Three-Dimensional Framework of Graphene Nanomeshes Shell

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3D Framework of Graphene Nanomeshes-Shell/CoO 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13290 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

3D Framework of Graphene NanomeshesShell/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

KEYWORDS: Graphene frameworks, bifunctional electrocatalyst, oxygen reduction reaction, oxygen evolution reaction, co-doping, electrocatalysis, zinc-air batteries

ABSTRACT. The synthesis of durable and low-cost electrocatalyst is crucial but challenging. Here, we developed one-pot pyrolysis approach towards the preparation of heteroatom-doped hierarchical porous 3D graphene frameworks decorated with multi-layer graphene shell coated

ACS Paragon Plus Environment

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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 Znair 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 high-performance practical electrocatalyst for metal-air batteries and fuel cells at low cost.

1. INTRODUCTION 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 of 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 recognised 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,


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the high cost, low stability and sluggish kinetics have restricted noble metal catalysts in practical applications.9 Therefore, growing interests have been attracted in exploring the earth-abundant elements, specifically, cobalt based oxides and alloys, as promising replacement with high activity, long-term stability and cost-effectiveness for the promising practical applications.10-17 However, their catalytic potency is still mainly 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, the pristine graphene have 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, sulphur, 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 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 favourable: since the introduced micropores could enrich the defects population, which are expected to act as the reasonable anchor sites for heteroatom doping;


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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 synthesized by template-directed chemical vapour deposition (CVD),35 hydrothermal procedure36,37 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 multi-layer graphene/cobalt oxide nanocrystals (denoted as GM-Co-B-N). The new hierarchical material was synthesized by rapid pyrolysis of cobalt-oleate complex and graphene nanomeshes at 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. 2. EXPERIMENTAL SECTION Chemicals. Graphite (99.95% purity) was obtained from Qingdao Huarun Graphite Co., Ltd., Chemically pure ferrous sulphate (FeSO4), hydrochloric acid (HCl, 37%), sodium oleate (NaOleate), oleic acid (OA), cobalt chloride hexahydrate (CoCl2·6H2O), ethanol, hexane and


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melamine were purchased from Sinopharm Chemical Reagent Co., Ltd. All aqueous solutions were prepared by deionized water. All chemicals were used without further purification. 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 hours 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 hour. 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 was 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 hours under stirring. The reactant was then washed with distilled water in a reparatory funnel several times and evaporated off hexane to get a thick waxy cobalt-oleate complex. 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 cobaltoleate 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 quartz tube furnace for 2 hours respectively. As controlling samples, undoped and pristine graphene coated with cobalt oxide NPs (G-Co), boron doped graphene with cobalt oxide NPs (G-Co-B), nitrogen doped graphene with cobalt oxide NPs (G-Co-N) and


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boron/nitrogen co-doped graphene with cobalt oxide NPs (G-Co-B-N) were also prepared with the similar methodology, taking the introduction of boron, nitrogen dopants and graphene nanomesh as variables. Instrumental characterization. Scanning electron microscopy (SEM) images were obtained on a field-emission SEM 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 acceleration voltage of 200 kV. HAADF-STEM 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 X-ray 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 excitation source. The Brunauer−Emmett−Teller (BET) specific surface area were deduced from the N2 physical adsorption measurement data that were obtained using an ASAP 2010 Accelerated Surface Area and Porosimetry System. 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, US). The catalyst ink was prepared by mixing the catalyst


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(12 mg) with 3 ml ethanol-water (1:1) and 10 µL Nafion (5 %) with the assist of bath sonication. Subsequently, the catalyst was loaded on the surface of GC electrode surface (diameter: 5 mm) by drop casting and dried in air. It is noted that special care was taken to maintain the loading of the catalysts the same in all samples. All the experiments were conducted at room temperature, in an O2 or N2-saturated 0.1 M KOH aqueous solutions as electrolyte. All samples were tested for 5 times for the consistence and commercial Pt/C catalyst (20 wt. %) was used for comparison. CV curves were measured at a scan rate of 50 mvs-1 and LSV curves were at 10 mvs-1 at the range of +0.2 to -1.0 V for ORR and 0.0 to 1.0 V for OER. 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 the zinc foil and the air electrode as the anode and the cathode, respectively, was fabricated. The air electrode was fabricated as follows: a certain amount of catalyst (GM-Co-B-N or Pt-C) with 5 wt. % Nafion was sonicated and drop casted on carbon cloth, then dried at 60°C overnight. It is noted that the catalyst loading was kept at 0.5 mg cm2 for all batteries. The electrolyte was 6M 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. 3. RESULTS AND DISCUSSION 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 graphenenanomesh sheet, Co3O4 nanocrystal coated by graphene-like layers co-doped by B, N elements was co-synthesized by one-pot annealing. Before the process, GM as backbone of 3D framework


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was prepared in advance. 20~30 µm large literal size of exfoliated graphene sheets (Figure S1) were synthesized by an established electrochemical approach.39 The raw graphene exhibited flatsheet morphology with random wrinkles and clear six-fold rotational symmetry featured the high 2D crystalline structures. Next, graphene was mixed with etching agent ferrous sulphate (FeSO4) and 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 homogenous distribution of nanopores with 2-4 pore size in graphene sheet, as shown in Figure 2(a).

Figure 1. Schematic graph of the preparation of 3D framework of B, N co-doped graphene nanomeshes-shell/Co3O4 nanocrystals (GM-Co-B-N). After that, the cobalt-oleate complex was mixed with as-produced GM powder and boron/nitrogen dopants (boric acid/melamine) using pestle and mortar as 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 up to 600°C under argon atmosphere, the oleate chain was further reacted with dopants of melamine and boric acid, and


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was then pyrolyzed into carbon layer co-doped by N,B. The cobalt may catalyse the carbon crystallization into graphene-like layer on large graphene sheets support.42 The gas pressure in 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 as-produced GMCo-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 to the presence of the elements carbon, cobalt, nitrogen and boron, respectively, indicating the homogenous distribution of doped heteroatoms and NPs, as shown in Figure 2.

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


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We used scanning electron microscopy (SEM) to record the effects of heating rates on the framework formation. The GM-Co-B-N samples were obtained at different heating rates, 5 °C min-1, 10 °C min-1 and 20 °C min -1, as shown in Figure S2 (a), (b) and (c), respectively. At 10°C min-1heat rate, the most regular 3D framework featured with 1-3 µm macroscopic pores and cavities could be obtained, while the other heating rate samples didn’t own 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,B onto 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 free space gallery among GM sheets. Therefore, the gas release rate and resulted inner pressure would be the key factors for the formation of 3D frameworks of GM sheets. Lowest heating rate led to too low pressure to form macro-pores of 3D frameworks while 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 appreciate inner pressure to expand GM sheets stacked and maintain the interconnected 3D structure of 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


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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: the spacing of graphitic shell of Co3O4/C NPs. Figure 3(a) shows a typical TEM image of GM-Co-B-N where 10-15 nm nanoparticles were well dispersed and uniformly anchored on GM sheets. High resolution of TEM image clearly presented the nanoparticles are actually coated by a thin graphitic shell as core-shell nanostructure, as shown in Figure3 (b-c). In order to get more insight of this core-shell structure, the inter-layer distance (d-spacing) has been calculated from HRTEM images with the assistance of lattice fringe patterns. The d-spacing value is about 0.202 nm (Figure3c), 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 3(d), as the inter-layer spacing is about 0.35 nm, which could be attributed to the graphitization of cobalt-oleate complex at high temperature.


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Figure 4. High-angle annular dark field (HAADF) images of GM-Co-B-N. (a) lowmagnification HAADF image showing core-shell nanoparticles loaded onto graphene nanomeshes. (b,c) High-resolution (b) bright field and (c) dark field TEM image of single coreshell nanoparticle. (d) High-resolution HAADF image of the core cobalt oxide (Co3O4) of (c). To give a direct evidence of atomic structure of the materials synthesized, high-angle annular dark field (HAADF) imaging was performed in an AC-STEM. Figure 4(a) clearly presented the nanoparticles is a kind of two-phase of core-shell structure loaded onto graphene nanomeshes support. High-resolution images of bright field (Figure 4b) and dark field (Figure 4c) of single core-shell nanoparticle demonstrate good crystallinity of irregual nanoshell and the core nanocrystal. With evidence of 0.34 nm layer distance and fine lines in the right bottom part in Figure 4(b), the nanoshell can be thought as a kind of multi-layer graphene. The core is singlecrystalline Co3O4 nanocrystal. Figure 4d presented (400) plane of the core Co3O4 and its fast


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Fourier transform (FFT) image (inset). This crystalline graphitic layer (< 10 layers), or say, multi-layer graphene could encapsulate and confine the unstable inner cobalt oxide NPs from the direct contact with the harsh environments (electrolyte) to avoid corrosion and poisoning. Large literal sheet size of graphene nanomeshes may stimulate in-situ growth of multi-layer graphene coating co-doped by boron and nitrogen onto Co3O4 nanocrystals through rapid thermolysis with cobalt-oleate complex with controlling heating rate up to 600°C.37 The strong interaction between graphitic layers and cobalt oxide, specifically, the difference in their work functions could lead to the changes of the density of states and lower the local work function of graphene.44In addition, the potential synergistic effects of heteroatom doping45,

46

could be

greatly beneficial to the practical electrocatalysis process, as demonstrating in further electrochemical measurements.

(a)

(b) G Co3O4

Intensity (a.u.) G GM GM-Co-B-N

80

ID/IG=0.55

G

ID/IG=0.35

1000

1500

2000

2500

3000

-1

Wavenumber (cm )

2

189.85 m /g

GM-Co-B-N

-1 2

19.73 m /g

0.4

GM

500

2

0.2

ID/IG=1.19

(d)

98.15 m /g

0.0

GM-Co-B-N

0.6

0.8

1.0 0

Relative pressure (P/P )

3 -1

320 280 240 200 160 120 80 40 0

40 60 2theta (degree)

G

dV/dlog(D) (cm g nm )

3

D

GF-Co-B-N

Intensity (a.u.)

20

(c) Quantity adsorbed (cm /g)

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20

40 60 80 Pore width (nm)

100


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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. The X-ray diffraction (XRD) reveals characteristic peaks of high crystallinity of cobalt oxide (JCPDS No. 42-1467) in as-prepared GM-Co-B-N frameworks, as exhibited in Figure 5(a). The (002) peak at 2ɵ 26.6° may match with multi-layer graphene layer coating onto Co3O4. Raman spectra in Figure 5(b) provided the direct information about the structure and defects of carbon materials. The D band at 1343 cm-1 was originated from the structural defects (planar disorder, graphitic edges and sp3 bonding) and G band at 1560 cm-1 was an index of graphitization. The extent of defects in catalyst is quantified by calculating the relative integrated intensities of these two bands (ID/IG). The calculated ratio of these two bands are as following: G (0.35) < GM (0.55) < GM-Co-B-N (1.19).More defects in GM-Co-B-N probably resulted from multi-layer graphene coating co-doped by B,N from the decomposition of oleate part. These introduced defects would be beneficial for the electrocatalytic reactions as these defects could not merely be the catalytic active sites themselves, but the created nanopores and framework microstructures could also favourably anchor more heteroatoms and provide vital accessibility for mass transport. Brunauer-Emmett-Teller (BET) surface area and pore size distribution was measured by N2 adsorption-desorption analysis as shown in Figure5(c). The raw graphene after the same heat treatment as GM without etching agent exhibited the poorest surface area of less than 20 m2 g-1, which is due to the severely restacking of graphene sheets. The GM, which was intentionally introduced in-plane nanopores, has increased its surface area to 98.15 m2g-1, nearly four times larger than G. GM-Co-B-N frameworks exhibited the largest surface area of 189.85 m2g-1 with a type IV behaviour hysteresis loop, which is about twice of GM. This surface area increment could be attributed to microstructural construction of GM 3D frameworks by the rapid


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dopant decomposition. Mesoporous structure with nanopores size from 2 to 20 nm is illustrated in Figure 5(d). In our work, the hierarchical porous system contains the created nanopores in GM sheets and micropores from interconnected 3D frameworks. Therefore, the characteristics of high surface area and optimized construction of porous system would be beneficial for electrocatalytic activity.

(a)

(b)

(c)

N1s

C

Pyrrolic N

GM-Co-B-N Co

O

B 1s

Pyridinic N Graphitic N

GM-Co-B-N

N

GM-Co-B-N

B

N-oxide

B-O

G-Co-B-N

G-Co-B-N

G-Co-B-N C-BNO

C-BN/C-BO

G-Co-B

410

405

12

Percentage (at. %)

10

G-Co-B

1200 1000 800 600 400 200 Binding energy (eV)

0

395

390

G-Co-B

198

B.E.(eV)

(d) G-Co-N

400

Pyrrolic Graphitic

8 6 4 2 0

G-Co-N

G-Co-B-N GM-Co-B-N

195

(e)

N-oxide Pyridinic

10 Percentage (at. %)

Intensity (a.u.)

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192 189 B.E.(eV)

186

B-O C-BN/BO C-BNO

8 6 4 2 0

G-Co-B

G-Co-B-N GM-Co-B-N

Figure 6.(a) Long range XPS spectra of G-Co-B, G-Co-N, G-Co-B-N and GM-Co-B-N, (b) the deconvoluted N1s spectra of G-Co-N, G-Co-B-N and GM-Co-B-N, (c) the deconvoluted B1s spectra of G-Co-B, G-Co-B-N and GM-Co-B-N, (d) summary of doped nitrogen bonding configurations and (e) summary of doped boron bonding configurations. X-ray photoelectron spectroscopy (XPS) was carried out to elucidate surface elemental composition and bonding configuration of catalysts. The long-range spectrum of the GM-Co-B-


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N catalyst portrays the presence of C, N, B, O and Co. Both of the boron and nitrogen peaks were observed, indicating the successful B, N co-doping into graphitic structures. The peaks of cobalt, centred at 782 eV is the evidence of pyrolytic cobalt-oleate complex. The contents of above elements are 69.2 at. % (C), 9.8 at. % (O), 2.8 at. % (Co), 8.3 at. % (B) and 9.8 at. % (N), respectively. Three controlling samples, G-Co-B, G-Co-N and G-Co-B-N, representing raw graphene pyrolyzed with cobalt-oleate complex and boric acid, melamine and their mixture, respectively, were also prepared and examined, as summarized in Table S1. It has been widely studied that the heteroatom doping could contribute materials’ electrocatalytic performance significantly,22-29 therefore, a comprehensive exploration of bonding configurations of doped boron and nitrogen are crucial for understanding the electrocatalytic performances and probing their catalytic mechanism. The spectral deconvolutions of N1s peaks were primarily used to determine the bonding configuration of N in materials, as listed in Figure 6(b). The deconvolution peaks at about 398.7, 399.6, 400.4 and 403.0 eV corresponds to pyridinic N, pyrrolic N, graphitic N and N-oxide, respectively. For G-Co-N, G-Co-B-N and GMCo-B-N catalysts, the total amount of doped nitrogen was increased from 8.3 at. % to 8.5 at. % and 9.8 at. %, respectively, attributing to the enrichment of defects in samples, specifically, the nanopores on GM and large surface area of GM frameworks. More importantly, the ORRpromoted graphitic and pyridine N were all above 90% of the doped nitrogen, which could be the key factor for understanding the mechanism of their enhanced electrocatalytic performances. The contents of aforementioned bonding configurations were calculated in Figure 6(d). Comparably, the spectral deconvolutions of B 1s peaks could also be detailed, as shown in Figure 6(c). The deconvolution peaks at about 190.6, 192.3 and 194.7eV were assigned to C-BN/C-BO, C-BNO and B-O, respectively.47 The contents of boron bonding configurations were summarized in


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Figure 6(e). It is shown that the content of doped boron heteroatoms was increased gradually from 7.51 at. % to 8.27 at. %, so as the proportion of C-BNO bonding, which is the dominant configuration in all B doped samples. Therefore, the comprehensive discussion of the bonding configurations of doped heteroatoms could be significantly beneficial for the understanding of their electrocatalytic performance and probing the active sites accordingly to picture the catalytic mechanism.

(b)

2 1 0 -1 -2 N2-saturated

-3

O2-saturated

-4

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

-2 -1

2

-0.40 V -0.45 V -0.50 V -0.55 V

-2

-1

0.1 0.02

0.03 w

0.04-1/2 0.05

-1/2

0.06

(rpm )

400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2500 rpm

-4 -6 -8 -10

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V)

(e)100

17.2 %

70

50

0

Pt-C GM-Co-B-N 1000

2000 3000 t (s)

4000

5000

Percentage (%)

90

60

-6

-0.8 -0.6 -0.4 -0.2 0.0 0.2 Potential (V versus Ag/AgCl)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 G-Co G-Co-B G-Co-N-Co-B-N-Co-B-N G GM 110 100 90 80 Methanol added 70 60 50 40 Pt-C 30 GM-Co-B-N 20 0 50 100 150 200 250 300 350 t (s)

(f) 9.8 %

80

G-Co G-Co-B G-Co-N G-Co-B-N Pt GM-Co-B-N

-4

0.3 0.2

0

-2

(d)

Potential (V)

4 j (mA cm )

-2

Current density (mA/cm )

(c)

0

-2

3

Current Density (mA•cm )

4

Electron transfer number

-2

Current density (mA/cm )

(a)

Percentage (%)

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



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Figure 7. (a) CV curves of GM-Co-B-N in O2 and N2 saturated electrolyte, (b) polarization curves of G-Co, G-Co-B, G-Co-N, G-Co-B-N and GM-Co-B-N at 1600 rpm rotation speed in O2 saturated electrolyte, (c) LSV curves of GM-Co-B-N in different rotation speed, ranges from 400 to 2500 rpm, inset K-L plot of GM-Co-B-N, (d) electron transfer numbers of catalyst samples, (e) the durability of electrodes and (f) current density loss-time CA responses of GM-Co-B-N and Pt-C electrodes at -0.3 V in oxygen saturated electrolyte, rotating speed, 1200 rpm. The arrow indicates the addition of 3M methanol into electrochemical cell. Electrochemical performances of the samples synthesized were investigated comparatively. To adequately understand the influences of unique defect-rich framework structures and codoped heteroatoms as active sites toward ORR, CV curves were recorded. The resultant voltammograms of GM-Co-B-N framework catalyst is presented in Figure 7(a). It is shown that a distinct cathodic peak centred at -0.22 V (vs Ag/AgCl) can be observed in O2 saturated electrolyte, indicating its pronounced ORR electrocatalytic activity. In order to get a detailed insight on the kinetic aspects of the ORR process, an investigation with LSV using rotating disk electrode (RDE) was employed. A detailed LSV investigation of GM-Co-B-N along with controlling samples and commercial Pt-C (20 wt. %) are presented in Figure 7(b), thus the ORR activity can be ranked as GM-Co-B-N>G-Co-B-N>G-Co-N>G-Co-B>G-Co. 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 to onset potentials of -0.11 V 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


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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 35 mV enhancement on half-wave potential; moreover, the steeper curve and higher current density, specifies 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 was further used to reveal the catalytic mechanism and the number of electron transferred (n) during ORR process in alkaline medium. As shown in Figure 7(c), a set of LSV curves for GM-Co-B-N catalyst were measured from 400 to 2500 rpm, the current densities gradually increased with the increase of rotation speed, attributing to the shorten diffusion distance at higher rotation speed. The K-L plot for GM-Co-B-N is shown in Figure 7(c) inset, in which good linearity and nearly parallelism of the plot suggesting first-order reaction kinetics towards 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 direct formation of water. In contrast, the n values of other controlling samples were summarized in Figure 7(d) and Figure S6, demonstrating a combined two- and four-electron reduction pathway. It is noted that the catalytic pathway of the other co-doping G-Co-B-N was also close to the favourable four-electron transfer process. A comprehensive comparison of control samples with final catalyst GM-Co-B-N is presented in Figure S5. The as-desired four-


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electron reaction and most positive onset/half-wave potentials with the higher catalytic efficiency exhibited by GM-Co-B-N framework signifies that the strategy adopted here (combination of heteroatom co-doping and porous framework structure) has large advantages and can benefit towards devising cost-effective ORR electrocatalysts. The stability of catalyst and the resistance to crossover effect are important considerations for the practical applications. The stability of GM-Co-B-N catalyst with respect to commercial Pt-C was assessed through chronoamperometric (CA) measurements. As illustrated in Figure 7(e), the CA response for GM-Co-B-N exhibited very slow attenuation with high current retention of 90.2 % after 5000 s; while a commercial Pt-C catalyst showed a much faster degradation with 82.8 % retention, owing to the dissociation of Pt nanoparticles for carbon supports or inevitable aggregations. This remarkable stability can be ascribed to: (1) the stable mutual contact between Co3O4/C NPs and GM sheets to prevent severe aggregations; (2) protections by the graphitic shell of Co3O4/C NPs from direct contact with the harsh environment or poisons, and (3) the unique hierarchical porous framework, which can facilitate both the interlayer and intra-layer mass transport while preventing the agglomeration and restacking of GM sheets. Furthermore, the methanol crossover poisoning was also evaluated by methanol injection. It is shown that no obvious performance decay was identified for GM-Co-B-N after the methanol injection at about 150 s. In contrast, a sharp jump of current retention was appeared for Pt-C catalyst, indicating a much better electrocatalytic selectivity toward ORR and has an excellent ability for avoid crossover effects.


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(b)

-2

30 25 20 15

G-Co G-Co-B G-Co-N G-Co-B-N GM-Co-B-N Pt-C

10 5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Potential (V versus Ag/AgCl)

(a) Current Density (mA•cm )

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


271 mV dec

0.65

174 mV dec

148mV dec

0.60

Potential (V versus Ag/AgCl)

-1

G-Co G-Co-B G-Co-N G-Co-B-N GM-Co-B-N Pt-C

-1

-0.3

-1

81 mV dec

0.50 -0.6

205 mV dec

-1

0.55 103 mV dec

-1

-1

0.0

0.3

0.6

0.9

log (j)

Figure 8. (a) LSV curves of G-Co, G-Co-B, G-Co-N, G-Co-B-N and GM-Co-B-N in O2 saturated electrolyte for OER, (b) the corresponding Tafel plot of those OER catalysts. To further investigate GM-Co-B-N framework as bifunctional catalyst for oxygen electrocatalysis, OER activity of GM-Co-B-N together with controlling samples were evaluated, as shown in Figure 8(a). Rapid current density increase in relatively high potential range indicates the OER activity of all samples. It is clearly that GM-Co-B-N exhibited the smallest onset potential and largest current density, highlighting its superior activity for OER among those samples. Typically, the OER activity could also be evaluated by the potential required to oxidize water at a current density of 10 mA cm-2 (Ej10), which is a metric relevant to solar fuel synthesis. In our work, such potential for GM-Co-B-N is 0.63 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 the G-Co and G-Co-B showed the similar catalytic activity (the almost overlapping LSV curves), and so as G-Co-N and G-Co-B-N. The corresponding Tafel plots specifies GM-Co-B-N possessed 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


21


in Table S2, our co-doped GM-Co-B-N is on the top level among the reported nanocarbon/Co based bifunctional electrocatalysts.

(b)1.6

1.6 1.4

1.4

1.2

Voltage (V)

Open circuit voltage (V)

(a)1.8

GM-Co-B-N

1.0 0.8 0.6 0.4 0

(c)1.6

100

200

300

400

500

600

Time (s)

-2

10 mA cm

1.2

1.0 0

Pt-C GM-Co-B-N

10

20

30

40

50

60

Time (min)

(d) 10 mA cm

1.4

Pt-C

-2

GM-Co-B-N

2.0

10 mA cm

-2

Voltage (V)

1.2 Voltage (V)

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

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1.0 0.8 0.6 0.4 0.2 0

Pt-C GM-Co-B-N

1.5

1.0 Cycling:20 min discharge,20min charge

100 200 300 400 500 600 700

0

2

-1

Specific capacity (mAh g )

4 6 Time (h)

8

10

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-Co-B-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. Based on the excellent bifunctional electrocatalytic activity of GM-Co-B-N frameworks as 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


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V was observed, consisting with preciously reported values of such cells, as shown in Figure 9(a). Galvanostatic discharge measurements in Figure 9(b) revealed that the GM-Co-B-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 cut-off 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 9(c). It is noted that the whole discharge voltage was maintained above 1.40 V. Galvanostatic discharge–charge cycling was also performed at 10 mA cm−2 (20 min in each state). The charge–discharge voltage gap of GM-Co-B-N catalyst was as small as 0.68 V, which merely increased to 0.75 V after 10 hours’ cycling with a small overpotential of 70 mV. In contrast, the voltage gap of Pt-C based battery increased from 0.69 V to 0.83 V at the same condition, as illustrated in Figure 9(d). The lower overpotential exhibited by GM-Co-B-N framework catalyst indicated remarkable rechargability of zinc–air battery in addition to the outstanding ORR and OER activities in alkaline medium. 4. Conclusions In summary, 3D porous framework of graphene nanomeshes supported multi-graphene-like nanoshell coating Co3O4nanocrystals were synthesized by one-pot pyrolysis process of cobaltoleate complex with boron and nitrogen dopants and graphene nanomeshes. With controlling heating rate up to 600°C, graphene-based 3D porous framework interconnected with 1-3 µm macro-pores and < 20 nm nanopores and in-situ co-doping of boron and nitrogen were almost realized simultaneously. Large literal sheet size of electrochemically exfoliated graphene-based nanomeshes stimulated rapid thermolysis with cobalt-oleate complex to form 10-15 nm Co3O4


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nanocrystals and in-situ growth of multi-layer graphene coating co-doped by boron and nitrogen at 600°C.The resulting strong synergistic effects of heteroatoms-doping, multi-layer graphenelike nanoshells confined Co3O4 nanocrystals in 3D porous structure contribute to rich catalytic active sites and electronic/mass transport. The new material exhibited superior bifunctional electrocatalytic activity on ORR and OER to classic Pt/C with outstanding onset/half-wave potential, higher catalytic current density, efficient four-electron pathway, excellent crossover tolerance and long-term stability. The GM-Co-B-N new material-based Zn-air battery exhibited comparable performance with commercial Pt-C with even smaller overpotential after long-term cycling. We believe our present the strategy of composition control along with porous structure can be further extended to develop other 3D porous nanohybrid of graphene for various energy conversion and storage applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: TEM characterization of electrochemically exfoliated graphene nanosheets, SEM images of GMCo-B-N obtained at different heating rate, XRD and XPS of obtained graphene nanomesh, CV and LSV curves of control samples, K-L plots for controlling samples, XPS element contents details for all samples, discussions on K-L plot and comparison of newly reported Co based bifunctional electrocatalyst.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] (Y. Liu). *Email:[email protected] (J. Wang) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the financial support by grants from National Natural Science Foundation of China (51502320, 21373255, 21503259, U1662102), State Key Laboratory of Powder Metallurgy, Central South University, Natural Science Foundation of Shanxi Province (201601D021060), Shanxi Scholarship Council of China and Hundred Talent Program of Chinese Academy of Sciences (2015YCR001). REFERENCES (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting. Cells Chem. Rev. 2010,110, 6446–6473. (2) Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. Oxygen Electrochemistry as a Cornerstone for Sustainable Energy Conversion. Angew. Chem., Int. Ed. 2014, 53, 102–121. (3) Shao, Y.; Park, S.; Xiao, J.; Zhang, J.-G.; Wang, Y.; Liu, J. Electrocatalysts for Nonaqueous Lithium–Air Batteries: Status, Challenges, and Perspective. ACS Catal. 2012, 2, 844–857. (4) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886–17892. (5) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724–761.


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Three-Dimensional Framework of Graphene Nanomeshes Shell

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