A zinc ion hybrid capacitor based on sharpened pencil-like hierarchically porous carbon derived from metal–organic framework
Graphical abstract
Introduction
With the development of portable electronic equipment and electric vehicles, the ever-increasing requirement for both high energy and high power densities has promoted the search of new efficient energy storage technologies and devices [1], [2], [3]. Among of these, lithium-ion batteries (LIBs) are considered to be one of the most promising electrochemical energy storage devices due to their excellent energy density [4], [5], [6]. But the limited power output of LIBs is far behind the practical application demands [7], [8]. Supercapacitors, as another important energy storage device, are famous by its high power density and excellent cycle life span. However, the low energy density still restricts their widespread applications [9], [10]. By contrast, metal-ion hybrid capacitors, which integrate the high energy density of secondary rechargeable batteries and the high power output of supercapacitors, have sprung up and are considered as promising candidates for next-generation energy storage devices [11], [12], [13]. In recent years, lithium-ion hybrid capacitors (LIHCs) [14], [15], sodium ion hybrid capacitors (SIHCs) [16], [13] and potassium ion hybrid capacitors (PIHCs) [11], [17] have been extensively studied and significant advances have been received. However, most metal ion hybrid capacitors are assembled in organic electrolyte, that can cause the environmental impact and safety issues. In terms of these concerns, aqueous energy storage technologies and devices are the promising candidates due to the better safety, high ionic conductivity, lower toxicity and low cost contributed by aqueous electrolyte. Among various systems in aqueous electrolyte, zinc-based energy storage technology has been received extensive attentions due to the distinctive features of metallic zinc [18], [19], [20]. Metallic zinc has high theoretical capacity (820 mA h g−1), relatively low redox potential (-0.76 V vs. the standard hydrogen electrode), low price, non-toxicity, and good compatibility in aqueous [21]. Zinc ion hybrid capacitors (ZIHCs), featuring both the high energy density and the high power output due to integrating the battery-type Zn anode and capacitive-type carbon cathode in one device, are one of the most promising energy storage device. Recently, increasing research efforts have been devoted to developing the electrode materials of ZIHCs [22]. For example, Li et al. [23] used porous carbon derived from pencil sharpening as the electrode material to construct a ZIHC device with high energy density of 140.7 W h kg−1 (136.1 W kg−1). Yin et al. [24] invented a ZIHC with excellent performance based on an oxygen-rich porous carbon cathode, which could provide a maximum energy density of 104.8 W h kg−1 (58 W kg−1). Due to the limited charge storage capacitance of carbon electrode based on physical adsorption/desorption energy storage mechanism, however, the energy density of ZIHCs still has a room to improve. It can be found from these research that in order to improve the performance of ZIHC, an effective method is to develop carbon materials with stable structure, high specific surface area, suitable pore size distribution and heteroatom doping.
Metal-organic frameworks (MOFs) are a new class of porous materials with periodic network structure formed by self-assembly of multidentate organic ligands and metal ions. Due to its unique pore structure, large specific surface area and structural controllability, MOFs and MOFs-derived materials have been widely used in electrochemical energy storage [25], [26]. Specifically, MOF-derived nanoporous carbon materials largely retain the morphological characteristics of the precursor, have a unique hierarchical porous structure, could provide more active sites for ion adsorption, and shorten the diffusion path of electrolyte ions, showing excellent electrochemical performance [27]. Therefore, the application of MOF-derived nanoporous carbon materials for construction of ZIHCs should have a great potential.
Herein, a new type of sharpened pencil-like nanoporous carbon (MPC) is achieved by using metal–organic framework (MIL-47) as precursor combing the chemical activation method (Fig. 1). Using this MPC as cathode material, a high performance ZIHC device is assembled benefiting from the merits of its high specific surface area, hierarchically porous structure and rich oxygen-containing functional groups. As expected, the assembled ZIHC device could deliver an impressive specific capacitance of 289.2 F g−1 at a current density of 0.2 A g−1 and exhibit excellent cycle stability with over 96.7% capacitance retention after 10 000 cycles at a current density of 10 A g−1. Impressively, as-built ZIHC could achieve a high energy density up to 130.1 W h kg−1 at a power output of 180.3 W kg−1. At a maximum power density of 7.8 kW kg−1, the high energy density of 56 W h kg−1 is still delivered, suggesting the application potential of as-assembled ZIHC based on the MOF-derived porous carbon material.
Section snippets
Preparation of MIL-47 (V)
The precursor of MIL-47 (V) was prepared by a modified protocol according to the previously reported method [28]. The typical procedure as follows: 0.32 g terephthalic acid (H2BDC) and 0.43 g VOSO4 were added into 25 mL N, N-dimethylformamide (DMF). After stirred for 30 min, the mixture was transferred to a Teflon-lined stainless-steel autoclave and heated in an oven at 160 °C for 48 h. After cooling down to room temperature, the yellow powder was gathered via centrifugation and washed with
Morphology and structural analysis
The precursor of MIL-47 (V) was successfully synthesized, that demonstrated by the XRD patterns (Fig. S1a). The MIL-47 (V) presented a rod-like morphology similar to a sharpened pencil (Fig. S1b). After pyrolysis/etching and activation, the obtained samples inherited the sharpened pencil-like rod morphology of the MIL-47 (V). However, the roughness and the porosity were different with each other for three samples. Obviously, the rod-like structure became more and more porosity with the
Conclusions
In summary, an aqueous ZIHC with excellent performance was built by utilizing a hierarchically porous carbon derived from MOF. Benefiting from the merits including the high specific surface area, hierarchically porous structure and rich oxygen-containing functional groups, the advanced MPC-2 has sufficient space for ion/charge storage with fast ion/electron transportation rate. Consequently, MPC-2 not only exhibited excellent electrochemical performance in KOH aqueous electrolyte, but also
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 work was supported by the National Nature Science Foundations of China (Grants No: 22065021, 21867015), the Province Nature Science Foundations of Gansu (Grants No: 20JR5RA453) and Hongliu Outstanding Youth Teacher Cultivate project of Lanzhou University of Technology.
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