Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 3

Chapter In situ Grown CNTs on Carbon Paper 3.1 Introduction This chapter mainly focuses on the CVD synthesis method for the in situ CNT growth on carbon paper as well as the structural and compositional properties of the in situ grown CNTs. These in situ grown CNTs were synthesized directly onto carbon paper to serve as both the gas diffusion layer and catalyst layer simultaneously to provide high porosity and surface area for PEMFC electrodes. Comparing with other CNT growth methods, the general thermal CVD technique was chosen to grow the CNTs for its ease of being scaled-up and relatively low growth temperature [1]. The aim of this work was to optimize the synthesis process for CNTs grown on carbon paper, in order to enhance the effectiveness of CNTs as integrated GDL and CL for PEMFC applications. Previously, several research groups intended to grow CNTs directly onto carbon paper as catalyst support for PEMFC applications [2-5]. In 2004, Wang and coworkers first proposed the idea of in situ growth of CNTs on carbon paper via a CVD process [2]. They electrodeposited Co catalysts for CNT growth on one side of carbon paper by a three-electrode DC method in a wt% CoSO4 and wt% H3BO3 solution at room temperature. It was found that the Co catalysts were selectively deposited on the side of the carbon paper facing the electrolyte solution, due to the high hydrophobicity of the carbon paper. This one-side Co deposition allowed the selective growth of CNTs on one side of carbon paper. To grow CNTs on carbon paper, the Co coated carbon paper was placed in a CVD furnace at ambient pressure and heated to 550 °C in h under a 150 sccm N2 flow and 7.5 sccm H2 flow. The 63 carbon paper was maintained at these conditions for 30 and then subjected to CNT growth under a C2H2 flow of 7.5 sccm at 700 °C for h. This growth process was rather time-consuming for its long heating conditioning. In addition, it was observed that the in situ grown CNTs via this process showed a very low density on the carbon paper, probably due to the low Co loading obtained by the electrodeposition method. In their progressive work [3], they used Co−Ni bimetallic catalysts synthesized via similar electrodeposition process to improve CNT growth. However, the in situ grown CNTs were still not dense enough thus an additional VXC72R-based gas diffusion layer was applied on the backside of the CNT-based electrode to enhance electrode hydrophobicity. Later in 2006 Villers et al. [4] also reported their results of in situ grown CNTs on carbon paper as catalyst support. In their study, Co−Ni bimetallic catalysts were obtained by dipping the carbon paper into a mixture of ethanol (93vol%), water (6vol%) and silane (1vol%) solution containing 0.3 M Ni(NO3)2 and 0.3 M Co(NO3)2 for h. During the CNT growth, the carbon paper was heated at 600 °C for at 350 sccm Ar and 1.5 sccm H2 flow to obtain Co−Ni nanoparticles. Then the temperature was increased to 800 °C for for CNT growth under a C2H4 flow of 16 sccm. This method was more time-effective; however, it was found that the in situ grown CNTs were generally straight covering on carbon fibers that a VXC72R-based gas diffusion layer was still necessary for the CNT-based electrode to provide adequate gas diffusion porosity. In a more recent study by Saha et al. [5], a similar growth process was used to grow CNTs on carbon paper at 800 °C for 10 under a gas flow of 90% Ar, 5% H2 and 5% C2H4 also catalyzed by Co−Ni nanoparticles. It is noteworthy that the morphology of their in situ grown CNTs also showed a dendrite pattern that they could not provide sufficient porosity and an additional GDL is always needed on the backside of the carbon paper. 64 Although these in situ grown CNTs in previous studies may provide an advance as catalyst support for their high surface area, the overall effectiveness of their synthesis process and their porous morphology still require considerable optimization work for their applications in PEMFC electrodes. In this study, the in situ growth of CNTs on carbon paper was carried out via a general thermal CVD process using sputtered metal thin films as growth catalyst. Contrary to previous studies where metal catalysts were deposited by wet chemical reduction methods, the sputtered metal catalysts were directly deposited onto the carbon paper surface and the catalyst loading could be easily controlled regardless of the high inertness of the carbon fibers. In order to obtain in situ grown CNTs with high surface area and high porosity, a series of optimization studies were conducted on the influence of different growth conditions, including type of catalyst metal, sintering of catalyst, growth temperature, catalyst loading, growth duration and flow rate of C2H4. Results will be demonstrated and discussed in the following section. 3.2 Optimization of Growth Condition In this section, the experimental optimization studies on growth conditions for in situ grown CNTs will be mainly depicted based on the structure and morphology of the as-grown CNTs on carbon paper. A series of growth conditions were investigated on their influence toward the structure and morphology of the in situ grown CNTs to obtain a CNT-modified carbon paper surface with high surface area and high porosity. 65 3.2.1 Type of Catalyst Metal At the beginning of this study, transition metals Fe, Co and Ni were chosen as growth catalysts due to their high catalytic activity for CNT growth extensively reported in previous studies. The catalyst metals were deposited onto carbon paper separately by direct sputtering without any wet chemical process. The sputtering conditions were fixed at a 100 W output power and 10 mTorr Ar pressure. For each metal catalyst, the catalyst loading was controlled at around 16−18 nm in nominal thickness determined by measuring corresponding film thickness based on simultaneous deposition on Si wafers. The metal coated carbon paper was then transferred to the furnace CVD system for CNT growth. Initially the CNT growth process was carried out under conditions similar to those of the process reported by Saha et al. [5]. The system was firstly heated up to 750 °C at the rate of 15 °C min-1 under a carrier gas flow of 100 sccm Ar + 5vol% H2. Afterwards, 10 sccm C2H4 was introduced as the carbon feedstock gas when the temperature was maintained at 750 °C. After h CNT growth the system was cooled down to room temperature under the same carrier gas. To evaluate the influence of metal catalyst type on the CNT growth, SEM images of the as-grown CNTs were investigated as shown in Fig. 3.1. As can be seen in Fig. 3.1, successful CNT growth was obtained from all the metal catalysts that were sputter-deposited on the carbon paper. However, the as-grown CNTs showed different structure and morphology when Fe, Co and Ni were individually used as the growth catalyst. In Fig. 3.1 (a) and (a’), it was observed that a dense CNT layer was formed when a thin layer of Fe catalysts were sputter-deposited on the carbon paper. The in situ grown CNTs had a relatively small size distribution range and little amorphous 66 (a) (a’) (b) (b’) (c) (c’) Fig. 3.1 SEM images of CNTs grown on carbon paper with (a)&(a’) sputtered Fe catalysts, (b)&(b’) sputtered Co catalysts, and (c)&(c’) sputtered Ni catalysts. Growth temperature: 750 °C; growth duration: h; catalyst loading: 16-18 nm thin film; C2H4 flow rate: 10 sccm. carbon was observed in the CNT layer. By contrast, although the CNTs produced by Co and Ni catalysts had a smaller diameter and more curly structure, the in situ grown CNTs showed a notably lower density that the CNT layer could not fully cover the 67 carbon paper surface, in comparison with the Fe catalyzed growth. In addition, a considerable amount of amorphous carbon was observed for both Co and Ni catalyzed growth (see Fig. 3.1 (b’) and (c’)). Accordingly, we conducted the following optimization experiments solely based on Fe catalysts. 3.2.2 Growth Temperature After determination of catalyst type, further optimization studies were carried out by examining the effect of growth temperature for CNT growth. According to previous studies described in Section 3.1, the growth temperatures for the in situ grown CNTs on carbon paper were mostly in the range from 700 °C to 800 °C. To optimize the growth temperature for in situ CNT growth, a series of growth processes were carried out at 700 °C, 750 °C and 800 °C, respectively. As shown in Fig. 3.2, in situ grown CNTs are rarely seen on the carbon papers grown under 700 °C and 800 °C, in contrast to those grown under the initial growth temperature of 750 °C. As CNT growth temperature is associated to the type of carbon feedstock used, the unsuccessful CNT growth at 700 °C and 800 °C can be attributed to the fact that 700 °C may not be high enough to effectively activate the dissolution of C2H4 into Fe catalysts, limiting the CNT growth as well as the carbon graphitization (see Fig. 3.2 (a)). On the other hand, at a high growth temperature of 800 °C, thermal pyrolysis of C2H4 would take place to form amorphous carbon before it dissolves into Fe catalysts to form CNTs, thus it can be observed in Fig. 3.2 (c) that even fewer CNTs were obtained at this growth temperature compared with those grown at a lower temperature of 700 °C. Contrarily, the CNTs grown at 750 °C showed a vigorous growth that the carbon paper surface was completely covered by a dense CNT layer 68 (see Fig. 3.2 (b)). Therefore the growth temperature was fixed at 750 °C throughout the subsequent experiment in this study. (a) (b) (c) Fig. 3.2 SEM images of in situ CNT growth at (a) 700 °C, (b) 750 °C and (c) 800 °C. Growth duration: h; Fe catalyst loading: sputter-deposition; C2H4 flow rate: 10 sccm. 3.2.3 Growth Duration When the optimum growth temperature was determined as 750 °C, the effect of growth duration was investigated afterwards. In the above CNT growth processes, the growth processes were all initially carried out based on one-hour growth duration. It was presumed that growth duration is also an important factor for effective CNT growth thus an optimum growth duration is in need to explore for the in situ grown CNTs. To attain this important parameter, four in situ CNT growth processes were 69 performed under a series of growth periods: 15 min, 30 min, h and h. All the carbon papers were coated with sputter-deposited Fe catalysts and the flow rate of C2H4 was set at 10 sccm. The growth temperature was 750 °C, in accordance with previous optimization results. The SEM images of the as-grown CNTs are shown in Fig. 3.3 to reveal the effect of duration scale on CNT growth. It can be clearly seen that the density of the in situ grown CNTs increased with growth duration from 15 to h whereas such density increase was not so obvious when the growth duration was further raised to h. This tendency may probably be due to the CNT growth limit when the catalysts have lost their activity by being capsuled within the CNTs [8]. Accordingly, the optimum growth duration was determined as h. (a) (b) (c) (d) Fig. 3.3 SEM images of CNTs grown under growth duration of (a) 15 min, (b) 30 min, (c) h and (d) h. Growth temperature: 750 °C; Fe catalyst loading: sputterdeposition; C2H4 flow rate: 10 sccm. 70 3.2.4 Catalyst Loading The effect of Fe catalyst loading was investigated after the growth temperature and duration had been optimized. In the previous growth processes shown above, all the Fe-coated carbon papers were obtained via a sputter-deposition of Fe catalysts, which corresponds to a thin Fe film of 16−20 nm on a flat substrate. The specific sputtering rate was thus determined to be 4−5 nm min-1, according to the film thickness measurement by a surface profiler (Alpha-Step@ 500). In order to investigate the influence of catalyst loading on the CNT growth, a series of Fe-coated carbon paper were prepared by subjecting them to different sputtering duration, including min, min, and Fe sputter-deposition. The SEM images of the in situ grown CNTs from these catalyst loadings are illustrated in Fig. 3.4. It can be clearly seen that the different loadings of Fe catalysts resulted in different structure and morphology of the in situ grown CNTs. Comparing the results from 2, 4, and sputter-deposited Fe catalysts, it was found that CNTs grown from Fe catalyst were the most dense and uniform in structure and were comparatively longer and more curly to form a CNT network whereby the surface area and porosity of the carbon paper surface was greatly enhanced (see Fig. 3.4 (b)). For the Fe catalyst, the as-grown CNTs showed relatively smaller size and similar morphology as those from the Fe catalyst, whereas the growth was not uniform that some fibers were barely covered with CNTs as shown in Fig. 3.4 (a). Given that the carbon paper made of networks of carbon fibers has a high surface roughness, the thickness of the Fe catalysts sputter-deposited on carbon papers was considerably smaller than that on flat substrates. It is very likely that the sputter-deposition of Fe catalyst was not adequate to evenly disperse on the rough carbon paper surface, resulting in an uneven CNT growth (see Fig. 3.4 (a)). For the catalyst (see Fig. 3.4 (c)), it is 71 notable that the CNTs tended to grow in a bigger diameter and had a large size distribution range. Moreover, the in situ grown CNTs showed a visibly smaller length, which is probably due to the larger Fe particles formed as a result of the higher catalyst loading. This phenomenon agrees very well with the theory that CNT growth (a) (b) (c) (d) Fig. 3.4 SEM images of CNTs grown from different Fe catalyst loadings obtained by (a) min, (b) min, (c) and (d) sputter-deposition. Growth temperature: 750 °C; growth duration: h; C2H4 flow rate: 10 sccm. rate is roughly proportional to the inverse of CNT diameter. When the catalyst loading further increased to sputter-deposition, the CNT growth seemed impeded and a considerable amount of amorphous carbon was present on the carbon paper surface (see Fig. 3.4 (d)). This result can be attributed to the thick catalyst layer deposited on the carbon paper surface that the formation of Fe nanoparticles was hindered to catalyze the CNT growth. As such, the optimum catalyst loading for the 72 in situ CNT growth was found to be a sputter-deposition process for Fe, corresponding to a thin Fe film with thickness of 16−20 nm on a flat substrate. 3.2.5 Flow Rate of C2H4 Last but not least, besides investigation on growth temperature, growth duration and Fe catalyst loading, a set of CNT growth processes were performed under different C2H4 input by varying the C2H4 flow rate, to examine its effect on CNT growth. The flow rate of carbon feedstock gas was reported to be one of the most important factors that determine the structure and morphology of CNTs grown via CVD technique [9]. In this study, a series of C2H4 flow rates of sccm, 10 sccm, 15 sccm, 20 sccm and 25 sccm were investigated to reveal their influence on CNT growth, respectively. Other growth parameters, such as growth temperature, growth duration and Fe catalyst loading, were fixed according to the aforementioned optimization studies. The SEM images of the in situ grown CNTs at different C2H4 flow rate are shown in Fig. 3.5. As shown in Fig. 3.5 (a), prior to CNT growth the pristine carbon paper showed a weave structure with straight graphite fibers entangling together, where large holes and gaps were observed. While after the carbon papers experienced an optimized growth process based on the above optimization studies, they were all fully covered by a thick CNT layer whereby the holes and gaps were filled by the dense CNTs. However, it is noticeable that the CNT layers grown under different C2H4 flow rate appeared dissimilar from each other in their structure and morphology. With increasing C2H4 flow rate, it can be clearly seen that the as-grown CNTs on carbon paper tended to grow comparatively larger in length and smaller in diameter. 73 (a) (b) (c) (d) (e) (f) Fig. 3.5 SEM images of (a) pristine carbon paper, and CNTs grown under C2H4 flow rate of (b) sccm, (c) 10 sccm, (d) 15 sccm, (e) 20 sccm and (f) 25 sccm. Growth temperature: 750 °C; growth duration: h; Fe catalyst loading: sputter-deposition. Moreover, as the C2H4 flow increased, the in situ grown CNTs exhibited a more curly structure and they twisted together to form a highly porous CNT layer with little amorphous carbon impurities present. When the C2H4 flow increased up to 25 sccm (see Fig. 3.5 (f)), it can be observed that the pore size of the CNT layer reduced to 74 much less than 1um and its porosity was greatly enhanced with regard to that of the pristine carbon paper. Unlike previous studies where sparse or straight CNTs were vertically grown on carbon fibers [2-5], the CNT layer grown via the optimized CVD process at a high C2H4 flow rate above 15 sccm demonstrated extremely high density and high porosity by curling and coiling together. It indicated that increasing the flow ratio of carbon feedstock gas/carrier gas may lead to a faster CNT growth rate as well as a stimulated lateral growth for the CNTs to convolve with each other. As a result, a dense CNT layer with mesoporous structure was obtained on the carbon paper, and the carbon paper surface was modified by the dense CNT layer with tremendously enhanced surface area and porosity. This improvement in surface structure and morphology is particularly favorable for PEMFC electrodes as it can provide greatly refined gas diffusion channels, as well as a large increment of surface area for Pt deposition, thus yielding a considerably enlarged reaction area. However, the optimum C2H4 flow rate could not be determined solely based on the structure and morphology of the as-grown CNTs, in situ electrochemical evaluation is necessary to examine their influence on PEMFC performance, which will be demonstrated and discussed in the next chapter. 3.3 Characterization of in situ Grown CNTs Before carrying out in situ electrochemical evaluation on the in situ grown CNTs, physical characterizations such as BET surface area measurement and Raman spectroscopy were performed on the CNTs grown at different C2H4 flow rate as shown in Section 3.2.6. The BET surface area measurement was used to reveal the surface area increment of the CNT-modified carbon paper. Raman spectroscopy was also used to provide compositional analysis of the in situ grown CNTs. These 75 investigations on the physical properties of the in situ grown CNTs may help us understand better towards the integrated CNT-grown carbon paper as a potential electrode component for PEMFC applications. 3.3.1 BET Surface Area of in situ Grown CNTs In this study, BET surface area measurement was performed on different CNTgrown carbon papers obtained at a series of C2H4 flow rates. Experimental details are described in Section 2.3.2. Pure nitrogen was used as the adsorbate gas. In order for comparison, a pristine carbon paper was also measured for its BET surface area as a reference. The surface areas of the CNT-grown carbon papers obtained from different C2H4 flow rate are illustrated in Fig. 3.6, in which the trend in surface area change with C2H4 flow rate can be clearly observed. As shown in Fig. 3.6, the surface area of the pristine carbon paper was increasingly enhanced by the CNTs grown under increasing C2H4 flow rate. This result is expectable in that the in situ grown CNTs 3.5 BET surface area / m g -1 3.0 2.5 2.0 1.5 1.0 0.5 0.0 10 15 20 C2H4 flow rate / sccm Fig. 3.6 BET surface area of pristine carbon paper and CNT-grown carbon papers grown at C2H4 flow rate of sccm, 10 sccm, 15 sccm, and 20 sccm. 76 showed higher density and smaller size from sccm to 25 sccm C2H4 flow rate as demonstrated in the SEM images in Section 3.2.6. However, it was found that the surface area of the CNT-grown carbon paper from 25 sccm C2H4 flow rate could not be obtained due to the poor adsorption of N2 on the CNT surface. It is likely that the poor N2 adsorption may stem from the inert graphitic surface of the CNTs grown at 25 sccm C2H4 flow rate [10]. Table 3.1 gives the surface area of the CNT-grown carbon paper obtained at corresponding C2H4 flow rate, as well as the weight ratio of the as-grown CNT layer to the carbon paper. As can be seen in Table 3.1, the surface area of the pristine carbon paper almost increased 10 times from 0.37 m2 g-1 to 3.16 m2 g-1 when a CNT layer was grown onto it at a 20 sccm C2H4 flow rate. As the weight ratio of the asgrown CNT layer to the carbon paper was approximately around 1−2%, it was assumed that the surface area of the carbon paper is negligible compared to that of the in situ grown CNTs. Thus the surface area of the in situ grown CNTs can be estimated by dividing the surface area increment of the CNT-grown carbon paper by their weight ratio to the pristine carbon paper. The corresponding values of the estimated surface areas of the CNTs grown at different C2H4 flow rate are listed in Table 3.1. According to this approximation, the surface area of the in situ grown CNTs at 20 sccm C2H4 flow rate is up to 176.58 m2 g-1, which is slightly lower to the typical surface area (250 m2 g-1) of carbon black VXC72R. The large surface area of the in situ grown CNTs suggests that the CNT layer grown on carbon paper may be a promising catalyst support material for the subsequent Pt sputter-deposition. 77 Sample sccm 10 sccm 15 sccm 20 sccm Surface area of CNT-grown carbon paper / m2 g-1 0.37 0.79 1.54 1.87 3.16 Weight ratio of CNT to carbon paper − 1.04% 1.19% 1.24% 1.58% 40.38 98.32 120.97 176.58 Estimated surface area of in situ grown − CNTs / m2 g-1 Table 3.1 Estimated BET surface areas of the in situ grown CNTs obtained from C2H4 flow rate of sccm, 10 sccm, 15 sccm and 20 sccm. 3.3.2 Raman Spectra of in situ Grown CNTs Normalized Raman spectra of carbon black VXC72R, carbon paper TGPH090 and the in situ grown CNTs at different C2H4 flow rate were obtained to probe the sp2 (ordered) and sp3 (disordered) hybridized C Raman peaks of these carbon materials, which are shown in Fig. 3.7. In Raman spectra of carbon materials, typically, the G band corresponds to sp2 hybridization of the ordered graphite state, whereas the D and D´ band are derived from the disorder-induced features of the carbon structure due to the finite particle size effect or lattice distortion [11, 12]. As shown in Fig. 3.10, the Raman spectra of the in situ grown CNTs at different C2H4 flow rate all revealed a typical Raman peak pattern of multi-walled carbon nanotubes, in which two characteristic peaks representing the D band and the G band were notably identified at 1345 and 1581 cm-1 Raman shift [13]. The D´ band was also observed around 1615 cm-1, exhibited as a broadened G band. By contrast, the commercial carbon black VXC72R showed relatively small D and G peaks, suggesting that the graphite content of this carbon support material was very low. In addition, the Raman shift of the 78 carbon paper TGPH090 showed a sharp G band peak at 1581 cm-1 and a low D band peak, indicating a highly graphitized structure of the carbon fibers making up the carbon paper. D band G band Intensity / a.u. D' band CNT (5 sccm) CNT (10 sccm) CNT (15 sccm) CNT (20 sccm) CNT (25 sccm) carbon paper VXC72R 1000 1200 1400 1600 Raman shift / cm 1800 2000 -1 Fig. 3.7 Raman spectra of carbon black VXC72R, carbon paper TGPH090, and in situ grown CNTs at C2H4 flow rate of sccm, 10 sccm, 15 sccm, 20 sccm and 25 sccm. A more in-depth compositional analysis on the in situ grown CNTs was carried out by calculating the D-to-G band intensity ratio ID/IG, as listed in Table 3.2. The Dto-G band intensity ratio ID/IG is commonly used as an effective means to estimate the graphitization of CNTs [14]. It was found that the CNTs grown at C2H4 flow rate from sccm to 20 sccm showed high ID/IG values approximately in the range from 1.6−1.7. This result reveals that the in situ grown CNTs on carbon paper consisted of a large portion of disordered sp3 state on the CNT surface. The high ID/IG ratio may mainly be attributed to the defects formed during CNT growth. According to Hull’s study where Raman spectra of sonochemically oxidized CNTs were inspected [14], it 79 was found that the ID/IG ratio of the CNTs was increased from 1.02 to 1.25 with increasing oxidation time. It was also revealed in their study that a direct correlation existed between the uptake of the relative D-to-G band intensity and the density growth of various functional groups on an oxidized CNT surface with increasing treatment time. Therefore they reported that the amount of disordered carbon increases with the severity of surface oxidation. Based on their results, it is likely that the highly disordered carbon-contained surface of the in situ grown CNTs may be able to provide a stable Pt-CNT interface for direct Pt deposition without additional surface oxidation. By contrast, the pristine carbon paper showing high graphitization was reported to have a large surface tension and was thus difficult to coat with small metal catalyst particles via electrodeposition. On the other hand, it was noted that the CNTs grown at 25 sccm C2H4 flow rate had a comparatively low ID/IG value of 1.41, suggesting that the in situ grown CNTs have higher ordered sp2 carbon content. This may explain the poor N2 adsorption of this sample during the BET surface area measurement depicted in the last subsection, which was assumed as the consequences of the inert graphitic surface of the CNTs grown at 25 sccm C2H4 flow rate. Sample sccm 10 sccm 15 sccm 20 sccm 25 sccm ID/IG ratio 1.71 1.65 1.55 1.63 1.41 Table 3.2 D-to-G band intensity ratio ID/IG of the in situ grown CNTs obtained from C2H4 flow rate of sccm, 10 sccm, 15 sccm, 20 sccm and 25 sccm. 80 3.4 Summary In this chapter, a systematic study on the growth conditions for in situ grown CNTs on carbon paper is demonstrated. A set of optimized parameters have been obtained based on the structure and morphology of the CNTs grown under a series of types of growth conditions (see Table 3.3). After the optimized growth process, the carbon paper surface was fully covered by a dense CNT layer with enhanced surface area and porosity. In addition, the in situ grown CNT layers on carbon paper showed tunable diameter and surface porosity at different C2H4 flow rate. BET surface area characterization demonstrated that the in situ grown CNT layer had a much larger surface area than that of the pristine carbon paper. Raman spectroscopy indicated that the in situ grown CNTs had a large amount of defects on surface, which may provide various anchoring sites for Pt catalysts. In view of these favorable properties, the CNT layer grown on carbon paper is proposed as a promising candidate to serve as both gas diffusion layer and catalyst layer simultaneously for PEMFC electrodes. Growth condition Catalyst type Growth temperature Growth duration Catalyst loading Optimized parameter Fe thin film by sputtering 750 °C 1h 16−20 nm on Si wafer Table 3.3 Optimized growth conditions for the in situ grown CNTs on carbon paper via CVD growth process. 81 References [1] W. Z. Li, D. Z. Wang, S. X. Yang, J. G. Wen, and Z. F. Ren, Chem. Phys. Lett., 335, 141 (2001). [2] C. Wang, M. Waje, X. Wang, J. M. Tang, R. C. Haddon, and Y. S. Yan, Nano Lett., (2), 345 (2004). [3] X. Wang, M. Waje, and Y.S. Yan, Electrochem. Solid-State Lett., 8, A42 (2005). [4] D. Villers, S. H. Sun, A. M. Serventi, J. P. Dodelet, and S. 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Mater., 18, 1780 (2006). 82 [...]... towards the integrated CNT-grown carbon paper as a potential electrode component for PEMFC applications 3. 3.1 BET Surface Area of in situ Grown CNTs In this study, BET surface area measurement was performed on different CNTgrown carbon papers obtained at a series of C2H4 flow rates Experimental details are described in Section 2 .3. 2 Pure nitrogen was used as the adsorbate gas In order for comparison,... CNT layer grown on carbon paper may be a promising catalyst support material for the subsequent Pt sputter-deposition 77 Sample 0 5 sccm 10 sccm 15 sccm 20 sccm Surface area of CNT-grown carbon paper / m2 g-1 0 .37 0.79 1.54 1.87 3. 16 Weight ratio of CNT to carbon paper − 1.04% 1.19% 1.24% 1.58% 40 .38 98 .32 120.97 176.58 Estimated surface area of in situ grown − CNTs / m2 g-1 Table 3. 1 Estimated BET... 5 sccm, 10 sccm, 15 sccm and 20 sccm 3. 3.2 Raman Spectra of in situ Grown CNTs Normalized Raman spectra of carbon black VXC72R, carbon paper TGPH090 and the in situ grown CNTs at different C2H4 flow rate were obtained to probe the sp2 (ordered) and sp3 (disordered) hybridized C Raman peaks of these carbon materials, which are shown in Fig 3. 7 In Raman spectra of carbon materials, typically, the G band... treatment time Therefore they reported that the amount of disordered carbon increases with the severity of surface oxidation Based on their results, it is likely that the highly disordered carbon- contained surface of the in situ grown CNTs may be able to provide a stable Pt-CNT interface for direct Pt deposition without additional surface oxidation By contrast, the pristine carbon paper showing high graphitization... as-grown CNT layer to the carbon paper As can be seen in Table 3. 1, the surface area of the pristine carbon paper almost increased 10 times from 0 .37 m2 g-1 to 3. 16 m2 g-1 when a CNT layer was grown onto it at a 20 sccm C2H4 flow rate As the weight ratio of the asgrown CNT layer to the carbon paper was approximately around 1−2%, it was assumed that the surface area of the carbon paper is negligible... in situ grown CNTs 3. 5 2 BET surface area / m g -1 3. 0 2.5 2.0 1.5 1.0 0.5 0.0 0 5 10 15 20 C2H4 flow rate / sccm Fig 3. 6 BET surface area of pristine carbon paper and CNT-grown carbon papers grown at C2H4 flow rate of 5 sccm, 10 sccm, 15 sccm, and 20 sccm 76 showed higher density and smaller size from 5 sccm to 25 sccm C2H4 flow rate as demonstrated in the SEM images in Section 3. 2.6 However, it was... sccm, 15 sccm, 20 sccm and 25 sccm 80 3. 4 Summary In this chapter, a systematic study on the growth conditions for in situ grown CNTs on carbon paper is demonstrated A set of optimized parameters have been obtained based on the structure and morphology of the CNTs grown under a series of types of growth conditions (see Table 3. 3) After the optimized growth process, the carbon paper surface was fully covered... Campbell, J Appl Phys., 93, 4185 (20 03) [8] A Gohier, C P Ewels, T M Minea, and M.A Djouadi, Carbon, 46, 133 1 (2008) [9] Z Y Juang, J F Lai, C H Weng, J H Lee, H J Lai, T S Lai, and C H Tsai, Diamond Relat Mater., 13, 2140 (2004) [10] E Dujardin, T W Ebbesen, H Hiura, and K Tanigaki, Science, 265, 1850 (1994) [11] V Barbarossa, F Galluzzi, R.Tomaciello, and A Zanobi, Chem Phys Lett., 185, 53 (1991) [12] G Vitali,... gas In order for comparison, a pristine carbon paper was also measured for its BET surface area as a reference The surface areas of the CNT-grown carbon papers obtained from different C2H4 flow rate are illustrated in Fig 3. 6, in which the trend in surface area change with C2H4 flow rate can be clearly observed As shown in Fig 3. 6, the surface area of the pristine carbon paper was increasingly enhanced... of the carbon structure due to the finite particle size effect or lattice distortion [11, 12] As shown in Fig 3. 10, the Raman spectra of the in situ grown CNTs at different C2H4 flow rate all revealed a typical Raman peak pattern of multi-walled carbon nanotubes, in which two characteristic peaks representing the D band and the G band were notably identified at 134 5 and 1581 cm-1 Raman shift [ 13] The . silane (1vol%) solution containing 0 .3 M Ni(NO 3 ) 2 and 0 .3 M Co(NO 3 ) 2 for 2 h. During the CNT growth, the carbon paper was heated at 600 °C for 2 min at 35 0 sccm Ar and 1.5 sccm H 2 flow. 63 Chapter 3 In situ Grown CNTs on Carbon Paper 3. 1 Introduction This chapter mainly focuses on the CVD synthesis method for the in situ CNT growth on carbon paper as. as the carbon graphitization (see Fig. 3. 2 (a)). On the other hand, at a high growth temperature of 800 °C, thermal pyrolysis of C 2 H 4 would take place to form amorphous carbon before it