Three-Dimensional Framework of Graphene Nanomeshes Shell

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Three-Dimensional Framework of Graphene Nanomeshes Shell/ Co3O4 Synthesized as Superior Bifunctional Electrocatalyst for Zinc− Air Batteries Congwei Wang,†,‡ Zheng Zhao,†,§ Xiaofeng Li,‡ Rui Yan,†,§ Jie Wang,†,§ Anni Li,† Xiaoyong Duan,†,§ Junying Wang,† Yong Liu,*,‡ and Junzhong Wang*,† †

CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P. R. China § University of Chinese Academy of Sciences, Beijing 100049, P. R. China ‡

S Supporting Information *

ABSTRACT: The synthesis of durable and low-cost electrocatalyst is crucial but challenging. Here, we developed a onepot pyrolysis approach toward the preparation of heteroatomdoped hierarchical porous three-dimensional (3D) graphene frameworks decorated with multilayer graphene shell-coated cobalt oxide nanocrystal. Large literal sheet size of graphene nanomeshes may stimulate rapid thermolysis with cobalt− oleate complex to form Co3O4 nanocrystals and in situ growth of multilayer graphene coating co-doped by boron and nitrogen with controlling heating rate up to 600 °C. This new material worked as superior bifunctional electrocatalyst on oxygen reduction reaction and oxygen evolution reaction to commercial Pt/C with better onset potential/half-wave potentials, larger current density, better stability, and stronger methanol tolerance. The heteroatom co-doping into porous/curved graphene confined nanocrystals in 3D porous walls provided adequate accessibility of created catalytic active sites and ideal mass transport route for the excellent catalytic activity on redox reaction of oxygen. The synthesized material-based Zn−air battery further confirmed its superior electrolytic activity with high specific capacity and smaller overpotential. This one-pot pyrolysis method shows a great potential of scalable synthesis of high-performance practical electrocatalyst for metal−air batteries and fuel cells at a low cost. KEYWORDS: graphene frameworks, bifunctional electrocatalyst, oxygen reduction reaction, oxygen evolution reaction, co-doping, electrocatalysis, zinc−air batteries

1. INTRODUCTION

impaired by reduced electronic conductivity and poor interfacial interactions with conventional supporting materials. Graphene-based nanocarbons are ideal candidates for electrocatalyst supports and even catalysts due to their wide availability, environmental acceptability, distinctive physicochemical properties, and electronic structural tunability.18−21 However, pristine graphene has shown limited electrocatalytic activity due to its zero-band gap nature, chemical inertness, and few catalytic sites. Heteroatom doping is one solution to manipulate the band gap and tailor the electronic properties, which could cause electron modulation to provide desirable electronic structure for catalytic process.22−26 The doping with heteroatoms (such as nitrogen, sulfur, boron, phosphorus, etc.) has been carried out recently for exploiting the improved electrocatalytic performance through the strong synergistic effects between doped atoms, providing further space and

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the fundamental electrode processes for a range of sustainable energy conversion and storage devices such as metal−air batteries, fuel cells, and water splitting.1−3 Both ORR and OER processes involve multiple electron transfer process; however, their associated reaction mechanisms are distinctive,4,5 making the design of bifunctional electrocatalysts challenging. Platinum (Pt)-based metals are recognized as highly effective electrocatalyst for ORR, their OER performance are oppositely limited;6,7 meanwhile, iridium (Ir) and ruthenium (Ru) oxides are well known for superior OER properties, their ORR activities are relatively poor.3,8 Moreover, the high cost, low stability, and sluggish kinetics have restricted noble metal catalysts in practical applications.9 Therefore, growing interests have been attracted toward exploring the earth-abundant elements, specifically cobalt-based oxides and alloys, as promising replacement with high activity, long-term stability, and cost-effectiveness for promising practical applications.10−17 However, their catalytic potency is still mainly © XXXX American Chemical Society

Received: September 2, 2017 Accepted: November 6, 2017 Published: November 7, 2017 A

DOI: 10.1021/acsami.7b13290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces inspirations for the performance optimization.25−29 However, the electrocatalytic performances as bifunctional electrocatalyst based on doped graphene or other carbons are not so competitive yet. Along with the strategies to alter the catalytic properties through transition metal replacement and heteroatoms doping, the optimized chemical composition and microstructure of electrocatalysts is vital.30−32 Owing to the nature of ORR and OER, a hierarchical porous microstructure is more favorable because the introduced micropores could enrich the defects population, which are expected to act as the reasonable anchor sites for heteroatom doping; moreover, the constructed macropores (micrometer scale) could further facilitate the diffusion and mass transport of ORR- and OER-related species (O2, H+, OH−, and H2O) and liquid electrolyte by making the active sites more accessible to get boosted output in the desired reactions.33,34 Three-dimensional (3D) graphene materials with good performances have been synthesized by template-directed chemical vapor deposition (CVD),35 hydrothermal procedure,36,37 or freeze drying;38 however, these methods have their drawbacks, such as high cost, time consuming, or/and limited throughput. Moreover, the correlation between the hierarchical porous heteroatom doping structure and the associated electrocatalytic performance on ORR and OER still needs to be fundamentally addressed. Herein, we present a facile and scalable approach to synthesize a new bifunctional electrocatalyst of 3D graphene framework of boron and nitrogen co-doped nanomeshes/ curved multilayer graphene/cobalt oxide nanocrystals (denoted as GM-Co-B-N). The new hierarchical material was synthesized by the rapid pyrolysis of cobalt−oleate complex and graphene nanomeshes (GM) in the presence of boron and nitrogen sources. The product exhibited much enhanced ORR and OER catalytic activity, including comparable onset/half-wave potential with commercial Pt/C, larger current density, better stability, and strong methanol-tolerant capability. The zinc− air devices using the material as electrode show good performance.

washed with distilled water in a reparatory funnel several times and evaporated off hexane to get a thick waxy cobalt−oleate complex. 2.3. Synthesis of Boron/Nitrogen Co-Doped GM 3D Frameworks Coated with Cobalt Oxide Nanocrystals (GM-Co-B-N). In a typical synthesis, 0.15 g GM was mixed with 0.15 g cobalt−oleate complex, 1.5 mL oleic acid, 0.6 g melamine, and 0.45 g boric acid by using a pestle and mortar. The slurry was then heated to 600 °C at a heating rate of 5, 10, and 20 °C min−1 under argon atmosphere in a quartz tube furnace for 2 h respectively. As controlling samples, undoped and pristine graphene coated with cobalt oxide nanoparticle (NPs) (G-Co), boron-doped graphene with cobalt oxide NPs (G-CoB), nitrogen-doped graphene with cobalt oxide NPs (G-Co-N), and boron/nitrogen co-doped graphene with cobalt oxide NPs (G-Co-BN) were also prepared with the similar methodology, taking the introduction of boron, nitrogen dopants, and graphene nanomesh as variables. 2.4. Instrumental Characterization. Scanning electron microscopy (SEM) images were obtained on a field-emission scanning electron microscope JSM-7001F (FESEM) operating at 10 kV. X-ray diffraction (XRD) was recorded from 5 to 80° at a scan rate of 0.02° s−1 using the Cu Kα (1.5406 Å) radiation. Transmission electron microscopy (TEM) images were taken with FEI, TECNAI G2 F20 microscope at the acceleration voltage of 200 kV. High-angle annular dark field scanning transmission electron microscopy (HAADFSTEM) was performed on a JEM ARM200F equipped with double aberration correctors in Institute of Physics, Chinese Academy of Sciences, and a cold field emission gun operated at 200 kV. STEM images were recorded using a HAADF detector with a convergence angle of 25 mrad and a collection angle between 70 and 250 mrad. Under these conditions, the spatial resolution is ca. 0.08 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed using Thermo ESCALAB 250 spectrometer, employing an Al-KR Xray source with a 500 μm electron beam spot. Raman spectra were recorded using Jobin-Yvon HR-800 Raman system with 532 nm line of Ar laser as the excitation source. The Brunauer−Emmett−Teller (BET) specific surface area was deduced from the N2 physical adsorption measurement data that were obtained using an ASAP 2010 Accelerated Surface Area and Porosimetry System. 2.5. Electrochemical Measurements. The electrochemical performances were investigated using a set of electrochemical methodologies, such as cyclic voltammetry (CV), rotating disk electrode (RDE), in a three-electrode electrochemical cell fitted with platinum wire as the counter electrode and Ag/AgCl as the reference electrode on a Autolab electrochemical analyser (PGSTAT204) and a MSR electrode rotator (PINE). The catalyst ink was prepared by mixing the catalyst (12 mg) with 3 mL ethanol−water (1:1) and 10 μL Nafion (5%) with the assistance of bath sonication. Subsequently, the catalyst was loaded on the surface of glassy carbon electrode surface (diameter: 5 mm) by drop casting and dried in air. It is worth noting that special care was taken to ensure that the loading of the catalysts was the same in all of the samples. All of the experiments were conducted at room temperature in an O2- or N2-saturated 0.1 M KOH aqueous solutions as the electrolyte. All of the samples were tested for five times for consistency, and commercial Pt/C catalyst (20 wt %) was used for comparison. The CV curves were measured at a scan rate of 50 mV s−1 and the linear sweep voltammetry (LSV) curves at 10 mV s−1 at the range of +0.2 to −1.0 V for ORR and 0.0−1.0 V for OER. The chronoamperometric (CA) curves were tested at −0.3 V in O2-saturated 0.1 M KOH at a rotation speed of 1200 rpm. A homemade zinc−air single battery with a zinc foil and an air electrode as the anode and cathode, respectively, was fabricated. The air electrode was fabricated as follows: a certain amount of catalyst (GMCo-B-N or Pt/C) with 5 wt % Nafion was sonicated and drop casted on the carbon cloth and then dried at 60 °C overnight. It is noted that the catalyst loading was kept at 0.5 mg cm2 for all of the batteries. The electrolyte was a 6 M KOH aqueous solution with 0.2 M zinc acetate. Measurements were performed on the as-built battery cell at room temperature with a LAND multichannel battery testing system.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Graphite (99.95% purity) was obtained from Qingdao Huarun Graphite Co., Ltd. Chemically pure ferrous sulfate (FeSO4), hydrochloric acid (HCl, 37%), sodium oleate (Na-Oleate), oleic acid (OA), cobalt chloride hexahydrate (CoCl2·6H2O), ethanol, hexane, and melamine were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the aqueous solutions were prepared by deionized water. All of the chemicals were used without further purification. 2.2. Synthesis of Graphene Nanomesh (GM) Nanosheets and Oleate−Cobalt Complex. Raw graphene was fabricated from graphite paper using electrochemical exfoliation approach. The asproduced graphene aqueous dispersion (5 mg mL−1) was mixed with FeSO4 solution under magnetic stirring with a mass ratio of graphene/ FeSO4 of 1. After sonication under ambient condition (120 W) for 2 h and drying into solid, the mixture was then heated to 900 °C at a heating rate of 10 °C min−1 under the argon atmosphere by using a quartz tube for 1 h. The product was then washed with 10% (volume ratio) HCl solution and water for several times sequentially, and at last freeze-dried into graphene nanomesh powder. The cobalt−oleate complex was prepared by reacting cobalt chloride and sodium oleate. Briefly, 3.12 g CoCl2 and 10.55 g sodium oleate were dissolved in a mixture solvent composed of 18 mL distilled water, 24 mL ethanol, and 42 mL hexane. The resultant solution was heated to 70 °C and kept at this temperature for 4 h under stirring. The reactant was then B

DOI: 10.1021/acsami.7b13290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic graph of the preparation of 3D framework of B and N co-doped graphene nanomeshes shell/Co3O4 nanocrystals (GM-Co-BN).

Figure 2. (a) TEM image of GM. Scale bar 20 nm. The inset shows the size distribution of nanopores. (b) SEM image of GM-Co-B-N. Scale bar 500 nm. Energy-dispersive spectroscopic (EDS) color mapping of (c) nitrogen, (d) boron, (e) cobalt, and (f) carbon, respectively.

3. RESULTS AND DISCUSSION

(FeSO4 ) and a solid reacted at the high-temperature thermolysis to form graphene nanomeshes (GM).40,41 It is noted that most of pyrolyzed iron content could be removed by acid leaching, as shown in Figure S3. The GM featured homogeneous distribution of nanopores with a 2−4 pore size in the graphene sheet, as shown in Figure 2a. After that, the cobalt−oleate complex was mixed with the asproduced GM powder and boron/nitrogen dopants (boric acid/melamine) using pestle and mortar as the wrapping coating. The grinded fine dark-brown slurry was further rapidly heated to 600 °C under argon flow. As shown in Figure 1, liquid phase of cobalt−oleate−melamine−boric acid was reacted by programmed thermal annealing. Colloidal cobalt oxide nanocrystal coated oleate chain formed as the synthesis of traditional quantum dots.41 With the temperature increase of up to 600 °C under argon atmosphere, the oleate chain was further reacted with dopants of melamine and boric acid and then pyrolyzed into carbon layer co-doped by N and B. The

The synthesis of 3D graphene-based frameworks of boron/ nitrogen co-doped graphene nanomeshes supported nanoparticles of Co3O4 nanocrystal coated graphene shells was carried out by one-pot rapid pyrolysis of the liquid−solid mixture of organic and inorganic compounds and graphene nanomeshes (GM), as is illustrated in Figure 1. On the support of graphene-nanomesh sheet, Co3O4 nanocrystal coated by graphene-like layers co-doped by B and N elements was cosynthesized by one-pot annealing. Before the process, GM as the backbone of the 3D framework was prepared in advance. Large literal size of exfoliated graphene sheets (20−30 μm, Figure S1) were synthesized by an established electrochemical approach.39 The raw graphene exhibited a flat-sheet morphology with random wrinkles and a clear sixfold rotational symmetry featured high two-dimensional crystalline structures. Next, graphene was mixed with an etching agent ferrous sulfate C

DOI: 10.1021/acsami.7b13290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 3. TEM images of GM-Co-B-N in (a) low and (b) high magnifications. HRTEM image of Co3O4/C NPs. HRTEM images and d-spacing of the (c) core and (d) shell. Inset shows the spacing of graphitic shell of Co3O4/C NPs.

Figure 4. High-angle annular dark field (HAADF) images of GM-Co-B-N. (a) Low-magnification HAADF image showing the core−shell nanoparticles loaded onto graphene nanomeshes. (b, c) High-resolution, (b) bright field, and (c) dark field TEM images of a single core−shell nanoparticle. (d) High-resolution HAADF image of the core cobalt oxide (Co3O4) of (c).

to the presence of the elements carbon, cobalt, nitrogen, and boron, respectively, indicating the homogeneous distribution of doped heteroatoms and NPs, as shown in Figure 2. We used scanning electron microscopy (SEM) to record the effects of heating rates on the framework formation. The GMCo-B-N samples were obtained at different heating rates, 5, 10, and 20 °C min −1, as shown in Figure S2a−c, respectively. At 10 °C min−1 heat rate, the most regular 3D framework featured with 1−3 μm macroscopic pores and cavities could be obtained,

cobalt may catalyze the carbon crystallization into a graphenelike layer on large graphene sheets support.42 The gas pressure in a graphene−liquid mixture from the decomposition of organic and inorganic compounds induced the formation of 3D frameworks.43 To verify the structure and composition of the as-produced GM-Co-B-N frameworks, energy-dispersive spectroscopic (EDS) mapping was carried out for examining the heteroatoms doping and cobalt oxides NPs distribution. Chemical mapping confirmed that the bright spots correspond D

DOI: 10.1021/acsami.7b13290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Chemical composition characterizations of (a) XRD, (b) Raman spectra, and (c) N2 adsorption/desorption isotherm and (d) pore width of GM-Co-B-N.

whereas the other heating rate samples did not show such structural features. The liquid phase was the mixture of cobalt oleate, melamine, and boric acid. With heating, the liquid chemicals could react with each other to release the gases (such as H2O, COx, CNy). At the same time, C−N, B−N, and B−C− N bondings could be formed in the solid residue. Some of the solid is the graphene-like coating in situ co-doped by N and B onto the cobalt oxide nanocrystals. The SEM images of control samples are listed in Figure S7. As schematically shown in Figure 1, the liquid chemicals were wrapped among the GM sheets. During the heat treatment, the gas pressure from liquid decomposition probably resulted in the free space gallery among the GM sheets. Therefore, the gas release rate and the resulting inner pressure would be the key factors for the formation of 3D frameworks of the GM sheets. Lowest heating rate led to too low pressure to form the macropores of 3D frameworks, whereas fastest heating rate resulted in the “explosion” effect of gases to destroy the interconnected microstructure of GM. Therefore, the optimized heating rate at 10 °C min−1 could generate significant inner pressure to expand the GM sheets stacked and maintain the interconnected 3D structure of the GM sheets. Moreover, the 3D structure of graphene nanomeshes shells constructed by one-pot pyrolysis method seems be more reliable and more effective to create active sites than conventional freeze drying or hydrothermal method, and cheaper than most of CVD synthesis.35−38 Figure 3a shows a typical TEM image of GM-Co-B-N in which 10−15 nm nanoparticles were well dispersed and uniformly anchored on the GM sheets. High resolution of the TEM image clearly shows that the nanoparticles are actually coated by a thin graphitic shell as a core−shell nanostructure, as shown in Figure 3b,c. To get more insight into this core−shell structure, the interlayer distance (d-spacing) has been

calculated from the high-resolution TEM (HRTEM) images with the assistance of lattice fringe patterns. The d-spacing value is about 0.202 nm (Figure 3c), which matches well with the d-spacing value corresponding to the (400) plane of cobalt oxide (JCPDS No. 42-1467). The crystalline graphitic shell is shown in Figure 3d, as the interlayer spacing is about 0.35 nm, which could be attributed to the graphitization of the cobalt− oleate complex at high temperature. To give a direct evidence of the atomic structure of the materials synthesized, a high-angle annular dark field (HAADF) imaging was performed in an AC-STEM. Figure 4a clearly shows that the nanoparticles are kind of two phases of the core−shell structure loaded onto the graphene nanomeshes support. High-resolution images of bright field (Figure 4b) and dark field (Figure 4c) of a single core−shell nanoparticle demonstrate good crystallinity of irregular nanoshell and the core nanocrystal. With evidence of 0.34 nm layer distance and fine lines in the right bottom part in Figure 4b, the nanoshell can be thought as a kind of multilayer graphene. The core is a single-crystalline Co3O4 nanocrystal. Figure 4d shows the (400) plane of the core Co3O4 and its fast Fourier transform image (inset). This crystalline graphitic layer ( G-Co-B-N > G-Co-N > G-Co-B > GCo. Undoped and nonporous G-Co exhibited poorest onset potential (Figure S4) of −0.20 V due to the lack of active sites and limited surface area and accessibility. Single doped G-Co-B and G-Co-N exhibited 90 and 120 mV improvement in the onset potentials of −0.11 and −0.08 V, respectively. This activity improvement can be attributed to the successful modification in the local charge densities induced by the doped heteroatoms. The co-doped G-Co-B-N displayed an onset potential of −0.04 V, confirming the co-doping could prominently enhance the electrocatalytic activity. Moreover, the correspondingly increased working current density also suggested its improved reactivity. The porous co-doped frameworks, GM-Co-B-N, demonstrated an almost identical onset potential with commercial Pt/C, but a 35 mV enhancement in the half-wave potential; moreover, the steeper curve and a higher current density specify its outstanding catalytic activity. This superior performance could be credited to (1) its unique building block of GM, providing abundant nanopores as anchor sites for both heteroatoms and homogeneous distribution of Co3O4/C NPs; (2) the strong synergistic effects between co-doped heteroatoms, pomegranate-like Co3O4/C NPs and GM sheets; (3) interconnected 3D framework combined with nanopores in GM engineered the optimized hierarchical porous system to facilitate both mass and electron transfer to enhance the electrocatalytic kinetics. Koutecky−Levich (K−L) plots were further used to reveal the catalytic mechanism and the number of electron transferred (n) during the ORR process in an alkaline medium. As shown in Figure 7c, a set of LSV curves for GM-Co-B-N catalyst was recorded from 400 to 2500 rpm, with the current densities gradually increasing with the increase in the rotation speed due to the shortening of the diffusion distance at a higher rotation speed. The K−L plot for GM-Co-B-N is shown in Figure 7c inset, wherein a good linearity and a near parallelism of the plot suggest first-order reaction kinetics toward the electrochemical reduction of O2. Herein, the calculated electron transfer number is 4.02 at potentials from −0.40 to −0.55 V, suggesting a more preferred four-electron pathway and a direct formation of water. In contrast, the n values of other controlling samples are summarized in Figures 7d and S6, demonstrating a combined two- and four-electron reduction pathways. It is H

DOI: 10.1021/acsami.7b13290 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 9. (a) Open-circuit voltage of GM-Co-B-N-based Zn−air battery. (b) Typical galvanostatic discharge curves of Zn−air batteries with GM-CoB-N and Pt/C as cathode catalysts at 10 mA cm−2 current densities. (c) Long-term galvanostatic discharge curves of Zn−air batteries and (d) galvanostatic discharge−charge cycling curves, current density 10 mA cm−2, 20 min for each state.

mV. In contrast, the voltage gap of Pt/C-based battery increased from 0.69 to 0.83 V under the same condition, as illustrated in Figure 9d. The lower overpotential exhibited by GM-Co-B-N framework catalyst indicated remarkable rechargability of the zinc−air battery in addition to the outstanding ORR and OER activities in an alkaline medium.

V, which is the best performance among samples. It is noted that the order of OER electrocatalytic activity rearranged the trend of ORR activity, as G-Co and G-Co-B showed similar catalytic activity (almost overlapping LSV curves) as G-Co-N and G-Co-B-N. The corresponding Tafel plots specifies that GM-Co-B-N possesses the smallest OER Tafel slope of 81 mV dec−1, intrinsically explaining that GM-Co-B-N could express the highest OER electrocatalytic activity. As shown in Table S2, our co-doped GM-Co-B-N is on the top level among the reported nanocarbon/Co-based bifunctional electrocatalysts. Based on the excellent bifunctional electrocatalytic activity of GM-Co-B-N frameworks as the air electrode, a rechargeable Zn−air full battery, composed of an air electrode, a separator, and a zinc anode, was constructed and evaluated, as shown in Figure S8. A Pt/C-based air electrode with the same catalyst loading was also tested for comparison. An open-circuit voltage of ∼1.45 V was observed, consisting of the previously reported values of such cells, as shown in Figure 9a. Galvanostatic discharge measurements in Figure 9b revealed that the GM-CoB-N frameworks exhibited high voltages of 1.43 V at a discharge current density of 10 mA cm−2, which is highly comparable to that of Pt/C catalyst. Normalized to the mass of consumed Zn during the long-term galvanostatic discharge process, the specific capacity of the battery made with GM-Co-B-N frameworks was calculated to be ∼632 mAh g−1 at a cutoff voltage of 0.9 V and discharge density of 10 mA cm−2, which was slightly lower than that of commercial Pt/C (651 mAh g−1), as exhibited in Figure 9c. It is noted that the whole discharge voltage was maintained above 1.40 V. Galvanostatic discharge−charge cycle was also performed at 10 mA cm−2 (20 min in each state). The charge−discharge voltage gap of GMCo-B-N catalyst was as small as 0.68 V, which merely increased to 0.75 V after 10 h of cycling, with a small overpotential of 70

4. CONCLUSIONS In summary, 3D porous framework of graphene nanomeshes supported multigraphene-like nanoshell coating Co3O4 nanocrystals was synthesized by one-pot pyrolysis process of the cobalt−oleate complex with boron and nitrogen dopants and graphene nanomeshes. By controlling the heating rate up to 600 °C, graphene-based 3D porous framework interconnected with 1−3 μm macropores and