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Enhanced electrocatalytic activity of Pt/3D hierarchical bimodal macroporous carbon nanospheres Ratna Balgis, W. Widiyastuti, Takashi Ogi, and Kikuo Okuyama ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05873 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

Enhanced electrocatalytic activity of Pt/3D hierarchical bimodal macroporous carbon nanospheres Ratna Balgis, †* W Widiyastuti, ‡ Takashi Ogi, †* and Kikuo Okuyama† †

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University,

1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan ‡

Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember, Kampus ITS

Sukolilo, Surabaya 60111, Indonesia KEYWORDS: Nanostructured Pt/C; spray pyrolysis; fuel cell; Pt deposition; high-dispersibility.

ABSTRACT

Proton exchange membrane fuel cells require electrocatalysts with high platinum (Pt) loading, a large active surface area, and a favorable hydrodynamic profile for practical applications. Here we report the design of 3D hierarchically bimodal macroporous carbon nanospheres with an interconnected pore system, which are applied as an electrocatalyst support. Carbon-supported Pt (Pt/C) catalysts were prepared by aerosol spray pyrolysis followed by microwave chemical deposition. The hierarchical porous structures not only increased the dispersion of Pt nanoparticles but also improved catalytic performance. A hierarchical bimodal macroporous Pt/C catalyst with a mixture of 30- and 120-nm pore sizes showed the best performance. The

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electrochemical surface area and mass activity values of this support were 96 m2 g−1-Pt and 378 mA mg−1-Pt, respectively at a Pt loading of 15 wt%.

1. INTRODUCTION The deposition of platinum nanoparticles (Pt NPs) on the surface of supporting particles has given rise to a new generation of industrial high-performance electrocatalysts, with particular relevance for proton exchange membrane fuel cells (PEMFCs).1-5 Currently, most commercial PEMFCs catalysts use carbon NPs as a support, because of their high specific surface areas 6-9; however the surface of carbon features a microporous structure, which disrupts the hydrodynamic profile and limits parameters such as fluid diffusion.10 Reduced diffusion negatively influences catalytic activity, and reduces selectivity and the lifetime of the catalyst.11 The use of three-dimensional (3D) hierarchical porous carbon, which features a wide range of porosities (micro-, meso-, and macropores), offers an effective solution to achieve faster migration of guest molecules within a catalyst framework.12-16 Porous carbon structures combine the catalytic features of microporous structures and feature improved accessibility and increased molecular transport owing to different levels of porosities within a single body. The presence of mesopores offers a lower resistance to ion transport through the porous carbon and provides a large surface area for ion adsorption. The incorporation of macropores into a mesoporous architecture provides ion-buffering reservoirs, which guarantee a shorter ion diffusion distance and facilitate rapid transport of ions and fluid within the structure.17-24 The deposition of Pt NPs on the surface of 3D hierarchical porous carbon particles (Pt/C) has also been shown to be an effective way to improve electrical conductivity.25-27 Although many hierarchically structured carbon particles have been successfully developed, challenges remain


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relating to optimizing the porous structure within the carbon material so that it can accommodate a high Pt loading and provide a good hydrodynamic profile. The high Pt loading is necessary for mobile applications because it reduces the use of catalyst powder to limit the size and weight of PEMFC modules. To date, a Pt loading of ~40 wt% is commonly used for Pt/C commercial electrocatalysts and high Pt loadings have only been successfully achieved on the surface of carbon NPs, which have high surface areas. However, such Pt/C NPs easily become agglomerated during operation and the reduced lifetime of the catalyst lifetime is a considerable drawback for commercial applications. There have been only a few reports of Pt deposition on the surface of 3D nanostructured carbon particles, which promise better durability than currently used carbon NPs.8 The amount of Pt loading was also not so high to keep the good performance of the electrocatalyst. This may be because this 3D structure offers fewer deposition sites for Pt than available on carbon NPs. Hierarchical structures require optimized connectivity between pores of various sizes to allow for effective catalysis. Thus, hierarchically structured carbon particles require careful design to produce large pore structures with good connectivity between meso- and micro-pores. In this study, we showed that a hierarchical bimodal macroporous structure can improve the surface area available to accommodate Pt loading without sacrificing the active site accessibility. We thoroughly evaluated the effects of different sizes of secondary macropores in the catalyst supports on the dispersion of Pt NPs and catalytic activity. 2. EXPERIMENTAL Hierarchical porous carbon particles were prepared from an aqueous solution of 0.25 wt% of phenolic resin (Sumitomo Bakelite Co., Ltd., Tokyo, Japan) and negatively charged polystyrene


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latex (PSL) particles by a spray method.27,28 Functionalization of the nanostructured carbon particles was performed by Pt deposition on the nanostructured carbon surfaces though a microwave chemical deposition method. The as-prepared materials were characterized with a zetasizer, field-emission scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and N2 adsorption– desorption measurements. The electrochemical properties were characterized by cyclic voltammetry (CV) and a rotating disk electrode. Full experimental details of the material preparation and characterization are described in the Supporting Information.

3. RESULTS AND DISCUSSION 3.1 Effect of high Pt-loading on unimodal porous carbon particles. In our previous study, we found that porous carbon particles with a pore size of approximately 120 nm showed an optimum morphology for Pt deposition (15 wt%) and featured higher electrocatalytic performance than that of other structures (i.e. dense and hollow), as shown in Supporting Information Figure S1(a,b).27 Therefore, this porous carbon particle structure was further investigated in this study for mobile PEMFC catalyst applications which usually have a high Pt loading. To evaluate the ability of the porous carbon particles to accommodate a high Pt loading, in this study, various amounts of Pt NPs (i.e., 15, 30, and 40 wt%) were deposited onto the particles (indicated as Pt/CSP 15 wt%, Pt/CSP 30 wt%, and Pt/CSP 40 wt%, respectively). Our SEM and TEM studies, shown in Supporting Information Figure S1(c-h), indicated that the Pt NPs tended to agglomerate at higher Pt loading. In the case of a 15-wt% Pt loading, the Pt NPs were welldispersed, as shown in Supporting Information Figure S1(c,f). These results indicate that the


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carbon featured an adequate active surface area for Pt NP deposition up to 15 wt%; however, when the amount of loaded Pt NPs was increased further, agglomeration occurred, as shown in Supporting Information Figure S1(d,g) and (e,h). High-resolution TEM images, inset in Supporting Information Figure S1(f), (g), and (h), revealed that the size of individual Pt NPs increased as the Pt loading increased owing to Pt agglomeration, which in turn decreased the active surface area for electrochemical catalytic reactions. This result was confirmed from XRD results, as shown in Supporting Information Figure S1(i). The electrochemical characteristics of the prepared Pt/C catalysts were investigated by CV and oxygen reduction reaction (ORR) polarization curves measured under the same conditions. The CV adsorption–desorption characteristics of the Pt/CSP catalysts with various Pt loadings are shown in Supporting Information Figure S2(a). The calculated electrochemical surface areas (ECSAs) were 102, 64, and 45 m2 g−1-Pt for the Pt/CSP 15 wt%, Pt/CSP 30 wt%, and Pt/CSP 40 wt% catalysts, respectively. The Pt/CSP 15 wt% catalyst had the highest ECSA value, despite also having the lowest Pt-loading. This observation confirmed that Pt agglomeration decreased the active surface area of the Pt NPs. Theoretically ECSA can be calculated by ECSA = 6/(ρ x dc), where ρ and dc are the platinum density and particle diameter, respectively, assuming spherical particles. The active surface area of unimodal porous carbon with Pt-loading of 15, 30, and 40 wt.% were determined to be 81%, 64%, and 50%, respectively, as calculated from the ratio of the real and theoretical ECSAs. Typical ORR polarization curves for the unimodal porous Pt/CSP 15 wt%, Pt/CSP 30 wt%, and Pt/CSP 40 wt% catalysts, measured after 50 CV cycles, at a rotation speed of 1600 rpm, are shown in Supporting Information Figure S2(b). At an electrode rotational speed of 1600 rpm, the diffusion-limiting current density (Jlim) of materials that support direct four-electron transfer, i.e., Pt-based electrodes, is typically in the range of −5 to −6 mA cm−2. Pt/CSP catalysts


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with various Pt loadings reached the diffusion-limiting current. The Pt/CSP 15 wt% catalyst shows the highest Jlim at −5.6 mA cm−2, which was close to the theoretical diffusion-limiting current (−5.7 mA cm−2), followed by Pt/CSP 30 wt%, and Pt/CSP 40 wt% catalysts, respectively. The onset potentials of the catalyst were also slightly different at 0.98, 0.99, and 1 V for the Pt/CSP 15 wt%, Pt/CSP 30 wt%, and Pt/CSP 40 wt% catalysts, respectively. The ORR activity tests were performed at different rotation speeds (i.e., 400, 900, 1600, 2500, and 3600 rpm) to obtain Koutecký–Levich plots, as shown in Supporting Information Figure S2(c). The target potential for calculation of the mass and specific activities was E = 0.9 V. The number of electrons involved in the ORR was calculated from the Koutecký–Levich equation; four electrons were involved for all the unimodal porous Pt/C catalysts. The Pt mass activity (MA) values were calculated by normalizing the Pt loading on the disk electrode. As expected from the ORR polarization curves, the Pt/CSP 15 wt% catalyst showed the highest mass activity of 253 mA mg−1-Pt. The mass activity value of Pt/CSP 30 wt% was about 242 mA mg−1-Pt. At a higher Pt loading, the mass activity of Pt/CSP 40 wt% greatly decreased to half that of Pt/C15 wt% and Pt/C30 wt%, at approximately 145 mA mg−1-Pt. Theoretically, Pt loading is proportional to the electrocatalytic activity, because Pt is the main catalyst for electron formation. However, the dispersion of Pt NPs, which is influenced by the morphology of the supporting particle, also has a considerable effect on electrocatalytic performance. A low Pt loading of 15 wt% showed the best charge transfer and highest number of active Pt sites. In other words, the number of active Pt sites was reduced as Pt loading increased because of agglomeration. Nevertheless, these values were higher than or comparable to the MA value of a commercial Pt/C catalyst (Pt 46.1 wt%, Tanaka Kikinzoku Kogyou Co., Ltd.).26 This result may


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be attributed to carboxyl groups on the surfaces of the carbon materials, which improve binding between the Pt NPs and the carbon surface to enhance electrocatalytic activity.5 3.2 Modified hierarchical bimodal macroporous Pt/C catalysts. In the previous section, we showed that the structure of the catalyst support and dispersion of Pt are critical for ion and fluid transport to and from catalytic sites. Furthermore, these factors also determine the amount of active Pt sites formed in the catalyst. Porous carbon has been shown to have better performance and durability than commercial Pt/C catalysts, which use carbon NP catalyst supports.26 However, to date, this enhanced performance was only apparent at low Pt catalyst loading. When the amount of deposited Pt NPs was increased, the active surface area was not sufficient to ensure even Pt deposition and agglomeration of Pt occurred. This agglomeration decreased the active surface area of the Pt. Thus, a new porous configuration was necessary to accommodate a larger amount of Pt without blocking the flow of fluid.

Figure 1 Schematic of the experimental procedure for formation of (a) unimodal and (b) bimodal macroporous carbon nanospheres.


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Hierarchical bimodal macroporous carbon particles may address the issue. Large macropores may facilitate rapid transport of fluid and Pt penetration into the interior of carbon particles, while smaller macropores increase the total surface area available for Pt deposition. Such porous carbon particles can be obtained from the use of negatively charged PSL particles of different sizes, as templates during synthesis. Phenolic resin and negatively charged PSL particles will independently move and scatter in the droplet. In the case of PSL particles of one size, the crosslinked phenolic resin will pack and self-assemble in a droplet through solvent evaporation. The resulting phenolic resin-PSL composite will feature half of the PSL particles on the surface of the composite, as shown in Figure 1(a). When smaller PSL particles are added into the precursor mixture, the small PSL particles can fill the voids between the larger PSL particles. This phenomenon creates a chain-like configuration, in which one large PSL is surrounded by several small PSL particles to produce layered porous carbon particles after the final pyrolysis process, as shown in Figure 1(b).29 To evaluate the effect of bimodal pore sizes, small PSL spheres with sizes of 40, 90, or 120 nm were mixed with the larger 230-nm PSL spheres and the obtained carbon particles were named MP40, MP90, and MP120, respectively. Figure 2 shows that PSL particles of different sizes greatly influenced the morphology of the resulting carbon particles. The MP40 sample showed irregular interstices surrounding large spherical pores on the surface of the spherical carbon particles, as shown in the SEM image in Figure 2(a). The presence of interstices was confirmed from the TEM image in Figure 2(d). Interestingly, the obtained porous carbon particle showed a hollow morphology with a rough shell and consisted of aggregated nanoparticles. Figure 2(b) clearly shows small spherical pores surrounding larger spherical pores over the surface of the MP90 carbon particles. The TEM image in Figure 2(e) confirmed the formation of


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hierarchical porous carbon and the formation of flower-like spherical carbon particles. This morphology may provide a low resistance to ion and fluid transport during electrocatalytic processes as well as a higher specific surface area, compared with that of MP40. The addition of larger secondary PSL particles (~120 nm, MP120) resulted in spherical carbon particles. This result may be explained by the total volume of the two large PSL spheres occupying more of the droplet space, such that the phenolic resin could only move into a limited space between the PSL particles in the droplet. The phenolic resin could only form a thin layer between the template PSL particles and after the decomposition of PSL, porous carbon particles with thin layers were formed, as shown in Figure 2(c). A TEM image of Figure 2(f) shows that the hierarchical porous particles formed with overlapping pores and some pores were not completely spherical owing to intersection with one another. These channel pores are critical, because this permeable layer allows fluid to be transported between the porous core and the exterior of particles, thus enlarging the three-phase boundary among the catalyst, gas phase, and electrolyte. The structural properties of the obtained carbon particles were also evaluated by N2 adsorption–desorption measurements as shown in Figure 2(g). The isotherm curves of both the unimodal and bimodal porous carbon particles showed an initial rapid rise in the adsorbed gas volume with increasing relative pressure, followed by a slow increase, i.e., type II isotherm curves. The inset in Figure 2(g) shows the inflection point corresponding to both completion of monolayer coverage and pore filling by capillary condensation. The MP90 carbon particle adsorbed the highest volume of N2, followed by the MP40, SP, and MP120 carbon particles. The isotherm curve of the unimodal and bimodal porous carbon showed type A hysteresis, which was attributed to the presence of cylindrical pores open at both ends.30 The specific surface areas


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(SBET) of the particles were calculated to be 70, 132, 139, and 61 m2 g−1 for the SP, MP40, MP90, and MP120 carbon particles, respectively.

Figure 2. SEM and TEM images of bimodal porous C particles with PSL sizes of: (a, d) 230 and 40 nm, MP40, (b, e) 230 and 90 nm, MP90, (c, f) 230 and 120 nm, MP120; (g) N2 adsorption– desorption isotherm curves of unimodal (SP) and bimodal porous C particles; and (h) mesopore size distributions of unimodal (SP) and bimodal porous C particles by Barrett-Joyner-Halenda (BJH) method.31


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The meso- and small macro-pore size distribution curves of various hierarchical porous carbon particles are calculated using BHJ model based on the Kelvin model of pore filling which is the most applicable for the measurement of mesopore size range and the results are shown in Figure 2(h).31 The SP carbon particles showed large pore volumes from pores < 5 nm in diameter, which formed during the phenolic resin decomposition, and pores of ~100 nm, which were likely related to the template particles (i.e. 120 nm of PSL). The MP40 carbon particles showed formation of hierarchical mesopores along with secondary pores of ~70 nm induced by the 40-nm PSL template. The MP90 carbon particles showed the widest ranging hierarchical pore structure. These particles featured pores distributed in the mesoporous (25–35 nm) and macroporous (>50 nm) size regimes. Both these peaks featured high intensities and contributed a considerable volume. Similar to SP carbon particles, MP120 carbon particles featured a relatively narrow pore size distribution, a high contribution to the pore volume from pores < 5 nm, and their secondary pores formed at around 90 nm owing to the incorporation of 120 nm PSL particles. 3.3 Electrocatalytic activities of hierarchical bimodal porous Pt/C catalysts. The obtained hierarchical bimodal macroporous carbon particles were functionalized by depositing a large amount of Pt (40 wt%) to form Pt/CMP40, Pt/CMP90, and Pt/CMP120 catalysts, as shown in Figure 3 (a-f). The Pt NPs were well dispersed on the surface of MP90 as shown in Figure 3(b). However, the Pt NPs tended to agglomerate on some parts of the MP40 and MP120 surfaces as shown in Figure 3(a) and (c), respectively. To analyze the dispersion of Pt we performed TEM and high resolution TEM analysis. Figure 3 (d-f) shows that the Pt NPs were dispersed over the entire surface of the hierarchical bimodal macroporous carbon particles and also coated cavities. High resolution TEM images, shown inset in Figure 3 (d-f), confirmed that Pt deposited on the surface


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of MP90 was well-dispersed; however, Pt showed a tendency to agglomerate on the surfaces of MP40 and MP120. The MP40 particles may provide a high specific surface area for Pt deposition; however, smaller pores may have trapped the Pt NPs leading to agglomeration. Unlike the MP40 particles, the MP90 particles featured a path that allowed for Pt penetration into the core of the particles through large pores and featured a high internal surface area, from smaller pores, for Pt deposition. The small pores were larger than the mesopore scale and the surface of the deposited Pt NPs was activated. In the case of MP120, although Pt penetrated into the core of the particles, there was not enough space for Pt deposition owing to the relatively low specific surface area; therefore, the particles agglomerated. Figure 3(g) shows the CV adsorption–desorption characteristics of the MP40, MP90, and MP120 Pt/C catalysts. The calculated ECSAs were 34, 68, and 41 m2 g−1-Pt for the Pt/CMP40, Pt/CMP90, and Pt/CMP120 catalysts, respectively. The ECSA value of the Pt/CMP40 catalyst was lower than that of Pt/CMP120, and that of the Pt/CMP90 catalyst was the highest, almost twice as large as those of the other catalysts. The effects of Pt agglomeration occurring in Pt/CMP40 and Pt/CMP120 likely made a small contribution to the lower ECSA values; however, the lack of an interconnected bimodal pore system also contributed to this result. Typical ORR curves for those Pt/CMP catalysts, recorded after 50 CV cycles at a rotation speed of 1600 rpm, are shown in Figure 3(h). Although the three catalysts reached the diffusionlimiting current, the half-wave potential (E1/2, the point half way between zero current and the diffusion-limited current density plateau Jlim) of the Pt/CMP40 and Pt/CMP120 was less than 0.9 V. This shows that interference from mass-transport losses occurred below E = 0.9 V. Therefore, the mass activity values of Pt/CMP40 and Pt/CMP120 were low, corresponding to values of about 90


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and 81 mA mg−1-Pt, respectively. The mass activity of Pt/CMP90 was greater than those of other catalysts at about 347 mA mg−1-Pt.

Figure 3. SEM and TEM images of bimodal macroporous Pt/C catalyst with Pt loading of 40 wt% and PSL sizes of: (a, d) 230 and 40 nm, Pt/CMP40, (b, e) 230 and 90 nm, Pt/CMP90 (c, f) 230 and 120 nm, Pt/CMP120; (g) CV of the catalysts after 50 cycles in oxygen-free 0.1 M HClO4 (cycling between 0 and 1.2 V at a sweep rate of 50 mV∙s−1); (h) ORR polarization curves at rotation rate of 1600 rpm after 50 cycles in oxygen-saturated 0.1 M HClO4 at a sweep rate of 10 mV∙s−1.


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The low catalytic activity of Pt/CMP40 could be attributed to partial blocking of active catalytic sites, likely caused by microbubbles formed by cavitation during the ORR activity tests. As mentioned previously, the Pt/CMP40 catalyst featured more micro- and meso-pores. The microbubbles likely blocked some of the micropores, which affected the hydrodynamics of the Pt/CMP40 catalyst. The microbubbles remained unbroken because of the strong convective force in the axial direction at high angular velocities. In addition, low charge transfer as a result of the low overpotential (ߟ) likely contributed to microbubble preservation.27 The low catalytic activity in the case of Pt/CMP120 can be attributed mainly to Pt agglomeration. These results suggest that the hierarchical morphology of the carbon catalyst support plays a crucial role in determining the electrocatalytic performance. Porous carbon with appropriate hierarchical macro-pores showed the best charge transfer and featured more Pt NP active sites than any other type of catalyst. The hierarchical porous nanostructure ensured fast ion diffusion through a reduced diffusion pathway. This structure also featured macroporous frameworks that acted as ion-buffering reservoirs, mesoporous walls that allowed for fast ion transmission, and a microporous texture for charge accommodation. Furthermore, these particles had continuous electron pathways, which are also important for achieving high-rate performances. 3.4 Effects of Pt loading on electrocatalytic activity. The MP90 carbon particle showed the greatest potential for high Pt loading hence we attempted to further optimize the performance of this catalyst support under various Pt loadings. The electrocatalytic activities of Pt/CMP90 catalysts with Pt loading of 15, 30, and 40 wt% were evaluated. Figure 4 shows the SEM and TEM images of the Pt/CMP90 with various Pt loadings. The dispersion of Pt NPs was inversely proportional to the Pt loadings as the particles tended to agglomerate with increasing Pt loading.


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The sizes of individual Pt NPs were approximately 2.2, 3.8, and 3 nm for Pt/CMP90 15, 30, and 40 wt%, respectively. Figure 5(a) shows the CV adsorption–desorption characteristics of the three Pt/CMP90 catalysts. Among these curves, that for Pt/CMP90 with a Pt loading of 15 wt% showed the largest Pt–H adsorption and H2 evolution peaks. The ECSA was calculated to be 96, 62, and 68 m2 g−1Pt for Pt loading of 15, 30, and 40 wt%, respectively. The highest ECSA was obtained at a Pt loading of 15 wt%, showing that optimal contacts among the gas fuel, catalyst, and membrane were obtained under these conditions. The ECSA values of the two Pt/CMP90 catalysts with Pt loadings of 30 and 40 wt% were similar and although not as high as that of Pt/CMP90 15wt%, the values were acceptable.

Figure 4. SEM and TEM images of bimodal-porous Pt/C catalyst (Pt/CMP90) with Pt loadings of: (a, d) 15 wt%, (b, e) 30 wt%, and (c, f) 40 wt%. Figure 5(b) shows the oxygen reduction polarization curves measured after 50 CV cycles at a rotation speed of 1600 rpm. All three catalysts showed a similar mixed kinetic–diffusioncontrolled region, at E = 0.9 V. However, Pt/CMP90 with a Pt loading of 30 wt% showed the


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lowest diffusion-limited current density plateaus, indicating some contributions from O2 diffusion through the Nafion film in the testing of this catalyst. The Koutecký–Levich plots are shown in Figure 5(c) and were used to calculate the mass and specific activities. The mass activity values quantified at E = 0.9 V for Pt loading of 15, 30, and 40 wt% were 378, 320, and 347 mA mg−1-Pt, respectively, as shown in Figure 5(d).

Figure 5. (a) CV of Pt/CMP90 with various Pt loading after 50 cycles in oxygen-free 0.1 M HClO4 (cycling between 0 and 1.2 V at a sweep rate of 50 mV∙s−1) (b) ORR polarization curves, (c) Koutecký-Levich plots measured after 50 cycles in oxygen-saturated 0.1 M HClO4 at a sweep rate of 10 mV∙s−1 and (d) ECSA and mass activity values.


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The above results show that the ECSA and MA of Pt/CMP90 with high Pt loadings of 30 and 40 wt% were relatively stable and slightly lower than that of Pt/CMP90 15 wt%. All these values were higher that of unimodal porous Pt/C at the same Pt loading, suggesting that the bimodal porous network may have a larger surface area available for Pt deposition. These features contribute to enhanced electrocatalytic performance through a larger active surface area and undisrupted hydrodynamic flow. 4. CONCLUSION Hierarchical bimodal macroporous carbon nanospheres with an interconnected pore system were synthesized in this study and applied as a 3D catalyst for PEMFCs. The Pt/C catalysts were prepared by a hybrid method involving hierarchically nanostructured porous carbon particles prepared by aerosol spray pyrolysis followed by microwave-assisted Pt deposition. The effects of various hierarchical porous structures on the Pt deposition and the catalytic performance of Pt/C electrocatalysts were evaluated. Electrochemical characterization of the catalysts showed that a hierarchical bimodal-porous Pt/C catalyst with a mixture 30- and 120-nm pores (MP90) gave higher performance than that of other bimodal-porous and porous Pt/C catalysts. Furthermore, the ECSA and MA values of Pt/CMP90 with high Pt loadings of 30 and 40 wt% were slightly lower than that of Pt/CMP90 15 wt% but showed good stability. The ECSA and MA values for Pt/CMP90 40 wt% were approximately 68 m2 g−1-Pt and 347 mA mg−1-Pt, and 96 m2 g−1-Pt and 378 mA mg−1-Pt for Pt/CMP90 15 wt%, respectively. These results indicate that our bimodal macroporous network may allow for high Pt loading and enhance electrocatalytic performance by providing a greater active surface area and supporting hydrodynamic flow. ASSOCIATED CONTENT


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Supporting Information Details about experimental procedure; SEM, TEM, XRD, and electrocatalytic properties of Pt deposited on unimodal porous carbon particles (Pt/CSP); and comparison of electrocatalytic properties of Pt deposited on bimodal macroporous carbon particles (Pt/CMP) and commercial Pt/C are available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *Address: Department of Chemical Engineering, Hiroshima University, Higashi Hiroshima 7398527, Japan. Phone: +81-82-424-7850. Fax: +81-82-424-7850. E-mail: [email protected] and [email protected]. ACKNOWLEDGMENT This research was supported by a Grant-in-Aid for Young Scientists B (15K18257) and Grant-inAid 26709061 and 16K13642 sponsored by the Ministry of Education, Culture, Sports, Science, and Technology of Japan. This work is partly supported by the Center for Functional Nano Oxide at Hiroshima University. The authors would like to thank Mr. Takahiro Mori for experimental assistance and Sumitomo Bakelite Co., Ltd., for supplying the phenolic resin. REFERENCES (1)

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