Elsevier

Carbon

Volume 161, May 2020, Pages 25-35
Carbon

Nitrogen-rich porous carbon in ultra-high yield derived from activation of biomass waste by a novel eutectic salt for high performance Li-ion capacitors

https://doi.org/10.1016/j.carbon.2020.01.045Get rights and content

Abstract

High performance N-doping porous carbon (NDPC) are ideal electrode materials for Li-ion capacitors (LICs). However, the practical application of NDPC is extremely limited, which is mainly attributed to the typical methods for the scale preparation NDPC are time-consuming, high cost and low yield. Herein, we have developed a new route for high efficient, environment friendly, low cost and high yield fabrication of NDPC, using biomass waste as the carbon source and a novel eutectic salt as the activation agent. After a series of comparative experiments, the application of eutectic salt herein not only reduce the process time and cost, but also obviously enhance the yield, the specific surface area (SSA) and nitrogen-doping content of NDPC sample (labelled as NDPC-0.5). In combination of the rich N-doping level, larger SSA and interconnected porous structure, the NDPC-0.5 sample exhibit an excellent electrochemical performance as both cathode and anode materials for a LIC, with specific discharge capacities of ∼60 and 290 mAh g−1 at a current density of 5 A g−1. The resultant NDPC-0.5//NDPC-0.5 LIC device delivers a high energy density of 116.9 Wh kg−1 at 500 W kg−1, with a capacity retention of 81% after 8000 cycles at 2 A g−1.

Introduction

Due to the expeditious development of potable electronics, electric vehicles, and uninterrupted power supplies, the requirement of high-energy and high-power storage system increasing daily [1]. Lithium-ion batteries (LIBs) and supercapacitors (SCs), which are extensively applied in these field at present, cannot simultaneously meet the high energy and power densities, because LIBs usually possess high energy density but poor power density, and the SCs generally achieve high power density but suffer from low energy density [2,3]. Under these cases, it is very important to open up advanced energy storage apparatuses that combine the complementary characteristics of both LIBs and SCs. Lithium ion capacitors (LICs), originated from the hybrid device of LIBs and SCs, not only provide excellent energy and power density, but also achieve long cycling life [4]. As a result, they have been considered to fill up the gap between the LIBs and ECs and become one of the most promising candidates for meeting the growing demand in energy storage field and also received much more attention in the very recent years [4,5]. In order to improve the electrochemical performance of LICs, many materials possessing unique composition and/or novel structure, such as insertion-type materials (Li4Ti5O12 [6,7], TiO2 [8,9], Nb2O5 [10], TiNb2O7 [11], Li3VO4 [12], and graphite [13,14]) and conversion-type or alloying reaction materials (such as Fe3O4 [15,16], MnFe2O4 [17], MnO [18], Sn/C [19], and Si/C [20]) for anode materials, as well as active carbon (AC) [6,8,10,14,16,21], carbon nanotube (CNT) [19,22], graphene [23,24] and heteroatom-doped porous carbons (HDPCs) [[25], [26], [27], [28], [29]] as cathode materials have been widely explored and successfully utilized for LICs during the past two decades or so. Although great achievements have been achieved in developing high performance LICs, the mismatch of the rate performance between the negative electrode and positive electrode make the full use of the high specific capacity electrode very difficult, resulting in high energy density LICs generally be achieved by sacrificing their power density [30]. As a result, it is still a tremendous challenge to search for idea anode materials with superior rate capability and the cathode materials with higher specific capacities towards constructing high performance LICs [1,30].

In very recent years, LICs with two electrodes using the same materials have just began to attain special interest. Among them, porous carbonaceous materials with novel structure and unique composition are most commonly investigated as both positive and negative materials for the manufacture of high performance LICs, because of their tunable porosity, stable electrochemical property, and rich reserves in nature [[31], [32], [33], [34], [35], [36]]. Among these carbonaceous materials, heteroatom-doped porous carbons (HDPCs), especially for nitrogen-doping porous carbons (NDPCs), with a lot of attractive features (high electronic conductivity, adjustable the interlayer distance, improved wettability of electrolyte and enhanced capacitance performance induced by the rapidly pseudocapacitive reaction and/or rich active sites), exhibited a great potential applications for high performance LICs [[32], [33], [34], [35], [36]]. Thus, the structural design and scalable preparation of NDPCs from a green and low cost route are imperative and necessary.

Biomass and biomass wastes, as cheap and sustainable precursors for the NPCs have been exploited for energy storage apparatuses (such as SCs, LIBs and LICs) in many researchers [[37], [38], [39], [40]]. There are generally two routes for the preparation of NDPCs using the biomass and/or biomass wastes: the first is in-situ direct pyrolysis of the precursor with nitrogen sources and activation agents (such as CaCl2 [41], NaOH [42], KOH [43], K2CO3 [44], and ZnCl2 [41,45]) by one-step, and the other is two-step synthesis method using the pyrolysis procedure firstly to achieve intermediate product and activation of the as-obtained intermediate product secondly [[46], [47], [48]]. For the first route, it is difficult to uniformly disperse the activation agent and nitrogen source in the mixed precursors and also usually cause some highly toxic substances (KOCN and/or KCN) during the activation process [37,44]. For the second route, it is very time-consuming and usually generate NDPCs with low nitrogen level. Meanwhile, low yield (generally less than 25 wt.% based on the biomass) is achieved and tedious washing is required to obtain pure product in both synthesis routes. Although substantial advancement in the preparation of NDPC-based electrode materials, obvious disadvantages such as high energy consumption, tedious procedures and toxic characteristics still hinder their large-scale application in “sustainable and low-cost” energy storages. Thus, a novel route for efficient synthesis of NDPCs for LICs employing eco-friendly and naturally plentiful biomass and/or biomass wastes is worth encouraging.

Herein, we reported a novel route for the preparation of NDPC in high yield (>40 wt% based on the mass of bagasse), making use of sugar cane bagasse as the carbon precursor and a new eutectic salt as both the activation agent and nitrogen sources. We choose bagasse as the carbon precursor herein because of its abundance, sustainability and low cost, which has also been successfully applied as the carbon precursor for the fabrication of porous carbon-based materials for energy storage devices [41,43,49]. The new eutectic salt, directly prepared via the mixing of ZnCl2, urea and KCl in a mass ratio of 3:2:0.5 at 110 °C, is also environment friendly and low cost. The as-prepared NDPC sample (labelled as NDPC-0.5) with high specific surface area (SSA, 1505.9 m2 g−1), moderate total pore volume (Vt = 0.88 cm3 g−1), rich nitrogen-doping content (up to 9.5 at.%) and hierarchical porous structure shows obviously enhanced electrochemical performance as both positive and negative materials for LICs. Based on these unique features, thus, a LIC (NDPC-0.5//NDPC-0.5) assembled by the NPDC-0.5-based cathode and anode exhibits superior electrochemical properties, delivering a high energy density of 87.7 Wh kg−1 at a high power density of 10,000 W kg−1.

Section snippets

Preparation of nitrogen-doping porous carbons (NDPC)

In a representative experiment, ZnCl2 (6.0 g), urea (4.0 g) and KCl (1.0 g) were firstly mixed in a bake and then heated at 110 °C for 1 h to generate a deep eutectic salt (DES), after that, sugar cane bagasse (2.0 g) was immersed into the DES liquid and stirred for another 30 min, then the mixture was transferred into a tube furnace, heated up to 800 °C under N2 atmosphere with a ramp rate of 5 °C min−1 and maintained this temperature for additional 2.0 h to carbonize and activate the mixed

Synthesis process

Previous investigation suggested that different mass ratios of ZnCl2 and urea can form deep eutectic solvents (salts) at low temperature [51,52], which allows them possible to act as solvents for uniformly mixing the raw materials firstly and then act as activators for generating porous structure during the preparation of NPCs materials. Furthermore, it can act as an efficient nitrogen source for the preparation of NPCs. After a series of comparative experiments via the change of the chemical

Conclusions

In summary, we have developed a cheap and high efficient route for the synthesis high performance NDPCs in high yields with rich N-doping level and adjustable SSAs and pours structure, using novel DESs as activation agents. By optimized the chemical compositions of the DESs, the as-prepared NDPC-0.5 material, with rich N-doping level, high SSA, moderate Vt and hierarchically porous structure, exhibits high specific capacity, superior rate performance and excellent cycle capability as both

CRediT authorship contribution statement

Kaixiang Zou: Conceptualization, Formal analysis, Investigation, Writing - original draft. Zixing Guan: Resources, Validation. Yuanfu Deng: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Guohua Chen: Supervision, Project administration.

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in. In word, the manuscript entitled “Nitrogen-rich porous carbon in ultra-high yield derived from activation of biomass waste by a novel eutectic salt for high performance Li-ion

Acknowledgements

This work was supported by the National Natural Science Foundation of China, China (Grant No. 21875071), the National Natural Science Foundation of China-Hong Kong Research Grant Council (NSFC-RGC) Joint Research Scheme (Grant No. 21661162002 and N_HKUST601/16).

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