 278 224 352 441 267 314 207 103 225 (257a)
Data from Cabot.
Fig. 1. Adsorption–desorption isotherm and pore distribution of (a) MPAC and (b) Vulcan XC-72R.
into a Teflon plate and sealed by silicone [11]. Carbons were filled in the cavity with 1 mm diameter and 0.5 mm depth. Cyclic voltammetric curves were measured with μAutolab Potentialstat/Golvanostat Type III instrument in two kinds of electrolytes: 1 M H2SO4, to assess the capacitance effect and 1 M H2SO4 with 10 mM K3[Fe(CN)6], to measure the electrochemical activity. An Ag/AgCl electrode (Argenthal, 3 M KCl, 0.207 V versus standard hydrogen electrode (SHE) at 25 °C) was used as reference electrode. 2.6. Characterization of Pt/C The morphology was characterized by high-resolution fieldemission scanning electron microscope (FESEM, JEOL-FESEM6700) and transmission electron microscope (TEM, Philips Tecnai 20). For measuring Pt loading amount, carbon support was combusted and then the residual was dissolved in aqua regia. The Pt concentration in diluted aqua regia solution was analyzed by inductively-coupledplasma atomic-emission spectrometry (ICP-AES). 3. Results and discussion 3.1. Surface area and pore structure Table 1 shows the results of surface areas of Vulcan XC-72R and ACs prepared by polymer blend method. Within the range of process parameters, both the carbonization temperature and the polymer
J.Y. Hwang et al. / Diamond & Related Materials 18 (2009) 303–306
blend composition showed more pronounced effects than the gas flow rate on the surface area of ACs. The largest surface area was obtained at a gas flow rate of 100 sccm and a carbonization temperature of 650 °C. Meanwhile, Raman analyses showed that the ratio of the intensity of G-band and D-band increased further with the carbonization temperature, indicating better structural order and possible enhancement of conductivity. However, the surface area decreased dramatically above a carbonization temperature of 650 °C, at which hydrogen atoms were gradually eliminated from the network of carbon polymeric chains and the electrical conductivity increased rapidly as the polymeric chains became an interconnected conducting network. Above that temperature, the porous properties became poor though the conductivity improved. Therefore, unless indicated otherwise, a gas flow rate of 100 sccm and a carbonization temperature of 650 °C were applied. The composition of blended polymers also showed a strong effect on the resultant surface area. When the composition of FA:PEG:Pyrrole was kept at 2:4:1, the resultant carbon exhibited a mesoporous structure and a surface area of 441 m2/g, nearly twice the corresponding value of Vulcan XC-72R (225 m2/g). For convenience, the term mesoporous active carbon (MPAC) refers to the carbon material prepared under this particular condition hereafter. Fig. 1 is the nitrogen adsorption–desorption isotherms and pore size distribution (inset) of MPAC and Vulcan XC-72R. The isotherm
305
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Fig. 3. Voltammetric curves measured at (□) 50 mV/s, ( ) 30 mV/s and (—) 10 mV/s in electrolyte of 1 M H2SO4 plus 10 mM K3(Fe(CN)6). Upper inset graph shows the relation of oxidation peak current and square root of scan rate. Lower inset graph presents the definition of peak current.
shape of MPAC can be categorized as type 4, which possesses a hysteresis loop [12], usually associated with an adsorption process of mesoporous solids wherein capillary condensation has occurred. In contrast, the isotherm shape of Vulcan XC-72R displays type 2, characteristic of physical adsorption of gases by non-porous solids. 3.2. Electrochemical properties The CV curves of MPAC and Vulcan XC-72R are shown in Fig. 2 (a) and (b). In only sulfuric solution (solid lines), the larger area enclosed indicates the larger capacitance. In other words, ions are more strongly adsorbed on the surface of electrode and this might interfere with the occurrence of specific reactions. With the presence of solvated K3[Fe(CN)6] (dash lines), the redox couple appears as electrons transfer at the interface of electrode and electrolyte. For an ideal electrode, the potential difference between the oxidation peak and reduction peak is 59 mV [13]. The measured potential difference is usually larger than that. Also, the higher the peak current is, the more the transferred electrons, implying the better electrochemical activity. In Fig. 2 (a), MPAC shows extremely low current performance in sulfuric solution compared to that in solution with solvated K3[Fe (CN)6], and the potential difference is close to 100 mV. There are no other reaction peaks present in both electrolytes. The results shown in Fig. 2 suggest that MPAC is a good electrode material for electrochemical applications. In contrast, for Vulcan XC-72R (Fig. 2 (b)), though the total current performance is higher than that of MPAC, its capacitance effect is significant. Besides, the potential difference is smaller than 59 mV, due to the adsorption effect [13], i.e., the current contribution of redox
Table 2 Pt content measured by ICP-AES
Designated ratio MPAC MPAC-O2 MPAC-O3 MPAC-HNO3 MPAC-fun XC-O3 XC-HNO3 XC-fun Fig. 2. Cyclic voltammetry analyses of (a) MPAC and (b) Vulcan XC-72R. Measurements were conducted in electrolytes of 1 M H2SO4 (solid line) and 1 M H2SO4 plus 10 mM K3(Fe(CN)6) (dash line) with scan rate of 10 mV/s.
a
Treatment
Pt wt.%
Efficiency (%)
None 10% O2 at 350 °C for 5 h Ozone oxidationa HNO3 at 80 °C for 4 h Activation and functionalizationb Ozone oxidationa HNO3 at 80 °C for 4 h Activation and functionalizationb
18.7 0.03 0.74 ± 0.04 0.82 ± 0.04 2.54 ± 0.05 10.82 ± 0.2 1.35 ± 0.02 2.11 ± 0.04 2.84 ± 0.05
0.16 3.96 4.38 13.58 57.86 7.22 11.28 15.19
Flowing O3 of 40 mg/h in ambience for 5 days. Heat treatment at 350 °C for 5 h in 10% O2 in Ar flow, followed by immersion in NaClO. b
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an acid-treated carbon black. In this study, several funtionalization processes were performed on MPAC and Vulcan XC-72R. The Pt loading amount on these treated carbon blacks is shown in Table 2. For Vulcan XC-72R, the Pt loading amounts are about 2 wt.%, similar to those reported by Yasuda et al. [6]. On the other hand, the Pt loading amount in MPAC after functionalization is over 10 wt.%, a basic loading amount reported for use in fuel cells [17]. Though acid treatments are conventionally used to form a large number of carboxylic acid groups, the surface area slightly decreased according to the BET measurements. The oxygen activation, however, could only form weak acid functional groups, but the surface area increased in contrast. This pre-treatment makes the MPAC more hydrophilic and therefore the following NaClO functionalization dramatically increased the number of functional groups, even stronger than nitric acid treatment [4]. The combination of oxygen activation and NaClO functionalization hence substantially increased the Pt loading amount. Subpanels (a) and (b) of Fig. 4 are SEM and TEM, respectively, images of Pt-loaded MPAC with 10 wt.% Pt. It is quite clear that PtNPs (averaged at 2 nm) are uniformly distributed. This demonstrates that the functional groups on the surface of carbon blacks effectively enhance the dispersion and also suppress the coagulation of PtNPs. 4. Conclusion
Fig. 4. Electron microscope images of Pt/MPAC. (a) SEM. (b) TEM.
couple comes from reactants adsorbed on the electrode surface rather than fresh reactants continuously diffusing from electrolyte. This property is unwanted for an electrode material. Furthermore, in both electrolytes, the rising part at high potential represents the formation of oxygen, and the arrow-marked peak symbolizes the reduction of formed oxygen [14]. In K3[Fe(CN)6] electrolyte, there is another small redox couple observed at ~ 0.1 V. These phenomena might be caused by the functional groups or impurities in Vulcan XC-72R. Fig. 3 is the CV curves of MPAC in sulfuric solution with solvated K3 [Fe(CN)6] measured at three different scan rates, and lower inset shows how the peak current was determined. The peak current and square root of scan rate (upper inset), show a linear relation with a correlation coefficient of 0.9944, suggesting that the electrochemical reaction is diffusion-controlled [15,16]. In other words, if the uniformity of electrolyte can be improved, i.e., the lower the concentration gradient during reaction, the performance could even be enhanced further. 3.3. Ion-exchange Yasuda et al. [6] reported that the Pt loading using the ionexchange method is dependent on the amount of carboxylic acid on the surface of carbon blacks and 37.4 wt.% Pt loading was obtained on
With polymer blend method, the properties of carbon blacks were optimized by controlling the concentration of blended chemicals and carbonization parameters. As corroborated by BET and CV measurements, a mesoporous active carbon with larger surface area (441 m2/g), low capacitance effect, and electrochemical activity has been prepared successfully. In contrast, though commercial carbon black, Vulcan XC-72R, shows electrochemically active, the large capacitance and adsorption make it an inferior electrode material. Highly dispersed PtNPs on carbon supports were prepared by ionexchange method, but the loading amount strongly depended on the functionalization treatments. After oxygen oxidation at 350 °C followed by NaClO functionalization, a Pt loading amount of 10 wt.% and exchange efficiency of 59% have been obtained in MPAC. References [1] [2] [3] [4]
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
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