Ultrasonic-assisted preparation and characterization of hierarchical porous carbon derived from garlic peel for high-performance supercapacitors

Ultrasonics - Sonochemistry 60 (2020) 104756

Contents lists available at ScienceDirect

Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Ultrasonic-assisted preparation and characterization of hierarchical porous carbon derived from garlic peel for high-performance supercapacitors

T

⁎

Zhaocai Teng, Kuihua Han , Jinxiao Li, Yang Gao, Ming Li, Tongtong Ji School of Energy and Power Engineering, Shandong University, Jinan 250061, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultrasonic-assisted impregnation Biomass Porous carbon Pore structure Electrochemical performance

Ultrasonic-assisted impregnation is used to synthesize physically modified garlic peel-based 3D hierarchical porous carbons (PCs), and the effect on PCs is investigated by changing ultrasonic time. The results show that ultrasonic waves can effectively peel off surface attachments of the carbonized product, so that activator has a better mass transfer process and create more active sites. The connectivity of 3D pore network is enhanced as well, so the structure and electrochemical properties of garlic peel-based porous carbon (GBPC) are improved. The ultrasonic disperser is used as an ultrasonic generator, specific conditions are as follows: ultrasonic frequency is 40 kHz, ultrasonic power is 500 W, and ultrasonic time is 0, 3, 6, and 9 min, respectively. With the increase of ultrasonic time, impurities again block the pore structure during dynamic movement, resulting in a decrease in electrochemical performance. Specifically, the performance of GBPC-6 is the most excellent, the specific surface area (SSA) increases from 2548 m2 g−1 to 3887 m2 g−1, the specific capacitance increases from 304 F g−1 to 426 F g−1 at a current density of 1 A g−1 in a two-electrode test system. Energy density and cycle performance are also improved at the same time, which are attributed to rational structure. In addition, the effectiveness of the strategy of ultrasonic-assisted synthesis has been confirmed on another precursor material–scallion, meaning that this work proposes a new and simple modification method for improving the performance of biomass-based PCs.

1. Introduction Supercapacitor is a new type of energy storage device that is between ordinary capacitor and battery in terms of performance, and fundamental difference is the charge transfer system [1,2]. The control of charge transfer system is divided into two levels [3]: (1) designing a porous structure for constructing a fast ion conducting channel within the electrode materials (EMs); (2) establishing a strong conductive network around the EMs. This article focuses on the first level of research. EMs of supercapacitors mainly include carbon materials [4], conductive polymers [5], metal oxides [6] and composite electrodes [7,8], etc. Among them, porous carbons (PCs) are considered to be ideal adsorbent and active materials due to their well-developed pore structure and large SSA [9], especially their rich macropores (< 2nm), mesopores (2–50 nm) and micropores (> 50 nm). Specifically, a large number of micropores provide a rich accumulation space for the charge; mesopores reduce ions diffusion distance and resistance, and make ions easily penetrate into the internal micropores; macropores act as a buffer ion storage layer [10,11]. While the problem is that excess micropores simultaneously limit the movement of ions, resulting in a decrease in

⁎

SSA utilization [12,13]. Common PCs are prepared by carbonization and activation of carbon-rich materials. In recent years, biomass has become a research hotspot of PC raw materials due to its low cost and easy recycling [14]. The common method for preparing PCs is chemical activation, activators are mainly classified into acid, alkali, salt, etc, H3PO4, KOH and ZnCl2 are typical representatives, and KOH activation is the most widely used [10,15–17]. In the laboratory research, KOH activation process usually needs to mix the alkali and carbon in a certain condition (time, temperature, and ratio, etc). The carbonized product is rich in impurities, its pore wall strength is low and easy to collapse. Some of impurities form deposits during carbonization covering the surface of the carbon while blocking small pore channels, which greatly affects the subsequent impregnation process, so impregnation time is generally 2–24 h [18,19]. Ultrasonic waves are widely used to increase the rate of chemical reactions in liquids, such as cleaning, emulsification, atomization, etc. due to their cavitation effect. Cavitation means the generation, vibration, migration, and destruction of microbubbles in liquid media [20]. At present, scholars have applied ultrasonic technology to the regeneration of activated carbons [21], rapid synthesis of foam-like mesoporous carbon

Corresponding author at: 17513 Jingshi Road, Lixia District, Jinan, Shandong Province 250061, China. E-mail address: [email protected] (K. Han).

https://doi.org/10.1016/j.ultsonch.2019.104756 Received 31 May 2019; Received in revised form 25 August 2019; Accepted 26 August 2019 Available online 31 August 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

2.3. Physical characterization

monolith and ultrasonic-assisted H3PO4 impregnation to prepare wooden activated carbon which were used for adsorbent materials [22–24]. However, there are few studies on the electrochemical performance of carbon materials modified by ultrasonic-assisted KOH impregnation and the mechanisms that result from the enhancement of ultrasound, so we try to introduce ultrasonic waves in the impregnation process to shorten impregnation time while expecting to improve the pore structure of PCs and explore its mechanism. At present, biomass includes seaweed [4], coconut shell, elm samara, rice husk, soybean, etc. has been extensively studied because of its natural porous structure. But the problem is that there are few studies on their modification to increase energy density, and most focus on the exploration of new materials. Unlike woody plants, herbaceous plants and their wastes contain less lignified cells. Their stems are weak and easily modified to obtain an ideal 3D scaffold structure. At the same time, the cost of herbaceous plants is relatively low. Therefore, according to the principle of adaptation to local conditions, we choose garlic peel, a by-product of mass production in Shandong Province, as a raw material. Based on previous research [25], an ultrasonic-assisted KOH impregnation strategy to synthesize garlic peel-based porous carbon (GBPC) was proposed. By controlling the variable of ultrasonic treatment time and then characterizing the morphology, pore structure and electrochemical performance of GBPC, the mechanism and influence law of ultrasonic waves are explored, which provides a better strategy for the high-quality application of garlic peel wastes and explores the universality of modification of PCs.

A nitrogen adsorption/desorption isotherm was obtained by using a SSA and pore size analyzer (JW-BK132F, Beijing, China). Prior to the adsorption test, the sample was first degassed for 5 h (300 °C). The SSA and pore characteristics of the sample were obtained by analysis of the isotherms: the micropore volume (Vmicro) was calculated by the t-plot method and the pore volume (VT) and SSA were separately calculated by the Barrett-Joyner-Halenda (BJH) model and the Brunauer-EmmettTeller (BET) method. The pore size distribution was determined by nonlocal density functional theory (NLDFT). The surface morphology and surface element distribution of PCs were analyzed by a SUPRATM 55 scanning electron microscope (Carl Zeiss AG, German) equipped with an energy dispersive X-ray spectrometer (EDS). Surface features were investigated by using a Thermo/ESCALAB 250 XI X-ray Photoelectron Spectroscopy (XPS). Raman spectroscopy was performed using a Renishaw/RM2000 and 514 nm laser. The phase composition of samples was characterized by powder X-ray diffraction (Rigaku D/MAX2500PC, λ = 1.5406). 2.4. Electrochemical measurement The electrode slurry was prepared by weighing the GBPC with conductive graphite and the polytetrafluoroethylene (PTFE) emulsion (60 wt%) in an 8:1:1 ratio and mixing in an appropriate amount of anhydrous alcohol. They were then uniformly mixed by ultrasonic dispersion treatment for 30 min, and the obtained slurry was uniformly coated on a foamed nickel having a diameter of 16.2 mm, and the mass load of the EM on each electrode sheet is about 4 mg. Thereafter, the electrode was dried in a vacuum oven at 80° C for 12 h and then pressed at 15 MPa for 1 min. Finally, in a two-electrode system with 6 M KOH as an electrolyte, two electrode sheets of similar EM mass were separated by glass fiber filter paper (Waterman, GF/B) and assembled into a button capacitor. The electrochemical performance of the supercapacitor was measured by using an electrochemical workstation (CHI 660D, Shanghai Chenhua, China). Specifically, it included galvanostatic charge-discharge (GCD) test, cyclic voltammetry (CV) test and electrochemical impedance spectroscopy (EIS) test. The specific capacitance C (F g−1) of the supercapacitor can be calculated by the following Eq. (1). The energy density E (Wh Kg−1) and power density P (W Kg−1) of the supercapacitor can be obtained from Eqs. (2) and (3), respectively:

2. Experimental 2.1. Materials The thin garlic peel wrapped around outside of the garlic was used as the raw material, and it was obtained from Shanghe, Shandong province. The chemicals used in this study (KOH, HCl, H2SO4, HNO3, absolute ethanol, etc.) were analytically pure reagents bought from Tianjin Kemer Chemical Reagent Co., Ltd. At the same time, high-purity nitrogen (purity ≥99.999%) provided by Jinan Deyang Special Gas Co., Ltd. was used as inert protective gas throughout the process, and an ultrasonic disperser of Hangzhou Success Ultrasonic Equipment Co., Ltd. model YPS11B-HB was used for assisting impregnation.

2.2. Preparation and optimization process of GBPC First, the garlic peel was washed with distilled water, dried in an oven at 120 °C for 12 h, and placed in a pulverizer (LBH-400Y, 32000 rpm) for 3 min, and sieved to obtain a garlic peel powder having a particle diameter of less than 178 μm. Using the optimized parameters in the previous research [25], the garlic peel powder was carbonized at 600 °C for 2 h in a tube furnace to obtain a preliminary carbon product. The KOH and the carbonized product were mixed into a solution at a mass ratio of 4:1, and placed in an ultrasonic disperser (40 kHz, 500 W) for ultrasonic impregnation for different time (0, 3, 6, 9 min). The sum of the ultrasonic-assisted and the static impregnation time was kept constant for 30 min, and entire impregnation process was kept at 65 °C. Similarly, the activation process used the optimized parameters to maintain the resulting mixture in an activated atmosphere furnace at 800 °C for 2 h. Both heating rates of two furnaces were 5 °C min−1 and flow rate of inert gas were both 0.04 m3 h−1. The obtained sample was washed with 1.0 M HCl to pH 8–9 to wash away impurities, and then thoroughly washed with distilled water of 80 °C to neutrality. Finally, the mixture was placed in an oven at 105 °C for 24 h, the obtained PC was named GBPC-X finally, X represents the time of ultrasonic-assisted impregnation, while the static impregnation time is 30-X.

C=

4I Δt mΔV

(1)

E=

1 C (ΔV )2 8

(2)

P=

E Δt

(3)

where I means the discharge current (A), Δt in Eq. (2) means the discharge time (s) while in Eq. (3) means the discharge time (h); m means PC mass of two electrode sheets (mg); ΔV means the electrical potential difference (V). 3. Results and discussion 3.1. KOH activation mechanism of GBPC The most commonly held theory of KOH activation assumes that C and KOH begin to be a solid-solid reaction followed by a solid-liquid reaction of C with KOH and other intermediates. The specific process is as follows [26]: First, the reaction of Eq. (4) occurs at 400–600 °C, when the reaction temperature is 600 °C, the KOH is completely consumed. Next, the K2CO3 formed in the Eq. (4) starts to undergo a decomposition reaction (Eq. (5)) at a temperature higher than 700 °C, and the reaction 2

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 1. (a) Nitrogen adsorption-desorption isotherms of the GBPC-0, GBPC-3, GBPC-6 and GBPC-9 samples. (b) Pore size distributions of the GBPC-0, GBPC-3, GBPC-6 and GBPC-9 samples.

assisted impregnation tends to increase first and then decrease. The SSA of GBPC-6 is highest. This increase may be attributed to the fact that during the ultrasonic-assisted impregnation process, the microbubbles in the liquid oscillate on the contact surface of the carbonized sample and rupture to generate local shear stress, causing the latter surface attachments to peel off while making the agent evenly distributed. The impurities in the thin wall are cleaned and the pores are opened as well. As a result, potassium vapor can have a better mass transfer during activation and create more active sites. Therefore, the SSA increases during activation process, while excessively long ultrasonic time may cause the clogging of the bundle structure by movement of impurities, thereby affecting the entire activation process, resulting in a decrease in SSA again. At the same time, it should be noted that the SSA ratio (SSA of micropores/SSA) of micropores decreased from 83.6% of GBPC-0 to 74.0% of GBPC-6, indicating a significant increase in mesopores, which is consistent with results shown in Fig. 1 and the most probable pore size presented in Table 1. It has long been believed that excess micropores and excessive specific surface area have no contribution to the capacitance [30,31]. Recently, Chmiola et al. [32] found that anomalous increase in carbon capacitance at pore size less than 1 nm (the specific value is 0.5–1 nm, this value varies depending on the electrolyte), studies have shown that within the micropores, ions are desolvated to form a single row can still store charges efficiently, which is called “superionic state” [33–35], therefore it is expected that GBPC-3 and GBPC-6 may exhibit good electrochemical performance with the increase of micropores with a pore diameter of 0.58–0.7 nm and small mesopores. Fig. 2 shows the morphological changes of the samples during the preparation characterized by SEM. Fig. 2a clearly presents the tube bundle structure obtained after preliminary carbonization of garlic peel. Fig. 2b–c are respectively the pore structure diagram of tube bundle section after local amplification of GBPC-0 and GBPC-6 by 30,000 times. In contrast, the pores of GBPC-6 are more abundant and regular, which is beneficial to the transport of electrolyte ions [36]. Fig. 2d depicts a pore structure diagram of the same partial magnification of 50,000 times, which suggests that ultrasonic waves have a certain effect of enhancing the connectivity of the slit-type pores during the auxiliary impregnation process and reduce mass transfer resistance of potassium vapor. So that the most probable pore size is enlarged, which is in contrast to the ultrasonic assisted extraction process [37,38] but the mechanism of the reaction is consistent. Locally generated shear and mechanical effects during cavitation bubble collapses may cause cracks in the cell wall in the tube bundle structure to promote the immersion of the activator while enhancing pore connectivity [39]. The phase composition of GBPCs was analyzed by XRD pattern (Fig. 3a). It is obvious that the four samples show a broad peak of 002 at 2θ = 21.7°, and a weak 101 peak at 2θ = 44.3°, suggesting that the

is completed at 800 °C, during which reactions of the Eqs. (6)–(8) are simultaneously carried out. The potassium vapor formed by the Eq. (7) and the Eq. (8) enters the lattice gap of the GBPC, and after washing away K and its salt compound, many pores are formed.

2C + 6KOH → 2K + 2K2 CO3 + 3H2

(4)

K2 CO3 → K2 O + CO2

(5)

CO2 + C → 2CO

(6)

K2 CO3 + 2C → 2K + 3CO

(7)

K2 O + C → 2K + CO

(8)

3.2. Characterization of the GBPCs Fig. 1a shows the N2 isotherm adsorption-desorption curve of prepared GBPCs. It is apparent that isotherms of four samples belong to type IV-B (according to IUPAC classification), a sharp rise in the adsorption curve at low relative pressure (P/P0 < 0.1) indicates the presence of a large number of micropores [27,28]. As the relative pressure increases (0.3 < P/P0 < 1), obvious adsorption hysteresis occurs, that is, the isothermal adsorption curve shows a hysteresis loop, which reveals that capillary condensation occurs in this relative pressure interval. Compared with GBPC-0, GBPC-3 and GBPC-6 have a more pronounced hysteresis loop, which suggests an increase in mesoporous ratio [29]. The pore size distribution of each PC sample is shown in Fig. 1b. Using NLDFT theoretical analysis, it can be seen that the micropores of GBPC-3 and GBPC-6 increase between 0.58 nm and 0.64 nm, mesoporous growth occurs at 2.01 nm, while entire pore size distribution has no fluctuations. This indicates the ultrasonic-assisted impregnation process is only a physical modification process, and most of pores don’t collapse during the sonochemical process and subsequent activation. From the specific pore structure parameters of four samples shown in Table 1, we can see that SSA of the samples synthesized by ultrasonicTable 1 Specific pore structure parameters of the samples. Sample

GBPC-0 GBPC-3 GBPC-6 GBPC-9

SSA

SSAmicro

VT

Vmicro

D

(m2 g−1)

(m2 g−1)

(cm3 g−1)

(cm3 g−1)

(nm)

2548 3438 3887 3099

2129 2578 2876 2451

1.587 1.995 2.242 1.726

1.149 1.316 1.452 1.216

2.058 2.752 2.855 2.085

SSA: specific surface area; SSAmicro: SSA of micropores; VT: total pore volume; Vmicro: pore volume for micropores; D: the most probable pore size. 3

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 2. SEM images of the preliminary carbonization sample (a), the GBPC-0 (b), the low magnification (c)and the high magnification (d)of GBPC-6 sample.

shows D-band, G-band and 2D-band around 1346 cm−1, 1590 cm−1 and 2870 cm−1. The D-band is generally considered to be the disordered vibration peak of carbon materials, indicating the defect or edge of samples; the G-band is caused by the in-plane vibration of the hybrid carbon atom, and the 2D-band corresponds to the two-phonon resonance, suggesting that the overall GBPC tends to be ordered [40]. The ratio of D-band intensity to G-band intensity (ID/ IG ) is often used to characterize the amorphous or defect degree of carbon materials [25]. The ID/ IG values of GBPC-0, GBPC-3, GBPC-6, GBPC-9 are 0.89, 0.86, 0.83, 0.88, respectively. This ratio decreases first and then increases with the increase of ultrasonic time. It is because the introduction of

sample consisted mainly of amorphous carbon [28]. Moreover, the diffraction 002 peak of samples synthesized by ultrasonic assisted impregnation is getting stronger while other disorganized peaks are less, which indicates that the impurities generated by the carbonization process are peeled off during the impregnation process, so that they are easier to be cleaned and the samples are purer. It should be noted that the peaks of GBPC-3 and GBPC-6 at the initial angle of scattering are lower than the other two, implying that the proportion of micropores becomes smaller, which corresponds with that shown in Table 1 above. Fig. 3b presents a Raman spectrum of several samples used to further characterize the internal chemical structure, which respectively

Fig. 3. (a) XRD patterns and (b) Raman spectra of the GBPC-0, GBPC-3, GBPC-6 and GBPC-9 samples. 4

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 4. XPS results of the GBPC-0, GBPC-3, GBPC-6 and GBPC-9 samples. (a) XPS survey, (b–e) C1s spectra, (f–i) O1s spectra.

3.3. Electrochemical properties of GBPC electrodes

ultrasound for a short time enhances the order of the GBPC while when ultrasonic time is 9 min, the originally stripped impurities block the tube bundle structure and affect the subsequent activation process, which affects the structural order of GBPC. This also suggests that introduction of ultrasonic waves does not cause an increase in the degree of graphitization, but only makes the structure more orderly. The narrowing of the width of the D-band and the floating change of the 2Dband also prove the view above. The surface characteristics of GBPCs were characterized by XPS spectra (Fig. 4). The data shows that C, N, and O contents of GBPC-0 are 93.06%, 0.82%, and 6.12%, respectively. And three elements are observed under the binding energies of 284.8 eV, 400.0 eV, and 533.0 eV, which correspond to the C1s, N1s, and O1s spectra, respectively. N has no obvious peaks in Fig. 4a which can be negligible. The C1s spectrum (Fig. 4b–e) can be deconvoluted into sp2 C]C (284.6 eV), sp3 CeC (285.1 eV), CeO & CeN (286.1 eV), OeC]O (289.1 eV). After the introduction of ultrasound, the proportion of C]C bond (sp2 hybridization) increases significantly, which corresponds to the increase of the G band in the Raman spectrum. The O1s spectrum (Fig. 4f–i) can be deconvolved to C]O (531.5 eV), CeO (CeOH & CeOeC) (532.4 eV), OeC]O (533.8 eV), chemically adsorbed oxygen (H2O&O2) (535.5 eV), compared with the control sample, the specific gravity of CeOH, CeOeC and other functional groups increase after the introduction of ultrasound, which increases the hydrophilicity of the EM. At the same time, the chemical adsorption oxygen decreases and the C/ O ratio increases, which is beneficial to improve the electrochemical performance of active materials [41].

Electrochemical performance tests of GBPCs were carried out at room temperature (25 °C) using a two-electrode system with 6.0 M KOH as the electrolyte which is shown in Fig. 5. It can be calculated from Fig. 5a according to Eq. (1), the specific capacitances of four samples at a current density of 1A g−1 are 305, 386, 427, 342 F g−1, respectively. Fig. 5b shows a CV curve obtained at a scan rate of 50 mV s−1. The curve presents an approximately rectangular shape, indicating that there is no pseudo capacitance. The four EMs are typical electric double layer capacitor materials, which is consistent with results obtained from GCD curve. This means that the sample synthesized by ultrasonic-assisted impregnation has higher partial micropores and mesopores content than the original 3D multi-layer carbon skeleton structure, and the structure is more orderly, which creates a more ideal migration channel for ions during charge and discharge [42]. Therefore, the electrolyte ions adsorb more charge to form an electric double layer, which promotes the improvement of specific capacitance. Fig. 5c presents the specific capacitance of four samples measured at different current densities. The final specific capacitances of the four samples at a current density of 50 A g−1 are 192, 247, 305, 191 F g−1 respectively. When the ultrasonic treatment time is less than 6 min, the rate performance is more and more superior. The capacitance retention of GBPC-6 is 71.5%, which has the best rate performance. Fig. 5d shows the Nyquist plot of supercapacitors based on four EMs in the frequency range of 10−3–105 Hz. It is obvious that the Nyquist plot of four materials in the low-frequency region are close to the X-axis while in the high-frequency region have obvious semi-circular arcs, and X-intercept

5

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 5. Electrochemical performance tests of supercapacitors with a two-electrode system of 6.0 M KOH as an electrolyte. (a) GCD curves of GBPC-0, GBPC-3, GBPC-6, and GBPC-9 samples at a current density of 1 A g−1.(b) CV curves of GBPC-0, GBPC-3, GBPC-6, and GBPC-9 samples at a scan rate of 50 mV s−1.(c) Specific capacitance for GBPC-0, GBPC-3, GBPC-6, and GBPC-9 samples at current densities from 0.1 A g−1 to 50 A g−1.(d) Nyquist plot of GBPC-0, GBPC-3, GBPC-6, and GBPC-9 samples.

an ideal EM; on the other hand, the pore structure of ultrasonic-assisted synthesis guarantees high speed operation of charge transport system. The power density and energy density calculated by Eqs. (2) and (3) are plotted as Fig. 6d, and it is apparent that the performance of GBPC-6 is superior to that of the control sample. When the power density is 190.06 W Kg−1, the energy density is 59.57 Wh Kg−1, and the energy density of 49.18 Wh Kg−1 can be guaranteed when the power density increases to 16.24 KW Kg−1. Compared with ordinary capacitors and batteries, GBPC-6-based supercapacitor achieves a balance between power density and energy density to a certain extent, which is a promising energy storage device.

of curves are all small, which shows the excellent electrical conductivity and cycle performance of the garlic peel-based electrode. Judging from above intercept, the equivalent series resistance (ESR) of GBPC-6 is the lowest, the value is 0.22 Ω. The four samples in the intermediate frequency region have obvious 45° Warburg impedance region, proving that there is diffusion resistance, and the length of the impedance region of GBPC-3, GBPC-6 and GBPC-9 is shortened. GBPC-6 is the most obvious, which means the increase of ion mobility [43], and enhancement of the pore connectivity and development of transmission networks result in the reduction of diffusion resistance. As can be seen from Fig. 5, GBPC-6 shows the most excellent electrochemical performance. In order to evaluate its potential of practical application, we will conduct further tests which can be seen in Fig. 6. The specific capacitance of GBPC-6 at a current density of 0.5A g−1 is 443F g−1, which is 393F g−1 at a current density of 10A g−1. The capacitance retention is 88.7%. It can be seen from Fig. 6b that GBPC-6 always maintains a good rectangular characteristic when the scan rate is increasing to 200 mV/s, meaning that the active material still has fast ion responsiveness at high potential [44]. The pore structure can satisfy the diffusion and transport of electrolyte ions inside the material, showing good capacitance characteristics, which is consistent with the result presented in Fig. 5d. Fig. 6c appears the comparison of cycle performance between GBPC-0 and GBPC-6. The two samples are subjected to 10,000 cycles of charge and discharge at a current density of 5A g−1, and the capacitance retention shows a trend of increasing first and then decreasing. At the beginning, the temperature rises due to long-term constant current charge and discharge, so that the ion migration is accelerated, and the attenuation occurs at the later stage because the irreversible capacitance loss due to structural destruction of GBPCs. The specific capacitance after 10,000 cycles is retained over 94.7% and 97.1%, respectively. This means that on one hand, GBPC is

3.4. Mechanism and universality of ultrasound-assisted synthesis strategies Ultrasonic waves can play an auxiliary role in the impregnation process after carbonization: on one hand, the impregnation time is reduced greatly; on the other hand, the impurities in the carbonized sample crack and surface are exposed and washed in this process. The distribution of the activator KOH is more uniform, which promotes the subsequent pore-forming of activation process. Fig. 7 shows the microscopic process evolution diagram of ultrasonic-assisted synthesis of PC and conventional activated synthetic PC. The reaction mechanism of this process can be attributed to the combined effects produced by two different processes: on one hand, the local shear force triggered by cavitation effect and mechanical effect cause partial crack rupture and surface impurities fall off, thus improving the pore connectivity; on the other hand, the ultrasonic capillary effect (UCE), which refers to the increase of depth and velocity of penetration of liquid into canals and pores under some conditions of sonication [45,46], promotes the diffusion of ions in the improved structure and subsequent escape of potassium vapor. 6

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 6. Further electrochemical performance testing of the GBPC-6 sample. (a) GCD curves of the GBPC-6 sample at current densities from 0.5A g−1 to 10A g−1. (b) CV curves of the GBPC-6 sample at scan rates from 5 mV s−1 to 200 mVs−1. (c) Cycle performance of GBPC-0, GBPC-6 samples. (d) Ragone plot of GBPC-0, GBPC-6 samples.

groups, one group was the control sample (Control), and the other group was the PC synthesized by ultrasonic assisted impregnation for 3 min (USPC). The rest experimental conditions were the same as above. Fig. 8 presents a comparison of the characterization of the Control sample and the USPC sample. Fig. 8a–b show SEM images of the

Chong, C et al. [29] have studied enormous potential of herbs like elm samara and garlic peel in energy storage materials, so another promising precursor material–scallion which has a different structure [47] was selected to further validate the effectiveness of the ultrasonicassisted synthetic PCs strategy. The experiment was divided into two

Fig. 7. Microscopic process of GBPC synthesis. 7

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Fig. 8. Characterization of the Control sample and the USPC sample. (a) SEM image of the Control sample after 20,000 times magnification (b) SEM image of the USPC sample after 20,000 times magnification. (c) Pore size distribution of the Control sample and the USPC sample. (d) GCD curves of the Control sample and the USPC sample at a current density of 1A g−1.

Control sample and the USPC sample at 20,000 times magnification, respectively. Fig. 8c–d show the comparison of pore size distribution and GCD curves of the two samples. It is obvious that the pore structure of the ultrasonic-assisted synthesis group is more developed and the activation process is more complete. It can be seen from the graph and pore size distribution that the content of micropores (0.5–0.8 nm) and small mesopores increases. The excellent pore structure improves the electrochemical performance of USPC [48,49], and the specific capacitance increases from 335 F g−1 to 365 F g−1 at a current density of 1A g−1, confirming the effectiveness of this strategy.

Declaration of Competing Interest

4. Conclusion

Appendix A. Supplementary data

Based on previous research, the strategy of ultrasonic-assisted synthesis of GBPC was proposed. The results show that the structure and electrochemical properties of 3D layered PC are significantly improved after ultrasonic-assisted impregnation for a period of time, which is best at 6 min. Ultrasonic waves cause the surface adhesion of the carbonized product to fall off due to its cavitation, so that potassium vapor can have a better mass transfer during activation and create more active sites. The increase of the content of partial micropores (0.5–0.8 nm) and small mesoporous can increase the migration rate while ensuring a large amount of ion storage, resulting in the improvement of SSA, specific capacitance, rate performance, and cycle performance of EMs. This strategy can be extended to the large-scale production of energy storage and adsorbent materials because of its advantages of greatly shortening the impregnation time and improving the performance of biomass-based EMs.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ultsonch.2019.104756.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement The authors would like to acknowledge funding support by the Natural Science Foundation of Shandong, China (ZR2017MEE010), and the Fundamental Research Funds of Shandong University (2016JC005).

References [1] B. Dunn, H. Kamath, J.M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334 (2011) 928–935. [2] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin? Science 343 (2014) 1210–1211. [3] Y. Wang, X. Fu, M. Zheng, W.H. Zhong, G. Cao, Strategies for building robust traffic networks in advanced energy storage devices: a focus on composite electrodes, Adv. Mater. 31 (2019) 1804204. [4] S. Li, K. Han, J. Li, M. Li, C. Lu, Preparation and characterization of super activated carbon produced from gulfweed by KOH activation, Microporous Mesoporous Mater. 243 (2017) 291–300. [5] X. Wu, Z. Han, X. Zheng, S. Yao, X. Yang, T. Zhai, Core-shell structured Co3O4@ NiCo2O4 electrodes grown on flexible carbon fibers with superior electrochemical properties, Nano Energy 31 (2017) 410–417. [6] H. Liu, D. Zhao, Y. Liu, P. Hu, X. Wu, H. Xia, Boosting energy storage and electrocatalytic performances by synergizing CoMoO4@ MoZn22 core-shell structures,

8

Ultrasonics - Sonochemistry 60 (2020) 104756

Z. Teng, et al.

Chem. Eng. J. 373 (2019) 485–492. [7] D. Zhao, H. Liu, X. Wu, Bi-interface induced multi-active MCo2O4@ MCo2S4@ PPy (M= Ni, Zn) sandwich structure for energy storage and electrocatalysis, Nano Energy 57 (2019) 363–370. [8] D. Zhao, M. Dai, H. Liu, L. Xiao, X. Wu, H. Xia, Constructing high performance hybrid battery and electrocatalyst by heterostructured NiCo2O4@ NiWS nanosheets, Cryst. Growth Des. 19 (2019) 1921–1929. [9] B.D. Mccloskey, Expanding the ragone plot: pushing the limits of energy storage, J. Phys. Chem. Lett. 6 (2015) 3592. [10] T. Liu, F. Zhang, Y. Song, Y. Li, Revitalizing carbon supercapacitor electrodes with hierarchical porous structures, J. Mater. Chem. A 5 (2017) 17705–17733. [11] A. Volperts, G. Dobele, A. Zhurinsh, D. Vervikishko, E. Shkolnikov, J. Ozolinsh, Wood-based activated carbons for supercapacitor electrodes with a sulfuric acid electrolyte, New Carbon Mater. 32 (2017) 319–326. [12] O. Barbieri, M. Hahn, A. Herzog, R. Kötz, Capacitance limits of high surface area activated carbons for double layer capacitors, Carbon 43 (2005) 1303–1310. [13] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, J. Power Sources 157 (2006) 11–27. [14] Y. Zhang, X. Liu, S. Wang, L. Li, S. Dou, Bio-nanotechnology in high-performance supercapacitors, Adv. Energy Mater. 7 (2017) 1700592. [15] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828–4850. [16] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520–2531. [17] J. Li, K. Han, S. Li, Porous carbons from Sargassum muticum prepared by H3PO4 and KOH activation for supercapacitors, J. Mater. Sci. Mater El 29 (2018) 8480–8491. [18] R. Baccar, J. Bouzid, M. Feki, A. Montiel, Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions, J. Hazard. Mater. 162 (2009) 1522–1529. [19] S. Timur, I.C. Kantarli, E. Ikizoglu, J. Yanik, Preparation of activated carbons from Oreganum stalks by chemical activation, Energy Fuel 20 (2006) 2636–2641. [20] J. Rooze, E.V. Rebrov, J.C. Schouten, J.T. Keurentjes, Dissolved gas and ultrasonic cavitation-a review, Ultrason. Sonochem. 20 (2013) 1–11. [21] J. Lim, M. Okada, Regeneration of granular activated carbon using ultrasound, Ultrason. Sonochem. 12 (2005) 277–282. [22] Z. Zhang, X. Liu, D. Li, T. Gao, Y. Lei, B. Wu, J. Zhao, Y. Wang, W. Ling, Effects of the ultrasound-assisted H3PO4 impregnation of sawdust on the properties of activated carbons produced from it, New Carbon Mater. 33 (2018) 409–416. [23] D. Guo, W. Li, W. Dong, G. Hao, Y. Xu, A. Lu, Rapid synthesis of foam-like mesoporous carbon monolith using an ultrasound-assisted air bubbling strategy, Carbon 62 (2013) 322–329. [24] E. Şayan, Ultrasound-assisted preparation of activated carbon from alkaline impregnated hazelnut shell: an optimization study on removal of Cu2+ from aqueous solution, Chem. Eng. J. 115 (2006) 213–218. [25] Q. Zhang, K. Han, S. Li, M. Li, J. Li, K. Ren, Synthesis of garlic skin-derived 3D hierarchical porous carbon for high-performance supercapacitors, Nanoscale 10 (2018) 2427–2437. [26] J. Wang, S. Kaskel, KOH activation of carbon-based materials for energy storage, J. Mater. Chem. 22 (2012) 23710–23725. [27] P. Kleszyk, P. Ratajczak, P. Skowron, J. Jagiello, Q. Abbas, E. Frąckowiak, F. Béguin, Carbons with narrow pore size distribution prepared by simultaneous carbonization and self-activation of tobacco stems and their application to supercapacitors, Carbon 81 (2015) 148–157. [28] C. Long, L. Jiang, X. Wu, Y. Jiang, D. Yang, C. Wang, T. Wei, Z. Fan, Facile synthesis of functionalized porous carbon with three-dimensional interconnected pore structure for high volumetric performance supercapacitors, Carbon 93 (2015) 412–420. [29] C. Chong, D. Yu, G. Zhao, B. Du, T. Wei, S. Lei, S. Ye, F. Besenbacher, Y. Miao, Three-dimensional scaffolding framework of porous carbon nanosheets derived

[30]

[31]

[32] [33]

[34]

[35]

[36]

[37]

[38] [39]

[40]

[41] [42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

9

from plant wastes for high-performance supercapacitors, Nano Energy 27 (2016) 377–389. K. Kierzek, E. Frackowiak, G. Lota, G. Gryglewicz, J. Machnikowski, Electrochemical capacitors based on highly porous carbons prepared by KOH activation, Electrochim. Acta 49 (2004) 515–523. X. Fu, Y. Wang, J. Tuba, L. Scudiero, W.H. Zhong, Small molecules make a big difference: a solvent-controlled strategy for building robust conductive network structures in high-capacity electrode composites, Small Meth. 2 (2018) 1800066. J. Chmiola, Anomalous increase in carbon capacitance at pore sizes less than 1 nanometer, Science 313 (2006) 1760–1763. S. Kondrat, N. Georgi, M.V. Fedorov, A.A. Kornyshev, A superionic state in nanoporous double-layer capacitors: insights from Monte Carlo simulations, PCCP 13 (2011) 11359. V. Ruiz, C. Blanco, R. Santamaría, J.M. Juárez-Galán, A. Sepúlveda-Escribano, F. Rodríguez-Reinoso, Carbon molecular sieves as model active electrode materials in supercapacitors, Microporous Mesoporous Mater. 110 (2008) 431–435. C. Merlet, C. Péan, B. Rotenberg, P.A. Madden, B. Daffos, P.L. Taberna, P. Simon, M. Salanne, Highly confined ions store charge more efficiently in supercapacitors, Nat. Commun. 4 (2013) 2701. C. Zhu, T. Liu, F. Qian, T.Y. Han, E.B. Duoss, J.D. Kuntz, C.M. Spadaccini, M.A. Worsley, Y. Li, Supercapacitors based on three-dimensional hierarchical graphene aerogels with periodic macropores, Nano Lett. 16 (2016) 3448–3456. F. Dranca, M. Oroian, Optimization of ultrasound-assisted extraction of total monomeric anthocyanin (TMA) and total phenolic content (TPC) from eggplant (Solanum melongena L.) peel, Ultrason. Sonochem. 31 (2016) 637–646. G.L. Chahine, A. Kapahi, J. Choi, C. Hsiao, Modeling of surface cleaning by cavitation bubble dynamics and collapse, Ultrason. Sonochem. 29 (2016) 528–549. S. Chemat, A. Lagha, H. AitAmar, P.V. Bartels, F. Chemat, Comparison of conventional and ultrasound-assisted extraction of carvone and limonene from caraway seeds, Flavour Frag. J. 19 (2004) 188–195. A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401. L. Hao, X. Li, L. Zhi, Carbonaceous electrode materials for supercapacitors, Adv. Mater. 25 (2013) 3899–3904. T.E. Rufford, D. Hulicova-Jurcakova, E. Fiset, Z. Zhu, Q.L. Gao, Double-layer capacitance of waste coffee ground activated carbons in an organic electrolyte, Electrochem. Commun. 11 (2009) 974–977. M.G. Sullivan, R. Kötz, O. Haas, Thick active layers of electrochemically modified glassy carbon. Electrochemical impedance studies, J. Electrochem. Soc. 147 (2000) 308–317. Y. Gong, D. Li, C. Luo, Q. Fu, C. Pan, Highly porous graphitic biomass carbon as advanced electrode materials for supercapacitors, Green Chem. 19 (2017) 4132–4140. F. Chemat, N. Rombaut, A. Sicaire, A. Meullemiestre, A. Fabiano-Tixier, M. AbertVian, Ultrasound assisted extraction of food and natural products. Mechanisms, techniques, combinations, protocols and applications. A review, Ultrason. Sonochem. 34 (2017) 540–560. A. Sicaire, M.A. Vian, F. Fine, P. Carré, S. Tostain, F. Chemat, Ultrasound induced green solvent extraction of oil from oleaginous seeds, Ultrason. Sonochem. 31 (2016) 319–329. J. Yu, L. Gao, X. Li, C. Wu, L. Gao, C. Li, Porous carbons produced by the pyrolysis of green onion leaves and their capacitive behavior, New Carbon Mater. 31 (2016) 475–484. Y. Gao, L. Li, Y. Jin, Y. Wang, C. Yuan, Y. Wei, G. Chen, J. Ge, H. Lu, Porous carbon made from rice husk as electrode material for electrochemical double layer capacitor, Appl. Energy 153 (2015) 41–47. Y. Li, S. Roy, T. Ben, S. Xu, S. Qiu, Micropore engineering of carbonized porous aromatic framework (PAF-1) for supercapacitors application, PCCP 16 (2014) 12909–12917.