MetalâOrganic Coordination Polymer to Prepare Density Controllable

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Metal-Organic Coordination Polymer to Prepare Density Controllable and High Nitrogen Doped Content Carbon/ Graphene for High Performance Supercapacitors Jin-wei Luo, Wenbin Zhong, Yubo Zou, Changlun Xiong, and Wantai Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10201 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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ACS Applied Materials & Interfaces

Metal-Organic Coordination Polymer to Prepare Density Controllable

and

High

Nitrogen

Doped

Content

Carbon/Graphene for High Performance Supercapacitors Jinwei Luo,† Wenbin Zhong,*,† Yubo Zou,† Changlun Xiong,† Wantai Yang‡ †

College of Materials Science and Engineering, Hunan University, Changsha, 410082,

P. R. China. ‡

Department of Polymer Science, Beijing University of Chemical Technology,

Beijing, 100029, P. R. China. *E-mail: [email protected]

ABSTRACT Design and preparation of carbon-based electrode material with high nitrogen doping ratio and appropriate density attract much interest for supercapacitors in practical application. Herein, three porous carbon/graphene (NCGCu, NCGFe and NCGZn) with high doping ratio of nitrogen have been prepared via directly pyrolysis of graphene oxide (GO)/metal-organic coordination polymer (MOCP) composites, which were formed by reacting 4, 4'-bipyridine (BPD) with CuCl2, FeCl3, and ZnCl2, respectively. As-prepared NCGCu, NCGFe and NCGZn showed high nitrogen doping ratio of 10.68, 12.99 and 11.21 at.%; and high density of 1.52, 0.84 and 1.15 g cm-3, respectively. When as-prepared samples were used as supercapacitor electrodes, NCGCu, NCGFe and NCGZn exhibited high gravimetric specific capacitances of 369, 298.5, 309.5 F g-1, corresponding to high volumetric specific capacitances of 560.9, 1

ACS Paragon Plus Environment


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250.7, 355.9 F cm-3 at a current density of 0.5 A g-1, as well as good cycling stability, nearly 100% of the capacitance retained after 1000 cycles even at a large current density of 10 A g-1. It is expected that the provided novel strategy can be used to develop electrode materials in high performance energy conversion/storage devices. KEYWORDS: Metal-organic coordination polymer; High nitrogen doping ratio carbon/graphene;

Density

controllable;

Volumetric

specific

capacitance;

Supercapacitor

1. INTRODUCTION Supercapacitors have been considered as one of the most attractive energy storage devices owing to their high power density, ultralong cycling stability and fast charge/discharge rates.1-3 Nowadays, variety of porous carbon are served as electrode materials for electrical double-layer capacitors (EDLCs) attributed to their high specific surface area, good chemical stability and excellent electrical conductivity.2 However, further improvement of the specific surface areas of those porous carbon materials is quite limited based on the current technology. To break through this barrier, another way has been tried is to introduce heteroatom, such as nitrogen, into carbon network. The introduction of nitrogen will lead psedocapacitance and increases up the specific capacitance. It will also improve the electronic conductivity and surface wettability at the same time.2 The common methods to prepared nitrogen doped (N-doped) porous carbon materials are carbonization, chemical activation and directly

chemical

activation

nitrogen-containing

precursors

including

polyacrylonitrile,4 polyaniline,5,6 polypyrrole,7 biomass or biomass derivatives,8 and 2


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etc. The prepared N-doped porous carbons exhibited enhanced specific capacitances (over 300 F g-1) and large specific surface areas, but the pseudocapacitances did not improve much due to their relatively low nitrogen content. On the other hand, N-doped porous carbons can also be achieved by directly carbonizing high nitrogen-containing polymers such as polyaniline9 and polypyrrole,10 etc. The porous carbon materials prepared by this way do have improvement in nitrogen content, but the achieved specific capacitance is still not good enough because of their relatively low specific surface areas. Therefore, it is still in need of development of advanced carbon materials with both large specific surface area and high nitrogen doped content. Graphene, a two-dimensional material comprised of sp2 bonded carbon atoms, is considered as a great candidate of electrode material for supercapacitor because its high electrical conductivity, large specific surface area, and excellent mechanical property.11-15 The theoretical capacitance of graphene is about 550 F g-1.11,12 However, the capacitance of graphene that is generated from graphene oxide (GO) is generally as low as 100-200 F g-1, which has been proved the restacking graphene led to the decreased accessible specific surface area.13,14 In order to enhance the gravimetric specific capacitance of graphene, N-doped graphene has been involved in complicated structures with relative high specific surface area including hydrogels,16 aerogels,17 and other three-dimensional structures18. However, these graphene-based materials always possess low densities assigned to their porous structures, which may lead to less competitive in volumetric capacitances although they possess outstanding 3



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gravimetric capacitances. Moreover, Gogotsi and Simon, et al. recommended that volumetric capacitance is more reliable than gravimetric when it is used to evaluate the real potential of the electrode materials for supercapacitors.19 To design and prepare high volumetric capacitance carbon electrode materials should mainly control its volumetric capacity including pore structure, density and specific surface area, and surface chemistry. Recently, high-density graphene-based materials have been prepared by various routes. Densely packed graphene nanomesh-carbon nanotube hybrid film prepared by vacuum-assisted filtration method and graphene electrode prepared by capillary compression method exhibit high density of 1.126 and 1.33 g cm-3 and volumetric capacitance of 331 and 261.3 F cm-3, respectively.20,21 Compactly porous graphene macroform prepared by “evaporation-induced drying” and reduced graphite oxide fabricated by precipitation-assisted method possess a high density of 1.58 g cm-3 and 1.40 g cm-3, and exhibit volumetric capacitance of 376 F cm-3 and 255 F cm-3, respectively.22,23 Additionally, the boron and oxygen co-doped carbon nanofiber free-standing films with a packing density of 0.93 g cm-3 and high volumetric capacitance of 179.3 F cm-3 has been also prepared.24 Although these carbon

materials

have

high

density

and

volumetric

capacitance,

their

electrolyte-accessible surface areas are greatly reduced, hence sacrificing their gravimetric specific capacitances, which make them still less competitive in volumetric capacitances. Metal-organic coordination polymers (MOCP) or metal-organic frameworks (MOFs), as emerging multifunctional materials are consisted of metal ions 4


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coordinated with rigid organic molecules, have attracted many attentions due to their large specific surface area, tunable porosity, and potential applied in different fields, such as gas storage and separation, energy storage, catalysis and so on.25-27 Benefit from these advantages, MOCP/or MOFs can also be used as precursors/templates to prepare different porous carbon materials through thermal conversion. Especially, when nitrogen-containing organic molecules were chosen as ligands, the inherent nitrogen atoms in ligands which would be well kept in the final product after pyrolysis, thus N-doped porous carbons can be obtained.28-33 For example, hierarchical N-doped porous carbon,29 highly graphitized nanoporous carbons,30 nanoporous carbons,31-33 etc., can be prepared by pyrolysis of nitrogen-containing MOCP/or MOFs precursors, including isoreticular metal-organic framework (IRMOF-3), zeolitic imidazolate frameworks (ZIF-67, ZIF-8), and so on. However, the gravimetric specific capacitances of these porous carbon derived from MOCP/or MOFs are still unsatisfied (usually less than 250 F g-1), and their volumetric capacitances were rarely be investigated, which may attribute to the relatively low nitrogen content of their precursors. In order to obtain high gravimetric and volumetric capacitance electrode materials, preparing nitrogen-enriched carbon/graphene with high controllable-density as well as suitable porous structure is a good choose. Herein, a novel strategy is provided to prepare carbon/graphene (NCGCu, NCGFe and NCGZn) with high nitrogen doping ratio via directly pyrolysis of the composites of graphene oxide and nitrogen-enriched MOCPs, which formed by reacting 4, 4'-bipyridine (BPD) with CuCl2, FeCl3, and 5



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ZnCl2, respectively, and then removing metal species. MOCPs served as nitrogen and carbon sources in this method. Moreover, N-doped graphene with excellent electrical conductivity not only can act as a structural scaffold for carbons derived from MOCP but also can provide pseudocapacitance. As-prepared NCGCu, NCGFe and NCGZn possess high doping ratio of nitrogen as 10.68, 12.99, and 11.21 at.%, controllable high density of 1.52, 0.84, and 1.15 g cm-3, and exhibit high gravimetric specific capacitances of 369, 298.5, 309.5 F g-1 and high volumetric specific capacitances of 560.9, 250.7, 355.9 F cm-3, respectively.

2. EXPERIMENT SECTION 2.1. Materials. Natural graphite (325 mesh, Qingdao Henglide Graphite Co., Ltd., China) was used as obtained. 4, 4'-bipyridine (BPD, 98%) was purchased from Meryer Chemical Technology Co., Ltd. (Shanghai, China). Potassium permanganatepre (KMnO4), ferric chloride hexahydrate (FeCl3·6H2O), hydrochloric acid (HCl), hydrogen peroxide (30% H2O2), sulfuric acid (H2SO4, 98%), cupric chloride dihydrate (CuCl2·2H2O), sodium nitrate (NaNO3) and zinc chloride (ZnCl2) were obtained from Sinopharm Chem. Reagent Co., Ltd. (Beijing, China). All chemicals were of analytical reagent grade and were used directly without any further treatment. 2.2. Synthesis of graphene oxide (GO). A modified Hummers method was employed to prepare GO from natural graphite.34,35 Wherein 10 g of Graphite and 5 g of NaNO3 () were mixed with 240 mL of concentrated H2SO4 in an ice bath. Under vigorous stirring, KMnO4 (30 g) was 6


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gradually added in the suspension. After 2 h, ice bath was removed, and the mixture was stirred at 35 ℃ for 3h. With the reaction proceeding, the mixture became thick paste gradually, and its color turned into light brownish. And then, 480 mL of deionized water was slowly dropped into the paste under vigorous agitation. The temperature of above mixture was quickly increased to 95℃ and its color changed to yellow. After 30 min, the reaction was terminated by addition of 720 mL deionized water and 30% H2O2 aqueous solution (80 mL). For purification, the mixture was filtered and washed with 1 L of 10 wt% HCl aqueous solution to remove metal ions, followed by washing consecutively with deionized water to remove acid until the pH value of filtrate reached neutral. After filtration and dry under vacuum (40℃), the resulting gray powder were re-dispersed in ethanol by ultrasonication for 4 h to make an dispersion, and the dispersion was centrifugation at 5000 rpm for 10 min to remove any aggregates. Finally, the homogeneous yellow-brown color GO ethanol dispersions were obtained with concentration of 3.5 mg mL-1. 2.3. Synthesis of nitrogen-enriched doped carbon/graphene (NCG). All the NCG samples were prepared through a composite and pyrolysis process. A typical preparation of NCGCu is descripted as follow: Firstly, 8.6 mL GO (3.5 mg mL-1) was mixed with 11.4 mL CuCl2 solution (0.197 g CuCl2·2H2O dissolved in 11.4 mL anhydrous ethanol) in a three-necked flask. After 15 minutes stirring, 10 mL BPD ethanol solution (30 mg mL-1) was dropped into the mixed solution and the color of mixture gradually turned into navy blue. The mass ratio of GO/BPD was 1:10, and the molar ratio of BPD/CuCl2 was 1:0.6, the GO concentration was 1.0 mg mL-1, and the 7



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entire volume of solution was 30 mL. After continued stirring for another 2 h, the collection of the resultant products was centrifuged at 5000 rpm for 10 min. Thus the carbon precursor was obtained by drying the navy blue precipitate in a vacuum at 90℃ for 24 h. Afterwards, the as-prepared carbon precursor was annealed in a horizontal tubular furnace under N2 atmosphere with a heating rate of 5℃ min-1 to 650℃ and held for 2 h. The sample prepared was washed with 4 M HNO3 solution and deionized water to remove copper species. Finally, the nitrogen-enriched doped carbon/graphene was obtained by drying the sample in a vacuum oven at 90℃for 24 h, and named as NCGCu. For comparison, NCCu was synthesized in the same conditions but without GO. NCGFe and NCGZn were prepared in the same way to NCGCu, including the same mass ratio of GO/BPD and molar ratio of BPD/CuCl2 ，but CuCl2·2H2O were substituted by FeCl3·6H2O and ZnCl2, respectively. Apart from this, the pyrolysis temperature for NCGFe was 600℃, and the iron and zinc species for NCGFe and NCGZn, respectively, were removed by 4 M HCl solution and washed repeatedly with deionized water. For comparison, NCFe and NCZn were prepared in the same conditions but without GO. 2.4. Characterizations. The structures and morphologies of the as-prepared samples were measured by transmission electron microscope (TEM, FEI, Titan G2 60-300) and scanning electron microscope (SEM, Hitachi, S-4800). X-ray diffraction (XRD) patterns were collected on Brucker D8-Advance diffraction with Cu Kα radiation. The measurements of 8


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X-ray photoelectron spectroscopy (XPS) were conducted on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) that equipped with a 1486.6 eV Al Kα X-ray source. The measurements of Raman spectra were conducted on a Raman spectrometer (LR-3, Varian, USA) that equipped with a 633 nm laser light in a range of 300-3000 cm-1. Pore structure of the as-prepared samples was measured by physical adsorption of N2 at 77 K (Micromeritics ASAP 2020) after being vacuum-dried at 200℃ overnight. The specific surface area, micropore surface area and pore size distribution (PSD) were obtained by the Brunauer–Emmett–Teller (BET) method, t-plot method and density functional theory (DFT) method, respectively. 2.5. Electrochemical measurements. The working electrodes were fabricated by mixing active material, poly (tetrafluoroethylene) and carbon black in a mass ratio of 8:1:1 to form slurry, which was then coated on a stainless steel (1 cm2), compressed for 5 min under the pressure of 20 MPa and dried in a vacuum oven at 60 ℃ for 24 h. The weight of the active materials for working electrodes was about 2.5 mg. All electrochemical measurements were tested in a three-electrode cell, in which the prepared electrode was used as the working electrode, saturated calomel electrode (SCE) as reference electrode, and platinum foil as the counter electrode. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were measured on a CHI 660C electrochemical workstation (Chenhua, Shanghai) at room temperature, wherein 1 M H2SO4 electrolyte was applied. The potential range was controlled -0.2‒0.8 V, and electrochemical impedance spectroscopy (EIS) was 0.01 Hz‒100 kHz with an alternating current (AC) 9



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amplitude of 5 mV. The gravimetric specific capacitances (Cg) were obtained from GCD curves using the equation Cg = (I△t)/(m△V), where I is the current (A); △t is the discharge time (s); △V is the potential range (V) including the IR drop; and m is the total mass of active materials in the working electrode (g). The volumetric capacitances (Cv) of the samples were calculated based on the following equations (1) ρ = {Vtotal+1/ρcarbon}-1, (2) Cv = ρ Cg, where ρ is the density of the material (g cm-3), Vtotal is the total pore volume (cm3 g-1) and ρcarbon is true density of carbon (2 g cm-3). 3. RESULT AND DISCUSSION N-doped carbon/graphene with high nitrogen content and controllable density (NCGCu, NCGFe and NCGZn) were prepared through directly pyrolysis of the composites of GO/MOCP. Herein 4, 4'-bipyridine (BPD) was chosen as nitrogen and carbon resource due to its high nitrogen content up to 17.95 wt.%. The composites of GO/MOCP were formed through the coordination reactions of BPD and metal chlorides (CuCl2, FeCl3 and ZnCl2, respectively) in the presence of graphene oxide. During their reaction process, the lone pair of electrons from nitrogen atom in BPD, from oxygen atom of the hydroxyl and carboxyl groups in GO can both donate to a metal cation, thus forming their coordination complex structures.28,36 It is noted that the H-bonding, π-π stacking, and some other types of chemical or physical interactions between BPD and GO also played important roles in forming the well combined composites. Moreover, the carbons derived from MOCP can act as “spacer” to prevent the restacking of graphene caused by the strong π-π interactions during the pyrolysis process, and its porous structures can be further formed by removing the 10


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metal species with acids after the pyrolysis. The interior microstructures of the as-prepared carbon materials were investigated by using SEM and TEM. As shown in Figure 1a, b, NCGCu presents three-dimensional (3D) hierarchical network structures constructed with wrinkled sheet, and the wrinkled structures can be further confirmed from its TEM images (Figure 2a, b). While NCCu exhibits a 3D porous coral-like crosslinked structure (Figure 1c, d). NCGFe demonstrates a similar network structure with NCGCu (Figure 1e, f). Moreover, the highly wrinkled-sheets and folded structures of NCGFe can be further confirmed by its TEM images (Figure 2b, c). Compared with NCGFe, NCFe has a different angular block-like structure assembled by irregular carbon matrix derived from polymer (Figure 1 g, h). NCGZn possesses a hierarchical porous network structure constructed with wrinkled plates (Figure 1i, j), and the average diameter of open-pores is about a few hundred nanometers that is further confirmed in Figure 2e and f. NCZn shows a macroporous structure with the diameter about hundreds of micron. The macropores in NCZn may be caused by more seriously melting process of its precursor than experienced by NCCu and NCFe, leading to the release of gaseous products (such as CO2 and CO) during the pyrolysis process. Based on the above results, it can be found that NCGCu, NCGFe and NCGZn possess wrinkled plates network structures and the significant differences from NCCu, NCFe and NCZn are attributed to the introduced GO that can act as nucleation and growth sites for seeding pyrolysis product of MOCP.

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Figure 1. SEM images of (a, b) NCGCu, (c, d) NCCu, (e, f) NCGFe, (g, h) NCFe, (i, j) NCGZn and (k, l) NCZn at different magnifications.

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Figure 2. TEM images of (a, b) NCGCu, (c, d) NCGFe and (e, f) NCGZn at different magnifications (Insets are high magnification results). The XRD patterns of the as-prepared carbon materials are showed in Figure 3a. all samples exhibit a broad peak at about 25.3° and a weak peak at approximately 43.0°, attributing to the diffraction peak of (002) and (100) crystal planes of graphite carbons, respectively, which also suggest they have the low degree of crystallinity structures.35,37 However, the full-width at half-maximum (FWHM) of NCGCu, NCGFe and NCGZn has smaller values than that of NCCu, NCFe and NCZn, respectively, 14


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indicating relative high degree of crystallinity and graphitization in NCGCu, NCGFe and NCGZn. This result can be attributed to the introduced graphene increasing the ordered structure in the as-prepared carbon materials, and this was also confirmed by their TEM images (Inset images of Figure 2b, d and e). Figure 3b shows the Raman spectra of the as-prepared carbon materials, and all samples exhibits two prominent peaks at about 1350 cm-1 and 1590 cm-1 assigned to D and G bands, respectively. The D band corresponds to the structure defects and disorder carbon in graphite and the G band represents the vibration of sp2-hybridized carbon.37,38 The ratio of integrated intensities of D and G band (ID/IG) reflects the graphitization degree of carbon material,38 and the ID/IG values of NCGCu, NCCu, NCGFe, NCFe, NCGZn and NCZn are 1.76, 2.04, 1.97, 2.05, 2.07 and 2.31, respectively (Table S1). This result indicates as-prepared samples have low graphitization degree, which is also consisted with XRD analysis (Figure 3a). Compared with NCCu, NCFe, and NCZn, the ID/IG values of NCGCu, NCGFe, and NCGZn are low, which is attributed to the introduction of graphene that can help to increase the ordered structures of them. In additional, compared to NCCu, NCFe and NCZn, the D peak of NCGCu, NCGFe and NCGZn all shift a little towards low wave-numbers, which may ascribe to the interaction of prepared N-doped graphene and carbon.

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Figure 3. (a) XRD patterns and (b) Raman spectra of NCGCu, NCCu, NCGFe, NCFe, NCGZn and NCZn. Nitrogen adsorption-desorption isotherms were used to determine the specific surface areas and pore structures for the as-prepared carbon products. As shown in Figure 4a, NCGCu and NCCu exhibit type II isotherms indicating that the samples is non-porous structure materials, while the abrupt increase in the high relative pressure region (P/P0>0.9) indicates the existence of macropores. NCGFe displays type IV nitrogen adsorption isotherms, wherein an obvious sharp increase of adsorption appears at very low relative pressure (P/P0

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