graphene with intercalated carbon nanotube spacers introduced by electrostatic self-assembly for fuel cells

Materials Chemistry and Physics 225 (2019) 371–378

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Pt/graphene with intercalated carbon nanotube spacers introduced by electrostatic self-assembly for fuel cells

T

Zhen Liua, Ali Ahmed Abdelhafizb, Yangcheng Jianga, Chong Qub, Ikwhang Changb, Jianhuang Zenga,∗∗, Shijun Liaoa, Faisal M. Alamgirb,∗ a b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangdong Key Lab for Fuel Cell Technology, Guangzhou, 510641, China School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA, 30332-0245, United States

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

on graphene is separated • Ptby supported intercalated and self-assembled CNTs.

spacers are wedged between • CNT graphene, resulting in 3-D sandwiched structure.

efficiency and stability is enhanced • Ptrelative to a commerical Pt/C.

A R T I C LE I N FO

A B S T R A C T

Keywords: Self-assembly Pt/rGO Carbon nanotube Stability

Although graphene exhibits an appealing array of properties that are ideally suited to their use as catalyst supports for fuel cells, they suffer from a serious issue of graphitic restacking due to van-der-Waals force and π-π interaction, making much of the catalyst surface inaccessible to reaction. In this work, highly active and stable platinum nanoparticles supported on graphene with intercalated carbon nanotubes (CNTs) are prepared by electrostatic self-assembly for fuel cell applications. CNTs, which functioned as spacers, are wedged between graphene sheets via electrostatic self-assembly resulting in interconnected 3-D sandwiched structure. The intercalated CNTs inhibit restacking and lead to increased Pt utilization efficiency through improved mass transport. An enhancement factor of 114% for Pt utilization efficiency and electrochemical surface area (ESA) is found for the optimized Pt/rGO-CNT relative to that of Pt/rGO. Compared with the state-of-the-art Pt/C, the hybrid catalyst show markedly enhanced stability, i.e., with an ESA retention of 62% after cycling the catalyst in 0.5 M H2SO4 from −0.2–1.0 V up to 2000 scans, whereas the ESA retention for commercial Pt/C is only 19%. These results indicate our unique approach is promising to employ graphene as Pt catalyst support aiming for high activity and stability.

1. Introduction Environmentally friendly and sustainable energy resources rather

∗

than fossil fuels are desirable in order to meet our future power requirements [1]. Polymer Exchange Membrane Fuel Cells (PEMFCs) are one of the ideal devices to address these stringent energy issues since

Corresponding author. Corresponding author. E-mail addresses: [email protected] (J. Zeng), [email protected] (F.M. Alamgir).

∗∗

https://doi.org/10.1016/j.matchemphys.2018.12.100 Received 23 August 2018; Received in revised form 24 December 2018; Accepted 31 December 2018 Available online 03 January 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.

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CNT intercalation showed markedly improved activity and stability relative to that of a commercial Pt/C benchmark.

they are pollution-free and can generate power with high efficiency [2,3]. Commercial deployment of PEMFCs is still limited by their high cost since the commonly used state-of-the-art Pt or Pt-based electrocatalysts still suffer from low noble metal utilization efficiency and low catalytic activities as well as poor stability. Platinum-group-metal free electrocatalysts are definitely ideal, but their catalytic performances are still far from desirable based on current research. Therefore, intensive research efforts up-to-date are devoted to improve Pt efficiency and stability by various approaches. Amongst them, one of the intensively studied methods is to load Pt or Pt-based nanoparticles onto the newly emerged graphene support [4–7]. Graphene, a carbon monolayer packed into a two-dimensional honeycomb lattice, has many appealing properties, such as high theoretical surface area (2630 m2 g−1), high conductivity, excellent mechanical/thermal stability, and unique graphitized basal plane structure [8,9]. Dispersing high surface-to-volume ratio metal nanoparticles onto graphene is a promising method to maximize metal surface area available for the fuel cell reaction and make low loading catalyst feasible [10–13]. As an alternative to nanoparticles, atomically thin catalyst films push the number of surface atoms to near the theoretical limits, where ligand effects from the support can be tuned [14–18], has also recently been demonstrated on graphene [19]. Graphene itself is chemically inert and hydrophobic, instead, graphene oxide (GO) with affinity surfaces is often used as electrocatalyst support [20–23]. Unfortunately, the reduction of GO during Pt deposition is unavoidably accompanied by substantial restacking of separated graphene sheets due to van der Waals force and π-π interaction, leading to serious blocking of the active surface area [24,25], undoing the efforts made to improve metal utilization efficiency. To address this problem, one of the effective strategies is to introduce “spacers” of different types in graphene sheets. Carbon black [1,24,26,27], carbon nanotube [25] and boron carbide [20] have been reported to be intercalated into Pt/graphene or PtRu/graphene with successful application in fuel cells. By simply adding different amount of carbon black to the pre-formed Pt/graphene [1] or Pt on boron doped graphene [27], Pt catalysts with enhanced electrochemical surface areas were prepared. 40 wt% PtRu particles supported on the mix of graphene oxide and carbon black were used as catalysts for methanol oxidation reaction and were reported to out-perform PtRu on graphene and PtRu on carbon black catalysts [24]. Sanli et al. employed graphene-carbon black hybrid support to prepare platinum electrocatalysts using a polyol method and they found that carbon black acted as a spacer and intercalating agent, preventing graphene from restacking [26]. Restacking of individual graphene sheet was also reported to be effectively inhibited by introducing one-dimensional carbon nanotubes (CNTs) to form a 3-D porous microstructure for PtRu catalysts [28]. Therefore, it can be concluded unambiguously that the addition of intercalated materials into graphene can prevent restacking, leading to improved catalytic performances of the resulting materials. Unfortunately, the addition of the intercalated materials in these reported studies was implemented by simply mixing graphene sheets together with “spacers” without any control over their interactions. This would definitely result in local segregation of either graphene sheets or intercalated materials, leading to limited achievements. In our previous work, we have successfully fabricated PtxRuy nanoparticles through electrostatic self-assembly for methanol oxidation reaction [29]. In order to achieve ordered assembly between Pt/rGO and CNTs to form a sandwiched structure, electrostatic self-assembly technique was introduced in this work, i.e., oppositely charged Pt/rGO and CNTs are self-interacted into ordered nanostructures. Specifically, negatively charged CNTs are introduced to the pre-formed positively charged Pt/graphene in the solvent and the spontaneous electrostatic self-assembly resulted in Pt/graphene intercalated by CNTs. As a result, the Pt/graphene with CNT intercalation demonstrates high electrochemically active surface area and activity using methanol oxidation reaction as a probe reaction. In particular, the catalyst with 10 wt %

2. Experimental 2.1. Synthesis of graphene oxide Graphene oxide was synthesized according to a modified Hummer's method [30,31]. In a typical experiment, 2 g of graphite and 10 g of KMnO4 were slowly added into a mix solution containing 200 mL sulfuric acid and 80 mL phosphoric acid in a three-necked flask at 50 °C for 20 h under magnetic stirring. After that, the flask was transferred into an ice-bath and 15 mL H2O2 (30 wt %) was added dropwise into the suspension from the side arm until the slurry turned from black into golden yellow. The suspension was washed with 10 M HCl and deionized water, respectively, until the pH reached to neutral. GO was finally obtained after drying in a freezer dryer. 2.2. Functionalization of CNTs Commercial pristine CNTs (BET surface area of 95 m2 g−1, outter diameter 30–50 nm) used in this work were supplied by Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. Pristine CNTs (1 g) were refluxed with 300 mL H2SO4 (97%) and 100 mL HNO3 (68%) at 100 °C for 1 h. After that, CNTs were collected by repeated centrifugation and washing with de-ionized water for several times and dried for usage. 2.3. Synthesis of Pt/rGO-CNT(x) catalysts In a typical experiment, 320 mg of graphite oxide was dispersed in 200 mL ethylene glycol (EG) by sonication for 30 min, and then 10.8 mL of H2PtCl6/EG (0.038 mol L−1) was added. The pH of the solution was adjusted to be greater than 10 using KOH/EG solution. After that, and the mixture was subjected to microwave irradiation for 30 s under magnetic stirring. The mixture (Pt/rGO in EG) was cooled down to room temperature and was divided into 4 equal portions. Functionalized CNTs, which were homogenized in EG, was then added into the Pt/rGO. The sample, which was denoted as Pt/rGO-CNT(x), was collected by centrifugation (10000 r.p.m, 20 min) and washed with deionized water for 6 times. The pre-determined amount of CNTs added is 5 mg, 10 mg and 15 mg, which approximately corresponds to x wt.% of Pt/rGO where x = 0.0, 5.0, 10.0, 15.0. Finally, the precipitate was lyophilized and stored for further applications. 2.4. Material characterizations Transmission electron microscopy (TEM) measurements were made on a FEI Tecnai G2 F20 S-Twin with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images of the electrocatalysts were on a S4800 (Hitachi, Japan) operated at 15 kV. X-ray diffraction (XRD) was performed on a TD-3500 powder diffractometer (Tongda, China) using Cu-Kα radiation operated at 30 kV and 20 mA. The specific surface area (SBET) was determined according to the Brunauer–Emmett–Teller (BET) method in the relative pressure of 0.05–0.20 by nitrogen adsorption on an automatic volumetric sorption analyzer (TriStar II 3020 analyzer, Micromeritics, USA). The FT-IR analysis was conducted with a Tensor 27 (Bruke, Germany) in the spectral range from 4000 to 400 cm−1. The net charge at the surface was measured by Zeta Potential Analyzer (ZetaPALS/90plus). The exact loading of the catalysts were obtained using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Prodigy, Leeman America). 2.5. Electrochemical characterizations A conventional three-compartment electrochemical cell was used to 372

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graphene sheets in Fig. 1f. Therefore high relative amount of CNTs in Pt/rGO-CNT(15) would not contribute to the restacking of graphene and the relative amount of CNT in the catalysts was tentatively optimized at 10 wt% in this work. It is anticipated that the extended conjugated network made by the electrostatically self-assembled rGO/ CNT/rGO sandwiched structure would make most of Pt nanoparticles accessible to fuel cell reactions. The network structure for the as-prepared Pt/rGO-CNT(x) could be beneficial for mass transport through fast electronic and ionic conducting channels [24]. Representative SEM images of Pt/rGO-CNT(10) are shown in Fig. 1h–g. The intercalated CNTs would function as spacers to separate graphene sheets, making more active catalyst surfaces available.

evaluate the catalysts by cyclic voltammetry (CV). An Autolab PGSTAT30 served as the potentiostat/galvansotat. A rotating disk electrode (RDE, Pine Instrument Company, Model AFMSRCE) was used as the working electrode. The catalyst layer on the RDE was fabricated by casting 5 μL catalyst ink by micropipette (5 mg catalysts +1 mL 0.25 wt %) Nafion 117 (diluted by ethanol) onto a 5 mm diameter vitreous glassy carbon disk electrode. A Pt wire and a Ag/AgCl in 3 M KCl were used as the counter and the reference electrodes respectively while 0.5 M CH3OH in 0.5 M H2SO4 was the electrolyte for catalytic activity test. All electrochemical tests were recorded at room temperature. 3. Results and discussion 3.1. Electrostatic self –assembly of the catalysts

3.3. Material characterization of the catalysts

The schematic preparative process for the Pt/rGO-CNT(x) was illustrated in Fig. s1. Graphene oxide sheets were suspended homogeneously in the EG solution, then, platinum precursor was completely reduced together with the partially reduced GO via microwave reaction to form Pt/rGO. The functionalized CNTs, which were pre-homogenized in EG, were added in the mixed solution to obtain the resultant catalyst Pt/rGO-CNT(x). The isoelectric point of the pristine CNTs was adjusted through surface functionalization [32] and the point of zero charge (PZC) of the treated carbon nanotubes is measured to be 5.4 in this work. It is known that zeta potential (ζ) of dispersing system represents the surface charges, which can be fine-tuned by surface functional groups, hydrated ions and ionic species, etc. The charged Pt nanoparticles on the reduced graphene oxide in the solution are imparted by ethylene glycol as well as OH− ions, whereas charged CNTs are made by surface functionalization. The zeta potential of the as-prepared Pt/rGO and CNTs is measured to be −7.5 mV and +10.8 mV in Fig. s2, an indication of positive charges at the Pt/rGO and negative charges at the CNT surface. Upon mixing of the two dispersing systems, Pt/rGO and CNTs were interacted with each other due to electrostatic self-assembly. Experimentally, we observed that the precipitation of the Pt/rGO-CNT was much faster than that of Pt/rGO alone. Obvious catalyst precipitation could be observed for the Pt/rGO-CNT in 72 h, whereas no appreciable sedimentation could be found for the as-prepared Pt/rGO in the solution after one week (Fig. s3). Strong interactions between the πconjugated aromatic domains in graphene's basal plane or the surface functional groups of Pt nanoparticles with the functionalized CNTs should have taken place upon electrostatic self-assembly [33,34].

XRD patterns of the natural graphite used for the preparation of GO, the as-prepared GO and the electrocatalysts are shown in Fig. 2. There is a sharp diffraction peak at 2θ of 26.7° for natural graphite, which is characteristic of graphitic C (002) peak. Upon chemical oxidation, this peak shifted to 10.0° in 2θ, an indication that the stacked layers of graphite has been laterally exfoliated by the incorporated oxygenated functional groups [1,36]. Disappearance of this peak for the Pt/rGO and Pt/rGO-CNT(10) during Pt deposition signified the formation of graphene sheets during the ethylene glycol reduction. The face-centered cubic structure of Pt in both Pt/rGO and Pt/rGO-CNT(10) is confirmed by the diffraction peaks at 39.5° for (111), 46.3° for (200) and 67.7° for (220), respectively [37]. The diffraction peaks at 2θ angles at 25° for Pt/rGO-CNT(10) was mostly attributed to the existence of CNTs [38]. As expected, pristine CNTs possess no functional groups. After treatment, characteristic eC]O stretch centered at 1637 cm−1 and eOH stretch at 3454 cm−1 are observed in the infra-red (IR) spectrum shown in Fig. 3a. In Fig. 3b, the IR spectrum for the natural graphite is also featureless and successful fabrication of GO by the modified Hummer's method is evidenced by the bending vibration of water molecules and OH stretching mode of the physically adsorbed water molecules at 3400 cm−1, carbonyl/carboxyl at 1727 cm−1, aromatics at 1625 cm−1, carboxyl at 1384 cm−1, epoxy at 1261 cm−1 and alkoxy at 1085 cm−1, respectively1. It can be found from the IR spectra for the electrocatalysts, most of these functional groups disappeared upon reduction by ethylene glycol via microwave irradiation. However, GO was only partially reduced since Pt/rGO and the electrocatalysts with different amount of CNT showed the presence of similar functional groups, i.e., OeH, C]C, C]O and CeO20, as shown in Fig. 3c. C 1s XPS spectra for Pt/rGO and Pt/rGO-CNT(10) are shown in Fig. 4a and Fig. 4b, respectively. The C 1s spectra of Pt/rGO and Pt/ rGO-CNT(10) consist of CeC (sp3, 284.8 eV), C]C (sp2, 284.2 eV), OeCeO (286.6 eV) and OeC]O (288.6 eV) [39,40]. After reduction via microwave irradiation, the O/C ratio decreases considerably in graphene support, indicating that a large majority of the oxygenated species has been removed. In addition, a peak centered at 289.4 eV in Fig. 4b for Pt/rGO-CNT(10) was identified as a shake-up satellite due to π-π* transitions [41]. These results showed that GO was readily partially converted into graphene when the environment-friendly EG was used as a reducing agent under microwave irradiation [25]. Pt 4f spectra for Pt/rGO and Pt/rGO-CNT(10) are shown in Fig. s4, indicating successful deposition of Pt metals on the supports. Nitrogen adsorption–desorption isotherms were measured and presented in Fig. 5. The isotherms of the samples were consistent with type-Ⅳ hysteresis loop, an indication of lamellar mesoporous materials. BET surface area of the GO synthesized in this work was measured to be 184 m2 g−1. While Pt/rGO registered a surface area of 63 m2 g−1 due to Pt deposition and GO reduction and agglomeration, Pt/rGO-CNT(10) showed much higher surface area (144 m2 g−1) owning to the intercalated CNTs.

3.2. Structure and morphology A piece of GO sheet is shown in the TEM image in Fig. 1a. It is to be noted that the GO sheets are folded due to the surface affinity [35]. Pt nanoparticles are uniformly distributed on the reduced graphene oxide (graphene oxides are also concomitantly partially reduced in 30 s) during microwave irradiation in ethylene glycol in Fig. 1b. From the particle size distribution histogram of Pt/rGO in Fig. 1c, it can be found that the particle size of Pt/rGO varies from 2.2 nm to 4.0 nm with an average size of 3.0 nm. TEM images of the electrostatically self-assembled electrocatalyst Pt/rGO-CNT(5) in Fig. 1d, Pt/rGO-CNT(10) in Fig. 1e and Pt/rGO-CNT(15) in Fig. 1f showed intercalated carbon nanotubes with increased presences of CNTs in the order Pt/rGOCNT(5) < Pt/rGO-CNT(10) < Pt/rGO-CNT(15). It might be noted that the majority of Pt nanoparticles, if not all, should have been deposited on graphene since Pt/rGO was prepared by an impregnation method. The addition of CNT might help harvest those isolated Pt nanoparticles in the solution since some Pt nanoparticles are also dotted on the carbon nanotubes from the TEM images in Fig. 1d–f. Besides the CNTs wedged between graphene sheets, some excess CNTs are obviously bundled with each other, without intercalating in the layers of 373

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Fig. 1. TEM images of a) GO, b) Pt/rGO and c) particle size distribution histogram for Pt/rGO, d) Pt/rGO-CNT(5), e) Pt/rGO-CNT(10), f) Pt/rGO-CNT(15), g-h) SEM image of Pt/rGO-CNT(10).

region where only capacitive process occurs. In comparison, the hydrogen sorption/desorption peaks for the Pt/rGO are less prominent with a bun-like shape, most probably due to the blockage of the active sites by the re-stacked graphene. It is hypothesized that the CNTs might have sandwiched in the catalyst layer, contributing to highly exposed Pt surfaces. The electrochemical surface area (ESA) of the Pt catalysts could be estimated from the charges associated with hydrogen adsorption on Pt facets in the potential range from −0.2 to 0.1 V after the conventional correction for the pseudo-capacity seen in the double layer region and using the stoichiometry of one adsorbed H atom per Pt atom. ESA in m2 g−1 is then calculated assuming a correspondence value of 0.21 mC cm−2 Pt. On the other hand, assuming a monodisperse distribution of spherical particles, the theoretical surface area of Pt particles based on purely geometry considerations could be obtained to be 93 m2 g−1 using the mean Pt diameter (3 nm from Fig. 1d). Pt utilization efficiency can then be obtained after dividing ESA by the theoretical surface area. Table 1 summarizes the ESA and Pt utilization efficiency for all the catalysts. Higher ESA and increased Pt utilization efficiency are observed for the Pt/rGO-CNT(x) relative to that of Pt/ rGO, verifying our hypothesis in which CNTs acts as spacers intercalating in graphene layers. Enhancement in ESA with the addition of CNT follows a typical “volcano-type” plot in which Pt/rGO-CNT(10) displays the highest ESA (85.6 m2 g−1) with a Pt utilization efficiency of 92%. The catalytic activities towards room temperature methanol oxidation reaction in 0.5M CH3OH+0.5 M H2SO4 solution measured by cyclic voltammetry at a scan rate of 20 mV s−1 (Fig. 6 b). A commercial Pt/C catalyst from Johnson Matthey with 20 wt % Pt loading is also

Fig. 2. XRD patterns of the graphite, GO and the electrocatalysts.

3.4. Electrochemical surface areas and methanol oxidation activities During the catalyst preparation via microwave irradiation, concomitant partial conversion of graphene oxide into graphene occurs together with platinum precursor reduction. Therefore, the exact platinum loading in the resulting catalysts differs from the pre-determined value and has been obtained by ICP-AES. Fig. 6a show the cyclic voltammograms (CV) of the Pt/rGO and Pt/rGO-CNT(x) catalysts recorded in 0.5 M H2SO4 at room temperature. The current has been normalized to the geometric surface area of the electrode (0.196 cm−2). Obvious hydrogen sorption/desorption region for the Pt/rGO-CNT(x) catalysts are located between −0.2 and 0.1 V, followed by the double layer 374

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Fig. 3. FT-IR spectra of a) the functionalized and pristine CNT, b) graphite and graphene oxide and c) the electrocatalysts.

included as a benchmark. The catalytic activity (which has been measured by the peak current density) during the positive-going scan at 0.64 V is 19 mA cm−2 (or 648 A g−1), 10 mA cm−2 (or 392 A g−1) and 6 mA cm−2 (or 189 A g−1) for Pt/rGO-CNT(10), Pt/C-JM and Pt/rGO, respectively. Alternatively, the mass activity of Pt/rGO-CNT(10) is 1.7 and 3.4 times than that of a commercial Pt/C and Pt/rGO, respectively. Noted that the onset methanol oxidation potential followed in the order Pt/rGO-CNT(10) (0.19V) < Pt/C-JM(0.30V) = Pt/rGO(0.30V). 3.5. Accelerated stability tests To investigate the stability of the electrocatalysts, accelerated stability test (AST) has been conducted to assess ESA decrease with repeated potential cycling. Cyclic voltammetric curves of Pt/rGOCNT(10), Pt/rGO and Pt/C-JM have been recorded up to 2000 cycles in 0.5 M H2SO4 at a scan rate of 50 mV s−1 and the results are displayed in Fig. 7a–c. Well-defined hydrogen adsorption/desorption peaks are found for Pt/rGO-CNT(10) even after AST. Significant ESA retention has been found for Pt/rGO-CNT(10) and Pt/rGO relative to that of Pt/CJM. Catalytic activities towards MOR are also measured before and after AST in Fig. 7d–f. After AST, the MOR activity (peak current in the forward scan) for Pt/rGO-CNT(10) deactivates from 2.65 mA to

Fig. 5. Nitrogen adsorption–desorption isotherms.

1.05 mA, whereas Pt/C-JM and Pt/rGO lost their MOR activity completely. The loss of ESA with cycling number is plotted in Fig. 7g–i. After 2000 cycles, the ESA retention of Pt/rGO-CNT(10) is as high as 61%, whereas that for Pt/rGO and Pt/C-JM is merely 20% and 19%, respectively. It might be noted that catalytic stability is a critical issue to be considered for commercial application of a PEMFC. The as-

Fig. 4. C 1s XPS spectra for a) Pt/rGO and b) Pt/rGO-CNT(10). 375

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Fig. 6. a) CV curves obtained for the Pt/rGO, Pt/rGO-CNT(5), Pt/rGO-CNT(10), Pt/rGO-CNT(15) and b) Catalytic activities for MOR of the catalysts in 0.5 M H2SO4+0.5M CH3OH solutions at 20 mV s−1.

nanofiber (43% ESA retention) [42] and other catalysts reported in the literature under similar conditions. For example, ESA retention of 62.4% after 1000 CV cycles was found for 3D hierarchical Pt supported on nitrogen doped graphene [43] and less than 60% was found for Pt supported on nitrogen-doped CNTs with varied nitrogen content [44]. Systematic study of the Pt-graphene interface [19] has shown that the interface involves strong PteC interactions that lead to higher stability for Pt during electrochemical cycling. TEM images after AST for the Pt/ rGO-CNT(10), Pt/rGO and Pt/C-JM are shown in Fig. 8. An obviously less moderate Pt agglomeration has been found for Pt/rGO-CNT(10) compared with that of Pt/rGO and Pt/C-JM.

Table 1 Electrochemical surface area and Pt utilization efficiency of the catalysts. Catalyst

Pt loading analyzed by ICPAES (%)

Electrochemical surface area (m2 g−1)

Pt utilization efficiency (%)

Pt/rGO Pt/rGO-CNT(5) Pt/rGO-CNT(10) Pt/rGO-CNT(15)

25.0 23.4 22.6 21.2

40.2 62.4 85.6 68.8

43 67 92 74

prepared Pt/rGO-CNT(10) shows extremely high ESA retention in the harsh AST compared with the Pt catalysts supported on carbon

Fig. 7. Comparisons of durability of the Pt/rGO-CNT(10) and Pt/C-JM. a–c) CV curves for a) Pt/rGO-CNT(10), b) Pt/rGO and c) commercial Pt/C-JM catalyst before and after different cycles of CVs. The durability tests were carried out at room temperature in 0.5 M H2SO4 solutions with a sweep rate of 50 mV s−1. d-f) Catalytic activities for MOR for d) Pt/rGO-CNT(10), e) Pt/rGO and f) commercial Pt/C-JM catalyst after 50 and 2000 cycles of CV at room temperature in 0.5 M H2SO4 + 0.5 M CH3OH at 50 mV s−1. g-i) ESA and loss of ESA of g)Pt/rGO-CNT(10), h) Pt/rGO and Pt/C-JM with the cycling numbers of CVs. 376

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Fig. 8. TEM images of a) Pt/rGO-CNT(10), b) Pt/rGO, c) Pt/C-JM after AST.

4. Conclusions

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