3D Hierarchical Pt-Nitrogen-Doped-Graphene ... - ACS Publications

Research Article www.acsami.org

3D Hierarchical Pt-Nitrogen-Doped-Graphene-Carbonized Commercially Available Sponge as a Superior Electrocatalyst for Low-Temperature Fuel Cells Lei Zhao, Xu-Lei Sui, Jia-Long Li, Jing-Jia Zhang, Li-Mei Zhang, and Zhen-Bo Wang* School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West-Da Zhi Street, Harbin, 150001 China S Supporting Information *

ABSTRACT: Three-dimensional hierarchical nitrogen-doped graphene (3D-NG) frameworks were successfully fabricated through a feasible solution dip-coating method with commercially available sponges as the initial backbone. A spongy template can help hinder the graphene plates restacking in the period of the annealing process. The Pt/3D-NG catalyst was synthesized employing a polyol reduction process. The resultant Pt/3D-NG exhibits 2.3 times higher activity for methanol electro-oxidation along with the improvement in stability as compared with Pt/G owing to their favorable features including large specific surface area, high pore volume, high N doping level, and the homogeneous dispersion of Pt nanoparticles. Besides, Pt/3D-NG also presents high oxygen reduction reaction (ORR) performance in acid media when compared with Pt/3D-G and Pt/G. This work raises a valid solution for the fabrication of 3D functional freestanding graphene-based composites for a variety of applications in fuel cell catalysis, energy storage, and conversion. KEYWORDS: graphene, commercial sponge, 3D hierarchical structure, Pt-based electrocatalyst, fuel cells

1. INTRODUCTION The extensive use of automobiles throughout the world has aroused and continues to arouse a series of severe problems such as atmospheric pollution, greenhouse gas emissions, and rapid depletion of fossil fuel resources. In recent decades, electric vehicles, hybrid electric vehicles, and fuel cell vehicles have been explored to take the place of conventional vehicles owing to their high efficiency and being environmentally friendly and safe.1−3 Compared with a chemical battery, fuel cells possess unique advantages including high energy density, fast start-up, zero-emission, and low temperature operation.4−6 Therefore, research and development of automobile fuel cell technologies has become a hot topic in the research of newenergy vehicles. © 2016 American Chemical Society

Nevertheless, large-scale commercialization of fuel cells demands intensive research and development (R&D) to meet several challenges: lifetime, reliability, and cost.7−9 To maximize the utilization of the noble metal and cut the cost, Pt nanoparticles (NPs) are generally deposited on the supporting materials of high surface area. Carbon materials are widespread applied as supporting materials for fuel cells.10−13 Graphene, a monolayer graphite plate with a hexagonal packed lattice, has generated enormous interest since the first discovery by Geim et al. in 2004.14−17 Despite its wide range of potential Received: March 23, 2016 Accepted: June 7, 2016 Published: June 7, 2016 16026

DOI: 10.1021/acsami.6b03520 ACS Appl. Mater. Interfaces 2016, 8, 16026−16034

Research Article

ACS Applied Materials & Interfaces

Figure 1. Illustration of the preparation procedure for the 3D-NG.

2. EXPERIMENTAL SECTION

applications and very promising features relative to other carbon materials, it is still yet unclear whether graphene possesses a high value of practical application in electrochemical energy storage and conversion.18 For instance, a graphene sheet shows a strong tendency to form irreversible agglomerates caused by van der Waals force and π−π stacking interactions when graphene suspended in solutions is dried; therefore, the surface area of graphene will decrease dramatically, which greatly hampers its application in electrode materials.19−22 To confront such a problem, three-dimensional (3D) structured graphene has been proposed and prepared by various methods successfully, such as self-assembly,23 substratebased deposition,24 aerosolization,25 chemical vapor deposition (CVD),26 etc. In comparison to graphene, the outstanding characteristics of 3D graphene including a large available surface area, multidimensional electrical conduction, excellent mechanical strength, and unique porous architecture render it a promising support candidate for fuel cells.27−29 Wang et al. report Pt-loaded 3D hybrids constructed from g-C3N4 and graphene nanosheets as anode catalysts with excellent electrocatalytic performance for methanol oxidation.30 In another work, the 3D graphene network is fabricated by CVD and served as the support for Pt NPs, and the resulting Pt NP/3Dgraphene catalyst presents a remarkable electrocatalytic performance in regard to high catalytic activity and enhanced stability toward methanol electro-oxidation.31 However, there is an urgent need to propose a convenient and effective method to fabricate 3D graphene with high surface area, pore structure, and efficient heteroatoms doping to serve as support for Pt NPs in fuel cell applications. Herein, with commercial polyurethane (PU) sponge as the raw material, we report an efficient and convenient method to fabricate 3D hierarchical nitrogen-doped graphene. With favorable mass transport through the hierarchical porous architecture, the highly accessible surface area, the high N doping level, and the homogeneous dispersion of Pt NPs, the resulting 3D N-doped-graphene can substantially enhance the electrocatalytic activity and stability for methanol oxidation as well as for oxygen reduction when using a support for the Pt NPs. We believe that this facile and low-cost synthetic protocol can be appropriate for the synthesis of functionalized 3D graphene based composites, which reveal the great potential applications for advanced energy storage.

2.1. Fabrication of the 3D N-Doped Graphene (3D-NG). Graphite oxide (GO) was prepared via the modified Hummers method with graphite powder as raw material.32 3D-NG was fabricated via a facile solution dip-coating method with a commercial sponge as the template. Thirty milligrams of GO and 300 mg of urea were thoroughly dissolved into 30 mL of deionized water by ultrasonic treatment for 30 min, and a piece of the commercial polyurethane (PU) sponge was fully soaked in the solution. After ultrasonic treatment for about 10 min to fully immerse it, the sponge was taken out and dried at 60 °C, repeatedly. In this process, GO sheets and urea were attached on the PU foam framework. Then the sponge was calcinated at 900 °C in the horizontal tube furnace in Ar atmosphere for 1 h. 3D graphene (3D-G) is synthesized by the same procedure, but no urea was added. Moreover, common graphene was also synthesized by directly annealing GO at 900 °C in argon atmosphere for 1 h. The synthetic strategy of the 3D-NG supported Pt NPs hybrids is schematically illustrated in Scheme S1. 2.2. Synthesis of Pt/3D-NG. The Pt/3D-NG catalyst with the Pt content of 20 wt % was prepared through a microwave-assisted polyol process (MAPP).33 Briefly, 20 mg of prepared 3D-NG was mixed with 60 mL of ethylene glycol (EG) by ultrasonication for 1 h. Subsequently, the calculated amount of H2PtCl6/EG solution was joined into the dispersion and stirred for 3 h. The pH value of the solution was adjusted to 12.0 by using 1 mol L−1 NaOH/EG solution, followed by consecutive microwave heating for 55 s. Final products were collected after having been washed with deionized water repeatedly followed by vacuum drying. As a benchmark, Pt/3D-G and Pt/graphene (Pt/G) were synthesized using the same method. ICP analysis indicates the Pt loadings in Pt/G, Pt/3D-G, and Pt/3DNG catalysts are 16.3, 17.5, and 18.3 wt %, respectively. 2.3. Characterization. The morphologies of all samples were characterized by a scanning electron microscope (SEM, Hitachi S4800), a field emission transmission electron microscope (TEM), and high resolution TEM (HRTEM, FEI Tecnai G2 F20). X-ray diffraction (XRD) analysis was carried out on the D/max-RB diffractometer. Xray photoelectron spectroscopy (XPS) analysis was collected using a physical electronics PHI model 5700 instrument. The specific surface areas of samples were investigated by N2 adsorption−desorption with Brunauer−Emmett−Teller (BET) methods using a QUADRASORB SI analyzer. Raman spectra of samples were obtained on a Renishaw1000 Raman microscope. The Pt content in each sample was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Electrochemical measures were performed on a CHI 650E electrochemical analysis instrument using a standard three-electrode cell at an ambient temperature, with a glassy carbon disk electrode covered with a prepared catalyst, a platinum wire, and Hg/Hg2SO4 (0.68 V relative to reversible hydrogen electrode, RHE) 16027


Research Article

ACS Applied Materials & Interfaces as working, counter, and reference electrodes, respectively. In this study, all potentials are given versus RHE. More detailed information about the process of electrode preparation, physical characterization, and electrochemical measurement is provided in the Supporting Information.

porous network was constituted with layered graphene sheets (Figure 3d). Such a 3D-structured framework could provide them a highly accessible surface area, which provides not only more exposed nucleation sites for the anchoring of Pt precursors but also more accessibility for reactants to the catalysts to guarantee a sufficient mass transfer.36,37 As a contrast experiment, the graphene synthesized without a sponge template, as shown in Figures 3f, g, displays intensive agglomeration, suggesting the strong tendency to restack graphene sheets. Thereby, it is believed that the PU sponge is of great importance in preventing the agglomeration of graphene during the annealing process.38 The structural changes of initial GO to G after the annealing process were investigated by XRD (Figure 4a). The dominant (002) diffraction peak of GO appearing at 2θ = 10.0° reveals that GO was obtained through the efficient oxidation of the raw graphite.39 This characteristic peak vanishes, and a broad peak appears at around 26.2° after the annealing process, demonstrating the valid reduction of GO and the emergence of G or 3D-G.40 Nitrogen adsorption−desorption measurement was applied to characterize the structure of 3D-G samples. The obtained pore structure parameters of G and 3D-G are summarized in Table S1. The surface area of the 3D-G is 101.8 m2 g−1, which is much greater than that of the directly annealed GO (only 5.2 m2 g−1) gained by using the Brunauer− Emmett−Teller (BET) theory (Figure 4b, c), indicating the introduction of the PU sponge template can effectively inhibit the restacking of graphene nanosheets, which is in line with the previous SEM results. Furthermore, the total pore volumes for 3D-G and G are about 0.21 cm3 g−1 and 0.05 cm3 g−1, respectively. The higher pore volume of 3D-G is very beneficial for providing rich active sites and accelerating electron or mass transfer.41 FT-IR spectra of GO and 3D-NG are displayed in Figure S1. GO presents characteristic peaks centered at 1720, 1400, 1220, and 1050 cm−1, which could be attributed to carboxyl (−COOH), hydroxyl (−OH), epoxy (−C−O−C−), and alkoxy (C−O), respectively.42 On the contrary, for the 3DNG sample, these representative bands decayed significantly in intensity, implying the efficient reduction of the oxygen functional groups due to the well-known thermal reduction process. Furthermore, two peaks appeared at 1560 and 1150 cm−1, belonging to the vibration of CN and C−O/C−N bonds, respectively,43 confirming N doping in the graphene structure. XPS analysis was used to interrogate the element states on the surface of the samples. The N 1s peak located at ≈401 eV is clearly seen in 3D-NG materials, indicating the presence of nitrogen (N) in the 3D-NG with the atomic percentage of 6.19 at %. (Figure 4d). High-resolution C 1s XPS spectra of GO, G, 3D-G, and 3D-NG are shown in Figure S2. One main peak at 284.6 eV is ascribed to the C−C bonds, and the peaks at 285.7, 286.2, 287.6, and 289.0 eV can be attributed to C−N, C−O, CO, and O−CO bonds.44 After the annealing treatment, the peaks of oxygen functional groups decrease considerably in intensity, indicating the efficient deoxygenation by the thermal reduction process. The properties of doped N can be further investigated by the highresolution XPS analysis. The N 1s spectra of the 3D-NG (Figure 4e) can be deconvoluted into mainly four peaks at ≈398.2, 399.4, 400.9, and 402.7 eV, which could be sequentially attributed to the pyridine-like N, pyrrole-like N, graphite-like N, and oxidized N. 45,46 Theoretical calculations have corroborated that the import of N species in carbon materials will promote the interaction between Pt and the supports, thus

3. RESULTS AND DISCUSSION The procedure for the fabrication of 3D N-doped graphene (3D-NG) is illustrated in Figure 1. A commercial polyurethane (PU) sponge was chosen as the raw material for the 3D-NG network. The GO nanosheets with urea were first covered on the skeleton of a PU sponge through a simple immersion route. In this design, the 2D structure of GO nanosheets and watersoluble urea can be efficiently supported by the interconnected and continuous 3D framework of the sponge. Subsequently, the composite monolith was annealed under argon to transform the sponge-shaped composite into the 3D-NG framework at argon atmosphere. At a high temperature (900 °C), GO sheets were turned into reduced graphene oxide (rGO) through a wellknown thermal reduction process;34 at the same time, N atoms were doped into the graphene structure through decomposition of urea (Scheme S2).35 For comparison, graphene was also synthesized via directly annealing GO in Ar atmosphere at 900 °C for 1 h. The morphology of as-synthesized materials was first interrogated by a scanning electron microscopy (SEM). As seen from Figure 2a, without GO, the pristine PU sponge

Figure 2. SEM images of the PU sponge (a) and the PU sponge loaded with GO (b).

exhibits many macropores interconnected with each other. After having been saturated with GO solution, it could be obviously seen that sheet-like GO are adhered on the skeleton of PU sponge. It is proposed that the agglomeration of graphene during the annealing process can be efficiently alleviated owing to the presence of PU sponge. After annealing, the 3D hierarchical architecture of the sponge is well preserved. The optical image demonstrates that 3D-G is a self-supported macroscopic cube around 1.0 cm in side-length (Figure 3a). In contrast, the directly annealed GO nanosheets are obtained in the usual RGO powder. SEM images reveal a porous 3D graphene network with well-defined interconnected macropores in the micrometer size range (Figure 3b, c). The high magnification SEM image shows clearly that the synthesized 3D 16028


Research Article


Figure 3. Optical image of 3D-G (a); SEM images of 3D-G (b-d); optical image of the G powder (e); SEM images of G (f, g).

Figure 4. XRD patterns of GO and 3D-G (a); nitrogen adsorption−desorption isotherms of G (b) and 3D-G (c); XPS spectra of G, 3D-G, and 3DNG (d); high-resolution N 1s XPS spectra of 3D-NG (e); Raman spectra of G, 3D-G, and 3D-NG (f).

impeding the aggregation of precious metal particles.47 Figure 4f shows the Raman spectra for G, 3D-G, and 3D-NG. The peak intensities ratio of the D and G band in 3D-NG is slightly higher in regard to that in G or 3D-G owing to the structural distortion caused by nitrogen doping, which could enhance the electrocatalytic performance of the 3D-NG sample.48,49 Pt NPs were loaded on the as-prepared graphene-based materials via a MAPP. The XRD pattern confirmed the formation of face-centered cubic (fcc) crystalline Pt in the hybrid (Figure S3). TEM images in Figure 5 show that the

directly annealed GO nanosheets are opaque and thick, suggesting the graphene sheets are intensively restacked together; by contrast, the graphene sheets in 3D-NG are transparent and thin, implying their restacking-free property.37 The uniformly distributed Pt NPs with 2.12 nm in average particle size appear on the surface of 3D-NG, showing almost no agglomeration (Figure 5b). Additionally, an interplanar distance of 0.227 nm is observed under the high-resolution TEM examination (Figure 5c), which is highly consistent with the fcc crystalline Pt (111) plane.50 It could be found that the 16029


Research Article


catalytic activity for methanol eletrooxidation when compared with Pt/G and Pt/3D-G, which is in accordance with the ECSA results and electrochemical impedance spectroscopy (EIS) results (Figure 6d). Besides, Pt/3D-NG also presents an outstanding oxygen reduction reaction (ORR) performance including a higher diffusion-limiting current, a more positive onset potential, and a half-wave potential in acid media than Pt/3D-G and Pt/G catalysts (Figure 6e), highlighting the importance of 3D nitrogen-doped graphene support for enhancing the ORR performance. Such a phenomenon can be ascribed to the import of N atoms into the graphene substrate, which could not only improve the dispersion of Pt NPs but also polarize the neighboring carbon atoms and expedite the dissolution of water molecules to produce OH, thus resulting in the efficient removal of intermediate species and a boost in catalytic performance for methanol electrooxidation.37,51 Regarding the oxygen reduction reaction, the performance enhancement could be assigned to the increased electron density caused by the N species incorporation, which changes the atomic charge state, elevates the Fermi energy, and reduces the band gap, hence enabling N-doped graphene to present higher activity for ORR.52,53 Overall, the improved activity of both MOR and ORR must originate from the increased surface area, 3D framework structure, homogeneous dispersion, and narrower size distribution of Pt NPs and the mutual interaction between Pt and 3D-NG, which enhances the utilization of Pt. Our findings can also be extrapolated to a PtRu catalyst supported on 3D-NG. The PtRu/3D-NG catalysts were synthesized in the same method. Compared with the commercial PtRu/C, PtRu/3D-NG catalysts exhibit much higher catalytic activity toward methanol electro-oxidation (Figure S4), highlighting the crucial significance of 3D nitrogen-doped graphene support for promoting the MOR performance. To evaluate the catalytic stabilities of the catalysts, chronoamperometric measurements were carried out in the solution of 0.5 mol L−1 H2SO4 and 0.5 mol L−1 CH3OH at 0.65 V (Figure 6f). The current densities on all the samples display a rapid decline in the beginning stage, probably because of the deterioration by the carbonaceous intermediates forming during methanol electooxidation.54,55 Nevertheless, the current density on the Pt/3D-NG is higher than those of Pt/3D-G and Pt/G catalyst electrodes over the whole testing time. At the end of the 3600s test, the current density on the Pt/3D-NG electrode is still 7 times higher than those on the other two electrodes. These findings indicated that the Pt/3D-NG catalyst presents high catalytic stability for the methanol electrooxidation. The long-term stability of the catalysts is of critical significance to affecting their practical application, which was examined in a solution of 0.5 mol L−1 H2SO4 and 0.5 mol L−1 CH3OH by consecutive CVs.56 The forward peak current densities on three catalyst electrodes decrease gradually with the continuous scan, as obtained from Figure 7. As calculated, for Pt/3D-NG, a 72% forward peak current remains after 1000 CV cycles, but, for Pt/3D-G and Pt/G, only 61% and 60% are retained. It is remarkable that the current density on Pt/3D-NG is much higher than those on Pt/3D-G and Pt/G catalyst electrodes (Figure 7d) during the whole durability testing process, manifesting the excellent long-term stability of Pt/3DNG. The ECSA before and after the stability tests is shown in Figure S6. The results show the ECSA decreases by 55.0, 54.5, and 37.6% for Pt/G, Pt/3D-G, and Pt/3D-NG after the

Figure 5. TEM micrographs (a, b) and the corresponding size distribution (d) of Pt/3D-NG; HRTEM image of Pt/3D-NG (c); TEM images of Pt/3D-G (e); and Pt/G (f).

Pt NPs deposited on G and 3D-G samples appear aggregated in some degree, while Pt NPs on the 3D-NG samples are quite uniform. This may be attributed to the fact that the interaction between Pt and the N-doped supports can efficiently inhibit Pt NPs migration and maintain Pt NPs in a more dispersed state.37,47 As a starting point, the electrochemical performance of Pt/ 3D-NG was screened by means of cyclic voltammograms (CVs) in 0.5 mol L−1 H2SO4 solution. Figure 6a displays CV curves of the Pt/3D-NG, Pt/3D-G, and Pt/G catalysts obtained at room temperature. Significantly, the Pt/3D-NG catalyst gives a higher ECSA (52.2 m2 g−1) than both the Pt/3D-G (29.2 m2 g−1) and Pt/G (18.4 m2 g−1) catalysts by integrating the Coulombic charge for hydrogen adsorption. It manifests that Pt/3D-NG is propitious for electrocatalytic reactions, thus offering more available active sites. The electrocatalytic behavior toward the methanol electro-oxidation reaction (MOR) was further performed in a solution containing 0.5 mol L−1 H2SO4 and 0.5 mol L−1 CH3OH. As seen from Figure 6b, a high mass activity of 551.5 mA mg−1 is achieved for Pt/ 3D-NG, which is 1.8 and 2.3 times that of the Pt/3D-G (305.7 mA mg−1) and Pt/G (242.1 mA mg−1) electrodes, respectively. Furthermore, as displayed in Figure 6c, the corresponding potential on Pt/3D-NG is much lower than these on Pt/3D-G or Pt/G catalysts at a given oxidation current density. This result indicates that Pt/3D-NG behaves better with electro16030


Research Article


Figure 6. CV of Pt/G, Pt/3D-G, and Pt/3D-NG catalysts in 0.5 mol L−1 H2SO4 (a) and in a solution of 0.5 mol L−1 H2SO4 + 0.5 mol L−1 CH3OH (b, c). Scanning rate: 50 mV s−1, test temperature: 25 °C; Nyquist plots of EIS for Pt/3D-NG, Pt/3D-G, and Pt/G recorded in 0.5 mol L−1 H2SO4 + 0.5 mol L−1 CH3OH. Polarization potential: 650 mV; test temperature: 25 °C (d). Polarization curves of Pt/3D-NG, Pt/3D-G, and Pt/G for ORR on RDE at 1600 rpm in oxygen-saturated 0.5 mol L−1 H2SO4 at 25 °C with a scan rate of 10 mV s−1 (e). Chronoamperometric curves for Pt/G, Pt/ 3D-G, and Pt/3D-NG catalysts in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH at a fixed potential of 0.6 V vs RHE (f).

Figure 7. CV of Pt/G (a), Pt/3D-G (b), and Pt/3D-NG (c) in a solution of 0.5 mol L−1 H2SO4 containing 0.5 mol L−1 CH3OH during the stability test. Scanning rate: 50 mV s−1; test temperature: 25 °C. Mass activities of Pt/G, Pt/3D-G, and Pt/3D-NG catalysts with cycle numbers during a stability test (d).

16031


Research Article


Figure 8. TEM images and the size distributions of Pt/3D-NG (a, b), Pt/3D-G (c, d), and Pt/G (e, f) before (a, c, e) and after (b, d, f) a stability test.

stability tests, respectively. Clearly, the Pt/3D-NG behaves better with higher stability compared to Pt/G and Pt/3D-G. TEM micrographs with Pt NPs size distributions of Pt/3D-NG, Pt/3D-G, and Pt/G catalysts before and after stability tests were displayed in Figure 8. In accordance with the durable CVs response, the TEM images show that the particle size of Pt NPs on all three catalysts grows after 1000 CV in line with their reduced current densities. With respect to the TEM images after stability tests, it can evidently be obtained that the particle sizes of Pt/G, Pt/3D-G, and Pt/3D-NG grow to 4.06, 3.90, and 3.24 nm, increasing by 80.4, 78.9, and 52.8%, in comparison with those before CVs, respectively. Noteworthily, Pt NPs on 3D-NG samples are free of agglomeration after the long-time electrochemical test, while drastic agglomeration of Pt NPs appears on 3D-G and G samples after the potential cycling, indicating that Pt/3D-NG is quite stable during the test process. Our results indicate that the Pt/3D-NG catalyst has effectively prevented the agglomeration of Pt NPs and remains the excellent electrocatalytic activity for methanol electrooxidation. According to the above findings, it is worth mentioning that 3D-NG used as a support for the Pt NPs can effectively promote the electrocatalytic performance for methanol electrooxidation due to their unique features (Scheme 1). First, the interconnected 3D porous structure provides abundant accessibilities and facilitates an effective transportation of reactants and products.27,57 Second, the presence of large quantities of doped N species will reinforce the interaction between the Pt and the support, enabling it to maintain Pt NPs in a uniform dispersion state.37,58 Third, the three-dimensional structure of graphene framework guarantees the high-speed electronic conductivity.59,60 Finally, the uniform distribution of Pt NPs with small particle size on the unique 3D N-doped graphene architecture will make the most use of Pt NPs.

Scheme 1. Schematic Illustration for the Exceptional Performance of 3D-NG as Catalyst Supports

Therefore, the resultant Pt/3D-NG nanocomposites possessing remarkably high and stable catalytic activity hold great potential applications in fuel cells.

4. CONCLUSIONS In conclusion, we have proposed a convenient and efficient method for fabrication of the 3D nitrogen-doped graphene network by using commercial sponges as the template and graphene oxide as the building block. With this design, 3D-NG exhibits 2.3 times higher activity for methanol electro-oxidation as well as the 10% increase in stability when using a support for the Pt NPs as compared with Pt/G. Besides, Pt/3D-NG also presents a high ORR performance in comparision with Pt/3D16032


Research Article


Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4, 1289−1295. (12) Negro, E.; Stassi, A.; Baglio, V.; Aricò, A. S.; Koper, G. J. Electrocatalytic Activity and Durability of Pt-Decorated NonCovalently Functionalized Graphitic Structures. Catalysts 2015, 5, 1622−1635. (13) Liu, J.; Liu, C.-T.; Zhao, L.; Zhang, J.-J.; Zhang, L.-M.; Wang, Z.B. Effect of Different Structures of Carbon Supports for Cathode Catalyst on Performance of Direct Methanol Fuel Cell. Int. J. Hydrogen Energy 2016, 41, 1859−1870. (14) Novoselov, K.; Geim, A. K.; Morozov, S.; Jiang, D.; Katsnelson, M.; Grigorieva, I.; Dubonos, S.; Firsov, A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (15) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils. Science 2009, 324, 1312−1314. (16) 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. (17) Cui, X.; Li, Y.; Bachmann, S.; Scalone, M.; Surkus, A.-E.; Junge, K.; Topf, C.; Beller, M. Synthesis and Characterization of Iron− Nitrogen-Doped Graphene/Core−Shell Catalysts: Efficient Oxidative Dehydrogenation of N-Heterocycles. J. Am. Chem. Soc. 2015, 137, 10652−10658. (18) Raccichini, R.; Varzi, A.; Passerini, S.; Scrosati, B. The Role of Graphene for Electrochemical Energy Storage. Nat. Mater. 2015, 14, 271−279. (19) Carrera-Cerritos, R.; Baglio, V.; Aricò, A. S.; Ledesma-García, J.; Sgroi, M. F.; Pullini, D.; Pruna, A. J.; Mataix, D. B.; Fuentes-Ramírez, R.; Arriaga, L. G. Improved Pd Electro-Catalysis for Oxygen Reduction Reaction in Direct Methanol Fuel Cell by Reduced Graphene Oxide. Appl. Catal., B 2014, 144, 554−560. (20) Antolini, E. Graphene as a New Carbon Support for LowTemperature Fuel Cell Catalysts. Appl. Catal., B 2012, 123−124, 52− 68. (21) Xia, B.; Yan, Y.; Wang, X.; Lou, X. W. D. Recent Progress on Graphene-Based Hybrid Electrocatalysts. Mater. Horiz. 2014, 1, 379− 399. (22) Neri, G.; Scala, A.; Barreca, F.; Fazio, E.; Mineo, P.; Mazzaglia, A.; Grassi, G.; Piperno, A. Engineering of Carbon Based Nanomaterials by Ring-Opening Reactions of a Reactive Azlactone Graphene Platform. Chem. Commun. 2015, 51, 4846−4849. (23) Xu, Y.; Shi, G.; Duan, X. Self-Assembled Three-Dimensional Graphene Macrostructures: Synthesis and Applications in Supercapacitors. Acc. Chem. Res. 2015, 48, 1666−1675. (24) Cao, X.; Yin, Z.; Zhang, H. Three-Dimensional Graphene Materials: Preparation, Structures and Application in Supercapacitors. Energy Environ. Sci. 2014, 7, 1850−1865. (25) Sudeep, P.-M.; Narayanan, T.-N.; Ganesan, A.; Shaijumon, M.M.; Yang, H.; Ozden, S.; Patra, P.-K.; Pasquali, M.; Vajtai, R.; Ganguli, S.; Roy, A.-K; Anantharaman, M.-R.; Ajayan, P.-M. Covalently Interconnected Three-Dimensional Graphene Oxide Solids. ACS Nano 2013, 7, 7034−7040. (26) Yoon, J.-C.; Lee, J.-S.; Kim, S.-I.; Kim, K.-H.; Jang, H.-H. ThreeDimensional Graphene Nano-Networks with High Quality and Mass Production Capability via Precursor-Assisted Chemical Vapor Deposition. Sci. Rep. 2013, 3, 1788. (27) Hu, C.; Cheng, H.; Zhao, Y.; Hu, Y.; Liu, Y.; Dai, L.; Qu, L. Newly-Designed Complex Ternary Pt/PdCu Nanoboxes Anchored on Three-Dimensional Graphene Framework for Highly Efficient Ethanol Oxidation. Adv. Mater. 2012, 24, 5493−5498. (28) Mao, S.; Lu, G.; Chen, J. Three-Dimensional Graphene-Based Composites for Energy Applications. Nanoscale 2015, 7, 6924−6943. (29) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z. Heteroatom-Doped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207−5234.

G and Pt/G. We believe that such an inexpensive, scalable, facile method might contribute to the rational fabrication of 3D functional free-stacking graphene-based composites, providing broad prospects for various applications in fuel cells, batteries, environmental treatment, and energy conversion.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03520. Detailed information on these catalysts including certain reagents, GO synthesis, synthetic strategy for samples, XRD pattern, pore structure parameter, and electrochemical measurements (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-451-86417853. Fax: +86-451-86418616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research is financially supported by the National Natural Science Foundation of China (Grant No. 21273058), China postdoctoral science foundation (Grant No. 2012M520731 and 2014T70350), and Heilongjiang postdoctoral foundation (LBH-Z12089).

■

REFERENCES

(1) Sabri, M. F. M.; Danapalasingam, K.; Rahmat, M. A Review on Hybrid Electric Vehicles Architecture and Energy Management Strategies. Renewable Sustainable Energy Rev. 2016, 53, 1433−1442. (2) Tamayao, M.-A. M.; Michalek, J. J.; Hendrickson, C.; Azevedo, I. s. M. Regional Variability and Uncertainty of Electric Vehicle Life Cycle CO2 Emissions across the United States. Environ. Sci. Technol. 2015, 49, 8844−8855. (3) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43−51. (4) Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88, 981−1007. (5) Kraytsberg, A.; Ein-Eli, Y. Review of Advanced Materials for Proton Exchange Membrane Fuel Cells. Energy Fuels 2014, 28, 7303− 7330. (6) Fu, X.; Choi, J.-Y.; Zamani, P.; Jiang, G.; Hoque, M. A.; Hassan, F. M.; Chen, Z. Co−N Decorated Hierarchically Porous Graphene Aerogel for Efficient Oxygen Reduction Reaction in Acid. ACS Appl. Mater. Interfaces 2016, 8, 6488−6495. (7) Tiwari, J. N.; Tiwari, R. N.; Singh, G.; Kim, K. S. Recent Progress in the Development of Anode and Cathode Catalysts for Direct Methanol Fuel Cells. Nano Energy 2013, 2, 553−578. (8) Saleh, F. S.; Easton, E. B. Diagnosing Degradation within Pem Fuel Cell Catalyst Layers Using Electrochemical Impedance Spectroscopy. J. Electrochem. Soc. 2012, 159, B546−B553. (9) Parvez, K.; Yang, S.; Hernandez, Y.; Winter, A.; Turchanin, A.; Feng, X.; Müllen, K. Nitrogen-Doped Graphene and Its Iron-Based Composite as Efficient Electrocatalysts for Oxygen Reduction Reaction. ACS Nano 2012, 6, 9541−9550. (10) 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. (11) Zhou, X.; Qiao, J.; Yang, L.; Zhang, J. A Review of GrapheneBased Nanostructural Materials for Both Catalyst Supports and Metal16033


Research Article

ACS Applied Materials & Interfaces (30) Huang, H.; Yang, S.; Vajtai, R.; Wang, X.; Ajayan, P. M. PtDecorated 3D Architectures Built from Graphene and Graphitic Carbon Nitride Nanosheets as Efficient Methanol Oxidation Catalysts. Adv. Mater. 2014, 26, 5160−5165. (31) Qiu, H.; Dong, X.; Sana, B.; Peng, T.; Paramelle, D.; Chen, P.; Lim, S. Ferritin-Templated Synthesis and Self-Assembly of Pt Nanoparticles on a Monolithic Porous Graphene Network for Electrocatalysis in Fuel Cells. ACS Appl. Mater. Interfaces 2013, 5, 782−787. (32) Shao, Y.; Wang, J.; Engelhard, M.; Wang, C.; Lin, Y. Facile and Controllable Electrochemical Reduction of Graphene Oxide and Its Applications. J. Mater. Chem. 2010, 20, 743−748. (33) Zhao, L.; Wang, Z.-B.; Li, J.-L.; Zhang, J.-J.; Sui, X.-L.; Zhang, L.M. A Newly-Designed Sandwich-Structured Graphene−Pt−Graphene Catalyst with Improved Electrocatalytic Performance for Fuel Cells. J. Mater. Chem. A 2015, 3, 5313−5320. (34) Zhang, P.; Wang, R.; He, M.; Lang, J.; Xu, S.; Yan, X. 3D Hierarchical Co/CoO-Graphene-Carbonized Melamine Foam as a Superior Cathode toward Long-Life Lithium Oxygen Batteries. Adv. Funct. Mater. 2016, 26, 1354−1364. (35) Lin, Z.; Waller, G.; Liu, Y.; Liu, M.; Wong, C.-P. Facile Synthesis of Nitrogen-Doped Graphene via Pyrolysis of Graphene Oxide and Urea, and its Electrocatalytic Activity toward the Oxygen-Reduction Reaction. Adv. Energy Mater. 2012, 2, 884−888. (36) Sun, T.; Zhang, Z.; Xiao, J.; Chen, C.; Xiao, F.; Wang, S.; Liu, Y. Facile and Green Synthesis of Palladium Nanoparticles-GrapheneCarbon Nanotube Material with High Catalytic Activity. Sci. Rep. 2013, 3, 2527. (37) Qin, Y.; Chao, L.; Yuan, J.; Liu, Y.; Chu, F.; Kong, Y.; Tao, Y.; Liu, M. Ultrafine Pt Nanoparticle-Decorated Robust 3D N-Doped Porous Graphene as an Enhanced Electrocatalyst for Methanol Oxidation. Chem. Commun. 2016, 52, 382−385. (38) Yang, Z. Y.; Jin, L. J.; Lu, G. Q.; Xiao, Q. Q.; Zhang, Y. X.; Jing, L.; Zhang, X. X.; Yan, Y. M.; Sun, K. N. Sponge-Templated Preparation of High Surface Area Graphene with Ultrahigh Capacitive Deionization Performance. Adv. Funct. Mater. 2014, 24, 3917−3925. (39) Wang, H.; Xu, Z.; Yi, H.; Wei, H.; Guo, Z.; Wang, X. One-Step Preparation of Single-Crystalline Fe2O3 Particles/Graphene Composite Hydrogels as High Performance Anode Materials for Supercapacitors. Nano Energy 2014, 7, 86−96. (40) Guo, H.-L.; Wang, X.-F.; Qian, Q.-Y.; Wang, F.-B.; Xia, X.-H. A Green Approach to the Synthesis of Graphene Nanosheets. ACS Nano 2009, 3, 2653−2659. (41) Ai, W.; Jiang, J.; Zhu, J.; Fan, Z.; Wang, Y.; Zhang, H.; Huang, W.; Yu, T. Supramolecular Polymerization Promoted in Situ Fabrication of Nitrogen-Doped Porous Graphene Sheets as Anode Materials for Li-Ion Batteries. Adv. Energy Mater. 2015, 5, 1500559. (42) Kumar, N. A.; Choi, H.-J.; Shin, Y. R.; Chang, D. W.; Dai, L.; Baek, J.-B. Polyaniline-Grafted Reduced Graphene Oxide for Efficient Electrochemical Supercapacitors. ACS Nano 2012, 6, 1715−1723. (43) Zhang, H.; Kuila, T.; Kim, N. H.; Yu, D. S.; Lee, J. H. Simultaneous Reduction, Exfoliation, and Nitrogen Doping of Graphene Oxide via a Hydrothermal Reaction for Energy Storage Electrode Materials. Carbon 2014, 69, 66−78. (44) Hou, Y.; Cui, S.; Wen, Z.; Guo, X.; Feng, X.; Chen, J. Strongly Coupled 3D Hybrids of N-doped Porous Carbon Nanosheet/CoNi Alloy-Encapsulated Carbon Nanotubes for Enhanced Electrocatalysis. Small 2015, 11, 5940−5948. (45) Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis. ACS Nano 2011, 5, 4350−4358. (46) Gu, J.-M.; Kim, W.-S.; Hwang, Y.-K.; Huh, S. Template-Free Synthesis of N-Doped Porous Carbons and Their Gas Sorption Properties. Carbon 2013, 56, 208−217. (47) He, D.; Jiang, Y.; Lv, H.; Pan, M.; Mu, S. Nitrogen-Doped Reduced Graphene Oxide Supports for Noble Metal Catalysts with Greatly Enhanced Activity and Stability. Appl. Catal., B 2013, 132-133, 379−388.

(48) Xue, Y.; Liu, J.; Chen, H.; Wang, R.; Li, D.; Qu, J.; Dai, L. Nitrogen-Doped Graphene Foams as Metal-Free Counter Electrodes in High-Performance Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2012, 51, 12124−12127. (49) Huang, H.; Ye, G.; Yang, S.; Fei, H.; Tiwary, C. S.; Gong, Y.; Vajtai, R.; Tour, J. M.; Wang, X.; Ajayan, P. M. Nanosized Pt Anchored onto 3d Nitrogen-Doped Graphene Nanoribbons Towards Efficient Methanol Electrooxidation. J. Mater. Chem. A 2015, 3, 19696−19701. (50) Zhang, C.; Sandorf, W.; Peng, Z. Octahedral Pt2CuNi Uniform Alloy Nanoparticle Catalyst with High Activity and Promising Stability for Oxygen Reduction Reaction. ACS Catal. 2015, 5, 2296−2300. (51) Huang, H.; Yang, S.; Vajtai, R.; Wang, X.; Ajayan, P. M. PtDecorated 3D Architectures Built from Graphene and Graphitic Carbon Nitride Nanosheets as Efficient Methanol Oxidation Catalysts. Adv. Mater. 2014, 26, 5160−5165. (52) Terrones, M.; Ajayan, P.; Banhart, F.; Blase, X.; Carroll, D.; Charlier, J.-C.; Czerw, R.; Foley, B.; Grobert, N.; Kamalakaran, R.; Kohler-Redlich, P.; Rühle, M.; Seeger, T.; Terrones, H. N-doping and Coalescence of Carbon Nanotubes: Synthesis and Electronic Properties. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 355−361. (53) Wong, W.; Daud, W. R. W.; Mohamad, A. B.; Kadhum, A. A. H.; Loh, K. S.; Majlan, E. Recent Progress in Nitrogen-Doped Carbon and its Composites as Electrocatalysts for Fuel Cell Applications. Int. J. Hydrogen Energy 2013, 38, 9370−9386. (54) Wang, Z.; Shi, G.; Zhang, F.; Xia, J.; Gui, R.; Yang, M.; Bi, S.; Xia, L.; Li, Y.; Xia, L.; Xia, Y. Amphoteric Surfactant Promoted ThreeDimensional Assembly of Graphene Micro/Nanoclusters to Accomodate Pt Nanoparticles for Methanol Oxidation. Electrochim. Acta 2015, 160, 288−295. (55) Li, Y.; Tang, L.; Li, J. Preparation and Electrochemical Performance for Methanol Oxidation of Pt/Graphene Nanocomposites. Electrochem. Commun. 2009, 11, 846−849. (56) Chu, Y. Y.; Wang, Z. B.; Jiang, Z. Z.; Gu, D. M.; Yin, G. P. A Novel Structural Design of a Pt/C-CeO2 Catalyst with Improved Performance for Methanol Electro-Oxidation by β-Cyclodextrin Carbonization. Adv. Mater. 2011, 23, 3100−3104. (57) Cao, X.; Yin, Z.; Zhang, H. Three-Dimensional Graphene Materials: Preparation, Structures and Application in Supercapacitors. Energy Environ. Sci. 2014, 7, 1850−1865. (58) Zhou, Y.; Neyerlin, K.; Olson, T. S.; Pylypenko, S.; Bult, J.; Dinh, H. N.; Gennett, T.; Shao, Z.; O’Hayre, R. Enhancement of Pt and Pt-Alloy Fuel Cell Catalyst Activity and Durability Via NitrogenModified Carbon Supports. Energy Environ. Sci. 2010, 3, 1437−1446. (59) Cheng, J.; Li, Y.; Huang, X.; Wang, Q.; Mei, A.; Shen, P. K. Highly Stable Electrocatalysts Supported on Nitrogen-Self-Doped Three-Dimensional Graphene-Like Networks with Hierarchical Porous Structures. J. Mater. Chem. A 2015, 3, 1492−1497. (60) Chen, S.; Duan, J.; Jaroniec, M.; Qiao, S. Z. Three-Dimensional N-Doped Graphene Hydrogel/Nico Double Hydroxide Electrocatalysts for Highly Efficient Oxygen Evolution. Angew. Chem., Int. Ed. 2013, 52, 13567−13570.

16034
