Improved Durability of Electrocatalyst Based on Coating of Carbon

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Improved Durability of Electrocatalyst Based on Coating of Carbon Black with Polybenzimidazole and their Application in Polymer Electrolyte Fuel Cells Tsuyohiko Fujigaya, Shinsuke Hirata, Mohamed R. Berber, and Naotoshi Nakashima ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01316 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 28, 2016

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ACS Applied Materials & Interfaces

Improved Durability of Electrocatalyst Based on Coating of Carbon Black with Polybenzimidazole and their Application in Polymer Electrolyte Fuel Cells Tsuyohiko Fujigaya*,†,‡,  Shinsuke Hirata † Mohamed. R. Berber,‡, § and Naotoshi Nakashima*,†,‡ †

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744

Motooka, Nishi-ku, Fukuoka, 819-0395, Japan. ‡

The World Premier International Research Center Initiative, International Institute for Carbon

Neutral Energy Research (WPI-I2CNER), Kyushu University 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan. §

Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt

JST-PRESTO,

4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan

KEYWORDS; Carbon Black, Polybenzimidazole, Electrocatalyst, Fuel cells, Durability

ACS Paragon Plus Environment

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Abstract Improvement of durability of the electrocatalyst has been the key issue to be solved for the practical application of polymer electrolyte membrane fuel cells (PEMFCs). One of the promising strategies to improve the durability is to enhance the oxidation stability of the carbon-supporting materials. In this report, we describe in details the mechanism of the stability improvement of carbon blacks (CBs; Vulcan and Ketjen) by coating with polybenzimidazole (PBI). Nitrogen adsorption experiments reveals that the PBI coating of CBs results in the capping of the gates of the CBmicropores by the PBI. Since the surface of the micropores inside the CBs are inherently highly oxidized, the capping of such pores effectively prevents the penetration of the electrolyte into the pore and works to avoid the further oxidation of interior of the micropore, which is proved by cyclic voltammogram measurements. Above mechanism agrees very well with the dramatic enhancement of the durability of the membrane electrode assembly (MEA) fabricated using Pt on the PBI-coated CBs as an electrocatalyst compared to the conventional Pt/CB (PBI-non coated) catalyst.

Introduction Polymer electrolyte membrane fuel cells (PEMFCs) have been receiving a great deal of attention as an energy source in the next generation due to their high-energy conversion efficiencies and high power densities.1-2 One of the key issues to be solved toward the commercialization of the PEMFCs is to improve the durability of the PEMFCs. 3 Especially, it has been pointed out that degradation of the PEMFC electrocatalyst severely limits the durability of the PEMFCs.4-7 Hence,


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the present challenge is to develop an electrocatalyst with a high stability in the PEMFC environment. Currently, the electrocatalysts composed of platinum nanoparticles (Pt-NPs) and carbon blacks (CBs)8-9 have been widely used,10 in which sintering and/or the dissolution of the Pt-NPs as well as the oxidative corrosion of the CBs are the main reasons for the catalyst degradation.3-6,

11-15

Since sintering and dissolution of the Pt-NPs are often triggered by the

corrosion of the supporting materials,16-17 durability of the electrocatalysts largely depends on the stability of the CBs. Basically, CBs are composed of a graphitic structure (sp2 carbon) with defect sites (sp3 carbons) covalently bonded to hydrophilic functional groups, such as epoxy, –COOH and –OH. It is recognized that the electrochemical oxidation of the carbon occurs more easily at the sp3 carbons than that at the sp2 carbons, leading to a performance decrease by provoking sintering and dissolution of the Pt-NPs.18-19 One of the strategies to improve the oxidation stability of the CBs is graphitization of CBs by thermal treatment under an inert atmosphere 20-21 or the substitution of the CBs by highly graphitized materials, such as carbon nanotubes or graphene. 22 However, since hydrophilic groups play an important role to bind Pt-NPs for stable loading on the carbon supports, the use of the highly graphitized carbon causes an inhomogeneous loading of the Pt-NPs on such supports.23 In this study, we explore a novel approach to improve the oxidation stability of the CBs by polymer coating in which polybenzimidazoles (PBIs) are used as the polymer. The PBI is chosen as the coating polymer owing to the following unique features; namely, i) it interacts with both graphitic surfaces and hydrophilic functional groups, such as –COOH and –OH to form a stable coating layer on the surfaces of the CBs, ii) it strongly interacts with Pt ions through coordination and can serve as the binding sites of Pt-NPs,24-25 and iii) it is stable in both strong acidic and alkaline conditions and utilized for both acid- and alkaline-type PEMFCs.26 In this study, the


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mechanism of the improvement of the durability was investigated using two different CBs, i.e., Vulcan and Ketjen, having a different degree of oxidation. Finally, we describe the fabrication of a membrane electrode assembly (MEA) using the PBI-coated CBs as the supporting materials and demonstrate the dramatic improvement in the durability of the PEMFC.

Results and discussion Structure and properties. The coating of Vulcan by PBI was carried out by a simple mixing of the Vulcan and PBI in N,N-dimethylacetamide (DMAc) (Fig. 1a).27 Successive vigorous washing with DMAc after filtration removed the unbound PBI to obtain only the PBI-wrapped Vulcan (Vulcan/PBI). Although Vulcan was well dispersed in water (Fig. 1b, left) because it carries hydrophilic moieties, such as carboxylic acid and hydroxyl group, dispersibility of the Vulcan/PBI in water became very poor, suggesting the successful coating of Vulcan by hydrophobic PBI (Fig. 1b, right). The X-ray photoelectron spectroscopy (XPS) diagram of the Vulcan/PBI shows a new peak at 400 eV that originated from the N1s of PBI after the coating (Fig. 1c). The thermogravimetric analysis (TGA) curve of the Vulcan/PBI shows a one-step weight loss similar to that of Vulcan (Fig. 1d). Since the estimation of PBI content in the Vulcan/PBI was difficult unlike the PBI-coated CNTs that showed the clear two-step weight loss corresponding to the thermal decomposition of the PBI and CNTs,25 the ratio was estimated by elemental analysis (see Supporting Information, Table S1). By considering the all the nitrogen was come from the PBI and carbon was come from both PBI and Vulcan, PBI content was determined to be 6.3 wt%. The PBI-coated Ketjen (Ketjen/PBI) was prepared in a similar manner and the PBI content was determined to be 22.5 wt% (see Supporting Information, Table S1). The difference of the PBI


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content can be explained by the difference of the surface area of Vulcan and Ketjen.28 Direct observation of the PBI coating using the electron microscope technique is now ongoing.

Electrochemical durability of PBI-coated CB. Electrochemical durability of the PBI-coated CBs (CB/PBI) was measured by a repeated potential cycling from/to 1.2 to/from 1.7 V to accelerate corrosion of the carbon materials. As shown in Figs. 2a and 2c, after the durability tests, the double layer capacitance of the Vulcan and Ketjen dramatically increased with increasing of the cycle numbers. In sharp contrast, as shown in Figs. 2b and 2d for the Vulcan/PBI and Ketjen/PBI, no such change was recognized (for enlarged current scales, see Supporting Information, Fig. S1). In Fig. 2, the peaks observed at 0.60  0.65 V in the forward scan and 0.40  0.45 V in the reverse scan are attributed to the generation of quinone moieties on the CBs and their reduction, respectively.29-31 Such peaks are pronounced for the non-coated Vulcan and Ketjen (see Supporting Information, Fig. S2), clearly indicating that the oxidation durability of the Vulcan and Ketjen are dramatically improved after the PBI coating. As can be seen in Figs. 2a and 2c, the Ketjen showed a more enhanced current than that of the Vulcan, which is due to higher surface areas of the Ketjen (1270 m2/g) derived from the greater number of micropores generated by oxidation 28 than that of the Vulcan (254 m2/g).28 The XPS of the Vulcan, Vulcan/PBI, Ketjen and Ketjen/PBI before and after the durability tests were measured. As shown in Figs. 2e and 2g, for the Vulcan and Ketjen, the peak at around 289.4 eV that originated from the C 1s of the COO moiety was observed after the durability tests, whereas no such peak was observed for the Vulcan/PBI and Ketjen/PBI (Figs. 2f and 2h). 32 In addition, the peak at 285.0 eV originated from the C-O become much pronounced for Ketjen (Fig. 2g, red line) due to the more sp 3-rich structure


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of Ketjen. These results clearly indicated that the electrochemical oxidation of the Vulcan and Ketjen was significantly suppressed by the PBI-coating. However, despite the clear difference observed in the XPS results, the TEM observations after the durability tests did not provide any significant morphological difference (see Supporting Information, Fig. S3), suggesting that even after the durability test, corrosion of these materials did not severely occur under the given experimental conditions. To explore the mechanism of the durability enhancement in the PBI-coated CBs, the surface structures of the Vulcan and Ketjen with and without the PBI-coating were investigated using gas adsorption measurements. Figs. 3a and 3b show the N 2 adsorption and desorption profiles of the 4 different materials, in which the surface areas of the Vulcan (206 m2/g) and Ketjen (1338 m 2/g) agreed well with the reported values. 28 The surface areas of Vulcan/PBI and Ketjen/PBI were found to decrease due to the PBI-coating, and their values were 87 and 1126 m 2/g, respectively. The pore size analysis based on the HorvathKawazoe (HK) method 33 revealed that the decrease in the surface areas after the PBIcoating was mainly caused by the decrease in the micropore areas, especially below 1.0 nm as shown in Figs. 3c and 3d. Quantitative analysis of the micropore area based on the t plots method34 provided that the micropore areas decreased after the PBI-coating from 59 to 10 m2/g (decreased by 83%) and 919 to 715 m 2/g (decreased by 22%) for the Vulcan and Ketjen, respectively, while the rest of surface (= mesopore + outer surfaces) decreased after the PBI-coating from 147 to 77 m 2/g (decreased by 47%) and 419 to 411 m 2/g (decreased by 2%) for the Vulcan and Ketjen, respectively (Figs. 3e and 3f). When we postulate that the PBI homogeneously coated the entire surfaces of the Vulcan and Ketjen, the thickness of the PBI layer was estimated to be 0.23 and 0.13 nm for the


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Vulcan/PBI and Ketjen/PBI, respectively, using the density of 1.33 mg/cm 3 of PBI. However, these values were lower than the monolayer thickness of the PBI and thinner than those of PBI coated on the CNTs (1.2 nm) and graphene (1.1 nm) as previously reported. 3536

Based on this analysis, we provide a schematic drawing for the PBI-coated CBs (Vulcan

and Ketjen) in Fig. 3g, in which the PBI preferentially coated the outer surfaces and the interior of mesopore surface of the CBs, but did not coat the micropore surface . As the consequence, the gate of the micropore was covered by PBI. This model well explains the significant decrease in the micropore volume as already discussed. The capping of the gate of the narrow pore was often observed in the porous membrane systems,37 which strongly supports the schematic drawing shown in Fig. 3g. Since the degree of defect on the surfaces of the inner micropores was reported to be higher than that of the outer ones, 38 the capping is quite effective to protect the inner micropores from the penetration of the electrolyte and to prevent oxidation of the CBs. Electrochemical durability for CB/PBI/Pt. In order to investigate the effect of the improved stability of CB-support on real PEMFC durability, we loaded Pt-NPs on the Vulcan/PBI (donated Vulcan/PBI/Pt, Fig. 4a) using the conventional polyol method as previously reported. 25 Vulcan/PBI was chosen for further investigation owing to their superior durability than Ketjen/PBI. Similar to Pt-NPs directly loaded on the Vulcan (Vulcan/Pt, Fig. 4b), the Vulcan/PBI/Pt showed a homogenous loading of uniform Pt-NPs (Fig. 4c) since PBI has an excellent binding property through its chelating effect of benzimidazole structure that facilitate the homogeneous growth of Pt-NPs similar to the oxidized surfaces of the carbon materials (for XPS N1s narrow scans, see Supporting Information, Fig. S4).24-25


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Indeed, the loading amount of Pt-NPs on Vulcan/PBI determined from the residual amount at 900 °C in TGA was 41.9%, manifesting the quantitative amount of fed Pt ion was loaded as Pt-NPs similar to the loading on Vulcan/Pt showing 44.0 wt% loading (see Supporting Information, Fig. S5a)27 The size of the Pt-NPs on the Vulcan/PBI/Pt was 2.9 ± 0.8 nm, which was close to that of the Vulcan/Pt (2.8 ± 0.5 nm). In addition, the XPS scans (Figs. S5 b-d) of the doublet peaks at 71.1 and 74.4 eV attributed to the Pt 4f 7/2 and Pt 4f5/2, respectively, exhibited similar composition ratios of Pt(0), Pt(II) and Pt(IV) for both the Vulcan/Pt (57.8/26.2/16.0) and Vulcan/PBI/Pt (52.9/22.9/24.2) as summarized in Fig. 4d.39 Therefore, it is worth emphasizing that the comparisons were carried out under the fair conditions in terms of the Pt size and the electronic state (for X-ray diffraction (XRD) profiles, see Supporting Information, Fig. S6). Fig. 5 shows the CV curves of the Vulcan/Pt (Fig. 5a) and Vulcan/PBI/Pt (Fig. 5b) catalysts. The electrochemically active surface area (ECSA) was calculated from the area of the characteristic peaks in the negative region (from 0.0 to 0.3 V vs. RHE) attributed to atomic hydrogen adsorption of the Pt-NPs on the catalysts based on the following equation. 40

ECSA = QH/(210  Pt loading on electrode)

(1)

where QH is the charge exchanged during the electro-adsorption of hydrogen on the surface of Pt-NPs. ECSA value of the Vulcan/PBI/Pt (46.0 m 2/gPt) was similar to that of the Vulcan/Pt (47.1 m 2/gPt), which were increased gradually in the beginning 100-200 cycles probably due to the cleaning of the Pt surface. Evidently, the Vulcan/PBI/Pt showed a


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slower decreasing rate of ECSA (from 53 to 28 m 2g-1/2000 cycles) compared to that of the Vulcan/Pt (from 48 to 33 m 2g-1/2000 cycles) (Fig. 5c) as we previously reported.41 Similar trend was also recognized in the slower decreasing of the reduction of Pt-oxide peak at around 0.7 V (vs. RHE) for Vulcan/PBI/Pt than Vulcan/Pt in Figs. 5a and 5b. Judging from the TEM images of the Vulcan/PBI/Pt and Vulcan/Pt after durability showing the only slight aggregation for both samples (see Supporting Information, Fig. S7), we considered that the major ECSA deterioration under the present condition (1.2  1.7 V) was not the Ostwald ripening but the oxidative carbon decomposition and/or Pt dissolution. To evaluate the Pt dissolution, quantitative analysis of the electrolyte solution after the durability test was necessary. Fuel cell performance measurements. PEMFC durability tests using membrane-electrode assemblies (MEAs) were carried out based on the protocol of Fuel Cell Commercialization Conference of Japan (FCCJ). 42 The MEA composed of the Nafion and Vulcan/Pt as the membrane and electrocatalyst, respectively (Nafion-MEAnon-coat), shows a faster drop of the cell voltage upon the potential cycles between 1.0 – 1.5 V (green circles in Fig. 5d), whereas the MEA composed of the Vulcan/PBI/Pt (Nafion-MEAcoat) shows the slower deterioration for the cell voltages (red circles in Fig. 5d). 43 Since it is known that the Pt dissolution and the aggregation were often triggered by the corrosion of the carbon support materials, 16-17 the observed improved durability of the Vulcan/PBI/Pt was attributed to the improvement of the stability of the Vulcan by the PBI-coating as discussed above. Indeed, TEM images of Vulcan/Pt and Vulcan/PBI/Pt after ADT showed the large decrease of the particle number of the Pt-NP.44 Thus, we assumed that the oxidative corrosion of the Vulcan accelerated the oxidative dissolution of Pt-NP. Although, clear correlation between the


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results obtained using this protocol and actual PEMFC durability under typical operating cycles is not clear yet, the difference is quite obvious. Chen et al. reported the durability improvement of CB/Pt by coating with polyaniline (PANI) polymerized on the surface of CB,44 and described that the electron delocalization between the d orbital of Pt and the PANI was important for the stabilization of Pt. Such an electronic interaction betwee n PtNPs and polymer would also contribute to durability of our PBI-coated catalysts. We previously reported that significant loss of Pt-NPs after the durability testing was observed for the Nafion-MEAnon-coat, whereas the Nafion-MEAcoat showed only slight increase of PtNP size.43 One of the possible explanation for such difference would be due to the strong binding between the PBI and the Pt-NPs through the coordination of the Pt-N. The present study suggested one more possible mechanism; namely, the PBI serves to prevent the oxidation of the Vulcan and prevent the loss of Pt, leading to a prolonged durability. XPS spectra of the electrocatalyst before and after the durability testing showed the C -C peak at 284.2 eV in the Vulcan/Pt decreased from 65% to 50%, while that of the Vulcan/PBI/Pt remained almost constant (from 58% to 61%, see Supporting Information, Fig. S8 and Table S2). These results clearly indicated that the carbon in the Vulcan/Pt was severely oxidized to form C-O (285.2 eV), C=O (286.0 eV) and COO (289.4 eV) species compared to the Vulcan/PBI/Pt. In the next generation PEMFCs, the temperature and humidity of the cell operation is expected to shift to high temperatures over 100 °C and non-humidified condition since such condition is promising to lower the poisoning effect by CO and simplify the PEMFC systems due to easy water management.45 To explore the possibility of the present finding for the next generation PEMFC, we incorporated Vulcan/PBI/Pt in MEA using a


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phosphoric acid (PA)-doped PBI as the electrolyte membrane since the PA-doped PBI showed a high proton conductivity at the condition and often used for high-temperature PEMFCs.46-47 It is pointed out that the higher temperature operation decreased the durability of the material especially for the carbon supporting material due to the more severe oxidative condition. 48 Thereby, the durability of the carbon supporting becomes more important. The durability testing of the MEAs were carried out based on the protocol of FCCJ42 under a non-humidified condition at 120 °C. As the results, the MEA using the Vulcan/PBI/Pt (PA-MEAcoat) exhibited a slower decrease in the cell potential compared to the MEA using the Vulcan/Pt (PA-MEAnon-coat) (see Supporting Information, Fig. S9). Since PBI is not expensive and the amount of PBI used this coating is not large, it is also attractive in terms of the cost. Therefore, we concluded that the simple PBI-coating leads to a dramatic durability-enhancement of the MEA without increasing cost. Detail analysis of the loading state in the pore is now investigating using 3D TEM tomography.

Conclusion We found that the oxidation stability of Vulcan and Ketjen upon potential cycling under acidic conditions was dramatically improved by a simple PBI-coating of the carbon supporting materials. The nitrogen gas adsorption analysis of the Vulcan and Ketjen with and without the PBI-coating revealed that the PBI-coating was effectively capped the gates of the micropores of the Vulcan and Ketjen. Since the degree of the oxidation on the micropores inside the Vulcan and Ketjen is known to be very high, the capping effectively prevents the penetration of the electrolyte to avoid oxidation of the inner micropores. As a result, we successfully improved the durability of the MEA having Pt-NP loaded on the Vulcan/PBI not only for Nafion-based MEA for low-temperature operation but also PA-based MEA for high-temperature operation. The proposed polymer-coating


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method is very simple and thus a promising way to improve the lifetime of PEMFC in low cost, which impacts the developments of the next generation PEMFCs. Recently, an increasing of an activity of oxygen reduction reaction by a ‘shelling’ of CB by nitrogen-containing layer was also reported by Noto et al.49-51 and lowering of Pt amount by PBI coating was reported by our group.52 Thus we can conclude that such a surface engineering of CB is promising for the practical PEMFC applications.

Methods Materials.

N,N-dimethylacetamide

(DMAc),

ethylene

glycol

(EG),

hydrogen

hexachloroplatinate hexahydrate (H 2PtCl6·6H2O), and phosphoric acid 85% were purchased from Wako Pure Chemical, Ltd., and used as received. The Vulcan (XC -72R) and Ketjen (ECP600JD) were purchased from Cabot Chemical Co., Ltd., and Lion, respectively. PBI was synthesized according to a previously reported method. 53 Phosphoric acid doped-PBI was prepared by a previously reported method. 40 Characterization. The X-ray photoelectron spectroscopy (XPS) spectra were measured using an AXIS-ULTRADLD instrument (Shimadzu, Co., Japan). The thermogravimetric (TGA) diagrams were measured using an EXSTAR TG/DTA6300 (SEIKO Instruments, Inc) at the heating rate of 10 ºC/min under a 200 mL/min air flow. The STEM and TEM micrographs were measured using an SU9000 (Hitachi High-Technologies) operated at 30 kV and a JEM-2010 (JEOL) operated at 120 kV, respectively. A copper grid with a carbon support (Okenshoji) was used for the STEM and TEM observations. The specific surface area and the micropore distribution were determined by the Brunauer-Emmett-Teller (BET) method and Horvath-Kawazoe (HK) method (applying the


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carbon-N2 interaction parameter at 77 K), respectively, based on the N 2 adsorption isotherm measurements (77 K, 10-8 < P/Po < 1) using a BELSORP-mini (BEL Japan, Inc.) The gas adsorption measurements were conducted after pre-treatment at 300 oC over 12 h under high vacuum. The electrochemical measurements were performed using a potentiostat (ALS 760D, ALS Co., Ltd.) connected in a three-electrode configuration at room temperature. A Pt wire and an Ag/AgCl were used as the counter and reference electrodes, respectively. The Ag/AgCl reference electrode was calibrated against the reference hydrogen electrode (RHE) potential in an aq. 0.1 M HClO4. All potentials are indicated against RHE. XRD profiles were measured using a Smart-Lab (Rigaku Corporation), in which Cu Kα (1.54056 Å) was used as the X-ray source. CHN elemental analysis were performed by CHN Corder MT-6 (Yanaco). Synthesis of Vulcan/PBI. Vulcan (20 mg) was added to PBI (0.4 mg/mL) dissolved in DMAc (20 mL), and sonicated for 1 h using a bath-type sonicator (Branson 5510). The formed composite was collected by filtration using a 0.2 μm membrane (Millipore), washed several times with DMAc to remove any unbound PBI, then dried under vacuum to provide a black powder (Vulcan/PBI, 21.3 mg). Ketjen/PBI was also prepared using a similar procedure to provide a black powder (24.5 mg), in which 0.8 mg/mL of the PBI dissolved in DMAc (20 mL) was used. Synthesis of Vulcan/PBI/Pt and Vulcan/Pt. Typically, the Vulcan/PBI (5.0 mg) was dispersed in a 60% aqueous EG solution (30 mL) by sonication to which a 60% aqueous EG solution of H 2PtCl6·6H2O (1.4 mM) was added. The resultant mixture was refluxed at 140 °C for 8 h. After cooling, the obtained material (donated as Vulcan/PBI/Pt) was filtered using a 0.2 μm PTFE membrane, washed several times with Milli-Q water, then dried under


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vacuum to provide the Vulcan/PBI/Pt as a black powder (7.3 mg). In a similar manner, Vulcan/Pt was synthesized using Vulcan.54 Durability tests of Vulcan, Ketjen, Vulcan/PBI and Ketjen/PBI. A gas diffusion layer (GDL) (SGRACET gas diffusion media, GDL 25 BC, SGL Carbon Group) was used as the working electrodes. The sample powder (1.0 mg) was ultrasonically dispersed in an 80% 2propanol aqueous solution (2.0 mL) in the presence of Nafion (25.0 wt% for Vulcan and 50.0 wt% for Ketjen) to form a homogeneous suspension. The sample loading on the electrodes was controlled at 3.0 mg cm-2 for the Vulcan and Vulcan/PBI and at 1.5 mg cm -2 for the Ketjen and Ketjen/PBI. in which the amount of the Ketjen was reduced because we found the large nonFaradaic current of Ketjen caused the overflow of the current. The triangle potential sweeps were cycled between 1.2 and 1.7 V at a rate of 0.5 V/sec. The CV curves were recorded every 100 cycles. Durability tests of Vulcan/Pt and Vulcan/PBI/Pt. A glassy carbon electrode (GC) with the geometric surface area of 0.196 cm2 was used as the working electrodes. The sample powder (1.0 mg) was ultrasonically dispersed in an 80% 2-propanol aqueous solution (2.0 mL) in the presence of 0.5 wt% of Nafion (5.43 L) to form a homogeneous suspension. The sample loading on the GC electrodes was controlled at 14 μgPt cm-2. The triangle potential sweeps were cycled between 1.2 and 1.7 V at a rate of 0.5 V/sec. The CV curves were recorded every 100 cycles.

Fuel cell testing. The electrocatalyst composites (Vulcan/PBI/Pt and Vulcan/Pt) were ultrasonically dispersed in a 60% EG aqueous solution, then deposited on a GDL by vacuum filtration to prepare a gas diffusion electrode (GDE). Pt loading was fixed to 0.45 mg/cm2. For low-temperature operation, Nafion (Nafion 117) was laminated between two GDEs to fabricate


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the Nafion-MEAs. For high-temperature operation, the PA-doped PBI membrane was laminated between two GDEs to fabricate the PA-MEAs, which was then hot-pressed with the 5 MPa for 30 s at 120° C. The Nafion-MEAs and PA-MEAs were evaluated at 80 ºC under R.H = 100% and 120 ºC under non-humidified conditions, respectively, using a computer-controlled fuel cell test system (Model 890e, Scribner Associate, Inc.). The polarization curves were recorded under atmospheric pressure of hydrogen flowing at 100 mL/min and dry air flowing of 200 mL/min at the anode and cathode, respectively. Durability test of MEA. The assembled MEAs were subjected to the potential-accelerating durability tests based on the protocol provided by the Fuel Cell Commercialization Conference of Japan (FCCJ).42 Typically, the potential sweeps were cycled between 1.0 and 1.5 V at the scanning rate of 0.5 V/s at 70°C under 50% relative humidity (RH) for Nafion-MEAs and at 120 °C under non-humidified conditions for PA-MEAs. H2 and N2 were fed to the anode and the cathode, respectively. The I-V curves were recorded every 1,000 cycles after switching the cathode gas from N2 to air.


15


a

b

PBI DMAc Vulcan

Vulcan/PBI

100

c

d

Weight %

80

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Vulcan /PBI

60 40 20

Vulcan

Vulcan Vulcan/PBI

0 405

400 395 Binding Energy / eV

200 400 600 o Temperature / C

800

Figure 1. (a) Schematic drawing of the preparation of Vulcan/PBI. (b) Photographs of aqueous dispersions of Vulcan (left) and Vulcan/PBI (right). (c) XPS N 1s spectra of Vulcan (black) and Vulcan/PBI (red). (d) TGA curves of Vulcan (black) and Vulcan/PBI (red).


16

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8

0 initial 400 cycle 800 cycle 1200 cycle 1600 cycle 2000 cycle

-4

0

0.2 0.4 0.6 0.8 1 Potential / V vs. RHE

c

2

0 initial 100 cycle 200 cycle 300 cycle 400 cycle

0


Intensity (a.u.)

e

C=C

0 initial 400 cycle 800 cycle 1200 cycle 1600 cycle 2000 cycle

-4

0


d

0 initial 100 cycle 200 cycle 300 cycle 400 cycle

-2 0


C=C

Vulcan /PBI

290 285 280 Binding Energy / eV

h

Ketjen /PBI

Intensity (a.u.)

Ketjen

1.2

C-F

295


1.2

Ketjen /PBI

2

f

C-F

g

1.2

Vulcan /PBI

Vulcan

COO-

295

b 4

-8

1.2

Ketjen

-2

Current Density / mA cm-2

4

-8

Current Density / A cm-2

Vulcan

Current Density / A cm-2

a

Intensity (a.u.)

Current Density / mA cm-2

8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


295


295


Figure 2. CVs of the (a) Vulcan, (b) Vulcan/PBI, (c) Ketjen and (d) Ketjen/PBI measured in 0.1 M HClO4 aqueous solutions. XPS profiles of (e) Vulcan, (f) Vulcan/PBI, (g) Ketjen and (h) Ketjen/PBI before (black) and after (red) the durability test. The C 1s peak derived from the C-F bonding originated from the Nafion used as the electrolyte.


17


1000

1600

800

Va / cm3 (STP) g-1

Va /cm3 (STP) g-1

a

b

1200

600

Vulcan Vulcan/PBI

400 200

0

0.2

0.4

0.6 p /p0

0.8

Ketjen Ketjen/PBI

800 400

0

0 0

1

0.2

0.4

0.6 p / p0

0.8

1

0.6

d

c 0.5 dVp/ ddp

dVp/ddp

0.2

Vulcan Vulcan/PBI

0.1

0.4 0.3

Ketjen Ketjen/PBI

0.2 0.1

250 -1

e

206

Micropore Mesopore + Outer Surface

1600

2

2

200

0 0

4

-1

1 2 3 Pore Diameter / nm

Surface Area / m g

0 0

Surface Area / m g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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59 150 87

100 147 50

10 77

1400

f

1338

1200

4

Micropore Mesopore + Outer Surface 1126

1000 800

919

715

600 400 200

0

1 2 3 Pore Diameter / nm

419

411

Ketjen

Ketjen/PBI

0 Vulcan

Vulcan/PBI

mesopore

g

outer surface

Figure 3. Absorption-desorption isotherms of N 2 for (a) Vulcan (black) and Vulcan/PBI (red) and (b) Ketjen (black) and Ketjen/PBI (red). The pore diameter distribution evaluated based on the HK method for (c) Vulcan (black) and Vulcan/PBI (red) and (d) Ketjen (black) and Ketjen/PBI (red). Bar graphs displaying the outer and micropore surface areas of (e)


18

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vulcan (left) and Vulcan/PBI (right) and (f) Ketjen (left) and Ketjen/PBI (right) evaluated based on the t-plot method. (g) Schematic drawing of the proposed structure of the PBIcoated CBs (Vulcan and Ketjen), showing a capping of the micropore gate by PBI coating.

a

H2PtCl6·6H2O

Vulcan/PBI

EG aq. 140 ˚C, 6 h

b

Vulcan/PBI/Pt

c

d Vulcan/Pt Diameter [nm] Loading [wt%] Pt(0)/Pt(II)/Pt(IV) ECSA

[m2/g]

2.8

0.5

Vulcan/PBI/Pt 2.9

0.8

44.5

41.9

52.9 / 22.9 / 24.2

57.8 / 26.2 /16.0

47.1

46.0

Figure 4. (a) Schematic drawing for the preparation of a Vulcan/PBI/Pt. (b, c) TEM images of (b) Vulcan/Pt and (c) Vulcan/PBI/Pt in the same magnifications. (d) List of the diameter, loading ratio, valence ratio, and ECSA values of the Vulcan/Pt and Vulcan/PBI/Pt.


19


a

20 0 -20 initail 400 cycle 800 cycle 1200 cycle 1600 cycle 2000 cycle

-40 -60 0


0 -20 initial 400 cycle 800 cycle 1200 cycle 1600 cycle 2000 cycle

-40 -60 0


1.2

-2

Cell voltage @ 0.2 A cm / V

0.9

c

50 -1

20

1.2

55

45 40 35 30 25

b

40

Current / A/gPt

Current / A/gPt

40

2 ECSA / m g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

0

500 1000 1500 Cycle Number

2000

d

0.8 0.7 0.6 0.5 0.4 0.3 0.2

0

20 40 60 80 100 Number of cycles x1000

Figure 5. CV curves of (a) Vulcan/Pt and (b) Vulcan/PBI/Pt measured during the potential cycling. (c) Plots of the ECSA values calculated from Figs 5a and 5b for the Vulca n/Pt (black line) and Vulcan/PBI/Pt (red line) as a function of the cycle number. (d) Plots of the cell voltages at 0.2 A/cm 2 for the Nafion-MEAnon-coat (green) and Nafion-MEAcoat (red). Reprinted in part with permission from ref. 43. Copyright 2014 Nature Publishing Group.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX CVs and TEM images of Vulcan, Vulcan/PBI, Ketjen and Ketjen/PBI. TGA, XPS, XRD of Vulcan/Pt and Vulcan/PBI/Pt before durability test. TEM and XPS of Vulcan/Pt and Vulcan/PBI/Pt after durability test.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions T.F. and N.N. conceived and designed the experiments. S.H. and M. R. B. carried out the experiments. T.F. and N.N. wrote the paper. Notes The authors declare no competing financial interest.

Acknowledgements We thank to the Service Centre of the Elementary Analysis of Organic Compounds in Kyushu University for CHN elemental analysis. This work was supported in part by the Low-Carbon Research Network (LCnet), the Nanotechnology Platform Project (Molecules and Materials Synthesis) and a Grant-in-Aid for Scientific Research on Innovative Areas "π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions" from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by the Japan Science and Technology Agency (JST) through its “Center of Innovation Science and Technology-based


21


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Radical Innovation and Entrepreneurship Program” (COI Program) and the Advanced Low Carbon Technology Research and Development Program (ALCA).

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Table of Contents Graphic

Carbon Black

Polymer-coated Carbon Black

Polymer Coating

Current Density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Stable

Oxidized 0


1.2

0


1.2


25

Improved Durability of Electrocatalyst Based on Coating of Carbon

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