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. 2026 Oct 6. Online ahead of print. doi: 10.1039/d6ra08120g

Different chain length hydrophilic ionic liquids on the surface properties of activated carbon

Junjie Wu a, Kangjie Wu a, Qingshan Liu a,✉, Mingxing Zhao a, Yanxia Qi a
PMCID: PMC13639872  PMID: 42840805

Abstract

Based on issues such as poor interfacial wettability when using activated carbon (AC) as a supercapacitor electrode material, this study employs two hydrophilic ionic liquids with different chain lengths, 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4] and [Bmim][BF4]); commercial activated carbon with larger pore size (1.73 nm) and higher mesopore fraction (11.63%) was modified by the impregnation method to prepare ILs-AC composite materials. The study found that the ionic liquids were successfully loaded onto the surface of activated carbon through physical adsorption. Although some micropores were blocked, resulting in a decrease in specific surface area, the hydrophilicity of the material was significantly enhanced, effectively reducing charge transfer resistance and markedly lowering the contact angle of AC, thus improving the interfacial wettability between the electrode and the electrolyte. Electrochemical tests demonstrated that AC modified with the shorter-chain ionic liquid [Emim][BF4] had a specific capacitance of 92.99 F g−1 (an increase of approximately 6.7%), whereas AC modified with the longer-chain ionic liquid [Bmim][BF4] exhibited a specific capacitance of 94.39 F g−1 (an increase of approximately 8.3%). Regarding long-term cycling performance(10 000 cycles), the capacitance retention rates of the modified materials were 96.64% and 92.83%, respectively, both higher than that of the original material (capacitance retention rate of 91.73%).

Dynamic process of contact angle testing. NEAT AC (a–c), 0.25%[Emim][BF4]-AC (d–f), 0.5%[Bmim][BF4]-AC(g–i).graphic file with name d6ra08120g-ga.webp

1. Introduction

Apart from biofuels, most renewable energy sources are harvested or utilized in the form of electricity, and there has been a great demand for reliable electrochemical storage platforms, including batteries and electrochemical supercapacitors.1 Although the energy storage and conversion mechanisms of these three systems are different, they are all based on similar electrochemical thermodynamics and kinetics, namely, the energy supply process occurs at the phase boundary of the electrode/electrolyte interface with independent electron and ion transport.2 As a new type of electrochemical energy storage device with high power density, supercapacitors are widely used in electronic devices, new energy transportation and industrial equipment due to their high efficiency, high power performance and high charge–discharge cycle life.3–5

The main development trend of supercapacitors is to improve their energy and power density. The electrode material selected for a supercapacitor largely determines the charge storage and capacitance, and designing materials with high capacitance is one of the key research areas. Currently, researchers are using a variety of materials to prepare electrodes for SCs, including activated carbon (AC),6,7 graphene,8 carbon nanotubes,9 conductive polymers,10 metal oxides,11 and composite materials.12,13 Carbon-based materials have become the main electrode materials used in commercial capacitors due to their large specific surface area, excellent conductivity, and abundant raw materials.14–17 Among them, activated carbon has become one of the most widely used carbon-based materials in commercial applications due to its low cost and large specific surface area. Despite the above advantages, the double-layer energy storage mechanism limits its capacitance performance and energy density.18

One of the mainstream strategies for improving activated carbon performance is heteroatom doping such as N, O, and S to introduce additional pseudocapacitive performance.19 Dong et al. prepared nitrogen-doped coal-based activated carbon by KOH/H2O co-activation method on lignite. The test results showed that at a current density of 0.5 A g−1, the doped sample was about 64.8% higher than the undoped sample, reaching as high as 323 F g−1.20

Ionic liquids (ILs) have become a research hotspot for supercapacitors due to their unique physicochemical properties, such as wide voltage window, low volatility, and high thermal stability.21–23 The properties of ionic liquids can be changed by adjusting the combination of cations and anions.24 Due to their tunable cation and anion structures, they are currently used as ideal precursors or doping sources for heteroatom-doped carbon materials. Braham S et al. activated fern shells with KOH and used [Emim][DCN] as a nitrogen precursor to prepare nitrogen-doped AC by pyrolysis. When tested in a three-electrode system test at 0.5 A g−1, the best sample showed a high specific capacitance of 567 F g−1.25 Liu et al. prepared nitrogen and phosphorus co-doped porous carbon microspheres by carbonizing [Mimm][DMP]-lignin. In 1 mol L−1 H2SO4 electrolyte system, the specific capacitance reached 338.2 F g−1 at a current density of 0.8 A g−1.26

Although heteroatom doping effectively improves the electrochemical performance of activated carbon, the interfacial wettability between the electrode material and the electrolyte remains one of the key factors restricting the rate performance of SCs. In research, the method of using ILs to modify the surface of carbon-based materials to improve their wettability has been widely used.27 Iessa et al. reported that combining ionic liquids with graphene nanosheets effectively prevented the re-stacking of graphene nanosheets, improved hydrophilicity, and provided active sites for their combination with polyaniline nanorods.28 However, existing research on ionic liquid modification of carbon-based materials mainly focuses on materials such as graphene and carbon nanotubes, while there are few reports on the modification and regulation of activated carbon.

Existing studies indicate that the carbon chain length of imidazolium-based ionic liquid cations can influence their molecular size, interfacial structure, transport behavior, and electrochemical response in porous carbon.29,30 In this regard, we selected the cations [Emim]+ and [Bmim]+ to construct a system for comparing the effects of carbon chain length. To further eliminate the influence of anions, [BF4]− was chosen as the anion. Current research on porous carbon shows that [Emim][BF4], [Bmim][BF4], and [Pmim][BF4] exhibit different interfacial and electrochemical behaviors.31 However, this study employs an aqueous electrolyte, and the potential corrosiveness and contamination caused by [BF4]− require further discussion. In aqueous environments, [BF4]− is not entirely stable and undergoes hydrolysis to form boron- and fluorine-containing species. This process is kinetically limited and affected by pH, temperature, and time.32 Lozada et al. investigated the hydrolytic behavior of 20 mM NaBF4 in aqueous solutions at different pH values and found that [BF4]− underwent rapid, reversible solvolysis under strongly acidic conditions (pH < 3), reaching an apparent equilibrium within minutes, whereas its decomposition was slow and irreversible at pH ≥ 7.4, with >80% of the initial [BF4]− remaining after 17 000 min at pH 14 (1 M NaOH) and 21 ± 2 °C; The hydrolysis involves partially hydroxylated boron-fluorine species and ultimately produces [F]− and [B(OH)4]− upon quenching.33 In this study, the two ILs were loaded onto the AC surface by impregnation at relatively low doping ratios, so the introduction of [BF4]− was relatively limited. Even if a certain degree of hydrolysis occurs, the total amount of potentially released boron- and fluorine-containing species is constrained by the initial doping ratio.

Therefore, to further explore the modification and regulation mechanism of ionic liquids on activated carbon, this paper selects two ILs, [Emim][BF4] and [Bmim][BF4], and modifies the AC surface with ILs via an impregnation method to prepare ILs-AC composite materials. By modifying the AC surface with ILs, the aim is to improve the wettability of the electrode material and electrolyte, reduce charge transfer resistance, and compare the effects of different cation carbon chain lengths on the energy storage mechanism in mesoporous activated carbon.

2. Experiments

2.1. Materials

All chemical reagents were of analytical grade and used as received without further purification. AC (XFP01, Jiangsu Xianfeng Nanomaterials Technology Co., Ltd) was used as the carbon substrate, exhibiting an average pore diameter of 1.74 nm with a micropore-dominated structure and a mesopore proportion of 11.63%. [Emim][BF4] (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences), [Emim][BF4] is synthesized from [Emim]Cl and NaBF4via a two-step method, with a purity of 99% and a water content of 2000 ppm. [Bmim][BF4] (Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences), [Bmim][BF4] is synthesized from [Bmim]Cl and NaBF4via a two-step method, with a purity of 99% and a water content of 3000 ppm. Prior to use, both ionic liquids were vacuum dried at 100 °C for 24 h under a vacuum of approximately −0.1 MPa. Carbon Black, N-methylpyrrolidone (NMP, Shanghai Maclean Biotechnology Co., Ltd, China), polyvinylidene fluoride (PVDF, Shanghai Maclean Biotechnology Co., Ltd, China), nickel foam (Suzhou Shengernuo Technology Co., Ltd, China), acetone (Chengdu Kelong Chemicals Co., Ltd, China), potassium hydroxide (KOH, Shanghai Maclean Biotechnology Co., Ltd, China), deionized water (Hangzhou Wahaha Group Co., Ltd), anhydrous ethanol (Shanghai Maclean Biotechnology Co., Ltd, China).

2.2. Material preparation

The pretreated ionic liquids were used to prepare the ILs-AC composites by the impregnation method. ILs-AC composites were prepared by the impregnation method, as follows: the loading ratio of IL was calculated based on mass percentage. The IL was accurately weighed and added to 20 ml of acetone. The mixture was placed in a sealed container and magnetically stirred at room temperature for 1 h to ensure complete dissolution. AC with the corresponding modification gradient was added to the solution, and then the mixture was magnetically stirred for 4 h until the acetone is completely evaporated. After stirring, the mixture was placed in a drying oven and dried at 60 °C for 12 h to thoroughly remove the acetone solvent, ultimately obtaining the ILs-AC composite material.

The loading ratio of [Emim][BF4] were 0.25%, 0.5%, 1%, and 1.25%; the loading ratio of [Bmim][BF4] were 0.25%, 0.5%, 1%, and 1.25%. The materials with different loading ratio were named as follows: 0.25%[Emim][BF4]-AC, indicating a composite of [Emim][BF4] and AC with a modification gradient of 0.25%.

Typically, 1.5 g PVDF was dissolved in 100 ml NMP solution. Stir magnetically in a sealed container for 48 h to obtain a PVDF-NMP mixed solution of 15 mg ml−1, which is used as a binder for the material. The mixed solution is dried and stored under sealed conditions.

2.3. Electrode preparation

The nickel foam sheet was cut into (1 × 2) cm2 pieces. The cut nickel foam sheets were placed in a container, submerged in deionized water, and sonicated for 10 min. The deionized water was then drained, and the sheet was submerged in anhydrous ethanol and sonicated for another 10 min. This process was repeated three times to thoroughly clean the nickel foam sheet. The cleaned nickel foam sheet, carbon black, and activated carbon composite material were placed in a vacuum oven and dried overnight at 60 °C.

Activated carbon material, carbon black, and PVDF-NMP mixed solution were weighed at a mass ratio of 8 : 1 : 1. The carbon black and PVDF-NMP mixed solution were first mixed in a sealed container at a mass ratio of 1 : 1 using magnetic stirring for 10 min. Then, the corresponding mass ratio of activated carbon composite material was added, and the mixture was magnetically stirred for 4 h to obtain the electrode slurry. 25 µL of the slurry was pipetted and evenly coated onto a (1 × 1) cm2 area below the nickel foam sheet. After coating, the electrode was placed in a vacuum oven and dried overnight at 60 °C.

2.4. Electrochemical testing

Electrochemical tests were performed using an electrochemical workstation (CHI760E, Shanghai Chenhua Instrument Co., Ltd) in both three-electrode and two-electrode systems. In the three-electrode system, the Hg/HgO electrode was used as the reference electrode, the platinum sheet electrode as the counter electrode, and the nickel foam electrode supported on activated carbon composite material as the working electrode. The specific capacitance of each electrode material in the three-electrode system were calculated using formula (1):

2.4. 1

In the two-electrode system, both electrodes are nickel foam electrodes loaded with activated carbon composite material with the same modification gradient as working electrodes. The specific capacitance of the device was calculated using formula (2):

2.4. 2

where C (F g−1) is the specific capacitance; I (A) is the current; Δt (s) is the discharge time; m (g) is the mass of the active material; and ΔV (v) is the voltage window.

2.5. Material characterization

The microstructure and elemental distribution of the materials were characterized using scanning electron microscopy (SEM, ZEISS Sigma 360, Germany) and its accompanying X-ray energy dispersive spectroscopy (EDS). Phase and crystal structure analysis was performed using X-ray diffraction (XRD, Rigaku Corporation), with a test range of 10–60° and a scan rate of 5° min−1. The specific surface area and pore size distribution of the samples were determined using a fully automated specific surface area and porosity analyzer (BET, Micromeritics ASAP 2460, USA). Characteristic functional groups and chemical bonds were identified using Fourier transform infrared spectroscopy (FT-IR, Thermo Fisher Nicolet iS50) in the wavenumber range of 4000–500 cm−1. The chemical state and relative abundance of elements on the material surface were determined using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, USA). Using a contact angle meter (SDC-350KS, Kunshan Shengding Industrial Intelligent Technology Co., Ltd), the dynamic contact angle data of the material was measured.

The electrochemical performance of the prepared electrodes was assessed using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests, all conducted on an electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, CHI 760E). Furthermore, to evaluate the aging behavior and cycle life of the electrode materials, long-cycle stability testing was performed using the Landian battery testing system (Wuhan Landian Electronics Co., Ltd, CT 3004A).

3. Results and discussion

3.1. Electrochemical performance

The electrochemical performance of the prepared samples was tested using 1 M KOH as the electrolyte.

First, GCD tests were conducted at a current density of 1 A g−1. Fig. 1a and b show the average specific capacitance obtained from multiple repeated GCD tests for [Emim][BF4]-AC and [Bmim][BF4]-AC, respectively. Fig. 1c and d present the results of GCD tests for samples with different doping concentrations. From Table 1., it can be seen that the specific capacitance of NEAT AC is 87.14 ± 4.14 F g−1. Using NEAT AC as the reference, this study further set up a series of loading radio for in-depth investigation. As shown in Table 1 and Fig. 1, a comparison was made of different modification concentrations in the three-electrode system, the optimal gradient for [Emim][BF4]-AC is 0.25%, with a specific capacitance of 92.99 ± 5.83 F g−1; for [Bmim][BF4]-AC, the optimal gradient is 0.5%, with a specific capacitance of 94.39 ± 5.45 F g−1.

Fig. 1. Galvanostatic charge–discharge performance, cyclic voltammetry and overall electrochemical performance under three-electrode system. (a) GCD error plots for [Emim][BF4]-AC at different doping ratios, (b) GCD error plots for [Bmim][BF4]-AC at different doping ratios, (c) GCD curves of [Emim][BF4]-AC at different doping ratios, (d) GCD curves of [Bmim][BF4]-AC at different doping ratios, (e) comparison of CV curves for all samples at a fixed scan rate, (f) comparison of GCD curves for all samples at a fixed current density.

Fig. 1

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Table 1. Electrochemical data of ILs-AC in three-electrode system (1 A g−1).

Sample Specific capacitance (F g−1)
Neat AC 87.14 ± 4.14
 
[Emim][BF 4 ]-AC
0.25% 92.99 ± 5.83
0.5% 84.54 ± 3.92
1% 85.49 ± 4.43
1.25% 84.77 ± 4.23
 
[Bmim][BF 4 ]-AC
0.25% 86.92 ± 2.78
0.5% 94.39 ± 5.45
1% 84.56 ± 4.56
1.25% 87.59 ± 3.44
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Compared with NEAT AC, the specific capacitance of [Emim][BF4]-AC and [Bmim][BF4]-AC is improved, mainly due to the modification of the AC electrode surface by ILs. After modification with ILs, ILs are adsorbed on the AC surface, enhancing interfacial wettability and facilitating the movement of electrolyte ions. In addition, IL modification reduces charge transfer resistance, allowing ions to make fuller use of the micropores, resulting in an increase in specific capacitance.

It is worth noting that the specific capacitance of ILs-AC exhibits fluctuations. This is due to the low loading ratios of ILs during the preparation process, which leads to slight variations during the impregnation process. To minimize the impact of this error, this study prepared and tested the samples multiple times, thereby enhancing the stability of the test results. In addition, the loading ratios of ILs also affect the specific capacitance results. At low doping ratios, ILs fully exert the role of surface modifiers, improving the hydrophilicity of the activated carbon surface and allowing electrolyte ions to make full use of the deep micropores, thereby enhancing the specific capacitance. As the IL content increases, ILs aggregate outside the micropores, making it difficult for electrolyte ions to diffuse into the micropores, which leads to a decrease in specific capacitance. When the ILs are excessive, the newly formed IL interface can facilitate ion transport. Therefore, the specific capacitance of the samples exhibits fluctuations. As [Bmim]+ has a longer butyl chain, its adsorption capacity and hydrophilicity are stronger than those of [Emim]+, resulting in differences in their doping ratios.

ILs themselves can exhibit interfacial capacitance by forming a double layer with the electrode interface. Current research has demonstrated that [Emim][BF4] has a differential capacitance of 9.9 µF cm−2.34,35 In the trace modification system of this study, the differential capacitance of ILs themselves is almost negligible. The enhanced electrochemical performance of ILs-AC is mainly achieved through the surface modification effect of ILs on AC.

To analyze the capacitive properties of the composite materials, CV tests were conducted on the three materials at a scan rate of 10 mV s−1, as shown in Fig. 1e. The results indicate that the enclosed area of the CV curve for 0.5% [Bmim][BF4]-AC was the largest, followed by 0.25% [Emim][BF4]-AC, while NEAT AC exhibits the smallest integrated area, which is consistent with the GCD test results.

Previous studies have indicated that carbon material electrodes based on imidazolium ILs primarily exhibit electric double-layer capacitance behavior. Zhang et al. used porous carbon electrodes and [Emim][BF4] electrolyte, and the test results showed a quasi-rectangular CV profile, indicating that no redox reaction occurred.36 Kong et al. tested porous carbon electrodes based on [Bmim][BF4]/AN electrolyte, showing rectangular curves, exhibiting standard electric double-layer capacitance behavior.37 Therefore, the CV curves of the composite materials prepared in this study showed no redox peaks, indicating that the introduction of ILs did not induce redox reactions.

To evaluate the stability of the composite materials under different current densities, we tested NEAT AC, [Emim][BF4]-AC, and [Bmim][BF4]-AC at current densities of 0.5, 1, 2, 3, and 5 A g−1. Fig. 2a shows the average specific capacitance obtained from repeated GCD measurements at different current densities, and Fig. 2b–d presents the GCD curves of the three materials at different current densities. The GCD test results are shown in Table 2. Increasing the alkyl chain length can enhance the adsorption affinity of imidazolium cations toward activated carbon, thereby promoting its interaction with the carbon material surface.38 The smaller-sized [Emim]+ has higher micropore accessibility, which facilitates rapid ion transport; whereas the longer alkyl chain of [Bmim]+ strengthens interfacial interactions with the carbon surface, contributing to the formation of a more stable interfacial ion structure. At lower current densities, the electrode has sufficient time to be infiltrated by the electrolyte and complete ion transport. Due to the smaller ion size and lower diffusion resistance of [Emim]+, the interior micropores of AC can be fully utilized, resulting in higher specific capacitance. As the current density increases, the ion diffusion time within the electrode shortens. Since [Bmim]+ tends to adsorb more on the AC surface, it improves the wettability of the electrode surface by the electrolyte and facilitates rapid charge storage in the outer surface regions.

Fig. 2. Rate performance in three-electrode system. (a) Specific capacitance of all samples at different current densities, (b) GCD curves of neat AC at different current densities, (c) GCD curves of 0.25%[Emim][BF4]-AC at different current densities, (d) GCD curves of 0.5%[Bmim][BF4]-AC at different current densities, (e) CV curves of neat AC at different scan rates, (f) CV curves of 0.25%[Emim][BF4]-AC at different scan rates, (g) CV curves of 0.5%[Bmim][BF4]-AC at different scan rates.

Fig. 2

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Table 2. Rate performance of ILs-AC in three-electrode system.

0.5 A g−1 1 A g−1 2 A g−1 3 A g−1 5 A g−1
NEAT AC 255.85 87.14 38.02 21.73 6.43
0.25% [Emim][BF4]-AC 273.34 92.99 39.02 24.33 11.69
0.5% [Bmim][BF4]-AC 265 94.39 38.27 21.78 6.54
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Additionally, as shown in Fig. 2e–g, CV tests were conducted at scan rates of 2, 5, 10, 20, 50, and 100 mV s−1, and all three samples exhibited quasi-rectangular shapes under different scan rates. This indicates the possible presence of both electric double-layer capacitance and pseudocapacitance in the samples, but no obvious redox peaks were observed, suggesting that the charge storage behavior of the electrodes is primarily adsorption affinity of imidazolium cations toward activated carbon, thereby promoting its interaction with the carbon material surface.39 The smaller-sized [Emim]+ has higher micropore accessibility, which facilitates rapid ion transport; whereas the longer alkyl chain of [Bmim]+ strengthens interfacial interactions with the carbon surface, contributing to the formation of a more stable interfacial ion structure. At lower current densities, the electrode has sufficient time to be infiltrated by the electrolyte and complete ion transport. Due to the smaller ion size and lower diffusion resistance of [Emim]+, the interior micropores of AC can be fully utilized, resulting in higher specific capacitance. As the current density increases, the ion diffusion time within the electrode shortens. Since [Bmim]+ tends to adsorb more on the AC surface, it improves the wettability of the electrode surface by the electrolyte and facilitates rapid charge storage in the outer surface regions. Derived from electric double-layer capacitance. With increasing scan rate, at a scan rate of 100 mV s−1, the curve of [Emim][BF4]-AC remained smooth, while the CV curve of [Bmim][BF4]-AC exhibited slight distortion at high scan rates, indicating that due to the steric hindrance of the long chain, ion diffusion in the pores is slightly more restricted in [Bmim][BF4]-AC than in [Emim][BF4]-AC.

Electrochemical impedance spectroscopy (EIS) was performed to investigate the charge transfer and ion diffusion behavior of the materials and to fit the equivalent circuit diagram (Fig. 3a) to analyze the related characteristics of charge transfer and ion diffusion dynamics. For the Nyquist diagram, in the high-frequency region, the semicircle diameter is closely related to the charge transfer resistance (Rct). The smaller the semicircle diameter, the less resistance the electron transfer process encountered at the interface between the electrode and the electrolyte, and the smoother the electron conduction. In the low-frequency region, the larger the slope, the higher the ion migration rate, that is, the diffusion of ions in the material and between the electrode and the electrolyte is more efficient.40 As shown in Table 3 and Fig. 3, compared with the original material, the AC samples modified with the two ILs showed a significant reduction in Rct at low doping concentrations, and the surface charge transport was enhanced. This improvement can be attributed to the improved wettability of the AC surface after IL modification. It is worth noting that the Rct value increases with the increase of loading, indicating that excessive ILs will further block the pore structure of AC and hinder the transport of ions in the porous structure.

Fig. 3. Electrochemical impedance spectroscopy. (a) Equivalent circuit diagram, (b) Nyquist plots of [Emim][BF4]-AC with different doping ratios, (c) Nyquist plots of [Bmim][BF4]-AC with different doping ratios, (d) comparative EIS curves of NEAT AC, 0.25%[Emim][BF4]-AC, and 0.5%[Bmim][BF4]-AC, (e) relationship diagram of impedance, capacitance, and gradient characteristics of [Emim][BF4]-AC, (f) relationship diagram of impedance, capacitance, and gradient characteristics of [Emim][BF4]-AC, (g) relationship diagram of impedance, capacitance, and gradient characteristics of [Bmim][BF4]-AC, (h) relationship diagram of impedance, capacitance, and gradient characteristics of [Bmim][BF4]-AC.

Fig. 3

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Table 3. Electrochemical impedance spectroscopy fitting parameters for different samples.

Sample R s (Ω) Error (%) R ct (Ω) Error (%)
NEAT AC 1.598 2.256 14.66 4.096
 
[Emim][BF 4 ]-AC
0.25% 1.435 1.818 6.899 2.947
0.5% 1.475 2.299 14.16 5.371
1% 1.613 2.067 13.03 4.61
1.25% 1.397 2.113 9.379 3.972
 
[Bmim][BF 4 ]-AC
0.25% 1.564 2.219 10.13 5.723
0.5% 1.439 1.961 7.863 3.452
1% 1.47 2.008 9.613 5.566
1.25% 1.59 1.968 10.79 4.55
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We believe that the trend of Rct corresponds to the three stages of ILs in the micropores of AC. When the doping ratio is low, the modification effect of ILs reduces the interfacial tension, allowing the electrolyte to enter the microporous regions of AC; as the amount of ILs increases, the excess ILs aggregate outside the micropores, increasing the difficulty of ion transport; When there is an excessive amount of ILs, they accumulate outside the micropores to form new interfaces, accelerating the ion transport process.

3.2. Electrochemical performance of the device

To further test the materials, the three materials were assembled into a symmetrical supercapacitor, and a systematic evaluation was conducted after electrochemical testing.

Firstly, GCD tests were performed on the assembled SCs. As shown in Fig. 1, the capacitances are arranged from largest to smallest as 0.5% [Bmim][BF4]-AC, 0.25% [Emim][BF4]-AC, and NEAT AC. In the GCD tests of assembled SCs, 0.25% [Emim][BF4]-AC showed higher specific capacitance than 0.5% [Bmim][BF4]-AC. This can be attributed to the smaller cation size of [Emim]+ and its lower diffusion resistance, which results in a more efficient ion transport process under symmetric electrode testing. Notably, in Fig. 4, the IR drop of NEAT AC is approximately 0.25 V, whereas the IR drops for 0.25% [Emim][BF4]-AC and 0.5% [Bmim][BF4]-AC are approximately 0.12 V and 0.15 V, respectively. Further comparison reveals that the IR drop of 0.25% [Emim][BF4]-AC is smaller than that of 0.5% [Bmim][BF4]-AC, which is related to the cation chain length. In conclusion, as the IR drop reflects the magnitude of the equivalent series resistance and charge transfer internal resistance, this indicates that the hydrophilicity of AC has been effectively enhanced after IL modification, and the interfacial wettability between the material and the electrolyte has also improved. As shown in Fig. 4e–g, the CV test results continue to maintain a good rectangular shape without significant distortion.

Fig. 4. Supercapacitor device performance. GCD curves: (a) NEAT AC, (b) 0.25%[Emim][BF4]-AC, (c) 0.5%[Bmim][BF4]-AC; CV curves: (d) NEAT AC, (e) 0.25%[Emim][BF4]-AC, (f) 0.5%[Bmim][BF4]-AC; (g) cycling stability over 10 000 cycles at 1 A g−1.

Fig. 4

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Furthermore, long-cycle testing was conducted on the assembled SCs. As shown in Fig. 4g, all three materials underwent 10 000 charge–discharge cycles in a two-electrode system. The NEAT AC sample retained 91.73% of its capacitance, the 0.25%[Emim][BF4]-AC retained 96.64%, and the 0.5%[Bmim][BF4]-AC retained 92.83%, all exhibiting excellent stability. This indicates that the modification of AC by ILs remains stable after multiple cycles. To more comprehensively and accurately evaluate its performance, we summarized and compared existing research results, revealing certain performance advantages of the modified materials, as shown in Table 4.

Table 4. Comparison of existing research findings.

Electrode materials Electrolyte Current density Number of times Retention rate References
AC//Zn 2M ZnSO4 4 A g−1 10 000 95.1% 41
AC/CNT/RO 1M LiClO4 0.2 A g−1 1000 75% 42
NiCo2P@CSA//AC 6M KOH 0.3 A g−1 10 000 75% 43
NEAT AC 1M KOH 1 A g−1 10 000 91.73% This work
0.25%[Emim][BF4]-AC 1M KOH 1 A g−1 10 000 96.64% This work
0.5%[Bmim][BF4]-AC 1M KOH 1 A g−1 10 000 92.83% This work
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3.3. Structural characterization

Based on the electrochemical results, it was hypothesized that the addition of ionic liquids could improve the wettability between the electrode and the electrolyte, and that ILs would partially block the AC pore structure. To verify this hypothesis, we conducted in-depth characterization analysis on the optimal samples (0.25% [Emim][BF4]-AC and 0.5% [Bmim][BF4]-AC) and the original samples under the three-electrode system test.

The morphologies of NEAT AC, 0.25% [Emim][BF4]-AC, and 0.5% [Bmim][BF4]-AC were characterized using scanning electron microscopy (SEM), and the results are shown in Fig. 5. The surface of Neat AC is generally smooth, maintaining a relatively intact surface structure. After modification, the 0.25% [Emim][BF4]-AC sample exhibits increased surface roughness, while the 0.5% [Bmim][BF4]-AC displays a more pronounced layered stacking structure. This phenomenon indicates that the modification treatment led to the formation of a richer layered structure and higher surface roughness, which is beneficial for ensuring adequate contact between the electrolyte and the electrode material.

Fig. 5. Morphology, crystal structure, pore structure, and chemical bond analyses. (a) SEM image of NEAT AC, (b) SEM image of 0.25%[Emim][BF4]-AC, (c) SEM image of 0.5%[Bmim][BF4]-AC, (d) XRD patterns of all samples, (e) BET adsorption–desorption isotherm, (f) DFT aperture distribution map, (g) FT-IR spectra in the range 500–1300 cm−1, (h) FT-IR spectra in the range 500–800 cm−1.

Fig. 5

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To further analyze the structure of the samples, X-ray diffraction tests were performed, as shown in Fig. 5d. Both the NEAT AC and the modified samples maintained a typical amorphous carbon structure, exhibiting (002) and (100) planes at 22° and 44°.44,45 This indicates that the overall structure of AC remains unchanged after modification with the two ionic liquids. Notably, the 002 peak is slightly shifted to higher angles from the (002) crystal plane. According to the Bragg equation, the diffraction angle θ is inversely proportional to the interplanar spacing d. The increase in θ leads to a decrease in d, indicating that ILs modification results in a smaller interlayer spacing (d002) in AC. This may be due to π–π interactions between ILs and AC.

Further BET testing was conducted on the materials. As shown in Fig. 5e, all samples exhibited type I adsorption isotherms, displaying standard AC characteristics, indicating that the material porosity is predominantly microporous. As shown in Fig. 5f, IL modification affected the AC porosity. As shown in Table 5, the NEAT AC samples possessed a high BET specific surface area (SBET) of 1839.15 m2 g−1 and a micropore specific surface area (Smicro) of 1711.58 m2 g−1. After the introduction of ionic liquids, both SBET and Smicro of the samples showed an overall decreasing trend. Specifically, for 0.25% [Emim][BF4]-AC and 0.5% [Bmim][BF4]-AC, their SBET decreased to 1742.69 m2 g−1 and 1756.35 m2 g−1, respectively, accompanied by a corresponding decrease in micropore volume (Vmicro). This trend strongly demonstrates that the ionic liquid cations ([Emim]+ and [Bmim]+) have successfully occupied or partially blocked the microporous channels of the carbon substrate. It is noteworthy that although the loading of the [Bmim][BF4] sample was higher than that of the [Emim][BF4] sample, the decreasing trend was different. This is because the smaller [Emim]+ is more likely to enter the microporous structure, while the larger [Bmim]+ tends to act on the external surface of AC, thus having a smaller effect on AC micropores and exhibiting a slightly higher SBET.

Table 5. BET test parameters for different samples.

Sample NEAT AC 0.25% [Emim][BF4]-AC 0.5% [Bmim][BF4]-AC
S BET (m2 g−1) 1839.15 1742.69 1756.35
S micro (m2 g−1) 1711.58 1591.03 1590.18
S ext (m2 g−1) 127.58 151.67 166.17
V total (cm3 g−1) 0.78 0.75 0.77
V micro (cm3 g−1) 0.68 0.63 0.64
D BET (nm) 1.74 1.75 1.78
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The performance of supercapacitors largely depends on the matching relationship between ion size and electrode pore size.46 The ion sizes of [Emim]+, [Bmim]+, and [BF4]− are 0.76, 0.9, and 0.33 nm, respectively. In this study, the DBET of NEAT AC is 1.74 nm, providing an ideal geometric space for ions of ILs to enter the pores. After modification with ILs, the DBET of the samples increases. This is because some micropores of AC are blocked by imidazolium cations entering the pores, while larger micropore channels are retained. The ILs modified on the surface of AC, together with the retained larger pores, facilitate ion transport.

To verify whether ILs successfully modified AC, the samples were characterized by FT-IR. As shown in Fig. 5g, the NEAT AC sample showed a straight line without obvious functional group peaks. Analyzing the ILs and their modified samples, compared with NEAT AC, the modified sample showed a [BF4]−related signal near 525 cm−1, but due to the low ionic liquid loading, the characteristic peak intensity in the modified sample was relatively weak. As shown in Fig. 5h, further narrowing the range, it can be found that the [Emim][BF4] peak is wider than that of [Bmim][BF4], and peaks of imidazole ring due to out-of-plane bending vibration or C–H out-of-plane deformation vibration appear near 650 cm−1.47 The samples were characterized by XPS to analyze their chemical states and elemental composition. First, as shown in Fig. 6a, both NEAT AC and the modified sample exhibited C 1s and O 1s peaks in the full spectrum. After ILs modification, the sample showed an N 1s peak; however, due to the low doping concentration, neither F 1s nor B 1s peaks were observed.

Fig. 6. XPS spectra (a–n) of the survey scan (a), 800–600 eV enlarged image (b), NEAT AC (c and d), 0.25%[Emim][BF4]-AC (e–g), 0.5%[Bmim][BF4]-AC (h–j), 1.25%[Emim][BF4]-AC (k and l), 1.25%[Bmim][BF4]-AC (m and n).

Fig. 6

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The chemical bonding information was further analyzed by high-resolution spectra. In order to compare the F peak positions, we magnified the Survey scan. XPS The enlarged view at 800–600 eV is shown in Fig. 6b. In the O 1s spectrum of NEAT AC (Fig. 6c), it can be fitted as C–O (∼531.68 eV) and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼533.08 eV), while in the C 1s spectrum (Fig. 6d), it can be fitted as C–C (∼284.78 eV), C–O (∼285.88 eV) and O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼289.28 eV), indicating that the original material is rich in oxygen-containing functional groups.48 After the addition of ILs, the O 1s spectrum (Fig. 6e) of the 0.25% [Emim][BF4]-AC sample can be fitted as C–O (∼531.78 eV) and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼533.18 eV), and the O 1s peak shifted. This phenomenon may be because the effect of ILs on AC is more on the surface. After the addition of ILs, the interlayer spacing of AC changes, affecting the oxygen-containing can be fitted as C2N–H/C2N (∼399.17 eV) and C–N3 (∼401.19 eV). In the 0.5% [Bmim][BF4]-AC spectrum, the O 1 s spectrum (Fig. 6h) can be fitted to C–O (∼531.48 eV) functional groups. The C 1s spectrum (Fig. 6f) can be fitted as C–C (∼284.88 eV), C–N (∼285.58 eV), C–O (∼286.78 eV), C Created by potrace 1.16, written by Peter Selinger 2001-2019 N/C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼288.48 eV) and O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼289.78 eV); the N 1s spectrum (Fig. 6g) and C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼532.98 eV), the C 1s spectrum (Fig. 6i) can be fitted to C–C (∼284.78 eV), C–N (∼286.01 eV), C–O (∼287.6 eV), C Created by potrace 1.16, written by Peter Selinger 2001-2019 N/C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼289.26 eV), and O–C Created by potrace 1.16, written by Peter Selinger 2001-2019 O (∼291.19 eV), while the N 1s spectrum (Fig. 6j) can be fitted to C2N–H/C2N (∼398.98 eV) and C–N3 (∼400.88 eV). It should be noted that in the optimal AC-IL sample, due to the low loading ratio of ILs, F 1s and B 1s cannot be observed in the survey spectrum. To verify the successful loading of ILs onto AC, 1.25% [Emim][BF4]-AC and 1.25% [Bmim][BF4]-AC were tested. Unlike the optimal AC-IL sample, distinct F 1s signals can be observed in the survey spectra of these high-concentration samples. Specifically, the high-resolution spectra exhibit obvious characteristic peaks of B 1s (∼191.9 eV) and F 1s (∼684.9 eV) for 1.25% [Emim][BF4]-AC (Fig. 6k and l), as well as B 1s (∼192.3 eV) and F 1s (∼685.1 eV) for 1.25% [Bmim][BF4]-AC (Fig. 6m and n). This directly confirms the successful incorporation of both ionic liquids onto the AC. The new signals together confirm the successful loading of imidazole cations into AC.49–54

From the analysis of the test results, ILs were successfully loaded onto AC, and the shift of oxygen-containing functional groups may be caused by the surface modification of AC by ILs. This phenomenon is consistent with the XRD and electrochemical test results. This successfully demonstrates that ILs and AC primarily interact through physical adsorption via π–π stacking interactions, van der Waals forces, and other interactions.55–57

After modification with ILs, their effects are not only reflected in the regulation of surface wettability but also involve the interactions at the electrode/electrolyte interface as well as the improvement of ion transport processes.58 Therefore, changes in contact angle need to be analyzed in conjunction with the characteristics of the porous structure and the dynamic spreading process of the droplet. To more intuitively assess the impact of ILs on the wettability of AC, dynamic contact angle tests were conducted on NEAT AC and the modified samples. The results are shown in Fig. 7 and Table 6, recording the dynamic process from the initial contact of the droplet with the material surface, through the beginning of spreading, to the completion of spreading. The data show that when the droplet contacts the material surface, the contact angles of NEAT AC, 0.25% [Emim][BF4]-AC, and 0.5% [Bmim][BF4]-AC samples are 44.3°, 45.73°, and 32°, respectively. As the droplet spreads, the contact angle of the NEAT AC sample eventually stabilizes at 14.33°, while the contact angles of the IL-modified samples stabilize at 7.8° and 6.65°. This phenomenon indicates that the wettability of AC has been successfully improved after IL modification.

Fig. 7. Dynamic process of contact angle testing. NEAT AC (a–c), 0.25%[Emim][BF4]-AC (d–f), 0.5%[Bmim][BF4]-AC (g–i).

Fig. 7

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Table 6. Time vs. contact angle for different samples.

Sample Time (s) Contact angle (°)
NEAT AC 0 44.30
3 16.62
6 14.33
0.25% [Emim][BF4]-AC 0 45.73
3 9.10
6 7.80
0.5% [Bmim][BF4]-AC 0 32.00
3 9.29
6 6.65
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It is noteworthy that the initial contact angle of the 0.25% [Emim][BF4]-AC sample (45.73°) is slightly higher than that of NEAT AC, which does not imply that IL modification has failed to improve the surface wettability of AC. The wetting behavior of porous or rough surfaces is influenced by surface chemical properties, structural characteristics, and the dynamic migration of the liquid within the pores. Therefore, the final stable contact angle better reflects the liquid's ability to penetrate the porous electrode rather than merely changes in surface hydrophilicity.59 Although the initial contact angle of the 0.25% [Emim][BF4]-AC sample did not significantly decrease, the stable contact angle was improved, indicating that IL modification facilitates the spreading and penetration of the liquid on the AC surface.

0.5% [Bmim][BF4]-AC exhibits lower initial and stable contact angles, which may be related to changes in interfacial properties caused by its longer alkyl chain structure. The increase in the side chain length of imidazolium ionic liquid cations can alter their film-forming behavior and spreading ability on solid surfaces. Longer alkyl chains are beneficial for enhancing the interfacial adhesion and two-dimensional spreading behavior of ionic liquids on certain solid surfaces, thereby improving apparent wettability.60

4. Conclusion

In this study, AC was successfully modified with ILs using a simple impregnation method. By adjusting the IL cations ([Emim]+ and [Bmim]+), a systematic comparison was made to verify their surface modification capability and effects on AC. Electrochemical test results indicate that both ILs improve the electrochemical performance of AC. The optimal gradient for [Emim][BF4]-AC is 0.25%, with a specific capacitance of 92.99 ± 5.83 F g−1 (an approximate 6.7% increase compared to NEAT AC); the optimal gradient for [Bmim][BF4]-AC is 0.5%, with a specific capacitance of 94.39 ± 5.45 F g−1 (an approximate 8.3% increase compared to NEAT AC), demonstrating excellent rate capability. During long-term cycling tests (10 000 charge–discharge cycles), relative to NEAT AC (capacitance retention of 91.73%), the capacitance retention of 0.25% [Emim][BF4]-AC was 96.64%, and that of 0.5% [Bmim][BF4]-AC was 92.83%, fully demonstrating that ILs enhance the cycling stability of AC. Finally, through characterization methods such as SEM, XRD, and XPS, we found that the effect of ILs on AC is based on a surface modification mechanism. Although ILs modification on the AC surface results in pore blockage and a decrease in specific surface area, it improves the hydrophilic of AC, reduces the contact angle, and successfully improves the interfacial wettability between AC and electrolyte.

Author contributions

All authors contributed equally to the write-up and revision of this work.

Conflicts of interest

There are no conflicts to declare.

Acknowledgments

This work was supported by the Program of Study Abroad for Young Scholar sponsored by China Scholarship Council (202509710014); the Huzhou Natural Science Foundation (2022YZ39); A Project Supported by Scientific Research Fund of Zhejiang Provincial Education Department (Y202248530).

Data availability

The data is available within the paper.

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