Electrochemical Properties of Trimethylammonium Tetrafluoroborate in Electrochemical Double-Layer Capacitors

Electrochemical double layer capacitors (EDLCs), also known as supercapacitors, are promising energy storage devices, especially when considering high power applications [1]. EDLCs can be charged and discharged within seconds [1], feature high power (10 kW·kg-1) and an excellent cycle life (>500,000 cycles). All these properties are a result of the energy storage process of EDLCs, which relies on storing energy by charge separation instead of chemical redox reactions, as utilized in battery systems. Upon charging, double layers are forming at the electrode/electrolyte interface consisting of the electrolyte’s ions and electric charges at the electrode surface. In state-of-the-art EDLC systems activated carbons (AC) are used as active materials and tetraethylammonium tetrafluoroborate ([Et4N][BF4]) dissolved in organic solvents like propylene carbonate (PC) or acetonitrile (ACN) are commonly used as the electrolyte [2]. These combinations of materials allow operative voltages up to 2.7 V - 2.8 V and an energy in the order of 5 Wh·kg-1[3]. The energy of EDLCs is dependent on the square of the operative voltage, thus increasing the usable operative voltage has a strong effect on the delivered energy of the device [1]. Due to their high electrochemical stability, ionic liquids (ILs) were thoroughly investigated as electrolytes for EDLCs, as well as, batteries, enabling high operating voltages as high as 3.2 V - 3.5 V for the former [2]. While their unique ionic structure allows the usage of neat ILs as electrolyte in EDLCs, ILs suffer from low conductivity and high viscosity increasing the intrinsic resistance and, as a result, a lower power output of the device. In order to overcome this issue, the usage of blends of ionic liquids and organic solvents has been considered a feasible strategy as they combine high usable voltages, while still retaining good transport properties at the same time. In our recent work the ionic liquid 1-butyl-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide ([Pyrr14][TFSI]) was combined with two nitrile-based organic solvents, namely butyronitrile (BTN) and adiponitrile (ADN), and the resulting blends were investing regarding their usage in electrochemical double layer capacitors [4,5]. Firstly, the physicochemical properties were investigated, showing good transport properties for both blends, which are similar to the state-of-the-art combination of [Et4N][BF4] in PC. Secondly, the electrochemical properties for EDLC application were studied in depth revealing a high electrochemical stability with a maximum operative voltage as high as 3.7 V. In full cells these high voltage organic solvent based electrolytes have a good performance in terms of capacitance and an acceptable equivalent series resistance at cut-off voltages of 3.2 and 3.5 V. However, long term stability tests by float testing revealed stability issues when using a maximum voltage of 3.5 V for prolonged time, whereas at 3.2 V no such issues are observed (Fig. 1). Considering the obtained results, the usage of ADN and BTN blends with [Pyrr14][TFSI] in EDLCs appears to be an interesting alternative to state-of-the-art organic solvent based electrolytes, allowing the usage of higher maximum operative voltages while having similar transport properties to 1 mol∙dm-3 [Et4N][BF4] in PC at the same time.

In state of the art electrochemical double layer capacitors (EDLCs), solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (AN) are used as electrolytes, while activated carbons (AC) are used as electrode active materials. These combinations of materials enable the realization of EDLCs with high power (up to 10 kW kg-1), extraordinary cycle life (>500.000 cycles) and energy in the order of 5Whkg-1. The operative voltages of these devices are of 2.7 to 2.8 V [1]. It is known that an increase of the EDLCs energy would allow the introduction of these high power devices in a larger number of applications [1-2]. Therefore, in the last years many efforts have been made to increase the energy of EDLCs. The energy E stored in an EDLC system is described by the equation E=1/2 CV 2, where C is the capacitance and Vthe operative voltage of the device. Considering this equation, it is evident that to increase the operative voltage in the most convenient strategy to increase the energy of these devices. It has been shown that when the material-electrolyte combination reported above is used, the use of operative voltages higher than 2.8 V results in a significant decrease of the devices’ cycle life [1-2]. Therefore, the development of innovative electrolytes appears therefore a key aspect for the realization of high energy EDLCs: in the future new solvents, new conducting salts as well as new ionic liquids need to be considered [1-2]. Together with the electrode material, the ions of the conducting salts are forming the double layer, effectively influencing charge storage in in EDLCs. The size, electrochemical and chemical stability of these ions have a dramatic impact on the storage process. Nevertheless, it is interesting (and somehow surprising) to notice that in the past only a relatively low number of studies focused on the development of new conducting salt for high voltage EDLCs. In this study we investigated the influence of conductive salts on the physical-chemical as well as electrochemical properties of PC based electrolytes in view of the realization of advanced high voltage EDLCs. We showed that the operative voltage of EDLCs containing PC-based electrolytes can be increased by a mere change in conductive salt. Nevertheless, anions and cations forming the conductive salt have to be carefully selected, as the choice of salt influences both electrochemical properties as well as ion transport properties of EDLC electrolytes, leading to an impact on energy and power storage in EDLC devices. While higher operative voltages are generally desirable, one has to consider and carefully balance the advantages and disadvantages of each electrolyte, as high values for viscosity or decreased ion mobilities can lead to decreased values for energy and power, even if operative voltages are increased [2]. Among the innovative electrolytes we investigated, those based on the salt N-methyl-N-butylpyrrolidinium tetrafluoroborate (Pyr14BF4) appear particularly promising. As a matter of fact, the electrolyte containing Pyr14BF4 as salt and in PC as solvent displays good conductivity and low viscosity and, thanks to its anion-cation combination, also allows the achievement of high operative voltages. As a consequence, its application enables the construction of EDLCs with higher energy and power compared to state-of-the-art devices (see Fig. 1) [3]. Furthermore, the use of these electrolytes makes also possible the realization of EDLCs exhibiting a high cycling stability [2-3].

Carbon based electrochemical double layer capacitors (EDLCs) display high specific power and have an extremely high cycle life (>500,000). Because of these characteristics, EDLCs are conveniently used in a large number of applications where rapid charge-discharge capability and reliability are required. [1,2]Since the most effective strategy to increase power as well as energy of an EDLC is to raise the operating voltage [3], many research efforts have been focused on the introduction of high voltage electrolytes.Among the electrolytes proposed so far, those containing mixtures of ionic liquids (ILs) and organic electrolytes, e.g. N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI) and Propylene Carbonate (PC), appear particular promising as they display low viscosity, high conductivity and electrochemical stability [4]. EDLCs containing the mixture PC-PYR14TFSI as electrolyte display high operative voltage (3.5 V) and a remarkable cycling stability [4]. It is interesting to notice that in these mixtures the IL merely acts as a conductive salt. Taking this point into account, it appears evident that the properties of the conducting salt, particularly its electrochemical stability and its solubility in the solvent, might play a crucial role for the development of high performance (and high voltage) EDLCs since they affect the physical-chemical properties of the electrolyte as well as double layer formation [5, 6,7].In recent work, we proposed the use of a new conducting salt, N-butyl-N-methylpyrrolidinium tetrafluoroborate (PYR14BF4). We showed that the electrolyte 2.3 M PYR14BF4 in PC displays a conductivity and viscosity comparable to conventional Et4NBF4 in PC, but exhibits a much broader electrochemical stability window, which makes the realization of EDLCs with an operative voltage of 3.2 V possible [8].In this work we reports an extended study of the use of PYR14BF4 among other compounds as conductive salts in EDLC electrolytes.The electrochemical properties of the considered electrolytes were determined by Cyclovoltametry (CV), electrochemical impedance spectroscopy (EIS), float voltage tests and long term charge discharge experiments.We found that several of the electrolyte’s electrochemical properties - especially the electrochemical stability – are influenced by the choice of salt. The use of PYR14BF4 as conductive salt results in higher operative voltages and thus higher values for energy and power compared to conventional EDLC systems.[1] R. Kötz, M. Carlen, Principles and applications of electrochemical capacitors, Electrochim. Acta, 45, 2000, 2483.[2] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources, 157, 2006, 11.[3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater., 7, 2008, 845.[4] A. Krause, A. Balducci, High voltage electrochemical double layer capacitor containing mixtures of ionic liquids and organic carbonate as electrolytes, Electrochem. Comm., 13, 2011, 814.[5] K. Xu, M. S. Ding, T. R. Jow, Quarternary Onium Salts as Nonaqueous Electrolytes for Electrochemical Capacitors, J. Electrochem. Soc., 148, 2001, A267-A274.[6] K. Chiba, T. Ueda, Y. Yamaguchi, Y. Oki, F. Saiki, K. Naoi, Electrolyte Systems for High Withstand Voltage and Durability II. Alkylated Cyclic Carbonates for Electric Double-Layer Capacitors, J. Electrochem. Soc., 158, 2011, A1320-A1327.[7] J. P. Zheng, T. R. Jow, The Effect of Salt Concentration in Electrolytes on the Maximum Energy Storage for Double Layer Capacitors, J. Electrochem. Soc., 144, 1997, 2417-2420.[8] S. Pohlmann, A. Balducci, A New Conducting Salt for High-Voltage Propylene Carbonate-Based Electrochemical Double Layer Capacitors, Electrochim. Acta, 110, 2013, 221-227.

Electrochemical double layer capacitors (EDLCs) are considered among the most promising electrochemical storage device for high power systems. EDLCs display extraordinary cycle life (>500.000 cycles), high power (up to 10 kW kg-1) and energy (in the range of 5 Wh kg-1), what makes them reliable energy sources for many applications. [1] In state-of-the-art EDLCs activated carbons (AC) are used as electrode active materials, while solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. These combinations of materials leads to EDLCs with operative voltages of 2.7-2.8 V. [1] Considering the formulas describing the energy (E=1/2 CV²) and power (P=V²/4R) of EDLCs, it is obvious that by increasing the operative voltage, energy and power values will be increased as well. For this reason, many efforts have been made towards the introduction of novel electrolytes. [1] Recently we showed that the use of alternative conductive salts, with respect to Et4NBF4, represents a promising strategy for the realization of high voltage EDLCs. As a matter of fact, using these salts in combination with PC, it is possible to realize EDLCs with operative voltages higher than 3 V. [2] It is well known that also the chemical-physical properties of the solvent have a strong influence on the operative voltage as well as on the performance, especially the power, of EDLCs. [1] Taking into account the lower viscosity of ACN compared to PC, the use of the former solvent in combination with the above mentioned alternative conductive salts appears therefore of great interest. Such combinations could allow the realization of high voltage and high power devices. In this work we report a systematic investigation about the chemical-physical properties of electrolytes containing the salts (tetraethylammonium tetrafluroroborate (Et4NBF4), tetraethylammonium bis(trifluoromethanesulfonyl)imide (Et4NTFSI), N-butyl-N-methylpyrrolidinium tetrafluoroborate (Pyr14BF4), 1-butyl-1-methylpyrrolidinium bis (trifluoromethanesulfonyl)imide (Pyr14TFSI)) and ACN as solvent. The ionic conductivity, viscosity, density and electrochemical stability windows (ESW) of these electrolytic solutions are considered in detail. Furthermore, the performance of EDLCs containing these advanced electrolytes is investigated and critically analysed. The results of this study indicate that the solvent-salt interaction has a strong influence not only on the chemical-physical properties of the electrolyte, but also on the energy, power and cycle life of the EDLC. Furthermore, they also indicate that the operative voltage of EDLCs containing alternative conductive salts is strongly influenced by the choice of the solvent. [3] References [1] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Advanced Materials, 26 (2014) 2219-2251 [2] S. Pohlmann, C. Ramirez-Castro, A. Balducci, J. Electrochem. Soc., 162 (2015) A5020-A5030. [3] J. Krummacher, C. Schütter, A. Balducci, manuscript in preparation Figure 1

In the state of the art electrochemical double layer capacitors (EDLCs), solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. Using these electrolytes in combination with activated carbons (AC) based electrodes is possible to realize EDLCs with operative voltages of 2.7-2.8 V and energy in the order of 5Whkg-1. The power of these devices is up 10 kW kg-1 and their cycle life higher than 500.000 cycles [1]. Presently the realization of high energy EDLCs is seen as one of the most important goal for an increase of the market of these devices [1]. The most straightforward strategy to increase the EDLC´s energy is to increase their operative voltage. Since such desirable increase is not possible when the state-of-the-art electrolytes are used, the introduction of new electrolytic solution is presently considered of great importance [1] In order to realize advanced electrolytes for EDLC, the introduction of new conducting salts and/or new solvents appears a viable strategy [1,2]. In the past years we showed that using non-conventional salts is possible to realize PC-based EDLCs with operative voltages higher than 3 V [1,2]. More recently, taking advantaged of computational screening [3], we identified cyano esters as new solvent for the realization of high voltage and high energy EDLCs [4,5]. We showed that cyano ester-based EDLCs display operative voltage as high as 3.5 V and good performance in term of specific capacitance and capacitance retention. After 500 h of float tests carried out at 3.2 V EDLCs containing this type of solvent are able to retain almost 80% of their initial capacitance [4]. It is well known that the ion-solvent interaction might have a dramatic effect on the behaviour of EDLCs [1,2]. While these interactions have been considered for conventional salt-solvent combinations, only little information is available on the case of non-conventional electrolyte components [2]. Herein, we report a systematic investigation about the chemical-physical properties of electrolytes containing the salts tetraethylammonium tetrafluroroborate (Et4NBF4), tetraethylammonium bis(trifluoromethanesulfonyl) imide (Et4NTFSI) and N-butyl-1-methylpyrrolidinium tetrafluoroborate (Pyr14BF4), in the state of the art solvent ACN, and in the alternative solvents Adiponitrile (ADN), 2-methylglutaronitril (MGN) and 3-cyanopropionic acid methyl ester (CPAME). The ionic conductivity, viscosity, density and electrochemical stability windows (ESW) of these electrolytic solutions are considered. Furthermore, the electrochemical performance of EDLCs containing these electrolytes are investigate in details. Finally, also the anodic dissolution of the aluminium current collector in these electrolytes is analysed and discussed [6].

In the state of the art electrochemical double layer capacitors (EDLCs), activated carbons (AC) are used as electrode active materials, while solutions of tetraethylammonium tetrafluroroborate (Et4NBF4) in propylene carbonate (PC) or acetonitrile (ACN) are used as electrolytes. These combinations of materials enable the realization of EDLCs with operative voltages of 2.7-2.8 V, extraordinary cycle life (>500.000 cycles), high power (up to 10 kW kg-1) and energy in the order of 5Whkg-1[1]. It has been shown that if the energy of EDLC could be increase up to 10Whkg-1 these devices could be introduced in many new applications, leading to a dramatic increase of their market [1]. For this reason, the development of high energy EDLCs is nowadays considered of crucial importance for the future of this technology. The most straightforward strategy to increase the EDLC´s energy is to increase their operative voltage. Since such desirable increase is not possible when the state-of-the-art electrolytes are used, the introduction of new electrolytic solution is presently consider of great importance [1]. To avoid time consuming “trial and error” experiments, it is desirable to “rationalize” this search for new electrolyte components. An important step in this direction is the systematic application of computational screening approaches. Via the fast prediction of the properties of a large number of compounds, for instance all reasonable candidates within a given compound class, such approaches should allow to identify of the most promising candidates for subsequent experiments [2]. In this paper we report about the use of computation screening for the identification of novel solvents for EDLCs. As an example application, the known chemical space of almost 70 million compounds is investigated in search of electrochemically more stable solvents. Cyano esters are identified as especially promising new compound class. As a matter of fact, the use of electrolytes containing cyano ester as solvent and Et4NBF4 as conductive salt allows for the realization of EDLCs with operative voltage as high as 3.5 V. These high voltage EDLCs display good performance in term of specific capacitance and capacitance retention. Furthermore, we showed that EDLCs containing this type of solvent are able to retain almost 80% of their initial capacitance after 500 h of float tests carried out at 3.2 V. This extraordinary capacitance retention, which is among the highest so far reported for high voltage EDLC, clearly indicate that cyano ester can be considered as a new and very interesting solvent in view of the realization of high voltage EDLC [3]. Taking these results into account, the use of computational screening appears as a very promising and novel strategy for a rational design of new electrolyte materials for EDLCs. It is important to remark that such approach can be applied not only to the search for new materials, but also to the optimization of mixtures and salt/solvent combinations.

A rapidly growing interest in renewable energy resources requires efficient energy storage systems as well as a need to elaborate a greater number of eco-friendly components. Electrochemical double-layer capacitors (EDLCs), also known as supercapacitors are a class of energy storage devices that store the energy due to the separation of oppositely charged ions at in the electrical field resulting the formation of electrical double layer (EDL) at the porous carbon electrode/electrolyte interface. EDLCs consist of two porous carbonaceous electrodes pre-soaked with electrolyte and separated with a membrane (separator). The simple electrostatic mechanism of energy storage, lack of chemical changes and faradaic transitions during operation results in high electrical capacitance with respect to classical capacitors, significantly higher power density in comparison to batteries, and practically unlimited life span. Currently, the commercially produced EDLCs typically rely on organic solvents such as acetonitrile or propylene carbonate with the addition of conductive salts. However, such drawbacks as their low conductivity, toxicity flammability and high cost, led to the growth of interest in aqueous electrolytes such as KOH, H2SO4 or simple inorganic salts. Among main advantages of aqueous solvents such features as their higher ionic conductivity, lower viscosity, increased safety, lower cost and ease of assembly under ambient atmosphere, should be underlined.Modern and technologically advanced charge storage devices often require flexible and deformable devices for specific applications. Therefore, a lot of research is held into the development of alternatives for currently used liquid (aqueous and organic) electrolytes, which suffer from two prominent drawbacks — (i) the possibility of electrolyte leakage and (ii) high standards of technology to safely encapsulate electrolyte in the device. To overcome these limitations, the solid-state EDLCs using an ionically-conductive polymer or hydrogel membrane, serving as both the separator and the electrolyte, are under the spotlight. In this respect, a cellulose, built of β-(1→4)-linked D-glucose units, is one of the most prevalent and easily degradable biopolymer. Albeit the wide availability, biodegradability and low cost, the usage of cellulose is limited due to insolubility in most common solvents. The recent alternative, to toxic and flammable organic compounds, such as N,N- Dimethylformamide/N2O4, N-methylmorpholine oxide (NMMO), are ionic liquids (ILs), that have been gaining lately a lot of attention in energy storage systems. Various ILs based on imidazolium, pyridinium and ammonium cation paired with strongly basic anion (e.g., OAc-, HCOO-) were also recently used to dissolve cellulose [1]. However, the requirements of high-purity syntheses and the cost of some of the cations/anions may affect a large scale application.Therefore, our research refers to an alternative route of chemical processing of microcrystalline cellulose, i.e. its dissolution using various wt% ratio of aqueous NaOH/urea mixtures, and further processing into a hydrogel membrane in the presence of cross-linking agents, such as glutaraldehyde and citric acid. The cellulose-based hydrogel membranes will be used as a support for various aqueous electrolytes, including H2SO4, KOH, K2SO4, i.e. most commonly used for aqueous EDLCs. Also, the alternative solutions will be used, i.e. based on polyoxometalates, or more precisely Keggin-type heteropolyacids (such as H4SiW12O40) which according to our recent results seem to be promising candidates to replace conventional acidic/neutral electrolytes [2]. The thickness of the membranes will be adjusted to introduce the minimum volume of electrolyte, i.e. necessary to fill the porosity and all void spaces in the electrode material and to avoid the electrolyte excess. A structure and morphology will be characterized with SEM/EDX, AFM, AT-FTIR, XRD and TGA. The ionic conductivity will be determined using impedance spectroscopy over wide range of temperatures. The designed systems will be compared, in terms of energy, power and cycleability, with their analogues using conventional polypropylene separator in the presence of excess of liquid electrolyte. Acknowledgement Financial support was provided by the National Centre for Research and Development (NCBR, Poland) under Techmatstrateg Grant no. 347431/14/NCBR/2018.[1] D. Kasprzak, I. Stepniak, M. Galinski, Electrodes and hydrogel electrolytes based on cellulose: fabrication and characterization as EDLC components, J. Solid State Electrochem. 22 (2018) 3035–3047, https://doi.org/10.1007/s10008-018-4015-y[2] N.H. Wisinska, M. Skunik-Nuckowska, S. Dyjak, P.J. Kulesza, Factors affecting performance of electrochemical capacitors operating in Keggin-type silicotungstic acid electrolyte, Appl. Surf. Sci. 530 (2020) 147273, https://doi.org/10.1016/j.apsusc.2020.147273.

Key parameters that influence the specific energy of electrochemical double-layer capacitors (EDLCs) are the double-layer capacitance and the operating potential of the cell. The operating potential of the cell is generally limited by the electrochemical window of the electrolyte solution, that is, the range of applied voltages within which the electrolyte or solvent is not reduced or oxidized. Ionic liquids are of interest as electrolytes for EDLCs because they offer relatively wide potential windows. Here, we provide a systematic study of the influence of the physical properties of ionic liquid electrolytes on the electrochemical stability and electrochemical performance (double-layer capacitance, specific energy) of EDLCs that employ a mesoporous carbon model electrode with uniform, highly interconnected mesopores (3DOm carbon). Several ionic liquids with structurally diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, and pyrrolidinium) were investigated. We show that the cation size has a significant effect on the electrolyte viscosity and conductivity, as well as the capacitance of EDLCs. Imidazolium- and pyridinium-based ionic liquids provide the highest cell capacitance, and ammonium-based ionic liquids offer potential windows much larger than imidazolium and pyridinium ionic liquids. Increasing the chain length of the alkyl substituents in 1-alkyl-3-methylimidazolium trifluoromethanesulfonimide does not widen the potential window of the ionic liquid. We identified the ionic liquids that maximize the specific energies of EDLCs through the combined effects of their potential windows and the double-layer capacitance. The highest specific energies are obtained with ionic liquid electrolytes that possess moderate electrochemical stability, small ionic volumes, low viscosity, and hence high conductivity, the best performing ionic liquid tested being 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.

Ionic liquids (ILs) have been emerged as the most promising class of electrolytes to achieve high energy density in electrochemical double layer capacitors (EDLCs) due to their unique properties. In this study, 1-butyl-4-methylpyridinium tetrafluoroborate ([BMPy][BF4]) was explored as the electrolyte for graphene based EDLC in presence of co-solvent. Highly viscous [BMPy][BF4] was diluted with two different organic solvents, namely acetonitrile (AN) and propylene carbonate (PC). Different weight ratios of [BMPy][BF4]: organic solvents were investigated and corresponding variation of EDLC’s performance was observed. Dynamic viscosity of these IL+solvent mixtures was also measured. Three electrochemical techniques, namely cyclic voltammetry, galvanometric charge discharge, electrochemical impedance spectroscopy were employed to analyse the effectiveness of these electrolyte–electrode systems. Maximum operating voltage 2.2 V was achieved for the four out of six studied systems. For PC based electrolytes, no effect of concentration of PC on operating potential window was observed. However with AN, electrochemical stability decreased with increase in solvent weight percentage. The highest specific energy (49 Wh kg−1) and power (4.13 kW kg−1) were obtained for 3:1 weight ratio of IL to AN. Reverse effect on specific capacitance and internal resistance was noticed for AN and PC based electrolytes.

High temperature solid-state supercapacitor designed with ionogel electrolyte

The design of interfaces between nanostructured electrodes and advanced electrolytes is critical for realizing advanced electrochemical double-layer capacitors (EDLCs) that combine high charge-storage capacity, high-rate capability, and enhanced safety. Toward this goal, this work presents a novel and sustainable approach for fabricating ionogel-based electrodes using a renewed slurry casting method, in which the solvent is replaced by the ionic liquid (IL), namely 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIFSI). This method avoids time-consuming and costly electrolyte-filling steps by integrating the IL directly into the electrode during slurry preparation, while improving the rate capability of EDLCs based on pure non-flammable ILs. The resulting ionogel electrodes demonstrate exceptional electrolyte accessibility and enable the production of symmetric EDLCs with high energy density (over 30 Wh kg-1 based on electrode material weight) and high-rate performance. These EDLCs could operate at temperatures up to 180 °C, far exceeding the limitations of traditional EDLCs based on organic electrolytes (e. g., 1 M TEABF4 in acetonitrile, up to 65 °C). Ionogel-type EDLCs exhibit remarkable long-term stability, retaining 88 % specific capacity after 10000 galvanostatic charge/discharge cycles at 10 A g-1 and demonstrating superior retention compared to conventional EDLCs (50 %), while also maintaining 92.4 % energy density during 100 h floating tests at 2.7 V. These electrochemical properties highlight their potential for robust performance under demanding conditions. This study highpoints the practical potential of ionogel-based electrodes to advance IL-based EDLC technology, paving the way for next-generation energy storage devices with high-temperature and high-voltage operational capabilities.

Electrochemical capacitors operating in an aqueous electrolyte solution have become ever-more popular in recent years, mainly because they are cheap and ecofriendly. Additionally, aqueous electrolytes have a higher ionic conductivity than organic electrolytes and ionic liquids. These materials can exist in the form of a liquid or a solid (hydrogel). The latter form is a very promising alternative to liquid electrolytes because it is solid, which prevents electrolyte leakage. In our work, hydrogel polymer electrolytes (HPEs) were obtained via photopolymerization of a mixture of acrylic oligomer Exothane 108 with methacrylic acid (MAA) in ethanol, which was later replaced by electrolytes (1 M Na2SO4). Through the conducted research, the effects of the monomers ratio and the organic solvent concentration (ethanol) on the mechanical properties (tensile test), electrolyte sorption, and ionic conductivity were examined. Finally, hydrogel polymer electrolytes with high ionic conductivity (σ = 26.5 mS∙cm−1) and sufficient mechanical stability (σmax = 0.25 MPa, εmax = 20%) were tested using an AC/AC electrochemical double layer capacitor (EDLC). The electrochemical properties of the devices were investigated via cyclic voltammetry, galvanostatic charge/discharge, and impedance spectroscopy. The obtained results show the application potential of the obtained HPE in EDLC.

A rapidly growing interest in renewable energy sources requires not only developing efficient energy storage systems but also incorporating a greater number of eco-friendly components. Electrochemical double-layer capacitors (EDLCs) are a class of energy storage devices capable to store the electrical charge due to the separation of oppositely charged ions in the electrical field which results in the formation of an electrical double layer (EDL) at the electrode/electrolyte interface. EDLCs consist in general of two porous carbon-based electrodes pre-soaked with electrolyte and separated with a membrane (separator). The simple electrostatic mechanism of energy storage, coupled with a lack of chemical changes and faradaic transitions during operation, results in high electrical capacitance compared to classical capacitors, significantly higher power density in contrast to batteries, and practically unlimited life span. Currently, commercial EDLCs typically rely on organic solvents, such as acetonitrile or propylene carbonate, with the addition of ionically-conductive salts. However, there are several drawbacks when it comes to practical applications involving particular low conductivity, toxicity, flammability and high cost. This resulted in an increased interest in aqueous electrolytes such as KOH, H2SO4 or simple inorganic salts, which although they have a limited potential window, exhibit many positive features including higher ionic conductivity, lower viscosity, increased safety, lower cost and ease of assembly under ambient atmosphere. Modern and technologically advanced charge storage devices often require high safety flexible and deformable devices for specific applications. However, at the current state-of-the-art, the EDLCs suffer from two prominent limitations (i) the possibility of electrolyte leakage and (ii) high standards of technology to safely encapsulate electrolytes in the device. Therefore, a lot of research is held to develop alternatives for currently used liquid (aqueous and organic) electrolytes. One of the solutions to overcome these limitations are solid-state EDLCs. Those systems use an ionically-conductive polymer or hydrogel membrane, which serves as both the separator and the electrolyte. Cellulose, built of β-(1→4)-linked D-glucose units, is one of the most prevalent and easily degradable biopolymers. Albeit, its wide availability, biodegradability and low cost, the usage of cellulose is limited due to insolubility in most common solvents. The recent alternative, to toxic and flammable organic compounds, such as N, N- dimethylformamide/N2O4, N-methylmorpholine oxide (NMMO), are ionic liquids (ILs), that have been gaining lately a lot of attention in energy storage systems. Various ILs based on imidazolium, pyridinium and ammonium cation paired with strongly basic anion (e.g., OAc-, HCOO-) were also recently used to dissolve cellulose. However, the requirements of high-purity syntheses and the cost of some of the cations/anions may affect a large scale application. Therefore, our research refers to an alternative route of chemical regeneration of microcrystalline cellulose, i.e. its dissolution using an aqueous mixture of NaOH/urea, and further processing into a hydrogel membrane in the presence of cross-linking agent epichlorohydrin. To improve the mechanical strength and electrolyte uptake, in-situ polymerized norepinephrine and agarose were subsequently incorporated obtaining an interpenetrating polymer network (IPN). The structure and morphology of the membranes were characterized with SEM/EDX, CP/MAS 13C-NMR, AT-FTIR, TGA, contact angle, and elementary analysis. The ionic conductivity was determined using impedance spectroscopy over a wide range of temperatures (5-60°C). The relation between stress and strain in the materials was also determined to diagnose the mechanical properties. The cellulose-based hydrogel membranes were further used as a support for various aqueous electrolytes, including H2SO4, Na2SO4, i.e. most commonly used for aqueous EDLCs. Also, the alternative electrolyte was used, i.e. silicotungstic acid, H4SiW12O40 which according to our recent results seems to be a promising candidate to replace conventional acidic electrolytes [1]. The designed systems were compared, in terms of energy, power and cycleability, with their analogues using conventional polypropylene separators and a liquid electrolyte.[1] N.H. Wisinska, M. Skunik-Nuckowska, S. Dyjak, P.J. Kulesza, Factors affecting the performance of electrochemical capacitors operating in Keggin-type silicotungstic acid electrolyte, Appl. Surf. Sci. 530 (2020) 147273, https://doi.org/10.1016/j.apsusc.2020.147273Acknowledgement Financial support was provided by the National Science Center under Preludium 19 grant no. 2020/37//N/ST4/01679. This work was implemented as a part of Operational Project Knowledge Education Development 2014–2020 co-financed by the European Social Fund, Project No POWR.03.02.00-00-I007/16-00 (POWER 2014-2020)

INTRODUCTION Recently, the fast-growing population is a great concern for our modern generation. As the population rise more the need for clean and renewable energy will also follow for various types of portable electronic devices. Among different types of electrical energy storage devices, supercapacitors, also known as electrochemical capacitors have garnered huge attention due to their many advantages, such as long cycle life, high power density, cost-effectiveness, light weight, eco-friendly, broad operating temperature range, high rate capability and good cycling stability. According to the charge storage mechanism Supercapacitors are basically of two types, electrochemical double layer capacitors (EDLCs) and pseudocapacitors. Compared to the EDLC materials (mainly carbon based materials) the Pseudocapacitive materials (such as metal oxides and conducting polymers) exhibited much higher capacitance due to fast and reversible redox reactions1. This inspired the search for an extensive variety of electrode materials for pseudocapacitor. In general, transition metal oxides, such as RuO2, MnO2 , NiO, Co3O4, SnO2, TiO2, V2O5, CuO, Fe2O3, and WO3 etc., have exhibited high specific capacitance values with their redox reaction performances. But high resistivity of the metal oxides electrodes seems to be a great challenge for their practical applications as high performance energy storage devices. Mixing one metal oxide material with other metal oxide materials has been well established and proved as one of the best solutions to improve the electrochemical conductivity. Literatures said that electrochemical performance can be enormously improved by adjusting the morphology of the nanomaterials. Among various metal oxide composite materials, NiMn2O4 nanoparticle are very promising electrode materials for pseudocapacitors due to their low-cost, eco-friendly, good electrochemical properties, and high cyclic life. Herein, we have synthesized and optimized NiMn2O4 nanocomposites with Ni:Mn molar ratio (1:2) and the cycling performance of NiMn2O4electrode was examined giving maximum Specific capacitance value of 936.8 F/gm in 1M KCl electrolyte shown in figure-1 (b,c). EXPERIMENTAL/THEORETICAL STUDY The specific capacitance (Cm) for NiMn2O4 electrode material was determined from the CV profiles measured at scan rate 2 mV/s, following the equation Cm= i/ 2mν , where m and ν are the mass of the electroactive material and potential scan rate and current (i) is obtained by integrating the area of the curves as it is defined by eq.(1),i(v) = ∫ i(v)dv/ (vc - va) ...... (1), where va and vc are the lowest and highest voltage of the potential range. The galvanostatic charge-discharge (GCD) cycling curves also have a nearly symmetric shape for various current densities [0.4mA/cm2 – 1mA/cm2], indicating that the composite has a good electrochemical capacitive characteristic and superior capacitive retention. RESULTS AND DISCUSSION NiMn2O4 nanoparticle was successfully synthesized by chemical process.Nanostructure studies were carried out using XRD and FTIR spectroscopy. The formation of the nanoparticle was inferred by field emission scanning electron microscopy (FESEM) and the average crystallite size has been observed 40 nm. The electrochemical response of the NiMn2O4electrodes, as an electrode material for supercapacitors, was found to be improved to a great extent. The maximum specific capacitance 936.8F/g is achieved in 1M KCl solution in 2mV/s scan rate with molar ratio of Ni/Mn 1:2. CONCLUSION The composite exhibits high Specific capacitance value and good cycling stability up to 5000 cycles (not shown in fig.). Hence, the synthesized NiMn2O4nanopartice was found to be the suitable promissing electrode material for energy storage applications. REFERENCES B.E. Conway, Electrochim. Acta, 38, 1249 (1993). ACKNOWLEDGMENTS Author Apurba Ray is grateful to the Department of Science and Technology (D.S.T),INSPIRE, Govt. of India for financial support. Figure 1

Our modern and technological society requests enhanced energy storage devices to tackle the current necessities. In addition, wearable electronic devices are being demanding because they offer many facilities to the person wearing it. In this manuscript, a historical review is made about the available energy storage devices focusing on super-capacitors and lithium-ion batteries, since they currently are the most present in the industry, and the possible polymeric materials suitable on wearable energy storage devices. Polymers are a suitable option because they not only possess remarkable mechanical resistance, flexibility, long life-times, easy manufacturing techniques and low cost in addition to they can be environmentally friendly, nontoxic, and even biodegradable too. Moreover, the electrical and electrochemical polymer properties can be tunning with suitable fillers giving to versatile conducting polymer composites with a good cost and properties' ratio. Although the advances are promising, there are still many drawbacks that need to be overcome. Future research should focus on improving both the performance of materials and their processability on an industrial scale, where additive manufacturing offers many possibilities. The sustainability of new energy storage devices should not