Highly Efficient and Stable Cellulose-Based Ion Gel Polymer

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A Highly efficient and stable Cellulose-based Ion Gel Polymer Electrolyte for Solid-state Supercapacitors Moshuqi Zhu, Lubing Yu, Shuaishuai He, Huachi Hong, Jian Liu, Lihui Gan, and Minnan Long ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01109 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019

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ACS Applied Energy Materials

A Highly efficient and stable Cellulose-based Ion Gel Polymer Electrolyte for Solid-state Supercapacitors Moshuqi Zhu †, Lubing Yu †, Shuaishuai He †, Huachi Hong †, Jian Liu†,††,†††*, Lihui Gan†, ††,†††*,

Minnan Long†

†College

of Energy, Xiamen University, Xiamen, 361102, China;

††Xiamen

Key Laboratory of Clean and High-valued Applications of Biomass, Xiamen

University, Xiamen 361102, China;

†††Fujian

Engineering and Research Center of Clean and High-valued Technologies for

Biomass, Xiamen University, Xiamen 361102, China

* Corresponding author. Tel.:+86-592-5952787; Fax:+86-5922188053 Email addresses: [email protected] (Jian Liu); [email protected] (Lihui Gan)

ACS Paragon Plus Environment

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ABSTRACT

In order to solve the current situation of low efficiency and instability of SCs, herein, the regenerated cellulose nanoparticles are applied on the electrolyte for the first time and a kind of solid-state SC with high performance is synthesized in a facile way. The electrolyte is prepared taking copolymer poly (vinyl alcohol) (PVA) as the polymer matrix, 1-Butyl-3methylimidazolium trifluoromethansulfonate (BmimCF3SO3) as the supporting electrolyte, graphene oxide as the ionic conducting promoter and regenerated cellulose nanoparticles as the regulator. This doped ion gel significantly improves the charge transfer resistance, because the homogeneously distributed regenerated cellulose nanoparticles make the ion transmission more orderly and stable and then reduce charge transfer resistance greatly. A model of the transmission of ions in the novel electrolyte is proposed. The cellulose-based gel electrolyte enables the SC to show good capacity retention of about 80%, and its charge/discharge efficiency maintains at 98% after 10,000 cycles. Those satisfactory performances are due to the high ionic conductivity, excellent compatibility with carbon electrodes and long-term stability of the doped ion gel. Attributed to the simple procedure and its components, the gel electrolyte is highly scalable, cost-effective, safe and nontoxic as well as has application potential in various energy storage and delivery systems.

KEYWORDS：Gel polymer electrolytes, Supercapacitors, Regenerated cellulose nanoparticles, Ionic liquids, Graphene oxide


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INTRODUCTION Recent years have seen many researches and development-based efforts to power sources which mainly fix on the electrochemical capacitors and cells, aiming to achieve highly specific energies including long-cycled life and highly specific powers as well as others based on the relative less expense. As a series of widely applied products, batteries are used as energy storage devices with high energy but low power. On the contrary, the supercapacitor (SC) acts as a device with low energy and high power and is ideal to meet high power pulse requirements

2,3.

Therefore, SCs

have been attracting attention to be the devices for energy storage as a result of their highly powerful density as well as lasting lifecycle

4,5.

The charge-storage-based mechanism endows

SCs with the category into the following: firstly, the capacitors with electric double layer (EDL), which stores the charge in the field of electricity upon the interface of the electrode/electrolyte; secondly, the pseudocapacitors, which make the energy storage in virtue of the reversibly rapid faradaic reaction upon the service of the electrode 6,7. Conventionally, a traditional SC comprises double electrodes (the collectors with active material bonds) with electrolyte and separator of liquid. Liquid electrolyte-based SC has the disadvantages like leakage of liquid, self-discharge, electrodes-based corrosion, great size and operation under the lower temperature as well as the toughness in the challenging shape designing. It is a disadvantageous application as a hand-held micro-electronically powered device 8. Therefore, flexible, secure and lightweight SCs are more attractive than traditional SCs for powering miniaturized electronic systems such as digital cameras, micro-robots, mobile phones and implantable medical devices 9,10. Afterwards, the gel polymer electrolytes (GPEs) which combine polymer separator and liquid electrolyte as the homogeneous gel phase, and exhibit stable electrochemical characteristics, higher ionic conductivity and excellent mechanical properties are created

11,12.

So far, electrolytes based on


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water are employed to make GPEs, with a weakness that their electrical properties will change over time as the water gradually evaporates. Besides, the latent window will limit the density of energy, which is often visible in the electrolytes made up of water. Amidst the backdrop, the application of ionic liquids (ILs), which work as organic salts in the form of room-temperature (RT) liquid with no solvents presence14, helps to make up electrolyte to tackle such occasion13 regarded as the ideal options. They solely comprise ions and are considered as ‘green’ materials with some interesting properties, because they are featured as the excellent solvents for various materials whether organic or inorganic, ranging from non-coordination to non-volatility as well as adjustable solvents and miscibilities and so on 15. At present, there is a variety of ILs with the combination of cations and anions, but imidazolium cation based-ILs are of particular interest due to their high conductivity. There are many advantages of using ILs as EDL supercapacitor electrolytes. For example, they are endowed fine materials with their wide voltage windows 16 as well as big intrinsically based capacitance, thus making for high-energy electrochemical facilities 17.

Some ILs are known to form a gel when being suspended in eco-friendly polymers, such as

poly (ethylene oxide), poly (vinyl alcohol) and poly (ethylene glycol)

18.

The in-situ

encapsulation of ILs involves the simultaneous formation of a three-dimensional (3D) network and an entrapment, which percolates throughout the matrix and is responsible for the solid-like behavior of the ion gel. Thus, the polymer network functions as a sponge with the holes full of ILs. The solid-formed gel scaffold restricts ILs, meanwhile, providing a concrete obstacle existing within the pair of electrodes thus preventing the occurrence of short circuit. On the one hand, the conduct lessens any risky leakage, and on the other hand, ILs is also a big helper to make plasticized polymer matrix or network, thus increasing the mobility of ions within electrolytes. Accordingly, it is unnecessary to implant a separator within the two electrodes


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during the fabrication of EDL capacitors via ion gel electrolyte. Therefore, there is the potential to reduce the cost of EDL capacitor cells 19. Due to these features, ILs do not have to work under the limitation of the high-capacity cells. Meanwhile, they can also be applied to the highlyfunctioned facilities including photovoltaic energy storages as well as tactile sensors20, 21, 22. Despite the fact that GPEs preform in a satisfying way, it has to be encountered with some issues left, like the high contact resistance between the electrolyte and electrode materials, and large fiber diameter of porous gel electrolyte, which limit the ion migration. Besides, considering the small number of the researches in biomedical field on GPEs, a new sort of well-biocompatibility gel electrolyte should be developed. In previous work, our group has prepared a kind of high-performance self-healing ion gels and discussed their mechanical properties and self-healing properties

23.

In order to further apply

these ion gels, here we studied their super capacitive performance. Herein, a new sort of regenerated cellulose nanoparticle (RCN) was first added in the electrolyte materials and then dispersed into ion polymeric matrix. After then, a series of gel electrolytes using different ionic liquids were prepared. On that basis, a kind of high-performance SC, BmimCF3SO3-PVA-GORCN with high specific capacity, good charge/discharge efficiency and excellent capacitance retention performance as well as a triple network was developed. We found that the addition of graphene oxide (GO) and RCN in the ion gel greatly improved the performance of SCs. A highdegreed transport channel like “highway” for continuous interconnectivity were formed on GO surface, which stemmed from homogeneous distribution of GO as a network in the ion gel matrix. However, the interconnected RCN matrix acts as a separation belt on the highway, which makes the ion transmission more orderly and stable and then reduces the charge transfer resistance greatly. Hence, the SC of BmimCF3SO3-PVA-GO-RCN results in a good capacity


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retention of about 80%, based on theoretical values. After 10,000 cycles, its charge/discharge efficiency is maintained at 98%.

RESULTS AND DISCUSSION

Figure 1. (a) The XRD graph of CE, CNC and RCN; (b) FTIR spectra of PVA, DMSO, BmimCF3SO3, BmimCF3SO3-PVA，BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-


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RCN. The SEM micrographs of (c) RCN, (d) original CE, (e) BmimCF3SO3-PVA, (f) BmimCF3SO3-PVA-GO, (g) BmimCF3SO3-PVA-GO-RCN and (h) is an enlarged view of the rectangular box in the (g) The XRD patterns of CE (cellulose), RCN, and CNC (cellulose nano-crystalline) were presented in Figure 1(a), in which the characteristic peaks appeared at 2θ =14.7°, 16.3°, 22.5° and 35.4° were assigned as the crystalline planes with Miller indices of (101), (101), (002) and (040) within the crystal structure of cellulose I

24.

The preservation of cellulose I structure in RCN

suggested that the crystal integrity of cellulose has been maintained. Compared with the original CE, the degree of crystallinity of RCN was improved due to the removal of amorphous parts in CE. PVA, DMSO (dimethyl sulfoxide), BmimCF3SO3 and hybrid materials were investigated with FTIR spectroscopy (Figure 1(b)). Table S1 summarized the assignment of some vibration modes of PVA, DMSO and BmimCF3SO3. In the spectrum of BmimCF3SO3, the peaks corresponding to –SO3 stretching (1259 cm-1 and 1169 cm-1) were detected. The two peaks faded away in the spectrum of BmimCF3SO3-PVA, but reappeared in those of BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN. It indicates that when the BmimCF3SO3 is presented alone in the PVA gel, the free ions lose easily and the electrochemical properties of the gel descends. Since GO and RCN can form hydrogen bonds with PVA, when they are uniformly dispersed in the PVA gel, a channel for ion free transport is established. It is also reflected in the subsequent SEM characterization and electrochemical performance. The morphology of RCN is characterized by SEM, as shown in Figure 1(c). The micrograph demonstrated short rod-shape with a size of 50-100 nm in diameter and 400-800 nm in length,


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and also with the smooth surface, which were quite different from the original CE (Figure 1(d)). The substantial changes could be attributed to the hydrolytic cleavage of glycosidic bonds of CE during the hydrolysis process. Small size RCN particles easily disperse in the polymer, which facilitates the transport of ions in the electrolyte. Unlike the smooth structure of BmimCF3SO3-PVA (Figure 1(e)), the morphology of BmimCF3SO3-PVA-GO becomes rough and exhibits web-like structure due to the presence of GO nanosheets (Figure 1(f)). The appearance of the structure of BmimCF3SO3-PVA-GO is attributed to the complete exfoliation and dispersion of GO into the polymer matrix. PVA can form hydrogen bonds with adjacent GO sheets, providing an additional bonding force for the gelation of GO 25. For BmimCF3SO3-PVA-GO-RCN (Figure 1(g) and (h)), both GO nanosheets and RCN are inter-dispersed uniformly and form a 3D porous web-like structure, with the pore diameter in nano scale. The pore walls of BmimCF3SO3-PVA-GO-RCN gel have rougher and more wrinkled textures than BmimCF3SO3-PVA-GO, and the pores distribution are more uniform and no agglomeration is found 26.


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Figure 2. (a) CV curves of SCs with EmimN(CN)2-PVA, BmimCF3SO3-PVA, BmimCl-PVA and NaCl-PVA at a scan rate of 100 mV s-1; Electrochemical performances of SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN: (b) CV curves of SCs at a scan rate of 100 mV s−1, (c) GCD curves of SCs at a current density of 0.5 A g-1, (d) Specific capacitance of SCs at different current densities, (e) Mass energy density and power density of SCs; Electrochemical performances of SC with BmimCF3SO3-PVA-GO-RCN: (f) CV curves of SC in different potential ranges at a scan rate of 100 mV s-1, (g) CV curves of SC at different scan rates, (h) GCD curves of SC at different current densities. Figure 2(a) displays the CV curves of EmimN(CN)2 (1 ‐ ethyl ‐ 3 ‐ methylimidazolium dicyanamide) -PVA, BmimCF3SO3-PVA, BmimCl (1-butyl-3-methylimidazolium chloride)PVA and NaCl-PVA SCs at a scan rate of 100 mV s-1. When solvent was EmimN(CN)2 or BmimCF3SO3, the voltage window of both curves can reach 2.5 V, much higher than that of NaCl-PVA (which only reached 0.5V). The CV curves of NaCl-PVA and BmimCl-PVA in different potential ranges at 100 mV s-1 are shown in Figure S1. The CV curve became irregular as the test voltage of NaCl-PVA reached 1.0 V and begun to fluctuate at 1.5 V. For BmimClPVA, 1.5V is the ideal test voltage. Furthermore, the SC of BmimCl-PVA exhibited a relatively high specific capacity of about 50 F g-1 in the first cycle at a current density of 1.0 A g-1. In contrast, the SC of NaCl-PVA exhibited a very low specific capacity of only about 8 F g-1 in the first cycle. In addition, although EmimN(CN)2-PVA had a large voltage window, BmimCF3SO3PVA showed a larger specific capacitance in the enclosed area. Therefore, our subsequent research is focused on the ionic liquid BmimCF3SO3 gels.


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In order to make the tests more stable, we chose the potential range of 0.0-2.2 V as the test potential of the BmimCF3SO3 SCs. The area covered by the CV curve of BmimCF3SO3-PVAGO-RCN was dramatically enhanced compared with those of BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA (Figure 2(b)). This enhancement was attributed to the presence of the GO and RCN, which greatly shortened the ion diffusion path, ensuring the high utilization of surface capacitive reactions in SC 27. The charge/discharge curve of the BmimCF3SO3-PVA -GO-RCN at a current density of 0.5 A g-1 was compared to those of the counterpart with BmimCF3SO3PVA and BmimCF3SO3-PVA-GO. All of them demonstrated a nearly triangular linear behavior as shown in Figure 2(c), which is in agreement with the capacitive charge/discharge mechanism of the ideal double-layer capacitor 28, just as the CV curves reveal. At the current density of 0.5 A g-1, the SC of BmimCF3SO3-PVA exhibited a comparable specific capacitance (89.07 F g-1) calculated based on galvanostatic charge-discharge (GCD) cycling, lower than those of BmimCF3SO3-PVA-GO (92.13 F g-1) and BmimCF3SO3-PVA -GO-RCN (103.23 F g-1). In addition, according to the explicit observation revealed, the Internal Resistance (IR) drop of the BmimCF3SO3-PVA-GO-RCN device (20 mV) was less than those of the BmimCF3SO3-PVA (140 mV) and BmimCF3SO3-PVA-GO (32 mV). The results are in relevance to the generalized SCs resistance, which exerts a major influence upon the electrochemical performance of the devices

29.

Figure. 2(d) shows the specific capacitance obtained for SCs with BmimCF3SO3-

PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN calculated using GCD cycling at different current densities. The SC with BmimCF3SO3-PVA-GO-RCN exhibited a good retention (50.6%) of specific capacitance at the current density from 0.5 A g-1 to 8.0 A g-1, whereas the SC with BmimCF3SO3-PVA SC showed a drastic drop in capacitance at a higher current density, resulting in a poor capacitance retention of 8.2%. The higher capacitance


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retention of SC with BmimCF3SO3-PVA-GO-RCN can be ascribed to the lower overall resistance

29.

It can be seen that the addition of GO and RCN increased the capacitance of SC.

Due to the abundant oxygen-containing functional groups on the surface and edge of GO sheets, the added GO in the ion gel interacted with the copolymer on the surface of GO so that an amorphous phase formed 30. Furthermore, since the GO distributes homogeneously in the ion gel matrix as a network, a highly interconnected and continuous transport channel is formed on the surface of GO as a “highway” for ion transport

31.

Moreover, the uniformly dispersed RCN

further improved the electrochemical performance of SC. This phenomenon can also be observed in SCs prepared by another ionic liquid, EmimN(CN)2 (Figure S3). Both power and energy densities are meaningful parameters of SCs, which are determined by certain electrochemical quantities. The mass energy density and power density at different current densities are shown in Figure 2(e). The BmimCF3SO3-PVA-GO-RCN SC demonstrated higher energy density and power density than the other two SCs. BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GORCN with GO added to the electrolyte possessed much higher energy density and power density than the SC with BmimCF3SO3-PVA. Higher power density can be attributed to fast ion transport. A high energy density of 34.3 Wh kg-1 with a power density of 550 W kg-1 in SC with BmimCF3SO3-PVA-GO-RCN was achieved at current density of 0.5 A g-1 and more importantly, the SC maintained 17.4 Wh Kg-1 and 8800 W Kg-1 even at 8 A g-1 as can be seen from ragone plot. In addition, we also tried to add the CNC into the gel electrolyte, but the electrochemical performance was not satisfactory (Figure S2). As seen from the CV curves, the areas of close curve of the BmimCF3SO3-PVA-GO-CNC was not as large as the original BmimCF3SO3-PVA. Moreover, although BmimCF3SO3-PVA-GO-CNC resulted in less charge transfer resistance (Rct) than BmimCF3SO3-PVA, the addition of CNC increased Rct compared with BmimCF3SO3-PVA-


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GO. This shows that CNC and GO do not cooperate to form a better structure to transport ions. In the reported solid electrolytes, the free space originated from the vibration of electrolyte molecule usually restricts the movement of ions greatly

32.

In some electrolytes, ions can only

transfer through holes, but some holes are not fully interconnected

33.

The bicontinuous two-

phase structure of RCN consists of water-rich phases and cellulose-rich phases

34,35.

In the

mixing process, RCN is dispersed in PVA. As hydrophilic substance, PVA chains initially diffuse into the water-rich phase and then penetrate into the cellulose-rich phase, making it possible to form the hydrogen bonds between PVA and RCN 36,37. Therefore, RCN and PVA will intimately contact with each other. When RCN was uniformly dispersed in the ionic gel, a developed 3D network structure was formed, through which the ions could easily transfer. Figure 2(f) exhibits CV curves of BmimCF3SO3-PVA-GO-RCN SC at different potential windows at a scan rate of 100 mV s-1. The fabricated symmetrical supercapacitor showed quasirectangular and symmetrical shapes, even at the potential window up to 2.0 V. However, when the voltage reached 2.5 V or more, the curve started to be slightly deformed, 2.2 V is selected as the subsequent test voltage. Figure 2(g) shows the CV curves of BmimCF3SO3-PVA-GO-RCN SC measured at scan rates of 5, 10, 20, 50, 80 and 100 mV s-1, respectively between 0.0 V and 2.2 V. It indicates an ideal capacitive behavior that the CV curves show rectangular‐like shapes and no obvious redox peaks are detected. Furthermore, even at a scan rate of 100 mV s-1, CV profiles still keep a relatively rectangular shape without any obvious distortion. It is a desirable fast charge/discharge property for the power devices. GCD curves of BmimCF3SO3-PVA-GORCN SC at 0.5, 1, 2, 5 and 8 A g-1, respectively exhibited a linear discharge behavior. It indicates an efficient power delivery for the existence of double layer capacitive performance with small IR drop (Figure 2(h)) 38, 39.


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Figure 3. (a) Nyquist plots of SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN. (b) Proposed schematic design showing ion transport in the gel electrolyte. As a versatile characterization technique, electrochemical impedance spectroscopy (EIS) is an efficient tool to investigate the behavior of electrode-electrolyte interface in electric double layer capacitors. Nyquist impedance plots of the SCs at RT are shown in Figure 3(a), performed in the frequency range of 100 kHz-0.01 Hz. The inset in Figure. 3(a) shows the impedance of three SCs at the high frequency region. The line perpendicular to the real-axis in the low-frequency region indicates the relatively ideal capacitance behavior of the electrode material. At intermediate frequencies, shorter Warburg impedance lines further confirm the rapid ion diffusion in materials 40, generally observed in the porous carbon-based SCs

41, 42.

The two kind

of resistive components (Rb and Rct) are determined by the semicircle intercepts of the real axis. The high-frequency intercept reflects the bulk resistance (Rb) of the polymer electrolyte membrane 43,44. Although it has been previously reported that the GO-doped ion gel significantly improves ionic conductivity compared with the GO-free controls

31,

no such phenomenon has

been observed in our system. However, interestingly, there is a significant decrease in charge


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transfer resistance (Rct) due to the addition of GO and RCN. As an important parameter related to the electrode-electrolyte interface, the Rct includes the electronic and ionic resistances in the SCs. Since the electrode material containing the aluminum foil current collector and electronic conductor is quite electronic-conducting, the electronic resistance is negligible. As the main resistance encountered by charge carriers (ions), the ionic resistance is obviously affected by the structure of pores in electrodes. Therefore, the ionic resistance is considered as the main Rct 8. A bigger semicircle is observed in the Nyquist plot of BmimCF3SO3-PVA than those of BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN, showing that there are some ion transport limitations at the interface of electrode-electrolyte in BmimCF3SO3-PVA and thus a high interface resistance exists. Nevertheless, in the case of BmimCF3SO3-PVA-GO, the GO sheets work as channels for ion transport with a lower interface resistance. Moreover, the Rct in BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN are 43.12 Ω, 29.30 Ω and 15.81 Ω, respectively, indicating that the RCN-added gel polymer electrolyte membrane has a better compatibility with the AC electrode. Hence, even a small Rct can enhance the performance of SCs significantly. This phenomenon can also be observed in SCs prepared by another ionic liquid, EmimN(CN)2 (Figure S3(d)). These results agree well with the CV analysis. Owning to its specific structure, the Rct of BmimCF3SO3-PVA-GO-RCN is maintained at a low level, which contributes to the ion transport. GO and RCN could improve power density effectively by providing such 3D channel networks filled with electrolytes in IL-DMSO medium. As shown in Figure 3(b), it is proposed that the structure models of BmimCF3SO3-PVA (model A) and BmimCF3SO3-PVA-GO (model B) with the illustrative cross section for gel electrolyte. BmimCF3SO3-PVA is like a winding road along which ions cannot move quickly (Model A). When GO added, the faster transfer rate of ions in the electrolyte leads to an increase in the


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significant capacitance of the SC. But Like cars passing through highways at a high speed are prone to traffic accidents, ions are likely to collide with each other when transporting in the BmimCF3SO3-PVA-GO electrolyte (Model B). However, BmimCF3SO3-PVA-GO-RCN consists of GO sheets and interconnected RCN backbones. The GO sheets work like highways which allow ions to pass quickly, and the RCN backbones work like separation belts which make transmission of ions regular and orderly (Model C). Positioned between the two electrodes, the gel electrolyte can not only keep the electrodes from contact, but also enable the free ionic transport with a perfect form of isolated electronic flow 45. The ionic conductivities of SCs were measured (Table 1). In the reported solid electrolyte, the free space originated from the vibration of electrolyte molecule often seriously restricts the movement of ions 32. Therefore, the ionic conductivity of solid polymer electrolyte is usually no more than 10-4 S cm-1 33. By contrast in our system, all ionic conductivity of SCs are greater than 10-3 S cm-1. Table 1. Electrical parameters of SCs based on the impedance analysis

Supercapacitor cell

Rb (Ω cm2)

Rct (Ω cm2)

σ（S·cm-1)

BmimCF3SO3-PVA

2.309

43.12

3.83×10-3

BmimCF3SO3-PVA-GO

5.022

29.30

1.76×10-3

BmimCF3SO3-PVA-GO-RCN

5.193

15.81

1.70×10-3


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Figure 4. Electrical characteristics of SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN. (a) Electrical impedance (b) Phase angle, and (c) Real and (d) Imaginary parts of complex capacitance vs. frequency; Plots of normalized active power, |P|/|S| and reactive power |Q|/|S| vs. frequency: (e) BmimCF3SO3-PVA, (f) BmimCF3SO3-PVA -GO and (g) BmimCF3SO3-PVA -GO-RCN; Cycling stabilities of SCs: (h) BmimCF3SO3-PVA, (i) BmimCF3SO3-PVA-GO, (j) BmimCF3SO3-PVA-GO-RCN, (k) Capacitance retention of SCs. Figure 4(a) represents the absolute value that the impedance |Z| concludes, which means the excitation frequency function. From the measurement, all three capacitors show a small resistance. |Z| is dependent of frequency at low frequencies, which indicates capacitive behavior (Z″). The formation of capacitive EDLs relies on the rearrangement of electric field-driven ion


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near the electrodes, but it is impossible at high frequencies. Moreover, |Z| is characterized by the resistive component (Z′) and independent of frequency at high frequencies

46,47.

Because of the

high electrical conductivity of the consisted negative and positive ions, the impedance of the BmimCF3SO3-PVA was found to be so low as 2.39 Ω at 100 kHz. The SCs with BmimCF3SO3PVA-GO and BmimCF3SO3-PVA-GO-RCN also showed a small impedance of 5.25 Ω and 5.55 Ω at 100 kHz, respectively. The values are very close to the Rb value calculated by fitting the Nyquist plot with software. The ideal impedance of electrolytes only consists of capacitance, and the corresponding phase angle is expected to be -90° 46. However, the results show the phase angles larger than -90° in the electrolytes of this research, because there are parasitic resistive parts related to the diffusion of ions or the internal resistance of electrolyte. The phase angles related to the frequency are shown in Figure 4(b). Comparing the performance of BmimCF3SO3-PVA and BmimCF3SO3-PVA-GO in the low frequency range, they are almost the same. The BmimCF3SO3-PVA, BmimCF3SO3PVA-GO and BmimCF3SO3-PVA-GO-RCN exhibited the phase angle of -68°, -71° and -77° at 0.01 Hz, respectively, which further highlighted the ideal capacitive nature of SC with BmimCF3SO3-PVA-GO-RCN. Because of the formation of the EDLs, the capacitive component determines the electrical characteristics in low frequency domains. Since the transfer of the ions is fairly slower than that of electrons, the impact on the EDL capacitance is negligibly slight in the high frequency domain 48. As the characteristic feature of the electrode and electrode/electrolyte interface, the real part of C′(ω) is plotted against the frequency (Figure 4(c)). The capacitance increased towards the lower end of frequency scale, because they were inversely related to each other. Corresponding to the mid-point of maximum capacitance, the relaxation time constant is calculated from the


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frequency. It is indicated that the addition of GO in the IL medium increases the capacitance of SC. The SC made from the RCN showed higher capacitance than that of BmimCF3SO3-PVA and it confirms the results from other electrochemical investigations. The plot of imaginary part of C′′(ω) against the frequencies for SCs is shown in Figure 4(d). A peak was observed within the given frequency range which showed the effective transportation of ions in pores

49.

The

frequency (fo) corresponds to the maximum capacitance was applied on the calculation of relaxation time constant (τo = 1/2πfo). τo is also known as the dielectric relaxation time for describing merit of a SC, representing one of its discharge characteristics. τo of SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN were 5.41 s, 4.27 s and 2.37 s, respectively. It is clearly indicated that the addition of GO and RCN improves the value of τo. The plots of power vs. frequency in the presence and absence of redox additives are shown in Figure 4(e)-4(g). The frequency dependence of two factors is opposite and the frequency corresponding to the insertion point is known as resonance frequency for calculating τo. The changeover of SC from the resistive behavior to the capacitive behavior occurs at resonance frequency

49.

The value of τo on SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and

BmimCF3SO3-PVA-GO-RCN are 5.04 s, 4.16 s and 2.36 s, respectively, which agrees with the previously discussed trend from the real and imaginary parts of complex capacitance. Long cycle life is another important requirement for supercapacitors. The cycling stability of SCs with BmimCF3SO3-PVA, BmimCF3SO3-PVA-GO and BmimCF3SO3-PVA-GO-RCN was investigated between 0.0 V and 2.2 V using the continuous galvanostatic charge/discharge method at a current density of 1.0 A g-1, as shown in Figure 4(h)-4(k). Although SC with BmimCF3SO3-PVA had a better capacitance retention, its charge/discharge efficiency was only 88% on average, and its specific capacitance was lower than those of BmimCF3SO3-PVA-GO


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and BmimCF3SO3-PVA-GO-RCN. For the SC with BmimCF3SO3-PVA-GO, the specific capacitance was increased compared with BmimCF3SO3-PVA, and the charge/discharge efficiency was also improved. However, just like cars passing through highways at a high speed are prone to traffic accidents, ions are likely to collide with each other when transporting in the BmimCF3SO3-PVA-GO electrolyte, resulting in a large fluctuation in the charging/discharging efficiency. And its capacitance retention also dropped sharply, remaining only about 60% at 10,000 circles. The addition of RCN in the gel electrolyte has greatly solved this problem. The SC with BmimCF3SO3-PVA-GO-RCN maintained a capacity retention of about 80% in the case of high specific capacitance, and its charge/discharge efficiency was 98% on average. The phenomenon can also be observed in SCs prepared by another ionic liquid, EmimN(CN)2 (Figure S5). When RCN was uniformly dispersed in the ionic gel, a 3D network structure was formed, through which the ions can easily transfer. RCN is like a separation belt on a highway which can maintain traffic order when the ions pass at a high speed. CONCLUSION To conclude, a kind of ion gel electrolytes were successfully synthesized with sustainable RCN in this study. Notably, the SC with BmimCF3SO3-PVA-GO-RCN exhibited high specific capacity, good charge/discharge efficiency (about 98%) and excellent capacitance retention (about 80%) performance. In this 3D ion gel electrolyte, the addition of GO and RCN has greatly improved the electrochemical performance of SC. Due to the homogeneous distribution of GO as a network in the ion gel matrix, high degree of continuous and interconnected transport channels were formed, which can be described as “highway” for the ion transport. In addition, the uniformly dispersed RCN acts as the separation belt on the highway, which makes the ion transmission more orderly and stable. Moreover, the addition of RCN greatly reduces the charge


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transfer resistance of SC. Hence, compared with other SCs, the charge/discharge efficiencies of BmimCF3SO3-PVA-GO-RCN see an increase of more than 30%. The proposed work provides a new method by using ionic liquids and RCN to prepare SC with excellent electrochemical performance and eco-friendly property, which can be a promising candidate for fabricating multifarious flexible and hand-held energy storage devices with high performance.

Figure 5. (a) Schematic illustration showing the fabrication of BmimCF3SO3-PVA -GO-RCN gel polymer electrolyte. (b) Schematic illustration showing the fabrication of a BmimCF3SO3PVA-GO-RCN symmetric Button supercapacitor cell assembly, Photographs BmimCF3SO3PVA-GO-RCN gel electrolyte: (c) Transparency and (d) Flexibility.


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METHOD Preparation of gel polymer electrode: A prepared AC electrode was given in accordance with the previous literature 8, 50. Preparation of gel polymer electrolyte Preparation of RCN suspension liquid: The preparation of 1.0 wt% RCN suspension was made with ionic liquid BmimCl in accordance with the prior literature 51. Preparation of cellulose nano-crystalline (CNC): The CNC was prepared with sulfuric acid by this reported method 35. Preparation of graphite oxide solution: The preparation of 0.1 mg/mL GO solution refers to our previous work 23. Preparation of gel polymer electrolyte: The experimental method refers to our previous work 23. In brief, the preparation of the integrated homogeneous solution based on ionic liquid (20 wt%) and DMSO (80 wt%). The combination of GO (1.0 mL, 0.1 mg/mL) with RCN (1.0 mL, 1 wt%) added in the mixture mentioned above (6.0 mL), and 30 min ultrasonic agitation contributed the attainment of homogeneous solution. After that, the PVA (2.0 g) was fed slowly and the mixture was heated to 95 ° C and kept in oil bath for 2 h with vigorous stirring and then poured into a Teflon mold (14 mm in diameter, 0.1 mm in thickness). After cooling down to RT, the obtained samples were subjected to a freezing/thawing cycle (12 h freezing at -10 °C followed by 12 h thawing at RT). The obtained ion gel was named as IL-PVA-GO-RCN. The gel without GO and RCN was named as IL-PVA, and the gel without RCN was named as IL-PVA-GO. NaCl was mixed with PVA to prepare the electrolyte of 1.0 wt% with respect to PVA and distilled water as


21


control

48,

Page 22 of 31

named as NaCl-PVA. The preparation of BmimCF3SO3-PVA-GO-RCN is shown in

Figure 5(a). Fabrication of supercapacitors: Electrodes coated with GPEs were assembled into button supercapacitors to form symmetric supercapacitors (Figure 5(b)). No additional separator or liquid electrolyte was added during the cell assembly. Electrochemical tests The electrochemical performance of as-prepared SCs was investigated by using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) in a two-electrode system. The electrochemical workstation (CHI660A) is made by Shanghai Chenhua corporation. The recorded CV curves were in the range of 0.0-2.5 V at scan rates of 5-100 mV s-1. The EIS was tested with the AC voltage at the frequency ranging from 100 kHz to 0.01 Hz in the potential amplitude of 5 mV. The GCD experiments were done between 0.0 V and 2.5 V at various current densities (0.5-8.0 A g-1) by using the equipment of multichannel battery test. The tests for the cycling stability of SCs were performed under a current density of 1.0 A g-1 at a constant chargedischarge rate for 10,000 cycles at RT (LAND CT2001A, Wuhan, China). The transparency and flexibility behaviors of the solid-state SC were tested via manual bending (Figure 5(c)-(d)). The calculation of the specified energy density (Esc, Wh kg-1), electrode capacitance (Csp, F g-1), as well as the power density (Psc, W kg-1) held by the super-capacitors is demonstrated in the equations as follows52, 53: 𝐶𝑆𝑃 = 4 ×

𝐼 𝑚

(∆𝑉∆𝑡)

(1)


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𝐸𝑠𝑐 =

𝐶𝑠𝑝 × 𝑉2

𝑃𝑠𝑐 =

8

1000

(2)

× 3600

𝐸𝑐𝑒𝑙𝑙 × 3600

(3)

∆𝑡

Where I (A) represents the discharge current; the discharge time is represented by Δt (s); the carbon mass held by the two symmetric electrodes is interpreted with m (g); ΔV (V) stands by the voltage difference in the duration of the discharge process excluding the IR drop. The electrical conductivity of gel polymer electrolyte was calculated by 𝐿

(4)

𝜎 = 𝑅𝑏 × 𝑆

where σ, L, Rb and S are the electrical conductivity, thickness, bulk resistance and contact area with electrode, respectively 50, 52. The following represents the impedance Z, |𝑍| = 𝑍𝑟𝑒2 + 𝑍𝑖𝑚2

(5)

Where the addition of Zre and Zim respectively denote the real part as well as the imaginary part included by impedance 46, 48. On the serial connection of the resistance and capacitance, here is the written electrical impedance as follows, 1

𝑍 = |𝑍|𝑒𝑖𝜃 = 𝑍𝑟𝑒 +𝑗𝑍𝑖𝑚 = 𝑅 + 𝑗𝜔𝐶

(6)

Where θ stands for the phase angle; j represents the imaginary number; ω refers to the angular frequency of excitation and C denotes the capacitance 48.


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For better understanding of the behavior of SC devices, real and imaginary capacitance harbor the variation within the applicable frequency range, which is respectively achieved through the relationship between (7) and (8) 54,55: 𝐶(𝜔) = 𝐶′(𝜔) ― 𝑗𝐶′′(𝜔)

(7)

Where C ′ (ω) stands for the real part; C ″ (ω) represents the imaginary part, both of which belong to the complex capacitance C(ω). It is illustrated as follows:, 𝐶 ′𝜔 = 𝜔ǀ𝑍( 𝜔)ǀ2

―𝑍′′ (𝜔)

(8)

𝑍 ′ (𝜔)

(9)

𝐶 ′′𝜔 = 𝜔ǀ𝑍( 𝜔)ǀ2

Where Z ′ (ω) constitutes the real part; Z ′ ′ (ω) refers to imaginary part, which are respectively attached to complex impedance Z(ω) in which ω represents the angular frequency offered by ω =2πf. It is at low frequency that C ′ (ω) is compatible with the capacitance of electrode materials; meanwhile, C ″ (ω) is compatible with the energy dissipation during an irreversible process which gives rise to a hysteresis

49.

Here is the calculated value of complex

power: 𝑆(𝜔) = 𝑃(𝜔) +𝑗𝑄(𝜔)

(10)

Where complex power P(ω) holds the real part; P(ω) is named as the active power; the imaginary part Q(ω) is entitled as the reactive power based on the following: 𝑃(𝜔) = 𝜔𝐶 ′

′ (𝜔)

+ ǀ∆𝑉𝑟𝑚𝑠ǀ2

𝑄(𝜔) = ―𝜔𝐶′(𝜔) + ǀ∆𝑉𝑟𝑚𝑠ǀ2

(11) (12)


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Where |ΔVrms|2=ΔVmax/ 2 with Vmax functions as the maximized amplitude of the signal of AC. Characterizations

The morphology of the samples was studied with scanning electron microscope (SEM, SUPRA 55, Zeiss Sigma, Germany). Fourier-transform infrared (FTIR) spectra were investigated with a spectrometer (Nicolet iS5, Thermo Fisher, USA). The crystal and variation of structure were measured by means of X-ray powder diffraction (XRD) with an Ultima IV spectrometer (Rigaku, Japan).

ASSOCIATED CONTENT

Supporting Information. General assignment of FIIR absorption bands (Table S1), CV curves and Cycling stabilities of SC with NaCl-PVA, BmimCl-PVA (Figure S1), Electrical characteristics of SCs with BmimCF3SO3 and EmimN(CN)2 (Figure S2, S3 and S4), Cycling stabilities of SCs with EmimN(CN)2 (Figure S5), Mechanical properties test of ion gels (Figure S6) AUTHOR INFORMATION

Corresponding Author [email protected] (Jian Liu)


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[email protected] (Lihui Gan) Present Addresses † College of Energy, Xiamen University, Xiamen 361102, China

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the Energy development Foundation of College of Energy, Xiamen University (No. 2018NYFZ03 & 2017NYFZ02). ABBREVIATIONS SC, supercapacitor; EDL, electric double layer; GPEs, gel polymer electrolytes; ILs, ionic liquids; RT, room-temperature; 3D, three-dimensional; RCN, regenerated cellulose nanoparticle;

GO, graphene oxide; PVA, poly (vinyl alcohol); CE, cellulose; CNC, cellulose nanocrystalline; EIS, impedance spectroscopy; CV, cyclic voltammetry.


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Highly Efficient and Stable Cellulose-Based Ion Gel Polymer

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