An asymmetric MnO 2 |activated carbon supercapacitor with highly soluble choline nitrate-based aqueous electrolyte for sub-zero temperatures

|activated carbon supercapacitors are attractive power devices that rival the electric double-layer capacitors (EDLCs) due to high reachable voltage. However, they greatly suffer from performance loss at low temperature as most of aqueous electrolytes freeze below ca. -10 ◦ C. Here, a concentrated choline nitrate-based (5 mol/L aqueous ChNO 3 ) electrolyte is applied to extend the working temperature range due to its eutectic-like properties. In such electrolyte, water acts as hydrogen bond donor for nitrate anion and low hydration energy for large choline cations favors ionic transport. The MnO 2 /CNT composite electrode with a hierarchical structure has been synthesized by hydrothermal process. The presence of CNTs as core component facilitates the electron conduction, while the two-dimensional MnO 2 flakes grown on the surface provide electrolyte transport pathways and improve the interfacial processes (pseudocapacitive charge/discharge). Thanks to the low hydration of choline cation, the individual activated carbon (AC, negative) and MnO 2 /CNT (positive) electrodes are charged symmetrically up to a cell voltage of 1.8 V. Overall, due to the wide electrochemical stability window (~2.0 V) and anti-freezing properties of ChNO 3 -based aqueous electrolyte and the hierarchical design of the MnO 2 /CNT composite, the asymmetric supercapacitor operates down to -40 ◦ C and displays excellent energy and coulombic efficiency with no loss of performance after several thousand cycles. This work provides a new possibility on the low temperature application of high voltage supercapacitors.


Introduction
Supercapacitors (SCs) are high-power devices that should be able to operate under harsh conditions with minimum risk to the environment.Electric double-layer capacitors (EDLCs) are the commercially available devices that use organic electrolytes with acetonitrile-based hazardous solvents and carbonaceous-based electrodes.EDLCs with organic electrolytes display high cell voltage up to 2.7-2.85V, however, with risks of fire and hazardous gases under emergency situations [1][2][3][4].Low cost and eco-friendly alternatives to these electrolytes are the aqueous ones with non-flammable and overall sustainable characteristics.A serious drawback with aqueous electrolytes based supercapacitors is the oxidation of positive carbon electrode that limits the voltage window to 1.5 or 1.6 V [5].One way to counter this issue is the use of concentrated electrolytes where scarcity of free-water prevents from oxidation processes and enables reaching high cell voltages ca.2.0 V [5,6].
Another strategy that solves the problem of carbon oxidation and enhances the cell voltage is to replace the positive carbon electrode with a MnO 2 pseudocapacitive electrode [7][8][9].The oxidation limit of the MnO 2 in aqueous electrolytes is ~1.2 V vs RHE which is much larger than the stability window of carbon electrodes and can result in cell voltages up to 2.0 V or 2.2 V. Nevertheless, at high cell voltage conditions and during long-term cycling, the structure and morphology of MnO 2 are compromised due to the Mn oxidation state changes (conversion of Mn 4+ to Mn 7+ at high potential) [10].This voltage push can seriously damage the electrode integrity and deteriorate the device's performance.Indeed, a cell voltage of 1.7-1.8V is reachable with MnO 2 | carbon device where a positive MnO 2 electrode only operates below its oxidation limit.This approach prevents the positive electrode from oxidation and makes it easy to achieve a long cycle life and a higher energy density than that of symmetric carbon/carbon supercapacitors.
An important aspect of MnO 2 |carbon supercapacitor in aqueous neutral electrolytes (pH ~ 7.0) is the overpotentials at both positive and negative electrodes that lead to a high cell voltage.In other words, the positive MnO 2 possesses the overpotential for aqueous electrolyte oxidation and the negative porous carbon electrode shows a high overpotential for hydrogen evolution [11].Hence, two electrochemical phenomena combined in a single cell make this family of SCs an attractive high energy storage option.Carbon nanotubes (CNTs) have been often used to stabilize MnO 2 and to increase the conductivity of pseudocapacitive materials, and these nanocomposite materials have been the subject of extensive research in the supercapacitor field [12][13][14].CNTs enable enhanced surface accessibility of MnO 2 , improve the bulk electrode conductivity as well as facilitate the ionic movement by providing facile pathways for the electrolyte.
The charging mechanism of MnO 2 has been widely discussed in recent years [15][16][17] with a broad agreement on its pseudocapacitive behavior.However, the charge compensation for the Mn 4+ /Mn 3+ valence state variation has been dedicated to alkali cations or H + ions in acidic medium.In the case of bulky anions and cations [18], such as in 1-Ethyl-3-methylimidazolium thiocyanate ionic liquids, anions are found to be responsible for compensating the charge during Mn 4+ /Mn 3+ conversion.The use of SCN − and PF 6 − has shown an influence on the MnO 2 capacitance as well, and therefore, the presence of larger cations with loose ion-pairing could enforce a different charge storage mechanism.It might be that a bulky cation, via better dissociation of ionic species, favors a charging of MnO 2 electrode with anions.Charging of MnO 2 electrode is also dependent on its structural properties: adsorption/desorption mostly occurs on amorphous MnO 2 , while insertion/de-insertion takes place in crystalline compounds.Despite several advantages of aqueous electrolytes, whether used in carbon/carbon symmetric or MnO 2 |carbon asymmetric SCs, their freezing at low temperatures limits the scope of application.Water/ alcohol-based binary mixtures exhibit low freezing temperature, however, owing to the reduce salt solubility in them, the total quantity of ionic species available for charging the electric double layer (EDL) is insufficient, leading to reduced capacitance.Furthermore, water and alcohols have different affinity for porous carbon electrode that disturbs the local ionic concentration in the bulk electrolyte and in confined nanopores [19].The electrolyte conductivity is another parameter that is important for achieving high SC power, and aqueous electrolytes often have a serious issue of freezing at low temperatures.On the other hand, high salt concentration is needed for the majority of ions to efficiently penetrate the pores and charge EDL at the negative carbon electrode, or to facilitate the pseudocapacitive charging at the positive MnO 2 electrode.Choline nitrate is highly soluble up to 30 mol/kg of water, and such a high concentration is possible due to the eutectic-like properties of choline salt [20], similar to the ones described for choline chloride/urea or choline chloride/ethylene glycol deep eutectic solvents [21][22][23].Previously, using choline salt-based aqueous electrolyte (5 mol/L ChCl), carbon/carbon supercapacitors have shown excellent electrochemical performance down to -40 • C [24].
Here, MnO 2 /CNT|carbon supercapacitor that operates effectively down to -40 • C with a voltage window of 1.8 V has been proposed.The MnO 2 /CNT nanocomposite displays improved performance as it facilitates the electrolyte accessibility, enhances the electronic conductivity of the bulk electrode and increases the charge storage characteristics.Then choline nitrate-based neutral aqueous electrolyte (at a selected concentration of 5 mol/L with highest ionic conductivity of 82 mS/cm) enables enhanced voltage at room temperature as well as its eutectic-like characterisitcs with water enables excellent low-temperature performance of the supercapacitor at sub-zero temperatures.

Hydrothermal synthesis
All the chemicals and materials used in this study were purchased at high quality from Sigma-Aldrich.The multiwalled CNTs (Sigma Aldrich, CAS-308068-56-6, d x l: 110-170 nm x 5-6 μm) were added to 10 wt.% HNO 3 and stirred at 80 • C for 12 h.Afterwards, the treated CNTs were filtrated and washed three times with deionized water and finally dried at 100 • C overnight and for 12 h at 120 • C in a vacuum.The MnO 2 /CNT nanocomposite was prepared according to the procedure proposed by Xia et al. [25], where 100 mg of acid-treated CNTs were mixed with ml of water under ultrasonic conditions (in a P120H Ultrasonic bath from ELMASONIC).This step was followed by the addition of 0.3 g of KMnO 4 to the mixture under stirring for 2 h.The mixture was transferred to teflon-lined, stainless steel autoclave where the treatment was carried out at 150 • C for 6 h.After careful removal of the prepared MnO 2 /CNTs nanocomposite, it was filtered and washed with deionized water and dried at 80 • C for 48 h.The MnO 2 was prepared as a control sample.0.3 g of KMnO 4 and 1.5 ml HCl (37 wt.%) was dissolved in 70 ml water to form a mixture.Then the mixture was transferred to the autoclave and heated at 150 • C for 4 h.The obtained two samples were black powder, which were then transferred for characterization with different techniques.

Physicochemical characterization
The morphology of the MnO 2 /CNT nanocomposite was investigated with scanning electron microscopy (Zeiss Supra 55 Scanning Electron Microscope (SEM) with primary energy of 15 keV) and the structural information of MnO 2 , CNTs and MnO 2 /CNTs with Raman spectroscopy at a laser wavelength of 645 nm by using.Raman measurements were performed by a Raman spectrometer (LabRam Evolution HR, Horiba Jobin Yvon) using 532 nm laser excitation with a power of 10 mW.X-ray powder diffraction (XRD) was carried out with a STOE Stadi P XRD diffractometer (50 kV, 40 mA) in Debye-Scherrer geometry with Mo Kα1 radiation (λ = 0.70932 Å).Thermogravimetric analysis (TGA) was realized through a thermogravimetric analyzer (STA 449C, Netzsch GmbH) under Ar/O 2 flow (30/10 ml min − 1 ) from 30 to 800 • C with a heating rate of 10 • C min − 1 .

Electrolyte and electrodes
Choline nitrate was prepared by a simple metathesis reaction of choline chloride, where choline hydroxide was derived via reaction with Ag 2 O (used as mild oxidizing agent), the precipitated silver chlorie was filtered out, and followed by the addition of HNO 3 to the filtrate until reaching a neutral pH [20,26].The stability window of 5 mol/L ChNO was investigated in a three-electrode beaker setup on a gold working electrode with a large platinum counter electrode and Ag + /AgCl reference electrode.The potential window was scanned at a rate of 10 mV/s between -1.6 V to +0.7 V.
For the construction of supercapacitors, self-standing pellet electrodes were prepared by mixing 90 wt.% of carbon (YP80F, Kuraray), or 85 wt.% MnO 2 /CNT nanocomposite with 5 wt.% poly(tetrafluoroethylene) binder (PTFE -60 wt.% dispersion in water, Sigma-Aldrich) and 5 or 10 wt.% carbon black (Super C65, Imerys) as conductivity enhancer.The three components for each type of electrode were mixed in ethanol (Sigma-Aldrich, 99.5%) until a dough was obtained.Then the dough was rolled into a homogenous electrode sheet that was dried under vacuum at 110 • C for 12 h with a final thickness ~100 μm.The pellets of electrodes with a diameter of 11 mm (8-10 mg of active materials) were punched out to assemble the supercapacitor cells.
S. Schrade et al.

Electrochemical characterization of supercapacitor cells
The electrochemical performance of full supercapacitors was tested in Teflon Swagelok® vessel at all temperatures from -40 • C to + 60 • C in a customized climatic chamber.The electrolyte was prepared from choline nitrate or lithium nitrate at a concentration of 5 mol/L for assembling the supercapacitor cells with current collectors made of stainless steel (type 304), and microfiber filter GF/A (WhatmanTM, thickness = 0.26 mm) was used as a separator, sandwiched between the electrodes.The two-electrode setup equipped with a reference electrode configuration was used to monitor the behaviour of the positive (MnO 2 ) and negative carbon electrodes in the cell (with an optimal active mass ratio of 1:1).In case of the three-electrode configuration, where a large microporous-carbon based counter electrode (30 times higher active mass than the working electorde) was used, the electrochemical properties of MnO 2 were investigated.In both types of cells, Ag + /AgCl in 3.0 mol/L KCl was used as reference electrode.The electrochemical performance of the two-and three-electrode cells with reference electrode was determined with a VMP3 (from Biologic) multichannel potentiostat/galvanostat.The gravimetric capacitance C (F g − 1 of active materials in two electrodes) was estimated from the integration of the galvanostatic discharge curves at 0.5 A g − 1 .The specific current used for testing is estimated from the total mass of active material in two electrodes.

Results and discussion
The physicochemical and electrochemical properties of MnO 2 /CNT nanocomposite material were investigated, while keeping in view the stability window of 5 mol/L ChNO 3 electrolyte.The charging behavior of MnO 2 and carbon electrodes in 5 mol/L aqueous ChNO 3 and LiNO 3 electrolytes is discussed.Then, the asymmetric (+)MnO 2 /CNT|YP80F(-) supercapacitor in aq.ChNO 3 is tested at 24 • C, down to -40 • C and at 60 • C. For high-temperature investigations, another supercapacitor cell with similar configuration was assembled, while room temperature and -40 • C measurements were conducted on the same cell.

Physicochemical characterization of MnO 2 /CNT composite electrode
The MnO 2 /CNT composite morphology is characterized and shown in Fig. 1a,b.The elongated MnO 2 crystalline layers are covering the CNTs structure forming a pattern via arrangement on the entire outer surface of nanotubes.The regular oblique layered structures with a diagonal length of approximately 100-150 nm can be seen.An equal distribution of MnO 2 onto the nanocomposite material indicates a homogeneous structure and suggests good connectivity of particles, which is crucial for the electron conductivity.The extended and uniform pillars of MnO 2 can provide a high available surface area, where the excess (reservoir) of electrolyte can be held and can improve the electrode/ electrolyte interface.attributed to the reflections (002), (100) and (004) of graphite [28].The XRD patterns of the pure MnO 2 powder can be indexed to the tetragonal α-MnO 2 (JCPDS 44-0141) [27].Regarding the MnO 2 /CNT composite, the MnO 2 can be indexed to the monoclinic birnessite type MnO 2 (JCPDS 80-1098) [25], and the diffraction peak from the CNTs at 12.3 • can still be observed.
The electrochemical stability windows in Fig. 2a were determined for a 5 mol/L ChNO 3 electrolyte on a gold working electrode between a potential range of -1.6 V to 0.7 V vs Ag + /AgCl (scan rate = 10 mV/s).The potential window is characterized by a flat current versus potential profile between -1.3 to 0.6 V, indicating a stable electrochemical behavior of the electrolyte.The positive scan towards 0.7 V is followed by an oxidation peak, while a strong reduction peak is visible at negative potentials down to -1.3 V.According to the presented scans in Fig. 2a, 5 mol/L ChNO 3 is electrochemically stable in the range of -1.2 to 0.6 V vs. Ag + /AgCl.Fig. 2b, c depicts the ionic species dissociation which is due to the hydrogen bond donor ability of water, similar to the effects observed in formation of deep eutectic solvents [21][22][23].We have observed similar depression of freezing point in case of 5 mol/L ChNO 3 determined by the differential scanning calorimetry data presented in Fig. 2d.

Charge storage in aqueous choline nitrate-and lithium nitrate-based electrolytes
The charge storage of MnO 2 /CNT compsite and carbon electrodes are investigated in a two-electrode cell using 5 mol/L LiNO 3 or ChNO 3 with a reference electrode setup.The cell voltage window is scanned up to a limited value of 1.0 V in order to preserve the electrolyte from oxidation and a low specific current of 0.1 A/g is used to make sure a slow electrode charging and a proper wetting of the electrodes.Fig.
shows that the positive electrode in aq.LiNO 3 displays low capacitance while the positive and negative electrodes in aq.ChNO 3 show nearly equal capacitance.In aq.LiNO 3 The positive MnO 2 electrode reaches a maximum potential of 0.7 V vs Ag + /AgCl while the maximum potential is 0.44 V in aq.ChNO 3 .Thereby, the positive electrode potential range ΔE + = 0.53 V and negative electrode potential range ΔE -= 0.14 V in aq.LiNO 3 , while the MnO 2 /carbon cell with aq.ChNO 3 where ΔE + = 0.32 V and ΔE -= 0.35 V (potential distribution is nearly equal between the electrodes.
The disparity of potential range and consequently the capacitance of each electrode in aq.LiNO 3 can be explained based on the ionic association between strongly hydrated lithium and the nitrate ions.At higher concentrations, lithium ions tend to form ion pairs and this property of lithium-based electrolytes strongly influence the charging of carbon nanopores [20,29].On the other hand, a loosely hydrated choline cation is better dissociated in the aqueous electrolyte, which results in better charging of both positive MnO 2 and negative carbon electrodes [20].It might be that the choline cation owing to its dissociation, allows the nitrate anion to compensate charges for the Mn 4+ /Mn 3+ oxidation state changes during charge/discharge.It has been previously reported that cations are the main species in aqueous lithium nitrate electrolytes that compensate for this redox reaction, which could arise from the strong association of lithium with the respective anion.On the other hand, in the case of large cations such as choline, the charging of MnO 2 would be different and controlled mainly by the anions, in this case the NO 3 − 1 ions.
In the charge/discharge curves of Fig. 4a, b, the positive electrode displays a noticeable change in charge/discharge slope at high potentials near 1.2 V vs. Ag + /AgCl, also indicated by the inceased current at high potential in the CV of Fig. 4c,.This is due to the fact that the MnO 2 electrode reaches the overpotential limit at high potential values, near the oxidation limit of electrode/electrolyte.In this asymmetric cell, the overall charge/discharge characteristics are determined by the pseudocapacitive MnO 2 -based electrode.This assumption follows the equation of capacitance for two capacitors connected in series 1/C cell = 1/ C YP80F + 1/C MnO2 , where the electrode with lower capacitance (MnO 2based one) determines the overall cell capacitance.Here, the capacitance differences between the two electrodes are minor, still the charge/ discharge pattern of the cell is similar to the positive MnO 2 electrode.
The cyclic voltammogram shown in Fig. 4c up to 1.8 V is characterized by a square-type shape indicating a supercapacitor-like behavior.The low cut-off voltage in this type of asymmetric supercapacitor cells is selected 0.3 V to avoid the positive MnO 2 electrode from operating below a certain potential window.In other words, the MnO 2 electrode is susceptible of operating below 0.2 V vs SHE if the cell is discharged below 0.3 V, leading to the reduction of Mn 4+ to Mn 2+ that can severely damage the electrode morphology and integrity [10].Likewise, the positive MnO 2 electrode should not operate beyond 1.2 V vs. SHE to avoid the oxidation of Mn 4+ to Mn 7+ , which can deteriorate the cell performance.Therefore, based on the two-electrode with reference electrode studies, a careful cell potential window is selected between 0.3 V to 1.8 V in this work.
The cyclic voltammogram in Fig. 4d is obtained in a three-electrode setup (half-cell) where MnO 2 /CNT electrode stability is demonstrated between 0.3 V to 1.0 V vs. Ag + /AgCl.Square-shape of the cyclic voltammogram indicates an ideal pseudocapacitive charge storage in 5 mol/L ChNO 3 .At higher potential, a relative increase of charging current suggests the surface changes related to the oxidation of Mn 4+ .One of the important aspect of MnO 2 /CNT|YP80F asymmetric supercapacitor is the low negative potential reached at the nanoporous carbon electrode, where electrolyte reduction causes hydrogen production, that is partially stored in the nanoporous carbon and partially extracted during repeated oxidation/reduction cycles.Thus, a reversible hydrogen adsorption/desorption in the nanoporous carbon electrode facilitates the enhanced cyclability of the supercapacitor cell.Overall, an optimized balancing of positive and negative electrodes leads to an optimized MnO 2 /CNT|YP80F asymmetric supercapacitor in choline nitrate-based aqueous electrolyte.C and the charge/discharge curves and cyclic voltammograms are presented in Fig. 5. Symmetric charge/discharge curves are obtained at 0.5 A/g and the CVs display square-type shape even at high scan rates, as presented in Fig. 5a and 5b and 5c, respectively.The squared shape of CVs is preserved at 80 mV/s indicating a capacitor-type behavior at high scan rate.Similarly, galvanostatic charge/discharge curves in Fig. 5d are collected at different specific currents (calculated per active mass of two electrodes, at 0.5, 1.0, 2.0 and 4.0 A/g) and even at high specific current, the charge/ discharge curves are not only symmetric but display very small ohmic loss.Hence, at high specific current and scan rates, reasonably high capacitance (38 F/g at 1 A/gspecific current calculated per total active mass in two electrodes) can be extracted from the MnO 2 /CNT|YP80F supercapacitor.
The MnO 2 /CNT|YP80F supercapacitor was cycled galvanostatically for 10000 charge/discharge cycles between 0.3 V and 1.8 V at a specific current of 1.0 A/g.The evolution of capacitance, energy efficiency (calculated from ratio of area under discharge versus charge curve) and coulombic efficiency (calculated from the ratio of discharge time versus charge time) is shown in Fig. 5e.A constant pattern of capacitance (~37 F/g), energy efficiency (~80%) and coulombic efficiency (~90%) Fig. 5. Electrochemical testing of MnO 2 /CNT|YP80F supercapacitor at room temperature and up to 1.8 V, (a) galvanostatic charge/discharge at 0.5 A/g, (b) cyclic voltamogramms at 5 mV/s, (c) cyclic voltammograms at various scan rates from 5 mV/s to 80 mV/s, the arrow shows the direction of increasing scan rate, (d) galvanostatic charge/discharge curves at various specific currents from 0.5 A/g to 4 A/g, (e) cell capacitance, energy efficiency and coulombic efficiency values during 10,000 galvanostatic charge/discharge cycling up to 1.8 V and specific current 1.0 A/g, and (f) charge/discharge curves collected after every 1000 cycles, the arrow indicates the direction of increasing number of cycles.
indicate a stable asymmetric supercapacitor device that can withstand high rate cycling at 1.8 V.The galvanostatic charge/discharge curves after every 1000 cycles, superimposed in Fig. 5f, show a slight decrease of specific capacitance with cycle number.A capacitance loss during long-term cycling could originate from several factors, including the surface changes upon interaction with electrolyte to the accumulation of corrosion products at the electrode/current collector interface.The capacitance during long term cycling varies from 38 F/g to 37 F/g while the energy efficiency from 80% to 77.5% and coulombic efficiency from 93% to 90%, showing an overall excellent cycling performance.
Asymmetric supercapacitor with MnO 2 /CNT|YP80F configuration was then tested at low temperatures.The supercapacitor performance was first investigated at 24 • C and then the temperature was gradually reduced down to -40 • C. Fig. 6 shows the electrochemical performance of MnO 2 /CNT|YP80F supercapacitor at various temperatures.It can be seen in Fig. 6a,b that the capacitance drops by nearly 64% from 24 • C to -40 • C. Another noticeable aspect is the gradual increase of ohmic loss (indicated in CV at the charging point around 0.35 V in Fig. 6b and at the start of discharge in Fig. 6d) with decreasing temperature.In particular, at -40 • C, the ohmic loss is ~0.45 V leading to low capacitance and also poor rate performance.CVs in Fig. 6b and galvanostatic charge/ discharge curves in Fig. 6d collected at 50 mV/s and 1.0 A/g, respectively show that, while at room temperature there is negligible ohmic loss, the shape of the curves indicates quite resistive behavior at very low temperatures.Nevertheless, one can notice a much better capacitance retention at -30 • C with almost symmetric charge/discharge curves and less resistive CVs.

Compraison of device performance parameters at different temperatures
Three performance key-parameters have been compared in Fig. 6, namely, capacitance (blue curves), energy efficiency (green curves) and coulombic efficiency (red curves) at various temperatures, were extracted from the galvanostatic charge/discharge at 0.5 A/g for the asymmetric MnO 2 /CNT|YP80F supercapacitor polarized up to 1.8 V.Both the capacitance and energy efficiency are strongly influenced and show a decreasing trend with decreasing temperatures.A noticeable decrease of energy efficiency from 83% at 24 • C to 58% at -40 • C shows that the temperature has a drastic effect on the energy and power performance of the supercapacitor.At 24 • C, on the other hand, by increasing the output voltage to 1.8 V the capacitance increases but the energy efficiency decreases (Fig. 7a).An increase in capacitance is commonly observed in hybrid supercapacitors containing carbon-based negative electrodes in aqueous electrolytes, which mainly originates from the reversible hydrogen adsorption/desorption in the carbon nanopores [11].As discussed in Section 3.2, water is electrochemically reduced to produce hydrogen at low negative potentials in the carbon nanopores, and a part of this hydrogen is stored in the pores while other hydrogen participates in the chemical reaction with carbon to reversibly adsorb and desorb.These faradaic reactions at negative electrode that contribute to the overall cell capacitance are suppressed when shifting the temperature from 24 • C to 0 • C. Upon further decreasing the temperature, the capacitance and efficiency values display nearly a constant trend indicating the absence of faradaic reactions at the negative electrode and consequently no additional capacitance contribution.Another reason for the lower capacitance at low temperature is related to the low ionic conductivity of the electrolyte and hindered ionic movement in the carbon pores (indicated by higher ohmic drop as discussed in Fig. 6), and also has been explained in our previous studies [24].
Fig. 6h shows the galvanostatic charge/discharge curves up to 1.8 V at room temperature, -30 • C, -40 • C and at 60 • C. One can also see a slightly higher capacitance at 60 • C in Fig. 6i (red dots), which is mainly due to the increased faradaic contributions at higher temperatures related to the reduction of the aqueous electrolyte at the negative carbon electrode (i.e.H2 evolution and adsorption into the carbon pores).
Overall, MnO 2 /CNT/YP80F asymmetric supercapacitor displays high capacitance, an improved performance due to the use of CNTs as electrode additive, which maintains increased electronic conductivity at low temperatures.The high solubility of choline nitrate in water makes

Fig. 2 .
Fig. 2. (a) Electrochemical stability window of 5 mol/L choline nitrate tested with gold (Au) working electrode, platinum (Pt) counter electrode and Ag + /AgCl in mol/L KCl as a reference electrode in a beaker cell.The potential window was scanned at 10 mV/s from lower values to the higher values with a gradual increase on both positive and negative polarities, (b, c) schemes showing the formation of eutectic mixture between choline nitrate and water (molar ratio of 1:11), where former acts as hydrogen bond acceptor (HBA) and the latter as hydrogen bond donor (HBD), (d) differential scanning claorimetry at 10 • C/min shows the absence of any thermal event when cooling down 5 mol/L choline nitrate solution from 25 • C to -100 • C.

Fig. 4b (Fig. 3 .
Fig. 4b (MnO 2 /CNT|YP80F) show the influence of CNTs on the electrochemical behavior of MnO 2 in 5 mol/L ChNO 3 .At a specific current of 0.2 A/g and a cell voltage up to 1.8 V, the positive MnO 2 electrode (Fig. 4a) shows symmetric charge/discharge curves.Although both positive and negative electrodes in this cell display a triangular shape of curve, the charge storage mechanism is different for the two electrodes.The negative carbon electrode stores charges at the electric double-layer

Fig. 4 .
Fig. 4. Galvanostatic charge/discharge testing of (a) MnO 2 and (b) MnO 2 /CNT electrodes at 200 mA/g in 5 mol/L choline nitrate at room temperature, (c) cyclic voltammetry of two-electrode with reference where MnO 2 /CNT is positive and YP80F is negative electrode, the red and blue lines show the potential excursions (potential ranges) of positive and negative electrodes, respectively (d) cyclic voltammogram of MnO 2 /CNT electrode in 5 mol/L choline nitrate in a selected potential window of 0.3 V to 1.0 V by using three-electrode setup where Ag + /AgCl (in 3 mol/L KCl) is used as reference electrode with large area activated carbon as counter electrode.

Fig. 6 .
Fig. 6.Electrochemical behavior of MnO 2 /CNT/ YP80F supercapacitor in 5 mol/L ChNO 3 up to 1.8 V at 24 • C, 0 • C, -20 • C, -30 • C and -40 • C, cyclic voltamogramms at different temperatures from room temperature to -40 • C by using scan rates of (a) 5 mV/s and (b) 50 mV/s, galvanostatic charge/ discharge curves at different temperatures from room temperature to -40 • C by using specific currents (c) 0.5 A/g and (d) 1 A/g (the specific current is calculated per total active mass present in the two electrodes).The direction of arrows indicate the trend for decreasing discharge capacitance.