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High performance spiro ammonium electrolyte for Electric Double Layer Capacitors
Journal of Power Sources 360 (2017) 41e47
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Short communication
High performance spiro ammonium electrolyte for Electric Double Layer Capacitors Donald DeRosa, Seiichiro Higashiya, Adam Schulz, Manisha Rane-Fondacaro*, Pradeep Haldar SUNY Polytechnic Institute e Colleges of Nanoscale Science and Engineering, 257 Fuller Road, Albany, NY 12203, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First reported use of APBF4 in an EDLC device. APBF4 electrolyte significantly outperformed SBPBF4 and TEABF4 in EDLCs. EDLC capacitance increased in devices with decreased cation size. EDLC resistance decreased in devices with decreased cation size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2017 Received in revised form 20 May 2017 Accepted 28 May 2017
The smallest spiro ammonium salt reported to date, 1 M 4-Axoniaspiro[3,4]octane tetrafluoroborate (APBF4), was successfully synthesized and investigated as the electrolyte with acetonitrile (AN) in an Electric Double Layer Capacitor (EDLC) for the first time. The electrochemical characteristics of EDLC devices containing 1 M APBF4/AN paired with commercial activated carbon electrodes were compared to devices containing popular EDLC electrolytes, 1 M 5-Azoniaspiro[4.4]nonane tetrafluoroborate (SBPBF4/ AN) and 1 M tetraethyl ammonium tetrafluoroborate (TEABF4/AN), using cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS). The average gravimetric capacitance of the 1 M APBF4 device (124.7 F g1) was found to be greater than the values measured for both the 1 M SBPBF4 device (108.6 F g1) and the 1 M TEABF4 device (99.2 F g1). The direct current equivalent series resistance (ESR) of the 1 M APBF4 device (383.4 mU cm2) was found to be substantially lower than the values measured for both the 1 M SBPBF4 device (501.0 mU cm2) and the 1 M TEABF4 device (710.8 mU cm2). These results demonstrate that APBF4, when compared to current commercial electrolytes, significantly enhances the energy storage properties of EDLC devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: EDLC Ultracapacitor Electrolyte SBPBF4 Spiro ammonium salt APBF4
1. Introduction Recently, organic electrolyte based Electric Double Layer Capacitors (EDLC) have attracted significant attention in the automotive industry with the growing need for high power, and durable energy storage devices that can endure the rigors of micro and mild
* Corresponding author. E-mail address: [email protected] (M. Rane-Fondacaro). http://dx.doi.org/10.1016/j.jpowsour.2017.05.096 0378-7753/© 2017 Elsevier B.V. All rights reserved.
hybrid vehicle systems [1e4]. The low equivalent series resistance (ESR), high power density, and high cycle life (>500,000 cycles) of EDLCs has been shown to substantially increase the efficiency and longevity of micro and mild hybrid vehicle systems, but adoption in these applications has been hampered by concerns regarding low energy density [5e8]. Researchers have strived to increase the energy density of organic electrolyte EDLCs, without compromising the cycle life or power density, predominately through innovations at the activated carbon electrode and the implementation of costly pure
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ionic liquids, while neglecting the incumbent electrolyte salt, tetraethyl ammonium tetrafluoroborate dissolved in acetonitrile (TEABF4/AN) [9e12]. Lately, 5-Azoniaspiro[4,4]nonane tetrafluoroborate (SBPBF4), a spiro quaternary ammonium salt, has garnered commercial interest as a possible candidate to replace TEABF4. The use of SBPBF4 in EDLCs at varying concentrations has been shown to increase the capacitance, decrease the equivalent series resistance (ESR), expand the potential window, and improve low temperature performance compared to devices containing TEABF4 [13e19]. Oxygen substituted SBP cation derivatives have shown similar performance increases as well [20]. The increase in capacitance for SBPBF4 salts has been attributed to the compact SBP cation, which enables higher surface area utilization by accessing smaller pores than the larger TEA cation. The increase in pore accessibility allows more cations to populate the double layer of the activated carbon electrode resulting in higher capacitance [21e24]. Additionally, the reduction in ionic resistance and by extension EDLC ESR in devices containing SBPBF4 salts has been attributed to the higher ionic conductivity of SBPBF4 compared to TEABF4 [25,26]. Investigations examining the performance of cations with varying structures suggest that ion size is a critical property that contributes to increasing ionic conductivity and pore diffusion [27,28]. 4-Axoniaspiro[3,4]octane tetrafluoroborate (APBF4) was synthesized to explore if a reduction in cation size with respect to SBP would result in further increases in EDLC capacitance and reductions in EDLC ESR while still maintaining the electrochemical stability window (ESW) necessary to operate at the typical EDLC voltage of 2.7 V despite the increased ring strain on the AP cation (Fig. 1). This is the first reported investigation of APBF4 as an electrolyte candidate to replace TEABF4 and SBPBF4 for EDLC applications. CV was employed to evaluate the ESW of 1 M APBF4/AN in contrast to 1 M SBPBF4/AN. GCD cycling to 2.7 V for 700 cycles was used to calculate the capacitance and ESR of devices containing 1 M AP BF4, 1 M SBPBF4, and 1 M TEABF4. EIS spectra were collected to confirm the ESR trend observed during GCD cycling and measure the electrolyte solution resistance in the devices.
2. Experimental 2.1. Materials Commercial activated carbon electrodes with a ~150 mm carbon layer and ~30 mm aluminum current collector were paired with 25 mm high porosity separators (3501 Celgard). The BrunaurEmmett_teller (BET) surface area of the commercial activated carbon electrodes, measured using a Quantachrome Nova 4200e, was 1188.4 m2/g while the Barret-Joyner-Halenda (BJH) calculated pore size distribution varied between 17 Å to 123 Å. The SBPBF4 and APBF4 salts were synthesized in house and dried under vacuum for 48 h. The TEABF4 salt (99.0% pure, EC- 86618) and electrolyte solvent, anhydrous acetonitrile (99.8% pure, EC-271004), were purchased from Sigma Aldrich. One molar electrolyte solutions were prepared in an argon filled glove box, and conductivities and viscosities of APBF4 (55.4 mScm1, 0.70 cP), SBPBF4 (54 mScm1, 0.69 cP), and TEABF4 (52.1 mScm1, 0.79 cP) were measured at 25 C. 2.2. Synthesis of APBF4 Pyrrolidine (142 g, 2.0 mol) was added to trimethylene chlorobromide (250 g, 1.0 mol) in an ether (250 ml) ice bath for 12 h to synthesize 3-(1-Pyrrolidy1)-propyl Chloride. 3-(1-Pyrrolidyl1)propyl Chloride (116.24 g, 0.80 mol) was added dropwise to boiling H2O (400 ml) and refluxed for 15 min. The resulting solution was washed with dichloromethane and evaporated to yield a crude chloride salt which was converted to a BF4 salt through the addition of 50% HBF4 (138.4 g, 0.80 mol) and ethanol (150 ml). The product was evaporated and vacuum dried to yield AP BF4 salt. Characterization and material property data has been reported by S. Higashiya [29]. 2.3. Synthesis of SBPBF4 Pyrrolidine (71.1 g, 1.0 mol), 1,4 dichlorobutane (127.0 g, 1.0 mol), and potassium carbonate (276.4 g) were added to acetonitrile
Fig. 1. Space filled models, chemical structures, and volumes generated by Schrodinger Jaguar Density Functional Theory (DFT) software using a hybrid B3YLP method and 6-31G** basis set for TEA, SBP, and AP cations. The atoms are denoted as follows: hydrogenewhite, carbonegrey, and nitrogeneblue. (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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(1.0 L) and refluxed for 18 h. The filtrate was vacuum dried to yield a crude chloride salt. An ion exchange was performed to convert the chloride salt to a BF4 salt with the addition 50% HBF4 (138.4 g, 0.80 mol) and ethanol. The subsequent salt was evaporated and vacuum dried to yield SBP BF4 [30].
The devices were tested after 24 h of assembly to allow the electrolyte to fully soak the electrodes and separator.
2.4. Device assembly
An Arbin BT2543 potentiostat was used to study the electrochemical characteristics of the EDLC devices containing 1 M APBF4, 1 M SBPBF4 and 1 M TEABF4. The CV method was employed to compare the ESW of devices containing 1 M APBF4 and 1 M SBPBF4 from 0 V to 4.0 V in 100 mV increments respectively at a scan rate of 1 mV s1. The CV method was further utilized to investigate the change in charge capacity and internal resistance at scan rates of 10 mV s1, 20 mV s1, 40 mV s1, 60 mV s1, and 100 mV s1. The GCD method was used to measure the capacitance and ESR after every 25th cycle of each device (containing 1 M APBF4, 1 M SBPBF4 and 1 M TEABF4) at a current density of 500 mA g1 and in a voltage range from 0 V to 2.7 V using the Maxwell 6 step procedure [31]. The devices were normally cycled at a current density of 2 A g1 to emulate the power density requirements of commercial EDLCs (Fig. 3a). An Ivium Vertex was used to collect EIS spectra of the devices prior to cell cycling at 57 frequencies between 1 MHz and 10 mHz and an amplitude of 5 mV.
A standard PAT test cell (EL-CELL electrochemical test equipment) was employed to fabricate EDLC devices in a two-electrode configuration using two activated carbon electrodes, a separator, and 100 mL of the electrolyte of interest. Prior to device assembly, the activated carbon electrodes were dried for 24 h at 115 C and cut into circular disks with an area of 2 cm2. Electrodes of identical masses were chosen in each EDLC cell to produce gravimetrically symmetrical cells. The separators were integrated into a PAT core insulation sleeve (EL-CELL). Using a micropipette, 100 mL of electrolyte was injected into the PAT core between the two electrodes. Stainless steel stubs and a polyethylene sealing ring were used to seal the core in the PAT cell base. The spring in the cell cap exerted approximately 40 N of force on the PAT cell core to maintain a constant distance between the two electrodes during device testing. The PAT cells were assembled in an Argon filled glove box.
2.5. Measurements
Fig. 2. Voltammograms with gravimetric current density for 1 M SBPBF4 from 0 V to 3.8 V(a), and 1 M APBF4 from 0 V to 3.4 V(b) all curves at a scan rate of 1 mV s1. Voltammograms with gravimetric capacitance at scan rates of 10 mV s1(red), 20 mV s1(blue), 40 mV s1(green), 60 mV s1(violet), and 100 mV s1(black) for 1 M SBPBF4(c), and 1 M APBF4(d). (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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3. Results and discussion 3.1. Cyclic voltammetry The CV method was employed from 0 V to 4 V at a scan rate of 1 mV s1 in 100 mV increments to identify and compare the breakdown voltage of devices containing 1 M APBF4 and 1 M SBPBF4. The voltammograms of these devices at (Fig. 2a and b) have a nearly rectangular shape at 1 V, 2 V, and 3 V, which is indicative of ideal EDLC behavior. The current responses for both devices begin to increase exponentially at cell voltages U 3.0 V, thereby potentially demonstrating EDLC breakdown. The faradaic ratio (R) was calculated on the third cycle at each voltage measured using Eq. (1):
. R ¼ Q charge Q discharge 1
(1)
to quantify EDLC breakdown (Qcharge is the charge accumulation during positive voltage sweeps and Qdischarge is the charge released during negative voltage sweeps). Ideally, the faradaic ratio is 0 and indicates the absence of redox reactions. When R > 0.1 a significant irreversible redox reaction has transpired within the EDLC, demonstrating the breakdown potential. The breakdown potential observed for the SBP BF4 device (3.8 V) was slightly higher than the APBF4 device (3.4 V). The lower breakdown potential and reduced ESW of APBF4 relative to SBP BF4 was attributed to the ring strain induced by the unfavorable bond angle (88.8 ) in the four membered ring of the AP cation. Both APBF4 and SBPBF4 devices were observed to have breakdown potentials significantly greater than the typical EDLC operational voltage of 2.7 V. The breakdown
potential of the SBPBF4 device (3.8 V) is sufficiently high to be considered a future electrolyte candidate for high voltage EDLC devices with a greater potential window (>2.7 V). CV was further utilized to observe the change in charge capacity and internal resistance of devices containing 1 M SBPBF4 and 1 M APBF4 at varying scan rates (Fig. 2c and d). The gravimetric capacitance (Cm) for each voltammogram was calculated according to Eq. (2):
Cm ¼ 4j=ðvmÞ
(2)
(where j is the current response in amps, v is the scan rate in mV s1, and m is the total active mass of both electrodes). The gravimetric capacitances at scan rates of 10 mV s1, 20 mV s1, 40 mV s1, 60 mV s1, and 100 mV s1 were plotted against the cell voltages for both devices. As the scan rate was increased from 10 mV s1 to 100 mV s1, the area of the voltammograms for both devices was reduced, as expected. The area reduction, corresponding to reduced cell capacity, occurs predominately at the inversion of the scan rate thereby indicating an increase in the device resistance [32]. The increased resistance was attributed to both electrode polarization effects and reduced electrolyte mobility in small pore regions. Despite the increased resistance and reduction in capacity with an increasing scan rate, devices containing APBF4 and SBPBF4 maintained comparable and near ideal EDLC behavior at high scan rates. 3.2. Galvanostatic charge/discharge The GCD method was used to determine the gravimetric capacitance and ESR of devices containing 1 M TEABF4, 1 M SBPBF4,
Fig. 3. GCD curves(a), gravimetric capacitance(b), ESR(c), and Ragone plot(d) for devices containing 1 M APBF4(blue-circle), 1 M SBPBF4(red-square), and 1 M TEABF4(green-triangle). (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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and 1 M APBF4 in a process consistent with commercial EDLC measurement. These devices were charged and discharged at a current density of 2 A g1 to a cell potential of 2.7 V for 700 cycles (Fig. 3a). The gravimetric capacitance of these devices was calculated in accordance with the Maxwell procedure every 25th cycle at a current density of 500 mA g1 according to Eq. (3):
Cm ¼ 4ðjDt=DVmÞ
(3)
(where j is the current, DV is the potential from Vmax to ½ Vmax, Dt is time, and m is the total activate materials mass) [31,33e37]. The ESR of these devices was calculated from the potential drop measured during a 5 s rest after discharge to half the cell potential (1.35 V) using Eq. (4):
. Rdischarge ¼ ðV1 V2 Þdrop Dj
45
(4)
(where V1 is the initial voltage before the rest period, V2 is the final voltage after the rest period, and Dj is the change in current from discharge to rest) [38]. After cycling, the change in gravimetric energy and power densities of these devices at current densities from 10 mAg1 to 8 A g1 were recorded to generate a Ragone plot(Fig. 3d). The average gravimetric capacitance of the 1 M APBF4 device (124.7 F g1) was found to be considerably higher than the values calculated for both the 1 M SBPBF4 device (108.6 F g1) and the 1 M TEABF4 device (99.2 F g1). The disparity in observed capacitance values for each device inversely corresponded with the differences
Fig. 4. EIS full spectra(a) and magnified spectra(b) for devices containing 1 M APBF4(blue-circle), 1 M SBPBF4(red-square), and 1 M TEABF4(green-triangle). (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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in cation volume for each electrolyte. The capacitance values increased (TEA < SBP < AP) as the cation size decreased (TEA > SBP > AP). This capacitance cation size relationship was attributed to the increased pore accessibility of smaller cations, which corresponded to greater available surface area for capacitive energy storage. The average calculated ESR of the 1 M APBF4 device (383.4 mU cm2) was found to be lower than the values calculated for both the 1 M SBPBF4 device (501.0 mU cm2) and the 1 M TEABF4 device (710.8 mU cm2). The highest calculated ESR was observed in the device containing 1 M TEABF4, the electrolyte with largest cation in this study. Conversely, the lowest calculated ESR was observed in the device containing 1 M APBF4, the electrolyte with the smallest cation in this study. The reduction in device ESR with the reduction in cation volume was attributed to the reduced ion packing density at the electric double layer, which enabled increased ion mobility and pore diffusion. 3.3. Electrochemical impedance spectroscopy EIS was employed to corroborate the ESR trend measured using GCD and observe the different electrolyte solution resistances (Rs) of devices containing 1 M TEABF4, 1 M SBPBF4, and 1 M APBF4. The EIS spectra for each device was collected prior to cycling at 57 frequencies between 1 MHz and 10 mHz. The Nyquist plot at low frequencies includes a straight line demonstrating ideal capacitor behavior for each device (Fig. 4a.). At high frequencies, a semi-circle was observed, representative of the contact resistance between the stainless steel stubs, aluminum current collector, and activated carbon material present in the devices. The 45 Warburg diffusion slope characterizing electrolyte ion mass transport was also identified for each device in the intermediate frequency range[39e42]. The electrolyte solution resistance, the first intersection to the real axis of in the high frequency range, of the 1 M APBF4 device (548 mU) was found to be lower than devices containing 1 M SBPBF4 (657 mU) and 1 M TEABF4 (767 mU). The magnitude of the electrolyte solution resistance was found to increase (AP < SBP < TEA) with cation ion size. The electrolyte solution resistance was found to correspond to the same trend as the measured conductivity of the electrolytes and GCD measured ESR of the assembled devices. 4. Conclusions The breakdown potential, capacitance, and direct current ESR for EDLC devices using APBF4, the smallest known spiro ammonium salt, as the electrolyte with commercial activated carbon electrodes were compared to devices containing 1 M SBPBF4 and 1 M TEABF4 using CV and GCD. Devices containing APBF4 and SBPBF4 displayed near ideal EDLC behavior at high scan rates while cycled at potentials from 0 V to 3 V. The breakdown potential observed for APBF4 (3.4 V) devices was lower than SBPBF4 (3.8 V), but was still sufficiently greater than the typical EDLC operation potential of 2.7 V. The average gravimetric capacitance of devices containing 1 M APBF4 (124.7 F g1), the electrolyte with the smallest cation in this study, was found to be significantly higher than devices containing 1 M SBPBF4 (108.6 F g1) and 1 M TEABF4 (99.2 F g1). The capacitance values observed increased (TEA < SBP < AP) as the size of the cation employed in the EDLC devices decreased (TEA > SBP > AP). Alternatively, the average ESR calculated decreased as the size of the cation employed in the EDLCs decreased. The ESR of EDLC devices containing 1 M APBF4 (383.4 mU cm2) was considerably lower than devices containing 1 M SBPBF4 (501.0 mU cm2) and 1 M TEABF4 (710.8 mU cm2). The electrolyte solution resistance in devices, measured with EIS,
containing 1 M APBF4 (548 mU) was also lower than devices containing 1 M SBPBF4 (657 mU) and 1 M TEABF4 (767 mU). As a result, devices containing APBF4 possessed the highest capacitance and lowest ESR compared to devices containing the industry incumbent electrolytes SBPBF4 and TEABF4. Future experiments involving the AP cation will focus on investigating the impact of electrode porosity and structure with AP salts on EDLC device performance.
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and self-discharge characterizations of ultracapacitors, in: http://www. maxwell.com/images/documents/1007239-EN_test_procedures_technote.pdf, 2015. W.G. Pell, B.E. Conway, Analysis of power limitations at porous supercapacitor electrodes under cyclic voltammetry modulation and dc charge, J. Power Sources 96 (2001) 57e67, http://dx.doi.org/10.1016/S0378-7753(00)00682-0. A. Burke, M. Miller, Testing of electrochemical capacitors: capacitance, resistance, energy density, and power capability, Electrochim. Acta 55 (2010) 7538e7548, http://dx.doi.org/10.1016/j.electacta.2010.04.074. M.D. Stoller, R.S. Ruoff, Best practice methods for determining an electrode material's performance for ultracapacitors, Energy Environ. Sci. 3 (2010) 1294e1301, http://dx.doi.org/10.1039/C0EE00074D. IEC 62391-1, Fixed Electric Double Layer Capacitors for Use in Electronic Equipment e Part 1: Generic Specification, first ed., 2006. €r, P. Przygocki, Q. Abbas, F. Be guin, Appropriate methods for evalA. Lahe€ aa uating the efficiency and capacitive behavior of different types of supercapacitors, Electrochem. Commun. 60 (2015) 21e25, http://dx.doi.org/ 10.1016/j.elecom.2015.07.022. F. Beguin, E. Frackowiak, Supercapacitors Materials, Systems, and Applications (Chapter 12), 2012. B.E. Conway, W.G. Pell, Power limitations of supercapacitor operation associated with resistance and capacitance distribution in porous electrode devices, J. Power Sources 105 (2002) 169e181, http://dx.doi.org/10.1016/S03787753(01)00936-3. M.E. Suss, T.F. Baumann, M.A. Worsley, K.A. Rose, T.F. Jaramillo, M. Stadermann, J.G. Santiago, Impedance-based study of capacitive porous carbon electrodes with hierarchical and bimodal porosity, J. Power Sources 241 (2013) 266e273, http://dx.doi.org/10.1016/j.jpowsour.2013.03.178. I. Yang, S.-G. Kim, S.H. Kwon, M.-S. Kim, J.C. Jung, Relationships between pore size and charge transfer resistance of carbon aerogels for organic electric double-layer capacitor electrodes, Electrochim. Acta 223 (2017) 21e30, http://dx.doi.org/10.1016/j.electacta.2016.11.177. M.E. Suss, T.F. Baumann, M.A. Worsley, K.A. Rose, T.F. Jaramillo, M. Stadermann, J.G. Santiago, Impedance-based study of capacitive porous carbon electrodes with hierarchical and bimodal porosity, J. Power Sources 241 (2013) 266e273, http://dx.doi.org/10.1016/j.jpowsour.2013.03.178. H.D. Yoo, J.H. Jang, J.H. Ryu, Y. Park, S.M. Oh, Impedance analysis of porous carbon electrodes to predict rate capability of electric double-layer capacitors, J. Power Sources 267 (2014) 411e420, http://dx.doi.org/10.1016/ j.jpowsour.2014.05.058.
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Short communication
High performance spiro ammonium electrolyte for Electric Double Layer Capacitors Donald DeRosa, Seiichiro Higashiya, Adam Schulz, Manisha Rane-Fondacaro*, Pradeep Haldar SUNY Polytechnic Institute e Colleges of Nanoscale Science and Engineering, 257 Fuller Road, Albany, NY 12203, United States
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
First reported use of APBF4 in an EDLC device. APBF4 electrolyte significantly outperformed SBPBF4 and TEABF4 in EDLCs. EDLC capacitance increased in devices with decreased cation size. EDLC resistance decreased in devices with decreased cation size.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 March 2017 Received in revised form 20 May 2017 Accepted 28 May 2017
The smallest spiro ammonium salt reported to date, 1 M 4-Axoniaspiro[3,4]octane tetrafluoroborate (APBF4), was successfully synthesized and investigated as the electrolyte with acetonitrile (AN) in an Electric Double Layer Capacitor (EDLC) for the first time. The electrochemical characteristics of EDLC devices containing 1 M APBF4/AN paired with commercial activated carbon electrodes were compared to devices containing popular EDLC electrolytes, 1 M 5-Azoniaspiro[4.4]nonane tetrafluoroborate (SBPBF4/ AN) and 1 M tetraethyl ammonium tetrafluoroborate (TEABF4/AN), using cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS). The average gravimetric capacitance of the 1 M APBF4 device (124.7 F g1) was found to be greater than the values measured for both the 1 M SBPBF4 device (108.6 F g1) and the 1 M TEABF4 device (99.2 F g1). The direct current equivalent series resistance (ESR) of the 1 M APBF4 device (383.4 mU cm2) was found to be substantially lower than the values measured for both the 1 M SBPBF4 device (501.0 mU cm2) and the 1 M TEABF4 device (710.8 mU cm2). These results demonstrate that APBF4, when compared to current commercial electrolytes, significantly enhances the energy storage properties of EDLC devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: EDLC Ultracapacitor Electrolyte SBPBF4 Spiro ammonium salt APBF4
1. Introduction Recently, organic electrolyte based Electric Double Layer Capacitors (EDLC) have attracted significant attention in the automotive industry with the growing need for high power, and durable energy storage devices that can endure the rigors of micro and mild
* Corresponding author. E-mail address: [email protected] (M. Rane-Fondacaro). http://dx.doi.org/10.1016/j.jpowsour.2017.05.096 0378-7753/© 2017 Elsevier B.V. All rights reserved.
hybrid vehicle systems [1e4]. The low equivalent series resistance (ESR), high power density, and high cycle life (>500,000 cycles) of EDLCs has been shown to substantially increase the efficiency and longevity of micro and mild hybrid vehicle systems, but adoption in these applications has been hampered by concerns regarding low energy density [5e8]. Researchers have strived to increase the energy density of organic electrolyte EDLCs, without compromising the cycle life or power density, predominately through innovations at the activated carbon electrode and the implementation of costly pure
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ionic liquids, while neglecting the incumbent electrolyte salt, tetraethyl ammonium tetrafluoroborate dissolved in acetonitrile (TEABF4/AN) [9e12]. Lately, 5-Azoniaspiro[4,4]nonane tetrafluoroborate (SBPBF4), a spiro quaternary ammonium salt, has garnered commercial interest as a possible candidate to replace TEABF4. The use of SBPBF4 in EDLCs at varying concentrations has been shown to increase the capacitance, decrease the equivalent series resistance (ESR), expand the potential window, and improve low temperature performance compared to devices containing TEABF4 [13e19]. Oxygen substituted SBP cation derivatives have shown similar performance increases as well [20]. The increase in capacitance for SBPBF4 salts has been attributed to the compact SBP cation, which enables higher surface area utilization by accessing smaller pores than the larger TEA cation. The increase in pore accessibility allows more cations to populate the double layer of the activated carbon electrode resulting in higher capacitance [21e24]. Additionally, the reduction in ionic resistance and by extension EDLC ESR in devices containing SBPBF4 salts has been attributed to the higher ionic conductivity of SBPBF4 compared to TEABF4 [25,26]. Investigations examining the performance of cations with varying structures suggest that ion size is a critical property that contributes to increasing ionic conductivity and pore diffusion [27,28]. 4-Axoniaspiro[3,4]octane tetrafluoroborate (APBF4) was synthesized to explore if a reduction in cation size with respect to SBP would result in further increases in EDLC capacitance and reductions in EDLC ESR while still maintaining the electrochemical stability window (ESW) necessary to operate at the typical EDLC voltage of 2.7 V despite the increased ring strain on the AP cation (Fig. 1). This is the first reported investigation of APBF4 as an electrolyte candidate to replace TEABF4 and SBPBF4 for EDLC applications. CV was employed to evaluate the ESW of 1 M APBF4/AN in contrast to 1 M SBPBF4/AN. GCD cycling to 2.7 V for 700 cycles was used to calculate the capacitance and ESR of devices containing 1 M AP BF4, 1 M SBPBF4, and 1 M TEABF4. EIS spectra were collected to confirm the ESR trend observed during GCD cycling and measure the electrolyte solution resistance in the devices.
2. Experimental 2.1. Materials Commercial activated carbon electrodes with a ~150 mm carbon layer and ~30 mm aluminum current collector were paired with 25 mm high porosity separators (3501 Celgard). The BrunaurEmmett_teller (BET) surface area of the commercial activated carbon electrodes, measured using a Quantachrome Nova 4200e, was 1188.4 m2/g while the Barret-Joyner-Halenda (BJH) calculated pore size distribution varied between 17 Å to 123 Å. The SBPBF4 and APBF4 salts were synthesized in house and dried under vacuum for 48 h. The TEABF4 salt (99.0% pure, EC- 86618) and electrolyte solvent, anhydrous acetonitrile (99.8% pure, EC-271004), were purchased from Sigma Aldrich. One molar electrolyte solutions were prepared in an argon filled glove box, and conductivities and viscosities of APBF4 (55.4 mScm1, 0.70 cP), SBPBF4 (54 mScm1, 0.69 cP), and TEABF4 (52.1 mScm1, 0.79 cP) were measured at 25 C. 2.2. Synthesis of APBF4 Pyrrolidine (142 g, 2.0 mol) was added to trimethylene chlorobromide (250 g, 1.0 mol) in an ether (250 ml) ice bath for 12 h to synthesize 3-(1-Pyrrolidy1)-propyl Chloride. 3-(1-Pyrrolidyl1)propyl Chloride (116.24 g, 0.80 mol) was added dropwise to boiling H2O (400 ml) and refluxed for 15 min. The resulting solution was washed with dichloromethane and evaporated to yield a crude chloride salt which was converted to a BF4 salt through the addition of 50% HBF4 (138.4 g, 0.80 mol) and ethanol (150 ml). The product was evaporated and vacuum dried to yield AP BF4 salt. Characterization and material property data has been reported by S. Higashiya [29]. 2.3. Synthesis of SBPBF4 Pyrrolidine (71.1 g, 1.0 mol), 1,4 dichlorobutane (127.0 g, 1.0 mol), and potassium carbonate (276.4 g) were added to acetonitrile
Fig. 1. Space filled models, chemical structures, and volumes generated by Schrodinger Jaguar Density Functional Theory (DFT) software using a hybrid B3YLP method and 6-31G** basis set for TEA, SBP, and AP cations. The atoms are denoted as follows: hydrogenewhite, carbonegrey, and nitrogeneblue. (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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(1.0 L) and refluxed for 18 h. The filtrate was vacuum dried to yield a crude chloride salt. An ion exchange was performed to convert the chloride salt to a BF4 salt with the addition 50% HBF4 (138.4 g, 0.80 mol) and ethanol. The subsequent salt was evaporated and vacuum dried to yield SBP BF4 [30].
The devices were tested after 24 h of assembly to allow the electrolyte to fully soak the electrodes and separator.
2.4. Device assembly
An Arbin BT2543 potentiostat was used to study the electrochemical characteristics of the EDLC devices containing 1 M APBF4, 1 M SBPBF4 and 1 M TEABF4. The CV method was employed to compare the ESW of devices containing 1 M APBF4 and 1 M SBPBF4 from 0 V to 4.0 V in 100 mV increments respectively at a scan rate of 1 mV s1. The CV method was further utilized to investigate the change in charge capacity and internal resistance at scan rates of 10 mV s1, 20 mV s1, 40 mV s1, 60 mV s1, and 100 mV s1. The GCD method was used to measure the capacitance and ESR after every 25th cycle of each device (containing 1 M APBF4, 1 M SBPBF4 and 1 M TEABF4) at a current density of 500 mA g1 and in a voltage range from 0 V to 2.7 V using the Maxwell 6 step procedure [31]. The devices were normally cycled at a current density of 2 A g1 to emulate the power density requirements of commercial EDLCs (Fig. 3a). An Ivium Vertex was used to collect EIS spectra of the devices prior to cell cycling at 57 frequencies between 1 MHz and 10 mHz and an amplitude of 5 mV.
A standard PAT test cell (EL-CELL electrochemical test equipment) was employed to fabricate EDLC devices in a two-electrode configuration using two activated carbon electrodes, a separator, and 100 mL of the electrolyte of interest. Prior to device assembly, the activated carbon electrodes were dried for 24 h at 115 C and cut into circular disks with an area of 2 cm2. Electrodes of identical masses were chosen in each EDLC cell to produce gravimetrically symmetrical cells. The separators were integrated into a PAT core insulation sleeve (EL-CELL). Using a micropipette, 100 mL of electrolyte was injected into the PAT core between the two electrodes. Stainless steel stubs and a polyethylene sealing ring were used to seal the core in the PAT cell base. The spring in the cell cap exerted approximately 40 N of force on the PAT cell core to maintain a constant distance between the two electrodes during device testing. The PAT cells were assembled in an Argon filled glove box.
2.5. Measurements
Fig. 2. Voltammograms with gravimetric current density for 1 M SBPBF4 from 0 V to 3.8 V(a), and 1 M APBF4 from 0 V to 3.4 V(b) all curves at a scan rate of 1 mV s1. Voltammograms with gravimetric capacitance at scan rates of 10 mV s1(red), 20 mV s1(blue), 40 mV s1(green), 60 mV s1(violet), and 100 mV s1(black) for 1 M SBPBF4(c), and 1 M APBF4(d). (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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3. Results and discussion 3.1. Cyclic voltammetry The CV method was employed from 0 V to 4 V at a scan rate of 1 mV s1 in 100 mV increments to identify and compare the breakdown voltage of devices containing 1 M APBF4 and 1 M SBPBF4. The voltammograms of these devices at (Fig. 2a and b) have a nearly rectangular shape at 1 V, 2 V, and 3 V, which is indicative of ideal EDLC behavior. The current responses for both devices begin to increase exponentially at cell voltages U 3.0 V, thereby potentially demonstrating EDLC breakdown. The faradaic ratio (R) was calculated on the third cycle at each voltage measured using Eq. (1):
. R ¼ Q charge Q discharge 1
(1)
to quantify EDLC breakdown (Qcharge is the charge accumulation during positive voltage sweeps and Qdischarge is the charge released during negative voltage sweeps). Ideally, the faradaic ratio is 0 and indicates the absence of redox reactions. When R > 0.1 a significant irreversible redox reaction has transpired within the EDLC, demonstrating the breakdown potential. The breakdown potential observed for the SBP BF4 device (3.8 V) was slightly higher than the APBF4 device (3.4 V). The lower breakdown potential and reduced ESW of APBF4 relative to SBP BF4 was attributed to the ring strain induced by the unfavorable bond angle (88.8 ) in the four membered ring of the AP cation. Both APBF4 and SBPBF4 devices were observed to have breakdown potentials significantly greater than the typical EDLC operational voltage of 2.7 V. The breakdown
potential of the SBPBF4 device (3.8 V) is sufficiently high to be considered a future electrolyte candidate for high voltage EDLC devices with a greater potential window (>2.7 V). CV was further utilized to observe the change in charge capacity and internal resistance of devices containing 1 M SBPBF4 and 1 M APBF4 at varying scan rates (Fig. 2c and d). The gravimetric capacitance (Cm) for each voltammogram was calculated according to Eq. (2):
Cm ¼ 4j=ðvmÞ
(2)
(where j is the current response in amps, v is the scan rate in mV s1, and m is the total active mass of both electrodes). The gravimetric capacitances at scan rates of 10 mV s1, 20 mV s1, 40 mV s1, 60 mV s1, and 100 mV s1 were plotted against the cell voltages for both devices. As the scan rate was increased from 10 mV s1 to 100 mV s1, the area of the voltammograms for both devices was reduced, as expected. The area reduction, corresponding to reduced cell capacity, occurs predominately at the inversion of the scan rate thereby indicating an increase in the device resistance [32]. The increased resistance was attributed to both electrode polarization effects and reduced electrolyte mobility in small pore regions. Despite the increased resistance and reduction in capacity with an increasing scan rate, devices containing APBF4 and SBPBF4 maintained comparable and near ideal EDLC behavior at high scan rates. 3.2. Galvanostatic charge/discharge The GCD method was used to determine the gravimetric capacitance and ESR of devices containing 1 M TEABF4, 1 M SBPBF4,
Fig. 3. GCD curves(a), gravimetric capacitance(b), ESR(c), and Ragone plot(d) for devices containing 1 M APBF4(blue-circle), 1 M SBPBF4(red-square), and 1 M TEABF4(green-triangle). (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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and 1 M APBF4 in a process consistent with commercial EDLC measurement. These devices were charged and discharged at a current density of 2 A g1 to a cell potential of 2.7 V for 700 cycles (Fig. 3a). The gravimetric capacitance of these devices was calculated in accordance with the Maxwell procedure every 25th cycle at a current density of 500 mA g1 according to Eq. (3):
Cm ¼ 4ðjDt=DVmÞ
(3)
(where j is the current, DV is the potential from Vmax to ½ Vmax, Dt is time, and m is the total activate materials mass) [31,33e37]. The ESR of these devices was calculated from the potential drop measured during a 5 s rest after discharge to half the cell potential (1.35 V) using Eq. (4):
. Rdischarge ¼ ðV1 V2 Þdrop Dj
45
(4)
(where V1 is the initial voltage before the rest period, V2 is the final voltage after the rest period, and Dj is the change in current from discharge to rest) [38]. After cycling, the change in gravimetric energy and power densities of these devices at current densities from 10 mAg1 to 8 A g1 were recorded to generate a Ragone plot(Fig. 3d). The average gravimetric capacitance of the 1 M APBF4 device (124.7 F g1) was found to be considerably higher than the values calculated for both the 1 M SBPBF4 device (108.6 F g1) and the 1 M TEABF4 device (99.2 F g1). The disparity in observed capacitance values for each device inversely corresponded with the differences
Fig. 4. EIS full spectra(a) and magnified spectra(b) for devices containing 1 M APBF4(blue-circle), 1 M SBPBF4(red-square), and 1 M TEABF4(green-triangle). (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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in cation volume for each electrolyte. The capacitance values increased (TEA < SBP < AP) as the cation size decreased (TEA > SBP > AP). This capacitance cation size relationship was attributed to the increased pore accessibility of smaller cations, which corresponded to greater available surface area for capacitive energy storage. The average calculated ESR of the 1 M APBF4 device (383.4 mU cm2) was found to be lower than the values calculated for both the 1 M SBPBF4 device (501.0 mU cm2) and the 1 M TEABF4 device (710.8 mU cm2). The highest calculated ESR was observed in the device containing 1 M TEABF4, the electrolyte with largest cation in this study. Conversely, the lowest calculated ESR was observed in the device containing 1 M APBF4, the electrolyte with the smallest cation in this study. The reduction in device ESR with the reduction in cation volume was attributed to the reduced ion packing density at the electric double layer, which enabled increased ion mobility and pore diffusion. 3.3. Electrochemical impedance spectroscopy EIS was employed to corroborate the ESR trend measured using GCD and observe the different electrolyte solution resistances (Rs) of devices containing 1 M TEABF4, 1 M SBPBF4, and 1 M APBF4. The EIS spectra for each device was collected prior to cycling at 57 frequencies between 1 MHz and 10 mHz. The Nyquist plot at low frequencies includes a straight line demonstrating ideal capacitor behavior for each device (Fig. 4a.). At high frequencies, a semi-circle was observed, representative of the contact resistance between the stainless steel stubs, aluminum current collector, and activated carbon material present in the devices. The 45 Warburg diffusion slope characterizing electrolyte ion mass transport was also identified for each device in the intermediate frequency range[39e42]. The electrolyte solution resistance, the first intersection to the real axis of in the high frequency range, of the 1 M APBF4 device (548 mU) was found to be lower than devices containing 1 M SBPBF4 (657 mU) and 1 M TEABF4 (767 mU). The magnitude of the electrolyte solution resistance was found to increase (AP < SBP < TEA) with cation ion size. The electrolyte solution resistance was found to correspond to the same trend as the measured conductivity of the electrolytes and GCD measured ESR of the assembled devices. 4. Conclusions The breakdown potential, capacitance, and direct current ESR for EDLC devices using APBF4, the smallest known spiro ammonium salt, as the electrolyte with commercial activated carbon electrodes were compared to devices containing 1 M SBPBF4 and 1 M TEABF4 using CV and GCD. Devices containing APBF4 and SBPBF4 displayed near ideal EDLC behavior at high scan rates while cycled at potentials from 0 V to 3 V. The breakdown potential observed for APBF4 (3.4 V) devices was lower than SBPBF4 (3.8 V), but was still sufficiently greater than the typical EDLC operation potential of 2.7 V. The average gravimetric capacitance of devices containing 1 M APBF4 (124.7 F g1), the electrolyte with the smallest cation in this study, was found to be significantly higher than devices containing 1 M SBPBF4 (108.6 F g1) and 1 M TEABF4 (99.2 F g1). The capacitance values observed increased (TEA < SBP < AP) as the size of the cation employed in the EDLC devices decreased (TEA > SBP > AP). Alternatively, the average ESR calculated decreased as the size of the cation employed in the EDLCs decreased. The ESR of EDLC devices containing 1 M APBF4 (383.4 mU cm2) was considerably lower than devices containing 1 M SBPBF4 (501.0 mU cm2) and 1 M TEABF4 (710.8 mU cm2). The electrolyte solution resistance in devices, measured with EIS,
containing 1 M APBF4 (548 mU) was also lower than devices containing 1 M SBPBF4 (657 mU) and 1 M TEABF4 (767 mU). As a result, devices containing APBF4 possessed the highest capacitance and lowest ESR compared to devices containing the industry incumbent electrolytes SBPBF4 and TEABF4. Future experiments involving the AP cation will focus on investigating the impact of electrode porosity and structure with AP salts on EDLC device performance.
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