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Carbon Nanofibers/Fabrics and Pulse Electrochemical Deposition Catalyst As a Gas Diffusion Electrode for Application in a Fuel Cell Jui-Hsiang Lin,* Tse-Hao Ko, and Miao-Yu Yen Department of Materials Science and Engineering, Feng Chia UniVersity, Taichung, Taiwan ReceiVed April 14, 2009. ReVised Manuscript ReceiVed June 16, 2009
A carbonaceous material (carbon nanofibers/fabrics, CNFs/CFs), that carbon nanofibers are directly grown upon carbon fabrics, is also used as one of the catalyst supporters in this experiment. It is a successful debut of the pulse electrochemical deposition method for decorating Pd particles on carbon fabrics. The ultrafine particle noble metal, acting as a catalyst in a fuel cell, is heterogeneously nucleated on the interphase between the solid surfaces of carbon fabrics and liquid electrolyte. By imposing the pulse potential energy, the method provides an energy barrier to limit the grain growth of cluster nuclei, while it has catalyst nucleation promotion. PdCl2 solution was directly used as the precursor on carbon fabrics (CFs) and CNFs/CFs by the pulse electrochemical deposition method. Field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray powder diffraction analyses (XRD), and electrochemical experiments are used to observe and measure the catalysts and substrate. The catalytic growth mechanism and performance on a fuel cell are discussed. When catalytic carbon nanofiber fabrics were assembled with Nafion 117 as half-cell modules, electrochemical half-cell analyses showed that the half-cell module had 10.3 mA/ cm2 in value.
1. Introduction Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) have high potentiality for use in electric power production, for consumer electronics, and portable power applications, due to their effective energy conversion efficiency and high power density.1 Recently, much research has focused on PEMFCs. The gas diffusion layer (GDL) and catalyst layer (CL) are critical materials in PEMFC studies. The GDL supports gaseous fuel transfer to the CL in a fuel cell. It should be electrically conductive to obtain current from the redox reactions at the CL. During fuel cell operation, water is produced by the redox process. Fuel cells have the potential to revolutionize energy conversion and distribution in the medium and long term. Vehicle propulsion, stationary cogeneration of heat and electricity, and power supply in mobile applications are current focus areas of fuel cell technology. Cost reduction can be achieved by using more effective supported catalysts with low precious metal loadings.2 Conductive carbon black powders, such as XC-72,3 are commonly used as membrane fuel cell catalyst support materials. However, these conventional carbon materials have been investigated for a long time and substantial information has already been accumulated. A recent development in the optimization of precious metal catalyst and high surface area support materials has been the investigation of carbon nanotubes (CNTs), * To whom correspondence should be addressed. Telephone: +886-4-24517250, ext 5303. Fax: +886-4-24518401. E-mail: gonvcat@ yahoo.com.tw. (1) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van Norstand Reinhold: New York, 1989. (2) Haug, A. T.; White, R. E.; Weidner, J. W.; Huang, W.; Shi, S.; Stoner, T.; Rana, N. J. Electrochem. Soc. 2002, 149, A280–A287. (3) Litster, S.; McLean, G. J. Power Sources 2004, 130, 61–76.
carbon nanofibers (CNFs), and carbon nanohorns4-10 to replace traditional carbon powders. Paoletti et al.11 reported that Pt deposits were obtained with a fine nanostructured surface, as well as uniform electrocatalyst distribution, on both carbon black (CB) and CNTs substrates. Electrodeposition parameters allow significant control of Pt particles morphology, leading to spherical, dendritic, and lamellar shapes. Hsu et al.12 reported that a carbon nanotubesupported PtRu anode catalyst for DMFCs applications has been successfully prepared using a modified polyol method and characterized to have a desired composition and excellent morphology. Tzeng et al.13 reported that CNFs with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition (CVD). Zhang et al.14 reported the idea of direct growth of CNTs on CFs and its use as a support for a fuel cell catalyst. Carbon nanofibers have few nanometers in mean diameter, and the cylindrical graphite walls can reduce more critical (4) Girishkumar, G.; Vinodgopal, K.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 19960–19666. (5) Tsai, M. C.; Yeh, T. K.; Tsai, C. H. Electrochem. Commun. 2006, 8, 1445–1452. (6) Yang, R.; Qiu, X.; Zhang, H.; Li, J.; Zhu, W.; Wang, Z.; Huang, X.; Chen, L. Carbon 2005, 43, 11–16. (7) Guo, J.; Sun, G.; Wang, Q.; Wang, G.; Zhou, Z.; Tang, S.; Jiang, L.; Zhou, B.; Xin, Q. Carbon 2006, 44, 152–157. (8) Niu, J. J.; Wang, J. N. Electrochim. Acta 2008, 53, 8058–8063. (9) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223–233. (10) Cameron, D. S.; Cooper, S. J.; Dodgron, J. L.; Harrison, B.; Jenkins, J. W. Catal. Today 1990, 7, 113–137. (11) Paoletti, C.; Cemmi, A.; Giorgi, L.; Giorgi, R.; Pilloni, L.; Serra, E.; Pasquali, M. J. Power Sources 2008, 183, 84–91. (12) Hsu, N. Y.; Chien, C. C.; Jeng, K. T. Appl. Catal., B 2008, 84, 196–203. (13) Tzeng, S. S.; Hung, K. H.; Ko, T. H. Carbon 2006, 44, 859–865. (14) Zhang, L.; Jiang, S. P.; Wang, W.; Zhang, Y. J. Power Sources 2007, 170, 55–60.
10.1021/ef900331c CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009
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Figure 1. FE-SEM images of CNFs/CFs in a CVD process.
nucleation size for catalysts. A carbonaceous material, carbon nanofibers/fabrics (CNFs/CFs), carbon nanofibers directly grown upon carbon fabrics, was also used as one of the catalyst supporters in the experiment. This experiment is a successful debut of the pulse electrochemical deposition method for decorating ultrafine particle noble metal on carbon fabrics. The above ultrafine particle noble metal, acting as catalyst in PEMFC, was heterogeneously nucleated on the interphase between solid surfaces of carbon fabrics and liquid electrolyte. By imposing the pulse potential energy, the method provides an energy barrier to limit the grain growth of cluster nuclei, while it has catalyst nucleation promotion. 2. Experimental Section The membrane electrode assembly (MEA) is obtained from DuPont (Nafion 117) and fabricated with an activated area of 4 cm2. The GDLs are manufactured from carbon fiber cloths. The carbon fabrics are examined CFs (0.46 mm, 253 g/m2). The gas diffusion electrode (GDE) is from E-TEK (0.5 mg cm-2 catalyst, Pt, supported on carbon cloth, E-TEK, Inc.). The CNFs/CFs was fabricated by CVD technique and purified by diluted acids. In this study, CNFs were grown directly on treated carbon fabrics (50 mm × 50 mm) via a thermal CVD process. Reagent grade NiCl2 solution (-500 mV vs Ag/AgCl) was directly used as the precursor on CFs by a pulse electrochemical deposition method, and the carbon fabrics were used for substrate without any purification and pretreatment. The pulse electrochemical deposition was a method for enhancing the adhesion and size control of Ni and the subsequently grown CNTs onto the fibers of CFs. Natural gas was used as the carbon source. The pressure inside the reaction furnace was maintained at about 1 atm during the whole deposition process. The prepared specimens were then placed into a tube furnace and thermally treated at 400 °C for 60 min and then at 540 °C under a mixed-gas atmosphere of H2 for 60 min. The furnace was then cooled down to ambient temperature under H2 atmosphere. The
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Figure 2. FE-SEM images of CFs-Pd by a pulse electrochemical deposition method.
CNFs were directly grown upon CFs (CNFs/CFs) and were also used as one of the catalyst supporters in this study. After activation, the sample was washed with dilute acid and deionized water to remove alkali compounds and impurities. The microphologies of the synthesized CNTs were examined by a field emission scanning electron microscope. The ultrafine particle noble metal, acting as catalyst in PEMFC, was heterogeneously nucleated on the interphase between solid surfaces of CFs and liquid electrolyte. By imposing the pulse potential energy, the method provides an energy barrier to limit the grain growth of cluster nuclei, while it has catalyst nucleation promotion. PdCl2 solution was directly used as the precursor on CFs and CNFs/CFs by a pulse electrochemical deposition method. Two kinds of finished products were examined: CFs-Pd and CNFs/CFs-Pd. Both kinds were manufactured from the same noble metal but with various substrates. This experiment is observed by field emission scanning electron microscopy (FESEM, S-4800, Hitachi); palladium catalysts did have nanosize morphologies on CNFs. Palladium catalysts had clear lattice images with mean spacing of d(111) about 0.22 nm which was observed by high-resolution transmission electron microscopy (HR-TEM, 1200EX, JEOL II). X-ray powder diffraction analyses (XRD, MXP3, MAC SCIENCE) showed that the average crystal size of palladium was between 13 and 14 nm, and it varied according to cyclic counts of deposition. Amounts of catalysts loading could be evaluated via open circuit potential of carbon fabric working electrodes versus cyclic counts of deposition. In the experiment, a three-electrode assembly half-cell15 was used in both pulse electrochemical deposition and electrochemical reaction analyses. The determination of electrochemical properties is performed in the experimental cell with a three-electrode system, electrolyte of 1 M H2SO4 solution, and a glassy paper as separator. Cyclic voltammetry (CV) is done in the potential range of 1.2 to ∼ -0.2 V at 5 mV/s by using a (15) Chen, S. K.; Ko, T. H.; Chung, C. Y.; Hung, K. H. Taiwan Patent I300278, 2006.
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Figure 3. FE-SEM images of CNFs/CFs-Pd with (a) 400, (b) 800, (c) 1200, and (d) 2400 cyclic counts by a pulse electrochemical deposition method.
CHI627B, CH Instruments. Adsorption and desorption reactions of hydrogen with palladium catalysts were analyzed via CV.
3. Results and Discussion In this study, NiCl2 solution was directly used as catalyst on carbon cloth by a pulse electrochemical deposition method and CNFs were directly grown on CFs by a CVD method. Figures 1-3 show the microphologies of the synthesized CNTs, CFs-Pd, and CNFs/CFs-Pd. As shown in Figure 1, parts a and b, the CNFs were directly grown on CFs by a CVD method. It can be seen that CNFs are isolated with 30-70 nm diameters, whereas it is difficult to make out the length of the CNFs from the FE-SEM observation due to their twisting. As parts a and b of Figure 2 show, the CFs-Pd fabricated by PdCl2 solution were used as the precursor in 5 and 50 cyclic counts pulse electrochemical deposition. In Figure 3b, the deposition exhibits a porous layer with accumulation on the carbon fiber surface. The Pd coating layer was rough, porous, and blanketed the surface of the carbon fibers. The CNFs/CFs fabricated by PdCl2 solution were used as precursor in 400, 800, 1200, and 2400 cyclic counts pulse electrochemical deposition, as shown in Figure 3a-d. It can be seen that the number of Pd particles increased with cyclic counts. Figures 4 and 5 are the TEM images of CNFs/CFs-Pd by a pulse electrochemical deposition method. In Figure 3a, the deposited spheres exhibit about 25 nm diameters and are sparsely distributed over CNFs/CFs; Figure 3b shows more numbers of Pd particles than Figure 3a, and the particles exhibit 10-50 nm diameters; in Figure 3c, it can be seen that Pd particles exhibit 10-50 nm diameters and are widely distributed over CNFs/CFs. Figure 3d shows a similar observation by FE-
Figure 4. TEM image of CNFs/CFs-Pd by a pulse electrochemical deposition method. The area marked with a white circle is for Pd particles.
SEM image with Figure 3c. However, Figure 3d has greater Pd particles size (25-50 nm in diameter) than Figure 3c. The result is caused by potential tendency in the electrochemical curve. HR-TEM images of Pd deposited on the CNFs/CFs by pulse electrochemical deposition methods are shown in Figure 4. Thereby, we can observe that white circles are Pd particles. According to Bragg low and the X-ray pattern, d(111) of Pd
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Figure 5. TEM image of CNFs/CFs-Pd about the d space.
illustrates 2.246 Å. The theory d(111) of graphite is 3.354 Å. HR-TEM images (Figure 5) show clear layer structure of CNFs/CFs-Pd. It is apparent that two red regions are expected to have Pd deposition. The average d(111) space of these two circle regions demonstrates 2.30 Å. This result confirms to theory d(111) of Pd and the effect of pulse electrochemical deposition methods. The electrochemical curve of CNFs/CFs was grown Pd particles by a pulse electrochemical deposition method in 2400 cyclic counts which was charged with 5 and 50 mA. The various charges cause the difference of the microstructure of CNFs/ CFs. CNFs have great electrochemical characterization and high crystal carbon structure. These properties cause great surface electric conductivity in the pulse electrochemical deposition stage and increase the rate of deposition. The pulse electrochemical deposition with 50 mA, the potential has a violent change. It can be evaluated that CNFs/CFs are widely and quickly nucleated over the surface in the earlier pulse electrochemical deposition stage. Figure 6 is the CV curve of CNFs/CFs-Pd with 400, 800, 1200, and 2400 cyclic counts. The results of the CV test of the CNFs/CFs-Pd electrode are shown in Figure 6. It can be observed that the current density of redox peaks of CNFs/ CFs-Pd 2400 cyclic counts is much larger than others. The steep current change at the switching potentials reflects quick charge propagation in the corresponding electrodes. The deviation from the imaginary rectangular emerged is due to the pseudocapacitive effects. It is evident that reversible redox transitions involving proton exchange occurred when the samples are polarized. The current density of redox peaks of CNFs/CFs-Pd 400, 800, 1200, and 2400 cyclic counts are 1.6, 1.9, 1.9, and 2.5 mA/cm2, respectively. The pattern of CNFs/ CFs-Pd 400, 800, and 1200 show the similar trend. The current density of redox peaks of CNFs/CFs-Pd 2400 cyclic counts has obvious variation. Figure 6b shows the CV curve of CNFs/ CFs-Pd with 400, 800, 1200, and 2400 cyclic counts at high voltage. The mass activity and specific activity of the catalyst are usually measured at high voltage above 0.8 V (vs NHE), because a low voltage such as 0.3 V (vs NHE, 0.1 V vs Ag/ AgCl) includes contributions from mass transfer and membrane resistance. This means that the electrochemical activity of CNFs/ CFs-Pd 2400 cyclic counts is higher than others. Therefore, the electron is easy to transport from CNFs to the CFs. CNFs/ CFs-Pd 2400 cyclic counts has a highest current density of 2.5 mA/cm2 in the hydrogen adsorption/desorption reaction. It has advantages of a highly uniform particle distribution and a
Figure 6. CV curve of CNFs/CFs-Pd with 400, 800, 1200, and 2400 cyclic counts.
Figure 7. Current density of CNFs/CFs-Pd 2400 cyclic counts and E-TEK samples.
simple fabricating process. Controlling the cyclic counts times and current also controls the Pd particle size. The half-cell analysis shows the values of specific activity (mA/cm2) and mass activity (A/g) to estimate for the catalyst performance of the CNFs/CFs-Pd 2400 cyclic counts (0.023 mg/cm2 catalyst, Pd, supported on CNFs/CFs) and E-TEK sample (0.5 mg/cm2 catalyst, Pt, supported on carbon cloth, E-TEK, Inc.). Both samples were also studied under the same experimental conditions. It can be observed from Figure 7 and Table 1 that the CNFs/CFs-Pd 2400 cyclic counts has 10.3 mA/cm2 and 447.8 A/g1, at about 0.1 V (vs Ag/AgCl). This result is consistent with that of various catalysts and surface-
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Table 1. Half-Cell Test of CNFs/CFs-Pd 2400 Cyclic Counts and E-TEK Samples
sample
catalyst loading (mg/cm2)
specific activity (mA/cm2)
mass activity (A/g)
E-TEK CNFs/CFs-Pd 2400 cyclic counts
0.5 0.023
41.4 10.3
82.8 447.8
activated area of catalysts. The CNFs/CFs-Pd 2400 cyclic counts sample shows lower specific activity than the E-TEK sample. However, it is shown that CNFs/CFs-Pd 2400 cyclic counts sample has higher mass activity than the E-TEK sample. The CNFs hybridized CFs should have wide applications in fuel cells, and the catalyst properties might be modified by forming a larger value of pulse electrochemical deposited catalyst. 4. Conclusion CNFs with 30-70 nm diameters have been directly grown on CFs by a pulse electrochemical deposition method using
NiCl2 solution as catalyst in a CVD system. The noble metal particles have been directly grown on CNFs/CFs by a pulse electrochemical deposition method using PdCl2 solution as catalyst. The palladium-coated CNFs/CFs-Pd 2400 cyclic counts had a highest current density of 2.5 mA/cm2 in the hydrogen adsorption/desorption reaction. The electrochemical half-cell analyses showed that the half-cell module had 10.3 mA/cm2 of specific activity and 447.8 A/g1 of mass activity. The CNFs/CFs-Pd 2400 cyclic counts sample shows exceptional mass activity as opposed to the E-TEK sample. Acknowledgment. The study support of the Ministry of Economic Affairs, ROC (97 Program no. 97-EC-17-A-08-S1-099), the National Science Council, and Feng Chia University is gratefully appreciated.
EF900331C
