Eco-friendly and thermally stable cellulose film prepared by phase inversion as supercapacitor separator
Introduction
In recent years, the rapid development of energy-driven mobile devices such as smart grids, mobile phones and electric vehicles has driven the development of lithium-ion batteries, fuel cells and supercapacitors, which have great potential in transportation electrification and large-scale energy storage [1]. Electrochemical energy storage technology has been recognized as a very promising way to solve fuel constraints and environmental issues to achieve a clean and sustainable development [2,3]. Compared to other types of electrochemical energy storage devices (such as traditional flat capacitors and batteries), supercapacitors exhibit excellent performance in many aspects, such as higher energy density and power density, strong durability and cycling test stability [[4], [5], [6]]. The assembly form of the important functional parts of the supercapacitor is similar to the structure of the battery [7]. It is mainly composed of electrodes, electrolyte and separator. Electrode as a component is a very important factor affecting the performance of supercapacitors [8,9]. Currently, a large amount of work is focused on the development of various electrode materials (polypyrrole, polyaniline, polythiophene, graphene, Mn-doped ZrO2 nanoparticles, Ge/TiO2 nanocomposite, Co-doped ZrO2 nanoparticles) [[10], [11], [12], [13], [14], [15], [16], [17]] and electrolytes (water electrolyte, organic electrolyte, ionic electrolyte) [[18], [19], [20]] to meet the specific requirements of different applications. Furthermore, the separator is mainly used to prevent short-circuiting of between the two electrodes due to physical contact. It is required to have a thin thickness and high porosity, electrolyte adsorption and retention, strength and thermal stability to meet practical applications. However, the development of separators as separate components from electrodes and electrolytes has received little attention.
A good separator should have good chemical stability, high tensile strength to meet the assembly requirements, high porosity and uniform pore size distribution to avoid local overheating, good electrolyte wettability and high electrolyte uptake to improve the ionic conductivity [21,22]. At present, the research on separators is mostly based on lithium-ion batteries (polyethylene, polypropylene, and their combination) [23,24] and fuel cells (the glass fiber-based membranes, composite films, and organic polymer-based nonwoven) [25,26]. There are fewer separators have been developed specifically for supercapacitors. Moreover, these separators have the disadvantages of low hydrophilixity on the surface, low porosity, and non-uniformity of pore sizes, which directly leads to low electrolyte absorption, liquid retention and low ionic conductivity. Therefore, it is imperative to develop suitable separators to take advantage of the characteristics of supercapacitors.
Cellulose, one of the most abundant natural polysaccharides in nature, is widely present in nature in different forms, such as wood, bamboo, cotton, crop straw, spider-based nanofibers, sea-silk based nanofibers and silkworm-based silk fibers [[27], [28], [29]]. As a renewable, degradable, non-toxic, biocompatible, chemically modified and low cost biomass, cellulose and its derivatives [30,31] are widely used in paper, textile, functional materials, health care and other fields [32,33]. The inherent properties of cellulose itself, such as low cost, excellent wettability, size/thermals/chemical stability and good mechanical property, can satisfy some of the key properties of an ideal supercapacitor separator. Some cellulose-based films (especially cellulose acetate, methyl cellulose, bacterial cellulose and cellulose-based composite) have been employed as separator materials in energy storage aspect for some time [[34], [35], [36], [37]]. However, their low porosity leads to low electrolyte uptake and liquid retention, which directly impair ionic transport through the separator, resulting in low ionic conductivity. The high porosity of the separator predicts the flowability of the ions between the positive and negative electrodes, which can increase the absorption and liquid retention of the electrolyte, thereby accelerating the flow of ions, avoiding the generation of overheating and waste of energy.
In recent years, the preparation of porous cellulose films has received extensive attention, including electrospinning (modified cellulose acetate film, MCA) [38], paper-making like process (mesoporous Cladophora cellulose film) [39], forcespinning (forcespinning cellulose acetate film, FS-CA) [40], and phase-inversion [41]. And more, a large amount of research work is currently focused on adjusting the porosity of the cellulose-based separator, the amount of electrolyte absorbed and the like by adding other components, such as Polydopamine/cellulose/polyacrylamide [42], cellulose acetate-titania separators [43], nanocomposite poly(vinylidene fluoride)/nanocrystalline cellulose seperators [44], aluminum oxide/poly vinylidenefluoride-hexafluoro propylene-carboxymethyl cellulose/polyethylene composite separators [45], polyvinylidene fluoride/polymethyl methacrylate/cellulose acetate separators [46]. Most of the researches have focused on the preparation of porous films based on cellulose derivatives and bacterial cellulose as a matrix, or blending cellulose with other components. The pore structure and other properties of these films need to be further improved. In addition to phase inversion and papermaking, other preparation methods often have complex procedures and poor control conditions, such as different requirements for temperature and extrusion rate in different steps of the extrusion molding process used to make polymer films. In addition, electrospinning has the disadvantage of high cost and safety issues due to high voltages. Phase-inversion can be regarded as a simple and feasible excellent method for preparing the cellulose film with higher porosity. The entire process of phase inversion is a simple multi-step combination, including dissolution, immersion, solvent evaporation, phase inversion and coagulation [47]. The phase-inversion porous materials are obtained through a certain sol-gel process, which involves the evaporation of solvent to convert a continuous-phase polymer solution system into a three-dimensional network gel polymer [48]. In view of the issues that are in urgent need of improvement such as safety, cumbersome manufacturing process and high cost, phase inversion has attracted much attention as an alternative method for preparing high porosity cellulose films [49].
There is relatively less research on the porous cellulose films for supercapacitor by simply adjusting the parameters in the film formation process by using cellulose as a film-forming substrate without adding other components or undergoing the modification process. Using cellulose as a raw material and ionic liquid as a solvent [50], porous cellulose materials (cellulose aerogels and cellulose carbon aerogels [51]) are obtained by adjusting parameters in the dissolution-regeneration (phase-inversion) process [52], which mainly used as electrode for supercapacitors [53] and adsorbent for wastewater purification [54,55]. Therefore, a more detailed and in-depth study on the preparation of cellulose films using a single component of cellulose as the matrix and preparing cellulose films by phase-inversion method should be conducted to achieve a better capacitance performance of the assembled supercapacitor with cellulose separators.
In this work, the transparent renewable porous cellulose separators (ACR) were proposed and investigated via phase-inversion method with 1-Allyl-3-methylimidazolium chloride (AMIM-Cl) as a solvent. The effects of the concentration of cellulose solution on the performance of the separator was investigated, including chemical structure (intermolecular interaction, crystallinity), microstructure, thermal stability, optical transmittance, mechanical properties, ionic conductivity, and electrolyte uptake. The assembled supercapacitor (electrode/separator/electrode) devices were tested to evaluate the performance of cellulose film as a separator in the field of supercapacitors. The cycle stabilities of the assembled supercapacitors were also evaluated. The electrochemical performances of the assembled supercapacitor with cellulose separator (ACR) were evaluated and the results were compared with those assembled by using two commercial films (TF4040, MPF30AC) as separators.
Section snippets
Materials and reagents
Microcrystalline cellulose (MCC, white powder, particle size: 90 μm, density: 1.5 g/cm3), potassium hydroxide (KOH), activated carbon (density: 1.0–1.2 cm3/g, granularity: 5–8 μm, specific surface area: 1800 ± 100 m2/g, powder), acetylene carbon black and polytetrafluoroethylene were provided by Aladdin Chemical Reagent Co., Ltd. (China). 1-Allyl-3-methylimidazolium chloride (AMIM-Cl) was purchased from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, China. Commercial
Properties of the ACR films
The dissolution of cellulose in AMIM-Cl solvents is due to the larger organic cation and another smaller inorganic anion of AMIM-Cl, which play a very important role in the dissolution of cellulose. The Cl− anion is bonded to the hydroxyl proton of the H–O–H bond, while the [Amim]+ cation is associated with the oxygen atom of the H–O–H bond, resulting in the breakage of intermolecular and intramolecular hydrogen bonding, which in turn causes the dissolution of cellulose [56]. After the
Conclusion
In summary, transparent and renewable porous cellulose films (ACR-3, ACR-5, ACR-7, ACR-9) with different concentrations of cellulose solution (3 wt%, 5 wt%, 7 wt%, 9 wt%) were proposed and investigated via a phase-inversion process with 1-Allyl-3-methylimidazolium chloride (AMIM-Cl) as the solvent. The ACR (ACR-3, ACR-5, ACR-7, ACR-9) films with high thermal stability, strong mechanical properties and excellent apparent properties are expected to be applied in the field of supercapacitors. The
CRediT authorship contribution statement
Daman Xu: Conceptualization, Writing - original draft. Genhui Teng: Formal analysis, Resources. Yingqi Heng: Data curation, Investigation. Zhizhong Chen: Methodology, Software, Validation. Dongying Hu: Supervision, Writing - review & editing.
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
This work was supported by the Natural Science Foundation of Guangxi (2018GXNSFBA138025).
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