A versatile Co-Activation strategy towards porous carbon nanosheets for high performance ionic liquid based supercapacitor applications
Graphical abstract
Introduction
Supercapacitor, a new class of renewable electrochemical energy storage device with high specific power density, superior rate capability and long cycle life, has attracted tremendous attention recently on account of its widespread popularity, especially in the areas of hybrid electric vehicles and portable electronic apparatus [[1], [2], [3]]. Currently, considerable research efforts have been devoted to explore various carbonaceous materials that can provide high energy density without sacrificing its power density due to the fact that carbon materials display extraordinary features, such as large specific surface area, moderate electrical conductivity, relatively low cost, and extensively availability [[4], [5], [6]]. Hitherto, activated carbons are the most commonly recognized electrode materials for commercial supercapacitors since its chemically inert nature, low cost and excellent cyclic stability [[7], [8], [9]]. The long and tortuous channels, however, suffered from ions kinetic problems related to the increased resistance for electrolyte ions penetrating into the inner pores, particularly at large current densities [10,11]. To circumvent this limitation, intense efforts have been focused on employing various nanocarbons, such as carbon nanofibers [12], nanotubes [13], nanosheets [[14], [15], [16], [17], [18]], nanomesh [19,20], mesoporous spheres [21], nanocages, etc., [22]. Among them, 2D sheet-like structures, especially 2D porous carbon nanosheets (PCNS), have emerged as one of the most promising functional materials for next-generation electrochemical energy storage and conversion systems owing to the fact that they can offer more exposed active sites than bulk electrodes and afford more effective surface area compared to graphene [23]. The unique structural features fulfil multiple functionalities by substantially shorten the ion-diffusion distance and thus significant improvements in power densities without sacrificing its energy densities. Until now, several routes, such as chemical vapor deposition, hard or soft templating, chemical or physical exfoliation, and self-assembly techniques have been previously developed to prepare PCNS with sizes of nano-to micrometers, and the potential application of these PCNS in high performance supercapacitors have also been extensively studied [14]. Nevertheless, these methods suffer from more or less-severe drawbacks, including the requirement of expensive carbon sources, elaborate procedures and special designed precursors. Therefore, the preparation of PCNS by a facile method still remains s highly sophisticated challenge.
To date, significant improvements in power density in various aqueous based electrolytes (e.g., KOH, H2SO4, Na2SO4) have been achieved, however, supercapacitors with no-aqueous electrolyte (e.g. organic electrolyte and ionic liquid) usually display drastically reduced power density together with large IR drop. Actually, organic electrolyte is dominating for commercial application due to their wide stable working voltage that can enhance the energy density substantially according to equation E = 1/2CV2. Benefiting from the higher potential window, ionic liquids (ILs) are good candidates as the electrolyte for high-energy supercapacitor applications [24,25]. Nonetheless, the large ionic size of ILs possesses intrinsically low ionic conductivity and high viscosity, which usually prevent from fast-ion transport in the inner pores, and therefore the unsatisfactory rate performance. For instance, the specific capacitance retention is generally less than 50% as the current density moderately increased from 1 to 20 A/g whereas this value is usually larger than 70% in aqueous electrolytes, and the specific capacitance retention approximated to 70% at 50 A/g are rarely reported in ILs based supercapacitors (Table S1). Thus, the large-energy density and outstanding rate performance in ILs, which are essentially required for their applications in the next-generation electrochemical energy electrochemical energy storage systems, still remains a huge challenge.
As is known, a favorable strategy for designing high performance ILs based supercapacitor is to explore novel functional nanocarbons that have well-built microporous channels for charge storage and short diffusion distances for fast ion transport, and meanwhile, the partially graphitized shells to guarantee rapid electron transfer. In the present work, we describe a versatile yet efficient synthesis strategy to prepare PCNS with distinct crumpled structures without additional soft/hard templates. The thickness of the as-obtained PCNS is less than 30 nm and the lateral size is about several micrometers. In this methodology, a kind of abundant petrochemical byproduct of petroleum asphalt is used as the carbon sources, and numerous pores are generated after chemical activation. In particular, the introducing of NaOH as the co-activating reagent is very crucial to achieve PCNS structures, and the morphology and textural properties of the products are highly dependent on the mass ratio between KOH and NaOH. The as-synthesized PCNS can be used as electrodes for ILs based high energy density supercapacitors with excellent rate capability. Our work clearly demonstrates that co-activation process is a promising way to direct transformation low-cost petroleum asphalt into PCNS for applications in electrochemical energy storage, which do not require additional templates and pre-synthesized precursors. We believed that the strategy demonstrated here opens up the possibility of synthesizing value-added nanocarbons from sustainable and cost-effective precursor for next-generation high energy & power density supercapacitors.
Section snippets
Experiment section
Petroleum asphalt was provided by Liaoning Mingqiang Chemical Co., Ltd. (China). The others chemicals used were of analytical grade and purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China).
Results and discussion
The schematic of the preparation of PCNS from alkali and petroleum asphalt is shown in Fig. 1. Through a combination of KOH and NaOH activation, a series of products are obtained by changing the mass ratio between KOH and NaOH. It is well known that the chemical reactions between KOH and carbon can generate highly microporous with large surface area [26,27]. On the contrary, NaOH activation usually gives much broader pore size distribution but relatively low surface area in comparation with KOH
Conclusions
To sum up, we have employed petroleum asphalt, which is low cost with high carbon yield, as the precursor for the fabrication of PCNS with excellent electrochemical capacitive performance in IL electrolyte through a versatile co-activation approach. We found the morphology and textural structural of the products are highly dependent on the mass ratio between KOH and NaOH. PCNS-2 has a high specific surface area (2053 m2/g), a unique crumpled structure with abundant micropores (0.6–0.8 nm),
Acknowledgements
The authors are grateful to the financial supports from the Key Scientific Research Projects at North Minzu University (Grant no. 2017KJ06), National Natural Science Foundation of China (Grant no. 51762001), CAS “Light of West China” Program (Grant no. XAB2017AW07) and Natural Science Foundation of Ningxia (Grant no. 2018AAC03108).
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