Multistage Self-Assembly Strategy: Designed Synthesis of N-doped Mesoporous Carbon with High and Controllable Pyridine N Content for Ultrahigh Surface-Area-Normalized Capacitance

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2021Multistage Self-Assembly Strategy: Designed Synthesis of N-doped Mesoporous Carbon with High and Controllable Pyridine N Content for Ultrahigh Surface-Area-Normalized Capacitance Liangliang Zhang†, Tao Wang†, Tu-Nan Gao, Hailong Xiong, Rui Zhang, Zhilin Liu, Shuyan Song, Sheng Dai and Zhen-An Qiao Liangliang Zhang† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Tao Wang† State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Tu-Nan Gao State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Hailong Xiong State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Rui Zhang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Zhilin Liu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 , Shuyan Song State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022 , Sheng Dai Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. and Zhen-An Qiao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012 https://doi.org/10.31635/ccschem.020.202000233 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Nitrogen doping could improve the performance of carbon materials in electrocatalysis, CO2 adsorption, and energy storage. [1]However, the control of the doping type and the amount of nitrogen (N)-doped in carbon materials in a simple and environmentally friendly way remains challenging. Herein, we report a facile, multistage, self-assembly strategy for the synthesis of two-dimensional N-doped mesoporous carbon (2D NMC) by using graphene oxide (GO) as a structure-directing agent. The resultant 2D [email protected] rendered quantitatively controllable mesopores (8–25 nm). The 2D [email protected] rendered quantitatively controllable mesopores (8–25 nm), high and controllable N content (up to 19 wt %), and the percentages of pyridine and pyridone/pyrrolic N atoms were as high as 49.9% and 35.3%, respectively. Due to these unique characteristics, the fabricated 2D [email protected] exhibited an ultrahigh surface-area-normalized capacitance of up to 90.6 μF cm−2, which was much higher than the theoretical electrochemical double-layer capacitance of activated carbon (15–25 μF cm−2). Moreover, our proposed multistage self-assembly strategy is versatile, and thus, could be extended to the synthesis of one-dimensional (1D) [email protected] and zero-dimensional (0D) [email protected] materials. Download figure Download PowerPoint Introduction Mesoporous carbon materials with tunable pore size, large surface area, and excellent chemical and thermal stability, have received widespread attention due to their outstanding performance in adsorption, catalysis, energy conversion, and storage.1–11 The introduction of heteroatoms into mesoporous carbon materials is an effective method for the modification of the electron donor/acceptor characteristics of carbon materials.12,13 Heteroatoms, such as N atoms in carbon frameworks serve as Lewis base sites to enhance the performance of carbon materials in many applications such as usage in supercapacitors, batteries, and oxygen reduction reactions (ORRs).14–18 In particular, pyridine (N-6) and pyridone/pyrrolic nitrogen atoms (N-5) in carbon frameworks are considered to play key roles in improving the surface-area-normalized capacitance via pseudocapacitance19–21 due to their reversible faradaic redox reactions in charge–discharge processes.22 Though many efforts have been devoted to the preparation of N-doped mesoporous carbon materials via the harsh NH3 activation of neat mesoporous carbon under high thermal reaction temperatures (usually ≥ 700 °C),23–30 the N content of these materials is limited typically to ∼ 10 wt %, of which the percentages of N-6 and N-5 are inadequate due to the low doping efficiency of ammoniated carbon materials. Besides, the introduction of N atoms into the carbonaceous structure via NH3 activation occurs mainly at the plane edges and on defect sites, resulting in both uneven distribution of N and uncontrollable N species.31,32 A molecular self-assembly method is highly efficient for the preparation of mesoporous carbon and heteroatom-doped mesoporous carbon materials via the formation of hydrogen bonds between resol-based polymers as carbon precursors and triblock copolymers as templates, because of the timesaving, environmentally friendly, and the succinct procedures involved.6,7,33–37 A series of N-doped mesoporous carbon materials have been facilely synthesized by this method via the use of N-containing resol as a precursor, or the use of N-rich organic molecules for copolymerization with resol.33,34,38–42 Although a uniform distribution of N in the carbon skeleton could be achieved, the products often have an inferior pore structure with poor thermal stability, and the N content remains very low.38 For other types of polymers with specific functional groups or doped elements, the lack of a matching driving force between the polymer precursor and surfactant micelles during the molecular self-assembly and polymerization processes generally leads to the failure of the formation of a dynamically stable mesostructure, and ultimately, results in a dense and nonporous structure. Two-dimensional (2D) carbon materials with abundant exposed sites and high mass transfer rates are emerging materials for use in energy storage and electrocatalysis.43–47 The construction of mesopores in 2D carbon materials is highly useful but generally depends on silica nanotemplates, which require not only time-consuming multistep procedures but also strong corrosive chemicals such as hydrofluoric acid.48,49 Furthermore, the sizes, mesostructures, and morphologies of the replicated carbon materials are limited to those of the parent templates. Therefore, the design of a rational self-assembly method for the synthesis of 2D carbon materials with precisely adjustable pore sizes and pore structures, high N content, and high percentages of N-6 and N-5 is extremely difficult and yet urgently needed. Herein, a facile multistage self-assembly strategy for the synthesis of 2D graphene oxide (GO)@N-doped mesoporous carbons ([email protected]) with tunable pore sizes (8–25 nm) is reported. In this method, 2,6-diaminopyridine (DAP) was used as a monomer inherent with a pyridine N atom, which was polymerized to polydiaminopyridine (PDAP) as a carbon precursor. The block copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO) is assembled into spherical micelles for mesopores, and graphene oxide was incorporated not only as a structure-directing agent for the 2D materials but also as a platform for enriching the composite micelles. This multistage self-assembly strategy began with the DAP adsorbed on the spherical PS-b-PEO micelles via hydrogen bonds, which further self-assembled closely on both sides of the graphene oxide via electrostatic interaction and hydrogen bonds. After the addition of a strongly oxidative reagent, ammonium persulfate (APS), DAP was polymerized into PDAP around the PS-b-PEO micelles, resulting in the formation of a 2D composite. Subsequently, the composite obtained carbonized under the N2 atmosphere to remove the surfactant and obtained mesoporous structures in the [email protected] The synthesized 2D [email protected] had a high and tunable N content (up to 19%) of which the percentages of N-6 and N-5 were as high as 49.9% and 35.3%, respectively. Additionally, the uniform mesopore size in the [email protected] could be adjusted precisely from 8 to 25 nm by varying the length of the polystyrene (PS) segment of the diblock copolymer. Benefitting from their abundant N sites with high percentages of N-6 and N-5, 2D structure, as well as controllable, uniform mesopores, the 2D [email protected] exhibited excellent performance as supercapacitor materials with ultrahigh surface-area-normalized capacitances of up to 90.6 μF cm−2, and also, excellent cyclic stability. Experimental Method Materials Cerium(III) nitrate hexahydrate [Ce(NO3)3·9H2O], ammonium peroxydisulfate (APS), and copper(I) bromide (CuBr) were purchased from Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). Graphene oxide (GO; 2 mg·mL−1) and carbon nanotubes (CNTs) were purchased from Suzhou Hengqiu Nanotechnology Co., Ltd. (Suzhou, China). 2,6-Diaminopyridine (DAP), monomethoxy poly(ethylene oxide) (Mw: 5000 g/mol, designated as PEO-5000), and α-Bromoisobutyryl bromide, were purchased from Sigma-Aldrich Corporation (Shanghai, China). Styrene, pyridine, and aluminum oxide (Al2O3) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Glacial acetic acid, ethanol, ethylene glycol, ether, petroleum ether, and tetrahydrofuran (THF) were purchased from Beijing Chemical Works, China. All reagents were used without further purificationthe detailed preparation of PS-b-PEO diblock copolymer was provided in Supporting Information. Preparation of [email protected] Exactly 30 mg of PSn-b-PEO117 copolymer was dissolved in 3 mL of THF, followed by a dropwise addition of a 9 mL mixture of deionized (DI) water and ethanol (v/v = 1∶2) into the THF solution under vigorous stirring to generate micelles. Afterward, 15 mL of DI H2O was added quickly to the reaction solution to quench micelle aggregation. Next, 24 mg of DAP was added to the reaction mixture and stirred continuously for 60 min. Subsequently, 0.9 mL of GO solution (2 mg·mL−1) was injected into the reaction mixture, and 0.3 mL of APS (0.334 g·mL−1) solution was added after 10 min. The total mixture was stirred for 4 h at 5 °C. The product was obtained by centrifugation at 8000 r/min for 3 min, washed with ethanol and water for one time and dried overnight at 60 °C. The [email protected]/PSn-b-PEO117-X obtained was heated gradually at 2 °C min−1, from room temperature to 350 °C, and kept at this temperature for 3 h under an N2 atmosphere. Then the temperature was raised by 5 °C/min to 700 °C and maintained at 700 °C for 1 h. In this work, the subscript "n" represents the degree of assembly of the PS block. PS131-b-PEO117, PS190-b-PEO117, PS230-b-PEO117, and PS305-b-PEO117 were used to prepare [email protected] with different mesopore sizes, and the products were denoted as [email protected], [email protected], [email protected], and [email protected] Two more products, [email protected] and [email protected], were acquired by adjusting the original final temperature of 700 °C to 600 °C and 800 °C, respectively. Reports of the experimental details, characterization methods, and synthesis of other samples are available in the Supporting Information. Results and Discussion The block copolymer PS-b-PEO was synthesized via atom transfer radical polymerization (ATRP) reaction and verified by nuclear magnetic resonance (NMR; Supporting Information Figure S1a) and gel permeation chromatography (GPC; Supporting Information Figure S1b).50–53 For the synthesis of [email protected], PS-b-PEO spherical micelles with varying PS cores and fixed PEO coronas were formed by the self-assembly of PS-b-PEO in tetrahydrofuran (THF)/H2O/EtOH mixed solvent, which exhibited an apparent Tyndall effect (Figure 1j). The hydrodynamic diameter distribution of the micelles in the mixed solution was measured by dynamic light scattering (DLS). As presented in Figure 1j, the PS131-b-PEO117, PS190-b-PEO117, PS230-b-PEO117, and PS305-b-PEO117 micelles, and denoted as M-1, M-2, M-3, and M-4, respectively, with the corresponding hydrodynamic diameters of ∼27, ∼34, ∼37, and ∼54 nm. A linear fitting equation, y = 0.15x + 5.70 (R2 = 0.94) was obtained to illustrate the linear relationship between the average PS-b-PEO micelle size and the PS block length (Figure 1k). The DAP monomers were assembled with the PEO of PS-b-PEO to form DAP/PS-b-PEO composite micelles. In the presence of GO, the DAP/PS-b-PEO composite micelles were anchored on the GO surface and cross-linked under a strong oxidation reaction with APS. As presented in Supporting Information Figure S2, the composite obtained using PS131-b-PEO117 as the template and denoted as 2D [email protected]/PS-b-PEO-1, exhibited a 2D nanosheet structure with the 2D morphology and mesostructure well maintained during the carbonization process, thereby, leading to the formation of a mesoporous 2D structure in the corresponding product, [email protected] As determined by scanning electron microscopy (SEM), [email protected] sustained the 2D nanosheet structure ( Supporting Information Figure S3a), and as determined by transmission electron microscopy (TEM), it had uniform mesopores of about 8 nm in diameter (Figure 1a). A high-resolution TEM (HRTEM) image revealed many micropores in the mesoporous wall (Figure 1b) that could have been provided interconnected channels among the mesopores. Atomic force microscopy (AFM) was employed to measure the thickness of the 2D samples. As presented in Figure 1c, [email protected] displayed a flat surface with a uniform thickness of ∼20 nm, which was almost twice the diameter of the mesopores and indicated a sandwich-like structure in which there was only one layer of mesopores on each side of the GO. According to the element mapping images of [email protected] (Figure 1d), the C, N, and O elements were distributed uniformly in the carbon frameworks. Thus, the [email protected] obtained was indeed a novel 2D-rich N-doped mesoporous carbon. Figure 1 | (a and b) TEM images of [email protected] (c) AFM image of [email protected] (d) HAADF-STEM image and corresponding EDX mapping of C, N, and O in [email protected] (e and f) SEM images of [email protected] TEM images of (g) [email protected], (h) [email protected], and (i) [email protected] (j) Hydrodynamic diameters of micelles measured by DLS; the inset is a photo of PS-b-PEO colloids that shows the presence of the Tyndall effect. (k) Relationship between the PS chain length of PS-b-PEO and the average micelle diameter (dmicelle) as determined by DLS, and the peak mesopore size of the [email protected] as estimated by the BJH method (dBJH) and the TEM images of the [email protected] (dTEM), respectively. TEM, transmission electron microscopy; AFM, atomic force microscopy; HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; EDX, energy-dispersive X-ray spectroscopy; SEM, scanning electron microscopy; DLS, dynamic light scattering; BJH, Barrett–Joyner–Halenda. Download figure Download PowerPoint Next, we explored the relationship between the mesopore size and the PS chain length by preparing three other PS-b-PEO samples (PS190-b-PEO117, PS230-b-PEO117, and PS305-b-PEO117) to synthesize a series of 2D-rich N-doped mesoporous carbons (denoted as [email protected], [email protected], and [email protected], respectively), which, according to SEM analysis had similar 2D structures ( Supporting Information Figures S3b and c; Figure 1f). Due to the large mesopores formed by PS305-b-PEO117, the closely packed pores on the surface of [email protected] were observed directly in a high-magnification SEM image (Figure 1e). As shown in Figures 1a and 1g–i, all the [email protected] were covered with closely packed mesopores, and the average pore size increased from 8 to 25 nm with increased PS chain length in the PS-b-PEO, ranging from 131 to 305. A linear fitting equation y = 0.101x − 6.43 (R2 = 0.94) was obtained to describe the relationship between the mesopore size and the PS chain length of the diblock copolymer (Nps). The function confirmed a quantitative control of the mesopore size via the variation of the PS chain length (Figure 1k and Table 1). Table 1 | Structural Parameters of the 2D [email protected] Samples Samples SBETa [m2 g−1] dTEMb [nm] dmb [nm] dBJHb [nm] [email protected]c 322 ∼ ∼ ∼ [email protected] 324 8 27 9 [email protected] 273 11 34 11 [email protected] 255 17 37 17 [email protected] 234 25 54 23 aThe Brunauer–Emmett–Teller surface areas (SBET) measured from the N2 adsorption–desorption isotherms. bThe average pore diameters as calculated by TEM images, DLS, and the BJH method, respectively. c[email protected] prepared in the absence of PS-b-PEO. Detailed porosities of the [email protected] were illustrated via N2 adsorption–desorption analysis (Figure 2a), using an automated ASAP 2420 apparatus system (Micromeritics, Shanghai, China). All the isotherms of the [email protected] exhibited type-IV curves, according to the classification of the International Union of Pure and Applied Chemistry (IUPAC), indicating the existence of abundant mesopores. With the increase of the PS chain length from 131 to 305, the capillary condensation step shifted gradually to a higher adsorption pressure, indicating an increase in the mesopore size,54 with a subsequent decrease in the Brunauer–Emmett–Teller (BET) surface area of the [email protected] from 324 to 234 m2 g−1 (Table 1) at a carbonization temperature of 700 °C. All the adsorption isotherms of the [email protected] revealed adsorption capacities at a low relative pressure (P/P0 < 0.05), also demonstrating abundant micropores in the frameworks. The micropore size distribution curve of [email protected] showed two dominant pore sizes of 0.6 and 0.8 nm ( Supporting Information Figure S4c). As the carbonization temperature was increased from 600 to 800 °C, the BET surface areas of the [email protected], [email protected], and [email protected] increased from 63 to 390 m2 g−1 (Figure 2c). Further, pore size distribution curves of the [email protected] were acquired based on the method of Barrett–Joyner–Halenda (BJH). We observed that, by increasing the PS chain length, the average pore diameters of the [email protected] increased from 9 to 23 nm, consistent with the TEM results (Figure 2d and Table 1). A linear function of the relationship between the average mesopore size and NPS was obtained (y = 0.085x − 3.13, R2 = 0.95), which confirmed further the quantitative control of the average mesopore size of [email protected] via the change of NPS (Figure 1k). Figure 2 | (a) N2 adsorption–desorption isotherms of [email protected], [email protected], [email protected], and [email protected], which are respectively offset by 250, 200, 90, and 50 cm3 g−1 for a clearer view. (b) Raman spectra of [email protected] with different carbonization temperatures. (c) N2 adsorption–desorption isotherms of [email protected] with different carbonization temperatures. (d) BJH pore size distribution curves of [email protected] (e) XPS spectra of [email protected] (f) Percentages of N species as calculated by XPS and CHN. BJH, Barrett–Joyner–Halenda; XPS, X-ray photoelectron spectroscopy. Download figure Download PowerPoint We acquired Raman spectra of [email protected] (Figure 2b), which fitted into three peaks ascribed to the D1 band (1350 cm−1), D3 band (1500 cm−1), and G band (1590 cm−1), corresponding to the defects in the graphitic lattice, the amorphous carbon, and the in-plane vibration of the sp2 carbon network, respectively.55 The G/D3 ratio of the band intensities (IG/ID3) could be used to reflect the degree of graphitization of carbon materials. The values obtained for the [email protected], [email protected], and [email protected] were 3.03, 3.61, and 6.76, respectively. Notably, the degree of graphitization of [email protected] increased with increasing carbonization temperature. Carbon, hydrogen, and nitrogen (CHN) elemental analysis were conducted using a CHN Analyzer 2400 (PerkinElmer, Jiangyan, China), to measure the compositions of the [email protected] nanostructures ( Supporting Information Table S1). As the carbonization temperature increased from 600 to 800 °C, the N content of [email protected] decreased from 22.4% to 14.4%. X-Ray photoelectron spectroscopy (XPS) was carried out to analyze further the elemental identifications of the [email protected] ( Supporting Information Figures S4a and S5). We investigated the types of N atoms in the [email protected] materials by fitting the N 1s into four typical peaks at 402.1, 401.0, 400.2, and 398.3 eV56,57 of the XPS spectrum (Figure 2e and Supporting Information Figure S4b), which to N, N, N-5 or N, and N-6 N, respectively. N-5 and N-6 were the dominant N for more than of N species in [email protected], which was much higher than in typical N-doped carbon materials. Moreover, as presented in Figure the of different of N atoms of the carbonized under varying were calculated based on the As the carbonization temperature increased from 600 to 800 °C, the content of N-6 decreased from to that of N-5 decreased from to and that of graphitic N increased from to and the abundant N sites, high percentages of N-5 and N-6 the and of [email protected] and thus, [email protected] be a for use in supercapacitors, oxygen reduction reactions and We to more light on a more of the and the formation by the multistage self-assembly strategy by the leading to the synthesis of 2D [email As presented in in the spherical micelles with PS cores and PEO coronas were formed by the self-assembly of PS131-b-PEO117 in the mixed The formation of the spherical micelles was confirmed by ( Supporting Information Figure the of which was ( Supporting Information Table DAP monomers were added and assembled with the PEO of PS131-b-PEO117 to form composite micelles via hydrogen to the of the spherical micelles formed by PS131-b-PEO117, that of the composite micelles was from to because of the of DAP on the after the addition of GO, the composite micelles self-assembled and anchored on the GO surface via between the functional groups of GO as and and the of DAP or the of the polymerization of DAP was by the addition of APS. The DAP monomers around the composite micelles and were into the of the micelles. As determined by the spectroscopy spectra of the the at 700 and to the PS block ( Supporting Information Figure which confirmed the formation of [email The of [email was which that PDAP was on both sides of the GO, and confirmed further the formation of a sandwich-like structure. after the of the spherical micelles of PS131-b-PEO117 via a mesoporous structure was formed in the 2D [email protected] We this mesoporous by preparing carbon without the addition of GO, and observed that this had only the of GO as the that in the assembly of PDAP and surfactant micelles ( Supporting Information Figure In the absence of PS-b-PEO in the the N2 adsorption isotherms in the obtained [email protected] exhibited a indicating the presence of only micropores in the and the absence of mesopores. This that PS-b-PEO as a in the formation of the mesopores (Figure and Supporting Information Figure increasing the PS chain length, the PS-b-PEO micelles resulting in a series of samples with mesopores. Moreover, [email protected] and were also obtained after the of GO with and respectively ( Supporting Information Figures and and thus, demonstrating the of the multistage molecular self-assembly 1 | of the for the of [email protected] Download figure Download PowerPoint Benefitting from their low large and excellent cyclic stability, carbon have for use in energy and storage. Moreover, 2D [email protected] are supercapacitor materials due to their abundant N sites with high percentages of N-6 and N-5, 2D structures, and controllable uniform mesopores. Therefore, we their supercapacitor by the 2D [email protected] as materials in a system with at a of to and with a oxide in energy and storage analysis 3 and Supporting Information Figure The area of the [email protected] curve was the among the products of different carbonization temperatures and 800 Figure and Supporting Information Figure and to the time at from to 10 A g−1 (Figure indicating the specific capacitance and the temperature of 700 °C. The curves of the [email protected] ( Supporting Information Figures and exhibited that were at high rates of up to which indicated an outstanding The of the [email protected] were to be almost with a change that from to 10 A g−1 (Figure and Supporting Information Figure thus, further an excellent and The specific capacitances of the [email protected] were calculated from the curves at different As presented in the Supporting Information Figure of all the [email protected] with similar N-5 and N-6 [email protected] had the mesopores, thus, exhibited the specific capacitance of up to g−1 at a of A to surface area, which, in provided the double-layer capacitance ( Supporting Information Figure a high of 10 A the specific capacitances of [email protected] and [email protected] were maintained than that of [email protected], as their mesopores mass Figure 3 | (a) of the and of [email protected] (b) curves of [email protected] at different (c) curves of [email protected], [email protected], and [email protected] at a of 5 (d) capacitances of [email protected], [email protected], and [email protected] at different (e) The surface-area-normalized capacitance of [email protected] at A g−1 in with those of other carbonaceous materials. electrochemical double-layer curves, curves, cyclic Download figure Download PowerPoint is an

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