Pt Nanoparticle-Loaded Graphene Aerogel Microspheres with

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Pt Nanoparticles Loaded Graphene Aerogel Microspheres with Excellent Methanol Electrooxidation Performance Miao He, Guoxia Fei, Zhuo Zheng, Zhengang Cheng, Zhanhua Wang, and Hesheng Xia Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00021 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Pt Nanoparticles Loaded Graphene Aerogel Microspheres with Excellent Methanol Electrooxidation Performance Miao He, Guoxia Fei, Zhuo Zheng, Zhengang Cheng, Zhanhua Wang*, Hesheng Xia State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, PR China.

ABSTRACT Platinum-decorated graphene aerogel microspheres were fabricated through a combined electrospraying, freeze-casting and solvothermal process. Platinum nanoparticles with a narrow size distribution are evenly anchored on the graphene aerogel microspheres without agglomeration benefitting from the distinct center-diverging microchannel structure of the graphene aerogel microspheres, which results in the as-prepared catalysts presenting excellent electrocatalytic performance including high electrocatalytic activity and high poison tolerance towards methanol electrooxidation, showing great potential for direct methanol fuel cell anode catalysts. In particular, the Platinum decorated graphene aerogel microspheres exhibit an extremely high mass activity of 1098.9 mA mg-1 towards methanol oxidation as well as excellent antipoisoning ability,

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which are dramatically enhanced compared with Pt particles dispersed on graphene oxide and commercial carbon black supports. 1. INTRODUCTION Facing the continuous deterioration of environment and fast depletion of fossil fuels, direct methanol fuel cell (DMFC) has received increasing interests due to its distinct advantages such as high energy transition efficiency, low pollution emission, simple device fabrication, and convenient storage and handling of the liquid methanol,1-5 and has been regarded as one of the most promising portable electronic devices.6-9 However, the absence of high efficiency and longterm durability anode catalysts for methanol oxidation reaction (MOR) has retarded the commercialization process. 5, 10, 11 To date, the most promising catalysts for MOR are Pt based materials.10, 12 To improve the catalytic activity and reduce the cost of catalysts, considerable efforts have been made to disperse the ultrasmall Pt nanoparticles on a conductive carbon matrix, such as Pt/carbon blacks13-15, Pt/carbon nanotubes16-18 and Pt/graphene19-22. In particular, Pt decorated graphene hybrids exhibit enhanced electrocatalytic performance compared with carbon blanks and carbon nanotubes owing to the distinctive advantages of graphene such as high specific surface area, flexible twodimensional structure, and good conductivity. However, irreversible agglomeration of graphene sheets through the intensive van der Waals force would decrease the high pristine specific area,23, 24 blocking the surface catalytically active sites as well as impeding mass transportation at the same

time25,

26.

Integrating graphene sheets into three-dimension aerogel can maximally retain the

accessible surface area thus promoting the deposition of Pt nanoparticles, and the strong crosslink of 3D graphene network can maximally avert the degradation of catalytic performance caused by stacking of graphene sheets.12,

22, 27

Compared with Pt-graphene hybrids, Pt nanoparticles


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decorated on graphene aerogel materials have demonstrated improved MOR catalytic performance in many of the recent works. 28-31 However, in order to ensure the whole aerogel is fully filled by the precursor solution, the monolithic graphene aerogel needs to be broken into smaller pieces by ultrasonic or mechanical stirring before the dispersing of Pt particles enabling the graphene sheets to be fully exposed to the Hexachloroplatinate solution, resulting in partly destroyed porous structure along with some crashed graphene nanosheets. The broken graphene aerogel with uncontrollable fragment diameter as well as some exfoliated graphene sheets tending to aggregate is adverse to the homodisperse of Pt particles resulting in a decrease in active reaction sites of methanol electrooxidation, which impede the maximum utilizing of precedingly mentioned advantages of graphene aerogel towards MOR. In order to get the utmost out of high specific surface area and porous structure of graphene aerogel, graphene aerogel microspheres with well-controlled morphologies and highly ordered structures come into our view. In our previous work, the reduced graphene oxide aerogel microspheres (rGOAMs) were successfully prepared by a combination method of electrospraying and freeze-casting with unique “center-diverging microchannel” dandelion-like structures which hold specific surface ca.140.4375 cm2 g-1 and a pore volume of 0.33 g cm-3.32 Inspired by the previous efforts in the development of Pt nanoparticles loaded on monolithic graphene aerogels to be used as DMFC anode catalysts, we consider that the as-prepared rGOAMs with unique “centerdiverging microchannel” dandelion-like structures may provide a possibility of enhancing the electrocatalytic performance of Pt nanoparticles towards MOR by forming well dispersed Pt nanoparticles loaded rGOAMs hybrids catalyst. The rGOAMs with highly ordered centerdiverging microchannels as well as micron size spheres are favorable to the disperse of Pt nanoparticles instead of breaking the aerogel like the bulk ones. In order to verify our hypothesis,


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herein, we prepared Pt decorated reduced graphene oxide aerogel microspheres (denoted as Pt/rGOAMs) serving as DMFC anode catalysts for MOR. The Pt particles have been successfully loaded on the supports by glycol solvothermal reduction and the as-prepared catalysts are evaluated by structural characterization covering XRD, SEM, TEM, and XPS as well as electrochemical measurements including cyclic voltammetry scanning and chronoamperometric (I−t) tests. The distinct center-diverging microchannels of individual microspheres enable the uniform dispersion of Pt nanoparticles which contribute to more exposure of accessible electroactive sites as well as easier mass transportation. As a consequence, the Pt/rGOAMs exhibit excellent catalytic activity and stability which are superior to those of Pt/Vulcan XC-72R (Pt/C) and Pt/graphene oxide (Pt/GO) when used as catalysts for methanol electrooxidation.

Scheme 1. Schematic illustration of the synthesis of the Pt/rGOAMs catalysts which involves: 1) preparation of GOAMs; 2) thermal reduction to rGOAMs; 3) controllable growth of Pt NPs on the surfaces of rGOAMs by ethylene glycol solvothermal reaction. 2. EXPERIMENTS 2.1. Materials


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Graphene oxide was obtained from Changzhou The Sixth element Co. Ltd (China) with the lateral size about 10 μm and thickness about 2 nm. Hexachloroplatinate (H2PtCl6) and Nafion117 solution (5%) were obtained from Shanghai Aladdin biochemical technology Co. Ltd (China). Ethylene glycol (EG) and ethanol were obtained from Chengdu Kelong Chemical Reagent Company (China). Commercial carbon black (Vulcan XC-72R) was obtained from Cabot Co. (America). 2.2.

Catalysts preparation

Scheme 1. illustrates the synthesis of the Pt/rGOAMs catalysts. The preparation of GOAMs can be referred to our previous work using the combination method of electrospraying and freezecasting.30 And the GOAMs were sequentially heated in a vacuum oven for 60 min at 120 ºC, 60 min at 150 ºC, and 120 min at 180 ºC, respectively, to obtain the rGOAMs. The Pt/rGOAMs were prepared through ethylene glycol reduction. In a typical procedure: 30 mg of rGOAMs were mixed with 2 mL of H2PtCl6-EG solution (1 mg mL-1) and 45 mL of EG solution. The mixture was then transferred to a flask and refluxed at 120 ºC for 10 h under mechanic stirring. The hybrids were separated through filtration and then washed repeatedly with ultrapure water. After freeze drying, the Pt/rGOAMs was acquired. The Pt/C and Pt/GO were prepared in a similar procedure while the mixtures were subjected to ultrasonic treatment for 0.5 h to form uniform ink before transferred to the flask. The Pt contents in the catalysts calculated from TGA (Thermogravimetric Analysis) were controlled to be at approximately 20 wt% (Supporting Information, Figure S1). 2.3.

Structural characterization The crystalline of the as-prepared catalysts were studied by X'Pert X-ray diffraction (XRD)

(Philip, Holland). The SEM images were recorded by using a scanning electron microscope (JSM5900LV, JEOL, Japan). TEM images were obtained through a Tecnai G2 F20 S-TWIN


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Transmission electron microscopy (FEI, American). XPS measurements were performed using an XSAM800 X-ray photoelectron spectroscopy (Kratos, Britain). TGA measurements were carried out from 25 to 800 ºC in an oxygen atmosphere at a heating rate of 10 ºC min-1 with a stare system thermogravimetric analysis (Mettler Toledo, Switzerland). 2.4.

Electrochemical measurements The electrochemical measurements were conducted on a CHI 600E electrochemical analysis

instrument with a three-electrode system, including a glass carbon electrode (GCE, 3 mm in diameter) used as working electrode, a Pt wire used as counter electrode, and an Ag/AgCl (sat. KCl) used as reference electrode. The working electrode was prepared as follows: 5 mg catalysts powder and 25 μL Nafion117 solution were ultrasonically dispersed in a mixture solution of 2 mL ethanol and 2 mL water. After that, 10 μL catalyst suspension was carefully coated on the surface of the well-polished glass carbon electrode and dried at ambient environment for 3 h. The electrochemically active surface area (ECSA) of the as-prepared catalysts was measured by the cyclic voltammetry (CV) scanning in a 0.5 M H2SO4 solution within a potential from 0 to 1.2 V. The ECSA was calculated from: ECSA =

𝑄𝐻 0.21 × 𝑀𝑃𝑡

in which the QH (mC) is the integral charge of the H adsorption peak after calibrating with the baseline of double layer area. 0.21 mC cm-2 is the approximate constant for the adsorption/desorption of single layer H atom from the unit area Pt surface. And MPt is the Pt mass loaded on the working electrode. The electrocatalytic activity of the catalysts for the MOR was measured by recording CV plots in a 0.5 M H2SO4 and 1 M CH3OH solution within a potential range from 0.2 to 1.2 V. To investigate the stability of the catalysts, chronoamperometric (I−t)


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tests was performed at a set potential at 0.8 V for a duration of 3600 s. Before all the electrochemical measurements, the electrolyte solution was saturated with high-purity N2 gas for 15 min. Serval CV scans were carried out before the reproducible voltammograms were obtained and only the last voltammogram was recorded. All the potentials reported in this study were referenced to the RHE. 3. RESULT AND DISCUSSION The as-prepared Pt/C, Pt/GO, and Pt/rGOAMs catalysts were initially characterized by the powder X-ray diffraction (XRD) shown in Figure 1 to identify the crystal structure. The sharp diffraction peak at 10.30° of GO (Supporting Information, Figure S2) move to higher angle around 24° in the patterns of Pt/GO, and Pt/rGOAMs, which indicates that the GO has been reduced into graphene in the thermal process. Meanwhile, the diffraction peaks located at 39.8°, 46.8°, 67.8°, and 81.4° correspond to the characteristic (111), (200), (220) and (311) crystalline planes of Pt with face-centered-cubic (fcc) crystal structure, respectively. All the invested catalysts possess sharper and more intense peak at 39.8° assigned to the poison-resistant and highly catalytically active Pt(111) lattice33,34. The Pt/C, Pt/GO, and Pt/rGOAMs hold 49%, 57%, and 56% of Pt(111) lattice among entire Pt crystal..

Figure 1. XRD patterns of Pt/C, Pt/GO, and Pt/rGOAMs.


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a)

b)

c)

d)

e)

f)

Figure 2. Typical SEM images of rGOAMs (a-c), Pt/rGOAMs (d, e) and Pt/GO (f). The morphology and the nanostructure of the prepared catalysts were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Typical SEM images of rGOAMs (Figure 2, a, b and c) show a unique “center-diverging microchannel structure as our previous work32, presenting perfect spherical shapes as well as highly porous network. Figure 2d-e show the morphology of the rGOAMs decorated with Pt nanoparticles. The Pt/rGOAMs retain the porous network after the EG reduction process benefiting from the thermal reduction of GOAMs while the spherical shapes and surface morphology are partly influenced due to the mechanical stirring and repeated freeze-drying. Figure 2e demonstrates the cellular interconnected pores on the surface of Pt/rGOAMs hybrids at a diameter of several micrometers providing a readily accessible channels for mass transfer. However, the Pt/GO hybrids (Fig 2f) present a totally different morphology in which the graphene sheets are seriously stacked, resulting in a decrease in accessible surface area. The Pt component is unable to be loaded on the imbedded graphene sheets and the re-stacking of Pt/GO can cause a large loss in active reaction sites accompanying with increasing pressure on mass transportation. The morphology and distribution of Pt


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a)

b)

c)

d)

e)

f)

g)

h)

i)

Figure 3. Morphological and nanostructural analysis of the Pt based catalysts. a-i) The TEM images and Pt nanoparticles distribution of Pt/C (a-c), Pt/GO (d-f), and Pt/rGOAMs(g-i). nanoparticles of Pt/C (Figure 3a-b), Pt/GO (Figure 3d-f), and Pt/rGOAMs (Figure 3g-i) were observed by TEM. From Figure 3a, it can be seen that dozens of conductive black carbon particles aggregate together, in which Pt nanoparticles are unevenly attached. The Pt particles possess an average diameter at 4.39 nm (Figure 3c) showing serious agglomeration in red circles of Figure 3b. The Pt nanoparticles on Pt/GO hybrids (Figure 3d) hold a larger average diameter at 5.19 nm (Figure 3f) showing less agglomeration while the Pt particles are not uniformly distributed on the graphene sheets (Figure 3e). Figure 3g, h clearly demonstrates the well-dispersed Pt nanoparticles of Pt/rGOAMs with average diameter of 4.72 nm (Figure 3i). The Pt nanoparticles present a nearly


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monodisperse morphology along with narrow particle size distribution between 4-5.5 nm. The uniform disperse of Pt nanoparticles indicates a strong anchoring effect between Pt particles and graphene sheets benefiting from the residual functional groups. The functional groups have strong attraction forces toward Pt particles as well as ion exchange capabilities acting as metal-anchoring sites35. The strong anchoring effect can prevent Pt particles aggregating and detaching thus providing more reaction active site at a fixed Pt content.

a)

b)

c)

d)

Figure 4. a) XPS wide scan of Pt/C, Pt/GO, and Pt/rGOAMs. b-c) C1s narrow scan of GO (b) and Pt/rGOAMs (c). d) Pt 4f narrow scan of Pt/rGOAMs. To further evaluate the chemical composition on the surface of the as-prepared catalysts, Xray photoelectron spectroscopy (XPS) measurements were carried out. As shown in Figure 4a, the C, O, and Pt peaks are clearly observed. Furthermore, the as-measured C1s spectrum can be


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deconvoluted into several signals centered at around 284.8 eV (sp2 C–C), 286.0 eV (C–OH), and 288.9 eV (O-C=O), and it is clear that the intensity of oxygenated functional groups on graphene sheets in both Pt/GO (Figure S3, supporting information) and Pt/rGOAMs (Figure 4c) was significantly declined compared with the initial graphite oxide (Figure 4b). Resulting from the removal of oxygenated groups, the conjugated carbon structure is well developed thus converting to graphene nanosheets. Meanwhile, the elemental ratio of C/O for GO calculated from XPS is 2.37, increasing to 5.37 for rGOAMs, 9.26 for Pt/GO, and 9.15 in Pt/rGOAMs, which further indicates that the GO has been transferred to graphene by removing the surface functional groups in the thermal processing. Additionally, the deconvoluted Pt 4f spectrum in Figure 4d present two chemical states of Pt in the catalysts: the metallic Pt at the double binding energies of 71.3 and 74.7 eV, and the 2+ oxidation state of Pt at the binding energies of 72.4 and 76.6 eV. The ratio of Pt2+ and Pt was calculated from Pt 4f narrow scan, indicating that more than 80% of Pt2+ have been reduced to metal Pt through EG reduction.6 The Pt/rGOAMs hybrids are expected to present excellent performance in the application of DMFC anode catalyst due to the attractive architectural features. The electrocatalytic properties of the as-prepared catalysts were initially investigated by means of cyclic voltammograms (CV) in 0.5 M H2SO4 solution shown in Figure 5a. The electrochemically active surface area (ECSA) calculated from the hydrogen adsorption Coulombic charge in Figure 5a is a crucial parameter for supporting materials which indicates the accessible electrochemically active sites of the catalyst. Remarkably, Pt/rGOAMs possess the highest ECSA of 44.74 m2 g-1 compared with the Pt/C (15.89 m2 g-1) and Pt/GO (26.78 m2 g-1) evaluated at the same condition with the similar Pt concentration (around 20 wt%). Unlike the conductive carbon black and graphene oxide which tend to aggregate together thus imbedding the reaction active site, the Pt/rGOAMs with distinct diverging


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microchannels can mostly prevent the graphene nanosheets from aggregating resulting in increased accessible reaction active sites which contributes to a significant improvement of the ECSA. The methanol oxidation electrocatalytic activity was then investigated by cyclic voltammograms (CV) in a mixed solution of 0.5 M H2SO4 and 1 M CH3OH. As depicted in Figure 5b, the Pt/rGOAMs electrode exhibits a remarkably highest mass current density of 1098.9 mA mg-1, which is almost 2.5-fold greater than that of Pt/GO (434.7 mA mg-1) and 5.5-fold greater than that of Pt/C (199.0 mA mg-1), respectively, revealing that the Pt/rGOAMs catalysts possess the most efficient catalytic activity towards methanol oxidation. Such an enhanced catalytic activity towards MOR can be ascribed to: (1) The ultra-uniform dispersion of Pt nanoparticles on Pt/rGOAMs. The Pt nanoparticles on Pt/rGOAMs present a nearly monodisperse morphology with almost none particle agglomeration, showing a totally different pattern from control samples. Well-dispersed Pt nanoparticles provide more accessible surface area at a fixed Pt content for fuel absorption and proton exchange (The CH3OH molecules are adsorbed on the surface of the Pt particles and dehydrogenate in multiple steps forming carbon-containing intermediate poisoning species. The oxygenated species generated from dissociated water then react with the carbon-containing intermediate poisoning species and eventually oxidize to CO2.). (2) The distinct center-diverging microchannels in the microspheres. The graphene sheets tend to aggregate together in Pt/GO result in embedding as well as agglomeration of Pt nanoparticles, while the porous aerogel structure in Pt/rGOAMs could maximally inhibit the aggregation providing more catalytically active Pt sites. In 2005, Duan et al. have successfully dispersed Pt nanoparticles on GOA (graphene oxide aerogel) which presented catalytic activity towards MOR of 876 mA mg-1 (2 times of commercial Pt/C (437 mA mg-1)28. While in this study, the as prepared Pt/rGOAMs present a rather enhanced activity towards MOR of 1098.9 mA mg-1 (5.5 times of Pt/C catalysts (199.0 mA mg-1), indicating


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the rGOAMs are preferable catalyst supports for DMFC anode catalysts. Furthermore, the asprepared Pt/rGOAMs also show a competitive catalytic activity compared with the newly developed Pt catalysts such as Pt/NGA5 at 507.5 mA mg-1, Pt/RuO2/G12 at 841.9 mA mg-1, and Pt/gahene-TiO236 at 423.3 mA mg-1.

Figure 5. Electrocatalytic properties of the Pt/C, Pt/GO, and Pt/rGOAMs. Cyclic voltammograms of the Pt/C, Pt/GO, and Pt/rGOAMs electrodes recorded in 0.5 M H2SO4 solution (a) and mixture of 0.5 M H2SO4 and 1 M CH3OH (b) at a scan rate of 50 mV s−1, indicating that Pt/rGOAMs have the largest ECSA values and highest electrocatalytic activity among all the investigated catalysts. c-d) Chronoamperometric (I−t) curves of the Pt/C, Pt/GO, and Pt/rGOAMs electrodes recorded in a solution of 0.5 M H2SO4 and 1 M CH3OH at 0.8 V. In order to further verify the structural priority of rGOAMs, monolithic graphene aerogels were prepared (Supporting Information, Figure S4) to serve as supports of Pt particles as a control


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sample (denote as Pt/rGOA). Some noticeable agglomeration of Pt nanoparticles on rGOA can be observed and the Pt particles are unevenly dispersed (Supporting Information, Figure S5). The Pt/rGOA present an ESCA at 30.3 m2 g-1 along with a catalytic activity towards MOR at 741.4 mA mg-1 in the electrocatalytic measurements (Supporting Information, Figure S6) which are not performing as well as Pt/rGOAMs. Compared with monolithic graphene aerogel, the rGOAMs with micron size and center-diverging microchannels are beneficial to the loading of Pt particles as well as mass transportation of fuel and protons which can demonstrate enhanced catalytic performance. The long-term antipoisonous ability of the anode catalysts is another challenge issue that largely retarded the commercialization process. In this study, I−t measurements were performed to evaluate the long-term stability of the as-prepared catalysts at a potential of 0.8 V for 3600 s (Figure 5, c and d). The catalytic activity of the three electrodes decreased gradually over time as the accumulation of the CO like poisoning species occupying the Pt reaction active sites. Remarkably, the current density of Pt/C decline to 9.5% over a period of 3600 s, and the Pt/GO decline to 19.76% while the Pt/rGOAMs hold 36.1% of the maximum current density showing a enhanced antipoisonous ability towards MOR. The distinct porous structure in the microspheres maximally prevent the aggregating of graphene nanosheets thus averting the decrease of catalytic activity due to the imbedding of the Pt nanoparticles, showing enhanced stability towards MOR. 4. CONCLUSION In summary, we have demonstrated the successful fabrication of Pt-decorated 3D architectures derived from reduced graphene oxide aerogel microspheres (rGOAMs) through a combined electrospraying, freeze-casting and solvothermal process. The rGOAMs retain the distinct “centerdiverging microchannel” dandelion-like structures by thermal reduction contributing to the


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homogenous dispersion of Pt particles. Benefiting from the distinct microstructure, the resulting Pt/rGOAMs hybrids present excellent electrocatalytic ability in terms of high electrocatalytic activity and high poison tolerance towards MOR compared with Pt/C and Pt/GO catalysts, showing outstanding performance to be used as anode electrocatalysts for DMFCs. This study together with our previous work disclose the potential of graphene aerogel microspheres applied as sensors, supercapacitors, photocatalysis, and batteries. ASSOCIATED CONTENT Supporting Information The Supporting Information is available. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the International Cooperative Research Program between National Natural Science Foundation of China and Italian Ministry of Foreign Affairs (51861135201). The author also thanks the financial support from State Key Laboratory of Polymer Materials Engineering of Sichuan University (Grant No. sklpme2017-3-04). REFERENCES


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(1).

Page 16 of 22

Ren, L.; Hui, K. S.; Hui, K. N., Self-assembled free-standing three-dimensional nickel nanoparticle/graphene aerogel for direct ethanol fuel cells. J. Mater. Chem. A 2013, 1, 5689-5694.

(2).

Huang, H.; Ma, L.; Tiwary, C. S.; Jiang, Q.; Yin, K.; Zhou, W.; Ajayan, P. M., Worm-Shape Pt Nanocrystals Grown on Nitrogen-Doped Low-Defect Graphene Sheets: Highly Efficient electrocatalysts for methanol oxidation reaction. Small, 2017, 13, 1603013.

(3).

Wachsman, E. D.; Marlowe, C. A.; Lee, K. T., Role of solid oxide fuel cells in a balanced energy strategy. Energy Environ. Sci. 2012, 5, 5498-5509.

(4).

Zhao, L.; Wang, Z.-B.; Li, J.-L.; Zhang, J.-J.; Sui, X.-L.; Zhang, L.-M., One-pot synthesis of a threedimensional graphene aerogel supported Pt catalyst for methanol electrooxidation. RSC Adv. 2015,

5, 98160-98165. (5).

Zhao, L.; Sui, X.-L.; Li, J.-Z.; Zhang, J.-J.; Zhang, L.-M.; Huang, G.-S.; Wang, Z.-B., Supramolecular assembly promoted synthesis of three-dimensional nitrogen doped graphene frameworks as efficient electrocatalyst for oxygen reduction reaction and methanol electrooxidation. Appl. Catal.

B: Environ. 2018, 231, 224-233. (6).

Huang, H.; Yang, S.; Vajtai, R.; Wang, X.; Ajayan, P. M., Pt-decorated 3D architectures built from graphene and graphitic carbon nitride nanosheets as efficient methanol oxidation catalysts. Adv.

mater. 2014, 26, 5160-5165. (7).

Huang, H.; Wang, X., Recent progress on carbon- based support materials for electrocatalysts of direct methanol fuel cells. J. Mater. Chem. A 2014, 2, 6266-6291.


16

Page 17 of 22 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

Langmuir

(8).

Debe, M. K., Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012,

486, 43-51. (9).

Lu, Y.; Du, S.; Steinberger-Wilckens, R., One-dimensional nanostructured electrocatalysts for polymer electrolyte membrane fuel cells—A review. Appl. Catal. B: Environ. 2016, 199, 292-314.

(10).

Tsai, M.-C.; Nguyen, T.-T.; Akalework, N. G.; Pan, C.-J.; Rick, J.; Liao, Y.-F.; Su, W.-N.; Hwang, B.-J., Interplay between Molybdenum Dopant and Oxygen Vacancies in a TiO2 Support Enhances the Oxygen Reduction Reaction. ACS Catal. 2016, 6, 6551-6559.

(11).

Zhao, L.; Wang, Z.-B.; Li, J.-L.; Zhang, J.-J.; Sui, X.-L.; Zhang, L.-M., Hybrid of carbon-supported Pt nanoparticles and three dimensional graphene aerogel as high stable electrocatalyst for methanol electrooxidation. Electrochimi. Acta 2016, 189, 175-183.

(12).

Huang, H.; Zhu, J.; Li, D.; Shen, C.; Li, M.; Zhang, X.; Jiang, Q.; Zhang, J.; Wu, Y., Pt nanoparticles grown on 3D RuO2-modified graphene architectures for highly efficient methanol oxidation. J. Mater.

Chem. A 2017, 5, 4560-4567. (13).

Kaluža, L.; Larsen, M. J.; Zdražil, M.; Gulková, D.; Vít, Z.; Šolcová, O.; Soukup, K.; Koštejn, M.; Bonde, J. L.; Maixnerová, L.; Odgaard, M., Highly loaded carbon black supported Pt catalysts for fuel cells. Catal. Today 2015, 256, 375-383.

(14).

Chen, S.; Wei, Z.; Qi, X.; Dong, L.; Guo, Y. G.; Wan, L.; Shao, Z.; Li, L., Nanostructured polyanilinedecorated Pt/C@PANI core-shell catalyst with enhanced durability and activity. J. Am. Chem. Soc. 2012, 134, 13252-13255.


17

Langmuir 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

(15).

Page 18 of 22

Chang, J.; Feng, L.; Liu, C.; Xing, W.; Hu, X., Ni2P enhances the activity and durability of the Pt anode catalyst in direct methanol fuel cells. Energy Environ. Sci. 2014, 7, 1628-1632.

(16).

Kitahara, T.; Nakajima, H.; Okamura, K., Gas diffusion layers coated with a microporous layer containing hydrophilic carbon nanotubes for performance enhancement of polymer electrolyte fuel cells under both low and high humidity conditions. J. Power Sources 2015, 283, 115-124.

(17).

Zhang, J.-J.; Wang, Z.-B.; Li, C.; Zhao, L.; Liu, J.; Zhang, L.-M.; Gu, D.-M., Multiwall-carbon nanotube modified by N-doped carbon quantum dots as Pt catalyst support for methanol electrooxidation. J. Power Sources 2015, 289, 63-70.

(18).

Zhao, L.; Wang, Z.-B.; Sui, X.-L.; Yin, G.-P., Effect of multiwalled carbon nanotubes with different specific surface areas on the stability of supported Pt catalysts. J. Power Sources 2014, 245, 637643.

(19).

Li, Y.; Gao, W.; Ci, L.; Wang, C.; Ajayan, P. M., Catalytic performance of Pt nanoparticles on reduced graphene oxide for methanol electro-oxidation. Carbon 2010, 48, 1124-1130.

(20).

Xia, B.; Yan, Y.; Wang, X.; Lou, X. W., Recent progress on graphene-based hybrid electrocatalysts.

Mater. Horiz. 2014, 1, 379-399. (21).

Zhao, L.; Wang, Z.-B.; Li, J.-L.; Zhang, J.-J.; Sui, X.-L.; Zhang, L.-M., A newly-designed sandwichstructured graphene–Pt–graphene catalyst with improved electrocatalytic performance for fuel cells.

J. Mater. Chem. A 2015, 3, 5313-5320. (22).

Huang, H.; Zhu, J.; Zhang, W.; Tiwary, C. S.; Zhang, J.; Zhang, X.; Jiang, Q.; He, H.; Wu, Y.;


18

Page 19 of 22 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

Langmuir

Huang, W.; Ajayan, P. M.; Yan, Q., Controllable Codoping of Nitrogen and Sulfur in Graphene for Highly Efficient Li-Oxygen Batteries and Direct Methanol Fuel Cells. Chem. Mater. 2016, 28 (6), 1737-1745. (23).

Bai, S.; Shen, X., Graphene–inorganic nanocomposites. RSC Adv. 2012, 2, 64-98.

(24).

Zhang, L. Y.; Zhao, Z. L.; Yuan, W.; Li, C. M., Facile one-pot surfactant-free synthesis of uniform Pd6Co nanocrystals on 3D graphene as an efficient electrocatalyst toward formic acid oxidation.

Nanoscale 2016, 8, 1905-9. (25).

Huang, Q.; Tao, F.; Zou, L.; Yuan, T.; Zou, Z.; Zhang, H.; Zhang, X.; Yang, H., One-step Synthesis of Pt Nanoparticles Highly Loaded on Graphene Aerogel as Durable Oxygen Reduction Electrocatalyst. Electrochimi. Acta 2015, 152, 140-145.

(26).

Yin, H.; Zhang, C.; Liu, F.; Hou, Y., Hybrid of Iron Nitride and Nitrogen-Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction. Adv. Func. Mater. 2014, 24, 2930-2937.

(27).

Zhao, L.; Sui, X.-L.; Li, J.-L.; Zhang, J.-J.; Zhang, L.-M.; Wang, Z.-B., Ultra-fine Pt nanoparticles supported on 3D porous N-doped graphene aerogel as a promising electro-catalyst for methanol electrooxidation. Catal. Commun. 2016, 86, 46-50.

(28).

Duan, J.; Zhang, X.; Yuan, W.; Chen, H.; Jiang, S.; Liu, X.; Zhang, Y.; Chang, L.; Sun, Z.; Du, J., Graphene oxide aerogel-supported Pt electrocatalysts for methanol oxidation. J. Power Sources 2015, 285, 76-79.

(29).

Kwok, Y. H.; Tsang, A. C. H.; Wang, Y.; Leung, D. Y. C., Ultra-fine Pt nanoparticles on graphene


19

Langmuir 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 22

aerogel as a porous electrode with high stability for microfluidic methanol fuel cell. J. Power

Sources 2017, 349, 75-83. (30)

Gao, Z.; Li, M.; Wang, J.; Zhu, J.; Zhao, X.; Huang, H.; Zhang, J.; Wu, Y.; Fu, Y.; Wang, X., Pt nanocrystals grown on three-dimensional architectures made from graphene and MoS2 nanosheets: Highly efficient multifunctional electrocatalysts toward hydrogen evolution and methanol oxidation reactions. Carbon 2018, 139, 369-377.

(31)

Yan, M.; Jiang, Q.; Zhang, T.; Wang, J.; Yang, L.; Lu, Z.; He, H.; Fu, Y.; Wang, X.; Huang, H., Three-dimensional low-defect carbon nanotube/nitrogen-doped graphene hybrid aerogelsupported Pt nanoparticles as efficient electrocatalysts toward the methanol oxidation reaction. J.

Mater. Chem. A 2018, 6, 18165-18172. (32)

Liao, S.; Zhai, T.; Xia, H., Highly adsorptive graphene aerogel microspheres with center-diverging microchannel structures. J. Mate. Chem. A 2016, 4, 1068-1077.

(33)

Xie, B.; Zhang, Y.; Du, N.; Li, H.; Hou, W.; Zhang, R., Preparation of preferentially exposed poisonresistant Pt(111) nanoplates with a nitrogen-doped graphene aerogel. Chem. Commun. 2016, 52, 13815-13818.

(34)

Adzic, R. R.; Tripkovic, A. V.; Ogrady, W. E., Structural effects in electrocatalysis. Nature 1982,

296, 137-138. (35)

Chien, C.-C.; Jeng, K.-T., Effective preparation of carbon nanotube-supported Pt–Ru electrocatalysts. MateR. Chem. Phys. 2006, 99, 80-87.


20

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Langmuir

(36)

Zhao, L.; Wang, Z.-B.; Liu, J.; Zhang, J.-J.; Sui, X.-L.; Zhang, L.-M.; Gu, D.-M., Facile one-pot synthesis of Pt/graphene-TiO2 hybrid catalyst with enhanced methanol electrooxidation performance. J. Power Sources 2015, 279, 210-217


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Pt Nanoparticles Loaded Graphene Aerogel Microspheres with Excellent Methanol Electrooxidation Performance Miao He, Guoxia Fei, Zhuo Zheng, Zhengang Cheng, Zhanhua Wang*, Hesheng Xia

State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute, Sichuan University, Chengdu 610065, PR China.

Email: [email protected]

Table of content:


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Pt Nanoparticle-Loaded Graphene Aerogel Microspheres with

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