Investigation of the microstructure on the nanoporous carbon based capacitive performance
Graphical abstract
Introduction
Supercapacitors have attracted much attention recently as they can bridge the gap between batteries and conventional capacitors due to their higher power density and longer cycle life [[1], [2], [3]]. Therefore, the development of high-performance and cost-effective supercapacitor electrode materials has been a hot topic in recent years. In the previous literature, different types of metal-organic framework (MOF) have been used to prepare porous carbon materials for supercapacitor applications because of their high porosity, large surface area, tunable morphologies and chemical tunability [[4], [5], [6]]. The MOF-derived porous carbon materials are particularly appealing for supercapacitors in virtue of several unique structural merits: firstly, the highly porous structure of NPC material provides an ideal substrate with a large specific surface area for sufficient electrolyte ion contact and storage to improve the electrochemical performance [7,8]. Although MOF derived porous carbon has presented outstanding performance, the size of the MOF-derived NPC is a key parameter to determine electric capacitor performance [9,10]. In large-sized NPC materials, more of the pores can be used for electrolyte ion storage, the well-conjugated structure can also increase the conductivity [11]. However, the inner pore structure may not be fully accessible during the limited charge/discharge time period, and the pathway for ion diffusion will be extended due to the large particle size, which will lower the usage of the specific surface area and decrease the supercapacitance. On the other hand, nano-sized nanoporous carbons have the typical advantages/disadvantages when compared to large-sized materials. The physical stacking of nanoparticles is not mechanically stable and will further increase the resistance due to poor and decreasing ohmic contact. So the combination of the advantages of both large and small sized nanoporous carbon will significantly increase the electrochemical performance of a NPC-derived supercapacitor device. Furthermore, other properties of MOF-derived NPC material can influence the supercapacitor behavior, for example heteroatom doping, degree of graphitization and the microporous and ion transfer tunnel structure inside the NPC materials. Thus it is indispensable to have an appropriate pore structure for an ideal supercapacitor electrode material.
Since the EDLC capacitance arises from the electric charges accumulated at the electrode interface, the storage, diffusion and ion transfer effect of electrolyte in the porous structure of NPC material is critically important [12]. According to previous research, a noticeable volume of micropores (<1 nm) can provide abundant adsorbing sites for the electrolyte ions, thus the fraction of micropores in the porous material is the cornerstone of supercapacitance [[13], [14], [15]]. Meanwhile, the ordered mesopore (2–50 nm) channels facilitate the electrolyte ion diffusion in the material, shorten the pathway of ion transfer distance during the charging-discharging process. A well-balanced micro-/mesopore ratio will significantly enhance the supercapacitor performance, especially the rate performance [[16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]. However, how to design this hierarchical porous carbon material to achieve high performance supercapacitor still remains a great challenge.
In this contribution, an interconnected nanoporous carbon material (NPC-950) with a proper ratio of micro-/mesopores was obtained by carbonization of nanosized Al-MOF. As control experiments, two disconnected nanoporous carbon materials, NPC-850 and NPC-850(950) were also selectively prepared under the same conditions. In the electrochemical results, the interconnected NPC-950 exhibits a much higher capacitive performance in both acidic and organic electrolyte solutions, which is much better than that of NPC-850 and NPC-850(950). Furthermore, varieties of organic electrolytes with different ion sizes were used to investigate the relationship between the pore size, ion size and the supercapacitance. It is found that the electrolyte BMIMBF4 with the most suitable ion size is the best choice for the prepared materials to achieve high supercapacitor performance.
Section snippets
Experimental section
Materials. 1,4-dicarboxybenzene (BDC), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), N,N-Dimethylformamide (DMF), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), lithium perchlorate (LiClO4), tetraethylammonium tetrafluoroborate (TEABF4), acetonitrile (AN), polycarbonate (PC), hydrochloric acid (HCl) were purchased from Sigma-Aldrich Chemical Co. All chemicals were directly used without further purification.
Results and discussion
The preparation processes of nano-sized Al-MOF and corresponding nanoporous carbon materials are illustrated in Fig. 1. Al(NO3)3•9H2O provides Al3+ ions and BDC provides the carboxylic groups for building the MOF nanocrystals [[31], [32], [33]]. The size of the as-prepared Al-MOF crystals is in the range of 20–50 nm (Fig. 2a), which is much smaller than reported for MOF-5, ZIF-67 or MIL-53 MOFs [[34], [35], [36]]. The nano-sized MOF crystals can be used for preparing the nano-sized porous
Conclusions
A interconnect structured well-designed porous carbon material was prepared by carbonization of nanosized Al-MOF through controlling the carbonization temperature, which produces co-existing micro- and mesopore structure within NPC framework. With the bimodal pore structure as well as higher electronic conductivity in the carbon framework, NPC-950 exhibited the highest capacitance values of 298 F g−1 and 233 F g−1 in the acidic and organic electrolyte, which was much higher than for NPC samples
CRediT authorship contribution statement
Shuai Zhang: Writing - original draft, parts of experimental work. Maciej Galiński: Resources, parts of experimental work. Xiaoguang Liu: Resources, parts of experimental work. Krzysztof Sielicki: Resources, parts of experimental work. Xuecheng Chen: Supervision. Paul K. Chu: Writing - review & editing. Rudolf Holze: Writing - review & editing. Ewa Mijowska: Conceptualization.
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.
Acknowledgments
The authors acknowledge the financial support from the National Science Centre, Poland, within BEETHOVEN-2016/23/G/ST5/04200.
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