Supercapacitors Based on Propylene Carbonate Solution Operating from -45 ºC to 100 ºC. A. Jänes, T. Thomberg, J. Eskusson, and E.

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1 ECS Transactions, 64 (0) (015) / ecst The Electrochemical Society Supercapacitors Based on Propylene Carbonate Solution Operating from -45 ºC to 100 ºC A. Jänes, T. Thomberg, J. Eskusson, and E. Lust Institute of Chemistry, University of Tartu, 14a Ravila Street, Tartu, Estonia The most promising organic electrolyte for medium temperature range supercapacitors (SC) seems to be a solution of (C H 5 ) 3 CH 3 NBF 4 in acetonitrile (AN), characterised by excellent conductivity, low viscosity and a relatively broad electrochemical stability window. However AN low boiling point means that the vapour pressure would be high in hot environments, thus, the high flammability and risk of explosion at higher temperatures, decreasing noticeably the safety issues for SC, are critical. Propylene carbonate was used as the most suitable solvent, based on a comprehensive consideration taking into account the good electrochemical stability, conductivity and performance at moderate temperature. The main aim of this paper is to study the electrochemical characteristics of propylene carbonate based electrical double layer capacitors using the microporous TiC-CDC electrodes within the wide region of temperature, i.e., from -45 ºC to 100 ºC. Introduction Supercapacitors (SC), also known as electric double layer capacitors or electrochemical capacitors are the electrochemical energy storage devices in which the electric charge is stored in the electrical double layer formed at the interface between electrode and an electrolyte solution. These devices can provide high power capability, excellent reversibility and long cycle life. Typically they exhibit times larger capacitance per unit volume or mass than conventional capacitors. The electrolyte used is typically one solvent or solvent mixture containing one or sometimes two or three salts. In many cases, the physical and electrochemical properties of the electrolyte are a key factor determining the internal resistance of the supercapacitor and the therefore the power density properties. The most promising organic electrolyte for medium temperature range SCs seems to be a solution of (C H 5 ) 3 CH 3 NBF 4 in acetonitrile (AN), characterised by excellent conductivity, low viscosity and a relatively broad electrochemical stability window. However, acetonitrile low boiling point means that the vapour pressure would be high in hot environments. The high flammability and risk of explosion at higher temperatures of supercapacitors significantly decreases the safety of their use. At room and higher T, the propylene carbonate (PC) was used as the most suitable solvent, based on a comprehensive consideration taking into account the good electrochemical stability, conductivity and performance at moderate temperature. The electrochemical stability 31 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

2 ECS Transactions, 64 (0) (015) window of tetraalkylammonium salts in organic solvents determined at microporous carbon electrodes suggests the possibility of operating voltages up to 3. V. However, analysis of unpublished results for SC based on carbide derived carbons in (C H 5 ) 3 CH 3 NBF 4 acetonitrile electrolyte demonstrated that for maximizing energy and power the pore size distribution, i.e., interplay between the effective diameter of partially desolvated ions with the pore medium diameter, discussed already since 000 (1-7), show that for increase of power density values the mass of negative electrode must be increased due to the higher effective area occupied by one adsorbed (C H 5 ) 3 CH 3 N + cation compared with BF - 4 (,6-8). The main conclusion that can be made is that the effective charge balance between negatively and positively charged electrodes depends strongly on the specific surface area and pore size of material under study. The main aim of this paper is to study the electrochemical characteristics of PC-based (melting temperature ºC, flash point 13 ºC, boiling temperature 4 ºC) electrochemical double layer capacitors based on the microporous TiC-CDC electrodes, within wide region of temperature, i.e., from -45 ºC to 100 ºC. Experimental All the experiments discussed were made inside the glove box MBraun Labmaster sp at very clean and dry conditions (O and H O concentration lower than 0.1ppm). The two-electrode Al test cell with two identical electrodes (flat cross section surface area.0 cm, thickness 100 ± 3 m) was assembled inside the glove box and between working electrodes the 5 m thick Nippon Kodoshi TF445 separator layer was used. The argon ( %) has been used for saturation of the ("Merck",Selectipur, moisture content by Karl Ficher titration method less than 0 ppm) electrolyte. PC has been stored over molecular sieves before used for preparation of the 1 M (C H 5 ) 3 CH 3 NBF 4 (Stella Chemifa, Japan, assay 99.9 %, moisture content < 50 ppm, additionally purified and dried) electrolyte. A microporous carbon powder was synthesized from titanium carbide (TiC, Alfa Aesar, m powder) by a gas phase reaction with Cl at T = 950 C for 4 h. After reaction of TiC with Cl, the system has been flowed by Ar % and thereafter H has been used for 8 h (at T = 800 ºC) to reduce all the chemically active functional groups from the surface of TiC-CDC. Thereafter Ar % for 1 h has been used to evacuate the adsorbed molecular H from the TiC-CDC carbon powder. Specific surface area, pore size distribution, micropore volume, micropore surface area and other parameters were measured using the Micromeritics ASAP 00 system and calculated according to the BET surface area and t-plot methods discussed more detail in (). Before assembling of the symmetrical SC cells, the Ti-CDC powder was heated at T = 300 C for 48 h under reduced pressure (p < 10-3 atm). The electrodes were constituted of the microporous TiC- CDC particles, visualized in Fig. 1, from Teflon (PTFE) binder and were roll-pressed together. Electrodes were covered by the pure Al layer from one side by the electro magnetron sputtering method (-10). 3 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

3 ECS Transactions, 64 (0) (015) Fig. 1. SEM (a) and HRTEM (b) images of TiC-CDC electrode material used for studies. The dynamic viscosity, η, of PC at fixed temperatures from -45 to 100 ºC was obtained using U-shaped viscosimeter with flow pipe diameter 0.8 mm. The viscosimeter was calibrated with MilliQ + water at 0.0 ± 1 C. Fig. demonstrates dependence of dynamic viscosity and specific conductivity ( of 1 M (C H 5 ) 3 CH 3 NBF 4 solution in carbonate PC on temperature applied. As it can be seen in Fig., nearly exponentially increases with the increase of temperature. Fig.. Dynamic viscosity and specific conductivity dependencies of 1 M (C H 5 ) 3 CH 3 NBF 4 propylene carbonate based electrolyte measured at different temperatures. Inset: Physical characteristics of PC and AN. The rectangular shape of the cyclic voltammograms expressed as capacitance vs. cell potential (C, E) plots, presented in Fig. 3, show that the so-called nearly ideal capacitive behaviour has been established at potential scan rates v 10 mv s -1 from -5 to 80 ºC (Fig. 3a). The potential scan rate at witch the so-called distortion effects can be seen in the region of potential switch over increases with the increase of temperature applied. Thus, the dependence of the shape of the current density, cell potential (j, E)-curves is mainly influenced by the viscosity and molar conductivity of electrolyte. At higher 33 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

4 ECS Transactions, 64 (0) (015) temperatures T 80 ºC partial charge transfer or some faradic reactions started at E.6 V (Fig. 3b). However, as it was demonstrated by Laheäär et al., and Jänes et al. (3-5), it is impossible to separate the practical charge transfer and faradic current components based on cyclic voltammetry data only. (a) (b) Fig. 3. Cyclic voltammograms expressed as capacitance vs. cell potential curves measured at different temperatures (at scan rate 10 mv s -1 ) (a), and at T = 100 ºC (at different scan rates) (b). It should be noted that the values of capacitance C can be calculated from j, E curves according to Eq. 1: 1 C j( d E / dt), [1] if we assume that the capacitance C is constant (C f( E)), or if the series resistance R s 0 or if the current j 0 (d( E)/dt = v is the potential scan rate) (-10). Thus, Eq. 1 can be used for calculation of the capacitance values only in the region of small potential scan rates if the values of current are very small (as the potential drop (jr drop) losses are negligible only at these conditions) and the current response is essential of a pure capacitor. In a symmetrical two-electrode system used the specific capacitance C m (farads per gram) of the activated carbon can be obtained from the capacitance of the cell by Eq. : C Cm m, [] where m is weight (g) of the TiC-CDC if, to the first approximation, we assume that the positively and negatively charged electrodes have the same capacitance at fixed E. Analysis of the experimental data demonstrates that within T from -45 to 100 ºC, the values of C are practically independent of v, if v 5 mv s -1. Thus, the nearly equilibrium values of capacitance corresponding to the adsorption/absorption equilibrium can be obtained only at slow potential scan rates for all temperatures studied. Cyclic voltammograms measured at different temperatures (Fig. 3a) indicate that only at T > 80 ºC, noticeable deviation of system from ideally polarisable system behaviour has been established at E > 3.0 V, where, some faradic processes started. With the decrease of temperature lower than 0 ºC noticeable decrease of capacitance takes place. Surprisingly, even at -45 ºC, quite remarkable capacitance values have been established (C > 50 F g -1 ). For more detailed analysis of cells the Nyquist plots have been measured within the range of ac frequencies from 1 mhz to 300 khz at fixed cell potentials and temperatures from -45 ºC to 100 ºC. According to the Nyquist plots data, given in Fig. 4a, for different temperatures, the fairly conventional behaviour for a two porous electrode system with 34 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

5 ECS Transactions, 64 (0) (015) uniformly distributed solution series resistance, R s, and double layer series capacitance, C s, is exhibited (except at T = -45 ºC and 100 ºC). (a) (b) (c) Fig. 4. Nyquist plots (a), phase angle (b) and series capacitance (c) vs. ac frequency plots ( E = 3.0 V) for the SC measured at different temperatures. At T = 80 ºC and 100 ºC, the high-frequency faradic processes are very well visible as minimum (at f = 5 khz) in, log f plots (Fig. 4b). Based on Nyquist plots analysis (Fig. 4b) the linear section with the phase angle value 45 within ac frequency region 0.01 < f < 10 Hz (usually called as the nanoporous section of the complex plane plot) has been observed. According to the model taking into account the so-called two-level (i.e. hierarchical) structure of the activated carbon electrodes for supercapacitor, there are two very well expressed regions in the phase angle, log f -plots at T = 80 ºC. The first plateau at frequencies from 1 khz to 1 Hz is characterizing the so-called adsorption and mass transfer processes in mesoporous region of porous electrodes and the second region at f < 0.01 Hz, characterises processes in microporous area. It is very important to stress that with decreasing temperature the mass transfer limitations expresses more clearly. Extrapolation of the high frequency part of the Z, Z curve to the condition Z = 0 gives the so-called equivalent series resistance of the cell (ESR) (R E = Z ( = R s ( ). R E values depend strongly on the temperature used (Fig. 4a, inset), being in an agreement with the molar conductivity data, given in Fig.. It should be mentioned that the complex impedance (Fig. 4a) of system studied depends noticeably on T applied and Z values decrease from -45 ºC to 80 ºC. At T = 100 ºC, Z values are ten times higher than at T = ºC indicating the blocking adsorption of faradic reaction products (H and O gases) inside of the porous carbon material. Similarly to the 35 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

6 ECS Transactions, 64 (0) (015) other non-aqueous electrolyte systems, noticeable increase of C s (Fig. 4c) at f < 1 Hz takes place indicating to the establishment of adsorption/desorption equilibrium for systems under study. Thus, at f Hz, long plateaus in C s, log f plots have been established, indicating that the limiting capacitance values are quite high. Low frequency C s ( 0) values depend very strongly on the temperature applied and C s ( 0) values increase from -45 ºC to ºC. Due to the physical adsorption C s ( values decrease from 60 ºC to 100 ºC. The values of the real and imaginary part of capacitance have been calculated according to Eqs. 3 and 4: C( C'( jc"(, [3] Z"( Z'( C'( C"( Z( Z(,, [4] where Z( is the complex impedance. It should be mentioned that the low frequency part of C for the supercapacitor cell corresponds to the so-called static capacitance, which is measured during the constant current discharge/charge steps, and C corresponds to the energy dissipation by an irreversible electrochemical processes. Fig. 5. C vs. ac frequency plots ( E = 3.0 V) for the SC measured at different temperatures. Inset: corresponding relaxation time constant R1 vs. temperature plot. According to the results presented in Fig. 5, the C, f- dependencies have a maximum at the so-called relaxation frequency, f R, determining the characteristic time constant R1 = ( f R ) -1 (Fig. 5, inset). R1 depends noticeably on the temperature applied and R1 increases with the decrease of the specific conductivity of the electrolyte. The values of complex power can be expressed as following (11): S( P( jq(, [5] where the real part of power is expressed as P( C"( Erms, [6] and the imaginary part of power as Q( C'( E rms, [7] Erms Emax with (E max is the maximal amplitude of ac potential). Ideal capacitor (a system with the ideal capacitive behaviour) has no real part as there is only the reactive contribution to the complex power, hence Eq. 5 simplifies: 36 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

7 ECS Transactions, 64 (0) (015) j Erms S( ) jq j C E rms. [8] Z" Systems with the ideal resistive behaviour have no imaginary part as this component only dissipates energy and the complex power takes the well-known form Erms S( Z'. [9] It should be noted that real supercapacitors balance between two states mentioned before: resistive at high frequencies ( and capacitive at low frequencies ( 0). Between these two states supercapacitor behaves like a resistance-capacitance (RC) transmission like circuit (10,11). The dependence of the normalised real part (P(/ S ) and imaginary part (Q(/ S ) for the complex power on ac frequency are presented in Fig. 6. In a good agreement with the data in Fig. 6, the relaxation time constants, obtained in Fig. 6 as the frequency of the intersection point of the (P(/ S ), logf - and (Q(/ S ), logf -plots ( R = ( f R ) -1 ) (Fig. 6 inset), are very different for SC cells measured at different temperatures. Thus, comparison of the data for the cells measured at different temperatures indicates the huge influence of the viscosity of the PC electrolyte at moderate and low ac frequencies on the limiting capacitance and time constant values calculated. The data for T = 100 ºC are totally different from data measured at T 80 ºC, indicating that at T 100 ºC, the role of faradic processes, as well as evaporation of residual H O (squeezing out of electrolyte from micropores and decomposition to H and O ) has a very important influence onto complex power and complex energy values. Fig. 6. Normalized reactive power Q / S and active power P / S vs. ac frequency plots for the SC, measured at different temperatures. Inset: corresponding relaxation time constant R vs. temperature plot. For the more detailed analysis, the constant current charging/discharging curves (Fig. 7a) have been integrated to obtain the charge densities accumulated during charging Q ch and discharging Q disch steps (Fig. 7b). The calculations show that in agreement with CV data measured at -45 T 70 ºC, the coulombic efficiency is very high ( 97.8 %) at E = 3.0 V. The significant decrease of Q ch and Q disch takes place at temperatures lower than -30 ºC ( 99.7 %) and low coulombic efficiency 94.4 % has been calculated at T = 100 ºC. 37 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

8 ECS Transactions, 64 (0) (015) Fig. 7. Constant current charging/discharging curves (a), and integrated surface charge (filled symbols) / discharge density (empty symbols) ( E = 3.0 V, j = 10 ma cm ) and coulombic efficiency vs. temperature dependences at different temperatures (b). For better comparison with our previous studies (-7), the maximum theoretical energy density (E max ) and power density (P max ) values at E = 3.0 V were calculated (Fig. 8a) according to Eqs. 10 and 11: Cs Sel E Emax m 3.6, [10] E Sel Pmax 4 R E m [11] In Eqs. (10) and (11) C s (F cm - ) is the series capacitance at f = 1 mhz, S el is the flat cross-section surface area of electrodes, m is the mass of two electrodes (g), R E (Ω cm ) is the high-frequency series resistance and 3.6 comes from conversion of time and mass units. Constant power charge/discharge tests have been made using two electrode SCs at different temperatures. The energy and power relationships (calculated taking into account the total active material weight of two electrodes) for the SCs at different temperatures have been calculated within the cell potential range from 3.0 V to 1.5 V. The results calculated based on the optimal power density values (P max 10 kw kg -1 ) are given in Fig. 8a. It can be seen that quite high energy density (~ 7-30 W h / kg -1 ) values at moderate power densities (~ 1 kw kg -1 ) have been measured. Data in Fig. 8a show that from -5 ºC to 80 ºC, the E max is practically independent of the temperature applied, caused by the practically constant values of C s and R s calculated for SC based on PC electrolyte within T from -5 ºC to 80 ºC. However, at fixed E = 10 W h kg -1, noticeable dependency of P max on the temperature applied has been observed (Fig. 8a). 38 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

9 ECS Transactions, 64 (0) (015) Very quick decrease of P max (Fig. 8b) has been established for SC tested at -35 ºC, explained by high series resistance values of electrolyte. Fig. 8. The maximum specific energy (E max ) and specific power (P max ) dependences ( E = 3.0 V) for the SC calculated at different temperatures (a). Ragone plots for the SCs completed using TiC-CDC electrodes in 1 M (C H 5 ) 3 CH 3 NBF 4 propylene carbonate solution, obtained from constant power tests within the cell potential range from 3.0 V to 1.5 V (b). The so-called floating tests, i.e., the repetitive holding of SC cells at constant potential E =.7 V for fixed time t = 5 h, thereafter applying constant current charge/discharge regimes with A g -1 within the cell potential range from 1.35 to.7 V and impedance spectroscopy tests at.7 V, have been applied to investigate the electrochemical stability of SC cells under study. The charge and discharge capacitances were calculated from data of the third cycle. The capacitance of the cell was obtained from the slope of the discharge curves according to Eq. 1: dt CCC j d( E), [1] where dt/d( E) is the slope of the discharge curve at applied constant current density j. Fig. 9. Results of floating tests for SC at T = 90 ºC and E =.7 V. After 45 hours of floating the specific capacitance (Fig. 9) calculated from constant current charge/discharge curves 64.9 F g -1 is lower due to the electrochemical degradation of SC cell at T = 90 ºC. The high temperature has been selected to accelerate the testing of system under study. At T 80 ºC, and especially at T -0 ºC, the stability of cells was noticeably better due to the absence of faradic processes observed at higher T 80 ºC. 39 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

10 ECS Transactions, 64 (0) (015) Conclusions The electrochemical characteristics of the supercapacitors (SC) consisting of the two identical mainly microporous titanium carbide derived carbon (TiC-CDC) electrodes in 1 M (C H 5 ) 3 CH 3 NBF 4 solutions in propylene carbonate (PC) at different temperatures have been studied using the cyclic voltammetry, constant current charge/discharge and electrochemical impedance spectroscopy methods. The limiting capacitance, characteristic time constants and complex power values calculated depend noticeably on the temperature applied, i.e., on the viscosity and specific conductivity of electrolyte solution used. At lower temperatures the more pronounced influence of T on the characteristic relaxation time R, C s and R s values, determining the energy density and power density of SC, respectively, has been observed. The supercapacitors based on PC with addition of 1 M (C H 5 ) 3 CH 3 NBF 4 salt can be applied even at T -45 ºC as a supercapacitor device with very high coulombic efficiency ( > 98 %) and surprisingly moderate energy and power density has been completed. The region of ideal polarisability E 3.0 V has been achieved for microporous TiC- CDC electrodes in 1 M (C H 5 ) 3 CH 3 NBF 4 electrolyte at -45 ºC T 80 ºC. At T -5 ºC, noticeable influence of series resistance on the shape of cyclic voltammetry plots has been observed. At T = 100 ºC, a noticeable deviation of SC from the ideal capacitive behaviour has been established explained by the evaporation of residual H O and squeezing out of electrolyte from micropores, as well as by the initiation of faradic processes (H and O ) at microporous TiC-CDC electrodes. Ragone plots have been measured for all SCs completed, except at T -35 ºC, where the power densities are very low (P max 0.1 kw kg -1 ) at high energy densities (E max 10 W h kg -1 ). At moderate power densities (P 1 kw kg -1 ) very quick decrease of E max has been established. Acknowledgements The present study was supported by The Estonian Centers of Excellence in Science: High Technology Materials for Sustainable Development, ETF Project 9184, European Regional Development Fund Project SLOKT1009 T and Project IUT0-13. References 1. G. Salitra, A. Soffer, L. Eliad, Y. Cohen and D. Aurbach, J. Electrochem. Soc., 147, 486 (000).. E. Lust, A. Jänes, T. Pärn and P. Nigu, J. Solid State Electrochem., 8, 4 (004). 3. A. Laheäär, H. Kurig, A. Jänes and E. Lust, Electrochim. Acta, 54, 4587 (009). 4. A. Laheäär, A. Jänes and E. Lust, Electrochim. Acta, 8, 309 (01). 5. A. Laheäär, A. Jänes and E. Lust, Electrochim. Acta, 56, 9048 (011). 6. A. Jänes, L. Permann, M. Arulepp and E. Lust, J. Electroanal. Chem., 569, 57 (004). 7. A. Jänes and E. Lust, Electrochem. Commun., 7, 510 (005). 8. A. Jänes and E. Lust, J. Electroanal. Chem., 588, 85 (006). 9. I. Tallo, T. Thomberg, H. Kurig, A. Jänes, K. Kontturi and E. Lust, J. Solid State Electrochem., 17, 19 (013). 10. H. Kurig, A. Jänes and E. Lust, J. Electrochem. Soc., 157, A7 (010). 11. H. Kurig, M. Vestli, A. Jänes and E. Lust, Electrochem. Solid State Lett., 14, A10 (011). 40 Downloaded on to IP address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).