nanomaterials Article Sustainable and Printable Nanocellulose-Based Ionogels as Gel Polymer Electrolytes for Supercapacitors Rosa M. González-Gil 1,2,† , Mateu Borràs 3,4,† , Aiman Chbani 3 , Tiffany Abitbol 5 , Andreas Fall 5 , Christian Aulin 5,6 , Christophe Aucher 3, * and Sandra Martínez-Crespiera 1, * 1 Applied Chemistry and Materials, ARTS Department, Leitat Technological Center, C/Pallars, 186-179, 08005 Barcelona, Spain; rosamaria.gonzalez@icn2.cat 2 Novel Energy-Oriented Materials Group, Catalan Institute of Nanoscience and Nanotechnology, ICN2 (CSIC-BIST), Edifici ICN2, Campus UAB, 08193 Barcelona, Spain 3 Energy and Engineering, ARTS Department, Leitat Technological Center, C/de la Innovació, 2, 08225 Barcelona, Spain; mateu@bioo.tech (M.B.); achbani@leitat.org (A.C.) 4 Arkyne Technologies SL (Bioo), C/de la Tecnología, 17, 08840 Barcelona, Spain 5 Bioeconomy and Health, RISE Research Institutes of Sweden, Drottning Kristinas väg 61, 114 28 Stock-holm, Sweden; tiffany.abitbol@ri.se (T.A.); andreas.fall@ri.se (A.F.); Christian.aulin@Holmen.com (C.A.) 6 Holmen Iggesund, 825 80 Iggesund, Sweden * Correspondence: caucher@leitat.org (C.A.); sandramartinez@leitat.org (S.M.-C.); Tel.: +34-93-788-23-00 (S.M.-C.) † These authors contributed equally to this work.  Abstract: A new gel polymer electrolyte (GPE) based supercapacitor with an ionic conductivity up to  0.32–0.94 mS cm−2 has been synthesized from a mixture of an ionic liquid (IL) with nanocellulose Citation: González-Gil, R.M.; Borràs, (NC). The new NC-ionogel was prepared by combining the IL 1-ethyl-3-methylimidazolium dimethyl M.; Chbani, A.; Abitbol, T.; Fall, A.; phosphate (EMIMP) with carboxymethylated cellulose nanofibers (CNFc) at different ratios (CNFc Aulin, C.; Aucher, C.; Martínez-Crespiera, S. Sustainable ratio from 1 to 4). The addition of CNFc improved the ionogel properties to become easily printable and Printable Nanocellulose-Based onto the electrode surface. The new GPE based supercapacitor cell showed good electrochemical Ionogels as Gel Polymer Electrolytes performance with specific capacitance of 160 F g−1 and an equivalent series resistance (ESR) of for Supercapacitors. Nanomaterials 10.2 Ω cm−2 at a current density of 1 mA cm−2 . The accessibility to the full capacitance of the device 2022, 12, 273. https://doi.org/ is demonstrated after the addition of CNFc in EMIMP compared to the pristine EMIMP (99 F g−1 10.3390/nano12020273 and 14.7 Ω cm−2 ). Academic Editors: Federico Cesano, Simas Rackauskas and Mohammed Keywords: nanocellulose; ionic liquid; ionogel; gel polymer electrolyte; renewable energy storage; Jsaim Uddin supercapacitors Received: 22 December 2021 Accepted: 12 January 2022 Published: 15 January 2022 1. Introduction Publisher’s Note: MDPI stays neutral From all the different types of energy storage systems (ESS), the interest in superca- with regard to jurisdictional claims in pacitors (SCs) has grown thanks to their energy storage mechanism by electrical double published maps and institutional affil- layer, which increases cycle life and allows the use of safer ionic conductors and electrode iations. materials than in traditional batteries [1]. Their promising properties such as high-power density, fast charge–discharge rate and long-cycle stability, have turned SCs into a poten- tially useful tool for application in flexible and wearable electronics [2–4]. In this sense, Copyright: © 2022 by the authors. the use of these quasi-solid electrolytes could play an important role in the production Licensee MDPI, Basel, Switzerland. of printed supercapacitors providing flexibility, transparency, stability, and safety to the This article is an open access article devices [5]. distributed under the terms and Different types of electrolytes can be used for ESS, including liquid, solid state elec- conditions of the Creative Commons trolytes (SSEs) [6] and gel polymer electrolytes (GPEs). GPEs consist basically of a liq- Attribution (CC BY) license (https:// uid electrolyte, that can be an ionic liquid (IL), or an ionic salt trapped inside a three- creativecommons.org/licenses/by/ dimensional (3D) polymer network [7,8]. ILs, generally a room temperature molten salt, 4.0/). Nanomaterials 2022, 12, 273. https://doi.org/10.3390/nano12020273 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2022, 12, 273 2 of 14 are usually used in GPEs to improve the properties of the ESSs, because of their great elec- trochemical stability over a wide electrochemical window with minimum decomposition, high ionic conductivity, non-volatility, non-flammability, and good thermal and chemical stabilities [9,10]. Moreover, ionogel electrolytes (IL based gels) present interesting advan- tages against liquid leakage and corrosion problems, increasing electrochemical stability, and facilitating packaging and manufacturing processes, due to both their liquid-like ionic conductivity and solid-like structural and mechanical features with great flexibility [10]. Recently, in response to an increased demand concerning the sustainable and cost- effective development of electrochemical energy storage devices, great efforts are being devoted to replacing the existing fossil energy resources by new ESSs based on green sus- tainable materials. As a class of green materials, nanocellulose (NC) has received extensive attention due to its natural abundance, recyclability, facile and diverse modification, large specific area, and dimensional stability [11,12]. NC can be classified into the following cate- gories depending on the source, preparation methods and fiber morphology: (1) cellulose nanofibers (CNF), which refers to the nanoscale cellulose fibrils obtained by fibrillating cellulose under the action of high pressure or mechanical forces, (2) cellulose nanocrystals (CNC) obtained by acid hydrolysis conditions to form nano-sized, rod shaped particles with high crystallinity, and (3) bacterial nanocellulose (BC) with a high aspect ratio obtained by a microbial fermentation process [12,13]. Compared with other biomass-derived green materials, such as lignin, chitin, etc., NC shows great advantages in the field of ESSs [14–20]: versatile surface modification via functionalization of its hydroxyl groups that allow the integration of other active materials; great thermal stability and extraordinary mechanical properties that can improve the safety properties of the ESSs, through formation of the previous mentioned gel polymer electrolytes (GPEs) [20]. Despite the NC advantages described above, the processing of cellulose-based ma- terials for practical application in energy storage fields, like GPEs, is limited due to their highly ordered structure and crystallinity. However, nanocellulose can be solubilized in some ionic liquids by the disruption of the hydrogen bonding network and the for- mation of new strong hydrogen bonds with the IL anions [21]. Hence, ILs anions play a predominant role during this process, different than cations with weaker interactions (van der Waals and/or electrostatic interactions) [21,22]. In this way, the use of NC or other sources of cellulose as polymeric matrix for ionogel preparation has been studied.to develop green and biofriendly electrolyte alternatives to fluorinated polymers. Thiemann et al., synthesized an ionic liquid GPE by gelation of microcellulose thin films with 1-ethyl- 3-methylimidazolium methylphosphonate [23]. These cellulose ionogels showed high specific capacitances (5–15 µF·cm−2 ), high transparency and flexibility. In another example, a cellulose-based dual network ionogel electrolyte was synthesized by Rana et al. by dissolving phosphorylated microcrystalline cellulose in 1,3-dimethylimidazolium methyl phosphate [DMIM][MeO(H)PO3 ] followed by post-polymerization. The as-synthesized ionogel electrolyte exhibited a high ionic conductivity (2.6–22.4 mS·cm−1 ) over a wide temperature range (up to 120 ◦ C) [22]. In this work, we have worked with 1-ethyl-3-methylimidazolium dimethyl phosphate (EMIMP) as the ionic liquid because of the well-known ability of phosphates, among other anions, to dissolve carbohydrates [21,22]. While ionogels from natural cellulose or microcel- lulose have already been obtained easily [21–24], we selected carboxymethylated cellulose nanofibers (CNFc) for our studies due to its superior properties: better mechanical perfor- mance, and processability; together with the presence of carboxylic groups on the surface, which can boost the strong inter- and intra- molecular interactions inside the resulting ionogel [25]. We present the combination of the EMIMP with CNFc as hosting renewable polymer to provide an easy-to-prepare transparent ionogel electrolyte with exceptional thermal and electrochemical capabilities. For that purpose, four different ratios of CNFc to IL were studied and tested in a coin-cell supercapacitor device as electrolyte (CNFc ratio from 1 to 4) to observe the change in electrochemical performance. Physicochemical characterizations (composition, morphology, rheology, and thermal stability) and electro- Nanomaterials 2022, 12, 273 3 of 14 chemical characterizations (capacity, ionic conductivity and cyclability) were performed to investigate its properties and understand the functionality of the novel nanocellulose-based ionogel electrolytes. 2. Materials and Methods 2.1. Materials Carboxymethylated cellulose nanofibers were produced by RISE Research Institutes of Sweden from a commercial sulfite softwood pulp (Domsjö Dissolving plus; Domsjö Fabriker AB, Domsjö, Sweden). Ethanol (Rectapur® ) was purchased from VWR (VWR AB International, Spånga, Sweden). Monochloroacetic acid (99%, ACS reagent, ClCH2 COOH), acetic acid (ACS reagent), iso-propanol (ACS reagent), sodium hydroxide (ACS reagent), sodium hydrogen carbonate (ACS reagent) and methanol (ACS reagent) were purchased from Sigma Aldrich (Stockholm, Sweden). 1-ethyl-3-methylimidazolium dimethyl phos- phate (>98% grade) (EMIMP) was purchased at Iolitec (Heilbronn, Germany) and used as received. Active Carbon YP50F (Kuraray, Kurashiki, Japan), conductive carbon TIMCAL C 65 (Cabot), 1-Methyl-2-pyrrolidone (NMP, Scharlau, Sant Feliu del Llobregat, Spain) and 5130 PVDF Solef (Solvay, Barcelona, Spain) were used in the formulation of the active layers of the supercapacitors. The fabricated electrodes were assembled in CR2032 coin cells from MTI by using cellulose separator Kodoshi (Nippon Kodoshi Corporation, Kochi, Japan) and 1-ethyl-3-methylimidazolium dimethyl phosphate (EMIMP, >98%, Iolitec) or EMIMP:CNFc mixtures (96:4 to 99:1 in ratio) as electrolyte. 5 samples electrolytes are reported including EMIMP pure electrolyte and 4 mixtures adding 1%, 2%, 3% and 4% of CNF in EMIMP ionic liquid. It has not been possible to produced jelly electrolyte with higher rate of CNF due to the loss of the mechanical properties not allowing to print and process the component. The preparation of the electrode is identic for each symmetric coin cell and described in Section 2.3. Each electrode is prepared by coating of the jelly electrolyte directly on the top of the dried carbon electrode and then the CR2032 coin cell is assembled with staking 2 symmetric electrode separated by Kodoshi nanocellulose separator (thickness 50 µm) without further addition of electrolyte or solvent. 2.2. Carboxymethylated Cellulose Nanofibers (CNFc) Fabrication CNFc fabrication was done according to the general methodology described in the literature [26]. Most commonly, CNFc is prepared with a total charge of ~600 µeq/g, corresponding to a carboxmethyl degree of substitution (DS) of 0.1 [27]. In this work, a higher charge DS 0.3 CNFc was produced as has been recently described [28]. Generally, resulted pulp is solvent exchanged to ethanol followed by carboxymethylation reaction, which begins by impregnating the pulp with a solution of monochloric acetic acid in isopropanol, followed by transfer of the pulp to a heated methanolic solution of NaOH in isopropanol and reaction under reflux for 1 h. Compared to the reaction conditions for DS 0.1, to achieve DS 0.3, the dosage of monochloroacetic acid is increased 4.4-fold and NaOH 2.7-fold. After reaction, the carboxymethylated pulp is washed with NaHCO3 to neutralize, and finally washed with water to remove excess reagents, until the wash has a conductivity below 10 µS/cm. To generate CNFc, the carboxymethylated pulp is microfluidized (Microfluidizer M-110EH, Microfluidics Corp., Newton, MA, USA) at 1700 bar (1 pass) to give a suspension of DS 0.3 CNFc at 1 wt%. The CNFc was then subjected to lyophilization and morturation before use in Leitat (Barcelona, Spain). 2.3. CNFc Based Ionogel Preparation In Figure 1 it is shown a scheme about the NC-ionogel preparation process. For this, freeze-dried CNFc from an aqueous dispersion (at 3 wt%) was mechanically mixed with different ratios of EMIMP (p/p, EMIMP/CNFc: 99:1, 98:2, 97:3 and 96:4, referred as EMIMP/CNFc 99:1, EMIMP/CNFc 98:2, EMIMP/CNFc 97:3 and EMIMP/CNFc 96:4) with heating (100 ◦ C) for 16 h to obtain a yellowish viscous gel (final appearance pictures are depicted in Figure S1 from Supplementary Materials). The as prepared ionogel could 22, 11, x FOR PEER REVIEW 4 of 15 different ratios of EMIMP (p/p, EMIMP/CNFc: 99:1, 98:2, 97:3 and 96:4, referred as EM- IMP/CNFc 99:1, EMIMP/CNFc 98:2, EMIMP/CNFc 97:3 and EMIMP/CNFc 96:4) with heating different Nanomaterials (100 of ratios 2022, 12, 273 °C)EMIMP for 16 h to obtain (p/p, a yellowish EMIMP/CNFc: 99:1,viscous geland 98:2, 97:3 (final appearance 96:4, referred aspictures EM- are 4 of 14 depicted in Figure S1 from Supplementary Materials). IMP/CNFc 99:1, EMIMP/CNFc 98:2, EMIMP/CNFc 97:3 and EMIMP/CNFc 96:4) with The as prepared ionogel could be used without heating (100 °C) any additional for 16 step h to obtain in ambient a yellowish air, showing viscous gel (finallong-term appearancestability since pictures areno de- stabilization depicted phenomena, in Figure S1 befrom like phase-separation, usedSupplementary without Materials). any additional have step inbeen The observed as prepared ambient after approx. ionogel air, showing could 2 years. be long-term stability since used without any additional step in ambient no destabilization air, showing phenomena, long-term stability like phase-separation, havesince been no de- observed after approx. stabilization phenomena, like phase-separation, have been observed after approx. 2 years. 2 years. Figure 1. Schematic preparation of CNFc-based ionogels. Figure 1. Schematic preparation of CNFc-based Figure 1. Schematic ionogels. preparation of CNFc-based ionogels. 2.4. Supercapacitor Preparation 2.4. Supercapacitor Preparation The active Preparation ink used in the supercapacitor application was fabricated by mixing NMP, 2.4. Supercapacitor active carboninkYP50F, The active inkcarbon conductive used inTiMCALC65 the supercapacitor application andfabricated binder PVDF was atfabricated dryNMP, weightby mixing ra- NMP, The active used in the supercapacitor application was by mixing tio of 80:10:10 active carbon by conductive YP50F, using a Dipermat conductive carbon (VMA-GETZMANN TiMCALC65 and binder PVDF at dry weight ratio active carbon YP50F, of 80:10:10 by carbon using a TiMCALC65 Dipermat and binderGmbH, (VMA-GETZMANN atReischsof, PVDF GmbH, dry weight Germany) ra- Germany) with Reischsof, tiowith a 3D printed of 80:10:10 by using head a 3D a reproducing Dipermat printed the shape of an industrial head(VMA-GETZMANN reproducing the shapeGmbH, planetary Reischsof, of an industrial Germany) planetary mixer but mixer but adapted adapted for lower with a 3D printed for volume head of reproducingink. The prepared ink was printed onto a 20 μm aluminum lower volume of the ink.shape of an industrial The prepared planetary ink was printed ontomixer a 20 µm butaluminum foil by foil byfor adapted bare coating lower volume bare(gap ofofink. coating 100 μm) The (gap andµm) dried ofprepared 100 inkfor and was2 hprinted dried at 2120 for h at°C in◦avacuum onto 120 C20inμm obtaining aluminum vacuum obtaininga dry a dry coating coating of 50 μm thickness foil by bare coating (gap of 50of µm and active 100thickness μm) andand material dried loading for 2material active of 1.8 h at 120loading mg °C in vacuum cm −2. For the of 1.8 mg obtaining− 2 cm . For CNFc-iono- a drythe CNFc-ionogel gel printing, coating of 50 μmheating thickness to and printing, 80–100 °Ctomaterial active heating allowed Cdirect 80–100 ◦ loading printing allowed ofdirect 1.8 mg using cm−2.aFor printing Doctor Blade Blade theaCNFc-iono- using Doctor technique technique (gap gel(gap of 150heating printing, μm bar) onto ofto150 µmabar) 80–100 carbon ontoelectrode °C allowed surface printingas directelectrode a carbon shown using surface in Figure aasDoctor shown Blade 2. technique in Figure 2. (gap of 150 μm bar) onto a carbon electrode surface as shown in Figure 2. 2. Directby Figureprocess Figure 2. Direct printing printing Doctorprocess Bladeby Doctor Blade technique technique of the of the CNFc-ionogel CNFc-ionogel onto the electrodes. onto the electrodes. Figure 2. Direct printing process by Doctor Blade technique of the CNFc-ionogel onto the electrodes. 2.5. Characterization 2.5. Characterization 2.5. Characterization X-ray diffraction (XRD) patterns were recorded for each sample on a diffractome- X-ray diffraction ter (XRD) (Malvern X-ray diffraction (XRD) patterns werewere PANalytical patterns recorded X’pert recorded Pro for each for MPD, each X-ray sample sample on a Facility, Diffraction diffractometer on a diffractometer Institut Català de (Malvern PANalytical X’pert Nanociència i Pro MPD, Nanotecnologia) X-Ray at room Diffraction temperature (Malvern PANalytical X’pert Pro MPD, X-Ray Diffraction Facility, Institut Català de Facility, from 10 Institut to 50 2θ.Català Fourier detransform Nanociència infrared spectroscopy i Nanotecnologia) at room were recorded temperature with a Shimadzu Iraffinity-1S spectrometer in- in ATR Nanociència i Nanotecnologia) at room temperature fromfrom10 to 1050 to 2Ɵ.50Fourier 2Ɵ. Fourier transform transform in- −1 frared mode. spectroscopy The were specimens recorded were measured directly with a scan range from in ATR4000 cm . 400 to frared spectroscopy were recorded withwith a Shimadzu a Shimadzu Iraffinity-1S Iraffinity-1S spectrometer spectrometer in ATR The thermogravimetric analysis (TGA) of the samples (5–10 mg) were carried out by using mode. mode. TheThe specimens specimens werewere measured measured directly directly with with a a scan scan range range from from 400 to 400 4000tocm4000 −1. cm−1. a TGA/DSC (Mettler-Toledo, University of Alicante, Alicante, Spain), equipped with au- TheThe thermogravimetric thermogravimetric analysis analysis (TGA)(TGA) ofsamples of the the samples (5–10 mg) carried were carried byout by using tosampler and gas-controlled unit, (5–10 mg) were from room temperature out to 900 ◦using C with heating rate of a TGA/DSC a TGA/DSC (Mettler-Toledo, (Mettler-Toledo, University 20 ◦ C/minUniversity under N2 of of Alicante, Alicante, conditions. Alicante, Alicante, Scanning Spain), ElectronSpain), equippedequipped Microscopy with (SEM) with au- wereau-performed tosampler tosampler andand gas-controlled in a Zeiss Merlin FE-SEM (Carl Zeiss Microscopy, Autonomous University ofofBarcelona, gas-controlled unit,unit, fromfrom room room temperature temperature to 900 to°C900 °C with with heatingheating rate of rate 20 20 °C/min °C/min under under N2 conditions. N2 Bellaterra, conditions. Scanning Scanning Spain) Electron Electron with samples Microscopy Microscopy mounted (SEM) directly on(SEM) werewith werestubs SEM performed performed in adhesive in carbon tape and metalized with Au thin film (E5000 Sputter Coater. Polaroid Equipment Limited. Wat- Nanomaterials 2022, 12, 273 5 of 14 ford). Images were recorded at different steps of magnification. The rheologic properties of EMIMP and all fabricated electrolytes were analyzed by using the rheometer Bohlin CVO 100–901 in the share rate range from 0.0517 to 888 s−1 . The electrochemical testing was performed with a multichannel potentiostat VMP3 (Bio-Logic) run with the EC-Lab software. Cyclic voltammetry was performed between 0 < V < 2.0, and with a scan rate between 10 < mV s−1 < 50, to investigate side reactions, evaluate the potential window of stability and the device behavior. Galvanostatic measure- ments at a constant areal capacity of mA·cm−2 and between 0 < V < 2.0 V) were used to calculate the specific capacity (mAh·g−1 of AC, energy (unit), power (unit), and equivalent series resistance (ESR) is calculated from the ohmic drop between of the developed su- percapacitors. Electrochemical impedance spectrum measurements were carried out with frequency range of 50 mHz to 1 MHz. All these analyses were carried out by VMP3 from Biologic. Finally, a long cycling at 1 mA·s−1 for 10,000 cycles were carried out by Neware cycler. The electrochemical CR2032 coin-cell type used for the tests is built from the stack of two symmetric electrodes separated by cellulose membrane separators and with 40 µL of EMIMP or 150 µm (wet) bar-coated EMIMP:CNFc gel as electrolyte ensuring it excess. The cell capacitance (Ccell ) of a symmetrical capacitor can be calculated according to Equation (1) [29]: Ce Ccell = ; Ce = 2Ccell (1) 2 where Ce = C+ = C− are the specific capacitance of the two electrodes in mAh·g−1 of activated carbon, and the cell capacitance of a supercapacitor can be calculated from the charge-discharge experiment Equation (2): I Ccell = (2) mdV/dt This can be then used, when combined with Ccell equation to obtain the electrode capacity, and subsequently obtain information on the capacity of an investigated material Equation (3): 4I Ce = (3) mdV/dt where I (A) is the discharge current, m (g) the total mass of active materials of two electrodes and dV/dt the slope of the discharge curve between 80% and 40% of the cut-off voltage (V). From the total specific cell capacitance of two-electrode system (Ccell ), the maximum energy (Emax. ) and maximum power (Pmax. ) of a supercapacitor cell can be calculated according to Equations (4) and (5): 1 Emax. = Ccell V 2 (4) 2 V2 Pmax. = (5) 4ESR where, V (V) is the cell voltage minus the ohmic drop, and ESR (Ω) is the total equivalent series resistance of the supercapacitor. From the EIS, the ionic conductivity of the different gels has been calculated using the following formula Equation (6): l σ= (6) R1 A where σ is the ionic conductivity (S m−1 ), l (m) is the thickness of the membrane, A (m2 ) is the contact area between the electrode and the electrolyte and R1 (Ω) is the ionic resistance at high frequency corresponding to the first interception of the circle in the Nyquist plots. 3. Results and Discussion An extensive characterization of the prepared CNFc-ionogels have been realized to study their composition, homogeneity, thermal and rheological properties. XRD patterns 3. Results and Discussion An extensive characterization of the prepared CNFc-ionogels have been realized to study their composition, homogeneity, thermal and rheological properties. XRD patterns Nanomaterials 2022, 12, 273 for each IL/CNFc ratio, pristine CNFc (lyophilized) and EMIMP are shown in Figure 3a.6 of 14 Broad signals are obtained in all cases, obtaining major intensities at 12° and 25° for all the EMIMP/CNFc mixtures, which are related to the ionic liquid EMIMP (triangle signals), for each IL/CNFc ratio, pristine CNFc (lyophilized) and EMIMP are shown in Figure 3a. with a minor signal Broadpresent as obtained signals are a broadinband at 15° all cases, corresponding obtaining to (002) major intensities nanocellulose at 12 ◦ and 25◦ for all the crystalline planeEMIMP/CNFc (star signals).mixtures, The major which are related to the ionic liquid EMIMPa (triangle component of ionic liquid ensures good ionic signals), conductivity and witha good a minor gel flexibility signal present due to their as a broad bandmainly ◦ amorphous nature. at 15 corresponding Fourier to (002) nanocellulose crystalline transform infrared plane (star spectroscopy signals). (FTIR) The major of each componentwere CNFc-ionogel of ionic alsoliquid ensuresand recorded a goodareionic conductivity and a good gel flexibility due to their depicted in Figure 3b, where the IR profiles of pure CNFc and EMIMP are also added. As mainly amorphous nature. Fourier transform infrared spectroscopy (FTIR) of each CNFc-ionogel were also recorded and are in case of previous XRD studies, the EMIMP has the major intensity, showing its typical depicted in Figure 3b, where the IR profiles of pure CNFc and EMIMP are also added. bands of -CH fromAs inthe case imidazolium of previous XRD ringstudies, (3060 and 1570 cmhas the EMIMP −1), the -CH2 st and -CH3 st the major intensity, showing its (2920–2970 cm−1typical ), -CH3bandsbendof(1450 cm−1)the -CH from and -CH2 bendring imidazolium (780 cm−1 (3060 ) from and 1570 the cm−ethyl 1 ), thechains, -CH2 st and the -C-N st at 1240 -CHcm st−1 and finally, (2920–2970 cm − 1 the ), -P-O-C -CH (930–1050 bend (1450 cm − cm 1 −1 ) ) and and -CH-P=O (1090–1200 bend (780 cm −1cm −1) the ) from 3 3 2 − 1 − 1 vibrations of theethyl chains, the -C-Ngroup. diethylphosphate st at 1240 CNFccm (highlighted and finally, the in-P-O-C (930–1050 blue) shows an cm -OH)bandand -P=O (1090–1200 cm −1 ) vibrations of the diethylphosphate group. CNFc (highlighted in blue) at 3400 cm−1, which increases with the higher ratio of CNFc in the mixtures, a -C=O peak shows an -OH band at 3400 cm−1 , which increases with the higher ratio of CNFc in the related with the carboxylated groups at 1600 cm−1, which appears slightly shifted to 1650 mixtures, a -C=O peak related with the carboxylated groups at 1600 cm−1 , which appears cm for all ionogel −1 slightlymixtures, shifted toand 1650acm -C-O-C −1 for allstreaching at 1000 ionogel mixtures, andcm −1 fromstreaching a -C-O-C CNFc glucose at 1000 cm−1 units, which canfromnotCNFc be appreciated glucose units, inwhich the mixtures can not bedue to overlaping appreciated with EMIMP in the mixtures P=O due to overlaping and P-O-C signals. with EMIMP P=O and P-O-C signals. (a) (b) (c) (d) Figure 3. (a) XRD; (b) FTIR; (c) TGA and (d) Rheological data from CNFc-EMIMP GPE, including pristine CNFc (gel form at 1%wt) and EMIMP. Nanomaterials 2022, 12, 273 7 of 14 Thermal properties (TGA) were studied for each CNFc-ionogel and are depicted in Figure 3c. The TGA profiles shown in all cases a two-step weight loss at temperatures below 100 ◦ C and above 250 ◦ C, which can be reasonably attributed to evaporation of adsorbed water (less than a 15% wt) and the degradation of CNFc and the EMIMP, respectively. This data shows the high thermal stability of the new gel electrolyte. DSC experiments have been also studied to determine the degradation temperatures for each ionogel in comparison to pure EMIMP (Figure S2 and Table S1, Supplementary Materials), obtaining similar results between them (290 ◦ C for EMIMP, to 293 ◦ C for EMIMP/CNFc 96:4). The addition of smalls amounts of nanocellulose has a minor affection on the decomposition temperatures, improving them slightly. ILs are mostly Newtonian fluids and more viscous than most common molecular solvents [30]. Their ionogels show similar viscoelastic properties under stress, providing a higher-quality electrode/electrolyte contact, in comparison with other rigid solid-state electrolytes [10,31]. In the case of this work, viscosity measurements (Figure S3, Supplementary Materials) reveal that almost all the prepared NC-ionogels have similar viscosity characteristics, except for the mixture EMIMP/CNFc 96:4, that reveals a thixotropic behavior, typical of NC. Regarding the apparent viscosity dependence of the applied shear stress (Figure 3d), all NC-ionogels, and CNFc (1% wt, gel), show a reduction in the viscosity, which is more pronounced in the ionogels with a higher CNFc content (EMIMP/CNFc 96:4 and EMIMP/CNFc 97:3), with no changes in case of pure EMIMP. This characteristic, named shear-thinning [32], is common of pseudoplastic fluids and consents on a more effective particles flow through the liquid phase, reducing the viscosity of the solution. It is intrinsic of the nanocellulose and its derivatives [33,34], and it is responsible for the good printability of the presented ionogels, while the desired mechanical properties were maintained. High resolution scanning electronic microscopy (FE-SEM) was done for the ionogel EMIMP/CNFc 96:4 to study its morphology, showing a smooth and homogeneous surface. Moreover, an EDS mapping of nitrogen, phosphorous and sodium elements that are present Nanomaterials 2022, 11, x FOR PEER REVIEW in the ionic liquid (N and P) and the nanocellulose (Na) was also performed to confirm 8 of 15 the homogeneity of the GPE. (See Figure 4). Figure 4. FE-SEM Figure image 4. FE-SEM of EMIMP/CNFc image 96:496:4 of EMIMP/CNFc gel gel andand its EDS mapping its EDS onon mapping N,N, NaNa and P elements. and P elements. In the same way, to use these NC-based ionogels as electrolytes for supercapacitors, their electrochemical performance and properties were first evaluated. Symmetric cells were assembled in CR2032 coin cell to test the electrochemical performance of the EMIMP electrolyte, alone and with the different CNFc ratios, using a cellulosic Kodoshi separator. Nanomaterials 2022, 12, 273 8 of 14 In the same way, to use these NC-based ionogels as electrolytes for supercapacitors, their electrochemical performance and properties were first evaluated. Symmetric cells were assembled in CR2032 coin cell to test the electrochemical performance of the EMIMP electrolyte, alone and with the different CNFc ratios, using a cellulosic Kodoshi separator. The cyclic voltammogram at 10 mV·s−1 (See Figure 5a) of the supercapacitor with EMIMP electrolyte shows a near rectangle shape, approximated to the ideal situation of double layer capacitor (or supercapacitor). There is no visible redox peak from Faradaic current over the potential region. However, a peak onset appears as 2 V is approached. At high ials 2022, 11, x FOR PEER REVIEW scan rates (See Figure 5b), the shape of the CV curve was distorted due 9toofan15increase of the internal resistance and the peak onset near 2 V was shifted to higher voltage, due to the increase in cell resistance at higher scan rate. Figure 5. CV (cyclic voltammetry) Figure 5. CV (cycliccurves at (a) 10 mV voltammetry) s−1; (b) curves 50 10 at (a) mV s−1 sscan mV rates −1 ; (b) 50 for mVeach EM- rates for each s−1 scan IMP/CNFc ionogel (99:1, 98:2, 97:3 and 96:4). Pristine EMIMP is also added for comparison. EMIMP/CNFc ionogel (99:1, 98:2, 97:3 and 96:4). Pristine EMIMP is also added for comparison. In order to clarifyWhenif the CNFcpeak onset observed is added to the EMIMP in theionic cyclic voltammetry liquid, the capacitywas ofcaused the device increases with by the electrolyte the addition degradation or of CNFc, of because theitspeak ownonset behaviornear (as 2 Vhas in the oxidation been zonein seen before becomes more evident and different IL electrolytes), [35]a new one between a floating test was0carried and 0.5out. V in[36] the The reduction zone appears supercapacitors (Figure 5a). At with high scan the different studied rates (Figure electrolytes were5b), the shape charged of the at 2V in aCV curve was stepwise rampdistorted and the due to an increase of the voltage was maintainedinternal resistance for a period of 24but h, to aslesser shown extent than Figure S4with from EMIMP alone and the Supplementary peaks onset near 2 V Materials. The leakage current obtained at each time was recorded and could be used to directlyand resistance and between 0 and 0.5 V disappear. However, no clear differences in capacity betweenofthe assess the importance theelectrolytes electrochemicalwith different ratios of degradation ofEMIMP/CNFc the electrolytes caninbethe appreciated fully from the cyclic voltammetry studies. charged state. The results showed that the leakage current of the EMIMP alone was 6 times higher than the one In order to clarify if the peak onset observed in the cyclic voltammetry was caused obtained by adding CNFc (i.e., 120 μA vs 20 μA). The data obtained by the electrolyte degradation or because of its own behavior (as has been seen before for the different EMIMP/CNFc electrolytes were low enough to determine that the peak in different IL electrolytes) [35], a floating test was carried out [36]. The supercapacitors onset showed in the cyclic voltammogram was not caused by electrolyte degradation. [37] with the different studied electrolytes were charged at 2V in a stepwise ramp and the Moreover, these results demonstrated that the addition of CNFc increase the EMIMP volt- voltage was maintained for a period of 24 h, as shown Figure S4 from Supplementary age resistance. The capacity Materials. Theand resistance leakage of the current supercapacitors obtained at each time withwasEMIMP recordedandand withcould be used the gel electrolytes (by the introduction of different rates of CNFc in the EMIMP) to directly assess the importance of the electrochemical degradation of the electrolytes in were evaluated by galvanostatic the fully chargedcharge-discharge state. The results tests.showed All these thatparameters the leakagewere calculated current of the EMIMP alone using the equations described in Section 2 and the results are showed was 6 times higher than the one obtained by adding CNFc (i.e., 120 µA in Table S1 in Sup- vs 20 µA). The plementary Materials and Figure data obtained 6. In for the the caseEMIMP/CNFc different of capacities calculated electrolytes from were cyclic low voltam- enough to determine metry, as shownthat Table the S4 peakfrom Supplementary onset showed in the Materials, the capacitance cyclic voltammogram tendency was not causedin allby electrolyte cases is decreasing at higher[37]. degradation scanMoreover, rate. Higher specific these resultscapacity demonstratedis measured that theafter addition addition of CNFc increase of CNFc, whichthe EMIMP is online withvoltage resistance. galvanostatic The capacity and charge-discharge resistance results. of the supercapacitors with The supercapacitors with CNFc in the EMIMP and with electrolyte exhibittheagel electrolytes smaller ESR in(by all the introduction intensity ranges ofthandifferent rates of CNFc in the super- capacitors with only EMIMP (4.0–10.2 Ω for different rates of EMIMP:CNFc and 14.7 Ω for EMIMP alone). These results also indicate that EMIMP/CNFc gel electrolytes (in all the studied ranges) have higher ionic conductivity than EMIMP alone, and this has been con- firmed by the electrochemical impedance spectroscopy showed in Figure 7. In addition, Nanomaterials 2022, 12, 273 9 of 14 the EMIMP) were evaluated by galvanostatic charge-discharge tests. All these parameters were calculated using the equations described in Section 2 and the results are showed in Table S1 in Supplementary Materials and Figure 6. In the case of capacities calculated from cyclic voltammetry, as shown Table S4 from Supplementary Materials, the capacitance tendency in all cases is decreasing at higher scan rate. Higher specific capacity is measured after addition of CNFc, which is online with galvanostatic charge-discharge results. The supercapacitors with CNFc in the electrolyte exhibit a smaller ESR in all intensity ranges than the supercapacitors with only EMIMP (4.0–10.2 Ω for different rates of EMIMP:CNFc and 14.7 Ω for EMIMP alone). These results also indicate that EMIMP/CNFc gel electrolytes (in all the studied ranges) have higher ionic conductivity than EMIMP alone, and this has been confirmed by the electrochemical impedance spectroscopy showed in Figure 7. In addition, according to Equations (2) and (3), the capacity for the electrolyte EMIMP and EMIMP/CNFc with different ratios were calculated to be 99 and 132–160 F g−1 of YP50F active material at current density of 1 mA cm−2 . The introduction of CNF in the electrolyte significantly increases the capacity of the electrode, and this increase on capacity and decrease on ESR is also significant between the four gel electrolytes with different CNFc s 2022, 11, x FOR PEER REVIEW ratios. Maximum of capacity is reached for EMIMP/CNFc 97:3 ionogel 10 of but 15 presenting similar capacity than the EMIMP/CNFc 96:4 ratio. However, the ESR of EMIMP/CNFc 96:4 is clearly lower than the other gel electrolytes that present similar values. In addition, we previously demonstrated that this increase on capacity does not come from the electrolyte (or CNFc) degradation by the floating test, rather this increment on capacity suggests that (or CNFc) degradation by the floating test, rather this increment on capacity suggests that part of the CNFcpart present inCNFc of the the electrolyte present in is displaced the toisthe electrolyte electrode displaced to surface (due to the electrode his (due to his surface high polarity), creating an extra high polarity), porous creating anstructure extra porouswhere morewhere structure ions are moreallocated ions are during allocated during the the charge process, and therefore, increasing the capacity of the device. charge process, and therefore, increasing the capacity of the device.It has not been It has not been tested tested above 96:4above due to thedue 96:4 poor mechanical to the of gel above poor mechanical of gelthis range. above this range. Figure 6. (a) Electrode capacity Figure and (b) internal 6. (a) Electrode capacityresistance (ESR) resistance and (b) internal of the supercapacitors with EMIMP with EMIMP (ESR) of the supercapacitors and EMIMP/CNFcand mixtures (99:1, 98:2, EMIMP/CNFc 97:3 and mixtures 96:4) (99:1, as97:3 98:2, electrolyte and 96:4)atasdifferent charge-discharge electrolyte at different charge-discharge current rates. current rates. The capacity The capacity variation with variation with the cycles is the cycles shown inisFigure shownS5.in Figure S5. The supercapacitors with The supercapacitors with EMIMP show a good capacity retention of 80% after 10,000 cycles at 1 mAatcm EMIMP show a good capacity retention of 80% after 10,000 cycles 1 mA cm−2 between −2 be- 0 < supercapacitors tween 0 < V <2. The V <2. The supercapacitors with CNFcwith CNFc had hadcapacity a quick a quick capacity decay indecay in the the first first 2000 cycles 2000 (~0.020 mAh g −1 cycle−1 of capacity decay) and then present a moderate degradation cycles (~0.020 mAh g−1 cycle−1 of−capacity decay) and then present a moderate degradation (~0.001 mAh g 1 cycle−1 of capacity decay) reaching the 10,000 cycles with a capacity (~0.001 mAh g cycle of capacity decay) reaching the 10,000 cycles with a capacity reten- −1 −1 retention below 50%. It is possible that during the cycles, the CNFc cannot be retained in tion below 50%. It is possible that during the cycles, the CNFc cannot be retained in the the matrix of the electrolyte and that creates a larger SEI in the first cycle, starting to cover matrix of the electrolyte and that creates a larger SEI in the first cycle, starting to cover part of the carbon porous and therefore limiting the capacity of the electrodes. part of the carbon porous and therefore limiting the capacity of the electrodes. To rule out a possible dissolution of CNFc to carboxymethyl cellulose (CMC) units, an ionogel with a 96:4 ratio EMIMP/CMC was prepared and assessed. Hence, in the case of use CMC instead CNFc, the electrochemical performance of these supercapacitors, −1 omaterials Nanomaterials 2022, 11, x FOR PEER 2022, REVIEW 12, 273 11 of 1510 of 14 Figure Figure 7. (a) 7. (a)plots Nyquist Nyquist plots of the of the different different EMIMP/CNFcEMIMP/CNFc gel electrolytes gel electrolytes and EMIMP and EMIMP ionic liquid; ionic liquid; (b) Equivalent circuit of the (b) Equivalent system. circuit of the system. To rule out a possible Finally, electrochemical dissolution impedance of CNFc to(EIS) spectroscopy carboxymethyl cellulose (CMC) for the supercapacitors units, an with EMIMP/CNFc gel electrolytes and EMIMP are shown in Figure 7a. The equivalent circuitcase of ionogel with a 96:4 ratio EMIMP/CMC was prepared and assessed. Hence, in the was madeuse withCMC instead CNFc, the electrochemical performance of these supercapacitors, shown EC-Lab (Figure 7b). The Nyquist plots show a semi-circle at high frequen- in Table S3, provides specific capacitance values of 123 F g−1 , being like EMIMP/CNFc cies region and a line perpendicular to the X-axis at low frequencies that correspond the 99:1 ionogel values. Moreover, it has been observed the formation of a solid precipitates by ionic diffusion and the double layer formation at the surface of the conductive solid phase. mixing CMC to EMIMP, presumably due to a lack of interaction between the CMC and IL R1 is mostly corresponding because to the of of the diminution ionic conductivity available -OH groups. in theAsdielectric, it has been both gel and the mentioned, mem-presence brane separator. R2 is referring to the charge transfer both ionic and of these hydroxyl groups plays an important role in the dissolution of cellulose in electronic in the bulkILs, due of the electrode. to they Q1 are and Q2 are constant the responsible of thephase element formation of newsimulating hydrogen the real behaviour bonds between theofNC a and pure capacitor. the ILsThe first[38,39]. anions thing to be considered is a charge transfer resistance (Rct, that in- cludes the ionic resistance), Finally, which could electrochemical be intensively impedance investigated spectroscopy in the (EIS) for thesemi-circle loop with supercapacitors appearingEMIMP/CNFc in the middlegel frequency electrolytesregion. and A decrease EMIMP in resistances are shown in Figure is 7a. shown in superca-circuit The equivalent was made pacitors with CNFcwith EC-Labto compared (Figure 7b). The Nyquist supercapacitors withplotsonlyshowEMIMP. a semi-circle at high in This decrease frequencies re- region sistances has andobserved been a line perpendicular above all intothe thevalue X-axisofatthe lowcharge frequencies thatresistance transfer correspond (Rthe ct). ionic These results are in line with the ones obtained in the CV and galvanostatic tests. The real R1 is diffusion and the double layer formation at the surface of the conductive solid phase. mostlycapacitance and imaginary corresponding to the versus theionic conductivity frequency is given in the dielectric, both in supporting gel and in Figure S7membrane and separator. R2 is referring to the charge transfer both ionic using the definition from P.L. Taberna and al. [40] The real part of the capacitance shows and electronic in the bulk of the electrode. Q1 and Q2 are constant phase element simulating the real behaviour of a the variation of the available stored energy with the frequency. The imaginary part of the pure capacitor. The first thing to be considered is a charge transfer resistance (Rct, that capacitance represents the losses that occur during charge storage. In these graphs, can includes the ionic resistance), which could be intensively investigated in the semi-circle observe that the maximum capacitance is reached faster at higher frequencies in the case loop appearing in the middle frequency region. A decrease in resistances is shown in of the blended electrolytewith supercapacitors containing CNFc versus CNFc compared EMIMP electrolyte. to supercapacitors with only NoEMIMP. real difference This decrease is observed within the different percentage of CNF used in this in resistances has been observed above all in the value of the charge transfer studied. No degradation resistance (Rct ). of the electrolyte is demonstrated These results are in line with bythe theones floating test, in obtained also thetheCV same active material and galvanostatic tests.and The real electrode and systems have been used for all electrolyte testing, then we imaginary capacitance versus the frequency is given in supporting in Figure S7 and assume no intrinsic supplementary capacity using the is reached definition from P.L. byTaberna addingand CNFcet al.to[40]. EMIMP butpart The real a faster of theaccess to thisshows capacitance maximal capacity is demonstrated. the variation of the available stored energy with the frequency. The imaginary part of the capacitance All the values ofrepresents the losses(σ, ionic conductivity that mS occur cm−1during charge storage. ) and resistances (R1 andIn these R2, Ω)graphs, are can observe that the maximum capacitance is reached faster included in Table 1. The increase in conductivity with the addition of CNFc was not ex- at higher frequencies in the case of the blended electrolyte containing CNFc versus EMIMP pected because of the important increment on the electrolyte viscosity (almost 0 to 60 Pa electrolyte. No real difference ·s), Figure 3d and Figure S3 from Supplementary Materials). However, as can be seen in Table 1, adding nanocellulose has helped to increase the ionic conductivity of the GPEs, Nanomaterials 2022, 12, 273 11 of 14 is observed within the different percentage of CNF used in this studied. No degradation of the electrolyte is demonstrated by the floating test, also the same active material and electrode systems have been used for all electrolyte testing, then we assume no intrinsic supplementary capacity is reached by adding CNFc to EMIMP but a faster access to this maximal capacity is demonstrated. All the values of ionic conductivity (σ, mS cm−1 ) and resistances (R1 and R2, Ω) are included in Table 1. The increase in conductivity with the addition of CNFc was not expected because of the important increment on the electrolyte viscosity (almost 0 to 60 Pa ·s), Figure 3d and Figure S3 from Supplementary Materials). However, as can be seen in Table 1, adding nanocellulose has helped to increase the ionic conductivity of the GPEs, causing a decrease in the internal resistance of the system (0.32–0.94 mS cm−1 for EMIMP/CNFc ionogels against 0.26 mS cm−1 for EMIMP). Plenty of polar functional groups in the system helped to increase the dielectric constant of the gel electrolytes, which is proportional to the capacity assuming all electrode surface area and distances between the two electrodes are the same for each cell assembled. This also increases the interaction between ions and polymer chains and helps to dissociate these ion pairs into separate ions. As a result, more free ions are generated and consequently, the ionic conductivity is enhanced. Moreover, the abundant polar functional groups supplied by cellulose offer an ionic conduction channel by aiding facile ion hopping onto nearby polar groups However no tendency could be measured with the increased ratio of CNFc. Table 1. Ionic conductivity and resistances of the different EMIMP/CNFc gel electrolytes and pristine EMIMP ionic liquid. Electrolyte σ (mS cm−1 ) R1 (Ω) R2 (Ω) EMIMP 0.26 ± 0.03 11.14 36.02 EMIMP/CNFc 99:1 0.57 ± 0.03 5.07 12.76 EMIMP/CNFc 98:2 0.32 ± 0.03 8.92 15.77 EMIMP/CNFc 97:3 0.94 ± 0.03 3.08 17.03 EMIMP/CNFc 96:4 0.44 ± 0.03 6.5 13 4. Conclusions Novel GPEs have been prepared by the addition of different CNFc ratios (from 1 to 4) to EMIMP, showing excellent thermal and mechanical properties. Contrary to the usual liquid electrolytes, these new NC based GPEs not only improve safety issues, but also show a good printability, very interesting for printed electronics applications. Moreover, enhanced electrochemical properties have also been measured. In case of specific capac- itance, EMIMP/CNFc ionogels have shown a significant increase (160 F g−1 ) compared to the pristine IL (99 F g−1 ), and its internal resistances have considerable decreased from 14.7 Ω cm−2 to 10.2 Ω cm−2 at a current density of 1 mA cm−2 . Although the viscosity (Figure 3d) of the electrolyte increases after CNF addition, which should promote lower ionic conductivity, all electrochemical measurements show both higher ionic conductivity and capacity with CNF addition. These data confirm that the addition of polar groups coming from nanocellulose play a beneficial role. However, one of the points to be im- proved in future experiments is to maintain the capacitance over the cycles (50 % decrease in capacitance in the first 2000 cycles). A drastic loss of the capacity is observed for all samples containing CNF after long cycling (Supplementary Materials Figure S5), although the leakage current test showed no electrolyte degradation (Figure S4). With these results we assume the addition of CNF is beneficial at device level, reducing the initial internal resistance and increasing the quality of the interface between the electrolyte and the elec- trode. But this effect disappears within the cycling, where samples with and without CNFc show similar capacity after 2.000 cycles A possible passivation of the electrode that it is reducing both capacity and ionic conductivity is not discarded and needs to be further Nanomaterials 2022, 12, 273 12 of 14 studied. Moreover, investigations about the use of CNFc vs CMC have been also performed, showing that the use of the nanofibrils of cellulose form (CNFc) provides better solubility in EMIMP and higher electrochemical performances. With all this results we can conclude that these new GPEs have shown great potential for applications in energy storage devices. Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/nano12020273/s1. Figure S1: Final aspect of the as-prepared ionogels pictures; Figure S2: Differencial scanning calorimetry thermogram (DSC) of the EMIMP/CNFc mixtures (99:1, 98:2, 97:3 and 96:4). Pristine CNFc and EMIMP were added for comparison purposes; Figure S3: Rheological results at room temperature vs. time of the EMIMP/CNFc mixtures (99:1, 98:2, 97:3 and 96:4). Pristine CNFc and EMIMP are added for comparison purposes; Figure S4: Current (µA) vs. time for supercapacitors with EMIMP and EMIMP/CNFc (99:1, 98:2, 97:3 and 96:4) as electrolytes for a period of 24h; Figure S5. The capacity variation with the cycles and for supercapacitors with EMIMP and EM-IMP/CNFc (99:1, 98:2, 97:3 and 96:4) as electrolytes; Figure S6: Galvanostatic charge-discharge curve of EMIMP and ionogels with different ratio of CNFc at i = 1 mA cm−2 ; Figure S7. (a) Real part of the capacitance vs Frequency and (b) Imaginary part of the capacitance versus Frequencies of EMIMP and ionogels with different ratio of CNFc. Table S1: Temperature decomposition for each EMIMP/CNFc mixtures (99:1, 98:2, 97:3 and 96:4) mixtures, obtained from differential scanning calorimetry thermograms (DSC). Pristine CNFc and EMIMP temperatures are also added; Table S2: Summary of the key performance parameters on different current density such as specific capacitance, ESR, Emax and Pmax; Table S3: Summary of the key performance parameters on different current density such as specific capacitance and ESR of EMIMP/CMC vs EMIMP/CNFc 99:1; Table S4: Summary of specific capacitance of EMIMP and ionogels with different ratio of CNFc at 10 mV s−1 and 50 mV s−1 . scan rate for CV. Author Contributions: Conceptualization, S.M.-C., R.M.G.-G. and M.B.; investigation, R.M.G.-G., M.B., A.C. and T.A.; data curation, R.M.G.-G., M.B. and A.C.; writing—original draft preparation, R.M.G.-G., M.B. and A.C.; writing—review and editing R.M.G.-G., M.B., A.C., T.A., A.F., C.A. (Christian Aulin), C.A. (Christophe Aucher) and S.M.-C.; visualization, R.M.G.-G., M.B., A.C., T.A., A.F., C.A. (Christian Aulin), C.A. (Christophe Aucher) and S.M.-C.; supervision, S.M.-C. and C.A. (Christophe Aucher); project administration, S.M.-C.; funding acquisition, S.M.-C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by European Union’s Horizon 2020 research and innovation program under grant agreement No 761000 (GREENSENSE project). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: Authors thank to Thermal Analysis Lab at the University of Alicante’s Technical Research Services (SSTTI) for your support in TGA Analysis. 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