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High-energy hybrid electrochemical capacitor operating down to −40 °C with aqueous redox electrolyte based on choline salts
Journal of Power Sources 427 (2019) 283–292
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High-energy hybrid electrochemical capacitor operating down to with aqueous redox electrolyte based on choline salts
40 � C
Patryk Przygocki, Qamar Abbas, Barbara Gorska, François B�eguin * Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Berdychowo 4, 60-965, Poznan, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon/carbon capacitor Choline salt Redox electrolyte Hybrid capacitor Low temperature
We report on a carbon/carbon hybrid electrochemical capacitor (hybrid EC) based on an aqueous redox-active electrolyte, which operates efficiently down to 40 � C. The electrolyte comprises choline iodide (ChI; 0.5 mol kg 1) as redox active component and choline nitrate (ChNO3; 5 mol kg 1) as supporting salt which en ables the liquid state to be extended down to 42 � C, as proved by differential scanning calorimetry. Interest ingly, the hybrid EC displays a high discharge capacitance of 81 F g 1 (at 0.1 A g 1; per total mass of electrodes), which is twice higher than its symmetric counterpart utilizing only an aqueous solution of ChNO3 (41 F g 1). The doubling of cell capacitance results from hybridization of the positive battery-type electrode (harnessing redox reactions of polyiodides trapped in carbon porosity) with the negative electrical double-layer (EDL) capacitive electrode. Even at 40 � C, the hybrid EC retains a high capacitance of 50 F g 1 and shows negligible ohmic drop. As its energy and power performance is in the same range as for EDL capacitors using tetraethylammonium tetrafluoroborate in acetonitrile, this new hybrid EC is a highly cost-effective and green alternative to these traditional systems.
1. Introduction The incorporation of renewables in the energy mix and the electri fication of transportation systems require the development of costeffective and environmentally friendly energy storage devices capable of adapting the delivery to the demand. In case of wind turbines and solar cells, these devices could store the excess energy generated at periods where it is not needed, and further deliver it in a controlled manner. Similarly, they would enable to recover the braking energy of vehicles, and then employ it to start and/or accelerate them, as well as to power trams (without catenary) or buses after being quickly recharged during periodical stops. Accordingly, harnessing effective energy stor age devices is essential to these technologies. To meet the challenge, innovations are required in traditional batteries, redox flow batteries and electrochemical capacitors, however they should be designed involving green engineering. To uptake energy pulses, high power devices capable of rapid charging and discharging, such as electrochemical capacitors (ECs), are applied alone or as systems fitted with batteries (external hybridization). At present, commercially available ECs are constructed from electrodes based on high-purity activated carbon (AC) [1] and an organic
electrolyte formulated of quaternary ammonium salt and environmen tally unfriendly acetonitrile (ACN) [2–5]. They provide moderate energy density, yet excellent power [2,6]. Owing to the extremely low viscosity of ACN, the electrolyte retains good transport properties even at low temperature [7,8], and as a result, such ECs display almost constant energy and power performance down to 40 � C [9], by contrast to e.g., lithium-ion batteries. Due to safety issues, there are attempts to replace the volatile and flammable ACN, e.g., with propylene carbonate (PC), however the devices with this solvent exhibit deteriorated performance below 0 � C [9]. Another drawback of ECs implementing organic elec trolyte is their high cost, due to the need of using ultra-pure and extra dried components, while assembling the devices under air and moisture free-atmosphere. At present, the development of safe, eco-friendly and low-cost ECs with competitive performance to the conventional devices is an impor tant challenge. In this context, the choice of aqueous electrolytes seems to be justified for solving environmental, economic and safety issues. Nevertheless, the presently known aqueous ECs with activated carbon electrodes exhibit moderate energy density (E) compared to their organic counterparts, due to the lower electrochemical stability of water restricting their operational voltage (U) and energy (E), accordingly to
* Corresponding author. E-mail address: [email protected] (F. B�eguin). https://doi.org/10.1016/j.jpowsour.2019.04.082 Received 14 February 2019; Received in revised form 17 April 2019; Accepted 19 April 2019 Available online 30 April 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 427 (2019) 283–292
the formula E ¼ ½ CU2 (C - capacitance). Typically, AC/AC electro chemical capacitors with tetraethylammonium tetrafluoroborate in ACN operate at 2.5–2.8 V [7], against 1.5–2.0 V for neutral aqueous solutions of lithium sulphate, depending on the type of current collectors (gold or stainless steel) [10,11]. Upon exceeding the “safe” voltage (preferably determined by potentiostatic floating or galvanostatic cycling), the maximum potential of the positive electrode is shifted beyond the oxy gen evolution potential, causing oxidation processes and electrode deterioration [12,13] together with corrosion of the positive current collector [14,15]. Other salts applied in aqueous electrical double-layer capacitors are alkali metal nitrates [16,17], acetates [18] and bis(tri fluoromethylsulphonyl)imide [19,20]. Among aqueous solutions, redox-active electrolytes are a taskspecific subgroup for enhancing energy. As the name suggests, they contain molecules or moieties (electroactive component) undergoing reversible redox reactions and providing a faradaic contribution, which in ideal case imposes battery-like operation of a porous carbon elec trode. Such electrode coupled with a capacitive one, operating through electrical double-layer (EDL) formation, enables internal cell hybridi zation [21,22]. The capacitance of such hybrid EC is mainly determined by the capacitance of the EDL carbon electrode, CHYB � CEDL, and is nearly twice higher than the capacitance of a symmetric cell imple menting the same porous carbon for both electrodes [22]. Accordingly, redox-active electrolytes have been proven to enhance the energy of carbon/carbon ECs through the enlargement of cell capacitance. Rec ognised redox-active electrolytes are hydroquinone (in sulphuric acid supporting electrolyte) or iodides (potassium salt), however the voltage of hybrid ECs is moderate, 0.8 or 1.2 V, respectively [23–27]. Further energy boosting of such hybrid ECs was accomplished by combining a redox-active component and a supporting salt, providing greater elec trolyte stability, and termed as bi-functional electrolyte [28–30], e.g., a
hybrid EC using 1 mol L 1 Li2SO4 þ 0.5 mol L 1 KI can operate up to 1.5–1.6 V [28,30], against only 1.2 V for the hybrid EC in 1 mol L 1 KI. Moreover, while hydroquinone gives rise to a high cell self-discharge due to shuttling between electrodes [31,32], this effect is not observed with iodides owing to their encapsulation in the pores of the positive electrode [29,33]. Generally, capacitors using neutral aqueous electrolytes exhibit good electrochemical performance at room temperature (RT), yet it de teriorates at low temperature as the cell resistance increases, while the electrolyte may even freeze. Until now, only few publications report about strategies to tentatively extend the low temperature range of cells in water medium. Organic additives such as methanol [34,35], ethylene glycol [36] or formamide [37] have been proposed, however they strongly reduce the salt solubility, leading consequently to lowering electrolyte conductivity. Recently, by implementing a neutral aqueous solution of choline chloride (ChCl; 5 mol kg 1) without organic additive, we have successfully designed a carbon/carbon ECs demonstrating good electrochemical performance down to 40 � C [38]. Choline salts are advantageous for their low cost, eco-friendly character, high solubility in water enabling the formulation of highly-conductive aqueous solu tions (e.g., 5 mol kg 1 ChCl displays a conductivity of 88 mS cm 1 at RT) of almost neutral pH (�6.5). Nevertheless, chloride anions entail risks of corrosion, which restricts the usage of stainless steel current collectors. Therefore, it is desirable to combine the choline cation with a non-corrosive anion. Taking into account the foregoing, this work presents the imple mentation of a bi-functional aqueous electrolyte made of choline nitrate (5 mol kg 1 ChNO3; supporting electrolyte) and choline iodide (0.5 mol kg 1 ChI; redox active component) to realize an environmen tally friendly carbon/carbon hybrid EC displaying good performance down to 40 � C (Fig. 1). The thermal analysis by differential scanning
Fig. 1. Schematic illustration of the electrolyte characteristics, hybrid EC assembly and its electrochemical investigations and properties: (a) structure of the choline cation, (b) DSC thermograms of 1 mol kg 1 ChNO3 (blue curve) and 5 mol kg 1 ChNO3 (green curve) in the range of 130 � C to RT, (c) Hybrid EC in 2-electrode Swagelok assembly with reference electrode and cyclic voltammograms (CVs) of the positive battery-type and negative EDL electrodes, (d) hybrid pouch cell components: positive and negative electrodes, porous membrane impregnated with the aqueous electrolyte (ChNO3þChI), and packaging, (e) fabricated hybrid pouch cell, (f) test station using a 4-lead cell connection, and (g) compared CVs (2 mV s 1) of hybrid and symmetric ECs at 20 � C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 284
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Chemicals; SDFT ¼ 1962 m2 g 1), 5 wt% polytetrafluoroethylene binder (PTFE 60% dispersion in water, Sigma-Aldrich) and 5 wt% carbon black (C-NERGY Super C65, Imerys). These components were mixed with a small amount of isopropanol (>99%, Sigma-Aldrich) and they were stirred at 70 � C until a homogenous suspension was obtained. After evaporation of the alcohol, the obtained electrode material in the form of dough was further rolled and calendared to give a sheet of a homo geneous thickness ca. 180 μm. The sheet was dried under vacuum at 120 � C for 12 h using Büchi Glass Oven B-585. Electrochemical cells were built in a Teflon Swagelok® vessel (Fig. 1c) by sandwiching a glassy microfiber separator (GF/A, What man™, thickness d ¼ 260 μm) between 10 mm diameter disk punched out from the dried electrode material, and stainless steel (type 316L) current collectors were used. An Hg/Hg2SO4; 0.5 mol L 1 K2SO4 (þ0.658 V vs Standard Hydrogen Electrode – SHE) reference electrode was implemented in order to measure the potential range of individual electrodes. For pouch cells realization, the previously calendared sheet of carbon material was attached to an etched stainless steel foil (316L from Interbelts, thickness d ¼ 10 μm) coated with carbon conductive ink (Electrodag™ PF-407A™, Acheson, thickness d ¼ 10 μm) and further slightly calendared to ensure the proper contact between the electrode and the stainless steel current collector. The obtained material on cur rent collector was dried under vacuum at 120 � C for 12 h using Vacu cell® drier, and then cooled down to room temperature. Two coated carbon electrodes (5 cm � 6 cm) were then cut and weighed, further soaked in the electrolyte and positioned to sandwich a 7 cm � 7 cm glassy microfiber separator (Whatman GF/A, thickness d ¼ 260 μm). Then, both the electrodes and the separator were introduced in a pouch case made of aluminium laminated film, and �1.0 ml of electrolyte was introduced inside the cell. Afterwards, each cell was degassed for 5 s under vacuum before closing and sealing by a Vacuum Packer (Fig. 1d and e). The electrochemical properties of the electrodes and cells in 5 mol kg 1 ChNO3 or in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI were evaluated by cyclic voltammetry (CV, 2 mV s 1), galvanostatic cycling with potential limitation (GCPL) and electrochemical impedance spec troscopy (EIS, frequency from 1 mHz to 100 kHz), using a VMP3 multichannel potentiostat-galvanostat (Biologic, France). All the twoelectrode pouch cells were slightly pressed between two aluminium plates, and their electrochemical properties investigated using a fourlead connection (Fig. 1f). For the low temperature investigations, the pouch cells were placed in an environmental chamber MK 53 (E2) (Binder GmbH, Germany, temperature variation�0.1 � C); the temperature was decreased stepwise and equilibrated for 30 min at each selected value before per forming the measurements. The state-of-health (SOH) of the pouch cells was evaluated at RT by galvanostatic cycling (0.5 A g 1) in the voltage range from 0.1 to 1.5 V. The cells were also discharged at constant power of 0.05, 0.1, 0.2, 0.5, 1.0, 1.5 W in the voltage range from 1.5 V to 0.75 V to determine their Ragone plot. All the values of current density (in A g 1), capacitance (in F g 1), energy and power density are expressed per device (total active mass of the two electrodes).
calorimetry (DSC) demonstrates that, during cooling, the electrolyte does not freeze down to 150 � C, while upon heating it exhibits cold crystallization followed by melting which is completed at 42 � C (Fig. 1b). The electrochemical characteristics of the hybrid EC have been optimized by employing a microporous carbon for enhancing the EDL capacitance of the negative electrode and a micro-/mesoporous carbon for more effectively encapsulating polyiodides in the porous network of the positive electrode. The electrochemical performance of the indi vidual electrodes has been investigated in a two-electrode Swagelok cell equipped with reference electrode (Fig. 1c). In order to validate the performance in a real device, prototype cells have been constructed in pouch cell assembly (Fig. 1d and e) and tested using a four-lead connection (Fig. 1f). We clearly demonstrate that the hybrid EC ex hibits twice higher capacitance (Fig. 1g) and energy density than the symmetric one owing to the hybridization of the battery-type and EDL electrodes in the chosen electrolyte. 2. Experimental 2.1. Preparation and properties of the aqueous electrolytes Choline iodide (ChI; 98%) was bought from Alfa Aesar. Choline ni trate (ChNO3) was synthesized through a simple metathesis reaction between choline chloride (98% from Sigma Aldrich) and sodium nitrate (99% from Sigma Aldrich). Choline chloride (0.2 mol) was dissolved in 500 mL of methanol (99.8% from Avantor) in a round bottom flask equipped with a condenser, sodium nitrate with 10% excess (NaNO3, 0.22 mol) was added to this solution, and the mixture was stirred for 24 h at 40 � C. Afterwards, the by-product in the form of white sodium chloride precipitate was removed by vacuum filtration. In order to enhance the conversion of the substrates into products, another portion of 0.2 mol NaNO3 was added to the filtrate, the mixture was stirred for another 24 h at 40 � C and the NaCl precipitate eliminated by filtration. This procedure was repeated until sodium chloride was fully precipi tated from the solution, while unreacted NaNO3 was removed by vac uum filtration. In the next step, methanol was evaporated at 50 � C in a rotary evaporator, and choline nitrate was dried in Glass Oven B-585 (Büchi) under vacuum at 50 � C for 48 h. The electrolyte solutions were prepared by dissolving either 5 mol kg 1 ChNO3 or 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI in deionized water (conductivity ¼ 0.05 μS cm 1 at 24 � C). The pH of these solutions was measured with Elmetron CPC-501 pH meter and a value of 6.2 was found both for 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI. The conductivity of the two electrolytes was obtained from the equivalent series resistance (ESR) determined by electro chemical impedance spectroscopy in a Swagelok-type cell equipped with a Teflon® ring determining an electrolyte volume of ca. 339 mm3; the temperature ranged from 24 � C to 40 � C using an environmental chamber MK 53 (E2) (Binder GmbH, Germany, temperature var iation�0.1 � C). For verification, measurements were also realized at 24 � C with Elmetron CPC-501 conductivity meter, and similar values as from the ESR were found. The low temperature thermal properties of the two electrolytes were investigated by differential scanning calorimetry (DSC) using Netzsch DSC 204 F1 Phoenix equipped with a liquid ni trogen cooling system enabling to cover the range from 25 � C to 150 � C. The samples (~5 mg) were introduced into hermetically sealed aluminium crucibles; cooling and heating was conducted with the scan rate of 10 K min 1 and this procedure was repeated twice for each sample.
3. Results and discussion 3.1. Conductivity and thermal behaviour of aqueous solutions based on choline salts In this study, the choline nitrate concentration was established arbitrarily to 5 mol kg 1 in analogy with our previous research using aqueous choline chloride for low temperature applications [38]. The temperature dependence of electrolytes (5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI) conductivity is shown in sup porting information in Fig. S1a. At 24 � C, the two electrolytes display conductivity values of 71 mS cm 1 and 89 mS cm 1, respectively. The conductivity remains at acceptable level down to 40 � C, reaching
2.2. Manufacturing and electrochemical investigations of electrodes and cells Carbon electrodes were prepared by mixing 90 wt% of physically activated carbon (named PAC) (YP–80F, Kuraray; SDFT ¼ 1735 m2 g 1) or KOH-activated carbon (named KAC) (Maxsorb, Kansai Coke and 285
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1.7 mS cm 1 and 4.2 mS cm 1, respectively. These values are in the same range as observed for 5 mol kg 1 choline chloride, i.e. 88 mS cm 1 at 24 � C and 4.8 mS cm 1 at 40 � C [38]. Owing to a higher number of charge carriers in the 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI medium, the conductivity is higher than for 5 mol kg 1 ChNO3. The deviation from linearity at low temperatures in the ln (conductivity σ) vs 1/T Arrhenius plot (Fig. S1b) for both salt solutions reveals glass forming liquids, which is consistent with the DSC results presented in the next paragraph. The differential scanning calorimetry analysis (DSC) realized down to 150 � C shows that the 5 mol kg 1 choline nitrate solution (Fig. 2a) does not crystalize upon cooling; only a glass transition appears at Tg ¼ 128 � C. During heating, the glass transition is also observed (Tg ¼ 125 � C) and is followed by cold-crystallization (Tonset ¼ 89 � C, Tpeak ¼ 84 � C, Toffset ¼ -80 � C) and melting in a wide temperature range (Tm; from Tonset ¼ -76 � C to Toffset ¼ -41 � C); at peak maximum (located at 44 � C), most of the solid is melted. Similarly, in the case of the 5 mol kg 1 choline nitrate þ0.5 mol kg 1 choline iodide solution (Fig. 2b), devitrification was detected during cooling (at 126 � C). Upon heating, the thermogram displays some resemblance to the mono-salt solution, with glass transition (at 125 � C), cold crystallization (Ton � � � set ¼ 90 C, Tpeak ¼ 89 C, Toffset ¼ -87 C) and melting (from Tonset ¼ 66 � C to Toffset 42 � C). However, in case of the choline nitrate þ choline iodide solution, the phase transitions are more complex than for 5 mol kg 1 choline nitrate; the main cold crystallization peak is preceded by a small peak (Tonset ¼ 99 � C, Tpeak ¼ 95 � C), whereas the thermal contribution illustrating the melting process consists of three peaks (Tpeak ¼ 61 � C, 50 � C and 45 � C). The devitrification process of aqueous solutions has been vastly discussed by Angell suggesting that, in aqueous solutions of salts incorporating an organic cation, the glass forming process depends on the cation, due to clathrate structures formed by the water molecules around the alkyl groups [39]. Here, this property is considered to be at the origin of the same Tg values of the mono- and bi-salt electrolyte solutions, as the cation is common for the two salts. In turn, the cold crystallization phenomenon is recognised to occur for glass-forming liquids, polymers or highly-viscous ILs incorporating voluminous ions. As the temperature decreases, the liquid viscosity increases causing a hindrance for the formation of crystal nuclei required for the crystalli zation process. Consequently, the compound does not crystalize as ex pected, and it remains in liquid state (super-cooled) far below the melting point. Taking into account that an energetically favourable conformation of the molecules is required for the crystallization, the later becomes possible during heating as their mobility increases, enabling the development of molecular structures favourable for this
Fig. 2. Thermograms of a) 5 mol kg
1
process, and appears as an exothermic peak [40]. The differences caused by the presence of choline iodide e.g., two peaks of cold crystallization and three of melting process, illustrates the formation of separate nanophases. Overall, both electrolytes are in liquid state above 40 � C, which fits with the previous investigations on 5 mol kg 1 choline chlo ride presented in Ref. [38]. 3.2. Performance of the symmetric and hybrid capacitors at room temperature Keeping in view that the 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI aqueous solutions described in the previous section display good conductivity values, it is certainly worth to investigate the cells using these electrolytes. In this section, the electrochemical per formance of two-electrode cells with reference electrode (swagelok-type assembly) and two-electrode pouch cells is investigated at room tem perature. The cells in ChNO3 implement a KOH-activated carbon (named KAC) for both electrodes, whereas an asymmetric configuration of electrodes with KAC for enhanced EDL capacitance of the negative electrode and physically activated carbon (named PAC) for effective polyiodides trapping in the positive electrode is used for the cells in ChNO3 þ ChI, similarly to the hybrid cell presented in Ref. [30]. The electrochemical behaviour of individual electrodes for the symmetric ( )KAC/KAC(þ) cell in 5 mol kg 1 aqueous ChNO3 and asymmetric ( )KAC/PAC(þ) hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI at room temperature is presented in Fig. 3. The potential extrema of the electrodes shown in Fig. 3a were determined during galvanostatic charge/discharge of the cell from 0.1 V up to various voltages in the range from 0.8 V to 1.5 V. The lower voltage value of 0.1 V was selected in order to compare this capacitor with the asym metric hybrid one, where the discharge had to be stopped at 0.1 V for avoiding the negative electrode to operate in the range of the iodide/ iodine equilibrium potential. For the cell based on 5 mol kg 1 ChNO3, the potential range of the two electrodes is comparable, which is generally typical for a symmetric carbon/carbon cell (Fig. 3a and b). Although the minimum potential of the negative electrode is lower than the hydrogen evolution potential ( 0.366 V vs. SHE) at high voltage values, the charging characteristics remain linear, which is attributed to the high over-potential of di-hydrogen evolution related with the local pH increase inside the carbon porosity [11,41]. By contrast, the slight deviation of the positive electrode characteristics from linearity at voltage higher than 1.1 V (Fig. 3b) during galvanostatic charging reveals a non-pure EDL performance. As the maximum potential of this elec trode becomes higher than the thermodynamic limit of water oxidation (at pH ¼ 6.2, the oxygen evolution potential, OEP, is 0.864 V vs. SHE) at
ChNO3 and b) 5 mol kg 286
1
ChNO3 þ 0.5 mol kg
1
ChI (scanning rate 10 � C min 1).
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Fig. 3. Comparison of the electrochemical properties of individual electrodes at 24 � C in (a, b) the ( )KAC/KAC(þ) cell using 5 mol kg 1 ChNO3 and (c, d) ( )KAC/ PAC(þ) cell using 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI. Data obtained by galvanostatic (0.1 A g 1) cycling with potential limitation (GCPL) of two-electrode cells with reference electrode [Hg/Hg2SO4 in 0.5 mol L 1 K2SO4 (þ0.658 V vs. SHE)]. The dashed lines in a) and c) indicate the oxygen evolution potential (OEP) and hydrogen evolution potential (HEP) estimated by applying the Nernst equation for aqueous solutions with pH ¼ 6.2.
voltage higher than 1.1 V (Fig. 3a), this deviation is the signature of an irreversible oxidation of the positive electrode. The simultaneous shift of E0.1 (electrode potential at voltage of 0.1 V) for the two electrodes in the voltage range from 1.2 to 1.5 V is attributed to this change of surface functionality of the positive electrode [42]. By contrast, in case of the asymmetric ( )KAC/PAC(þ) hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI, the potential range of the positive electrode is very narrow, which is typical for a battery-type electrode (Fig. 3c and d). According to the literature and to the Pourbaix diagram of iodine [43–45], such narrow potential range results from a few faradaic reactions related with electron transfer and formation of iodine and polyiodides. As a consequence of the electrolyte redox activity and micro-/mesoporosity of the carbon used for effective polyiodides trap ping [30], the maximum potential of the positive electrode at U ¼ 1.5 V is only 0.744 V vs. SHE (as compared to 1.003 V vs. SHE in the sym metric cell using ChNO3), which is much below the water oxidation limit (0.864 V vs SHE). Consequently, such low potential prevents from oxidation of the positive carbon electrode, which explains the absence of E0.1 deviation in this electrolyte. Thus, owing to the narrow potential range of the positive electrode and high capacity of this electrode, the potential range of the EDL-type negative electrode (ΔE-hyb ¼ 1.156 V, Fig. 3d) is almost twice higher than in the symmetric cell
(ΔE-sym ¼ 0.631 V). It suggests that the capacitance of the asymmetric hybrid cell should be ca. twice higher than the capacitance of the symmetric one [21]. However, by contrast with the symmetric cell, the minimum potential of the negative electrode in the asymmetric ( ) KAC/PAC(þ) hybrid cell at voltage of 1.5 V is 0.749 V vs SHE (Fig. 3c), which is around 380 mV below the electrolyte reduction potential. Consequently, one can observe a slight deviation from linear charging profile of the negative electrode in this cell (Fig. 3d). Nevertheless, high over-potential of di-hydrogen evolution caused by the presence of ChNO3 as supporting electrolyte enables this electrode to still operate within the optimized potential range. The cyclic voltammograms (CVs) and galvanostatic charge/ discharge characteristics of the asymmetric ( )KAC/PAC(þ) hybrid pouch cells in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI are presented in Fig. 4a and b, respectively. For all the voltage values in the range from 0.1 V to 1.5 V, the CVs (Fig. 4a) display the rectangular shape charac teristic for a capacitor. Similarly, up to 1.5 V, the galvanostatic charge/ discharge displays the linear profile of a capacitor (Fig. 4b). As advised in Ref. [46], in order to properly estimate the efficiency and capacitance of the cells, the surface area under the GCPL curves should be integrated. Thus, as a result of the iodides faradaic contribution, the asymmetric hybrid EC displays advantageously a capacitance of 81 F g 1 at 1.5 V 287
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Fig. 4. Electrochemical performance of the asymmetric ( )KAC/PAC(þ) hybrid pouch capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI: a) CVs at 2 mV s 1; b) GCPL curves at 0.1 A g 1. c) Capacitance and d) energy efficiency vs voltage of the ( )KAC/PAC(þ) hybrid capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI (green continuous line) and ( ) KAC/KAC(þ) symmetric capacitor in 5 mol kg 1 ChNO3 (red dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. a) Specific cell capacitance and relative resistance recorded at RT during 20,000 galvanostatic (0.5 A g 1) cycles from 0.1 V to 1.5 V for symmetric ( )KAC/KAC (þ) capacitor in 5 mol kg 1 ChNO3 (red dotted lines) and asymmetric ( )KAC/PAC (þ) hybrid capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI (green dashed lines). Comparison of the GCPL (0.5 A g 1) curves before and after the cycling tests on b) symmetric and c) asymmetric hybrid elec trochemical capacitors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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(Fig. 4c) which is approximately twice higher than 41 F g 1 given by the symmetric cell in 5 mol kg 1 ChNO3. It stays in accordance with the theoretical concept of hybridization developed by Conway and Pell which states that the capacitance of a hybrid cell is (ideally) doubled when compared to the capacitance of a symmetric one with the same carbon electrodes [21]. In addition, as shown in Fig. 4d, the energy ef ficiency of the hybrid cell is ca. 5% higher than for the symmetric one whatever the applied voltage, owing to optimized porosity of positive electrode related with confinement of polyiodides which are accom modated well in big micropores/small mesopores of PAC in hybrid cell, as explained in our previous work [30]. The state-of-health of both symmetric capacitor in ChNO3 and asymmetric hybrid capacitor in ChNO3 þ ChI during prolonged galva nostatic (0.5 A g 1) cycling up to 1.5 V was evaluated using pouch cells (Fig. 5a). The hybrid capacitor exhibits a stable and very high capaci tance of 75 F g 1 up to 3000 cycles; then the capacitance slightly de creases to reach 92% of its initial value after 20,000 cycles. The capacitance of the symmetric EC drops very slowly throughout the test, reaching also 92% of its initial value after 20,000 cycles. In parallel, the relative resistance of the two kinds of cells increases by only 8% after 20,000 cycles. Fig. 5b and c confirms that the galvanostatic charge/ discharge characteristics of both types of cells are almost unchanged after the cycling test. Similarly, excellent life span was shown during potentiostatic floating at 1.5V on the two cells in ChNO3 and ChNO3 þ ChI (Fig. S2). Overall, it can be concluded that the hybrid EC exhibits ca.
twice higher capacitance than the symmetric cell, and that it demon strates excellent capacitance retention at room temperature when charged up to 1.5 V. Due to the fact that ageing is generally hindered at low temperature owing to the lower kinetics of the ongoing faradaic processes, one can expect even better electrochemical stability at subambient values [34,38]. 3.3. Electrochemical performance of symmetric and asymmetric hybrid pouch cells at low temperature Taking into account the excellent performance of the electro chemical cells at room temperature, the promising conductivity values of the two electrolytes down to 40 � C (Fig. S1a) and their low melting point (Fig. 2a and b), the pouch cells were investigated at sub-ambient temperature. Fig. 6a presents the cyclic voltammograms of the sym metric capacitor in 5 mol kg 1 ChNO3 from RT down to 40 � C. When the temperature decreases, the current leaps related with hydrogen chemisorption (at high voltage during anodic scan) and desorption (at low voltage during cathodic scan) diminish progressively until almost completely disappearing at 40 � C, where the hydrogen chemisorption process is quenched [34]. However, a part of the high voltage current leap at RT might be also attributed to the oxidation of the positive electrode, as it was previously suggested when discussing Fig. 3a and b. Interestingly, the charge propagation remains almost the same in the whole temperature range, except at 40 � C where the CV is slightly less
Fig. 6. a, c) CVs at 2 mV s 1 and b, d) GCPL curves at 0.1 A g 1 of (a, b) symmetric ( ) KAC/KAC(þ) cell using 5 mol kg (þ) hybrid cell using 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI from RT down to 40 � C. 289
1
ChNO3 and (c, d) asymmetric ( )KAC/PAC
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rectangular (Fig. 7a). The discharge time in the GCPL curves (Fig. 6b) gradually decreases along with decreasing temperature, reflecting the effect of electrolyte conductivity on the electrochemical cell perfor mance. In case of room temperature, the non-linear trend of the charging curve at high voltage is due to the irreversible oxidation of the positive electrode as previously mentioned in the comments of Fig. 6a. In order to demonstrate the gradual change of charging characteristics at various cut-off voltages with decreasing temperature, CV and GCPL curves for symmetric capacitor in 5 mol kg 1 ChNO3 from 0.8 V up to 1.5 V are presented in supporting information in Fig. S3 (at RT and 20 � C) and Fig. S4 (at 30 � C and 40 � C). The asymmetric hybrid cell in ChNO3 þ ChI was also investigated by cyclic voltammetry (Fig. 6c) and galvano static charge/discharge (Fig. 6d) at various temperatures from 24 � C to 40 � C. A moderate decay of the capacitive current (in case of CVs) and discharge time (in case of GCPL curves) was observed, especially at 30 � C and 40 � C, and is attributed to i) the lowering of electrolyte conductivity and ii) kinetically hindered redox processes taking place at the positive electrode. Fig. S5 in supporting information shows the CV and GCPL curves at the voltages from 0.8 V to 1.5 V for asymmetric hybrid capacitor in ChNO3 þ ChI at 20 � C, 30 � C and 40 � C. Fig. 7 presents the temperature dependence of capacitance and en ergy efficiency (using the data obtained by integration of the surface area under the GCPL curves) for the ( ) KAC/KAC(þ) cell in ChNO3 and ( )KAC/PAC(þ) hybrid cell in ChNO3 þ ChI. Logically, for both sys tems, the capacitance decreases with temperature, reaching relatively high values of 50 F g 1 and 31 F g 1 at 40 � C for the asymmetric hybrid and symmetric devices, respectively. Also, it is worth to notice that the capacitance drops more rapidly in case of the hybrid capacitor, obvi ously as a consequence of kinetical limitations of the redox reactions (related with the iodine/iodide system). However, the asymmetric hybrid cell still demonstrates much higher capacitance at 40 � C than the symmetric one, showing that the redox processes are still in play at such low temperature. Furthermore, the capacitance of the ( )KAC/PAC (þ) hybrid cell (50 F g 1) at 40 � C is still higher than the 41 F g 1 displayed by the symmetric cell at room temperature. Interestingly, for both cells, the energy efficiency initially increases with decreasing the temperature (reaching a maximum of 90% in the range from 0 to 20 � C). This efficiency increase for both cells is attributed to the quenching of the redox processes associated with hydrogen chemi sorption in the negative electrode [34]. When the temperature is further decreased to 40 � C, the energy efficiency of the two systems drops to reach 84% and 77% for the symmetric and asymmetric hybrid cells,
respectively. This decrease is related to the hindered mobility of ions within the electrode porosity, which will be further confirmed by the electrochemical impedance spectroscopy data (Fig. 8a and b). However, the slightly lower energy efficiency demonstrated by the ( )KAC/PAC (þ) hybrid cell at very low temperature (from 20 � C down to 40 � C) is additionally due to the slow kinetics of the faradaic reactions associated with the iodide/iodine redox couple. The symmetric and asymmetric hybrid pouch cells in 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI, respectively, have been investigated by electrochemical impedance spectroscopy (EIS) at open-circuit voltage (OCV) in the temperature range from RT to 40 � C; the Nyquist plots are shown in Fig. 8a and b. From the high frequency region, presented as insets in Fig. 8a and b, it is visible that the equiv alent series resistance (ESR) and equivalent distributed resistance (EDR) for both cells increase as temperature decreases. Interestingly, owing to the use of high-quality electrodes and pouch cell construction, there is not any semicircle visible on the Nyquist plots, which confirms the absence of charge transfer between the electrodes and the stainless steel current collectors. The ESR and EDR values of the two kinds of cells at various tem peratures are listed in Table 1. The increase of ESR as temperature de creases for the two cells is related with the decrease of bulk electrolyte conductivity. When decreasing the temperature, the EDR values, which represent the extended intermediate frequency part of the Nyquist plots (also known as mass transport region governed by the diffusion of charged species within the electrode porosity), indicate that the re sistances related with the mass transport of ionic species increase in both asymmetric hybrid and symmetric cells. The slightly higher increase of EDR observed at very low temperature (from 20 � C down to 40 � C) for the asymmetric hybrid cell is attributed to the slow kinetics of the iodide/iodine redox reaction, as already suggested when considering the higher decrease of energy efficiency for this cell in Fig. 7. The Nyquist plots of the two cells mainly differ in the intermediate frequency region, where the symmetric cell in ChNO3 behaves as an EDL capacitor with vertical trend of the characteristics whatever the temperature (Fig. 8a), while the asymmetric hybrid cell in ChNO3 þ ChI (Fig. 8b) demonstrates a significant deviation from vertical trend in the temperature range from 20 � C down to 40 � C, confirming the progressively slower kinetics of the iodide/iodine redox reaction with decreasing temperature. The Ragone plots of the asymmetric hybrid and symmetric cells discharged at constant power in the voltage range from 1.5 V to 0.75 V are shown in Fig. 8c (discharge curves for these two cells at RT are shown in Fig. S6). For comparison, the RT plot of a symmetric EDLC in organic electrolyte (1 mol L 1 TEABF4 in acetonitrile) in the voltage range from 2.7 V to 1.35 V is also included. At room temperature, and whatever the specific power value, the specific energy of the asymmetric hybrid cell in ChNO3 þ ChI and EDLC in organic electrolyte is compa rable and ca. twice higher than for the symmetric cell in ChNO3. For example, at a power of 3.0 kW kg 1, the energy of the hybrid EC in ChNO3 þ ChI is 11.6 Wh kg 1 as compared to 4.9 Wh kg 1 for the symmetric EC in 5 mol kg 1 ChNO3, and 12.1 Wh kg 1 for the cell in organic electrolyte. Moreover, at high power of 1.5 kW kg 1, the asymmetric hybrid EC is able to preserve 16.8, 12.1, 3.5 and 0.8 Wh kg 1 at RT, 20 � C, 30 � C and 40 � C, respectively, confirming that down to 20 � C the electrochemical performance is of the same order as at RT, as already demonstrated by the Nyquist plots. Such high energy values obtained at low temperature (especially at 20 � C) for the asymmetric hybrid cell using the bi-functional aqueous electrolyte are extremely promising for the development of low cost and environmen tally friendly electrochemical capacitors. 4. Conclusion
Fig. 7. Temperature dependence of capacitance and energy efficiency for symmetric ( ) KAC/KAC(þ) capacitor in 5 mol kg-1 ChNO3 (dotted lines) and ( ) KAC/PAC(þ) hybrid capacitor in 5 mol kg-1 ChNO3 þ 0.5 mol kg-1 ChI (dashed lines), obtained from GCPL (0.1 A g-1) up to 1.5 V.
Taking into account the serious drawbacks of environmentally un friendly organic electrolytes, aqueous choline nitrate presents an actual opportunity towards the realization of eco-friendly, sustainable and low 290
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Journal of Power Sources 427 (2019) 283–292
Fig. 8. Performance of symmetric EC in 5 mol kg 1 ChNO3 and asymmetric hybrid EC in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI at various temperatures from 24 � C down to 40 � C: Nyquist plots at open-circuit voltage on a) symmetric EC and b) hybrid EC; c) Ragone plots where an EDLC in organic electrolyte at RT (black full line) is shown for comparison. The data for Ragone plots were obtained from discharge at constant power in the range from 1.5 V to 0.75 V for both symmetric and asymmetric ECs in aqueous electrolyte and from 2.7 V to 1.35 V for the EDLC in organic electrolyte. The power values during cell discharge correspond to current regimes from 0.1 A g 1 to 5.9 A g 1 (expressed for a two-electrode device).
cost electrochemical capacitors. We have demonstrated that the 5 mol kg 1 choline nitrate solution displays good conductivity, neutral pH and does not freeze down to 150 � C. Owing to these properties, it then becomes feasible to implement an aqueous bi-functional electrolyte with choline nitrate as supporting salt enabling a voltage extension up to 1.5 V and choline iodide as redox active component allowing hybridi zation of electrodes. Owing to this hybridization, the cell demonstrates square shaped cyclic voltammograms, linear charge/discharge charac teristics, stable capacitance and resistance down to 40 � C, while dis playing twice higher capacitance. Down to 20 � C, the performance of the hybrid EC is comparable to the values at room temperature, in terms of energy and power. Definitively, the use of aqueous choline nitrate þ choline iodide electrolyte enables to develop an environmentally friendly and low cost capacitor approaching the performance of organic electrolyte-based systems in a wide range of sub-ambient temperatures.
Table 1 Temperature dependence of the equivalent series resistance (ESR) and equiva lent distributed resistance (EDR) for the symmetric cell in 5 mol kg 1 ChNO3 and asymmetric hybrid cell in 5 mol kg 1 ChNO3þ 0.5 mol kg 1 ChI at open circuit voltage. Temperature/� C
24 0 10 20 30 40
Symmetric cell in 5 mol kg 1 ChNO3
Asymmetric hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg ChI
ESR/Ω
EDR/Ω
ESR/Ω
EDR/Ω
0.131 0.135 0.139 0.148 0.166 0.202
0.167 0.204 0.229 0.282 0.393 0.602
0.131 0.152 0.155 0.175 0.218 0.298
0.178 0.289 0.293 0.367 0.564 0.747
1
291
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Acknowledgements
[15] [16] [17] [18] [19] [20]
The Polish National Science Center (NCN – Narodowe Centrum Nauki) is acknowledged for funding the OPUS project UMO 2014/15/B/ ST4/04957. P.P, Q.A and B.G. thank the Ministry of Science and Higher Education of Poland for financial support under the project DS 03/31/ DSMK/0367/2018. Authors are grateful to Imerys for providing the carbon black C65 and Kuraray for the carbon YP-80F.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.04.082.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
High-energy hybrid electrochemical capacitor operating down to with aqueous redox electrolyte based on choline salts
40 � C
Patryk Przygocki, Qamar Abbas, Barbara Gorska, François B�eguin * Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Berdychowo 4, 60-965, Poznan, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Carbon/carbon capacitor Choline salt Redox electrolyte Hybrid capacitor Low temperature
We report on a carbon/carbon hybrid electrochemical capacitor (hybrid EC) based on an aqueous redox-active electrolyte, which operates efficiently down to 40 � C. The electrolyte comprises choline iodide (ChI; 0.5 mol kg 1) as redox active component and choline nitrate (ChNO3; 5 mol kg 1) as supporting salt which en ables the liquid state to be extended down to 42 � C, as proved by differential scanning calorimetry. Interest ingly, the hybrid EC displays a high discharge capacitance of 81 F g 1 (at 0.1 A g 1; per total mass of electrodes), which is twice higher than its symmetric counterpart utilizing only an aqueous solution of ChNO3 (41 F g 1). The doubling of cell capacitance results from hybridization of the positive battery-type electrode (harnessing redox reactions of polyiodides trapped in carbon porosity) with the negative electrical double-layer (EDL) capacitive electrode. Even at 40 � C, the hybrid EC retains a high capacitance of 50 F g 1 and shows negligible ohmic drop. As its energy and power performance is in the same range as for EDL capacitors using tetraethylammonium tetrafluoroborate in acetonitrile, this new hybrid EC is a highly cost-effective and green alternative to these traditional systems.
1. Introduction The incorporation of renewables in the energy mix and the electri fication of transportation systems require the development of costeffective and environmentally friendly energy storage devices capable of adapting the delivery to the demand. In case of wind turbines and solar cells, these devices could store the excess energy generated at periods where it is not needed, and further deliver it in a controlled manner. Similarly, they would enable to recover the braking energy of vehicles, and then employ it to start and/or accelerate them, as well as to power trams (without catenary) or buses after being quickly recharged during periodical stops. Accordingly, harnessing effective energy stor age devices is essential to these technologies. To meet the challenge, innovations are required in traditional batteries, redox flow batteries and electrochemical capacitors, however they should be designed involving green engineering. To uptake energy pulses, high power devices capable of rapid charging and discharging, such as electrochemical capacitors (ECs), are applied alone or as systems fitted with batteries (external hybridization). At present, commercially available ECs are constructed from electrodes based on high-purity activated carbon (AC) [1] and an organic
electrolyte formulated of quaternary ammonium salt and environmen tally unfriendly acetonitrile (ACN) [2–5]. They provide moderate energy density, yet excellent power [2,6]. Owing to the extremely low viscosity of ACN, the electrolyte retains good transport properties even at low temperature [7,8], and as a result, such ECs display almost constant energy and power performance down to 40 � C [9], by contrast to e.g., lithium-ion batteries. Due to safety issues, there are attempts to replace the volatile and flammable ACN, e.g., with propylene carbonate (PC), however the devices with this solvent exhibit deteriorated performance below 0 � C [9]. Another drawback of ECs implementing organic elec trolyte is their high cost, due to the need of using ultra-pure and extra dried components, while assembling the devices under air and moisture free-atmosphere. At present, the development of safe, eco-friendly and low-cost ECs with competitive performance to the conventional devices is an impor tant challenge. In this context, the choice of aqueous electrolytes seems to be justified for solving environmental, economic and safety issues. Nevertheless, the presently known aqueous ECs with activated carbon electrodes exhibit moderate energy density (E) compared to their organic counterparts, due to the lower electrochemical stability of water restricting their operational voltage (U) and energy (E), accordingly to
* Corresponding author. E-mail address: [email protected] (F. B�eguin). https://doi.org/10.1016/j.jpowsour.2019.04.082 Received 14 February 2019; Received in revised form 17 April 2019; Accepted 19 April 2019 Available online 30 April 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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Journal of Power Sources 427 (2019) 283–292
the formula E ¼ ½ CU2 (C - capacitance). Typically, AC/AC electro chemical capacitors with tetraethylammonium tetrafluoroborate in ACN operate at 2.5–2.8 V [7], against 1.5–2.0 V for neutral aqueous solutions of lithium sulphate, depending on the type of current collectors (gold or stainless steel) [10,11]. Upon exceeding the “safe” voltage (preferably determined by potentiostatic floating or galvanostatic cycling), the maximum potential of the positive electrode is shifted beyond the oxy gen evolution potential, causing oxidation processes and electrode deterioration [12,13] together with corrosion of the positive current collector [14,15]. Other salts applied in aqueous electrical double-layer capacitors are alkali metal nitrates [16,17], acetates [18] and bis(tri fluoromethylsulphonyl)imide [19,20]. Among aqueous solutions, redox-active electrolytes are a taskspecific subgroup for enhancing energy. As the name suggests, they contain molecules or moieties (electroactive component) undergoing reversible redox reactions and providing a faradaic contribution, which in ideal case imposes battery-like operation of a porous carbon elec trode. Such electrode coupled with a capacitive one, operating through electrical double-layer (EDL) formation, enables internal cell hybridi zation [21,22]. The capacitance of such hybrid EC is mainly determined by the capacitance of the EDL carbon electrode, CHYB � CEDL, and is nearly twice higher than the capacitance of a symmetric cell imple menting the same porous carbon for both electrodes [22]. Accordingly, redox-active electrolytes have been proven to enhance the energy of carbon/carbon ECs through the enlargement of cell capacitance. Rec ognised redox-active electrolytes are hydroquinone (in sulphuric acid supporting electrolyte) or iodides (potassium salt), however the voltage of hybrid ECs is moderate, 0.8 or 1.2 V, respectively [23–27]. Further energy boosting of such hybrid ECs was accomplished by combining a redox-active component and a supporting salt, providing greater elec trolyte stability, and termed as bi-functional electrolyte [28–30], e.g., a
hybrid EC using 1 mol L 1 Li2SO4 þ 0.5 mol L 1 KI can operate up to 1.5–1.6 V [28,30], against only 1.2 V for the hybrid EC in 1 mol L 1 KI. Moreover, while hydroquinone gives rise to a high cell self-discharge due to shuttling between electrodes [31,32], this effect is not observed with iodides owing to their encapsulation in the pores of the positive electrode [29,33]. Generally, capacitors using neutral aqueous electrolytes exhibit good electrochemical performance at room temperature (RT), yet it de teriorates at low temperature as the cell resistance increases, while the electrolyte may even freeze. Until now, only few publications report about strategies to tentatively extend the low temperature range of cells in water medium. Organic additives such as methanol [34,35], ethylene glycol [36] or formamide [37] have been proposed, however they strongly reduce the salt solubility, leading consequently to lowering electrolyte conductivity. Recently, by implementing a neutral aqueous solution of choline chloride (ChCl; 5 mol kg 1) without organic additive, we have successfully designed a carbon/carbon ECs demonstrating good electrochemical performance down to 40 � C [38]. Choline salts are advantageous for their low cost, eco-friendly character, high solubility in water enabling the formulation of highly-conductive aqueous solu tions (e.g., 5 mol kg 1 ChCl displays a conductivity of 88 mS cm 1 at RT) of almost neutral pH (�6.5). Nevertheless, chloride anions entail risks of corrosion, which restricts the usage of stainless steel current collectors. Therefore, it is desirable to combine the choline cation with a non-corrosive anion. Taking into account the foregoing, this work presents the imple mentation of a bi-functional aqueous electrolyte made of choline nitrate (5 mol kg 1 ChNO3; supporting electrolyte) and choline iodide (0.5 mol kg 1 ChI; redox active component) to realize an environmen tally friendly carbon/carbon hybrid EC displaying good performance down to 40 � C (Fig. 1). The thermal analysis by differential scanning
Fig. 1. Schematic illustration of the electrolyte characteristics, hybrid EC assembly and its electrochemical investigations and properties: (a) structure of the choline cation, (b) DSC thermograms of 1 mol kg 1 ChNO3 (blue curve) and 5 mol kg 1 ChNO3 (green curve) in the range of 130 � C to RT, (c) Hybrid EC in 2-electrode Swagelok assembly with reference electrode and cyclic voltammograms (CVs) of the positive battery-type and negative EDL electrodes, (d) hybrid pouch cell components: positive and negative electrodes, porous membrane impregnated with the aqueous electrolyte (ChNO3þChI), and packaging, (e) fabricated hybrid pouch cell, (f) test station using a 4-lead cell connection, and (g) compared CVs (2 mV s 1) of hybrid and symmetric ECs at 20 � C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 284
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Chemicals; SDFT ¼ 1962 m2 g 1), 5 wt% polytetrafluoroethylene binder (PTFE 60% dispersion in water, Sigma-Aldrich) and 5 wt% carbon black (C-NERGY Super C65, Imerys). These components were mixed with a small amount of isopropanol (>99%, Sigma-Aldrich) and they were stirred at 70 � C until a homogenous suspension was obtained. After evaporation of the alcohol, the obtained electrode material in the form of dough was further rolled and calendared to give a sheet of a homo geneous thickness ca. 180 μm. The sheet was dried under vacuum at 120 � C for 12 h using Büchi Glass Oven B-585. Electrochemical cells were built in a Teflon Swagelok® vessel (Fig. 1c) by sandwiching a glassy microfiber separator (GF/A, What man™, thickness d ¼ 260 μm) between 10 mm diameter disk punched out from the dried electrode material, and stainless steel (type 316L) current collectors were used. An Hg/Hg2SO4; 0.5 mol L 1 K2SO4 (þ0.658 V vs Standard Hydrogen Electrode – SHE) reference electrode was implemented in order to measure the potential range of individual electrodes. For pouch cells realization, the previously calendared sheet of carbon material was attached to an etched stainless steel foil (316L from Interbelts, thickness d ¼ 10 μm) coated with carbon conductive ink (Electrodag™ PF-407A™, Acheson, thickness d ¼ 10 μm) and further slightly calendared to ensure the proper contact between the electrode and the stainless steel current collector. The obtained material on cur rent collector was dried under vacuum at 120 � C for 12 h using Vacu cell® drier, and then cooled down to room temperature. Two coated carbon electrodes (5 cm � 6 cm) were then cut and weighed, further soaked in the electrolyte and positioned to sandwich a 7 cm � 7 cm glassy microfiber separator (Whatman GF/A, thickness d ¼ 260 μm). Then, both the electrodes and the separator were introduced in a pouch case made of aluminium laminated film, and �1.0 ml of electrolyte was introduced inside the cell. Afterwards, each cell was degassed for 5 s under vacuum before closing and sealing by a Vacuum Packer (Fig. 1d and e). The electrochemical properties of the electrodes and cells in 5 mol kg 1 ChNO3 or in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI were evaluated by cyclic voltammetry (CV, 2 mV s 1), galvanostatic cycling with potential limitation (GCPL) and electrochemical impedance spec troscopy (EIS, frequency from 1 mHz to 100 kHz), using a VMP3 multichannel potentiostat-galvanostat (Biologic, France). All the twoelectrode pouch cells were slightly pressed between two aluminium plates, and their electrochemical properties investigated using a fourlead connection (Fig. 1f). For the low temperature investigations, the pouch cells were placed in an environmental chamber MK 53 (E2) (Binder GmbH, Germany, temperature variation�0.1 � C); the temperature was decreased stepwise and equilibrated for 30 min at each selected value before per forming the measurements. The state-of-health (SOH) of the pouch cells was evaluated at RT by galvanostatic cycling (0.5 A g 1) in the voltage range from 0.1 to 1.5 V. The cells were also discharged at constant power of 0.05, 0.1, 0.2, 0.5, 1.0, 1.5 W in the voltage range from 1.5 V to 0.75 V to determine their Ragone plot. All the values of current density (in A g 1), capacitance (in F g 1), energy and power density are expressed per device (total active mass of the two electrodes).
calorimetry (DSC) demonstrates that, during cooling, the electrolyte does not freeze down to 150 � C, while upon heating it exhibits cold crystallization followed by melting which is completed at 42 � C (Fig. 1b). The electrochemical characteristics of the hybrid EC have been optimized by employing a microporous carbon for enhancing the EDL capacitance of the negative electrode and a micro-/mesoporous carbon for more effectively encapsulating polyiodides in the porous network of the positive electrode. The electrochemical performance of the indi vidual electrodes has been investigated in a two-electrode Swagelok cell equipped with reference electrode (Fig. 1c). In order to validate the performance in a real device, prototype cells have been constructed in pouch cell assembly (Fig. 1d and e) and tested using a four-lead connection (Fig. 1f). We clearly demonstrate that the hybrid EC ex hibits twice higher capacitance (Fig. 1g) and energy density than the symmetric one owing to the hybridization of the battery-type and EDL electrodes in the chosen electrolyte. 2. Experimental 2.1. Preparation and properties of the aqueous electrolytes Choline iodide (ChI; 98%) was bought from Alfa Aesar. Choline ni trate (ChNO3) was synthesized through a simple metathesis reaction between choline chloride (98% from Sigma Aldrich) and sodium nitrate (99% from Sigma Aldrich). Choline chloride (0.2 mol) was dissolved in 500 mL of methanol (99.8% from Avantor) in a round bottom flask equipped with a condenser, sodium nitrate with 10% excess (NaNO3, 0.22 mol) was added to this solution, and the mixture was stirred for 24 h at 40 � C. Afterwards, the by-product in the form of white sodium chloride precipitate was removed by vacuum filtration. In order to enhance the conversion of the substrates into products, another portion of 0.2 mol NaNO3 was added to the filtrate, the mixture was stirred for another 24 h at 40 � C and the NaCl precipitate eliminated by filtration. This procedure was repeated until sodium chloride was fully precipi tated from the solution, while unreacted NaNO3 was removed by vac uum filtration. In the next step, methanol was evaporated at 50 � C in a rotary evaporator, and choline nitrate was dried in Glass Oven B-585 (Büchi) under vacuum at 50 � C for 48 h. The electrolyte solutions were prepared by dissolving either 5 mol kg 1 ChNO3 or 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI in deionized water (conductivity ¼ 0.05 μS cm 1 at 24 � C). The pH of these solutions was measured with Elmetron CPC-501 pH meter and a value of 6.2 was found both for 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI. The conductivity of the two electrolytes was obtained from the equivalent series resistance (ESR) determined by electro chemical impedance spectroscopy in a Swagelok-type cell equipped with a Teflon® ring determining an electrolyte volume of ca. 339 mm3; the temperature ranged from 24 � C to 40 � C using an environmental chamber MK 53 (E2) (Binder GmbH, Germany, temperature var iation�0.1 � C). For verification, measurements were also realized at 24 � C with Elmetron CPC-501 conductivity meter, and similar values as from the ESR were found. The low temperature thermal properties of the two electrolytes were investigated by differential scanning calorimetry (DSC) using Netzsch DSC 204 F1 Phoenix equipped with a liquid ni trogen cooling system enabling to cover the range from 25 � C to 150 � C. The samples (~5 mg) were introduced into hermetically sealed aluminium crucibles; cooling and heating was conducted with the scan rate of 10 K min 1 and this procedure was repeated twice for each sample.
3. Results and discussion 3.1. Conductivity and thermal behaviour of aqueous solutions based on choline salts In this study, the choline nitrate concentration was established arbitrarily to 5 mol kg 1 in analogy with our previous research using aqueous choline chloride for low temperature applications [38]. The temperature dependence of electrolytes (5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI) conductivity is shown in sup porting information in Fig. S1a. At 24 � C, the two electrolytes display conductivity values of 71 mS cm 1 and 89 mS cm 1, respectively. The conductivity remains at acceptable level down to 40 � C, reaching
2.2. Manufacturing and electrochemical investigations of electrodes and cells Carbon electrodes were prepared by mixing 90 wt% of physically activated carbon (named PAC) (YP–80F, Kuraray; SDFT ¼ 1735 m2 g 1) or KOH-activated carbon (named KAC) (Maxsorb, Kansai Coke and 285
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1.7 mS cm 1 and 4.2 mS cm 1, respectively. These values are in the same range as observed for 5 mol kg 1 choline chloride, i.e. 88 mS cm 1 at 24 � C and 4.8 mS cm 1 at 40 � C [38]. Owing to a higher number of charge carriers in the 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI medium, the conductivity is higher than for 5 mol kg 1 ChNO3. The deviation from linearity at low temperatures in the ln (conductivity σ) vs 1/T Arrhenius plot (Fig. S1b) for both salt solutions reveals glass forming liquids, which is consistent with the DSC results presented in the next paragraph. The differential scanning calorimetry analysis (DSC) realized down to 150 � C shows that the 5 mol kg 1 choline nitrate solution (Fig. 2a) does not crystalize upon cooling; only a glass transition appears at Tg ¼ 128 � C. During heating, the glass transition is also observed (Tg ¼ 125 � C) and is followed by cold-crystallization (Tonset ¼ 89 � C, Tpeak ¼ 84 � C, Toffset ¼ -80 � C) and melting in a wide temperature range (Tm; from Tonset ¼ -76 � C to Toffset ¼ -41 � C); at peak maximum (located at 44 � C), most of the solid is melted. Similarly, in the case of the 5 mol kg 1 choline nitrate þ0.5 mol kg 1 choline iodide solution (Fig. 2b), devitrification was detected during cooling (at 126 � C). Upon heating, the thermogram displays some resemblance to the mono-salt solution, with glass transition (at 125 � C), cold crystallization (Ton � � � set ¼ 90 C, Tpeak ¼ 89 C, Toffset ¼ -87 C) and melting (from Tonset ¼ 66 � C to Toffset 42 � C). However, in case of the choline nitrate þ choline iodide solution, the phase transitions are more complex than for 5 mol kg 1 choline nitrate; the main cold crystallization peak is preceded by a small peak (Tonset ¼ 99 � C, Tpeak ¼ 95 � C), whereas the thermal contribution illustrating the melting process consists of three peaks (Tpeak ¼ 61 � C, 50 � C and 45 � C). The devitrification process of aqueous solutions has been vastly discussed by Angell suggesting that, in aqueous solutions of salts incorporating an organic cation, the glass forming process depends on the cation, due to clathrate structures formed by the water molecules around the alkyl groups [39]. Here, this property is considered to be at the origin of the same Tg values of the mono- and bi-salt electrolyte solutions, as the cation is common for the two salts. In turn, the cold crystallization phenomenon is recognised to occur for glass-forming liquids, polymers or highly-viscous ILs incorporating voluminous ions. As the temperature decreases, the liquid viscosity increases causing a hindrance for the formation of crystal nuclei required for the crystalli zation process. Consequently, the compound does not crystalize as ex pected, and it remains in liquid state (super-cooled) far below the melting point. Taking into account that an energetically favourable conformation of the molecules is required for the crystallization, the later becomes possible during heating as their mobility increases, enabling the development of molecular structures favourable for this
Fig. 2. Thermograms of a) 5 mol kg
1
process, and appears as an exothermic peak [40]. The differences caused by the presence of choline iodide e.g., two peaks of cold crystallization and three of melting process, illustrates the formation of separate nanophases. Overall, both electrolytes are in liquid state above 40 � C, which fits with the previous investigations on 5 mol kg 1 choline chlo ride presented in Ref. [38]. 3.2. Performance of the symmetric and hybrid capacitors at room temperature Keeping in view that the 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI aqueous solutions described in the previous section display good conductivity values, it is certainly worth to investigate the cells using these electrolytes. In this section, the electrochemical per formance of two-electrode cells with reference electrode (swagelok-type assembly) and two-electrode pouch cells is investigated at room tem perature. The cells in ChNO3 implement a KOH-activated carbon (named KAC) for both electrodes, whereas an asymmetric configuration of electrodes with KAC for enhanced EDL capacitance of the negative electrode and physically activated carbon (named PAC) for effective polyiodides trapping in the positive electrode is used for the cells in ChNO3 þ ChI, similarly to the hybrid cell presented in Ref. [30]. The electrochemical behaviour of individual electrodes for the symmetric ( )KAC/KAC(þ) cell in 5 mol kg 1 aqueous ChNO3 and asymmetric ( )KAC/PAC(þ) hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI at room temperature is presented in Fig. 3. The potential extrema of the electrodes shown in Fig. 3a were determined during galvanostatic charge/discharge of the cell from 0.1 V up to various voltages in the range from 0.8 V to 1.5 V. The lower voltage value of 0.1 V was selected in order to compare this capacitor with the asym metric hybrid one, where the discharge had to be stopped at 0.1 V for avoiding the negative electrode to operate in the range of the iodide/ iodine equilibrium potential. For the cell based on 5 mol kg 1 ChNO3, the potential range of the two electrodes is comparable, which is generally typical for a symmetric carbon/carbon cell (Fig. 3a and b). Although the minimum potential of the negative electrode is lower than the hydrogen evolution potential ( 0.366 V vs. SHE) at high voltage values, the charging characteristics remain linear, which is attributed to the high over-potential of di-hydrogen evolution related with the local pH increase inside the carbon porosity [11,41]. By contrast, the slight deviation of the positive electrode characteristics from linearity at voltage higher than 1.1 V (Fig. 3b) during galvanostatic charging reveals a non-pure EDL performance. As the maximum potential of this elec trode becomes higher than the thermodynamic limit of water oxidation (at pH ¼ 6.2, the oxygen evolution potential, OEP, is 0.864 V vs. SHE) at
ChNO3 and b) 5 mol kg 286
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ChNO3 þ 0.5 mol kg
1
ChI (scanning rate 10 � C min 1).
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Fig. 3. Comparison of the electrochemical properties of individual electrodes at 24 � C in (a, b) the ( )KAC/KAC(þ) cell using 5 mol kg 1 ChNO3 and (c, d) ( )KAC/ PAC(þ) cell using 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI. Data obtained by galvanostatic (0.1 A g 1) cycling with potential limitation (GCPL) of two-electrode cells with reference electrode [Hg/Hg2SO4 in 0.5 mol L 1 K2SO4 (þ0.658 V vs. SHE)]. The dashed lines in a) and c) indicate the oxygen evolution potential (OEP) and hydrogen evolution potential (HEP) estimated by applying the Nernst equation for aqueous solutions with pH ¼ 6.2.
voltage higher than 1.1 V (Fig. 3a), this deviation is the signature of an irreversible oxidation of the positive electrode. The simultaneous shift of E0.1 (electrode potential at voltage of 0.1 V) for the two electrodes in the voltage range from 1.2 to 1.5 V is attributed to this change of surface functionality of the positive electrode [42]. By contrast, in case of the asymmetric ( )KAC/PAC(þ) hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI, the potential range of the positive electrode is very narrow, which is typical for a battery-type electrode (Fig. 3c and d). According to the literature and to the Pourbaix diagram of iodine [43–45], such narrow potential range results from a few faradaic reactions related with electron transfer and formation of iodine and polyiodides. As a consequence of the electrolyte redox activity and micro-/mesoporosity of the carbon used for effective polyiodides trap ping [30], the maximum potential of the positive electrode at U ¼ 1.5 V is only 0.744 V vs. SHE (as compared to 1.003 V vs. SHE in the sym metric cell using ChNO3), which is much below the water oxidation limit (0.864 V vs SHE). Consequently, such low potential prevents from oxidation of the positive carbon electrode, which explains the absence of E0.1 deviation in this electrolyte. Thus, owing to the narrow potential range of the positive electrode and high capacity of this electrode, the potential range of the EDL-type negative electrode (ΔE-hyb ¼ 1.156 V, Fig. 3d) is almost twice higher than in the symmetric cell
(ΔE-sym ¼ 0.631 V). It suggests that the capacitance of the asymmetric hybrid cell should be ca. twice higher than the capacitance of the symmetric one [21]. However, by contrast with the symmetric cell, the minimum potential of the negative electrode in the asymmetric ( ) KAC/PAC(þ) hybrid cell at voltage of 1.5 V is 0.749 V vs SHE (Fig. 3c), which is around 380 mV below the electrolyte reduction potential. Consequently, one can observe a slight deviation from linear charging profile of the negative electrode in this cell (Fig. 3d). Nevertheless, high over-potential of di-hydrogen evolution caused by the presence of ChNO3 as supporting electrolyte enables this electrode to still operate within the optimized potential range. The cyclic voltammograms (CVs) and galvanostatic charge/ discharge characteristics of the asymmetric ( )KAC/PAC(þ) hybrid pouch cells in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI are presented in Fig. 4a and b, respectively. For all the voltage values in the range from 0.1 V to 1.5 V, the CVs (Fig. 4a) display the rectangular shape charac teristic for a capacitor. Similarly, up to 1.5 V, the galvanostatic charge/ discharge displays the linear profile of a capacitor (Fig. 4b). As advised in Ref. [46], in order to properly estimate the efficiency and capacitance of the cells, the surface area under the GCPL curves should be integrated. Thus, as a result of the iodides faradaic contribution, the asymmetric hybrid EC displays advantageously a capacitance of 81 F g 1 at 1.5 V 287
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Fig. 4. Electrochemical performance of the asymmetric ( )KAC/PAC(þ) hybrid pouch capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI: a) CVs at 2 mV s 1; b) GCPL curves at 0.1 A g 1. c) Capacitance and d) energy efficiency vs voltage of the ( )KAC/PAC(þ) hybrid capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI (green continuous line) and ( ) KAC/KAC(þ) symmetric capacitor in 5 mol kg 1 ChNO3 (red dashed line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. a) Specific cell capacitance and relative resistance recorded at RT during 20,000 galvanostatic (0.5 A g 1) cycles from 0.1 V to 1.5 V for symmetric ( )KAC/KAC (þ) capacitor in 5 mol kg 1 ChNO3 (red dotted lines) and asymmetric ( )KAC/PAC (þ) hybrid capacitor in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI (green dashed lines). Comparison of the GCPL (0.5 A g 1) curves before and after the cycling tests on b) symmetric and c) asymmetric hybrid elec trochemical capacitors. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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(Fig. 4c) which is approximately twice higher than 41 F g 1 given by the symmetric cell in 5 mol kg 1 ChNO3. It stays in accordance with the theoretical concept of hybridization developed by Conway and Pell which states that the capacitance of a hybrid cell is (ideally) doubled when compared to the capacitance of a symmetric one with the same carbon electrodes [21]. In addition, as shown in Fig. 4d, the energy ef ficiency of the hybrid cell is ca. 5% higher than for the symmetric one whatever the applied voltage, owing to optimized porosity of positive electrode related with confinement of polyiodides which are accom modated well in big micropores/small mesopores of PAC in hybrid cell, as explained in our previous work [30]. The state-of-health of both symmetric capacitor in ChNO3 and asymmetric hybrid capacitor in ChNO3 þ ChI during prolonged galva nostatic (0.5 A g 1) cycling up to 1.5 V was evaluated using pouch cells (Fig. 5a). The hybrid capacitor exhibits a stable and very high capaci tance of 75 F g 1 up to 3000 cycles; then the capacitance slightly de creases to reach 92% of its initial value after 20,000 cycles. The capacitance of the symmetric EC drops very slowly throughout the test, reaching also 92% of its initial value after 20,000 cycles. In parallel, the relative resistance of the two kinds of cells increases by only 8% after 20,000 cycles. Fig. 5b and c confirms that the galvanostatic charge/ discharge characteristics of both types of cells are almost unchanged after the cycling test. Similarly, excellent life span was shown during potentiostatic floating at 1.5V on the two cells in ChNO3 and ChNO3 þ ChI (Fig. S2). Overall, it can be concluded that the hybrid EC exhibits ca.
twice higher capacitance than the symmetric cell, and that it demon strates excellent capacitance retention at room temperature when charged up to 1.5 V. Due to the fact that ageing is generally hindered at low temperature owing to the lower kinetics of the ongoing faradaic processes, one can expect even better electrochemical stability at subambient values [34,38]. 3.3. Electrochemical performance of symmetric and asymmetric hybrid pouch cells at low temperature Taking into account the excellent performance of the electro chemical cells at room temperature, the promising conductivity values of the two electrolytes down to 40 � C (Fig. S1a) and their low melting point (Fig. 2a and b), the pouch cells were investigated at sub-ambient temperature. Fig. 6a presents the cyclic voltammograms of the sym metric capacitor in 5 mol kg 1 ChNO3 from RT down to 40 � C. When the temperature decreases, the current leaps related with hydrogen chemisorption (at high voltage during anodic scan) and desorption (at low voltage during cathodic scan) diminish progressively until almost completely disappearing at 40 � C, where the hydrogen chemisorption process is quenched [34]. However, a part of the high voltage current leap at RT might be also attributed to the oxidation of the positive electrode, as it was previously suggested when discussing Fig. 3a and b. Interestingly, the charge propagation remains almost the same in the whole temperature range, except at 40 � C where the CV is slightly less
Fig. 6. a, c) CVs at 2 mV s 1 and b, d) GCPL curves at 0.1 A g 1 of (a, b) symmetric ( ) KAC/KAC(þ) cell using 5 mol kg (þ) hybrid cell using 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI from RT down to 40 � C. 289
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ChNO3 and (c, d) asymmetric ( )KAC/PAC
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rectangular (Fig. 7a). The discharge time in the GCPL curves (Fig. 6b) gradually decreases along with decreasing temperature, reflecting the effect of electrolyte conductivity on the electrochemical cell perfor mance. In case of room temperature, the non-linear trend of the charging curve at high voltage is due to the irreversible oxidation of the positive electrode as previously mentioned in the comments of Fig. 6a. In order to demonstrate the gradual change of charging characteristics at various cut-off voltages with decreasing temperature, CV and GCPL curves for symmetric capacitor in 5 mol kg 1 ChNO3 from 0.8 V up to 1.5 V are presented in supporting information in Fig. S3 (at RT and 20 � C) and Fig. S4 (at 30 � C and 40 � C). The asymmetric hybrid cell in ChNO3 þ ChI was also investigated by cyclic voltammetry (Fig. 6c) and galvano static charge/discharge (Fig. 6d) at various temperatures from 24 � C to 40 � C. A moderate decay of the capacitive current (in case of CVs) and discharge time (in case of GCPL curves) was observed, especially at 30 � C and 40 � C, and is attributed to i) the lowering of electrolyte conductivity and ii) kinetically hindered redox processes taking place at the positive electrode. Fig. S5 in supporting information shows the CV and GCPL curves at the voltages from 0.8 V to 1.5 V for asymmetric hybrid capacitor in ChNO3 þ ChI at 20 � C, 30 � C and 40 � C. Fig. 7 presents the temperature dependence of capacitance and en ergy efficiency (using the data obtained by integration of the surface area under the GCPL curves) for the ( ) KAC/KAC(þ) cell in ChNO3 and ( )KAC/PAC(þ) hybrid cell in ChNO3 þ ChI. Logically, for both sys tems, the capacitance decreases with temperature, reaching relatively high values of 50 F g 1 and 31 F g 1 at 40 � C for the asymmetric hybrid and symmetric devices, respectively. Also, it is worth to notice that the capacitance drops more rapidly in case of the hybrid capacitor, obvi ously as a consequence of kinetical limitations of the redox reactions (related with the iodine/iodide system). However, the asymmetric hybrid cell still demonstrates much higher capacitance at 40 � C than the symmetric one, showing that the redox processes are still in play at such low temperature. Furthermore, the capacitance of the ( )KAC/PAC (þ) hybrid cell (50 F g 1) at 40 � C is still higher than the 41 F g 1 displayed by the symmetric cell at room temperature. Interestingly, for both cells, the energy efficiency initially increases with decreasing the temperature (reaching a maximum of 90% in the range from 0 to 20 � C). This efficiency increase for both cells is attributed to the quenching of the redox processes associated with hydrogen chemi sorption in the negative electrode [34]. When the temperature is further decreased to 40 � C, the energy efficiency of the two systems drops to reach 84% and 77% for the symmetric and asymmetric hybrid cells,
respectively. This decrease is related to the hindered mobility of ions within the electrode porosity, which will be further confirmed by the electrochemical impedance spectroscopy data (Fig. 8a and b). However, the slightly lower energy efficiency demonstrated by the ( )KAC/PAC (þ) hybrid cell at very low temperature (from 20 � C down to 40 � C) is additionally due to the slow kinetics of the faradaic reactions associated with the iodide/iodine redox couple. The symmetric and asymmetric hybrid pouch cells in 5 mol kg 1 ChNO3 and 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI, respectively, have been investigated by electrochemical impedance spectroscopy (EIS) at open-circuit voltage (OCV) in the temperature range from RT to 40 � C; the Nyquist plots are shown in Fig. 8a and b. From the high frequency region, presented as insets in Fig. 8a and b, it is visible that the equiv alent series resistance (ESR) and equivalent distributed resistance (EDR) for both cells increase as temperature decreases. Interestingly, owing to the use of high-quality electrodes and pouch cell construction, there is not any semicircle visible on the Nyquist plots, which confirms the absence of charge transfer between the electrodes and the stainless steel current collectors. The ESR and EDR values of the two kinds of cells at various tem peratures are listed in Table 1. The increase of ESR as temperature de creases for the two cells is related with the decrease of bulk electrolyte conductivity. When decreasing the temperature, the EDR values, which represent the extended intermediate frequency part of the Nyquist plots (also known as mass transport region governed by the diffusion of charged species within the electrode porosity), indicate that the re sistances related with the mass transport of ionic species increase in both asymmetric hybrid and symmetric cells. The slightly higher increase of EDR observed at very low temperature (from 20 � C down to 40 � C) for the asymmetric hybrid cell is attributed to the slow kinetics of the iodide/iodine redox reaction, as already suggested when considering the higher decrease of energy efficiency for this cell in Fig. 7. The Nyquist plots of the two cells mainly differ in the intermediate frequency region, where the symmetric cell in ChNO3 behaves as an EDL capacitor with vertical trend of the characteristics whatever the temperature (Fig. 8a), while the asymmetric hybrid cell in ChNO3 þ ChI (Fig. 8b) demonstrates a significant deviation from vertical trend in the temperature range from 20 � C down to 40 � C, confirming the progressively slower kinetics of the iodide/iodine redox reaction with decreasing temperature. The Ragone plots of the asymmetric hybrid and symmetric cells discharged at constant power in the voltage range from 1.5 V to 0.75 V are shown in Fig. 8c (discharge curves for these two cells at RT are shown in Fig. S6). For comparison, the RT plot of a symmetric EDLC in organic electrolyte (1 mol L 1 TEABF4 in acetonitrile) in the voltage range from 2.7 V to 1.35 V is also included. At room temperature, and whatever the specific power value, the specific energy of the asymmetric hybrid cell in ChNO3 þ ChI and EDLC in organic electrolyte is compa rable and ca. twice higher than for the symmetric cell in ChNO3. For example, at a power of 3.0 kW kg 1, the energy of the hybrid EC in ChNO3 þ ChI is 11.6 Wh kg 1 as compared to 4.9 Wh kg 1 for the symmetric EC in 5 mol kg 1 ChNO3, and 12.1 Wh kg 1 for the cell in organic electrolyte. Moreover, at high power of 1.5 kW kg 1, the asymmetric hybrid EC is able to preserve 16.8, 12.1, 3.5 and 0.8 Wh kg 1 at RT, 20 � C, 30 � C and 40 � C, respectively, confirming that down to 20 � C the electrochemical performance is of the same order as at RT, as already demonstrated by the Nyquist plots. Such high energy values obtained at low temperature (especially at 20 � C) for the asymmetric hybrid cell using the bi-functional aqueous electrolyte are extremely promising for the development of low cost and environmen tally friendly electrochemical capacitors. 4. Conclusion
Fig. 7. Temperature dependence of capacitance and energy efficiency for symmetric ( ) KAC/KAC(þ) capacitor in 5 mol kg-1 ChNO3 (dotted lines) and ( ) KAC/PAC(þ) hybrid capacitor in 5 mol kg-1 ChNO3 þ 0.5 mol kg-1 ChI (dashed lines), obtained from GCPL (0.1 A g-1) up to 1.5 V.
Taking into account the serious drawbacks of environmentally un friendly organic electrolytes, aqueous choline nitrate presents an actual opportunity towards the realization of eco-friendly, sustainable and low 290
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Fig. 8. Performance of symmetric EC in 5 mol kg 1 ChNO3 and asymmetric hybrid EC in 5 mol kg 1 ChNO3 þ 0.5 mol kg 1 ChI at various temperatures from 24 � C down to 40 � C: Nyquist plots at open-circuit voltage on a) symmetric EC and b) hybrid EC; c) Ragone plots where an EDLC in organic electrolyte at RT (black full line) is shown for comparison. The data for Ragone plots were obtained from discharge at constant power in the range from 1.5 V to 0.75 V for both symmetric and asymmetric ECs in aqueous electrolyte and from 2.7 V to 1.35 V for the EDLC in organic electrolyte. The power values during cell discharge correspond to current regimes from 0.1 A g 1 to 5.9 A g 1 (expressed for a two-electrode device).
cost electrochemical capacitors. We have demonstrated that the 5 mol kg 1 choline nitrate solution displays good conductivity, neutral pH and does not freeze down to 150 � C. Owing to these properties, it then becomes feasible to implement an aqueous bi-functional electrolyte with choline nitrate as supporting salt enabling a voltage extension up to 1.5 V and choline iodide as redox active component allowing hybridi zation of electrodes. Owing to this hybridization, the cell demonstrates square shaped cyclic voltammograms, linear charge/discharge charac teristics, stable capacitance and resistance down to 40 � C, while dis playing twice higher capacitance. Down to 20 � C, the performance of the hybrid EC is comparable to the values at room temperature, in terms of energy and power. Definitively, the use of aqueous choline nitrate þ choline iodide electrolyte enables to develop an environmentally friendly and low cost capacitor approaching the performance of organic electrolyte-based systems in a wide range of sub-ambient temperatures.
Table 1 Temperature dependence of the equivalent series resistance (ESR) and equiva lent distributed resistance (EDR) for the symmetric cell in 5 mol kg 1 ChNO3 and asymmetric hybrid cell in 5 mol kg 1 ChNO3þ 0.5 mol kg 1 ChI at open circuit voltage. Temperature/� C
24 0 10 20 30 40
Symmetric cell in 5 mol kg 1 ChNO3
Asymmetric hybrid cell in 5 mol kg 1 ChNO3 þ 0.5 mol kg ChI
ESR/Ω
EDR/Ω
ESR/Ω
EDR/Ω
0.131 0.135 0.139 0.148 0.166 0.202
0.167 0.204 0.229 0.282 0.393 0.602
0.131 0.152 0.155 0.175 0.218 0.298
0.178 0.289 0.293 0.367 0.564 0.747
1
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Acknowledgements
[15] [16] [17] [18] [19] [20]
The Polish National Science Center (NCN – Narodowe Centrum Nauki) is acknowledged for funding the OPUS project UMO 2014/15/B/ ST4/04957. P.P, Q.A and B.G. thank the Ministry of Science and Higher Education of Poland for financial support under the project DS 03/31/ DSMK/0367/2018. Authors are grateful to Imerys for providing the carbon black C65 and Kuraray for the carbon YP-80F.
[21] [22] [23] [24] [25]
Appendix A. Supplementary data
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