Investigation of Further Improvement of Platinum Catalyst Durability

Mar 27, 2008 - Investigation of Further Improvement of Platinum Catalyst Durability with Highly Graphitized Carbon Nanotubes Support. Jiajun Wang,Gepi...
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Investigation of Further Improvement of Platinum Catalyst Durability with Highly Graphitized Carbon Nanotubes Support Jiajun Wang, Geping Yin,* Yuyan Shao, Zhenbo Wang, and Yunzhi Gao Laboratory of Electrochemistry, Department of Applied Chemistry, Harbin Institute of Technology, P.O. Box 411, Harbin 150001, Peoples’ Republic of China ReceiVed: January 9, 2008; In Final Form: February 26, 2008

The highly graphitized multiwalled carbon nanotubes (HG-MWCNT) were obtained by heat treatment at 2800 °C upon the as-obtained chemically vapor deposited multiwalled carbon nanotubes (CVD-MWCNT). The graphitization behavior was studied by X-ray diffraction and Raman spectroscopy. The results indicated that the obtained HG-MWCNT had a high degree of graphitization (95.3%), while it was only 39.5% for the as-obtained CVD-MWCNT. Electrochemical investigation suggested that the HG-MWCNT had a lower corrosion rate than the original MWCNT, which can be attributed to the less surface defects on the HGMWCNT with the increase of the graphitization degree. The durability of the corresponding Pt/CNT catalyst was discussed. The results revealed that Pt/HG-MWCNT using the highly graphitized carbon nanotubes as the supporting material had a higher electrochemical stability, which is due to the lower corrosion rate of HG-MWCNT and the stronger interaction between metal and carbon support.

Introduction In recent years, the durability of proton exchange membrane fuel cells (PEMFCs) has been recognized as one of the most important issues to be addressed before the commercialization of the PEMFCs. Among the various aspects of the degradation, the degradation of carbon-supported Pt (Pt/C) catalyst due to carbon support corrosion is one of the most important factors that decrease the operation life of PEMFC.1 Despite no widely accepted mechanism for carbon surface oxide generation and conversion, carbon is known to undergo electrochemical oxidation to surface oxides and CO2 under fuel cell conditions. One proposed generic stepwise mechanism of surface oxide formation and CO2 evolution is shown schematically in the reaction2

R-Cs-H f R-Cs-OH f R-CsdO f R-CsOOH f R-H + CO2(g) If the support is oxidized to CO2, Pt may be lost from the support; therefore, the more the carbon support is oxidized, the more Pt is lost.3 If the support is partially oxidized to surface oxide, the presence of oxygen-containing groups can both decrease the conductivity of catalysts and weaken the interaction between the support and the catalytic metal nanoparticles, which results in an accelerated sintering of catalytic metal nanoparticles.4 Furthermore, oxidized carbon surface can also lower the necessary hydrophobicity impacting the water management critical to PEM fuel cell operation.5 Thus, carbon corrosion behavior affects the lifetime of fuel cell. The alternative more stable support for PEM fuel cell is desirable. Recently, carbon nanotubes have been proposed as promising support materials for fuel cell catalyst due to their unique characteristics, including high aspect ratio, high electron conductivity, and enhanced mass transport capability.6 More importantly, some studies have reported that carbon nanotubes * Corresponding author. Phone and Fax: +86-451-86413707. E-mail: [email protected].

are more resistant to electrochemical oxidation than carbon black.7 Carbon nanotubes are usually considered as the rolled graphene sheets with a coaxis and with less dangling bonds and defects than carbon black. It is difficult for oxidative atoms to attack CNTs’ closed structure, and consequently it is stable in oxidizing conditions. Our previous studies8 indicated that multiwalled carbon nanotubes (MWCNT) were more resistant to electrochemical oxidation than carbon black under aqueous sulfuric acid solution. As a result of high corrosion resistance, MWCNT show lower loss of Pt surface area and catalytic activity when used as fuel cell catalyst support.9 Using MWCNT as the supporting material for Pt/C catalyst has been attempted to increase the durability of the catalyst. However, MWCNT still have high corrosion rate during the long-term durability test, which cannot meet the requirement of the durability performance for Pt/C catalyst. Thus, the stability of MWCNT should be enhanced further. It has been proposed that carbon material with more graphitized component can be more thermally stable.10 To the best of our knowledge, no studies about the stability of highly graphitized carbon nanotubes (graphitization degree >95%) and the corresponding Pt/HGMWCNT (highly graphitized multiwalled carbon nanotubes) catalyst were reported. In this work, MWCNT obtained by lowtemperature chemical vapor deposition (CVD) method were treated at high temperature to increase the degree of graphitization and crystalline perfection. The results indicate that highly graphitized MWCNT present higher electrochemical stability. Using MWCNT with high degree of graphitization as the support was also found to improve the durability of the resultant catalyst (Pt/HG-MWCNT). Experimental Section Preparation of MWCNT. CVD-MWCNT (10-20 nm in diameters, specific surface area >233 m2 g-1) and highly graphitized MWCNT (10-20 nm in diameters, specific surface area >116 m2 g-1) used in this experiment were obtained from Chengdu Organic Chemicals Co., Ltd. China. MWCNT were

10.1021/jp800186p CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008

Pt Catalyst Durability with HG-CNs Support first produced by the low-temperature CVD method. HGMWCNT were obtained by heat treatment upon the original CVD-MWCNT at 2800 °C for 15 days in a high-purity argon atmosphere. Preparation of Catalyst. Carbon nanotubes (MWCNT or HG-MWCNT) were functionalized by heating at reflux with concentrated nitric acid (HNO3). Pt/MWCNT catalyst was prepared by impregnation-reduction method. Hexachloroplatinic acid (H2PtCl6) was used as precursor of the catalyst, and formaldehyde was used as the reducing agent. The detailed preparation process can be described as follows:11,12 Water (30 mL) and isopropanol (30 mL) were added to 50 mg of MWCNT (or HG-MWCNT), and then the suspension was ultrasonically stirred for 1 h. Thereafter, H2PtCl6 was added to the suspension and ultrasonically stirred for 2 h. The pH value of the suspension was adjusted to 12 with NaOH aqueous solution, and an excessive amount formaldehyde was added into the suspension. After the mixture was impregnated for 20 min, it was heated to 80 °C and the agitation continued for 3 h at the same temperature. The resulting catalyst was washed with ultrapure water (∼18.2 MΩ·cm, Mill-Q Corp.) until Cl- was not detected and then dried overnight at 110 °C in vacuum. The 10 wt % Pt nanoparticles supported on the carbon nanotubes (Pt/MWCNT and Pt/HG-MWCNT) were obtained. Preparation of Electrode. The carbon nanotubes electrode was prepared as follows: The MWCNT (or HG-MWCNT) were suspended in isopropanol and agitated in an ultrasonic water bath, and the mixed ink was sprayed onto a PTFE hydrophobized carbon paper (Toray, containing 10 wt % PTFE), so the working electrodes were formed. The loading of spraying layer (for both MWCNT and HG-MWCNT) was 3 mg cm-2. The catalyst electrode was prepared by the similar method. First, the catalyst (Pt/MWCNT or Pt/HG-MWCNT, 10 wt % metal) was mixed with 5 wt % Nafion ionomer solution (Electrochem. Corp.) and isopropanol in an ultrasonic bath. Then the resultant ink was sprayed onto the PTFE-hydrophobized carbon paper (20 wt % PTFE) under 40 °C. The Pt loading was 0.5 mg cm-2, and the content of Nafion ionomer in the catalyst layer was 10 wt % (relative to catalyst layer loading: Pt + C + Nafion ionomer). Corrosion Investigation of MWCNT. The corrosion investigation of carbon nanotubes was conducted in a three-electrode cell setup. A platinum foil and a reversible hydrogen electrode (RHE) were employed as a counter and reference electrode, respectively. The above-prepared working electrode (1.0 × 1.0 cm2) was held vertically in a chamber filled with 0.5 mol L-1 H2SO4 with the carbon support layer exposed to the electrolyte solution. For electrochemical oxidation experiments, a constant potential of 1.2 V was applied with a HA-501 potentiostat/ galvanostat (Hokuto Denko Ltd., Japan). The current density was expressed by the geometric area of the electrode. After the oxidation test for 120 h, the carbon support electrode was washed with ultrapure water several times to remove the H2SO4 solution for further physical characterization. Durability Investigation of Pt/MWCNT Catalyst. Durability investigation of Pt/C catalyst was carried out by accelerated durability test (ADT) method. The ADT cell consists of a threeelectrode system, which included a reference electrode, a platinum mesh counter electrode, and a catalyst-coated carbon paper as a working electrode. The above-prepared electrode (2.3 × 2.3 cm2) was held vertically in a chamber filled with 0.5 mol L-1 H2SO4 with the catalyst layer exposed to the electrolyte solution, which mimics the environment of the electrode membrane interface in PEMFCs. ADT was conducted by

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Figure 1. XRD patterns for MWCNT and HG-MWCNT.

potential cycling between 0.60 and 1.20 V. Cyclic voltammogramms (CV) was used to characterize the EAS, which can be determined by the charge of hydrogen adsorption/desorption region. Before ADT, the electrodes were potential-scanned between 0.05 and 1.2 V for five cycles to obtain a steady CV. After ADT, the electrodes were washed with ultrapure water several times to remove the H2SO4 solution for further physical characterization. Physical Characterization. The particle morphology, size, and size distribution of Pt nanoparticles dispersed on the surface of carbon nanotubes were characterized by transmission electron microscopy (TEM) using a FEI/Philips TCNAI G2 TEM with a spatial resolution of 1 nm. X-ray diffraction (XRD) analysis was carried out for the catalysts with a D/max-rB(Japan) diffractometer using a Cu KR X-ray source operating at 45 kV and 100 mA. The XRD patterns were obtained at a scanning rate of 4° min-1 with an angular resolution of 0.05° of the 2θ scan in the range of 10 and 90°. X-ray photoelectron spectroscopy (XPS) measurements are made using a Physical Electronics Quantum-5600 Scanning ESCA Microprobe. The Al X-ray source operated at 250 W. The sample to analyzer takeoff angle was 45°. Survey spectra were collected at pass energy (PE) of 187.85 eV over the binding energy range 0-1300 eV. High binding energy resolution Multiplex data for the individual elements were collected at a PE of 29.55 eV. Raman spectroscopy (Raman) was used for examining the structure of carbon nanotubes, using SPEX-1403 Spectrograph. The excitation source was 60 mW with a wavelength of 514.5 nm from an argon ion laser. Results and Discussion Physical Characterization of Highly Graphitized MWCNT. The MWCNT and HG-MWCNT were characterized by XRD and Raman. The XRD patterns of the two carbon nanotubes are shown in Figure 1. The result showed a very clear structure transition from the low graphitized phase to a highly ordered graphitized phase. The XRD pattern for the MWCNT sample showed a peak with a maximum at 2θ ) 26.14°(Figure 1a), which was equivalent to a d spacing of about 3.406 Å. After the sample was heat-treated at high temperature for a long time (15 days), a significant rearrangement in the graphite mode occurred and a higher order carbon phase formed. As to the HG-MWCNT, the corresponding XRD pattern showed a very strong and sharp peak at 2θ ) 26.58°, which was almost

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Figure 2. Raman spectra for MWCNT and HG-MWCNT.

consistent with the (002) diffraction peak of an ideal graphite phase reported in previous literature.13 Meanwhile, the interlayer spacing contracted and the d spacing for HG-MWCNT at 2θ ) 26.58° was calculated to be 3.358 Å, which was remarkably smaller than the d spacing of 3.406 Å for MWCNT but was much closer to the d spacing of 3.354 Å for an ideal graphitized structure.12 In addition, the XRD pattern for HG-MWCNT also showed other peaks at 2θ ) 42.6, 44.5, 54.8, 77.7, and 83.7°, which may be ascribed to the diffraction peaks for the planes of graphite.14,15 The graphitization extent (G) can be determined from the average d002 spacing according to the following equation13

G ) (3.440 - d002)/(3.440 - 3.354) The calculated G for MWCNT and HG-MWCNT was 39.5% and 95.3%, respectively. The above structural transformation with heat treatment at high temperature, from a low graphitized carbon phase to a highly ordered graphitized phase, can also be clearly observed by the corresponding Raman spectra as displayed in Figure 2. The vibrational band appearing at ca. 1350 cm-1 was assigned to the disorder structure carbon (i.e., the D-band), and the highfrequency peak at ca. 1580 cm-1 was ascribed to graphite mode (i.e., the G-band).15 The G-band reflects the structure of the sp2 hybridized carbon. After the MWCNT were treated at high temperature, both broad bands of the HG-MWCNT became narrower and sharper. Meanwhile, the G-band of graphite mode for the HG-MWCNT also showed increased intensity relative to that of the D-band. The extent of graphite mode in the carbon material can be quantified by the intensity ratio of the G- to-D bands (i.e., IG/ID). The ratios of IG/ID are 0.77 and 2.76 for MWCNT and HG-MWCNT, respectively. It can be seen clearly that the IG/ID ratio increases for the carbon nanotubes upon heat treatment, confirming the structural transformation from a disorder carbon phase to a more ordered graphitized carbon phase. The result is consistent with the XRD analysis. Corrosion Investigation of Carbon Nanotubes. The electrochemical oxidation of carbon nanotubes was investigated by applying a fixed potential of 1.2 V on both carbon electrodes. The applied potential of 1.2 V was selected in the experiment because the cathode potential of a PEMFC is close to 1.2 V under open circuit conditions where the carbon support is very prone to oxidation. To characterize the electrochemical oxidation of the support, the XPS analysis technique was used to determine

Figure 3. Survey XPS spectra of MWCNT (a) and HG-MWCNT (b) before and after electrochemical oxidation test.

the oxygen extent on the carbon nanotubes support during the corrosion test. Figure 3 shows the survey XPS spectra of MWCNT (a) and HG-MWCNT (b) before and after electrochemical oxidation test. From the spectra, it can be seen that a significant increase in O1s peak value appears in each of the oxidized carbon nanotubes, which is the result of higher content of surface oxides on carbon surface due to electrochemical oxidation. The surface oxygen content is determined from the O/C atomic ratio, which can be obtained by integrating the area under the high-resolution XPS O1s and C1s spectra peaks, followed by correction with their sensitivity factors.16 The result indicates that the O/C atomic ratio of MWCNT increases from 2.5 to 6.0% after oxidized treatment for 120 h. By comparison, the O/C atomic ratio increases from 1.2 to 3.3% for HGMWCNT under the same treatment conditions, which is smaller than that of MWCNT. Thus, HG-MWCNT is more stable under electrochemical oxidation conditions. MWCNT prepared by the CVD method involves simultaneous decomposition of a carbon source with a metal catalyst under moderate temperature. The process introduces many structure defects with the formation of carbon nanotubes. These defects include edges, dangling bonds, vacancies, dislocations, and steps.17,18 It is easy for oxygen atoms to attack these sites to form surface oxides, which result in a high corrosion rate for the carbon material under electrochemical oxidation. Meanwhile, TEM analysis of raw MWCNT suggests that the material

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Figure 5. Electrochemical active surface area as a function of cycle numbers on Pt/MWCNT and Pt/HG-MWCNT electrode.

Figure 4. Cyclic voltammograms recorded on Pt/MWCNT (a) and Pt/HG-MWCNT (b) electrode in 0.5 M H2SO4 solution during the ADT.

contains numerous defects along the walls and at the ends of the tubes.14 The outer nanotube layer containing many defects can be “unzipped” and peeled away under oxidation conditions. Once the outer layer is removed, new defect sites are exposed and the process continues.9 Thus, the as-received CVDMWCNT under low temperature has a high corrosion rate under electrochemical oxidation conditions. On the other hand, graphitization treatment at high temperature for CVD-MWCNT can remove most of the defect sites and produce more perfect HG-MWCNT.19 It is difficult for oxygen atoms to attack the closed and perfect structure. In our work, the CVD-MWCNT were heat-treated at 2800 °C for 15 days, and the obtained HGMWCNT were highly graphitized (the degree of graphitization >95%), resulting in a higher electrochemical stability compared with MWCNT. Durability Comparative Investigation of Pt/MWCNT and Pt/HG-MWCNT. To test the effect of graphitization degree of carbon support on the stability of the corresponding carbon supported Pt catalyst, the Pt/CNT catalysts were measured by ADT simulating PEMFC operation conditions. ADT investigation was carried out by measuring electrochemical active surface area (EAS) during the potential cycling test. The potential cycles were performed from 0.6 to 1.2 V at 50 mV s-1 in 0.5 mol L-1 H2SO4. It is generally believed that the performance degradation of electrodes in PEMFC is mainly due to the EAS decrease of the catalysts. Figure 4 shows the representative cyclic voltammogramms of Pt/MWCNT and Pt/HG-MWCNT with the

Figure 6. XRD patterns for Pt/MWCNT (a) and Pt/HG-MWCNT (b) catalysts before and after ADT.

increasing cycle number during the ADT test. It can be seen that a reduction of the hydrogen adsorption and desorption in the CV appeared on all catalysts with the potential cycling, which shows a decrease of the EAS for each catalyst. EAS of the catalysts could be estimated from the Coulombic charge for the hydrogen adsorption and desorption (QH) in the cyclic voltammograms. Some oxygen may be present in the electrolytic

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Figure 7. TEM images and particle distribution histograms of Pt/CNT catalysts: (a) original Pt/MWCNT, (b) oxidized Pt/MWCNT, (c) original Pt/HG-MWCNT, and (d) oxidized Pt/HG-MWCNT.

solution, and the changes in electrochemical area (H2 adsorption) were compared assuming that nonzero oxygen concentration was constant throughout the 10 000 cycles. The calculated result reveals that EAS of the Pt/MWCNT electrode (Figure 4a) decreases from 1464.5 to 590.1 cm2 (the EAS is expressed with regard to the electrode area 2.3 × 2.3 cm2 but not in cm2 mg-1 Pt. The reason is that Pt amount is unknown for post-ADT sample due to the Pt dissolution or detachment) after 10 000 potential cycles tests, a loss of 59.7%. In contrast, EAS of the Pt/HG-MWCNT (Figure 4b) decreases from 1452.5 to 885.6 cm2, a loss of 39.0%. The EAS as a function of the CV cycle number obtained for the Pt/MWCNT and Pt/HG-MWCNT catalysts are shown in Figure 5. XRD patterns of the Pt/MWCNT and Pt/HG-MWCNT catalysts before and after 10 000 potential cycling tests were shown in Figure 6 panels a and b, respectively. The XRD pattern of the initial sample was taken using the as-received catalyst. In the case of the oxidized sample, the XRD pattern was obtained using the powders scratched from the electrode. The diffraction peak at 2θ of 25° shown in the catalysts is associated with the (002) plane of the hexagonal structure characteristic of carbon nanotubes. The (002) lines sharpen and move to higher angles after heat treatment at high temperature, indicating the HG-MWCNT have high graphitization degree. Pt nanoparticles are crystalline, as indicated by the characteristic peaks in the pattern. The diffraction peaks at 2θ of 40, 47, and 67° are associated with the Pt (111), (200), and (220) planes, respectively.20 The mean particle sizes can be calculated according to Scherrer’s formula.21 The calculated average particle size according to the diffraction peaks of Pt(111) is 2.9, 7.2, 3.0, and 6.5 nm for original Pt/MWCNT, post-ADT Pt/MWCNT, original Pt/HG-MWCNT, and post-ADT Pt/HG-MWCNT, respectively. It can be seen that the Pt/HG-MWCNT catalyst presents lower sintering resistance. The Pt/MWCNT and Pt/HG-MWCNT catalysts before and after ADT were also examined by TEM (Figure 7). As to the original Pt/MWCNT and Pt/HG-MWCNT catalysts, small Pt

nanoparticles with diameters from 2 to 3 nm are uniformly dispersed on the two carbon nanotubes (Figure 7a,c). The particle size distributions of the metal in the supported catalysts are obtained by directly measuring over 200 particles from bright-field TEM micrographs. The mean diameter of the original Pt/HG-MWCNT is slightly bigger than that of Pt/ MWCNT, which may be due to the less specific surface area for HG-MWCNT. On the other hand, the Pt nanoparticle size in these catalysts increases greatly after the ADT. Specifically, the mean diameter of Pt/MWCNT catalyst increases from 2.8 to 7.4 nm. In contrast, the Pt nanoparticles only change from 2.9 to 6.6 nm for Pt/HG-MWCNT catalyst. The result is consistent with that of XRD analysis. The above results revealed that the Pt/HG-MWCNT catalyst with the highly graphitized carbon nanotubes as the support was more electrochemically stable. This can be explained as the following. It is well known that the corrosion behavior of carbon support affects the stability of Pt/C catalyst. Thus, the high stability of HG-MWCNT results in the high stability of the Pt/ HG-MWCNT catalyst. In addition, increasing the degree of graphitization leads to the increasing strength of π-sites (sp2hybridized carbon) on the support, which acts as anchoring centers for Pt,22 thus strengthen the metal-support interaction.4,23 So, there is an increase in the resistance to sintering of the platinum crystallites with increasing degree of graphitization of the support. Thus, the Pt/HG-MWCNT have a higher stability under electrochemical oxidation conditions. Conclusions The as-received MWCNT are heat-treated at high temperature to increase the extent of graphitization. Electrochemical investigation indicates that the HG-MWCNT have a higher electrochemical stability, compared with that of the as-prepared CVDMWCNT. O/C atomic ratio of MWCNT increases from 2.5 to 6.0% after oxidized treatment for 120 h, while the O/C atomic ratio increases from 1.2 to 3.3% for HG-MWCNT under same

Pt Catalyst Durability with HG-CNs Support treatment condition. Meanwhile, the EAS loss for the Pt/ MWCNT catalyst after 10 000 potential cycling tests is 59.7%, while that is only 39.0% for Pt/HG-MWCNT catalyst. The Pt/ HG-MWCNT catalyst has a higher electrochemical stability under the same oxidization conditions. It can be also expected that using highly graphitized carbon material can improve the stability of the corresponding Pt/C catalyst. Acknowledgment. This work was supported financially by Natural Science Foundation of China (Nos. 20476020 and 20606007) References and Notes (1) Roen, L. M.; Paik, C. H.; Jarvic, T. D. Electrochem. Solid-State Lett. 2004, 7, A19. (2) Kangasniemi, K. H.; Condit, D. A.; Jarvi, T. D. J. Electrochem. Soc. 2004, 151, E125. (3) Willsau, J.; Heitbaum, J. J. Electroanal. Chem. 1984, 161, 93. (4) Coloma, F.; Sepulveda-Escribano, A.; Fierro, J. L. G.; RodriguezReinoso, F. Langmuir 1994, 10, 750. (5) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley Chichester: New York, 1988. (6) Li, W. Z.; Liang, C. H.; Qiu, J. S.; Zhou, W. J.; Han, H. M.; Wei, Z. B.; Sun, G. Q.; Xin, Q. Carbon 2002, 40, 791. (7) Wang, X.; Li, W. Z.; Chen, Z. W.; Waje, M.; Yan, Y. S. J. Power Sources 2006, 158, 154. (8) Shao, Y. Y.; Yin, G. P.; Zhang, J.; Gao, Y. Z. Electrochim. Acta 2006, 51, 5853.

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