Resin Copolymer under Hypersaline Condition to High

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Schiff-Base/Resin Copolymer under Hypersaline Condition to HighLevel N-Doped Porous Carbon Nanosheets for Supercapacitors Danfeng Xue, Dazhang Zhu, Mingxian Liu, Hui Duan, Liangchun Li, Xiaolan Chai, Zhiwei Wang, Yaokang Lv, Wei Xiong, and Lihua Gan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01125 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Applied Nano Materials

Schiff-Base/Resin Copolymer under Hypersaline Condition to High-Level N-Doped Porous Carbon Nanosheets for Supercapacitors Danfeng Xue,† Dazhang Zhu,† Mingxian Liu,*, †,§ Hui Duan,† Liangchun Li,† Xiaolan Chai,† Zhiwei Wang,‡,§ Yaokang Lv,∥ Wei Xiong,# Lihua Gan*,†

†

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science

and Engineering, Tongji University, Shanghai 200092, P. R. China. ‡

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental

Science and Engineering, Tongji University, Shanghai 200092, P. R. China. §

Shanghai Institute of Pollution Control and Ecological Security, Tongji University, Shanghai

200092, P. R. China. ∥

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P.

R. China. #

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, 693

Xiongchu Road, Wuhan 430073, P. R. China.

*Corresponding Authors. E-mail: [email protected] (M. Liu), [email protected] (L. Gan)

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ABSTRACT: We develop a novel strategy to fabricate nitrogen-rich porous carbon nanosheets (N-PCNs) using Schiff-base/resin copolymer under hypersaline medium. Melamine-terephthalaldehyde Schiff-base containning high-bond-energy C=N covalent bonds effectively reduces the loss of the N species during carbonization and thus provide high nitrogen dopants, while the introduction of melamine-formaldehyde resin and ZnCl2 as a solubility enhancing stabilizer play a key synergistic effect making the formation of stable polymeric network. Besides, ZnCl2 also serves as a salt-templating and a foaming agent. The resultant N-PCNs have a rich N-doping of 6.55 wt %, a unique nanosheet morphology with a thickness of ~200 nm, and a large surface area of 1403 m2 g−1 along with rational porous architecture, which affords superior electrochemical performances for a supercapacitor electrode, such as high gravimetric capacitances of 362 and 268 F g−1 at 2.0 A g−1 in a three-electrode and a two-electrode system respectively using KOH electrolyte. Moreover, the electrode delivers remarkable energy density of 9.2 Wh kg−1 at the power density of 505 W kg−1 and a high cycling stability with 93.8% capacitance retention at 1.0 A g−1 after 10000 cycles. The present study provides a new avenue for facile and high efficient construction of N-enriched porous carbons for potential supercapacitor application. KEYWORDS: Porous carbon nanosheet; High nitrogen content; Hypersaline medium; Schiff-base/resin copolymer; Supercapacitor electrode.


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1. INTRODUCTION Carbon-based supercapacitors exhibit prominent characteristics of high power output, fast charging/discharging kinetics and long cycle stability, which meets the demands of commercial applications for hybrid electric vehicles, industrial energy management, mobile electronic devices, etc.1-8 Only electrical double-layer capacitance (EDLC) is available for carbon electroactive materials which store electrical energy via the aggregation of ionic charges at the carbon/electrolyte interface, the surface area thus is an important parameter to determine the electrochemical performance.9-11 However, the capacitance of carbon electrodes are still unsatisfactory (usually < 250 F g−1) even under a ultrahigh surface area (e.g., 3550 m2 g−1).12-13 which hinders their widespread applications. In addition to surface area, the target to give carbon electrodes better capacitive performance can be achieved through controlling over their pore architectures and geometries.14-15 For example, Wang et al. reported that ultra-thin porous carbon nanosheet with hierarchical pores exhibits excellent electrochemical properties.16

In

our

previous

work,

carbon

nanospheres

with

an

ultramicroporous@microporous core–shell structure were fabricated for high-performance supercapacitors.17 Heteroatoms like nitrogen doping into carbon matrix has proven to be an effective way to significantly improve the electrochemical properties of carbon-based electrodes, which triggers pseudocapacitance due to reversible redox reaction of the functional atoms.18 Besides, the surface wettability, electronic conductivity and accessible surface of the materials can also be optimized.19-21 However, N contents in the carbons are relatively low (typically < 5 wt %) because N atoms usually derived from low-bond-energy C–N single bonds (305 kJ mol−1) in



most nitrogen sources. The broken of such covalent bonds during carbonization results in the escape of majority of N species.22-23 Schiff-base offers great potential as effective precursors to fabricate N-enriched carbons because the existence of high thermally stable and high bond-energy C=N bonds (615 kJ mol−1) can reduce the escape of heteroatoms during pyrolysis.24-26 However, the immature pore structure and relatively low surface area of Schiff-base derived carbons has greatly hindered their application in supercapacitor owing to the high ion transport resistance and deficient ionic adsorption sites.27 To develop an adequate porosity in such carbons, widely techniques are employed such as hard and soft templating28-29 or activation methods,30 which generally suffer certain drawbacks: application of hazardous chemicals or an additional post-activation process and low yield of carbons.31 Recently, a new approach for fabricating porous carbons called salt templating32-33 was presented, a non-carbonizable inorganic salt is mixed with a carbon precursor which is condensed and scaffolded in the presence of molten salt at elevated temperatures. The salt phase is easily removed by simple washing with water and leaving behind micropores or mesopores . This method shows multiple advantages of sustainable, cost-effective, simple and energetic costs. A hypersaline strategy is an extension of the salt-templating route, where the salt not only saved as a template but also as a stabilizer and foaming agent to induce a 3D structural formation.34 By mixing polymer precursor with aqueous salt solution close to the saturation limit, the inorganic salts with sufficient polarizability and Lewis acid character can lower the partial pressure of water and serve as a stabilizing agent to promote cosolubilization of all components. Besides, the possibility of pore tuning is related to the Hofmeister series, gaining salt concentration and salt type as additional parameters for pore structure control.35-37


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For example, Yu group reported the polymerization of phenol–formaldehyde under hypersaline conditions to synthesize carbon aerogels with a low density and a high surface area.34 Herein, inspired by the advantages of Schiff-base chemistry and hypersaline route, we report a novel strategy to design high-level nitrogen-rich porous carbon nanosheets (N-PCNs) through the polymerization and carbonization of terephthalaldehyde-melamine-formaldehyde copolymer under hypersaline medium ZnCl2. The formation of melamine-terephthalaldehyde Schiff-base offers high content nitrogen in N-PCNs due to high bond energy of C=N covalent bonds. Besides, the introduction of melamine-formaldehyde resin plays a key role of a stablizer for the polymeric network. Furthermore, ZnCl2 salt, besides a salt-templating and a foaming agent, enables the effective cosolubilization of all organic monomers to form a stable copolymer. Benefited from unique morphology, high-level N doping, high surface area together with reasonable pore structure, N-PCN electrodes exhibit excellent electrochemical performances such as a high gravimetric capacitance, a superior long-term stability, and a high power density with reasonable energy density in KOH electrolyte. The simple and innovative synthetic strategy highlights new opportunities for highly efficient design and synthesis of carbon nanomaterials for energy storage.

2. EXPERIMENTAL SECTION Chemicals and Materials. Melamine, terephthalaldehyde, formaldehyde solution (37 wt %), zinc chloride and HCl (36−38 wt %) were of analytical grade without further purification and were purchased from Sinopharm Chemical Reagent Co. Ltd. Graphite was obtained from



Shanghai Colloid Chemical Plant. Polytetrafluoroethylene (PTFE, catalog no. FR301B) was provided by Shanghai 3F New Materials Co. Ltd. Nickel foam was purchased from Shanghai Hongxiang Plant. Synthesis of N-PCNs. In an ice bath, ZnCl2 (1~9 g), 1.261 g melamine (10 mmol) and 1.341 g terephthalaldehyde (10 mmol) were dissolved in 1.5 mL formaldehyde (20 mmol) solution to obtained a white viscous sol. Afterwards, the sol was sealed in a 50 mL Teflon-lined autoclave and heated at 160 °C for 8 h to yield deep brown gel. After dried at 100 °C for 24 h under vacuum, the resulting nitrogen-containing polymer (referred as NP-x in which x hereafter denotes to the mass of ZnCl2) was carbonized at 700−900 °C (2.5 °C min−1) for 1 h under nitrogen atmosphere to obtained N-PCNs (termed as N-PCN-x-y in which y reflects to the carbonization temperature, details of synthesis conditions and the denotation of samples are listed in Table S1). To remove porogen (ZnO), the obtained samples were purified in 1.0 M HCl (repeated twice) and finally filtrated and dried in vacuum for characterization. Characterization. The morphology and microstructure of the sample were characterized using a Hitachi S-4800 scanning electron microscopy (SEM) and a JEM-2100 transmission electron microscopy (TEM). N2 adsorption and desorption isotherms were achieved from an analyzer (Micromeritics) of ASAP 2460 at −196 °C. The specific surface area and the pore volume were calculated using the Brumauer-Emmett-Teller (BET) method within P/P0=0.05−0.25 and Dubinin−Radushkevich plot at P/P0 =0.995, respectively. The pore size distribution was obtained from the adsorption branches of the isotherms using nonlocal density functional theory (NLDFT) equilibrium model. X-ray photoelectron spectrometer (XPS, AXIS Ultra DLD) was utilized to analyze the surface characteristics of the samples.


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X-ray diffraction (XRD) measurement was conducted on a Bruker D8 diffractometer with Cu radiation (λ = 0.154 nm). Thermo Nicolet NEXUS spectroscopy was applied to obtain the Fourier-transform infrared (FT-IR) spectra. Inductively coupled plasma mass spectroscopy (ICP-MS) was measured on Optima 7300 DV to test the concentration of zinc. Raman spectrum under λexc = 514 nm laser excitation was conducted under the exposure time and excitation power of 10 s and 20 mW, respectively. Electrochemical Measurement. For three-electrode system, the working electrode was assembled by homogeneously mixing as-prepared carbon sample (~3.0 mg), graphite and PTFE with a mass ratio of 8:1:1. The mixture was dispersed in ethanol, and then pressed onto a Ni foam under 20 MPa and dried at 100 °C to obtain a circle working electrode (with a diameter of 0.5 cm and a thickness of 0.25 mm). Saturated calomel was used as a reference electrode and Ni foam as a counter electrode. With a potential between −1.0 and 0 V in KOH electrolyte (6 M), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and galvanostatic charging/discharging (GCD) were tested. The capacitance of a single electrode was calculated using the following equation: Cm =

I × ∆t m × ∆V

(1)

where Cm denotes the gravimetric capacitance (F g−1), I refers to the discharging current (A), ∆t is the discharging time (s), m reflects the mass of active materials (g), ∆V is the voltage range (V) during the discharging process. For a two-electrode configuration, CV and GCD were tested at a potential from 0 to 1.0 V in 6 M KOH solution. The working electrode was obtained by mixing of carbon sample (~2 mg), graphite and PTFE with the same mass ratio above. After drying at 100 °C overnight, the



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mixture was pressed on the nickel foam. Using glassy paper as a separator to partition two identical sample electrodes. The capacitance of the cell (Ccell, F g−1), energy density (E) and power density (P) of were calculated using the following equations: Cm = 4Ccell = E = P =

4I × ∆t m × ∆V

Ccell V2 7.2 E ∆t

(2) (3) (4)

3. RESULTS AND DISCUSSION The synthesis route of N-PCNs is presented in Figure 1. Melamine and terephthalaldehyde can build up a 3D Schiff-base polymeric network by the reaction between amino groups of melamine and the aldehyde groups of terephthalaldehyde. Meanwhile, melamine also has reactivity with formaldehyde to form a resin through a dehydration reaction. Moreover, ZnCl2 can facilitate the dehydration process, which benefits the formation of stable schiff-base/resin copolymer. FT-IR spectrum of the polymer shown in Figure S1a exhibits the characteristic peaks at 1221 and 790 cm−1 which are ascribed to the stretching vibration of the C−C benzenoid ring and the stretching vibration of C−N covalent bonds,38 while the stretching vibration at 1420 cm−1 and 3297 cm−1 reflect the triazine rings and O−H bonds, respectively.27, 39

The characteristic peak at 1587 cm−1 corresponds to C=N covalent bonds,40, 41 indicating

the formation of the Schiff-base. Besides, C=O stretching vibration peak inherited from the aldehydes was not observed around 1725 cm−1,42 which confirms the formation of Schiff-base/melamine-formaldehyde resin copolymer. The C=N double bonds still exist in the carbons whereas the majority of other functional groups are broken, suggesting high-temperature stability of C=N bonds. XRD patterns shown in Figure S1b indicates the


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existence of ZnO in the nitrogen-doped carbons because ZnCl2 acts as a porogen via the transformation into ZnO during carbonization (ZnCl2 + H2O → ZnO + 2 HCl↑). ZnO template can be easily removed by washing with diluted HCl (zinc contents in N-PCNs detected by ICP analysis are less than 0.01 wt %) and amorphous N-PCNs were obtained and pores generated.

Figure 1. Schematic synthesis route of N-PCNs based on Schiff-base/resin copolymer under hypersaline condition.

It was found that the synergistic effect of formaldehyde and hypersaline medium ZnCl2 enables the effective cosolubilization of all organic monomers. The mixture of melamine−terephthalaldehyde−formaldehyde with ZnCl2 forms a viscous slurry (Figure 2a), the polymerization of which results in homogeneous monolith (Figure 2d). Without ZnCl2 or formaldehyde solution (using equivalent water to dissolve melamine−terephthalaldehyde), the mixture (Figure 2b,c) and the resultant hydrothermal products (Figure 2e,f) occurred phase separation. The copolymer of melamine−terephthalaldehyde−formaldehyde obtained under ZnCl2 hypersaline condition expands to several times of its original volume after carbonization (Figure 2g, left to middle), while the carbonized sample prepared without ZnCl2 medium have no such volume change (Figure 2g, right). This result indicates that ZnCl2, besides salt-template, also serves as a foaming agent during carbonization. Furthermore, the



carbonized sample from polymer obtained without formaldehyde also does not show a volume expansion as N-PCP-800, as shown in Figure 2h, which suggests that synergistic effect of formaldehyde and hypersaline medium ZnCl2 is vital for achieving N-PCNs. More importantly, ZnCl2 as a stabilizer enhances the miscibility of all components, and the polymerization of formaldehyde and melamine also promotes the stability of the polymeric network through the formation of terephthalaldehyde−melamine−formaldehyde copolymer. Therefore, the synergistic action of ZnCl2 and formaldehyde (or melamine-formaldehyde resin) was the key concern for the successful synthesis of N-PCNs.

Figure 2. The mixture of melamine−terephthalaldehyde−formaldehyde with (a) and without (b) ZnCl2, melamine−terephthalaldehyde−water with ZnCl2 (c), and the corresponding hydrothermal products (d, e and f) and carbonized samples (g, middle and right, and h).

ZnO/N-PCN-6-800 prepared under ZnCl2 hypersaline condition is composed of unique carbon nanosheets (Figure 3a) with a thickness of ~200 nm. After removal of ZnO, the carbon nanosheets are retained in the N-PCN-6-800 (Figure 3b,c). Whereas N-PCP-z6-800 (obtained with ZnCl2 as a salt template) and N-PCP-800 (obtained without ZnCl2) consist of


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micrometer-scaled particles (Figure 3d,e). This result indicates that the synergistic action of hypersaline medium ZnCl2 and the formation of stable copolymeric network in the presence of formaldehyde is responsible for the generation of carbons with nanosheet morphology. Raman spectra of N-PCNs (Figure S2) exhibit two peaks at 1580 and 1350 cm−1, respectively assigned to G peak (showing the graphitic carbon) and D peak (reflecting the disordered carbon). The intensity ratio of D peak to G peak (ID/IG) reflects the graphitization degree of carbon-based materials.43-44 The ID/IG values for N-PCN-6-700, N-PCN-6-800 and N-PCN-6-900 are 0.90, 0.86 and 0.83, respectively, which suggest enhanced graphitization degree of N-PCNs owning to increased carbonization temperature.

Figure 3. SEM images of ZnO/N-PCN-6-800 (a) and N-PCN-6-800 (b), a typical TEM image of N-PCN-6-800 (c), and SEM images of N-PCP-z6-800 (d) and N-PCP-800 (e).

N2 sorption isothermals and pore size distribution curves of N-PCNs are depicted in Figure 4. The isotherm curves of N-PCNs show a steep increase in the adsorbed volume at P/P0

Resin Copolymer under Hypersaline Condition to High

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