Hierarchical porous nitrogen-doped carbon microspheres after thermal rearrangement as high performance electrode materials for supercapacitors
Graphical abstract
Hierarchical porous nitrogen-doped carbon materials were obtained by solvothermal method to prepare hierarchical hydroxyl-containing polyimides(6FAP-PMDA) microspheres, followed by thermal rearrangement at 450 °C and further carbonization, which showed good electrochemical characteristics for supercapacitors.
Introduction
In recent decades, the growing demand for high-performance mobile equipment and renewable resources has promoted the continuous development of energy storage equipments[1], [2], [3], [4], [5], [6]. Supercapacitors, also known as electrochemical capacitors, have the characteristics of high power density, long cycle life, fast charge and discharge, and wide operating temperature. Therefore, they have broad application prospects in the field of energy storage [7], [8], [9], [10], [11], [12]. Supercapacitors can be divided into two types. One type is an electric double layer capacitor (EDLC), in which pure electrostatic charge is accumulated at the electrode/electrolyte interface for adsorption to generate stored energy. In addition, the specific capacitance of EDLC largely depends on the surface area and pore size of the electrode material that can be transferred to the electrolyte ions[13], [14], [15]. The other type is Faraday quasi-capacitor, which mainly generates Faraday quasi-capacitor through reversible oxidation–reduction reaction on and near the surface of Faraday quasi-capacitor active electrode materials (such as transition metal oxide and polymer), thus realizing energy storage and conversion[17], [18], [19], [20]. Therefore, the electrochemical performance of supercapacitors depends to a large extent on the electrode materials.
At present, the electrode materials of supercapacitors mainly include: carbon materials, metal oxides and conductive polymers[15], [16], [17], [18], [19], [20], [21]. Among them, carbon materials not only have good electrical conductivity, high specific surface area, but also have the advantages of low cost and environmental protection, so they have been widely studied and used[22], [23], [24], [25]. Especially, three-dimensional (3D) carbon materials with a highly continuous network skeleton, porous structure and excellent structure stability, can not only provide efficient ion transfer and ion transmission pathways quickly, but also enhance accessible active centers[26], [27], [28], [29], [30]. Further more, nitrogen doping is another effective strategy for improving the electrochemical performance of carbon materials because nitrogen-doped porous carbon improved surface wettability, electronic conductivity and provided more active sites [31], [32], [33], [34], [35].
Polyimide (PI) is a kind of nitrogen-containing polymer, which has been used as a precursor to prepare nitrogen doped carbon materials[36], [37], [38]. Its advantages including: 1) structural designability; 2) high thermal stability and residual carbon content; 3) easy carbonization and graphitization; 4) abundant heteroatoms. Some literatures have reported that polyimides can be prepared by solvothermal method, which exhibit 3D structure, abundant pore and good electrochemical properties[39], [40], [41], [42], [43], [44]. Qinglan Zhao et al[39] reported that pyromellitic dianhydride (PMDA)-phenylenediamine (PPD) based polyimides with different crystallinity and morphology were synthesized by simple one-step hydrothermal method. The microflower-like polyimides with controllable hierarchical mesoporous structure were constructed from nanosheets, which experienced a two-step enolisation reaction with reversible insertion of two sodium ions during the redox electrochemical reaction. Additionally, the polyimide (PMDA-PPD)-derived carbons were prepared by further thermally treated at 650 °C and KOH activation, which exhibited high specific surface area and retained the original microflower-like nanosheet morphology of the polyimide precursor[40]. Qiong Wu et al[41] used PMDA and ethylenediamine (EDA) as monomers to prepare nitrogen-doped 3D flower-like carbon materials by hydrothermal method and subsequent thermal treatment. The polyimides as anode materials for LIBs exhibited good electrochemical performance, including a high reversible capacity, excellent rate performance and good cycling stability, which were attributed to the synergistic effect between 3D flower-like nanostructure and high nitrogen content. Zhixiao Xu et al[42] reported that 3D flower-like polyimides with different morphology and properties were obtained by changing the structure of monomer, solvent and concentration of polymer solution. After subsequent thermal treatment, nitrogen-doped carbon materials with a flower-like hierarchical architecture exhibited outstanding catalytic activity toward ORR and excellent capacitive and cycling performance as the electrode materials in supercapacitor. Thus, it can be seen that by carefully selecting monomers and varying the polymerization conditions, the assembly of polyimides will be easily regulated, yielding various hierarchical superstructures, including flower-like and lantern-shaped spheres.
Recently, thermally rearranged (TR) polymers derived from hydroxyl-containing polyimides (HPIs) have been reported to have abundant micropore structures and used as gas separation membranes[45]. After the thermal treatment above Tg, the HPIs undergo rearrangement reaction to transform into polybenzoxazole (PBO), and release small molecule gas CO2 in the solid state[46], [47]. However, the TR process is strongly affected by chemical structure of precursor, Tg, preparation method, film thickness, and heat treatment conditions[48], [49], [50], [51], [52]. Hence, the microporous TR polymers are expected to play an important role in gas separation, catalysis, electrode materials and other fields.
Herein, we firstly proposed a kind of 3D hierarchical porous carbon materials, which was prepared by solvothermal method to obtain 3D hierarchical HPI microspheres, followed by step by step thermal treatment. During the thermal treatment, HPIs were converted into TR polymers with hierarchical microsphere structure, and then the hierarchical porous nitrogen-doped carbon materials were obtained by further carbonization. However, our experimental results showed that most of the rearrangeable HPIs can not form the hierarchical microspheres by solvothermal method due to their irregular macromolecular structure containing –OH and C(CF3)2, and the solvent also affect the preparation of 3D HPIs because of its excellent solubility. Finally, 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) and pyromellitic anhydride (PMDA) were selected as monomers in m-cresol solvent to synthesize poly(amic acid) (PAA), which was used to perform the self-assembly process under high temperature and high pressure to prepare 3D HPIs. The effects of the concentration of polymer solution, time of thermal rearrangement and final carbonization temperature on the physical and electrochemical properties were investigated in detail. The method involving thermal rearrangement provides a new insight into the development and production of hierarchical porous nitrogen-doped carbon materials as electrode materials.
Section snippets
Preparation of hierarchical porous nitrogen-doped carbon microspheres
3D hierarchical HPI microspheres were prepared by solvothermal method. The PAA concentrations were designed as 30, 45, and 60 mg/ml, respectively. When the concentration of PAA was over 70 mg/ml, there was no solid product, which probably caused by the high viscosity of the solution. Taking 45 mg/ml as example, 3.85 mmol 2,2-bis (3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) was firstly dissolved in 50 mL m-cresol followed by adding equimolar pyromellitic anhydride (PMDA) under stirring at
Results and discussion
The FTIR spectra of five samples are shown in Fig. 1. HPI-45 from PAA with 45 mg/ml was prepared at 200 °C by solvothermal method, and all characteristic peaks of the imide group were observed. The asymmetry stretching vibration, symmetrical stretching vibration and bending vibration of the C = O in imide group were assigned at 1780 cm−1, 1720 cm−1, and 720 cm−1, respectively. The vibration absorption peak of benzene ring was near 1500 cm−1, and the peak at 1380 cm−1 was attributed to the
Conclusions
3D structure carbon materials were prepared by solvothermal method using 6FAP and PMDA to prepare 3D hydroxyl-containing polyimides (HPIs), followed by thermal rearrangement at 450 °C and further carbonization. When the concentrations of PAA were 45 and 60 mg/ml, HPIs with 3D hierarchical microshperes structure assembled with nanosheets were prepared. After thermal treatment, HPIs underwent thermal rearrangement and carbonization, obtaining 3D porous carbon materials. The rearrangement time of
CRediT authorship contribution statement
Ying Liang: Investigation, Software, Writing - original draft. Yunhua Lu: Conceptualization, Methodology, Writing - review & editing, Investigation. Guoyong Xiao: Software, Data curation, Methodology. Jianhua Zhang: Supervision, Data curation. Haijun Chi: Validation, Supervision. Yan Dong: Validation, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially supported by Natural Science Foundation of Liaoning Province (No. 20180550439), the National Natural Science Foundation of China (No. 21878033) and the Talent Project of Liaoning University of Science and Technology (No. 601011507-17).
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