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Article

A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range

by
Ezgi Yurttas
1,2,
Yavuz Gokce
1,
Nazife Isik Semerci
3,
Emine Yagmur
1 and
Zeki Aktas
1,*
1
Department of Chemical Engineering, Faculty of Engineering, Ankara University, Tandogan, Ankara 06100, Turkey
2
Akana Engineering and Trade Inc., Sincan, Ankara 06909, Turkey
3
Department of Energy Engineering, Faculty of Engineering, Ankara University, Golbasi, Ankara 06830, Turkey
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1057; https://doi.org/10.3390/pr13041057
Submission received: 16 February 2025 / Revised: 14 March 2025 / Accepted: 26 March 2025 / Published: 2 April 2025
(This article belongs to the Special Issue Recent Advances in Conductor Materials for Energy Storage and Electrocaloric Applications)
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Versions Notes

Abstract

:
The wide operating temperature and voltage window are favourable properties that increase the practical applications of supercapacitors. Ionic liquids (IL) are suitable electrolytes that allow supercapacitors to be used in wide operating ranges. In this study, triethylammonium tetrafluoroborate (Et3NHBF4) is tested as a new IL to operate supercapacitors in a wide temperature range (−40 °C, 25 °C, and 80 °C) in the presence of commercial activated carbon. The performance of Et3NHBF4 is compared to two different commercial ILs. This study also investigates the application of heat treatment to determine suitable activated carbon surface characteristics for ILs. The results indicate that heat treatment enhances the electrode–electrolyte interaction, and the electrochemical performances of the supercapacitors prepared from the heat-treated activated carbon are significantly higher than the original commercial activated carbon. Electrochemical tests show that the synthesised Et3NHBF4 (with propylene carbonate) can be used over a wide temperature range and has a better energy storage performance, especially at −40 °C (specific capacitance of 42.12 F/g at 2 A/g), compared to the other two commercial ionic liquids.

1. Introduction

The increasing demand for energy, driven by rapid population growth and technological development, has led to the rapid depletion of traditional energy resources and accelerated the shift to renewable resources. The continuous demand for energy has necessitated the development of energy storage devices that can be used on demand. Supercapacitors (SCs), batteries, and fuel cells are the currently available energy storage systems.
Electrochemical double layer capacitors (EDLCs), or supercapacitors, store energy (power) depending on electrostatic interactions between the ions contained in the electrolyte and charged electrodes. These interactions allow thousands of charge/discharge cycles without deterioration [1,2,3]. EDLCs are possible systems that can be used as energy storage devices for automotive, microelectronic, and renewable energy applications due to their high power and energy density, low cost, and high efficiency [4,5,6,7,8]. They are effective in harsh temperature environments (with low internal resistance) while presenting a special possibility for military [9,10] and electric vehicle [11,12] applications where they need to meet high power and energy density demands [13].
The electrochemical properties and performances (capacitance, energy/power, and cycling stability) of SCs are associated with electrolyte and electrode materials [1,14,15]. Activated carbons are commonly used electrode materials in EDLCs and have been extensively studied [16,17,18,19]. The specific surface area, pore size distribution, and conductivity of activated carbon considerably affect the properties and performance of EDLCs. In addition, the properties of the electrolyte, such as solution stability, conductivity, ionic mobility, and diffusivity, also affect the EDLC’s performance [13,20].
Electrolytes are mainly classified as liquid electrolytes, solid/semi-solid-state electrolytes, and redox-active electrolytes. Organic and aqueous electrolytes are frequently used as liquid electrolytes. Aqueous electrolytes allow applications that do not require more than 1 V due to the narrow operating range, whereas with organic electrolytes, the voltage can reach up to 2.7 V [1,21]. Many studies have focused on the development of various electrolytes. However, the perfect electrolyte has not been developed to meet all requirements because every electrolyte has its own advantages and disadvantages [22,23]. Commercial SCs based on organic electrolytes can operate in a temperature range of −30 °C to 80 °C [1]. However, today, there is a need for supercapacitors that can be used at lower and higher temperatures, such as −50 °C and 100 °C. These capacitors are used in tough conditions, such as military, aerospace, and automotive applications. These difficulties could be overcome using ILs instead of conventional electrolytes [24].
The ILs are salts (composed of ions only) that dissolve at low temperatures with a wide liquid phase range [25]. These electrolytes are of great interest for use in SCs due to their low flammability, low volatility, low corrosivity, high chemical and electrochemical stability, wide operating voltages (>3.5 V), and wide temperature range. Furthermore, the physical and chemical properties of ILs can be adjusted by making a wide variety of cation and anion combinations. The ILs can be divided into two groups, which are protic (PILs) and aprotic ionic liquids (AILs). The equimolar amounts of a Brønsted base and a Brønsted acid are generally used to synthesise the PILs. Studies in the presence of PILs have focused on high-temperature applications [24,25,26,27,28,29]. The significant disadvantages of the ILs are their low conductivity and high viscosity at room temperature [30]. For SC applications, the conductivity of the ILs can be increased either by operating at high temperatures or by adding a significant amount of organic solvent to the electrolyte [31]. Jarosik et al. [32] reported that the IL–solvent mixtures are appropriate combinations for better conductivity and viscosity because the solvent has a more destructive influence on the ion pairing.
Various studies have been conducted on the use of IL–solvent mixtures in SCs. The studies have indicated that many parameters affect electrochemical performance in the presence of ionic liquids. The ion–pore size ratio, ion shape, viscosity, and conductivity of the IL affect the electrochemical performance [33]. Studies on the use of ILs and their mixtures [(1-butyl-1-methylpyrrolidinium tetrafluoroborate in propylene carbonate (PC), PYR14 BF4/PC, 1-butyl-1-methylpyrrolidinium bis-(trifluoromethyl sulfonyl) imide in PC, PYR14 TFSI/PC, 1-propyl-1-methylpyrrolidinium-bis(fluoro sulfonyl)imide in γ-butyrolactone, PYR13FSI/GBL, 1-butyl-1-methylpyrrolidinium-bis(trifluoromethane sulfonyl)imide in γ-butyrolactone, PYR14TFSI/GBL, tetraethylammonium tetrafluoroborate in PC, Et4NBF4/PC)] at different temperatures (−50 °C to 100 °C) in SCs are available in the literature. The researchers reported that the viscosity of ILs decreased with increasing conductivity as the temperature increased [24,33,34]. Leyva-Garcia et al. [33] stated that PYR14TFSI electrolyte performed well when dissolved in PC due to its lower viscosity and increased conductivity. Ruiz et al. [34] emphasised that the conductivity of the ionic liquid (N-methyl-N-butylpyrrolidinium bis- (trifluoromethane) sulfonyl imide) increased when mixed with an appropriate nitrile or carbonate-based solvent. Studies with electrolytes containing different anions and cations revealed that electrolytes containing PYR14+ and TFSI− had higher electrochemical stability [35], and also, the chemical and thermal stability of the FSI− anion was lower than TFSI− [24]. Pohlmann et al. [35] observed that the electrolyte 1 M PYR14BF4 in PC exhibited good conductivity, low viscosity, and allowed high operative voltages. The cyclic voltammetry (CV) analysis showed that the electrochemical stability of PYR14+- and TFSI−-containing electrolytes was higher. Dagousset et al. [24] used organic solvent with IL and reported that FSI−-based IL (PYR13FSI) deteriorated after ~1500 cycles and TFSI−-based ILs can be used for more than 10,000 cycles at 100 °C without any significant reduction in capacitance. They emphasised that the capacitance decreased by 20% and the internal resistance increased by 100% at high voltages (at 100 °C and −50 °C). Leyva-Garcia et al. [33] indicated that when activated carbon with a high specific surface area and an average pore size of 1.4 nm was used, high capacitance values could be obtained at three different temperatures (20, 40, and 60 °C) with 1M Et4NBF4/PC, 1M PYR14 BF4/PC, and 1M PYR14 TFSI/PC electrolytes. They reported that if the ion is adsorbed as a monolayer on the pore surface, the distance between the pore wall and the ion centre is the shortest, and a higher specific capacity could be provided.
The utilisation of supercapacitors at extreme temperature values necessitates the selection of an optimal electrolyte. In this regard, studies employing diverse ILs for the fabrication of high-performance and stable supercapacitor cells can be found in the literature. The present study represented an investigation into the potential use of Et3NHBF4 (with propylene carbonate) as an electrolyte over a wide temperature range for the first time, and its performance was compared with two different commercial electrolytes.

2. Materials and Methods

Et4NBF4 (Sigma-Aldrich, Darmstadt, Germany) and 1-Butyl-1-methylpyrrolidinium bis(trifluoromethane sulfonyl) imide (PYR14TFSI, Solvionic, Toulouse, France) ionic liquids were utilised as commercial ILs. Tetrafluoroboric acid solution and triethylamine (Sigma-Aldrich, Darmstadt, Germany) were used as initiator chemicals, and propylene carbonate (Sigma-Aldrich, Darmstadt, Germany) was used as a solvent. Sodium carboxymethyl cellulose (Sigma-Aldrich, Darmstadt, Germany) and carbon black (N-220, Kremenchuk, Ukraine) were employed as the binder and the conductivity additive. Titanium 30 (DreamWeavear, Charlotte, NC, USA) and a wood-based commercial activated carbon (H3PO4 activated at 400 °C) were provided as the separator and the electrode material.
The commercial activated carbon (AC) was directly exposed to a further heat treatment in a furnace at 400, 600, and 800 °C under a N2 atmosphere (for 1 h; flow rate: 0.5 L/min). The temperature selection was made with reference to the production temperature of commercial activated carbon. To investigate the effect of heat treatment on the structural and chemical properties of activated carbon, the samples were subjected to heat treatment separately, first at the production temperature (400 °C) and then at two higher temperature values (600 °C and 800 °C). The main purpose of this approach was to gradually remove the functional groups from the activated carbon surface, modify the pore structure, and characterise the changes in electrochemical performance. The treated samples were labelled as AC-400, AC-600, and AC-800 and used as electrode material in supercapacitor cells. The BET surface area and pore size distributions (the Non-Local Density Functional Theory, NLDFT, and Barrett–Joyner–Halenda, BJH, methods) of the samples were determined using a Quantachrome NOVA 2200, Quantachrome Instruments, Boynton Beach, FL, USA, series volumetric gas adsorption instrument. Particle size distribution analyses were performed using the laser particle size analyser (Malvern Hydro 2000 MU, Malvern Instruments Ltd., Malvern, UK).
Surface functional groups of the samples were detected by a Shimadzu FTIR-8040 (Fourier Transform Infrared Spectroscopy, Shimadzu Corporation, Kyoto, Japan) spectrometer. Each spectrum was recorded in the wavelength range of 400–4000 1/cm. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a Thermo Scientific (Waltham, MA, USA), K-Alpha, X-ray photoelectron spectrophotometer with an Al Kα X-ray source (1486.6 eV). The binding energy scales of the high-resolution spectra were calibrated by shifting the C1s peak position to 284.8 eV. Peak deconvolution was carried out using Gaussian–Lorentzian curves. Thermogravimetric analyses (TGAs) were carried out using a Perkin Elmer Pyris 1 thermal analyser (PerkinElmer, Dresden, Germany). The AC and the AC800 samples were heated from 25 to 950 °C at a heating rate of 10 °C/min under a N2 (100 mL/min) atmosphere.
Protic ionic liquid (PIL) triethylammonium tetrafluoroborate, Et3NHBF4, was synthesised by the reaction of the equimolar amounts of triethylamine and tetrafluoroboric acid based on a previously reported work [20]. Following the synthesis, the electrolyte was dissolved in deuterated dimethyl sulfoxide to verify the completion of the synthesis by 1H-NMR analysis. The data were recorded with a Bruker 300 MHz spectrometer. The 1H-NMR spectrum of Et3NHBF4 is shown in Figure S1. Chemical shifts are given as δH (300 MHz, DMSO-d6)/ppm: 6.69 (s, HBF4−), 3.10 (q, 6H, N–CH2), and 1.20 (t, 9H, N–CH2–CH3).
Electrode slurries were prepared by mixing the activated carbon sample (80% w/w), sodium carboxymethyl cellulose (10% w/w as a binder), and carbon black (10% w/w as a conductive additive). Water was used as the solvent for the binder. Aluminium foil (2 cm × 2 cm) was used as the current collector. A doctor blade was used to coat the surface of the current collector with slurry. The electrodes were then dried in an oven at 100 °C overnight to remove water. The electrodes and separator were wetted by immersion in the electrolyte (1 M) under vacuum, then combined and placed in an antistatic bag. The supercapacitor cell was then vacuumed and sealed with a hot press.
Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) techniques were employed to test the capacitive performance of the supercapacitor cells. The tests were carried out with a two-electrode cell configuration using Gamry Interface 5000P potentiostat/galvanostat (Gamry Instruments Inc., Warminster, PA, USA). Low, medium, and high temperature tests (−40 °C, 25 °C, and 80 °C) were performed in the climatic test cabinet (NUVE ID 300). The CV analyses were performed in the voltage range of 0 V to 2 V at different scanning rates (5, 10, 50, and 100 mV/s). The impedance plots were recorded with an amplitude of 5 mV rms in a frequency range of 20,000–0.01 Hz at an applied DC voltage of 0 V. The GCD tests were performed at constant current densities of 0.25, 0.5, 1, 2, and 5 A/g between the 0 V and 2 V potential range. The specific capacitance values of the supercapacitor cells were calculated by GCD analysis according to Equation (1):
C s = I ∆ t m ∆ V
where Cs (F/g) is the specific capacitance of the cell, I (A) is the applied current at discharge step, Δt (s) is the discharge time, m (g) is the total mass of the electrode material, and ΔV (V) is the applied potential difference (excluding IR drop). Energy and power densities were determined from Equation (2) and Equation (3), respectively:
E = C s ∆ V 2 2 × 3.6
P = 3600 × E ∆ t
where E (Wh/kg) is the energy density of the cell, and P (W/kg) is the power density of the cell.

3. Results

3.1. Characterisation of Activated Carbon

3.1.1. Surface Area and Pore Size Distribution

The surface area and pore size distributions of the activated carbon samples were determined from the nitrogen adsorption/desorption data, and the isotherms are shown in Figure S2. The hysteresis loops of the isotherms correspond to the presence of mesopores in the samples. The isotherms resulting from the hysteresis loop are classified as Type IV according to the IUPAC [36]. The BET surface areas, the total pore volumes at P/P0 ≈ 0.99, and the micro- and mesopore volumes of all samples are tabulated in Table 1. The mesopore volume (Vmesopore) was calculated by subtracting the micropore volume from the total volume of Vt (at P/P0 ≈ 0.99).
The pore size distributions of the samples were determined by the NLDFT and BJH methods (Figures S3 and S4). Figure S3 clearly shows that almost all activated carbon samples have pores smaller than 20 nm. Sharp peaks between 2 and 3 nm and between 1 and 2 nm indicate that these pores were predominant. There was no significant difference in the mesopore size distributions (2–50 nm), which were determined by the BJH method (Figure S4). The micropore and mesopore volume values in Table 1 were consistent with these observations. The data in the table demonstrate that subjecting the samples to a heat-treatment process resulted in a slight reduction in the BET surface area and pore volume of the activated carbon samples. The largest change was observed for AC-800; however, the micropore and mesopore volumes demonstrated minimal variation. The slight decrease in textural properties was apparent at elevated temperatures and can be ascribed to the progressive removal of surface functional groups with increasing temperature.

3.1.2. Particle Size Distribution

The particle size distributions of the samples were determined using a laser particle size analyser (Figure S5). The particle size parameters (d10, d20, d50, d90, and d(3,2)) are tabulated in Table S1. The particle size parameters, d10, d20, d50, and d90 indicate diameters (μm) at percentage points; 10%, 20%, 50%, and 90%, respectively. The particle sizes of the heat-treated samples at 600 and 800 °C (AC-600 and AC-800) were slightly finer than the AC sample (untreated commercial AC). This result was expected because the mass of the activated carbon obtained after heat treatment was lower than the initial loading mass. After heat treatment, the particles lose mass as the gasified compounds. The distribution curves of all samples were not shown in the figure for clarity as the curves are very close.

3.1.3. FTIR Analysis

Figure S6 illustrates the FTIR spectra of the commercial and heat-treated activated carbon samples. The samples exhibited comparable absorption bands within analogous wavenumber regions. However, the intensity of the bands decreased as the temperature increased from 400 °C to 800 °C. As the temperature increased, functional groups were removed from the surface of the activated carbon. The broad band observed at wavelengths of 3600–3300 1/cm was attributed to O–H stretching vibrations, which were caused by phenolic hydroxyl, alcohol groups, and adsorbed moisture on the structure. These bands shifted to higher wavenumbers due to the lower moisture content of the heat-treated samples. The absorption bands within the 3000–2850 1/cm wavenumber range were attributed to the aliphatic C–H stretching. The band observed in the commercial activated carbon sample around 1750–1700 1/cm wavenumber was attributed to the presence of the lactone and carboxylic groups. As the heat-treatment temperature was increased, the band underwent a transformation, splitting into two distinct peaks at 1744 1/cm (lactone groups) and 1700 1/cm (carboxylic groups). In the case of AC-800, these bands exhibited a notable reduction in intensity. Similar alterations were noted in the absorption bands observed within the 1680–1530 1/cm range (the quinone groups and aromatic C = C stretching vibrations) and the 1200–1000 1/cm range (combined peak of C–H in-plane deformation, C–O–C, C–C and P–O stretching vibrations, C–OH bending vibrations, and O–C tensile vibration in P–O–C (aromatic) linkage and P–OOH bond) [37,38,39].

3.1.4. TGA

TGA was performed on the AC and heat-treated activated carbon (AC-800) samples (Figure 1). Mass loss was observed in both samples due to moisture removal at 100 °C [40]. The AC sample lost 8% of its mass, while it was 3% for the AC-800 sample. The mass loss of the AC sample gradually decreased up to 525 °C, at which temperature the mass loss was approximately 12%. From 525 °C to 950 °C, the mass loss was sharper, and it was 35.2%. On the other hand, the total mass loss of the heat-treated AC-800 sample was approximately 14%. This was an expected result as the AC-800 sample had already been heat-treated at 800 °C.

3.1.5. XPS Analysis

XPS analysis was employed to investigate the surface chemical properties of the AC and AC-800 samples, with a particular focus on the nature of the surface functional groups and any alterations in their abundance. The surface compositions (atomic %) of the activated carbon samples were determined by calculating the peak area of each element and the total peak area obtained. The major elements detected on the surface were carbon, oxygen, and phosphorus. The oxygen content was indicative of the hydrophilic nature of the surface. The elemental composition and O/C ratios of the samples obtained from the XPS analysis are presented in Table 2. It is evident from the table that the oxygen content of the activated carbon sample was decreased by the heat treatment. This was primarily due to the removal of oxygen-containing surface functional groups. The low O/C ratio may be interpreted as indicative of a reduced hydrophilicity of the AC-800 sample.
Figure 2 illustrates the XPS spectra of AC and AC-800. The spectra display an asymmetric shape, indicative of the presence of diverse carbon species. The deconvolution of the carbon spectra yielded six distinct peaks with varying binding energies. The deconvolution of the C1s spectra allows for the identification and quantitative determination of the carbon-containing surface functional groups. The observed peaks at 284.6 ± 0.3 eV were attributed to sp2 hybridisation in the aromatic and aliphatic carbon structures. In addition to sp2 hybridisation, the samples also exhibited the presence of sp3 hybridisation (285.5 ± 0.3 eV) on their surfaces. The sp2 hybridisation and relatively mild sp3 hybridisation of carbon atoms were assigned to the degree of graphitisation or highly graphitic structure with limited defects and disorder [16].
The signal obtained at 286.6 ± 0.2 eV was attributed to the presence of –C–O-type structures in the form of hydroxyl, phenol, ether, or phosphorus complexes. The peak at 287.4 ± 0.1 implied the occurrence of carbon–oxygen double bonds in the form of quinone or ketone groups on the surface. On the other hand, the signal at 288.9 ± 0.3 eV was attributed to carboxylic-, ester-, anhydride-, or lactone-type surface groups. The signal observed at 291.2 ± 0.2 eV originated from the π-π* shake-up satellite in the aromatic rings for the samples. In consideration of the nature and relative content of functional groups (Table S2), it was observed that there was a decrease in oxygenated groups following heat treatment. The XPS results are in good agreement with the FTIR results.

3.2. Electrochemical Tests

3.2.1. Effects of Heat Treatment

This study compared the electrochemical performance of heat-treated AC samples to determine the most suitable electrode material. For this purpose, 1M Et4NBF4/PC (commercial electrolyte) was selected as the electrolyte, and the tests were conducted at a temperature of 25 °C. As illustrated in Figure 3a, the CV curves of the samples were examined at a scan rate of 50 mV/s, and it was observed that the AC-800 sample exhibited a symmetrical rectangular CV curve. It was noted that the deviation from a rectangular shape decreased in proportion to the increase in the heat-treatment temperature. This phenomenon was primarily attributed to the reduction in resistance within the supercapacitor cells at elevated temperatures. The findings from the FTIR and XPS analyses indicated that the oxygen-containing groups, including the hydroxyl and carboxyl groups, were eliminated from the activated carbon structure at elevated temperatures. The nature and quantity of these groups influence the degradation or enhancement of the electrical conductivity of an electrode material. The removal of oxygen-containing functional groups from the surface typically enhances conductivity, leading to an increased graphitic structure due to a reduction in surface defects [41]. The degree of graphitisation of the samples can be represented by the sp2/sp3 hybridisation ratio, and the presence of sp2 hybridised carbons is essential for better electrical conductivity [42,43]. Furthermore, the area under the CV curve is indicative of the electrochemical performance of the cell. Consequently, it can be posited that the AC-800 sample exhibited the maximal performance. The enhancement in electrochemical performance, as a consequence of heat treatment, is substantiated by GCD analyses. As illustrated in Figure 3b, the GCD results at a current density of 1 A/g demonstrate a substantial enhancement in the IR drop. While the heat treatment led to a reduction in the BET surface area, it concomitantly induced notable alterations in the physical and chemical properties of the activated carbon samples, thereby affecting their electrochemical performance. Consequently, the resulting pore structure became more conducive to the efficient diffusion of electrolyte ions, thereby mitigating the IR drop. The lowest IR drop and the highest specific capacitance values were obtained for the AC-800 sample. The charge and discharge exhibited near-symmetrical characteristics, indicative of the ideal EDLC properties of the sample. The specific capacitance values of the cells containing AC, AC-400, AC-600, and AC-800 were 14.29 F/g, 23.38 F/g, 25.41 F/g, and 30.71 F/g, respectively. The specific capacitance values clearly revealed the effects of heat treatment on electrochemical performance. The Nyquist curves (Figure 3c) revealed the impact of surface carbon content and electrolyte ion transfer on the resistances within the cell, with the semicircles observed in the high-frequency region denoting the charge transfer resistance of the cells. The limited electrical conductivity and the number of accessible pores resulted in the formation of large semicircles. The results demonstrated that the diameter of the semicircle decreased with an increasing annealing temperature, and the AC-800 exhibited the lowest charge transfer resistance. The electrochemical analysis results indicated that the AC-800 sample exhibited the best performance, thus substantiating its selection as the electrode material for testing the cell performance at various operating temperatures in the presence of three distinct ILs.

3.2.2. Electrolyte Performance at −40 °C, 25 °C, and 80 °C

The fabrication of supercapacitor cells was undertaken in the presence of AC-800 as the electrode material and three different electrolytes (1 M Et4NBF4/PC, 1 M PYR14TFSI/PC, and 1 M Et3NHBF4/PC). The electrochemical performance of the cells was examined at different temperatures (−40 °C, 25 °C, and 80 °C). The cell’s CV curves at 50 mV/s are displayed in Figure 4. As is evident from the figure, the cells exhibited EDLC-type CV curves in the presence of three electrolytes at all temperatures. The symmetrical and rectangular nature of the CV curves is a defining characteristic of EDLCs. The cells exhibited a near-rectangular shape at 25 °C and 80 °C, while a distortion in the shape was observed at −40 °C. This observation is attributed to the temperature-dependent viscosity of the electrolytes.
It is well established that the mobility of electrolyte ions is constrained and their diffusion into the pores is diminished at low temperatures due to high viscosity [1,42]. Furthermore, the CV curves can be used to derive additional information regarding the electrochemical performance of the cells. The area under the CV curve has been shown to be directly related to cell performance, with a higher area indicating better performance. The results demonstrated that cell performance at varying temperatures is dependent on the type of electrolyte employed. The investigation revealed that the cell with the most optimal performance at −40 °C was the Et3NHBF4/PC-based cell. No significant variation in cell performance was observed among electrolytes at 25 °C.
As demonstrated in Figure 5, a comparison of the effects of varying temperatures and electrolytes on the specific capacitances of the cells was conducted. The results indicated that supercapacitor cells exhibited high-rate capabilities in the presence of all electrolytes at 25 °C and 80 °C.
However, an inverse relationship was observed between the specific capacitance values and the current density, with a decrease occurring from 0.25 A/g to 5 A/g at −40 °C. For instance, the specific capacitance of the supercapacitor cell for the 1 M Et4NBF4/PC electrolyte exhibited a gradual increase with increasing temperature. The investigation further revealed that while the increase in current density exerted no adverse effect on the specific capacitance at 25 °C and 80 °C, it led to a decline of approximately 50% in performance at −40 °C. This decline was also observed in the 1 M PYR14TFSI/PC electrolyte. However, it showed the optimal performance at 25 °C. In contrast to the other electrolytes, the 1 M Et3NHBF4/PC-used cell exhibited the highest performance at −40 °C, with an approximate 25% increase in specific capacitance at 1 A/g. This cell exhibited electrochemical performance similar to that of the cell using 1 M PYR14TFSI/PC at 25 °C and 80 °C. While it demonstrated the highest performance at −40 °C, its performance was found to decrease with increasing temperature values. The decrease in performance may be attributed to the electrode–electrolyte interaction. The weakened interaction may change the reaction kinetics at the electrode surface with increasing temperature. In particular, the interactions between the AC-800 and the Et3NHBF4/PC may behave differently at high temperatures, resulting in a loss of electrochemical performance. The IR drop values for the Et3NHBF4/PC electrolyte were also recorded at −40 °C, 25 °C, and 80 °C (at 2 A/g). The values were 0.279 V, 0.133 V, and 0.198 V, respectively. A comparison of the performance of the electrolytes reveals that Et3NHBF4/PC (synthesised in our laboratory) is a suitable candidate for supercapacitor applications over a wide temperature range and performs comparably to other commercial ionic liquids.
As illustrated in Figure 6, the Nyquist plots obtained from the EIS analyses offer a clear depiction of the impact of temperature on the internal resistances of the cells. Nyquist plots represent the bulk solution resistance with the x-intercept in the highest-frequency region, the interface resistance between the electrode and the bulk solution with the semicircle in the middle-frequency region, and the tail in the low-frequency region representing the ion transport resistance within the particle pores. The semicircle is attributed to ion migration across the bulk electrolyte–electrode interface, while the sloping curve in the low-frequency region represents the combined effect of ion transport within the pores and the formation of double layers. Equivalent series resistance (ESR) is attributed to the contact resistance of the active materials and the current collector, the resistance between the active materials, and the electrolyte resistance.
The results indicate that there was a substantial decrease in ESR with an increase in the processing temperature from −40 °C to 80 °C for all electrolytes. The cell containing 1 M PYR14TFSI/PC exhibited the highest ESR at −40 °C, with a sharp decrease in resistance observed as the temperature increased, reaching its lowest value at 80 °C. The resistances of cells containing 1 M Et4NBF4/PC and 1 M Et3NHBF4/PC were lower than that of the cell containing 1 M PYR14TFSI/PC at −40 °C and higher at 80 °C. On the other hand, straight lines in the lower-frequency region were indicative of the diffusion resistance of the electrolytes into the pores. The charge transfer resistance and the adsorption performance were also evaluated using EIS analysis. In the low-frequency region, the 1 M Et3NHBF4/PC electrolyte exhibited a sloped line, indicating a diffusion-controlled electrode process. In contrast, the relatively steeper curves observed for the other two electrolytes suggest that ion diffusion and the establishment of the electrical double layer occur more easily within the cell. The fitting curve of the Nyquist plots and equivalent circuit model information were given in a revised version of the Supplementary Materials file (Figure S7). The cells with the 1 M Et3NHBF4/PC electrolyte exhibited the lowest ESR and diffusion resistance at −40 °C and 25 °C, suggesting its suitability for a broad range of applications in supercapacitors.
The electrochemical performances in terms of specific capacitance and the cyclic stabilities of the cells in the presence of three different electrolytes were investigated by GCD analyses. The long-term GCD analyses were tested at 2 A/g for 5000 cycles using the three IL electrolytes at −40 °C, 25 °C, and 80 °C (Figure 7). The results showed that the best performance was achieved with a different electrolyte at each temperature, and the electrochemical stability of the cells was temperature dependent. For example, the cell containing 1 M Et3NHBF4/PC had the best performance (42.12 F/g) at −40 °C. Although the long-term stability of this cell decreased during the analysis, it had the highest specific capacity at the end of 5000 cycles (36.39 F/g). The capacitive retentions of the cells in the presence of 1 M Et4NBF4/PC, 1 M PYR14TFSI/PC, and 1 M Et3NHBF4/PC at −40 °C were 102.43%, 95.86%, and 86.48%, respectively. The performance of the cell containing 1 M Et4NBF4/PC showed fluctuations through 5000 cycles, and the final cycle had slightly higher specific capacitance than the first cycle.
The 1 M PYR14TFSI/PC demonstrated the highest specific capacitance stability (96.14% capacitive retention) at 25 °C. While the initial performances of the cells containing 1 M Et4NBF4/PC and 1 M Et3NHBF4/PC were comparable to that of the 1 M PYR14TFSI/PC-containing cell, and their respective capacitive retention values were 82.55% and 76.30% at 25 °C. Conversely, the cell utilising 1 M Et4NBF4/PC exhibited its highest level (44.57 F/g) at 80 °C, though it also exhibited the maximum capacitive loss (39.13%). In contrast to the results at 25 °C and −40 °C, the most stable results were obtained at 80 °C for the cell containing 1 M Et3NHBF4/PC. The GCD analysis results revealed that the performance of 1 M Et3NHBF4/PC was much better at −40 °C compared to the performance of other ionic liquids.
Additionally, Ragone plots of the cells showed that the supercapacitor cell containing 1 M Et3NHBF4/PC had the most stable energy density values at −40 °C (Figure 8). The highest energy density value (28.58 Wh/kg) was obtained in the presence of 1 M Et4NBF4/PC at 80 °C. Although the cell containing 1 M Et3NHBF4/PC showed lower energy density than the other cells at 80 °C, its performance was better than the others at −40 °C and 25 °C. Therefore, it is suggested that 1 M Et3NHBF4/PC can be used as an appropriate electrolyte for supercapacitors operating in a wide temperature range (especially −40 °C and 25 °C).

4. Discussion

This study concluded that triethylammonium tetrafluoroborate (Et3NHBF4) can be used as a new electrolyte for supercapacitors, allowing them to operate over a wide temperature range. In this context, firstly, activated carbon surface properties were modified by different heat-treatment applications. When the heat-treatment temperature was increased from 400 °C to 800 °C, the carbon content of the sample increased from 89.2% to 94.2%, but the oxygen content decreased by 50%. This result indicated that the oxygen-rich compounds were removed from the structure by the heat-treatment process. This study revealed that the internal resistance diminished with an increasing heat-treatment temperature, and the supercapacitor cells exhibited ideal EDLC characteristics. The low resistance behaviour of the cell was attributed to the low oxygen content and the reduction in the roughness of the pore walls with an altered temperature. The specific capacitance value measured with AC-800 (30.71 F/g) exhibited an increase of more than twofold compared to untreated activated carbon (14.29 F/g). Subsequently, the electrochemical performance of three distinct ILs (commercial electrolytes: 1 M Et4NBF4/PC and 1 M PYR14TFSI/PC; synthesised electrolyte: 1 M Et3NHBF4/PC) was examined over a broad temperature range (−40 °C, 25 °C, and 80 °C) using AC-800 samples, revealing the optimal performance at elevated temperatures. The supercapacitor applications of Et3NHBF4 were assessed for the first time, demonstrating its efficacy across a wide temperature range, particularly at low temperatures (a specific capacitance of 42.12 F/g and an energy density of 16.89 Wh/kg at 2 A/g and −40 °C).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13041057/s1, Figure S1: NMR spectra of triethylammonium tetrafluoroborate; Figure S2: N2 adsorption / desorption isotherms of untreated activated carbon and heat-treated activated carbons at 400 °C, 800 °C at 600 °C; Figure S3: Pore size distributions according to NLDFT method; Figure S4: Pore size distributions according to BJH method; Figure S5: Particle size distribution graphs of AC and AC-800; Figure S6: FTIR analysis results of the samples; Figure S7: Curve fitting of Nyquist plots of Et3NHBF4/PC electrolyte (at −40 °C, 25 °C, and 80 °C) and EIS circuit model; Table S1: Particle size parameters of the AC and the heat treated ACs; Table S2: Type and the relative contents of the surface functionalities obtained by C1s XPS spectra. References [44,45,46,47,48,49,50] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, E.Y. (Ezgi Yurttas), Y.G., N.I.S. and Z.A.; methodology, E.Y. (Ezgi Yurttas), Y.G., N.I.S., E.Y. (Emine Yagmur) and Z.A.; validation, N.I.S. and Z.A.; formal analysis, E.Y. (Ezgi Yurttas) and Y.G.; investigation, E.Y. (Ezgi Yurttas), Y.G., N.I.S. and E.Y. (Emine Yagmur); data curation, E.Y. (Ezgi Yurttas), N.I.S. and Z.A.; writing—original draft preparation, E.Y. (Ezgi Yurttas), Y.G. and Z.A.; writing—review and editing, Y.G., E.Y. (Emine Yagmur) and Z.A.; visualization, E.Y. (Ezgi Yurttas); supervision, Z.A. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK)—Technology and Innovation Funding Programmes Directorate and Akana Engineering and Trade Inc. (TEYDEB-1501, Project No. 3170539).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to acknowledge to Hatice Gizem Şenyurt, Akana Engineering and Trade Inc., Turkey, for her assistance in the preparation and packaging of the suprcapacitor cells.

Conflicts of Interest

Author Ezgi Yurttas was employed by the company Akana Engineering and Trade Inc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The Akana Engineering and Trade Inc. had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. TGA graph of the AC and heat-treated activated carbon at 800 °C.
Figure 1. TGA graph of the AC and heat-treated activated carbon at 800 °C.
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Figure 2. C1s XPS spectra of (a) AC sample and (b) AC-800 sample.
Figure 2. C1s XPS spectra of (a) AC sample and (b) AC-800 sample.
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Figure 3. Comparison of the samples in the presence of 1 M Et4NBF4 /PC electrolyte at 25 °C. (a) CV analysis at a scanning rate of 50 mV/s. (b) GCD analysis at a 1 A/g current density. (c) Nyquist plots.
Figure 3. Comparison of the samples in the presence of 1 M Et4NBF4 /PC electrolyte at 25 °C. (a) CV analysis at a scanning rate of 50 mV/s. (b) GCD analysis at a 1 A/g current density. (c) Nyquist plots.
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Figure 4. CV curves at different temperatures (−40 °C, 25 °C, and 80 °C) and 50 mV/s.
Figure 4. CV curves at different temperatures (−40 °C, 25 °C, and 80 °C) and 50 mV/s.
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Figure 5. The effects of electrolyte type and temperature on the electrochemical performance of cells at different current densities (0.25–5 A/g).
Figure 5. The effects of electrolyte type and temperature on the electrochemical performance of cells at different current densities (0.25–5 A/g).
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Figure 6. Nyquist plots for all electrolytes at −40 °C, 25 °C, and 80 °C.
Figure 6. Nyquist plots for all electrolytes at −40 °C, 25 °C, and 80 °C.
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Figure 7. Specific capacitance values of the cells at −40 °C, 25 °C, and 80 °C at a 2 A/g current density for 5000 cycles.
Figure 7. Specific capacitance values of the cells at −40 °C, 25 °C, and 80 °C at a 2 A/g current density for 5000 cycles.
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Figure 8. Ragone plots of the cells at −40 °C, 25 °C, and 80 °C.
Figure 8. Ragone plots of the cells at −40 °C, 25 °C, and 80 °C.
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Table 1. Specific surface areas and pore volumes of the samples.
Table 1. Specific surface areas and pore volumes of the samples.
SampleBET Surface Area
(m2/g)		Vtotal
(cm3/g, P/P0 ≈ 0.99)		Vmicro (NLDFT)
(cm3/g)		Vmeso (cm3/g)Micropore Fraction
(%)		Mesopore Fraction
(%)
AC15391.5270.4671.06030.5869.42
AC-40015381.5550.4631.09229.7870.22
AC-60015021.5150.4511.06429.7770.23
AC-80013621.3750.4240.95130.8469.16
Vmeso = Vtotal − Vmicro.
Table 2. Elemental composition of the samples and O/C ratios.
Table 2. Elemental composition of the samples and O/C ratios.
Sample IDAtomic, %
COPO/C
AC89.29.81.00.11
AC-80094.24.90.90.05
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Yurttas, E.; Gokce, Y.; Semerci, N.I.; Yagmur, E.; Aktas, Z. A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range. Processes 2025, 13, 1057. https://doi.org/10.3390/pr13041057

AMA Style

Yurttas E, Gokce Y, Semerci NI, Yagmur E, Aktas Z. A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range. Processes. 2025; 13(4):1057. https://doi.org/10.3390/pr13041057

Chicago/Turabian Style

Yurttas, Ezgi, Yavuz Gokce, Nazife Isik Semerci, Emine Yagmur, and Zeki Aktas. 2025. "A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range" Processes 13, no. 4: 1057. https://doi.org/10.3390/pr13041057

APA Style

Yurttas, E., Gokce, Y., Semerci, N. I., Yagmur, E., & Aktas, Z. (2025). A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range. Processes, 13(4), 1057. https://doi.org/10.3390/pr13041057

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