Effect of removing potassium ions from activated carbon on its electrochemical performance for supercapacitors

Abstract

To produce pure activated carbon (AC) with a low potassium ion (K⁺) content for supercapacitors, coconut shell-derived AC activated by KOH was treated using a novel oxidation-ultrasound process based on hydrochloric acid (HCl) washing. The effects of different treatment conditions on the K⁺ content of AC were investigated, and the physical properties of the as-prepared samples were characterized by scanning electron microscopy (SEM) and N₂ adsorption–desorption isotherms. The electrochemical performance of AC as a supercapacitor electrode was characterized by cyclic voltammetry (CV), galvanostatic charge–discharge (GC) and electrochemical impedance spectroscopy (EIS). The results showed that the AC that was washed with 1.0 wt% HCl solution for 120 min and subsequently treated with 0.6 wt% H₂O₂ solution at 60 °C in an ultrasonic oscillator for 8 h possessed a K⁺ content of 46 mg kg⁻¹, which was much lower than the value of 417 mg kg⁻¹ found for the AC that did not undergo oxidation-ultrasound treatment. Moreover, the AC subjected to oxidation-ultrasound treatment possessed a large specific surface area and pore volume of 3460 m² g⁻¹ and 1.869 cm³ g⁻¹, respectively. In terms of the electrochemical performance, this exhibited a high specific capacitance of 306 F g⁻¹ at a current density of 1 A g⁻¹ in 1 M H₂SO₄ electrolyte and remained at 294 F g⁻¹ with a capacitance retention of 96% after 3000 cycles, indicating excellent stability and capacitive behavior of the AC electrode with respect to its use as a supercapacitor.

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1. Introduction

Supercapacitors, as new energy storage devices between batteries and conventional capacitors, have gained considerable attention in recent years due to their high power density and long cycle life.^1–4 They have been considered to be the most promising energy storage devices for applications such as hybrid electrical vehicles, brake energy recovery systems and digital telecommunication systems.^5,6 The electrode material is the most important factor affecting the performance of a supercapacitor.⁷ In general, a large surface area, an optimal pore size and a high-purity texture are necessary for an electrode material to achieve a high specific capacitance.

Recently, activated carbon (AC) has become the most widely used electrode material in supercapacitors, owing to its large surface area, good conductivity, excellent chemical stability and low cost.^8–11 However, due to the process limitations of KOH activation, the AC products show a high K⁺ content, generally above 3000 mg kg⁻¹, which can block some already-formed AC pores, lead to self-discharge of the AC electrode, and seriously influence the specific capacitance and cycle life of the supercapacitor. Therefore, it is necessary to remove K⁺ from AC, aiming to greatly improve the electrochemical performance of the supercapacitor.

K⁺ in AC usually spreads in the pores and is difficult to be removed effectively by HCl washing. However, ultrasound oscillation can contribute to the dispersion of K⁺ into the solution, while the oxidation process can weaken the force between K⁺ and the AC pore walls, allowing the AC samples to be purified. Herein, we report on a novel oxidation-ultrasound process for further reducing the K⁺ content of AC. The effect of the K⁺ content on the properties of AC has been systematically studied in terms of the physical and electrochemical performances.

2. Experimental

2.1 Preparation of AC sample

The AC in this study was prepared from coconut shell by KOH activation as follows. The coconut shell was first carbonized at 600 °C for 1 h and then crushed to particles sized of about 1 mm. Then, the coconut shell carbide was mixed with 50 wt% KOH solution with a KOH/C weight ratio of 4 : 1 in a stainless steel reactor. Furthermore, the mixture was moved into a muffle furnace for activation. During the activation process, the mixture was first pretreated at 350 °C for 2 h under air atmosphere, then heated up to 800 °C at a heating rate of 10 °C min⁻¹ and maintained at 800 °C for 1 h under sealed conditions. After cooling to room temperature, the resultant samples were subsequently washed with distilled water until a pH of approximately 6 was obtained. Finally, the prepared AC samples were dried at 120 °C for 10 h under vacuum.

2.2 Removal of K⁺ from the AC sample

The removal of K⁺ included two steps. In the first step, 2.0 g of the AC prepared above was mixed with 100 mL different weight ratios (0.3, 0.5, 0.7, 1.0, and 1.2 wt%) of HCl solution, and then stirred in an electro-thermostatic water cabinet at 80 °C for 30–150 min. In the second step, the purification of HCl-washed AC was continued by the oxidation-ultrasound process, in which 2.0 g HCl-washed AC was impregnated with 100 mL different weight ratios (0, 0.2, 0.4, 0.6, 0.8 and 1.0 wt%) of H₂O₂ solution in an ultrasonic oscillator at 10–80 °C for 2–12 h. The treated samples were then dried at 120 °C for 10 h under vacuum. Hereafter, we refer to the AC samples as AC-W, AC-W-H, and AC-W-H-U, referring to water, HCl and oxidation-ultrasound treatment of AC, respectively.

2.3 Sample characterization

The K⁺ content of AC was measured by inductively coupled plasma mass spectrometry (ICP-MS). The morphologies of the samples were investigated by scanning electron microscopy (SEM, S-3400, Hitachi). In addition, the porous properties of AC were characterized by N₂ adsorption–desorption isotherms at 77 K using a Micromeritics ASAP2020 apparatus. From these, the specific surface area was determined by the BET equation, the total pore volume was obtained from the N₂ adsorption at P/P₀ = 0.99, and the pore size distribution was analyzed with adsorption data based on the density functional theory (DFT).

2.4 Electrochemical characterization

Electrochemical performance measurements were carried out in a three-electrode system with 1 M H₂SO₄ as the electrolyte on an electrochemical workstation (VMP3B-2 × 2, Bio-Logic). The working electrode was prepared by mixing AC powders, acetylene black and 5 wt% PTFE with a weight ratio of 8 : 1 : 1, and the mixture was then casted onto a Ni-foam current collector. A platinum foil and Ag/AgCl were used as the counter and reference electrodes, respectively. The capacitive behavior of the AC electrode was characterized by cyclic voltammetry (CV), galvanostatic charge–discharge (GC) and electrochemical impedance spectroscopy (EIS) measurements. CV was performed from −0.2 to 0.8 V at various scan rates, GC curve was measured from −0.2 to 0.8 V at different current densities, and EIS was recorded at frequencies from 100 kHz to 10 mHz. All electrochemical measurements were carried out at room temperature.

3. Results and discussion

3.1 Factors influencing the removal of K⁺

3.1.1 Effect of HCl treatment on the K⁺ content of AC. The AC-W samples are first treated with HCl solution to remove K⁺. In this study, the effects of the mass fraction and the treatment time with HCl solution are investigated. Fig. 1(a) shows the relationship between the K⁺ content and the mass fraction of HCl solution for a fixed treatment time of 90 min. As seen from the figure, the K⁺ content displays an obvious decrease with an increasing mass fraction of HCl solution in the range of 0–1.0 wt%, and thereafter it presents an increasing trend. This can be attributed to the fact that an electric double layer is formed on the surface of K⁺ due to the excessive acid radical ions, resulting in the K⁺ becoming difficult to be removed from the AC. Therefore, 1.0 wt% is accepted as the optimal mass fraction of an HCl solution.

Fig. 1 Effect of mass fraction and treatment time with HCl solution on the K⁺ content of AC.

Furthermore, the relationship between the K⁺ content and the treatment time with HCl solution at a mass fraction of 1.0 wt% is shown in Fig. 1(b). It is observed that the K⁺ content significantly decreases from 1242 to 417 mg kg⁻¹ as the treatment time increases from 30 to 120 min, and thereafter it does not change significantly. Consequently, 1.0 wt% HCl solution and a treatment time of 120 min are chosen as the optimum treatment conditions for subsequent experiments, and the corresponding AC sample is marked as AC-W-H_{1%/120 min}.

3.1.2 Effect of oxidation-ultrasound process on the K⁺ content of AC. AC-W-H_{1%/120 min} is then subjected to an oxidation-ultrasound process to remove K⁺. The influence of the mass fraction of H₂O₂ solution, ultrasound temperature and ultrasound time on the K⁺ content of AC is studied. The results obtained are shown in Table 1 and Fig. 2. The effect of the mass fraction of H₂O₂ solution is studied in the range of 0–1.0 wt% at an ultrasound temperature of 60 °C and an ultrasound time of 6 h. It is observed from the data shown in Table 1 that the K⁺ content of AC without H₂O₂ treatment is 256 mg kg⁻¹, substantially lower than the value of 417 mg kg⁻¹ recorded for AC-W-H_{1%/120 min}. This is obviously due to the ultrasound effect; the ultrasonic waves can disperse K⁺, along with some uncarbonized substances inside the AC pores, into the solution and accelerate their mass transfer between the solid and liquid phases. In addition, as shown in Fig. 2(a), as the mass fraction of H₂O₂ solution increases from 0 to 0.6 wt%, the K⁺ content of AC decreases markedly from 256 to 61 mg kg⁻¹ and thereafter it remains substantially unchanged. Therefore, it can be stated that H₂O₂ treatment is effective in producing AC with a low K⁺ content. On the one hand, as an oxidizing agent, H₂O₂ solution can reduce the force between K⁺ and the AC pore walls, facilitating its transfer to the solution. On the other hand, K⁺ in AC can form chemical bonds with some organic groups, which can be destroyed by the presence of H₂O₂ solution, making it possible for K⁺ in AC to be effectively removed.^12,13

Table 1 Results of mass fraction of H₂O₂ solution, ultrasound temperature and ultrasound time on the K⁺ content of AC

Mass fraction of H2O2/wt%	0	0.2	0.4	0.6	0.8	1.0
K^+ content/(mg kg^−1)	256	184	92	61	64	75
Ultrasonic temperature/°C	10	20	40	60	70	80
K^+ content/(mg kg^−1)	157	124	96	61	74	91
Ultrasonic time/h	2	4	6	8	10	12
K^+ content/(mg kg^−1)	148	119	61	46	41	44

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Fig. 2 Effect of mass fraction of H₂O₂ solution, ultrasound temperature and ultrasound time on the K⁺ content of AC.

Fig. 2(b) exhibits the relationship between the K⁺ content and the ultrasound temperature in the range of 10–80 °C for an ultrasound time of 6 h, using 0.6 wt% H₂O₂ solution. It can be seen that the K⁺ content of AC decreases from 157 to 61 mg kg⁻¹ as the temperature increases from 10 to 60 °C, but increases with a further increase of temperature from 60 to 80 °C. This can be ascribed to the fact that the increasing temperature in an ultrasonic field can improve the energy and activity of K⁺, accelerating the movement of K⁺ into the solution. However, when the temperature is above 60 °C, the temperature can interfere with the ultrasonic field, which is not favorable for removing K⁺ from AC.

Fig. 2(c) shows the effect of ultrasound time on the K⁺ content of AC using 0.6 wt% H₂O₂ solution at an ultrasound temperature of 60 °C. It is clear that the K⁺ content drops from 148 to 46 mg kg⁻¹ as the ultrasound time increases from 2 to 8 h and it scarcely changes with a continuous increase in ultrasound time. Therefore, we consider 0.6 wt% H₂O₂ solution, ultrasound temperature of 60 °C and ultrasound time of 8 h as the optimum experimental conditions to remove K⁺ from AC, and the corresponding AC is marked as AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h}.

3.2 Sample characterization

3.2.1 Morphology characterization. Fig. 3(a)–(c) shows the surface morphologies of AC-W, AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} by SEM at 1000× magnification. It can be seen clearly that the morphologies of the AC samples display an obvious change on the surface. AC-W has large quantities of uncarbonized substances present on the surface. AC-W-H_{1%/120 min} has a relatively flat and smooth surface compared to AC-W, but there are still some residues on the surface. Moreover, as shown in Fig. 3(c), the surface of AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} was very clean and smooth, confirming that the oxidation-ultrasound process has a significant effect on the purification of AC, which is consistent with the abovementioned results of the K⁺ content.

Fig. 3 SEM images of (a) AC-W, (b) AC-W-H_{1%/120 min} and (c) AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h}.

3.2.2 Porous texture characterization. Considering that the specific surface area and pore size of AC are believed to be critical factors with respect to electrochemical capacitor applications,¹⁴ the N₂ adsorption–desorption isotherms and corresponding pore size distribution curves of the as-prepared AC are shown in Fig. 4. The detailed textural characteristics and specific capacitance of the samples are summarized in Table 2. As shown in Fig. 4(a), the presented isotherms are type I for all samples according to IUPAC classification.^15,16 AC-W exhibits the lowest isotherm and its N₂ adsorption volume achieves saturation at a low relative pressure of 0.1. This indicates that there are still many activating agents (KOH) blocking the AC pores, especially the mesopores, which are important in determining the electrochemical performance of an AC sample. AC-W-H_{1%/120 min} displays a significantly higher isotherm than AC-W and its N₂ adsorption volume shows a gradual increase until a relative pressure of 0.4 is reached, implying that most of the KOH remaining in the AC pores is removed by HCl washing. As for AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h}, it is clearly seen that this isotherm exhibits the highest adsorbed volumes and shows a hysteresis loop in the desorption branch at a relative pressure of 0.4. This fully proves that K⁺ along with other residues is removed from the AC pores by the oxidation-ultrasound process, significantly increasing the mesopore volumes.

Fig. 4 (a) N₂ adsorption–desorption isotherms and (b) the corresponding pore size distributions of the as-prepared samples.

Table 2 Textural characteristics and specific capacitance of the as-prepared samples^a

Samples	SBET (m^2 g^−1)	Vt (cm^3 g^−1)	Vmeso (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Dp (nm)	Cg (F g^−1)	Ash (wt%)	Yield (%)
a SBET: specific surface area; Vt: total pore volume; Vmeso: mesopore volume; Vmicro: micropore volume; Dp: average pore size; Cg: specific gravimetric capacitance; yield: the independent result of every step of the removal of K^+.
AC-W	2198	1.110	0.311	0.799	1.781	147	1.2	—
AC-W-H1%/120 min	3178	1.686	1.297	0.389	2.086	255	0.1	94.3
AC-W-H1%/120 min-U0.6%/60 °C/8 h	3460	1.869	1.615	0.254	2.228	306	0.01	97.5

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The pore size distributions of the as-prepared AC are shown in Fig. 4(b). It can be seen that the pore size of AC-W is distributed in the range of 0.6–2 nm, while that of AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} is concentrated at 0.5–2.0 nm and 2–4 nm, respectively. It is clearly seen that the mesopores (2–4 nm) gradually become the dominant type of pores when AC-W is purified with HCl and oxidation-ultrasound treatment. As shown in Table 2, AC-W exhibits a mesopore volume of 0.311 cm³ g⁻¹ and an average pore size of 1.781 nm, while AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} achieve mesopore volumes of 1.297 and 1.615 cm³ g⁻¹ and average pore sizes of 2.086 and 2.228 nm, respectively. Moreover, AC-W-H_{1%/120 min} has a significantly larger surface area and total pore volume than AC-W, increasing from 2198 to 3178 m² g⁻¹ and 1.110 to 1.686 cm³ g⁻¹, respectively, and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} possesses the largest surface area and pore volume of 3460 m² g⁻¹ and 1.869 cm³ g⁻¹. It is evident that the removal of K⁺ from AC samples creates some new micropores and simultaneously promotes the widening of existing micropores into mesopores, which can be accessible to electrolyte ions for electrical double-layer formation.¹⁷ Thus, the large surface area, high pore volume and low K⁺ content of the AC samples make them good candidates for electrode materials.

3.3 Electrochemical characterization

CV is used in the determination of the electrochemical performances of as-prepared samples. The typical CV results of AC-W, AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} at a scan rate of 10 mV s⁻¹ with a potential range of −0.2 to 0.8 V are shown in Fig. 5(a). It can be observed that the CV curves of all the AC electrodes display an approximately quasi-rectangular voltammogram shape, which is a characteristic of electrochemical double-layer capacitance.^18–20 Furthermore, the CV curve of the AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrode exhibits a bigger current response and a larger rectangular area than that of the AC-W and AC-W-H_{1%/120 min} electrode, demonstrating an obvious increase in capacity during the removal of K⁺. Fig. 5(b) shows the CV curves of the AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrode at different scan rates of 5–50 mV s⁻¹. As the scan rate increases, the CV curve gradually becomes tilted, but still maintains a rectangular-like shape even at 50 mV s⁻¹, indicating a small resistance in the accessible pores and an excellent capacitive behavior^21,22 of the AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrode.

Fig. 5 Cyclic voltammograms of the samples: (a) all AC electrodes at a scan rate of 10 mV s⁻¹ and (b) AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} with different scan rates.

GC measurements have also been conducted to investigate the electrochemical performances of as-prepared samples. Fig. 6(a) shows the GC curves of AC-W, AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} at a current density of 1 A g⁻¹ over a potential range of −0.2 to 0.8 V. It can be seen that all the samples present a virtually linear shape and isosceles triangle curve, indicating good reversibility and typical capacitive behavior of the AC electrodes.^23,24 According to the GC curves, AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} has a longer discharging time compared to AC-W and AC-W-H_{1%/120 min}, implying a larger specific capacitance, which is in good agreement with the abovementioned CV results. That is to say, it is proved that the removal of K⁺ from the AC samples has a significant effect on the electrochemical performance of the AC electrode.

Fig. 6 Galvanostatic charge/discharge curves of the samples: (a) all activated carbon electrodes at a current density of 1 A g⁻¹ and (b) AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} with different current densities.

The GC curves of AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} measured over a current density range of 0.5–5 A g⁻¹ are shown in Fig. 6(b). As seen from the figure, the discharging time drops as the current density increases and the curve still retains a typical triangle shape even at a high loading current density of 5 A g⁻¹, revealing that AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} as an electrode material is promising for use in a high performance supercapacitor. In addition, the specific capacitance of the AC electrodes can be calculated from the charge–discharge curves based on the following equation:^25,26

where C_g is the specific gravimetric capacitance (F g⁻¹), I is the current loaded, Δt is the discharge time (s), ΔV is the potential change during the discharge process, and m (g) represents the mass of AC. As shown in Table 2, AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} has the highest specific gravimetric capacitance of 306 F g⁻¹, which is more than twice as large as that of AC-W (147 F g⁻¹) and is about 20% greater than that of AC-W-H_{1%/120 min} (255 F g⁻¹). The enhanced specific capacitance of AC samples can be ascribed to the enhancement of the surface area, effective pore volume and purity, as confirmed by the abovementioned results of the K⁺ content and N₂ adsorption–desorption isotherms.

Fig. 7 shows the cyclic stability of the AC-W, AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrodes measured by galvanostatic charge–discharge at a current density of 1 A g⁻¹ for 3000 cycles. It can be seen that the specific capacitance of AC-W displays an obvious decrease from 147 to 125 F g⁻¹ after 1000 cycles and is maintained at about 116 F g⁻¹ with a capacitance retention of 79% after 3000 cycles. The specific capacitance of AC-W-H_{1%/120 min} is relatively stable and achieves a capacitance retention of 91% after 3000 cycles. The specific capacitance for AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} is found to be 306 F g⁻¹ in the first cycle and 294 F g⁻¹ after 3000 cycles with a coulombic efficiency of 96%, revealing the excellent stability and reversibility of the AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrode.

Fig. 7 Cyclic performances of the samples at a current density of 1 A g⁻¹.

EIS measurements are employed to obtain impedance performance information. Typical Nyquist plots for AC-W, AC-W-H_{1%/120 min} and AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} electrodes are presented in Fig. 8. All the three plots exhibit a semicircle in the high-frequency region and a straight line in the low-frequency region.²⁷ It is observed from the inset in Fig. 8 that the plot of the AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} sample displays a relatively smaller semicircle than that of the AC-W and AC-W-H_{1%/120 min} samples in the high-frequency region, indicating a lower charge transfer resistance. This may be ascribed to the larger mesopore volume and lower K⁺ content of AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h}, which can facilitate the rapid diffusion of electrolyte ions into the pores of the electrode materials. In addition, AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} shows a more vertical line leaning to the imaginary axis in the low-frequency region,²⁸ suggesting better capacitive behavior than AC-W and AC-W-H_{1%/120 min}.

Fig. 8 Nyquist plots of the as-prepared samples (inset: enlarged high-frequency region of Nyquist plots).

4. Conclusions

In this study, high performance AC for supercapacitors is prepared from coconut shell by KOH activation, using a novel oxidation-ultrasound process to remove K⁺ from AC. The experimental results demonstrate that the AC samples washed with 1.0 wt% HCl solution for 120 min and subsequently treated with 0.6 wt% H₂O₂ solution at 60 °C in an ultrasonic oscillator for 8 h possess a very low K⁺ content of 46 mg kg⁻¹. Compared to AC-W and AC-W-H_{1%/120 min}, AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} exhibits a larger surface area and pore volume of 3460 m² g⁻¹ and 1.869 cm³ g⁻¹. As an electrode material for electrochemical applications, AC-W-H_{1%/120 min}-U_{0.6%/60 °C/8 h} displays a high specific capacitance of 306 F g⁻¹ and a coulombic efficiency of 96% after 3000 cycles. Thus, the oxidation-ultrasound process has the potential to fabricate ACs with high performance for supercapacitor applications.

Acknowledgements

This research is financially supported by the projects in forestry public benefit research sector (201404610).

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