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Article

Electrochemical Evaluation of Choline Bromide-Based Electrolyte for Hybrid Supercapacitors

by
Zhanibek Ayaganov
1,
Fyodor Malchik
2,3,
Zhumabay Bakenov
4,
Zulkhair Mansurov
1,2,
Kaiyrgali Maldybayev
2,3,
Andrey Kurbatov
2,3,
Annie Ng
5,* and
Vladimir Pavlenko
1,5,*
1
Laboratory of Functional Nanomaterials, The Institute of Combustion Problems, Almaty 050009, Kazakhstan
2
Department of Chemistry and Chemical Technology, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan
3
Center of Physical-Chemical Methods of Research and Analysis, Al-Farabi Kazakh National University, Almaty 050012, Kazakhstan
4
Department of Chemical and Materials Engineering, Nazarbayev University, Astana 010000, Kazakhstan
5
Department of Electrical and Computer Engineering, Nazarbayev University, Astana 010000, Kazakhstan
*
Authors to whom correspondence should be addressed.
Energies 2024, 17(22), 5580; https://doi.org/10.3390/en17225580
Submission received: 17 October 2024 / Revised: 30 October 2024 / Accepted: 4 November 2024 / Published: 8 November 2024
(This article belongs to the Section D2: Electrochem: Batteries, Fuel Cells, Capacitors)
Browse Figures
Versions Notes

Abstract

:
Choline bromide (ChBr) has been less explored as an electrolyte material. This work demonstrates the promising potential of ChBr as a novel aqueous electrolyte for hybrid supercapacitors. At its optimized concentration of 3.5 M, ChBr solution exhibits a maximum conductivity of 79.56 mS cm−1 at room temperature, along with a viscosity of 3.15 mPas and a density of 1.14 g cm−3. A reduction in water activity of the optimized ChBr electrolyte concentration extends the electrochemical stability window (ESW), enabling operation up to 1.9 V for two-electrode cells. When the current densities increase from 0.5 to 5 A g−1, the hybrid supercapacitor based on ChBr electrolyte with the optimized mass ratio of electrodes composed of commercial microporous carbon (Maxsorb) demonstrates impressive specific energy and capacitance retention from 41 to 36 Wh kg−1 and from 330 to 300 F g−1 (per mass of one electrode), respectively. The experimental results obtained from this work demonstrate possibilities for further development and applications of ChBr-based hybrid systems in energy storage devices.

1. Introduction

Efficient and sustainable energy storage solutions are needed to meet the rapidly growing global energy demand and achieve carbon neutrality. The rapid technological advances in renewable energy, electric vehicles, and wearable electronics have fueled intensive research in this field. Electric double-layer capacitors (EDLCs) and hybrid supercapacitors (HSCs) show significant promise in addressing these challenges due to their high power density and long operational lifetimes [1,2]. The design of new electrode materials [3], optimization of the porous structure of carbon electrodes [4,5], refining capacitor designs, and exploring new electrolytes [6] are aimed at improving their performance. Recent advancements in energy storage solutions include the development of flexible and stretchable capacitors, as well as hybrid and flow supercapacitors, based on different ionogels, polymer electrolytes, and redox electrolytes [7].
Among them, HSCs have attracted particular attention for their ability to bridge the gap between batteries and capacitors, offering enhanced energy storage capabilities while maintaining high power density. HSCs typically consist of two types of electrodes: a battery-like (Faradaic) electrode and a capacitor-like (non-Faradaic) electrode, each possessing distinct specific capacitances and operating potential windows [8,9]. The battery-type electrode undergoes redox reactions, storing energy through faradaic processes, while the capacitor-type electrode stores energy through electrostatic processes, forming an electric double layer (EDL) at the interface with the electrolyte. The hybrid architecture allows HSCs to deliver higher specific capacitance and energy density compared to conventional EDLCs [10,11]. Electrolytes and electrodes containing redox components such as zinc, vanadium, copper, iron, iodine, bromine, etc. have shown potential in enhancing the performance of HSCs. These species, employed with aqueous electrolytes, offer significant advantages due to the ability of redox reactions occurring prior to or at the boundary of the water decomposition potential [12].
However, a key challenge is balancing the differing kinetic characteristics and capacitances of the two electrodes to optimize the overall device performance [13]. The basic principle of mass balancing involves ensuring that both electrodes have equal charge storage capacities, which is determined using Equation (1) [14,15]:
C − × Δ E − × m − = C + × Δ E + × m + ,
where C− and C+ are the specific capacitance (F g−1) of the negative and positive electrodes, respectively, ∆E− and ∆E+ are their corresponding working potential windows (V), and m− and m+ are the active mass of the positive and negative electrodes (g).
By rearranging Equation (1), the minimum mass ratio (m+/m−) for balanced charge storage is obtained, as shown in Equation (2) [8,14]:
m + / m − = ( C − × Δ E − ) / ( C + × Δ E + ) ,
This minimum mass ratio provides a baseline for balanced charge storage, ensuring efficient utilization of the wider potential window offered by asymmetric electrodes, which in turn supports a higher overall operating voltage and increased energy density. It also helps reduce potential imbalances that could otherwise lead to premature cell degradation or inefficient charge/discharge cycling performance [16].
The development of HSCs using alternative electrolytes, such as those based on choline salts, is currently of great interest due to their potential advantages over conventional inorganic salts [17,18]. While many choline-based compounds are under active investigation, the application of choline bromide (ChBr) as the HSC electrolyte remains unexplored. The investigation of ChBr in this work is driven by its distinctive chemical composition, which incorporates a redox-active bromide anion capable of undergoing redox reactions at the positive electrode. Specifically, the bromide ions (Br−) are oxidized to bromine (Br2) during the charging process, as described by the primary reaction [19,20]:
2 B r − ⇄ B r 2 + 2 e − ,   E 0 = 1.08   V   v s   S H E ,
In aqueous electrolytes, the bromine produced can further react with excess bromide to form polybromide ions, such as tribromide (Br3−) [20,21]:
B r 2 + B r − ⇆ B r 3 − ,
These redox reactions allow for efficient energy storage and release. The oxidation occurs at a potential of around 1.08 V vs. SHE, which is below the water oxidation potential, allowing the HSC to achieve high voltages without oxygen evolution [20,22]. The polybromide formation enhances the energy density by allowing more charge to be stored [23,24]. The general reaction equation can be expressed as:
2 B r − ⇄ B r 2 + 2 e − ⇄     B r − B r 3 − ,
The formation of B r 3 − in the electrolyte plays a critical role in these redox processes. Initially, B r 3 − rapidly dissociates into B r 2 and B r − , establishing a dynamic equilibrium when sufficient concentrations of B r 2 and B r − are present. This equilibrium is influenced by factors such as pH, total bromine content, and electrode potential, all of which are crucial for the electrochemical performance of the HSCs [25]. However, the subsequent reduction of B r 2 to 2 B r −   is a rate-limiting step due to its relatively slow kinetics [26,27,28]. On the other hand, the formation of B r 3 − is favored and plays a crucial role in preventing self-discharge and maintaining high Coulombic efficiencies (CEs) by confining the trihalides within the halide-rich phase. This confinement prevents the diffusion of bromide ions to the negative electrode, thus stabilizing the performance of HSCs [29]. In addition, the B r − / B r 3 − redox couple significantly enhances the capacitance of HSCs, showing that the presence of B r 3 − is beneficial for energy storage applications. The formation and equilibrium of B r 3 − not only support the redox dynamics but also contribute to the higher energy density and improved stability of the HSCs [23].
Unlike the bromide ion, the choline cation does not directly participate in redox reactions, but previous research has demonstrated its crucial role in facilitating charge accumulation within the EDL [17,26,30]. However, the high reactivity of bromine poses challenges like corrosion and self-discharge due to bromine diffusion. Mitigation strategies include using porous carbon materials to adsorb bromine and employing ion-selective membranes to confine bromides within the positive electrode region, thereby enhancing both capacitance and cycling stability [31]. Activated carbon with microporous structure such as Maxsorb can be particularly effective for bromine adsorption. The pore size of electrode materials also plays an important role in mitigating the shuttle effect in HSCs, as micropores can physically restrict the movement of redox substances, thereby reducing the shuttle effect [9,24,32]. Therefore, these HSCs can withstand long-term cycling by optimizing the electrolyte composition and porous structure, providing high performance and durability, without the need for ion-selective membranes to prevent the shuttle effect. These findings contribute to the advancement of energy storage devices by offering valuable insights into the utilization of the underexplored electrolyte composition.

2. Materials and Methods

The choline bromide (ChBr) (98%) was procured from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Before use, the salt was dried for 24 h in a vacuum oven at 100 °C. Additionally, the purity of the reagent was confirmed using nuclear magnetic resonance (NMR) spectroscopy, and the results are provided in Figure S1 in the Supplementary Information. Solutions were meticulously concocted by dissolving precise quantities of ChBr in the deionized water (Sartorius Arium 611DI, Sartorius AG, Göttingen, Germany, 18.2 MΩ.cm). Conductivity measurements of aqueous solutions with varying molarities were executed employing the YSI 3200 instrument (YSI Inc., Yellow Spring, OH, USA). The viscosity of the solutions was measured using an A&D Weighing SV-1A SV-A Series Sine-wave Vibro Viscometer (A&D Company Ltd., Tokyo, Japan). To thermostat the tested electrolyte at different temperatures, a cuvette containing the solution was placed in a flow cell connected to a Huber K6-NR refrigerated/heated circulator (Peter Huber Kältemaschinenbau SE, Offenburg, Germany). Throughout these investigations, unless specified otherwise, ambient conditions were maintained at a temperature of ~25 °C. Simultaneously, water activity measurements were determined through the WaterAct instrument. Raman spectra were measured on a Solver Spectrum, NT-MDT, Limerick, Ireland. A blue solid-state laser with wavelength of 473 nm and spectral resolution of 4 cm−1 was used for all measurements.
Commercially available microporous activated carbon (AC) (Maxsorb MSP-20X, Kansai Coke and Chemicals Co., Ltd., Hyogo, Japan), with a specific surface area (SBET) of 2306 m2 g−1, was employed for the fabrication of freestanding electrodes in combination with conductive carbon black (CB) (Super C45, Imerys, Paris, France) and a 60 wt.% aqueous solution of PTFE (Sigma-Aldrich, St. Louis, MO, USA) serving as a binder. The mixing process was performed in glassware under magnetic stirring, maintaining a mass ratio of 85:10:5 for AC, CB, and PTFE, respectively. To better facilitate the homogenization of components, a small amount of isopropyl alcohol, roughly twice the volume of the dry components, was added. The mixture was then subjected to a controlled heating to 70 °C with stirring followed by evaporation to obtain a pliable, plasticine-like mass. The resulting mass was rolled on a flat surface to a thickness of 80 μm. The resulting sheet was subjected to a drying process in a vacuum oven at 120 °C for 12 h. Finally, circular punches were used to extract electrodes with specified dimensions.
Electrochemical measurements with AC were performed using Swagelok polyvinylidene fluoride (PVDF) cells for two- and three-electrode configurations. A reference cell with a 3 mm diameter platinum working disk electrode (WE) and a platinum mesh counter electrode in 1M ChBr solution was used to determine the kinetics of redox reactions. To determine the electrochemical processes of interfacial interaction between microporous AC electrodes and the optimized concentration of the ChBr electrolyte (3.5 mol L−1), 99.9% pure titanium rods in a diameter of 12 mm, equipped with freestanding carbon composite electrodes were used as current collectors. The diameter of the WE was 6 mm, while the counter electrode was 12 mm, with both AC electrodes having a thickness of 80 μm. A saturated calomel electrode (SCE) was used as the reference electrode in the 3-electrode configuration measurement. To maintain a stable reference potential and prevent chloride ions from contaminating the tested electrolyte, we used a double salt bridge filled with ChBr solution to isolate the reference electrode from reaction products. A glass microfiber filter (Whatman GF/A, Cytiva, Shanghai, China) served as a separator. To enhance electrolyte wetting, the carbon electrodes were degassed in an electrolyte solution in a vacuum chamber prior to cell assembly.
The electrochemical cells were tested using a VSP multichannel potentiostat (Biologic Science Instruments, Seyssinet-Pariset, France) equipped with an electrochemical impedance spectroscopy (EIS) module, employing cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and EIS techniques. EIS measurements were carried out over a broad frequency range, from 10 mHz to 300 kHz. During the measurements, a low-amplitude sinusoidal perturbation of 5 mV was superimposed onto the system while maintaining a constant potential of 0 V vs. Open Circuit Potential (OCP) [33]. The system was allowed to equilibrate for 10 min at this potential before data acquisition, ensuring the establishment of steady-state conditions. The model equivalent circuit used in this study was adapted from [34]. The specific capacitance of the electrodes (F g−1) for three-electrode systems was calculated using Formula (6):
C = 1 2 × m w × v × ( E b − E a ) × ∫ E a E b I d E ,
where mw stands for the mass of the working electrode (g), v —is the scan rate (V s−1), ( E b − E a ) is the upper and lower potential limits during CV testing (V), and I represents the current (A).
The specific capacitance of the electrodes (F g−1) for two-electrode system was calculated using Formula (7):
C e l = 2 m t o t a l × v × ( V b − V a ) × ∫ V a V b I d V ,
where mtotal is the mass of the two electrodes (g), v is the scan rate (V s−1), ( V b − V a ) is the upper and lower voltage limits during CV testing (V), and I represents the current (A).
Charge–discharge curves offered an effective method to calculate resulting values of specific capacitance. The standard equation was adapted to nonlinear charge–discharge curves to obtain accurate results according to the protocol described elsewhere [35].

3. Results and Discussion

3.1. Physico-Chemical Characterization of ChBr Aqueous Solutions

Solutions of ChBr with different concentrations, from 0.5 mol L−1 to 5 mol L−1 were prepared to investigate the effect of concentration on the electrolyte properties. Determining the optimal concentration in electrolytes requires balancing conductivity with ESW, viscosity and reagent consumption, etc. It is well known that temperature has a strong effect on the viscosity of solutions, which in turn significantly affects their electrical conductivity [36,37,38]. The viscosities of ChBr solutions with varying concentrations were determined over a temperature range of 5–50 °C. A concentration–temperature color map as shown in Figure 1a summarizes the viscosity data, with color indicating the viscosity measured at each concentration and corresponding temperature. Analysis of the obtained viscosity values revealed that increasing the concentration of ChBr solution from 0.5 mol L−1 to 5 mol L−1 causes an increase in viscosity from 1.17 mPas to 6.70 mPas, respectively, at 25 °C. The viscosity measurements were also conducted from 5 °C to 50 °C to analyze the effect of temperature on ChBr solutions at different concentrations. As shown in Table S1 (Supplementary Information), increasing the temperature from 5 °C to 50 °C leads to a substantial decrease in viscosity (~50%–64%) across all concentrations tested.
The effect of concentration on the density of ChBr solutions was also studied. The results, presented in Figure 1b, show that solution density increases from 1.0335 g cm−3 at 0.5 mol L−1 to 1.2091 g cm−3 at 5 mol L−1. This demonstrates a significant increase in density with higher concentration and reveals a strong linear relationship between solution density and concentration, with a correlation coefficient of R2 = 0.99759.
Figure 2a demonstrates the dependence of conductivity on the concentration of ChBr solutions at room temperature. The conductivity reaches a maximum value of 79.56 mS cm−1 at a concentration of 3.5 M, above which a significant decrease in conductivity is observed. The electrical conductivity of a solution depends on the concentration and mobility of free ions in the solvent [39], which are influenced by the ionic concentration, ionic association and solvation [40]. In dilute solutions, cations and anions disperse sufficiently apart spatially, minimizing ionic association and solvation effects such that conductivity is primarily determined by the number of free ions. Consequently, as more ChBr salt is added to increase the concentration of free ions, the electrical conductivity of the ChBr solution rises accordingly. However, as the salt concentration continues to increase, the number of ions grows while the solvent volume remains constant, resulting in ions becoming more closely packed and thus ionic association and solvation effects beginning to significantly impact conductivity. In the case of ChBr solutions, when the concentration increases beyond 3.5 M, the ions of opposite charge (choline and bromide) become close enough for ion pairing to occur [41,42]. This results in an increase in the number of electrically neutral ion pairs formed through association as the ChBr concentration rises. The rising production of neutral ion pairs subsequently inhibits the electrical conductivity of the ChBr solution.
As shown in Figure 2a, the water activity (aw) of the ChBr electrolyte decreases with increasing salt concentrations, indicating a reduction in “free” water molecules available for hydrogen bonding [40]. The value of aw is defined as the ratio of the vapor pressure of water in a sample (P) to that of pure water (P0) at the same temperature, calculated as aw= P/P0. It ranges from 0 in completely dry samples to 1 in pure water [41]. This trend correlates with results from Raman spectroscopy (Figure 2b), which offer insights into the hydrogen bonding dynamics between water and dissolved ions. In ChBr solutions, the broad Raman band at 2700–3800 cm−1 corresponds to O-H stretching vibrations, sensitive to changes in hydrogen bonding [43]. Peaks below 3200 cm−1 reflect strong hydrogen bonds, while those above 3400 cm−1 indicate weaker bonds. As ChBr concentration increases, the intensity of peaks associated with strong hydrogen bonds, particularly the double-donor double-acceptor (DDAA) peak, decreases, indicating that bromide ions disrupt the water network by forming hydration shells around the ions. This disruption reduces the number of DDAA hydrogen bonds and leads to a right shift in the single-donor single-acceptor (DA-OH) peak due to strong polarization of water molecules by bromide ions, reflected by increasing the frequency of the O-H stretching vibration [44,45,46]. The disappearance of the free water peak (~3630 cm−1) is attributed to strong interactions between bromide ions and water molecules, which reduce the number of non-hydrogen-bonded water molecules. In addition, the emergence of peaks corresponding to C-H stretching vibrations (2900–3100 cm−1) as ChBr concentration increases reflects the growing number of CH bonds in the sample [47,48,49]. These spectral changes highlight the complex relationship between ion concentration and hydrogen bonding. Lowering the water activity (aw) can broaden the electrochemical stability window (ESW) by shifting the potential for the undesirable hydrogen evolution reaction (HER) to more negative voltages. A similar principle is used in “water-in-salt” (WIS) electrolyte systems to achieve a wider ESW, albeit at the expense of reduced ionic conductivity [49,50].
Based on the data obtained, a concentration of 3.5 mol L−1 ChBr was selected as the optimal electrolyte concentration for electrochemical studies. This concentration exhibits the highest conductivity at 79.56 mS cm−1, while also reducing water activity by 18% and maintaining a viscosity range from 4.86 to 2.16 mPas across temperatures from 5 to 50 °C. Increasing the concentration from 3 mol L−1 to 3.5 mol L−1 results in an average viscosity increase of 16%, and further increasing the concentration from 3.5 mol L−1 to 4 mol L−1 leads to a more pronounced average viscosity increase of 29% over the same temperature range.

3.2. Determination of Potential Electrolyte Stability Window

CV plots for ChBr-based electrolyte on bare platinum electrodes and porous AC with titanium current collectors are shown in Figure 3a and 3b, respectively. Figure 3a illustrates the electrochemical behavior of ChBr at a polished bare platinum electrode. The redox reaction peak is observed in the cathodic region at approximately +0.8 V vs. SCE [50]. The lack of hydrogen releases up to −1.0 V vs. SCE in Figure 3a indicates that the experimentally achievable potential window extends from −1.0 V to +0.9 V vs. SCE. Within this range, the CE is determined to be 93.51%. However, extending the potential window to +1.0 V relative to SCE results in a sharp decrease in CE to 64.99%, suggesting that beyond this potential there is an increase in irreversible reactions.
Figure 3b shows the CV curve for the AC electrode (the complete graph for all voltage ranges is provided in Figure S3a in the Supplementary information). In the negative potential region, charge accumulation occurs primarily through the EDL mechanism, while in the positive potential region, bromine redox reactions are observed according to Equation (5). In the negative potential region, decreasing the voltage below −1 V vs. SCE at the platinum electrode or below −1.1 V vs. SCE at the AC electrode leads to water decomposition through the hydrogen evolution reaction (HER), as shown in Figure 3a and Figure 3b, respectively.
The kinetics of redox reactions at platinum and AC electrodes, as presented in Figure 3, show certain differences. AC electrodes typically exhibit significant electric double-layer capacitance due to their high surface area and microporous structure [51]. Porous texture data of the carbon materials are summarized in Table S2. As a result, capacitive behavior is clearly observed alongside Faradaic responses [52]. In contrast, platinum electrodes show significant Faradaic capacitance due to the active involvement of surface platinum atoms in redox reactions, since the presence of platinum facilitates faster electron transfer reactions and a higher rate of Faradaic processes [53]. On the other hand, platinum electrodes generally have a smoother surface, which limits the available surface area compared to AC.
Figure 4 shows the specific capacitance and CE at different potential windows for the positive and negative electrodes, allowing determination of the optimal operating voltage range. The voltage window was determined to be 1.1 V vs. SCE in the negative potential region and 0.8 V vs. SCE for the positive region, as shown in Figure 4a and Figure 4b, respectively. When the potential window is increased to 0.8 V vs. SCE at the positive carbon electrode, CE remains above 90%, but a further increase in potential leads to a remarkable decrease in CE. This decrease can be caused by several factors, such as irreversible oxidation reactions of bromide ions due to overvoltage on the cell, interaction of bromine with the current collector or carbon electrode and their subsequent oxidation, as well as decomposition of water and release of oxygen at the positive electrode [54]. The decrease in CE in the negative potential region caused by the higher potential is due to electrolyte decomposition. This can be attributed to electrosorption of hydrogen in the carbon electrode pores since the curves still retain a quasi-rectangular shape, as previously shown in Figure 3b [26,55]. Thereby, in designing two-electrode cells, it is crucial to carefully select the appropriate voltage windows for both the positive and negative electrodes to ensure stable operation. When determining the working voltage window of the electrodes in a three-electrode configuration, the reaction reversibility threshold was set above 90%. This approach allows for further balancing of electrode masses according to their specific capacitances within the selected operating potential range. Using Equation (2), the optimum mass ratio of the positive and negative electrodes was determined to be approximately m + : m − = 0.6:1.0. This ratio provides balanced charge storage and optimum HSC performance at 1.9 V.

3.3. Electrochemical Investigations of 2-Electrode Cells: CV, GCD, and EIS

Two-electrode cells were assembled using mass-balanced porous AC electrodes with titanium current collectors and investigated through several electrochemical characterization techniques, including CV, GCD, and EIS measurements. Figure 5a shows the CV curve, where the separation between the capacitance caused by charge accumulation in the EDL only and the contribution of Faraday reactions to the cell capacitance at potentials above 0.6 V caused by oxidation of the Br− can be clearly seen. The HSC demonstrates a robust response to increasing scan rates up to 50 mV/s, maintaining a near-rectangular shape in the CV profile at initial voltages, which suggests low internal resistance. The specific capacitance was 323 F g−1 at a scan rate of 5 mV s−1 and 266 F g−1 at 50 mV s−1, reflecting a capacitance retention of 82.4%. The specific capacitance at higher scan rates is limited by slower Faraday processes. Evidence of these redox reactions can also be seen in the GCD curves (Figure 5b), where their presence leads to the nonlinearity of the discharge profiles. Figure 5c shows the impact of varying scan rates and current densities on the specific capacitance of HSCs using the ChBr electrolyte. It is noteworthy that at low scan rates, carbon micropores with large surface areas are known to contribute high volumetric capacitance due to the sufficient time available for ion penetration into the micropores. However, at higher scan rates/current densities, the reduced ion diffusion and charge transfer times limit the capability of the system to effectively store charges. With less diffusion time, the extent of ion adsorption/desorption at the electrode–electrolyte interface decreases, resulting in lower overall specific capacitance. Efficient charge transfer kinetics at the interface is necessary for fast ion transfer, but various factors such as electrolyte resistance and electrode surface area with complex pore channel system can hinder this process [56,57,58]. Figure 5d shows the dependence of CE and energy density on the current density. As can be observed, CE decreases at low current densities. Longer charge-discharge cycles at low currents allow more time for side reactions like electrolyte decomposition or oxidation of electrode materials, which reduce CE [59]. Changes in current density also affect the kinetics of the redox reaction according to the Nernst equation, leading to variations in the capacitance of the positive electrode and impaired mass balancing [60]. Additionally, the shuttle effect—where halide ions migrate between electrodes during cycling—becomes more pronounced during longer cycles at low current densities, further decreasing CE [61,62,63,64]. However, the well-developed microporous structure of the electrodes significantly mitigates the shuttle effect by restricting ion movement.
Cyclic stability tests were conducted at a current density of 2 A g−1 for 10,000 cycles. Based on the results of GCD tests shown in Figure 5c,d at various current densities, the decrease in specific capacitance and specific energy at 2 A g−1, compared to 0.5 A g−1, was minimal: 4.1% and 4.7%, respectively. Thus, the current density of 2 A/g was selected for cyclic stability tests in order to optimize the running time of measurements. Figure 5e plots the capacitance retention and CE over cycles. The capacitance initially increases up to 2500 cycles reaching a maximum. Similar increasing trends have also been observed in other HSCs [26,62]. It can occur when a viscous electrolyte gradually wets microporous electrode materials over time [65]. In the early cycles, the Br−/Br2− couple primarily contributes to pseudocapacitance, gradually reaching an equilibrium state, as evidenced by the plateau in capacitance and CE, and later converts to Br−/Br3− redox couples [66]. Even after 10,000 cycles, the capacitance retention and CE remain high at 98.3% and 95.2%, respectively.
Electrochemical impedance spectroscopy (EIS) was employed to assess the electrochemical performance of the ChBr-based HSC after long-term cycling. Figure 5f presents the Nyquist plot alongside the equivalent circuit model used in this study. The cell demonstrates a low equivalent series resistance (ESR, RS) of 0.3 Ω, indicative of high electrolyte conductivity. The Faraday reaction resistance (RF) is measured at 0.37 Ω, and QP represents the pseudocapacitance associated with this reaction. These components have been included in the equivalent circuit to encompass all relevant electrochemical processes occurring at the electrode-electrolyte interface. Bromide systems have a high OCP [66], for our system it was 0.74 V, which leads to the observed pseudocapacitance. These are observed in the Nyquist plot as two semicircles along with capacitance behavior (QDL). The charge transfer resistance (Rct), influencing the kinetics of the charge transfer process, is found to be 0.33 Ω. The Warburg resistance (Wd), or diffusion resistance, accounts for the resistances of the diffuse layers near the positive and negative electrodes. It appears as a non-vertical line at approximately 45 degrees in the intermediate frequencies and measures 0.4 Ω. The near-vertical rise in the low-frequency region correlates to the 331 F g−1 capacitance, aligning with the GCD measurements (Figure 5c). The total cell resistance, or equivalent distributed resistance (EDR), is determined by extrapolating the low-frequency region of the Nyquist plot to the real axis [67]. This value is calculated to be 1.4 Ω, consistent with the sum of all resistive components derived from the equivalent circuit model.
The results obtained in this work demonstrate the potential of using ChBr as a new redox-active material in aqueous electrolytes for HSCs. For comparison, Table 1 summarizes representative works using other choline salts as electrolytes, as well as HSCs based on KBr and other commonly used aqueous electrolyte systems, including neutral, acidic, and alkaline electrolytes. As the presented data shows, the ChBr based system reported in this work delivers a remarkable specific capacitance of 330 F g−1 and specific energy of 41 Wh kg−1 at 0.5 A g−1. These values outperform most of the established aqueous electrolyte systems. Moreover, the system exhibits a high working voltage of 1.9 V, exceeding the water decomposition threshold, and achieves a CE of 97%. It is noteworthy that after extended cycling at 2 A g−1, the system still retains 98.3% of its maximum capacitance. These exceptional characteristics make the system competitive not only with aqueous electrolytes based on KBr but also other choline-based salts. Compared to existing reports, our optimized ChBr electrolyte yields a well-balanced performance across key HSC metrics while achieving the wide voltage window at 1.9 V. This also emphasize the importance of careful physicochemical tuning of electrolytes for high-performance HSCs. The current work provide perspective for applying ChBr to develop efficient HSCs with high energy density and long lifetime. This opens new horizons for developing more efficient electrochemical devices, contributing to sustainable and advanced energy storage technologies.

4. Conclusions

This work conducted a comprehensive characterization of ChBr as a novel redox-active aqueous electrolyte for HSCs. The basic physicochemical properties, including viscosity, density, conductivity and water activity, were investigated for ChBr solutions prepared at concentrations ranging from 0.5 M to 5 M. From these analyses, 3.5 M ChBr was determined to be the optimized concentration for using as electrolyte. The electrochemical properties of optimized ChBr electrolyte were further studied, revealing redox reactions at the positive electrode and elucidating the mechanism of charge accumulation within the EDL at the negative electrode. Optimization of the electrode mass ratio and determination of the potential stability window enhanced the electrochemical characteristics of the system. The HSC in the two-electrode cell configuration exhibited a high CE of 97%, a specific energy of 41 W h kg−1 and a specific capacitance of 330 F g−1 at 0.5 A g−1. The HSCs also exhibit good cycling stability for 10,000 cycles at 2 A g−1. This work highlights the importance of parameter optimization for achieving optimal device performance with balanced characteristics and motivate further development of this new type of ChBr-based electrolyte for applications in electrochemical devices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en17225580/s1, Figure S1: Comparing the (a) 1H NMR and (b) 13C NMR spectra of choline bromide; Figure S2: (a) CV plots of different concentrations of ChBr electrolyte at 50 mV s−1 vs. SCE on a platinum working electrode (d = 3 mm) and (b) specific capacitance at different concentration; Figure S3: CV plots of AC WE at 2 mV s−1 vs. SCE with titanium current collectors in (a) 3.5 mol L−1 and (b) 4 mol L−1 ChBr; Table S1: Viscosity of choline bromide solutions as a function of concentration and temperature; Table S2: Porous texture data of the carbon materials [84].

Author Contributions

Conceptualization, A.N. and V.P.; methodology, V.P.; validation, Z.A., Z.B., Z.M. and A.K.; investigation, Z.A., F.M. and K.M.; writing—original draft preparation, Z.A.; writing—review and editing, Z.A., Z.B., Z.M., A.N. and V.P.; project administration, A.N. and V.P.; funding acquisition, V.P., Z.B. and A.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work is funded the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grants nos. AP14872549, BR21882402, 211123CRP1613, and 201223FD8801).

Data Availability Statement

The data presented in this study may be partially or fully provided upon request by contacting the corresponding authors.

Acknowledgments

V.P. acknowledges the financial support from the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Scientific Research Grant no. AP14872549). Z.B. acknowledges the financial support of the Ministry of Science and Higher Education of the Republic of Kazakhstan by the Targeted Program Funding BR21882402 “Development of new material technologies and energy storage systems for a green economy”. A.N. acknowledges the financial support from Nazarbayev University (Grant nos. 211123CRP1613 and 201223FD8801). The authors would like to thank Renata Nemkayeva from al-Farabi Kazakh National University for her help and support with Raman measurements. Special thanks are extended to Farkhad Tarikhov for NMR measurement and discussion.

Conflicts of Interest

The authors declare no conflicts of interest. The funders 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. Physicochemical characteristics of ChBr solutions: (a) viscosity as a function of concentration and temperature; (b) dependence of density on electrolyte concentration.
Figure 1. Physicochemical characteristics of ChBr solutions: (a) viscosity as a function of concentration and temperature; (b) dependence of density on electrolyte concentration.
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Figure 2. (a) Dependence of the conductivity and water activity on electrolyte concentrations and (b) Raman spectra of the ChBr electrolyte with varying concentrations. The purple-shaded region highlights the O-H stretching vibration band, which shifts and changes in intensity as the concentration of ChBr varies.
Figure 2. (a) Dependence of the conductivity and water activity on electrolyte concentrations and (b) Raman spectra of the ChBr electrolyte with varying concentrations. The purple-shaded region highlights the O-H stretching vibration band, which shifts and changes in intensity as the concentration of ChBr varies.
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Figure 3. CV plots of (a) bare platinum disk electrode at 50 mV s−1 in 1M ChBr and (b) AC electrode at 2 mV s−1 vs. SCE with titanium current collectors in 3.5 M ChBr.
Figure 3. CV plots of (a) bare platinum disk electrode at 50 mV s−1 in 1M ChBr and (b) AC electrode at 2 mV s−1 vs. SCE with titanium current collectors in 3.5 M ChBr.
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Figure 4. Determination of the operating voltage for the (a) negative and (b) positive electrode.
Figure 4. Determination of the operating voltage for the (a) negative and (b) positive electrode.
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Figure 5. The electrochemical properties of two-electrode cells at (a) different scan rates and (b) current densities; (c) specific capacitance; (d) CE and energy density as a function of the current density; (e) long-term cycling results at 2 A·g−1 and (f) Nyquist plot.
Figure 5. The electrochemical properties of two-electrode cells at (a) different scan rates and (b) current densities; (c) specific capacitance; (d) CE and energy density as a function of the current density; (e) long-term cycling results at 2 A·g−1 and (f) Nyquist plot.
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Table 1. Comparative characteristics of supercapacitors using choline salts and other common electrolytes at room temperature.
Table 1. Comparative characteristics of supercapacitors using choline salts and other common electrolytes at room temperature.
Electrode MaterialsSSA, m2 g−1ElectrolyteCell VoltageCurrent Density (A g−1)Energy Density (Wh kg−1)Power Density (W kg−1)C (F g−1)ΔC (%)CE (%)Ref.
Choline-based electrolytes
Kansai Coke, Maxsorb MSP-20X23063.5 mol L−1 ChBr1.90.54122633098% after 10,000 cycles at 2 A g−197%This work
5362297301
Poly (aniline-co-4-nitroaniline)-Choline formate/
2-hydroxyethyl cellulose = 3/1		0.60.5~30150594~90% after 5000 cycles at 5 A g−1~96% at 5 A g−1[68]
Interconnected hierarchical porous carbon (IHPC)34630.1 mol L−1 ChOH + 1 mol L−1 KOH1.30.5~20 203 462~91% after 30,000 cycles at 5 A g−1100% at 5 A g−1[69]
Activated Carbon1900ChCl/1, 2-butanediol = 1/42.01.0~16 1k116~87% after 10,000 cycles at 4 A g−1-[70]
(−) Kansai Coke, Maxsorb/(+)Kuraray, YP-80F1962/17355 mol kg−1 ChNO3 +0.5 mol kg−1 ChI1.50.1~123.0 k81
per total mass of electrodes		92% after 20 k cycles at 0.5 A g−1~86%
Energy eff.		[17]
MgO-templated hierarchical carbon~20005 mol kg−1 choline bis(trifluoromethylsulfonyl)imide in M0.75W0.251.60.5~85012881% after 20,000 cycles at 1 A g−1-[18]
DLC Supra 30 from Norit18695 mol kg−1 ChCl1.50.2--126~98% after 10,000 cycles at 1 A g−1~99%[71]
(+)MnO2/CNT|YP80F(−)-5 mol L−1 ChNO30.3–1.81.0--38
for cell		~97% after 10,00090%[72]
Redox active electrolytes
Fuzhou Yihuan Carbon Co., Fuzhou, China, YEC-8A18981 mol L−1 KBr2.00.25~33---~93%[30]
S-doped graphene2150.01 mol L−1 NH4VO3 + 1 mol L−1 H2SO41.63.032237036485% after 10,000 at 10 A g−199% at
10 A/g		[73]
AC/MgO templated carbon2315/19765 mol L−1 NaNO3 + 0.5 mol L−1 KBr1.80.527-23975% after 5000 cycles at 2 A g−194[74]
AC21801 mol L−1 Li2SO4 + 0.5 mol L−1 KI1.60.226~8030091% after floating for 120 h71%
energy efficiency		[75]
Kuraray, YP-80F21121 mol L−1 KBr1.91.01215 kW kg−1 at 15 A g−19281% after 10,000 cycles at 15 A g−1~98%[26]
Other common aqueous electrolytes
Neutral
AC from Salvia miltiorrhiza flowers17151 mol L−1 Na2SO41.80.5~2244819891% after 10,000 cycles at 10 A g−1 in 6 M KOH-[76]
Hierarchically porous carbons30031 mol L−1 Na2SO41.60.5~2140024091% after 2000 cycles at 5 A g−193%[77]
AC, Carbosino Co., Ltd., Shanghai, China25001 mol L−1 Li2SO41.60.25~1720019092% after 10,000 cycles at 1 A g−199%[78]
Acid
AC from waste water11031 mol L−1 H2SO41.01.0~15~937~123~66% after 6000 cycles at 5 A/g-[79]
AC9031 mol L−1 H2SO41.00.510490545--[80]
AC from Eichhornia crassipes6831 mol L−1 H2SO40.81 mA cm−2~9~315~44191% after 4000 cycles at 5 mA cm−2~81%[81]
Alkaline
AC35776 mol L−1 KOH1.00.1~10~2533095% after 10,000 at 1 A/g-[82]
B-doped graphene17020% KOH1.01.0~550228696% after 10,000 cycles at 20 A/g-[83]
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Ayaganov, Z.; Malchik, F.; Bakenov, Z.; Mansurov, Z.; Maldybayev, K.; Kurbatov, A.; Ng, A.; Pavlenko, V. Electrochemical Evaluation of Choline Bromide-Based Electrolyte for Hybrid Supercapacitors. Energies 2024, 17, 5580. https://doi.org/10.3390/en17225580

AMA Style

Ayaganov Z, Malchik F, Bakenov Z, Mansurov Z, Maldybayev K, Kurbatov A, Ng A, Pavlenko V. Electrochemical Evaluation of Choline Bromide-Based Electrolyte for Hybrid Supercapacitors. Energies. 2024; 17(22):5580. https://doi.org/10.3390/en17225580

Chicago/Turabian Style

Ayaganov, Zhanibek, Fyodor Malchik, Zhumabay Bakenov, Zulkhair Mansurov, Kaiyrgali Maldybayev, Andrey Kurbatov, Annie Ng, and Vladimir Pavlenko. 2024. "Electrochemical Evaluation of Choline Bromide-Based Electrolyte for Hybrid Supercapacitors" Energies 17, no. 22: 5580. https://doi.org/10.3390/en17225580

APA Style

Ayaganov, Z., Malchik, F., Bakenov, Z., Mansurov, Z., Maldybayev, K., Kurbatov, A., Ng, A., & Pavlenko, V. (2024). Electrochemical Evaluation of Choline Bromide-Based Electrolyte for Hybrid Supercapacitors. Energies, 17(22), 5580. https://doi.org/10.3390/en17225580

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