Integrated platinum carbon nanotube based electrocatalyst for high efficiency proton exchange membrane fuel cells 4

Chapter Integrated Pt/CNT-based Electrocatalyst for PEMFCs 4.1 Introduction Following our optimization studies on the in situ growth of CNTs on carbon paper shown in Chapter 3, the application of these in situ grown CNTs is presented in this chapter where fabrication and characterization of an integrated Pt/CNT-based electrocatalyst are intensively introduced. The integrated Pt/CNT-based electrocatalyst was fabricated for PEMFC electrodes via direct Pt sputter-deposition and their electrocatalytic performance on PEMFC reactions was characterized in a real PEMFC system. The aim of this work was to optimize the fabrication method for the integrated Pt/CNT-based electrode as well as to evaluate the effectiveness of this electrode as an integrated PEMFC component for high efficiency PEMFCs. It has been reported in a large number of studies that Pt/CNT-based electrocatalysts demonstrated higher electrochemical activity and stability than those of the conventional Pt/VXC72R electrocatalyst [1-8]. To exploit such improvement of Pt/CNT-based electrocatalysts, several research groups developed their Pt/CNT-based electrocatalysts based on in situ grown CNTs on carbon paper as the catalyst support [9-12]. The synthesis processes of the in situ grown CNTs in their studies were described in Section 3.1 and this section mainly introduces their synthesis methods for Pt deposition onto the in situ grown CNTs. In earlier study by Wang and coworkers [9], Pt catalysts were electrodeposited onto the in situ grown multi-walled carbon nanotubes (MWNTs) by a three-electrode DC method in mM H2PtCl6 and 0.5 M H2SO4 aqueous solution. The deposition process was carried out at a potential of V vs. SCE and the Pt loading was controlled by the total charge applied. It was reported 83 that a total Pt loading of 0.2 mg cm-2 was obtained by this electrodeposition method and the average Pt particle size was around 25 nm. However, the large Pt particles were found to be the major handicaps that caused a lower polarization performance compared to that of the conventional ink-process prepared electrode, where the Pt particle size was in the range of 2–4 nm. To reduce the Pt particle size, they used a chemical reduction method in their subsequent work to synthesis Pt/CNT-based electrocatalysts [10]. Prior to Pt deposition, the in situ grown CNTs were first surface functionalized by refluxing the CNT-grown carbon paper in N H2SO4 /4 N HNO3 for h. To deposit Pt catalysts, a certain amount of Pt precursor solution was sprayed onto the CNT-grown carbon paper under a 60−80 °C heating condition, and then followed by reduction in 20% H2 in N2 at 150 °C for h. The Pt precursor solution was a mixture of 250 mg wt% H2PtCl6, 60.8 mg wt% Nafion solutions and g isopropanol. After Pt deposition, approximately 0.15 mg cm-2 Pt was deposited onto the in situ CNTs as determined the weight difference of the CNT-grown carbon paper. According to the TEM micrograph of the deposited Pt catalysts, the Pt particle size was reduced to nm via this chemical reduction method and enhanced Pt utilization was attained to give higher polarization performance. However, it was found that the polarization performance of the Pt/CNT-based electrode remained very low without brushing an additional gas diffusion layer on the backside of the carbon paper. Based on Wang’s exploration on Pt catalysts supported on in situ grown CNTs, in 2006 Villers et al. also prepared a Pt/CNT-based electrocatalyst for PEMFC electrodes [11]. In their method, the in situ grown MWNTs were first immersed in a 1% silane solution diluted in 93% ethanol and 6% H2O, which consisted of 0.04 M PtCl4 and 0.04 M PtBr4. After h immersion, the MWNT-grown carbon paper with Pt 84 precursor were then air-dried and chemically reduced in H2 at 500 °C for 15 min. After reduction, they found that the deposited Pt catalysts were nanosized particles with an average particle size of 3−5 nm. However, chemical state analysis of the Pt nanoparticles by XPS revealed that there were still some Pt(II) (22%) and Pt(IV) (4%) present after Pt deposition. Moreover, it was found that one deposition process corresponded to a Pt loading of 0.16 mg cm-2 and increasing Pt loading was achieved by repetitive deposition, which indicated that this Pt deposition process could not provide efficient control of Pt catalyst loading. Later Saha and coworkers [12] claimed that a high Pt loading on in situ grown CNTs could be obtained using glacial acetic acid as the reducing agent. In their study, the MWNT-grown carbon paper was first chemically oxidized in a M HNO3 aqueous solution for h before Pt deposition. Afterward, the MWNT-grown carbon paper was washed with deionized water and dried in vacuum at 90 °C for another h. During the Pt deposition process, the MWNT-grown carbon paper was immersed into a mixture dispersion of Pt acetylacetonate (Pt(acac)2) and 25 ml glacial acetic acid and ultrasonicated for at room temperature. Then it was heated up to 110 °C for Pt reduction and held for h with constant stirring. At last, the Pt-deposited MWNT-grown carbon paper was rinsed with deionized water and dried at 90 °C overnight in a vacuum oven. The Pt loading obtained via this process was around 0.42 mg cm-2 determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES). Although the deposited Pt nanoparticles showed high density and small size distribution range (2−4 nm) on the in situ MWNTs, it should be noted that this deposition process was rather tedious and time-consuming. In addition, the wet-chemical deposition methods described above all revealed difficulties in control of Pt loading and Pt distribution, considering that simultaneous Pt deposition may occur on the carbon fibers in carbon paper, making 85 these Pt catalysts inaccessible to reactant gases. Furthermore, despite the enhanced polarization performance observed from the Pt/CNT-based electrocatalysts prepared by the above wet-chemical methods, it is worth noting that an additional VXC72Rbased gas diffusion layer was always needed on the backside of the carbon paper that adds further complexity to electrode preparation. In view of all these limitations, the effectiveness of the in situ grown CNTs as catalyst support is greatly undermined by the Pt deposition process. To resolve the above mentioned difficulties in Pt deposition onto in situ grown CNTs, controllable Pt deposition was conducted via direct sputter-deposition in this study. Sputter-deposition technique has the advantages of being able to directly deposit Pt catalyst with excellent control of deposition rate, as demonstrated in a number of studies [4, 13-17]. Moreover, as it is a surface deposition technique, it allows us to disperse the Pt catalyst highly localized at the surface of the CNT-grown carbon paper, leading to higher Pt utilization [4]. Last but not least, sputter-deposition technique was chosen for Pt deposition owing to its ability to directly deposit Pt catalysts onto the dense in situ grown CNT layer, thus additional CNT surface oxidation process can be eliminated. In the following sections, the fabrication process of the Pt/CNT-based electrocatalyst via sputter-deposition will be introduced and the overall effectiveness of this Pt/CNT-based electrocatalyst will be evaluated by a series of ex situ and in situ characterization studies. This chapter mainly concentrates on the electrochemical activity and mass transport properties of the Pt/CNT-based electrocatalyst for PEMFC electrodes. The electrochemical stability of this Pt/CNTbased electrocatalyst will be demonstrated and elaborated in the subsequent chapter. 86 4.2 Fabrication and Characterization of Integrated Pt/CNT-based Electrocatalyst This section presents the fabrication process of the sputter-deposited Pt catalysts, as well as the structural and compositional characterization towards them. The structural and compositional analysis is of much importance in evaluating the integrated Pt/CNT-based electrocatalyst in that it can provide valuable insights with regard to understanding the electrocatalytic performance of the catalyst in addition to the electrochemical characterization. 4.2.1 Fabrication of Integrated Pt/CNT-based Electrocatalyst In this study, direct sputter-deposition of Pt catalysts onto the in situ grown CNTs was carried out using a R.F. magnetron sputtering system (Denton Discovery18). In contrast to previous studies where wet-chemical deposition processes were used [9-12], several pieces of CNT-grown carbon papers were directly transferred into the sputtering chamber after CNT growth without surface oxidation. During the sputtering process, the Ar gas pressure was maintained at 10 mTorr while the specific deposition rate of Pt catalysts was varied by controlling the output sputter-power. The specific deposition rate of the Pt catalysts at a given output power was determined by weight difference of the CNT-grown carbon paper before and after Pt deposition. Controllable Pt deposition was then realized by varying the sputtering time based the calculated specific deposition rate. Initially a Pt loading of 0.04 mg cm-2 obtained at 100 W output sputter-power was used in this study, which is one tenth of the Pt loading in a commercial Pt/VXC72R-based electrode [18]. 87 4.2.2 Characterization of Integrated Pt/CNT-based Electrocatalyst In order to reveal the microstructure and morphology of the sputter-deposited Pt catalysts on the in situ grown CNTs, their TEM micrographs were examined using a high-resolution JEM-2010 FETEM system. Figure 4.1 (a) shows the TEM micrograph of the CNT grown on carbon paper at a C2H4 flow rate of 20 sccm. It can be seen that the CNTs grown on carbon paper by the thermal CVD process were multi-walled carbon nanotubes (MWNTs) with a typical outer diameter of about 30 nm and an inner diameter of around 10 nm. The Fe catalysts can also be observed as small particles buried inside the tube, which agrees well with previous studies [19, 20]. Moreover, it is noteworthy that the in situ grown MWNTs showed very coarse and incontinuous external graphitic walls, suggesting that various defects were generated on the CNT skin during growth. This result is in excellent agreement with the Raman spectra study of the in situ grown CNTs shown in Section 3.3.2. Nevertheless, the defects on the CNT surface may be a favorable feature for Pt deposition due to the presence of numerous anchoring sites for Pt particles, in contrast to the inertness of a perfect CNT skin that was reported unable to be wetted by liquids with a surface tension higher than 100–200 mN m-1 [21]. The in situ grown MWNT layer was then coated with 0.04 mg cm-2 Pt catalysts by sputter-deposition without any surface oxidation. Figure 4.1 (b) and (c) illustrate the TEM micrographs of the as-deposited Pt catalysts on in situ grown MWNTs. Contrary to previous studies where Pt catalysts prepared by various chemical reduction methods usually showed poor dispersion on untreated CNTs [10, 21], numerous nanosized Pt particles were observed to be densely dispersed on the CNT support via direct sputter-deposition as shown in Fig 4.1 (a). Instead of forming a Pt thin film, as was seen on the smooth Si substrate, the sputter-deposited Pt catalyst showed a scaly structure with a homogeneous 88 distribution on the CNT layer due to its high surface roughness and porosity. The average particle size of the Pt nanoparticles was approximately 2−3 nm. The Pt loading of 0.04 mg cm-2 corresponds to a Pt thin film of thickness about 20 nm on a smooth Si substrate. Given the extremely high roughness and porosity of the in situ grown MWNT layer, it is understandable that the Pt layer on MWNTs was of much lower thickness and thus Pt nanoparticles were formed and uniformly dispersed on the MWNT surface. According to Fig. 4.1 (c), the grain size distribution of the Pt nanoparticles on a single CNT shows a relatively small range mostly from 1−5 nm. TEM investigation demonstrates that well-dispersed Pt nanoparticles have been successfully deposited onto the in situ grown CNTs by direct sputter-deposition, which provides significant advances over the wet-chemical processes that Pt particle distribution is greatly enhanced and CNT surface oxidation is eliminated. (a) (b) 89 (c) Fig. 4.1 TEM micrographs of the in situ grown CNTs (a) before and (b) after Pt sputter-deposition, (c) Pt nanoparticles on a CNT support. Pt loading: 0.04 mg cm-2 Figure 4.2 shows the corresponding size distribution histogram based on Fig. 4.1 (c). The total number of the counted Pt nanoparticles was around 500−550 to ensure a statistically representative sampling. As observed in Fig. 4.2, most of the sputterdeposited Pt nanoparticles were less than nm in diameter, with the majority distribution in 2−3 nm. It was reported previously by Giordano et al. [22] that the 60 Frequency / % 50 40 30 20 10 0 Diameter / nm Fig. 4.2 Histogram of Pt particle size distribution based on Fig. 4.1 (c) 90 mass activity of Pt catalyst has a strong correlation to the Pt particle size and the maximum mass activity corresponds to a grain size around nm. Therefore the sputter-deposited Pt catalysts may give rise to a high mass activity for PEMFC reactions based on the particle size distribution demonstrated in Fig. 4.2. (a) Graphite layers (b) Pt polycrystalline MWNT Fig. 4.3 HRTEM micrographs of (a) an as-grown CNT tip and (b) a CNT tip with sputter-deposited Pt catalysts. 91 To further investigate the microstructure of the Pt/CNT-based electrocatalyst, high-resolution TEM (HRTEM) micrographs were obtained at magnifications up to 500,000. Figure 4.3 (a) shows the multi-graphitic walls on the tip of a CNT that was grown on carbon paper. The layered graphitic walls were clearly seen at the closed end with Fe catalysts buried inside, indicating a tip-growth mechanism for the in situ grown CNTs [19]. Considerable defects were also observed at the CNT surface, corresponding to the Raman results shown in the precious chapter. After Pt sputterdeposition, the TEM micrograph of a CNT tip with sputter-deposited Pt nanoparticles is illustrated in Fig. 4.3 (b). It is noticeable that the Pt nanoparticles exhibited a clear lattice fringe of crystallite structure, suggesting that the Pt catalyst produced via sputtering technique has a high crystallinity. This implies that the sputter-deposited Pt catalysts are probably in a pure metallic state with little oxide content. To examine the chemical state of the sputter-deposited Pt catalysts, XPS analysis was performed on the Pt/CNT-based electrocatalyst as shown in Fig. 4.4. Determination of Pt chemical state was carried out by means of spectrum deconvolution using a Gaussian/Lorentzian shape line modified by an asymmetric function. As can be seen in Fig. 4.4, the Pt 4f7/2 core level revealed a binding energy of 71.1 eV based on the Pt/CNT composite catalyst, with reference to 284.4 eV as the binding energy of C 1s. This result substantially supports the TEM results that the sputter-deposited Pt catalyst is mostly in the pure metallic state and has a fine crystallite structure. However, it is likely that a minute amount of oxidized Pt is present due to the anchoring oxidized groups on the CNT surface. By contrast, Villers found a noticeable amount of oxidized Pt catalysts on the in situ grown CNTs that were deposited by wet-chemical methods [11]. Therefore, a reduction process for the 92 Pt/VXC72R/CNT-based electrodes. The higher cell performance of the Pt/CNT-based electrode may probably arise from its larger active Pt surface area when Pt catalysts are sputter-deposited onto the highly porous in situ grown CNT layer. The polarization curves shown in Fig. 4.15 suggest that the support layer for sputterdeposited Pt catalysts has a major impact on cell performance whereas this assumption requires further verification. 1.0 1.0 Pt sputtered on in situ grown CNTs Pt sputtered on VXC72R GDL Pt sputtered on 50% commercial CNTs+50% VXC72R -2 0.8 Cell potential / V -2 Pt loading: 0.04 mg cm at cathode 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0.0 0.5 1.0 1.5 Current density / A cm 2.0 Power density / W cm 0.8 2.5 -2 Fig. 4.15 Polarization curves of MEAs with 0.04 mg cm-2 Pt sputterdeposited on different support layers at cathode: VCX72R, 50wt% VXC72R + 50wt% commercial CNTs, and in situ grown CNTs. A closer investigation on the above three support layers is presented based on their surface SEM images shown in Fig. 4.16. It is noticeable that the in situ grown CNT layer revealed a distinct surface structure from the VXC72R and VXC72R /CNT-based GDLs. Although the VXC72R-based GDL showed a high surface roughness, as was observed in Fig. 4.16 (a), its porosity was considerably restrained by the densely packed carbon black agglomerates. By blending 50 wt% CNT into the 118 VXC72R-based GDL, the GDL surface roughness and porosity were notably enhanced by the porous CNT frame filled with VXC72R particles (see Fig. 4.16 (b)), in good agreement with Kim and Kannan’s studies [3, 4]. On the other hand, the in situ grown CNT layer demonstrated a three-dimensional CNT clump where a much higher porosity was observed in Fig 4.16 (c). As a result, it is very likely that the sputtered Pt particles were able to deposit onto the CNT surface as well as penetrate into the layer whereby the overall active Pt surface area was greatly increased. It is well-known that the electrocatalytic activity of metal/support composite catalysts significantly depends on the size and dispersion of the metal nanoparticles as well as their interactions with support materials [44, 45]. Therefore the in situ grown CNT layer coated with sputter-deposited Pt catalysts could have shown enhanced cell performance compared with the other two support layers, corresponding to their polarization curve results. Given that a VXC72R-based GDL usually has a thickness up to several hundred microns [18], the in situ grown CNT layer showed a typical thickness of only 3−4 µm that could tremendously reduce the mass transport resistance for reactant gases and water (see Fig. 4.16 (d)). For this reason, the thin in situ grown CNT layer is able to effectively work as an integrated GDL and CL, in contrast to previous studies where CNT support layer or sputter-deposited Pt catalyst was used individually for PEMFC applications [3, 4, 10, 11]. 119 (a) (b) (c) (d) Fig. 4.16 SEM images of different Pt support layers: (a) VCX72R, (b) 50wt% VXC72R + 50wt% commercial CNTs, (c) in situ grown CNTs, and (d) cross-section of a CNT layer grown on carbon paper. Another attempt to verify the effectiveness of the in situ grown CNT layer was made by growing CNTs onto a mg cm-2 VCX72R-based GDL and a mg cm-2 VCX72R-based GDL. The in situ CNT growth was carried out using VXC72R-GDL spread carbon papers with sputter-deposited Fe catalysts. As illustrated in Fig. 4.17, dense CNT layers showing high surface area and porosity were successfully grown onto the VXC72R GDLs after growth, regardless of the VXC72R loading. The VXC72R GDLs with additional in situ grown CNT layers were observed having similar surface structure and morphology as the CNT layer directly grown on carbon paper. The aim of this study was to examine the requirement of incorporating 120 additional VXC72R-based GDLs into the Pt/CNT-based electrodes, as claimed in previous studies [9-12]. (a) (a’) (b) (b’) Fig. 4.17 SEM images of mg VXC72R GDL (a) before and (a’) after in situ CNT growth; and 10 mg VXC72R GDL (b) before and (b’) after in situ CNT growth. The polarization curves of the Pt/CNT/VXC72R-based electrodes are shown in Fig. 4.18. It can be clearly seen that, contrary to previous studies [9-12], the cell performance was visibly impaired by adding a VXC72R-based GDL into the Pt/CNTbased electrode, and this performance drop was deteriorated as the VXC72R loading increased, especially at large current densities. It was speculated that this phenomenon can be ascribed to the increased mass transport resistance of the Pt/CNT/VXC72Rbased electrodes introduced by the additional VXC72R GDLs. To verify this speculation, EIS studies were performed on the Pt/CNT/VXC72R-based electrodes as 121 shown in Fig. 4.19. It was found that both the Pt/CNT/VXC72R-based electrodes revealed water transport resistance semicircles in their Nyquist spectra even at the medium current density region of 0.6 V (see Fig. 4.19 (b)). It suggests that the additional VXC72R GDLs practically increased mass transport resistance as we speculated. This mass transport limitation was even more prominent when the cell was running at large current densities shown in Fig. 4.19 (c). It agrees very well with their polarization curves that the Pt/CNT/VXC72R-based electrode with mg cm-2 VXC72R GDL suffered from a dramatic voltage drop at the mass transport overpotential region. It has therefore validated the effectiveness of the in situ grown CNT layer as an integrated GDL and CL for its applications in high efficiency and low Pt loading PEMFC electrodes. 1.0 1.0 Pt/CNT catalyst at cathode Pt/CNT catalyst + 5mg VXC72R GDL at cathode Pt/CNT catalyst + 10mg VXC72R GDL at cathode 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 Power density / W cm Cell potential / V -2 Pt loading: 0.04 mg cm-2 at cathode 0.0 0.0 0.0 0.5 1.0 1.5 Current density / A cm 2.0 2.5 -2 Fig. 4.18 Polarization curves of MEAs with 0.04 mg cm-2 Pt sputter-deposited on different support layers at cathode: in situ grown CNTs, CNTs grown on mg VCX72R GDL , CNTs grown on 10 mg VXC72R GDL. 122 -0.7 (a) Pt/CNT catalyst Pt/CNT catalyst + mg VXC72R GDL Pt/CNT catalyst + 10 mg VXC72R GDL -0.6 -0.5 - Z'' / ohm -0.4 -0.3 -0.2 -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Z' / ohm -0.4 (b) Pt/CNT catalyst Pt/CNT catalyst + mg VXC72R GDL Pt/CNT catalyst + 10 mg VXC72R GDL - Z'' / ohm -0.3 -0.2 -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 Z' / ohm -0.6 (c) Pt/CNT catalyst Pt/CNT catalyst + mg VXC72R GDL Pt/CNT catalyst + 10 mg VXC72R GDL -0.5 - Z'' / ohm -0.4 -0.3 -0.2 -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Z' / ohm Fig. 4.19 Nyquist spectra of Pt/CNT and Pt/CNT/VXC72R-based cathodes at (a) 0.8 V, (b) 0.6 V, and (c) 0.4 V. 123 4.4 Integrated Pt/CNT-based MEA for PEMFCs In this section, we demonstrate the electrochemical performance of a Pt/CNTbased MEA with the Pt/CNT-based electrodes at both anode and cathode, to evaluate the overall effectiveness of the Pt/CNT-based electrode for PEMFC applications. The Pt/CNT-based MEA was first optimized for its preparation process, by probing the optimum Nafion impregnation loading for anode. Subsequently, electrochemical characterizations including polarization curve measurement and electrochemical impedance spectroscopy, were performed on the Pt/CNT-based MEA to provide a detailed analysis of this electrode configuration as both anode and cathode. 4.4.1 Optimization of MEA Preparation It has been demonstrated previously in Section 4.3.1 that there lies in an optimum Nafion impregnation loading for the Pt/CNT-based cathode, which was found to be 60 µl of 0.5 wt% Nafion solutions in correspondence to 0.04 mg cm-2 sputter-deposited Pt catalysts. To prepare a Pt/CNT-based MEA, it is a necessary step to find out the optimum Nafion impregnation loading for the Pt/CNT-based anode considering the distinct reactions at anode and cathode. Other parameters, such as sputter power, catalyst loading and CNT layer morphology, were consistent with the optimization results for Pt/CNT-based cathode shown in Section 4.3.1. Anode Nafion Impregnation Anode Nafion impregnation was investigated for MEA optimization based on the notion that the hydrogen oxidation reaction (HOR) at anode requires high proton conductivity within the anode catalyst layer, to ensure diffusion pathways for protons from catalyst layer to electrolyte membrane [24]. To determine the optimum Nafion 124 impregnation loading for Pt/CNT-based anode, a series of Nafion loadings were impregnated into several Pt/CNT-based electrodes by air-spraying 60, 80, 100 and 120 µl of 0.5 wt% Nafion solutions onto the electrode surface, respectively. The Pt/CNT-based electrodes were used as anode and hot-pressed into MEAs with identical Pt/CNT-based cathodes, where the Nafion loading was maintained at 60 µl 0.5 wt% Nafion solutions. Figure 4.20 shows the polarization curves of the Pt/CNTbased MEAs with various Nafion loading at anode. It was observed that a maximized cell performance of the Pt/CNT-based MEA was achieved by combining a 100 µl Nafion solution impregnated anode and a 60 µl Nafion solution impregnated cathode. 1.0 60ul 0.5wt% Nafion impregnated at anode 80ul 0.5wt% Nafion impregnated at anode 100ul 0.5wt% Nafion impregnated at anode 0.8 Cell potential / V 120ul 0.5wt% Nafion impregnated at anode 0.6 0.4 0.2 0.0 0.0 0.5 1.0 1.5 Current density / A cm 2.0 2.5 -2 Fig. 4.20 Polarization curves of Pt/CNT-based MEAs with various Nafion loadings at anode. Cathode Nafion loading: 60 µl of 0.5 wt% Nafion solutions. As shown in Fig. 4.20, the cell performance was visibly enhanced by the increased Nafion loading at anode from a nominal amount of 60 µl to 100 µl 0.5 wt% Nafion solutions, indicating increasingly enhanced proton conductivity in the anode catalyst layer. However, the cell performance was found exacerbated as the anode 125 Nafion loading was further increased to 120 µl Nafion solutions, exhibiting a notable voltage drop at medium to large current density regions. This performance deterioration can be attributed the covering of Pt catalysts by the excess Nafion ionomers that causes increased charge transfer and mass transport resistance in the electrode. Hence the optimum Nafion loading for anode was determined as 100 µl of 0.5 wt% Nafion solution to fabricate an integrated Pt/CNT-based MEA. 4.4.2 Electrochemical Characterization of Integrated Pt/CNT-based MEA To perform electrochemical characterization on the integrated Pt/CNT-based MEA, a symmetric MEA was prepared using two identical Pt/CNT-based electrodes at both the anode and cathode. The polarization curve of the symmetric MEA is depicted in Fig. 4.21, in comparison with the asymmetric MEA consisting of a Pt/VXC72R-based anode and a Pt/CNT-based cathode. It is noticeable that the symmetric MEA showed slightly lower performance at low and medium overpotential region whereas it outperformed the one with Pt/VXC72R-based anode at large current densities. As the symmetric MEA has a total Pt loading of only 0.08 mg cm-2, its maximum power density of 650 mW cm-2 is corresponding to a specific maximum power density of 8.1 W per mg Pt, which is much higher than that of 2.8 W per mg Pt for the MEA with Pt/VXC72R-based anode. It indicates that the Pt utilization is greatly enhanced when the Pt/CNT-based electrodes are used at both anode and cathode. Moreover, no dramatic potential drop was observed in polarization curve measurement for the symmetric MEA at mass transport overpotential region, suggesting excellent mass transport properties of the MEA where Pt/CNT-based electrode were used at both anode and cathode. 126 1.0 1.0 Pt/CNT-electrode at cathode Pt/CNT-electrode at anode & cathode 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0.0 0.5 1.0 1.5 Current density / A cm 2.0 Power density / W cm Cell potential / V -2 0.8 2.5 -2 Fig. 4.21 Polarization curves of symmetric Pt/CNT-based MEA and asymmetric MEA with Pt/VXC72R-based anode and Pt/CNT-based cathode. Further investigation on the MEA with symmetric Pt/CNT-based anode and cathode was conducted by means of EIS characterization (see Fig. 4.22). In agreement with the polarization curve results, the total impedance of the symmetric MEA was greater at 0.8 and 0.6 V; nevertheless, it showed visibly smaller impedance for each electrode process (charge transfer, oxygen diffusion and water transport) when the cell was under large current load. It was observed that the two MEAs demonstrated similar impedance characteristics with only differences in magnitude. Therefore all the deviations in their performance can be derived from their different anode configurations. First of all, the high frequency intersect Rohm for the symmetric MEA was slightly higher considering that both the anode and cathode were composed of highly hydrophobic CNTs thus leading to a higher ionic resistance of the membrane due to lower hydration. Moreover, recall that the Pt/VXC72R-based anode had a high Pt loading with well-impregnated Nafion ionomers while only a small amount of 127 Nafion ionomers were sprayed onto the surface of the Pt/CNT-based anode, it is likely that the more moist catalyst layer in the Pt/VXC72R-based anode may probably be in favor of reducing Rct by offering sufficient water for proton transport [46]. This mechanism explains the lower Rct of the Pt/VXC72R-based MEA at low and medium current density regions. When current density further increases, oxygen diffusion and water transport processes become dominant. Consequently, the effective diffusion length is limited by fast reaction and water transport is impeded by electrode flooding. Under these limitations, severe overpotential was overwhelmingly induced at large current densities as shown in Fig. 4.22 (c). However, this voltage loss was mitigated when both the anode and cathode were Pt/CNT-based electrodes, owing to the fact that the sputter-deposited Pt catalysts are mostly distributed at the electrodeelectrolyte interfaces. As demonstrated previously, the Pt/CNT-based electrode can provide higher the Pt utilization and lower mass transport resistance at large current densities where cell reactions take place very fast. As such, the symmetric MEA has lower impedances for charge transfer, oxygen diffusion and water transport processes at 0.4 V, compared to those of the MEA with a Pt/VXC72R-based anode. The EIS results provide a substantial support to the polarization curve results by revealing the impedance information of individual process in PEMFCs, and confirm that the Pt/CNT-based electrodes can give rise to enhanced charge transfer and mass transport properties when they are used at both anode and cathode. 128 (a) -0.8 Pt/CNT-electrode at cathode Pt/CNT-electrode at anode & cathode - Z'' / ohm -0.6 -0.4 -0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Z' / ohm -0.4 (b) Pt/CNT-electrode at cathode Pt/CNT-electrode at anode & cathode - Z'' / ohm -0.3 -0.2 -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 Z' / ohm -0.5 (c) Pt/CNT-electrode at cathode Pt/CNT-electrode at anode & cathode - Z'' / ohm -0.4 -0.3 -0.2 1000 Hz 13.3 Hz 2.1 Hz -0.1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Z' / ohm Fig. 4.22 Nyquist spectra of symmetric Pt/CNT-based MEA and asymmetric MEA at (a) 0.8 V, (b) 0.6 V, and (c) 0.4 V. 129 4.5 Summary In this chapter, an integrated Pt/CNT-based electrode has been developed for PEMFC applications via combined thermal CVD and sputter-deposition processes. The electrochemical performance of this Pt/CNT-based electrode has been optimized and comprehensively evaluated in a series of in situ electrochemical tests. It was found that the integrated Pt/CNT catalyst layer showed a remarkable improvement in polarization curve characterization compared with two commercial Pt/VXC72R catalysts. EIS studies further confirmed that the Pt/CNT-based electrode has superior charge transfer and mass transport properties that contribute to a much smaller electrode impedance in comparison to the reference electrodes. The improved charge transfer and mass transport properties of the Pt/CNT catalysts can be attributed to the high active surface area of the sputter-deposited Pt nanoparticles and the highly porous structure of the in situ grown CNT supports. Therefore, the combined electrode fabrication strategy can provide an efficient solution to prepare PEMFC electrodes with reliable catalytic performance and effective mass transport for high efficiency PEMFC applications. A flow chart diagram of this combined fabrication process is illustrated in Fig. 4.23 below. 130 Sputter-deposited a thin Fe layer onto two cm2 carbon papers as CNT growth catalyst Placed Fe coated carbon papers in a thermal CVD system and purged with Ar + 5vol% H2 Raised temperature to 750 °C and grew CNTs for h under 20 sccm C2H4 Brushed mg PTFE onto the backside of the CNT-grown carbon papers Sputter-deposited 0.04 mg cm-2 Pt onto CNTgrown carbon papers at 50 W, 10 mTorr Ar Sprayed 100 µl 0.5 wt% Sprayed 60 µl 0.5 wt% Nafion solutions onto the Nafion solutions onto the Pt/CN-based anode Pt/CN-based cathode Hot-pressed the Pt/CNT-based electrodes with Nafion 112 at 140 °C, 15 atm for 90 s Fig. 4.23 Flow chart diagram of combined fabrication process for a Pt/CNT-based MEA. 131 References [1] N. Rajalakshmi, H. Ryu, M. M. Shaijumon, and S. Ramaprabhu, J. Power Sources, 140, 250 (2005). [2] T. Onoe, S. Iwamoto, and M. Inoue, Catal. Commun., 8, 701 (2007). [3] A. M. Kannan, V. P. 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Soc.,139,765 (1992). 133 [...]... great potential for Pt deposition for PEMFC applications as demonstrated in previous studies [13-17] 20000 4f7/2, 71.1 eV 4f5/2, 74. 4 eV 16000 CPS 12000 8000 40 00 0 84 82 80 78 76 74 72 70 68 66 64 Binding energy / eV Fig 4. 4 XPS spectrum of Pt nanoparticles on CNT /carbon paper by sputter-deposition X-ray source: Al Kα 148 6.6 eV, pass energy: 20 eV 93 4. 3 Integrated Pt/CNT -based Cathode for PEMFCs As... sccm C2H4 flow Accordingly the in situ grown CNT layer obtained from 20 sccm C2H4 flow can provide an optimum surface morphology where both high surface area and large penetration depth of Pt catalysts are guaranteed 1.0 Pt/CNT -based cathode (5 sccm C2H4) Pt/CNT -based cathode (10 sccm C2H4) 0.8 Pt/CNT -based cathode (15 sccm C2H4) Cell potential / V Pt/CNT -based cathode (20 sccm C2H4) Pt/CNT -based cathode... calculated in situ ECSA for the Pt/CNT -based electrode was 17.2 m2 g-1, in comparison to 29.1 m2 g-1 for the 20wt% Pt/VXC72R -based electrode and 15.6 m2 g-1 for the 40 wt% Pt/VXC72R -based electrode The highest ECSA observed from the 20wt% Pt/VXC72R -based electrode may account for its well-dispersed Nafion ionomers throughout the catalyst layer The Pt/CNT -based electrode also showed relatively high ECSA owing... is usually negligible [ 24] As such, in this study we focus on the integrated Pt/CNT -based electrocatalyst for the ORR by examining their in situ electrochemical performance as the PEMFC cathode electrocatalyst 4. 3.1 Optimization of Electrode Preparation To actively verify the electrochemical performance of the integrated Pt/CNTbased for the ORR, electrode preparation process was first optimized in... the integrated Pt/CNT -based electrocatalyst was made into the cathode while the anode was a conventional VXC72R -based gas diffusion electrode with a commercial Hispec4000 catalyst (40 wt% Pt/VXC72R, Johnson-Matthey) The Pt loading at anode was maintained at 0.2 mg cm-2 and the electrode preparation process for the conventional VXC72R -based electrode was described in Section 2.2.3 For the Pt/CNT -based. .. Pt/CNT -based cathode (25 sccm C2H4) 0.6 -2 Pt loading: 0. 04 mg cm at cathode 0 .4 0.2 0.0 0.0 0.5 1.0 Current density / A cm 1.5 2.0 -2 Fig 4. 9 Polarization curves of Pt/CNT -based electrodes with different in situ grown CNT layers as catalyst support 102 4. 3.2 Electrochemical Characterization of Integrated Pt/CNT -based Cathode Subsequent to the optimization studies for electrode preparation, electrochemical... transport in Nafion electrolyte membrane Using D = 6 × 10-6 cm2 s-1 for a fully hydrated Nafion membrane [42 ] and 2.1 Hz for the characteristic frequency of the diffusion impedance, the corresponding diffusion length for water transport was calculated as 11.7 µm for all the three electrodes This result is consistent with the SEM result shown in Fig 4. 13 that clearly shows the membrane thickness of about... effectiveness for sputter-deposited Pt catalysts Figure 4. 15 presents the polarization curves of MEAs with 0. 04 mg cm-2 Pt sputterdeposited on the three support layers at cathode As shown in Fig 4. 15, the Pt/CNTbased catalyst exhibited a remarkable improvement in cell performance, compared to those of the sputter-deposited Pt catalysts based on VXC72R and VXC72R/CNT blend GDLs For the Pt/CNT -based electrode,... cm-2 for the Pt/VXC72R and 117 Pt/VXC72R/CNT -based electrodes The higher cell performance of the Pt/CNT -based electrode may probably arise from its larger active Pt surface area when Pt catalysts are sputter-deposited onto the highly porous in situ grown CNT layer The polarization curves shown in Fig 4. 15 suggest that the support layer for sputterdeposited Pt catalysts has a major impact on cell performance... case the performance has a doubled 112 Tafel slope dependence on electrode potential [31] Therefore, the Pt utilization of the Pt/VXC72R -based electrodes is estimated to be considerably lower than the Pt/CNTbased electrode at large current densities Pt/CNT catalyst layer Pt/VXC72R catalyst layer Fig 4. 13 Cross-section SEM image of MEA with Pt/CNT -based cathode and Pt/VXC72R -based anode The membrane thickness . methods [11]. Therefore, a reduction process for the 93 84 82 80 78 76 74 72 70 68 66 64 0 40 00 8000 12000 16000 20000 CPS Binding energy / eV 4f 7/2 , 71.1 eV 4f 5/2 , 74. 4 eV post-deposited. Fig. 4. 4 XPS spectrum of Pt nanoparticles on CNT /carbon paper by sputter-deposition. X-ray source: Al Kα 148 6.6 eV, pass energy: 20 eV. 94 4. 3 Integrated Pt/CNT -based Cathode for PEMFCs. electrode as an integrated PEMFC component for high efficiency PEMFCs. It has been reported in a large number of studies that Pt/CNT -based electrocatalysts demonstrated higher electrochemical