Pt-Covered Multiwall Carbon Nanotubes for Oxygen Reduction in Fuel

May 17, 2011 - Junhyung Kim, Seung Woo Lee, Christopher Carlton, and Yang Shao-Horn*. Electrochemical Energy Laboratory and Department of ...
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Pt-Covered Multiwall Carbon Nanotubes for Oxygen Reduction in Fuel Cell Applications Junhyung Kim, Seung Woo Lee, Christopher Carlton, and Yang Shao-Horn* Electrochemical Energy Laboratory and Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

bS Supporting Information ABSTRACT: Recently one-dimensonal (1-D) Pt nanostructures have shown greatly enhanced intrinsic oxygen reduction reaction (ORR) activity (ORR kinetic current normalized to Pt surface area) and/or improved durability relative to conventional supported Pt catalysts. In this study, we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in proton exchange membrane fuel cells (PEMFCs). The average ORR intrinsic activity of Pt NPs/MWNTs is ∼0.95 mA/cm2 Pt at 0.9 ViR-corrected versus reversible hydrogen electrode (RHE), ∼3-fold higher than a commercial catalyst 46 wt % Pt/C (Tanaka Kikinzoku Kogyo) in 0.1 M HClO4 at room temperature. More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt at 0.9 ViR-corrected vs RHE) is higher than other 1-D nanostructured catalysts and TKK catalysts. The enhanced intrinsic activity of 1-D Pt NPs/ MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs. SECTION: Energy Conversion and Storage

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esign of highly active catalysts for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) is one of the great challenges in the field of electrochemical energy storage and conversion.1 3 Recent advances have shown that the intrinsic activity (ORR kinetic current normalized to Pt surface area) of Pt-based catalysts can be enhanced by altering the surface electronic structure of Pt, which can be tuned using surface geometric (e.g., surface orientations,4 6 nanostructure shapes,7 9 and sizes10 12) and compositional1,13 18 (e.g., alloying with transition metals) effects. In particular, one-dimensonal (1-D) Pt nanostructured catalysts, which were pioneered by 3M19 21 and further researched by a number of groups, such as supportless Pt nanotubes,22,23 ultrathin Pt nanowires,8,24 and star-like Pt nanowires9 supported on carbon, have shown enhanced intrinsic ORR activity and/or improved durability19,21 as compared with supported Pt catalysts.3,25,26 Considering that surface edges, corners, and surface steps are less active sites than low-index atomic planes for ORR due to higher adsorption strength of oxygenated species (i.e., OH and O),3,10,27 the enhanced intrinsic activity of 1-D Pt nanostructures can be attributed to more surface sites located on low-index atomic planes and fewer corner and edge sites than Pt nanoparticles (NPs).19 21 However, the mass activity (ORR kinetic current normalized to Pt weight) of 1-D Pt nanostructures reported to date9,22,23 is lower than that of the state-of-the-art supported Pt catalysts (0.16 mA/gPt at 0.9 V vs reversible hydrogen electrode (RHE))3 despite their significant increase in specific activity for ORR.19 Therefore, developing 1-D Pt nanostructures with mass r 2011 American Chemical Society

activity at least 2 times greater than that of the state-of-the-art supported Pt catalysts is needed to meet the catalyst activity target set for PEMFCs.3 Here we report a simple synthetic route to create Pt-covered multiwall carbon nanotubes (Pt NPs/MWNTs) as promising 1-D Pt nanostructured catalysts for ORR in PEMFCs. The intrinsic ORR activity of Pt NPs/MWNTs (∼0.95 mA/cm2Pt with iRcorrection (∼0.53 mA/cm2Pt without iR-correction) at 0.9 V vs RHE at room temperature) is not only comparable to other 1-D Pt nanostructured catalysts reported previously,8,9,11,19 but is also ∼3 times greater than that of a commercial 46 wt % Pt/C catalyst (Tanaka Kikinzoku Kogyo, TKK). More significantly, the mass activity of Pt NPs/MWNTs measured (∼0.48 A/mgPt with iRcorrection (∼0.27 A/mgPt without iR-correction) at 0.9 V vs RHE) is higher than other 1-D nanostructured catalysts9,22,23 and commercial supported Pt catalysts.3 We utilized amine-functionalized MWNTs (Supporting Information and Figure S1 for details) as a template and a polyol process with ethanol to create Pt NPs on the MWNTs, which involved the reduction of chloroplatinic acid (H2PtCl6) by ethanol at 80 °C on the amine-functionalized MWNTs under N2 atmosphere (Air gas, 99.999%). Briefly, 15 mg of aminefunctionalized MWNTs was dispersed first in 80 mL of ethanol for 30 min. Chloroplatinic acid hexahydrate (H2PtCl6 3 6H2O, Received: April 19, 2011 Accepted: May 17, 2011 Published: May 17, 2011 1332

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Figure 1. Schematic of ethanol-based deposition of Pt NPs on the amine-functionalized MWNTs (Pt NPs/MWNTs).

Figure 3. ORR activity measurements of acid-treated, heat-treated Pt NPs/MWNTs (14.5 μgPt/cm2geo) at room temperature: (a) cyclic voltammogram in Ar-saturated 0.1 M HClO4, (b) oxygen reduction polarization curves collected from a positive-going potential sweep at a 10 mV/s scan rate in O2-saturated 0.1 M HClO4. (c) Koutecky Levich plots used to extract the kinetic ORR current, and (d) ORR intrinsic activity as a function of iR-corrected potential obtained at 1600 rpm after background and iR-correction (ohmic electrolyte resistance of ∼35 Ω).

Figure 2. TEM images of (a) as-prepared, (b) acid-treated, and (c) acid-treated, heat-treated Pt NPs/MWNTs.

0.08 mmol, 40 mg) in 20 mL of ethanol was then introduced into the dispersed amine-functionalized MWNTs ethanol solution under vigorous stirring. After 1 h of stirring, the mixture solution in a 250 mL flask was transferred into an oil bath and was heated under reflux to 80 °C in N2 atmosphere for 12 h. The precipitated heterogeneous catalyst powder was separated by filtration to

remove ethanol, which was further washed with 1000 mL of ethanol and 1000 mL of Milli-Q water by using vacuum filtration. The final catalyst powder was collected after drying at 50 °C in air for 12 h (Figure 1). Direct current plasma-atomic emission spectroscopy (DCP-AES) analysis revealed ∼31 wt % Pt for Pt NPs/MWNTs (details in Supporting Information), which could be translated to ∼1.5 mgPt/m2MWNT assuming a surface area value of 300 m2/g for MWNTs. Transmission electron microscopy (TEM) imaging showed that individual MWNTs were fully covered by Pt NPs, as shown in Figure 2a. TEM was performed at 200 KeV on a JEOL 2010F microscope equipped with a field emission gun. It is interesting to note that performing the same synthesis route with pristine MWNTs generated nonuniform Pt coverage on MWNTs (Figure S2). This observation suggests that amine functional groups, which can not only provide nucleation sites but also facilitate Pt precursors anchoring on the MWNT surface via a self-assembly process, are the key to obtaining a uniform coverage of Pt NPs on MWNTs. In addition, the distribution of Pt NPs on MWNTs remained uniform after an additional acid-treatment step in 6 M HCl solution8 for 2 h (Figure 2b) and an additional annealing step at 220 °C in air for 2 h (Figure 2c). However, it should be mentioned that average Pt grain size increases after an annealing step at 220 °C, which can reduce the number of grain boundaries within the Pt coating on the MWNT surface. ORR activity of Pt NPs/MWNTs was measured using rotating disk electrode (RDE) measurements of thin films of Pt NPs/ MWNTs dispersed on glassy carbon (0.196 cm2) in O2-saturated 0.1 M HClO4 at room temperature. Cyclic voltammetry measurements of Pt NPs/MWNTs were performed in O2-free, Arsaturated electrolyte (Figure 3a), from which the electrochemical active surface area (ESA) was obtained from integrating the charge associated with hydrogen adsorption/desorption (HAD) area in the potential range from ∼0.05 to ∼0.4 V vs RHE after 1333

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Figure 4. Intrinsic ORR activity comparison of acid-treated, heat-treated ∼31 wt % Pt NPs/MWNTs at 0.9 ViR-corrected vs RHE: (a) ORR specific and mass activity of Pt NPs/MWNTs and Pt/C collected from 1600 rpm in O2-saturated 0.1 M HClO4, where the error bars represent standard deviations obtained from three independent measurements; (b) cyclic voltammogram in Ar-saturated 0.1 M HClO4, having 14.5 μgPt/cm2geo and 51 m2Pt/gPt for Pt NPs/MWNTs and 9.9 μgPt/cm2geo and 94 m2Pt/gPt for TKK 46 wt % Pt/C; (c) diffraction-contrast image of a representative area of Pt NPs/ MWNTs; and (d) representative phase-contrast high-resolution TEM (HRTEM) imaging of the surfaces of Pt NPs on MWNTs.

double layer current subtraction (Supporting Information and Figure S3 for details).4 The specific surface area of acid-treated, heat-treated Pt NPs/MWNTs averaged from three independent measurements was 51 m2/gPt, which is somewhat lower than that of conventional supported Pt NP catalysts as expected.3 ORR polarization curves of Pt NPs/MWNTs in O2-saturated 0.1 M HClO4 were collected at different rotation rates from 100 to 2500 rpm (Figure 3b), from which ORR kinetic current in the positive-going sweep was extracted from the Koutecky Levich equation28 (Figure 3c and Supporting Information for details). Intrinsic ORR activity based on the Pt ESA of acid-treated, heat-treated Pt NPs/MWNTs was compared with that of Pt/C (TKK) as a function of potential in Figure 3d. The activity of acid-treated, heat-treated Pt NPs/MWNTs is ∼0.95 mA/cm2Pt with iR-correction (∼0.53 mA/cm2Pt without iR-correction) at 0.9 V vs RHE, which is comparable to other 1-D nanostructured Pt catalysts reported previously.8,9,11,19 It is interesting to note that the intrinsic ORR activity of acid-treated, heat-treated Pt NPs/MWNTs is considerably higher than that of as-prepared Pt NPs/MWNTs (0.40 mA/cm2Pt with iR-correction (0.24 mA/ cm2Pt without iR-correction)), and acid-treated (0.75 mA/cm2Pt with iR-correction (0.34 mA/cm2Pt without iR-correction)) Pt NPs/MWNTs at 0.9 V vs RHE (Figure S4). Although the ORR activity enhancement mechanism associated with the acid-treatment and annealing is not clearly understood, removing surface impurities that can block ORR active sides and small Pt clusters in the acid-treatment and decreasing the number of grain boundaries within the Pt film during annealing can contribute to the ORR activity increase of the Pt NPs/MWNTs. In addition, an increase in average Pt grain size after annealing step can be a

beneficial factor to ORR as explained by the particle size effect of Pt NPs from previous studies.3,10,11 The intrinsic activity of acid-treated, heat-treated, ∼31 wt % Pt NPs/MWNTs is about 3 times greater than that (∼0.36 mA/ cm2Pt at 0.9 ViR-corrected vs RHE) of 46 wt % Pt/C of 2.0 ( 0.6 nm (TKK),26,29 as shown in Figure 4a. It should be mentioned that without iR-correction, the intrinsic activity of Pt/C (TKK) was found to be 0.21 mA/cm2Pt, which is comparable to previous studies.3,11 As the intrinsic ORR activity depends on the geometric effects of surface Pt atoms,8,10,11,30 it is hypothesized that the enhanced ORR intrinsic activity of acid-treated, heat-treated Pt NPs/MWNTs can be attributed to increased coordination of surface Pt relative to Pt/C (TKK), which can reduce the binding of surface oxygenated species and promote ORR kinetics.6,10,11 This hypothesis is supported by the observation that the onset potential (Figure 4b) of adsorption of oxygenated species was shifted positively by ∼50 mV as compared to that of TKK Pt/C, indicating weak chemical adsorption of OHads-species on the surface of Pt NPs covering MWNTs.1,3 High-resolution TEM (HRTEM) imaging provides further support, where the surfaces of Pt NPs in acid-treated, heat-treated Pt NPs/MWNTs were found dominated by the (111)Pt and (200)Pt planes (Figure 4c,d). The lattice planes of Pt NPs in the HRTEM images were identified by measuring the interplanar distances and angles from the fast Fourier transformations (FFTs) of the HRTEM images, which were performed using the Gatan Digital Micrograph software (version 3.11.2, Gatan, Inc., Pleasanton, CA, U.S.A.). The mass activity of the acid-treated, heat-treated Pt NPs/ MWNTs was 0.48 ( 0.14 A/mgPt with iR-correction (0.27 ( 0.06 A/mgPt without iR-correction) (Figure 4a), which is ∼1.5 times higher than that of TKK Pt/C (0.33 ( 0.02 A/mgPt with 1334

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The Journal of Physical Chemistry Letters iR-correction (0.22 ( 0.02 A/mgPt without iR-correction)) at 0.9 V vs RHE (Figure S5). Moreover, the mass activities of acidtreated, heat-treated 1-D nanostructured Pt NPs/MWNTs with iRcorrection (0.48 A/mgPt) or without iR-correction (0.27 A/mgPt) were higher than those from previous studies such as supportless Pt nanotubes (0.09 A/mgPt),22 star-like Pt nanowires (0.14 A/mgPt),9 3 M NSTF catalysts (0.06 A/mgPt)11 and conventional supported Pt catalysts (0.1 0.2 A/mgPt).3 In conclusion, we utilized a simple ethanol-based polyol process to completely cover amine-functionalized MWNTs with uniform Pt NPs. The average ORR intrinsic activity of Pt-covered MWNTs is ∼0.95 mA/cm2Pt at 0.9 ViR-corrected vs RHE, ∼3-fold higher than a commercial catalyst (46 wt % Pt/C (TKK) in 0.1 M HClO4 at room temperature). The enhanced intrinsic activity of 1-D Pt NPs/MWNTs could be attributed to the weak chemical adsorption energy of OHads-species on the surface Pt NPs covering MWNTs. The average mass activity of Pt-covered MWNTs (∼0.48 A/mgPt) was found to be higher than that of the state-of-the-art commercial Pt/C catalysts.3

’ ASSOCIATED CONTENT

bS

Supporting Information. Synthesis of amine-functionalized MWNTs, DCP-AES analysis, X-ray photoelectron spectroscopy, electrode preparation, electrochemical measurements, determination of electrochemical surface area of Pt, and analysis of the kinetic current from RDE measurements. This material is available free charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Science Foundation MRSEC under Award Number DMR-0819762 and the U.S Department of Energy Grant Number DE-AC02-98CH10886 through Brookhaven National Laboratory. S.W.L. acknowledges a Samsung Scholarship from the Samsung Foundation of Culture. ’ REFERENCES (1) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493–497. (2) Zhang, J.; Sasaki, K.; Sutter, E.; Adzic, R. R. Stabilization of Platinum Oxygen-Reduction Electrocatalysts Using Gold Clusters. Science 2007, 315, 220–222. (3) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B: Environ. 2005, 56, 9–35. (4) Markovic, N. M.; Ross, P. N. Surface Science Studies of Model Fuel Cell Electrocatalysts. Surf. Sci. Rep. 2002, 45, 121–229. (5) Lee, S. W.; Chen, S.; Suntivich, J.; Sasaki, K.; Adzic, R. R.; Shao-Horn, Y. Role of Surface Steps of Pt Nanoparticles on the Electrochemical Activity for Oxygen Reduction. J. Phys. Chem. Lett. 2010, 1, 1316–1320. (6) Markovic, N.; Gasteiger, H.; Ross, P. N. Kinetics of Oxygen Reduction on Pt(hkl) Electrodes: Implications for the Crystallite Size

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