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Pulse electrodeposition of Pt nanocatalysts on graphene-based electrodes for proton exchange membrane fuel cells
Catalysis Communications 16 (2011) 220–224
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Pulse electrodeposition of Pt nanocatalysts on graphene-based electrodes for proton exchange membrane fuel cells Chien-Te Hsieh ⁎, Jyun-Ming Wei, Jiun-Shang Lin, Wei-Yu Chen Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 31 August 2011 Accepted 12 September 2011 Available online 2 October 2011 Keywords: Pt catalysts Pulse electrodeposition Graphene oxide Carbon paper Fuel cells
a b s t r a c t Pulse electrodeposition of Pt nanocluster catalysts on graphene oxide (GO)-carbon paper composite electrode for proton exchange membrane fuel cells has been investigated. The primary particle size was found to be 100–300 nm for the urchin-like cluster that was composed of a number of secondary grains. Experimental results show that the GO sheets decorated with the Pt clusters displays low-equivalent series resistance (~2.60 Ω) and high-power density (~1.3 kW g− 1 at 60 °C). Hence, such a catalyst electrode, consisting of the GO sheets and the metallic Pt clusters, is proved to have several advantages such as good dispersion of catalysts and high catalytic activity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted a lot of attention and have been extensively investigated due to their potential applications as a clean and environment-friendly power sources [1–4]. So far, supported Pt or Pt-based alloys have widely served as electrocatalysts because of their hydrogen oxidation and oxygen reduction capabilities [5–7]. To meet the increasing demand for high-performance PEMFCs, much effort has been devoted to exploring new designs of the catalyst electrodes. Recently, one singleatom-thick sheet of carbon lattice, namely graphene, has become popular due to its unique two-dimensional sheets of sp 2 hybridized carbon [8–10]. Basically, its extended honeycomb network is the basic building block of other crucial allotropes: it can be stacked to form 3-D graphite, rolled to form 1-D nanotubes, and wrapped to form 0-D fullerenes. The recent emergence of graphene has opened a new avenue for utilizing 2-D carbon materials as support in PEMFCs [11]. A number of advanced preparations such as pyrolysis of hybrid inorganic–organic precursor [12,13] and electroless deposition [6] for Pt-based electrocatalysts have been investigated. However, typical fabrication schemes for carbon-based PEMFC electrodes still involve the preparation of Pt nanoparticles on carbons via a chemical route, followed by a physical transfer (such as ink jet process) to a carbon fabric or a carbon paper (CP) [14]. This study proposes an alternative approach where graphene is deposited directly on CP, followed by
⁎ Corresponding author. Tel.: + 886 3 4638800x2577; fax: + 886 3 4559373. E-mail address: [email protected] (C.-T. Hsieh). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.09.030
Pt electrodeposition on the graphene-CP electrode. Besides being an efficient process, the electrodeposition method has a few advantages such as the high purity of the deposits [15–17]. This design of grapheneCP electrode is considered a promising support for heterogeneous catalysts if the Pt is well distributed throughout the graphene-CP supports. Previous studies have reported that the Pt nanoparticles are deposited on graphene sheets by chemical reduction [8,11,18]. However, to the authors' knowledge, there are few reports that examine (i) the activity of the electrodeposited Pt catalysts on the grapheneCP electrode, and (ii) the performance of a single cell, fabricated with the Pt-graphene electrodes. In this paper, a pulsed electrodeposition (PED) method was employed to deposit highly active Pt nanoclusters on graphene oxide (GO) sheets from an aqueous electrolyte. To inspect the effect of GO nanosheets, the Pt deposit on the CP substrate is also prepared to compare the catalytic activity and the cell performance. This result sheds some light on how the Pt catalyst, through the PED route, induces a high-power density and a commercial feasibility for PEMFC applications.
2. Experimental 2.1. Fabrication of GO-CP composites Natural graphite was oxidized to GO using the Hummer' method [19]. The GO powder is stored and dispersed in an aqueous solvent as needed. A filtration method was applied to coat the derived GO sheets over CP substrate (Toray Composites Inc.). The CP substrate was composed of irregular carbon fibers of 8–10 μm diameter. The
C.-T. Hsieh et al. / Catalysis Communications 16 (2011) 220–224
GO sheets were deposited over the CP filter after 24-h sedimentation. The GO-CP cake was then dried in an oven at 105 °C overnight. 2.2. PED synthesis of Pt catalysts A three-electrode configuration system was used for the deposition of Pt nanoparticles on the GO-CP composite. The Pt particles are electrochemically deposited on the carbon composite from the 1.0 mM PtCl4 + 1 M HCl solution. In the system, the working electrodes were constructed by pressing the GO-CP composites onto a stainless steel foil, which serves as the current collector. Pure Pt foil and saturated calomel electrode (SCE) electrodes served as counter and reference electrodes, respectively. The PED was carried out at a potential of −0.8 V vs. SCE. The deposition and rest periods were set at 0.5 s and 5 s, respectively. The cyclic count for the PE method was set at 125 cycles. Thereafter, the Pt-coated GO-CP samples were separated from the ionic solution and subsequently dried in a vacuum oven at 105 °C overnight. In comparison, the PED was also performed to deposit Pt on the fresh CP substrate. The Pt catalysts deposited over GO-CP and fresh CP were designated as Pt-GO and Pt-CP samples, respectively. 2.3. Characterization of Pt catalysts The microstructure of the Pt-GO composite was characterized by a field-emission scanning electron spectroscope (FE-SEM, JEOL JSM5600) and a high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2100). A thermogravimetric analyzer (TGA, Perkin Elmer TA7) was adopted to analyze the amount of catalysts deposited on the GO-CP composites. The Pt-loaded carbon composite was carefully cut into an area of 1 cm 2 and then placed in the TGA. The sample was heated to 1100 °C at a heating rate of 10 °C min − 1 in air atmosphere. To ensure the accuracy the Pt loading, the TGA analysis was carried out for each sample twice. The crystalline structure of the Pt catalysts was investigated by grazing incident X-ray diffraction (XRD) with Cu-Kα radiation, using an automated X-ray diffractometer
221
(Rigaku, D/MAX 2500). X-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique, and this technique was applied to characterize the chemical composition of the carbon supports. The XP spectra were recorded with a Fison VG ESCA210 spectrometer and Mg-Kα radiation. Cyclic voltammetry (CV) measurement was carried out under N2 atmosphere at ambient temperature using 1 M H2SO4 as the electrolyte solution using an electrochemical analyzer (CH Instrument, Inc., CHI 608). Each catalyst electrode was carefully cut into an area of 2 × 1 cm 2. In this work, the Pt wire and the Ag/AgCl electrode served as the counter and the reference electrodes, respectively. The working electrodes were constructed by pressing the CNT/CP composites onto a stainless steel foil, which served as the current collector. The electrochemical analyzer was also used to measure AC electrochemical impedance spectra of Pt catalyst electrodes. In this work, the potential amplitude of ac was equal to 5 mV, and the frequency was from 100 kHz to 1 mHz.
2.4. Performance of a single cell fabricated with Pt-GO electrodes A single-cell test fixture was applied for evaluating the catalyst performance for a H2/O2 based fuel cell. A membrane electrode assembly (MEA), consisting of two Pt-GO electrodes, was immersed in 5 wt.% Nafion solution for 0.5 h. The MEA was thermally pressed (135 °C, 140 atm, 2 min) with a Nafion 212 membrane (DuPont Inc., USA) between the two electrodes. The two electrodes pressed on the two sides of each MEA were identical. The MEA was then inserted between two graphite plates, which had a serpentine flow pattern. Two Teflon gaskets of 0.24 mm thickness were introduced between the membrane and the electrodes. Humidified hydrogen and oxygen gases were fed into the cell at a flow rate of 200 cm3 min− 1 and 400 cm3 min− 1, respectively. The back pressure of the humidified gases was kept at 4 atm. The temperature of the single cell maintained at 60 °C, and the polarization curve and the powder density of the cell were then characterized.
c
a 500 nm
b
d
Fig. 1. FE-SEM images of (a) CP substrate attached with GO nanosheets, (b) Pt-CP, and Pt-GO composites with (c) low (×10,000) and (d) high (×50,000) magnification. The inset is HR-TEM micrograph of GO sheets.
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3. Results and discussion 3.1. Characterization of Pt-GO catalysts Fig. 1(a) shows FE-SEM of the GO-CP composite after gravity sedimentation. As-derived GO nanosheets, prepared by the chemical oxidation of natural graphite powders, were smoothly covered over microscaled carbon fibers. The inset reveals the HRTEM image of fresh GO sheets, showing the multilayers with an area of several micrometers. This suggests the presence of an extensive conjugated hexagonal network of sp2 carbons in graphene [8]. Isolated small fragments of GO sheets and folded wrinkle-like features are observed on the surface of CP substrate. XPS analysis was applied for analysis of the surface chemical composition of GO nanosheets, indicating a high oxidation level, i.e., O/C atomic ratio: 31.5%. This means that the GO sheets consist of many aromatic conjugated domains modified with functional oxide groups [20]. Such high oxidation level results in functionalization by epoxy and hydroxyl groups on the graphene surface (i.e., basal plane), in addition to carbonyl and carboxyl groups at the edges of the graphene sheets. For comparison, the XPS analysis indicated that the O/C ratio of the pristine CP is only 4.3%, much lower than that of the GO sheets. The FE-SEM images of the electrodeposited Pt particles on fresh CP and GO-CP are shown in Fig. 1(b) and (c), respectively. Pt nanoclusters with particle size ranging from 100 to 300 nm are observed on the surface of the carbon fibers and the GO sheets. A magnifying picture, as shown in Fig. 1(d), proves that the primary urchin-like particles are composed of smaller triangle-like substructures. Basically, there is no major difference in the Pt morphology between Pt-CP and Pt-GO samples, and well-dispersed Pt clusters on both carbon substrates are observed. The shape of the Pt crystals becomes a nanocluster during the PED, thereby increasing the surface area of the catalyst. HR-TEM was applied to observe a single Pt cluster on GO sheet, as represented in Fig. 2(a). It can be seen that the Pt cluster with a number of a nanotips is attached to the basal planes and the edges of graphene sheets. The size of the primary nanostructure is approximately 120–150 nm, and the secondary nanotips are found to protrude out of the cluster. Fig. 2(b) shows an atomic resolution HR-TEM image of Pt cluster, which displays the lattice fingers of the cluster. The lattice fingers with a spacing of 0.22 nm are apparently visible in the nanograins, in which the (111) crystal plane of the Pt face-centered cubic (fcc) crystals can be viewed. The urchin-like cluster possesses a porous nature, thereby offering a favorable surface area and an active center for electrocatalysis. Based on the observation, it is proposed that the shape of the Pt cluster is dominated by the presence of sulfuric acid anions [16]. This is ascribed to the fact that the anions tend to adsorb preferentially on certain facets of metals with fcc crystal structure, especially for the Pt (111) plane [21]. Because the selective adsorption takes place on a specific Pt surface plane, the growth of the Pt deposits on some crystal planes would be distributed or inhibited, thus, leading to the anisotropic growth of the Pt cluster. Grazing incident XRD analysis was used to characterize Pt nanoparticles on carbon substrates, as shown in Fig. 3. Three well-defined peaks around 39.8, 46.2, and 67.5°, respectively, correspond to the diffraction peaks of crystal faces Pt (111), (200), and (220), which represent the typical character of the platinum fcc structure. Both Pt catalysts show stronger (111) peaks than the other two peaks. The calculated results reflect that the (111)/(220) intensity ratios of PtGO and Pt-CP are 6.43 and 4.09, respectively, whereas those of the (200)/(220) are 2.67 and 1.63, respectively. In comparison, the ratios of Pt-GO are larger than those of Pt-CP, indicating that the presence of GO alters the growth rate of Pt crystal in both crystallographic directions. This fast growth rate can be attributed to the fact that the strong interfacial interaction between the deposit species and the highly oxidized graphite plane favors the formation of nucleation sites with a lower energy barrier during the PED process [22].
a
50 nm
b [111]
0.22 nm
2 nm Fig. 2. HR-TEM micrographs of electrodeposited Pt nanocluster on GO sheet with (a) low and (b) high magnification, showing lattice fingers in Pt (111) plane.
3.2. Electrochemical surface area and cell performance Representative cyclic voltammograms (CVs) of Pt nanoclusters are recorded in 0.5 M H2SO4 electrolyte at 50 mV s − 1. Two potential regions are observed: (i) double-layer formation (0.2–0.4 V) and (ii) oxidation-reduction of Pt (0.4–0.8 V). The capacitive doublelayer behavior can be attributed to the presence of carbon substrate (i.e., CP and GO), which originates from the electrochemical surface area for the formation of a double layer. The voltammograms of the two PED Pt catalysts display the characteristics of a clean polycrystalline Pt [15]. One pair of reversible hydrogen adsorption–desorption peak is clearly identified in the potential region from +0.2 to −0.2 V vs. Ag/AgCl, i.e., Pt + H + + e − ↔ Pt–Hads. For a polycrystalline Pt electrode, the charge transfer associated with the above reaction is 210 μC cm − 2. The electrochemically active surface areas (ESA) of Pt-CP and Pt-GO are 42.1 and 94.9 m 2 g − 1, respectively, based on the normalized Pt loading on carbon substrate. The ESA value of Pt-GO is 1.83 times that of Pt-CP, indicating an improved catalytic activity of Pt on basal graphene sheets. To clarify inner resistance and catalytic activity, an AC impedance spectroscopy was used to characterize the catalyst electrodes. The equivalent serial resistance (Rt) for both catalyst electrodes can be estimated, i.e., Pt-CP (3.19 Ω) and Pt-GO (2.60 Ω). It is worth mentioning that there are no obvious increases of Rt value after 200 CV cycles, keeping low inner resistance and catalyst stability. This proves that the Pt deposits onto GO sheets still maintain electrochemically active area, inducing the excellent durability. The polarization curve of a single cell, fabricated with the Pt-GO electrodes, at 60 °C with H2/O2 is shown in Fig. 4(b), and is compared
C.-T. Hsieh et al. / Catalysis Communications 16 (2011) 220–224
(004)
(220)
(311) (311)
(100) (200)
(111)
(004)
(220)
(111) (100) (200)
Intensity (arbunit)
Pt
a 30
with the Pt-CP electrode. The Pt loadings on Pt-GO and Pt-CP are 0.26 and 0.32 mg cm − 2, respectively. The curves show that the Pt-GO cell displays a power density of 1.3 kW g − 1 which is much higher than the Pt-CP cell, i.e., 0.4 kW g − 1. The enhancement of power density is attributed to the following reasons: (i) the better electric conductivity of graphene sheets (1250 S m − 1) [8], as well as the specific interaction between Pt cluster and graphene, leads to the high electrocatalytic activity of Pt; (ii) the Pt nanoclusters possess many tips (e.g., Pt (111) crystalline plane), providing more accessible surface areas; (iii) the existence of graphene offers not only a smooth pathway for charge transfer but also a 2-D plane for good contact between the current collector and the proton exchange membrane (Nafion 117 in this study), rendering a better Pt utilization. Additionally, the Pt catalysts were electrochemically deposited onto twodimensional GO sheets, so that the isolation of carbons by Nafion can be prevented from the CP support. Thus, the Nafion layer tends to uniformly coat over the Pt clusters, inducing a continuous layer. Accordingly, the dispersion of the Pt catalysts on graphene has been found to be useful for achieving relatively better performance in fuel cell application.
C
b
40
50
60
70
223
80
2 theta (degrees) Fig. 3. Grazing incident XRD patterns of (a) Pt-CP and (c) Pt-GO samples.
4. Conclusions
8
a
Using an efficient PED method, porous Pt nanocluster catalysts with a high surface area were electrochemically deposited onto the GO-CP composite electrode. The primary particle size was found to be 100–300 nm for the urchin-like cluster that comprises trianglelike secondary grains. The electrochemical behavior of the Pt clusters onto the GO sheets has been well investigated using CV, AC impedance, and single-cell test. This study demonstrated that the presence of the GO sheets has a crucial role in improving the catalytic activity and the inner resistance of the Pt catalyst electrode. Such a catalyst electrode, consisting of GO sheets and metallic Pt nanoclusters, has advantages such as good dispersion of catalysts, low inner resistance, and high catalytic activity. The incorporation of the Pt-Go catalyst electrode by the EPD technique from a single-cell test has been satisfactory, with a promising potential for the novel MEA design in PEMFC application.
Current (A.g-1)
4
0
-4
-8
-12
Pt-GO Pt-CP
-16
Acknowledgments
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1 The authors are very grateful for the financial support from the National Science Council of the Republic of China under the contracts NSC 100-2120-M-155-001 and NSC 100-2221-E-155-031.
Potential (V vs. Ag/AgCl) 1.6
1
b Appendix A. Supplementary material
Potential (V)
1.2 0.6 0.8 0.4 0.4
Power density (kW.g-1)
0.8
0.2 Pt-GO Pt-CP 0 0
200
400
600
800
0 1000
Current density (mA.cm-2) Fig. 4. (a) Cycle voltammograms of catalyst electrodes at 50 mV s− 1, and (b) polarization curves of single cells, fabricated with different catalyst electrodes, under H2/O2 flows at 60 °C.
Supplementary data to this article can be found online at doi:10. 1016/j.catcom.2011.09.030. References [1] Z. Peng, J. Wu, H. Yang, Chemistry of Materials 22 (2010) 1098. [2] F. Ye, T.T. Wang, J.J. Li, Y.L. Wang, J.L. Li, X.D. Wang, Journal of the Electrochemical Society 156 (2009) 981. [3] B. Moreno, J.R. Jurado, E. Chinarro, Catalysis Communications 11 (2009) 123. [4] Çiğdem Güldür, Silver Güneş, Catalysis Communications 12 (2011) 707. [5] V. Stamenkovic, N.M. Markovic, P.N. Ross, Journal of Electroanalytical Chemistry 500 (2001) 44. [6] R.R. Adzic, J. Zhang, M.B. Vumirovic, M. Shao, Topics in Catalysis 46 (2007) 249. [7] S.M.S. Kumar, N. Hidyatai, J.S. Herrero, S. Irusta, K. Scott, International Journal of Hydrogen Energy 36 (2011) 5433. [8] Y. Si, E.T. Samulski, Chemistry of Materials 20 (2008) 6792. [9] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Electrochimica Acta 55 (2010) 3909. [10] J. Yao, X. Shen, B. Wang, H. Liu, G. Wang, Electrochemistry Communications 11 (2009) 1849. [11] B. Seger, P.V. Kamat, Journal of Physical Chemistry C 113 (2009) 7990. [12] V. Di Noto, E. Negro, Electrochimica Acta 55 (2010) 7564. [13] V. Di Noto, E. Negro, Fuel Cells 10 (2010) 234.
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[14] C.C. Chien, K.T. Jeng, Materials Chemistry and Physics 99 (2006) 80. [15] X. Chen, N. Li, K. Eckhard, L. Stoica, W. Xia, J. Assmann, M. Muhler, W. Schuhmann, Electrochemistry Communications 9 (2007) 1348. [16] H. Zhang, W. Zhou, Y. Du, P. Yang, C. Wang, Electrochemistry Communications 12 (2010) 882. [17] L. Sasithorn, T. Nisit, International Journal of Hydrogen Energy 35 (2010) 10464. [18] Y. Li, L. Tang, J. Li, Electrochemistry Communications 11 (2009) 846.
[19] W.S. Hummers, R.E. Offeman, Journal of the American Chemical Society 80 (1958) 1339. [20] X. Zhou, X. Huang, X. Qi, S. Wu, C. Xue, F.Y.C. Boey, Q. Yan, P. Chen, H. Zhang, Journal of Physical Chemistry C 113 (2009) 10842. [21] A. Kolics, A. Wieckowski, The Journal of Physical Chemistry. B 105 (2001) 2588. [22] J.J. Li, F. Ye, L. Chen, T.T. Wang, J.J. Li, X.D. Wang, Journal of Power Sources 186 (2009) 320.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Pulse electrodeposition of Pt nanocatalysts on graphene-based electrodes for proton exchange membrane fuel cells Chien-Te Hsieh ⁎, Jyun-Ming Wei, Jiun-Shang Lin, Wei-Yu Chen Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 28 July 2011 Received in revised form 31 August 2011 Accepted 12 September 2011 Available online 2 October 2011 Keywords: Pt catalysts Pulse electrodeposition Graphene oxide Carbon paper Fuel cells
a b s t r a c t Pulse electrodeposition of Pt nanocluster catalysts on graphene oxide (GO)-carbon paper composite electrode for proton exchange membrane fuel cells has been investigated. The primary particle size was found to be 100–300 nm for the urchin-like cluster that was composed of a number of secondary grains. Experimental results show that the GO sheets decorated with the Pt clusters displays low-equivalent series resistance (~2.60 Ω) and high-power density (~1.3 kW g− 1 at 60 °C). Hence, such a catalyst electrode, consisting of the GO sheets and the metallic Pt clusters, is proved to have several advantages such as good dispersion of catalysts and high catalytic activity. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) have attracted a lot of attention and have been extensively investigated due to their potential applications as a clean and environment-friendly power sources [1–4]. So far, supported Pt or Pt-based alloys have widely served as electrocatalysts because of their hydrogen oxidation and oxygen reduction capabilities [5–7]. To meet the increasing demand for high-performance PEMFCs, much effort has been devoted to exploring new designs of the catalyst electrodes. Recently, one singleatom-thick sheet of carbon lattice, namely graphene, has become popular due to its unique two-dimensional sheets of sp 2 hybridized carbon [8–10]. Basically, its extended honeycomb network is the basic building block of other crucial allotropes: it can be stacked to form 3-D graphite, rolled to form 1-D nanotubes, and wrapped to form 0-D fullerenes. The recent emergence of graphene has opened a new avenue for utilizing 2-D carbon materials as support in PEMFCs [11]. A number of advanced preparations such as pyrolysis of hybrid inorganic–organic precursor [12,13] and electroless deposition [6] for Pt-based electrocatalysts have been investigated. However, typical fabrication schemes for carbon-based PEMFC electrodes still involve the preparation of Pt nanoparticles on carbons via a chemical route, followed by a physical transfer (such as ink jet process) to a carbon fabric or a carbon paper (CP) [14]. This study proposes an alternative approach where graphene is deposited directly on CP, followed by
⁎ Corresponding author. Tel.: + 886 3 4638800x2577; fax: + 886 3 4559373. E-mail address: [email protected] (C.-T. Hsieh). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.09.030
Pt electrodeposition on the graphene-CP electrode. Besides being an efficient process, the electrodeposition method has a few advantages such as the high purity of the deposits [15–17]. This design of grapheneCP electrode is considered a promising support for heterogeneous catalysts if the Pt is well distributed throughout the graphene-CP supports. Previous studies have reported that the Pt nanoparticles are deposited on graphene sheets by chemical reduction [8,11,18]. However, to the authors' knowledge, there are few reports that examine (i) the activity of the electrodeposited Pt catalysts on the grapheneCP electrode, and (ii) the performance of a single cell, fabricated with the Pt-graphene electrodes. In this paper, a pulsed electrodeposition (PED) method was employed to deposit highly active Pt nanoclusters on graphene oxide (GO) sheets from an aqueous electrolyte. To inspect the effect of GO nanosheets, the Pt deposit on the CP substrate is also prepared to compare the catalytic activity and the cell performance. This result sheds some light on how the Pt catalyst, through the PED route, induces a high-power density and a commercial feasibility for PEMFC applications.
2. Experimental 2.1. Fabrication of GO-CP composites Natural graphite was oxidized to GO using the Hummer' method [19]. The GO powder is stored and dispersed in an aqueous solvent as needed. A filtration method was applied to coat the derived GO sheets over CP substrate (Toray Composites Inc.). The CP substrate was composed of irregular carbon fibers of 8–10 μm diameter. The
C.-T. Hsieh et al. / Catalysis Communications 16 (2011) 220–224
GO sheets were deposited over the CP filter after 24-h sedimentation. The GO-CP cake was then dried in an oven at 105 °C overnight. 2.2. PED synthesis of Pt catalysts A three-electrode configuration system was used for the deposition of Pt nanoparticles on the GO-CP composite. The Pt particles are electrochemically deposited on the carbon composite from the 1.0 mM PtCl4 + 1 M HCl solution. In the system, the working electrodes were constructed by pressing the GO-CP composites onto a stainless steel foil, which serves as the current collector. Pure Pt foil and saturated calomel electrode (SCE) electrodes served as counter and reference electrodes, respectively. The PED was carried out at a potential of −0.8 V vs. SCE. The deposition and rest periods were set at 0.5 s and 5 s, respectively. The cyclic count for the PE method was set at 125 cycles. Thereafter, the Pt-coated GO-CP samples were separated from the ionic solution and subsequently dried in a vacuum oven at 105 °C overnight. In comparison, the PED was also performed to deposit Pt on the fresh CP substrate. The Pt catalysts deposited over GO-CP and fresh CP were designated as Pt-GO and Pt-CP samples, respectively. 2.3. Characterization of Pt catalysts The microstructure of the Pt-GO composite was characterized by a field-emission scanning electron spectroscope (FE-SEM, JEOL JSM5600) and a high-resolution transmission electron microscope (HRTEM, JEOL, JEM-2100). A thermogravimetric analyzer (TGA, Perkin Elmer TA7) was adopted to analyze the amount of catalysts deposited on the GO-CP composites. The Pt-loaded carbon composite was carefully cut into an area of 1 cm 2 and then placed in the TGA. The sample was heated to 1100 °C at a heating rate of 10 °C min − 1 in air atmosphere. To ensure the accuracy the Pt loading, the TGA analysis was carried out for each sample twice. The crystalline structure of the Pt catalysts was investigated by grazing incident X-ray diffraction (XRD) with Cu-Kα radiation, using an automated X-ray diffractometer
221
(Rigaku, D/MAX 2500). X-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique, and this technique was applied to characterize the chemical composition of the carbon supports. The XP spectra were recorded with a Fison VG ESCA210 spectrometer and Mg-Kα radiation. Cyclic voltammetry (CV) measurement was carried out under N2 atmosphere at ambient temperature using 1 M H2SO4 as the electrolyte solution using an electrochemical analyzer (CH Instrument, Inc., CHI 608). Each catalyst electrode was carefully cut into an area of 2 × 1 cm 2. In this work, the Pt wire and the Ag/AgCl electrode served as the counter and the reference electrodes, respectively. The working electrodes were constructed by pressing the CNT/CP composites onto a stainless steel foil, which served as the current collector. The electrochemical analyzer was also used to measure AC electrochemical impedance spectra of Pt catalyst electrodes. In this work, the potential amplitude of ac was equal to 5 mV, and the frequency was from 100 kHz to 1 mHz.
2.4. Performance of a single cell fabricated with Pt-GO electrodes A single-cell test fixture was applied for evaluating the catalyst performance for a H2/O2 based fuel cell. A membrane electrode assembly (MEA), consisting of two Pt-GO electrodes, was immersed in 5 wt.% Nafion solution for 0.5 h. The MEA was thermally pressed (135 °C, 140 atm, 2 min) with a Nafion 212 membrane (DuPont Inc., USA) between the two electrodes. The two electrodes pressed on the two sides of each MEA were identical. The MEA was then inserted between two graphite plates, which had a serpentine flow pattern. Two Teflon gaskets of 0.24 mm thickness were introduced between the membrane and the electrodes. Humidified hydrogen and oxygen gases were fed into the cell at a flow rate of 200 cm3 min− 1 and 400 cm3 min− 1, respectively. The back pressure of the humidified gases was kept at 4 atm. The temperature of the single cell maintained at 60 °C, and the polarization curve and the powder density of the cell were then characterized.
c
a 500 nm
b
d
Fig. 1. FE-SEM images of (a) CP substrate attached with GO nanosheets, (b) Pt-CP, and Pt-GO composites with (c) low (×10,000) and (d) high (×50,000) magnification. The inset is HR-TEM micrograph of GO sheets.
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3. Results and discussion 3.1. Characterization of Pt-GO catalysts Fig. 1(a) shows FE-SEM of the GO-CP composite after gravity sedimentation. As-derived GO nanosheets, prepared by the chemical oxidation of natural graphite powders, were smoothly covered over microscaled carbon fibers. The inset reveals the HRTEM image of fresh GO sheets, showing the multilayers with an area of several micrometers. This suggests the presence of an extensive conjugated hexagonal network of sp2 carbons in graphene [8]. Isolated small fragments of GO sheets and folded wrinkle-like features are observed on the surface of CP substrate. XPS analysis was applied for analysis of the surface chemical composition of GO nanosheets, indicating a high oxidation level, i.e., O/C atomic ratio: 31.5%. This means that the GO sheets consist of many aromatic conjugated domains modified with functional oxide groups [20]. Such high oxidation level results in functionalization by epoxy and hydroxyl groups on the graphene surface (i.e., basal plane), in addition to carbonyl and carboxyl groups at the edges of the graphene sheets. For comparison, the XPS analysis indicated that the O/C ratio of the pristine CP is only 4.3%, much lower than that of the GO sheets. The FE-SEM images of the electrodeposited Pt particles on fresh CP and GO-CP are shown in Fig. 1(b) and (c), respectively. Pt nanoclusters with particle size ranging from 100 to 300 nm are observed on the surface of the carbon fibers and the GO sheets. A magnifying picture, as shown in Fig. 1(d), proves that the primary urchin-like particles are composed of smaller triangle-like substructures. Basically, there is no major difference in the Pt morphology between Pt-CP and Pt-GO samples, and well-dispersed Pt clusters on both carbon substrates are observed. The shape of the Pt crystals becomes a nanocluster during the PED, thereby increasing the surface area of the catalyst. HR-TEM was applied to observe a single Pt cluster on GO sheet, as represented in Fig. 2(a). It can be seen that the Pt cluster with a number of a nanotips is attached to the basal planes and the edges of graphene sheets. The size of the primary nanostructure is approximately 120–150 nm, and the secondary nanotips are found to protrude out of the cluster. Fig. 2(b) shows an atomic resolution HR-TEM image of Pt cluster, which displays the lattice fingers of the cluster. The lattice fingers with a spacing of 0.22 nm are apparently visible in the nanograins, in which the (111) crystal plane of the Pt face-centered cubic (fcc) crystals can be viewed. The urchin-like cluster possesses a porous nature, thereby offering a favorable surface area and an active center for electrocatalysis. Based on the observation, it is proposed that the shape of the Pt cluster is dominated by the presence of sulfuric acid anions [16]. This is ascribed to the fact that the anions tend to adsorb preferentially on certain facets of metals with fcc crystal structure, especially for the Pt (111) plane [21]. Because the selective adsorption takes place on a specific Pt surface plane, the growth of the Pt deposits on some crystal planes would be distributed or inhibited, thus, leading to the anisotropic growth of the Pt cluster. Grazing incident XRD analysis was used to characterize Pt nanoparticles on carbon substrates, as shown in Fig. 3. Three well-defined peaks around 39.8, 46.2, and 67.5°, respectively, correspond to the diffraction peaks of crystal faces Pt (111), (200), and (220), which represent the typical character of the platinum fcc structure. Both Pt catalysts show stronger (111) peaks than the other two peaks. The calculated results reflect that the (111)/(220) intensity ratios of PtGO and Pt-CP are 6.43 and 4.09, respectively, whereas those of the (200)/(220) are 2.67 and 1.63, respectively. In comparison, the ratios of Pt-GO are larger than those of Pt-CP, indicating that the presence of GO alters the growth rate of Pt crystal in both crystallographic directions. This fast growth rate can be attributed to the fact that the strong interfacial interaction between the deposit species and the highly oxidized graphite plane favors the formation of nucleation sites with a lower energy barrier during the PED process [22].
a
50 nm
b [111]
0.22 nm
2 nm Fig. 2. HR-TEM micrographs of electrodeposited Pt nanocluster on GO sheet with (a) low and (b) high magnification, showing lattice fingers in Pt (111) plane.
3.2. Electrochemical surface area and cell performance Representative cyclic voltammograms (CVs) of Pt nanoclusters are recorded in 0.5 M H2SO4 electrolyte at 50 mV s − 1. Two potential regions are observed: (i) double-layer formation (0.2–0.4 V) and (ii) oxidation-reduction of Pt (0.4–0.8 V). The capacitive doublelayer behavior can be attributed to the presence of carbon substrate (i.e., CP and GO), which originates from the electrochemical surface area for the formation of a double layer. The voltammograms of the two PED Pt catalysts display the characteristics of a clean polycrystalline Pt [15]. One pair of reversible hydrogen adsorption–desorption peak is clearly identified in the potential region from +0.2 to −0.2 V vs. Ag/AgCl, i.e., Pt + H + + e − ↔ Pt–Hads. For a polycrystalline Pt electrode, the charge transfer associated with the above reaction is 210 μC cm − 2. The electrochemically active surface areas (ESA) of Pt-CP and Pt-GO are 42.1 and 94.9 m 2 g − 1, respectively, based on the normalized Pt loading on carbon substrate. The ESA value of Pt-GO is 1.83 times that of Pt-CP, indicating an improved catalytic activity of Pt on basal graphene sheets. To clarify inner resistance and catalytic activity, an AC impedance spectroscopy was used to characterize the catalyst electrodes. The equivalent serial resistance (Rt) for both catalyst electrodes can be estimated, i.e., Pt-CP (3.19 Ω) and Pt-GO (2.60 Ω). It is worth mentioning that there are no obvious increases of Rt value after 200 CV cycles, keeping low inner resistance and catalyst stability. This proves that the Pt deposits onto GO sheets still maintain electrochemically active area, inducing the excellent durability. The polarization curve of a single cell, fabricated with the Pt-GO electrodes, at 60 °C with H2/O2 is shown in Fig. 4(b), and is compared
C.-T. Hsieh et al. / Catalysis Communications 16 (2011) 220–224
(004)
(220)
(311) (311)
(100) (200)
(111)
(004)
(220)
(111) (100) (200)
Intensity (arbunit)
Pt
a 30
with the Pt-CP electrode. The Pt loadings on Pt-GO and Pt-CP are 0.26 and 0.32 mg cm − 2, respectively. The curves show that the Pt-GO cell displays a power density of 1.3 kW g − 1 which is much higher than the Pt-CP cell, i.e., 0.4 kW g − 1. The enhancement of power density is attributed to the following reasons: (i) the better electric conductivity of graphene sheets (1250 S m − 1) [8], as well as the specific interaction between Pt cluster and graphene, leads to the high electrocatalytic activity of Pt; (ii) the Pt nanoclusters possess many tips (e.g., Pt (111) crystalline plane), providing more accessible surface areas; (iii) the existence of graphene offers not only a smooth pathway for charge transfer but also a 2-D plane for good contact between the current collector and the proton exchange membrane (Nafion 117 in this study), rendering a better Pt utilization. Additionally, the Pt catalysts were electrochemically deposited onto twodimensional GO sheets, so that the isolation of carbons by Nafion can be prevented from the CP support. Thus, the Nafion layer tends to uniformly coat over the Pt clusters, inducing a continuous layer. Accordingly, the dispersion of the Pt catalysts on graphene has been found to be useful for achieving relatively better performance in fuel cell application.
C
b
40
50
60
70
223
80
2 theta (degrees) Fig. 3. Grazing incident XRD patterns of (a) Pt-CP and (c) Pt-GO samples.
4. Conclusions
8
a
Using an efficient PED method, porous Pt nanocluster catalysts with a high surface area were electrochemically deposited onto the GO-CP composite electrode. The primary particle size was found to be 100–300 nm for the urchin-like cluster that comprises trianglelike secondary grains. The electrochemical behavior of the Pt clusters onto the GO sheets has been well investigated using CV, AC impedance, and single-cell test. This study demonstrated that the presence of the GO sheets has a crucial role in improving the catalytic activity and the inner resistance of the Pt catalyst electrode. Such a catalyst electrode, consisting of GO sheets and metallic Pt nanoclusters, has advantages such as good dispersion of catalysts, low inner resistance, and high catalytic activity. The incorporation of the Pt-Go catalyst electrode by the EPD technique from a single-cell test has been satisfactory, with a promising potential for the novel MEA design in PEMFC application.
Current (A.g-1)
4
0
-4
-8
-12
Pt-GO Pt-CP
-16
Acknowledgments
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1 The authors are very grateful for the financial support from the National Science Council of the Republic of China under the contracts NSC 100-2120-M-155-001 and NSC 100-2221-E-155-031.
Potential (V vs. Ag/AgCl) 1.6
1
b Appendix A. Supplementary material
Potential (V)
1.2 0.6 0.8 0.4 0.4
Power density (kW.g-1)
0.8
0.2 Pt-GO Pt-CP 0 0
200
400
600
800
0 1000
Current density (mA.cm-2) Fig. 4. (a) Cycle voltammograms of catalyst electrodes at 50 mV s− 1, and (b) polarization curves of single cells, fabricated with different catalyst electrodes, under H2/O2 flows at 60 °C.
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