Micro/mesopore carbon spheres derived from sucrose for use in high performance supercapacitors

SHI Jing, TIAN Xiao-dong*, LI Xiao, LIU Ye-qun, SUN Hai-zhen

（1. Analytical Instrumentation Center, State Key Laboratory of Coal Conversion, Institute of Coal Chemistry,Chinese Academy of Sciences, Taiyuan 030001, China;2. CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China;3. School of Chemistry and Materials Science, Ludong University, Yantai 264025, China）

Abstract： Micro/mesopore carbon spheres for use as the electrode materials of supercapacitors were prepared by hydrothermal carbonization followed by KOH/NaOH activation using sucrose as the carbon precursor. The effects of the KOH and NaOH activation parameters on the specific surface area, pore size distribution and electrochemical performance of the carbon spheres were investigated. Results indicate that the use of NaOH leads to the development of mesopores while the use of KOH increases the specific surface area and micropore volume. The pore size distribution of carbon spheres could be adjusted by varying the relative amounts of the reagents in the activation. Using a NaOH/KOH mass ratio of 2∶1 and a reagent/carbon sphere mass ratio of 3∶1, a good capacitance and rate performance of the supercapacitor electrode in both a 6 mol L?1 KOH aqueous electrolyte and a 1 mol L?1 MeEt3NBF4/propylene carbonate electrolyte was achieved. The prepared activated carbon gave a capacitance of 235 F g?1 at 0.1 A g?1 and a capacitance retention of 81.5% at 20 A g?1 in the 6 mol L?1 KOH aqueous electrolyte, and in a cell using the 1 mol L?1 MeEt3NBF4/propylene carbonate electrolyte, it gave the highest energy density of 30.4 Wh kg?1 and a power output of 18.5 kW kg?1.

Key words: Sucrose；Carbon sphere；Hydrothermal carbonization；Mixed alkali activation；Supercapacitor

1 Introduction

During the past decades, environmentally friendly energy storage devices, such as secondary batteries, supercapacitors (SCs) and fuel cells have gained intensive attention due to the growing requirement of sustainable energy and abatement of environmental pollution[1–3]. Owing to their high power output, rapid charge/discharge rate and excellent cycle stability, SCs are regarded as the most promising energy storage device for high power applications, such as wind turbines, digital communication devices,laptops and peak power sources[4]. To achieve high energy and high power output, it is curial to have a high specific capacitance even at a high charge/discharge rate.

Porous carbon materials are envisioned as the promising electrode materials for SCs owing to their adjustable pore size distribution (PSD), large specific surface area, good electrical conductivity and low cost[5]. Various biomass materials as the sustainable precursors, such as neem leaves[6], camellia petals[7],cornstalk[8], willow catkins[9]and prawn shells[10], have been used for the preparation of porous carbons via high temperature carbonization followed by physical/chemical activation[9–12]. As an important component of biomass, sucrose is widely used in most aspects of daily life including food industry, medical care, chemical industry, cosmetics, etc. It is well known that sucrose contains abundant carbon and oxygen, which are conductive to prepare activated carbons with good electrochemical performance[13]. In addition, compared with the direct carbonization method, the hydrothermal carbonization is a facile approach to prepare sphere-shaped carbon materials.Sphere-shaped sucrose-derived porous carbons can be obtained after further activation and annealing. For instance, the hydrothermal carbonized microspheres activated by KOH exhibits a high specific capacitance of 296.1 F g?1owing to its large amount of micropores centered at 0.6?0.8 nm[13], which are suitable for storing charge as revealed by the results from carbide-derived carbon (CDC) materials[14]. Nevertheless, the reported materials suffer from inferior capacitive performance at high current densities. Thus, a further pore texture improvement is required to attain an excellent electrochemical performance of sucrose-derived carbon materials. Fortunately, the researches on hierarchical porous carbons open a new avenue for fabricating high performance SCs[15–17]. By taking the advantages of micropores for charge storage of a high amount and mesopores for fast ion transport in the hierarchical porous carbons, porous carbon electrodes with a high specific capacitance and good rate capability can be obtained.

It is well demonstrated that micropore-dominated activated carbons can be prepared by KOH activation[18,19]. Although the template methods are usually needed to fabricate mesopore-dominated carbons, they are tedious and toxic[20]. Can a hierarchical porous structure be constructed via a simple method? Since the NaOH activation method can be used to prepared porous carbons with a certain amount of mesopore ranging from 2-5 nm[21], herein, a mixture of KOH and NaOH was adopted as activating agents for increasing the specific surface area and tuning the pore size distribution. The oxygen groups on the surface of the porous carbon spheres generated during the preparation process can improve the overall electrochemical performance by the enhancement of the electrode wettability and the extra pseudo capacitance. The hydrothermal carbonized sucrose was activated by NaOH and KOH activation to prepare spherical activated carbons, which exhibited impressive rate capability in both aqueous and organic electrolytes. This work provides an approach for high value-added use of sucrose.

2 Experimental

2.1 Synthesis of activated carbons

The hydrothermal carbonization of 0.1 mol L?1sucrose aqueous solution was carried out in a 100 mL Teflon-sealed autoclave at 180 °C for 24 h. Afterwards, the carbonaceous materials was collected and cleaned with deionized water for several times, ovendried at 80 °C for 12 h to obtain carbonized sample named as HTC. Then, the HTC was activated by KOH and/or NaOH activation in a tube furnace at 800 °C for 2 h under nitrogen atmosphere with a mass ratio of KOH+NaOH to HTC of 3 to yield samples designated as ATCx, wherexreferred to KOH and/or Na-OH. For example, when KOH alone was used, the sample was marked as ATCK.

2.2 Characterization

The samples were characterized by scanning electron microscopy (SEM, JEOL 7001F), X-ray diffraction (XRD, Bruker D8 Advance), Raman spectroscopy (Jobin Yvon LabRam HR800, excited by a 632.8 nm He/Ne laser), X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD) and N2adsorption (BET, Quantachrome Autosorb-iQ2). The surface area (SBET) was calculated according to the Brunauer-Emmett-Teller method and the PSD was calculated from desorption branch isotherms by the density functional theory (DFT) with a slit shaped pore assumption.

2.3 Electrochemical measurements

The cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) tests were carried out on the CHI 660C workstation. The cycling stability and rate capability were obtained with the LAND 2001B battery test system. The working electrode was prepared by blending the as-obtained samples, polytetrafluoroethylene, and carbon black (80∶10∶10 weight ratios) in 5 mL alcohol, and then the mixture was grinded by a mortar and coated onto a Ti foil current collector and dried at 80 °C overnight. In the three-electrode system, Pt sheet and Hg/HgO electrode were used as the counter electrode and reference electrode,respectively, using a 6 mol L?1KOH aqueous solution as the electrolyte. For the two-electrode configuration, two identical ATCK/Naworking electrodes were separated by a polypropylene film soaked in a 1 mol L?1MeEt3NBF4/propylene carbonate (PC) organic electrolyte. The specific capacitances of the single electrode (Cs, F g?1) and SC device (Ccell, F g?1)were calculated according to the Eq. (1) and (2). The energy density (E, Wh kg?1) and power density (P, W kg?1) of the device were obtained based on the Eq. (3)and Eq. (4):

whereiand Δtare discharge current (A) and discharge time (s), respectively, ΔVis the potential window after the internal resistance drop (V),mandmtotalrepresent the mass of single electrode materials (g)and the total mass of two electrode materials (g), respectively,Cs,Ccell,EandPrepresent the specific capacitance of single electrode (F g?1), the specific capacitance of a device (F g?1), energy density (Wh kg?1),power density (W kg?1), respectively.

3 Results and discussion

Fig. 1 SEM images of the samples using HTC∶KOH∶NaOH in mass ratios of (a) 1∶0∶0 (HTC), (b) 1∶3∶0 (ATCK),(c) 1∶0∶3 (ATCNa), (d) 1∶1∶2 (ATCK/Na).

SEM images of the samples at different synthetic conditions are given in Fig. 1. It can be seen that after hydrothermal carbonization, sucrose transforms into micrometer size carbon spheres with an average diameter of 5.3 μm (Fig. 1a). After activation, remarkable difference between activated carbons can be easily recognized. As shown in Fig. 1b, the activation of HTC with pure KOH leads to a complete change in morphology and bulk-sheet structure appears. Whereas, the sphere morphology can be maintained when the content of KOH is decreased, revealing that the etching ability of KOH is higher than that of NaOH.

The XRD patterns of the activated carbons(Fig. 2a) show similar reflections. The broad and low intensity diffraction peaks at 23.4° and 44.0° correspond to the diffraction of (002) and (100) plane of the graphite lattice, respectively, confirming the amorphous feature of the activated carbons[22]. Two obvious peaks at 1 338 cm?1(Dband) and 1 581 cm?1(Gband)can be detected in Raman spectra, which are assigned to defect carbons and sp2carbon in the crystalline graphite lattice, respectively[23]. Generally, the relative intensity ofDband andGband (ID/IG) is proportional to the graphitization degree of carbon materials[24]. TheID/IGvalues of ACTK, ACTNaand ACTK/Nashown in Fig. 2b are determined to be 1.11,1.01 and 1.05, respectively, suggesting that more defects existed in ACTK, which is in good agreement with previous SEM analysis.

Fig. 2 (a) XRD patterns and (b) Raman spectra of ATCK, ATCNa and ATCK/Na.

Fig. 3 (a) N2 adsorption isotherms and (b) PSDs of ATCK, ATCNa and ATCK/Na.

N2adsorption isotherms (Fig. 3a) of the obtained porous carbons show apparent distinction, indicating the different pore textures and specific surface areas.The ATCKexhibits a typical I isotherm according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which is related to microporous materials[18]. On the contrary, the ATCNashows an arc at the relative pressure rang of 0.05?0.20, suggesting the coexistence of a large amount of micropores and a small fraction of small size mesopores in ATCNa, which is confirmed by previous reports[25–27]. According to Linares-Solano’work, Na2O may be generated during the activating process[28], which can be transformed into NaCl by HCl washing. As for ATCK/Na, the isotherm displays the features of both ATCKand ATCNa. Over the entire relative pressure region, the more increment in nitrogen adsorption capacity of ATCKimplies more pores and higher specific surface area in this sample, which further demonstrates the higher etching efficiency of KOH in comparison with NaOH. The PSD curves(Fig. 3b) show that ATCKis a typical microporous material with a narrow size range of 0.4?1.6 nm,while the ATCNahas a large fraction of micropores centered at 0.64 nm and a few developed mesopores ranging from 2.0 to 3.7 nm. According to data listed in Table 1, the average pore diameter (Pav) of ATCNais up to 1.95 nm as compared with 1.79 nm of ATCK.Furthermore, the mesopore volumes ranging from 2.0?3.7 nm is 0.06 cm3g?1for ATCNaas compared with 0 of ATCK, revealing that NaOH is favorable to enlarge pore size, which is beneficial for improving rate capability. However, the smaller specific surface area of ATCNalimits its capacitance as compared with ATCK. Herein, a porous carbon (ACTK/Na) with both a high specific surface area and a proper micro/mesoporous structure is constructed by using mixture of KOH and NaOH as the activating agent. As shown in Fig. 3 and Table 1, the specific surface area of ACTK/Nais higher than that of ATCNaand its corresponding PSD is wider than ATCK. The high surface area can provide more active sites for charge storage,and the micro and mesoporous structure is beneficial to achieve a high capacitance with an improved rateperformance.

Table 1 Results of sorption tests and elemental compositions of activated carbons.

The activation not only results in developed porosity, but also affects the surface functionality. The XPS results shown in Fig. 4a reveal that oxygen-containing groups exist in all samples and their relative contents are listed in Table 1. The existence of oxygen-containing groups are considered to effectively enhance the hydrophilicity and wettability of the samples in electrolytes as well as to provide extra pseudocapacitance. As depicted in Fig. 4b-d, the chemical states of oxygen groups can be assigned to four peaks at 531.2, 532.3, 533.2, 535.2 eV, attributed by carbonyl/quinone (O I), phenolic/hydroxyl/ether (O II), carboxyl (O III) and chemisorbed oxygen/water (O IV), respectively[29,30]. More adsorbed oxygen/water content (8.3% for ATCKand 7.6% for ATCK/Na, while that of ATCNais 6.6%) can be detected in the KOH-activated sample, which means that a larger specific surface area contributes to more oxygen/water adsorption. These activated carbons were then tested as electrode materials for SCs.

The activated carbons were firstly tested in a 6 mol L?1KOH electrolyte with a three-electrode configuration. As shown in Fig. 5a, all samples exhibit an ideal rectangular-like shape with a slight hump at low potential region, suggesting the double layer capacitance is dominated with a small contribution of pseudocapacitance[31]. The oxygen-containing groups are commonly formed during activation. These functionalities can not only provide extra capacitance, but also improve the wettability of surface, resulting in good ion diffusion and surface utilization rate in electrolytes[32]. ATCKwith the largest specific surface area displays the largest capacitance. Fig. 5b gives the GCD plots at a low current density of 0.1 A g?1, from which, it can be found that the charge/discharge curves are symmetric, indicating a good electrochemical reversibility.

Fig. 4 X-ray photoelectron survey scanning spectra of (a) all samples, the deconvoluted O 1s peaks for (b) ATCK, (c) ATCNa and (d) ATCK/Na, respectively.

Fig. 5 The electrochemical performances of ATCK, ATCNa and ATCK/Na. (a) CV curves at 5 mV s?1, GCD plots at (b) 0.1 A g?1 and (c) 20 A g?1, (d) the IR drop, (e) specific capacitance as a function of current density and (f) Nyquist plots.

The symmetrical shape of GCD curves can still be maintained and no obvious IR drops can be observed even when the current density is increased up to 20 A g?1, revealing the good reversibility. As can be seen in Fig. 5d, the IR drops of ATCK, ATCNaand ATCK/Naat 20 A g?1are 64, 55 and 60 mV, which are quite smaller than previous reports[22,33,34]. A lower value of IR drop means a better ion transportation capability and superior reversibility. As depicted in Fig. 5e, although the capacitance decreases gradually with increasing current density, the samples (ATCNaand ATCK/Na) treated with NaOH show enhanced rate capability. For example, a capacitance retention rate of 74.6% is obtained for ATCK, while that of ATCNaand ATCK/Naare 80.1% and 81.5%, demonstrating that a porous carbon electrode with both a high capacitance and good rate performance should be made by adjusting the fraction of micropores and mesopores.Furthermore, the rate capability of ATCK/Nais superior to many activated carbons, such as petroleum cokebased porous carbon (59%, 50 to 1 000 mA g?1)[35],hierarchical porous carbon derived from parasol fluff(36.4%, 0.2 to 10 A g?1)[36], hollow mesoporous carbon spheres (87.2%, 0.5 to 10 A g?1)[37], porous carbon derived from waste plastics (60%, 1 to 20 A g?1).The Nyquist plots in Fig. 5f also verify good ion permeation after PSD is optimized by adjusting the activating agent composition. From the results of the threeelectrode system, it can be seen that ATCK/Nahas the best overall electrochemical performance (comprehensive specific capacitance and rate capability) due to its proper PSD and moderate accessible surface area. By adjusting the micropore and mesopore proportion and size, an optimal pore structure can be obtained to achieve both superior charge storage and better ion transportation or to balance the capacitance and rate performance of the as-obtained samples.

To investigate its electrochemical properties in real operating conditions, a symmetric SC device assembled from two identical ATCK/Naelectrodes was evaluated. As shown in Fig. 6a, a slight distortion of the rectangular shape can be evidenced upon the increase of the scan rate, indicating the fast ion incorporation. The GCD curves at 2, 5, 10 20 A g?1are nearly linear and symmetrical with small IR drops,suggesting the good capacitive property and electrochemical reversibility. The deviation from linearity can be attributed to pseudo faradic reaction and polarization during charge/discharge. The calculated capacitances of the device at 0.5, 1, 2, 5, 10, 20 A g?1are 30.3, 29.2, 27.9, 27.0, 25.9, 25.4 F g?1, respectively,with a retention rate of 83.8% at 20 A g?1. The capacitances based on single electrode at 0.5, 1, 2, 5, 10,20 A g?1are 121.2, 116.9, 111.7, 108, 103.9 and 101.7 F g?1, respectively. These results suggest that the interconnected micro/mesoporous structure provides efficient pathways for electrolyte ion movement throughout the carbon matrix, which can shorten ion diffusion and transport pathways during charge/discharge.

Fig. 6 The electrochemical performance of ATCK/Na electrode in organic electrolyte. (a) CV curves at various scan rates, (b) GCD plots and (c) Cs, Ccell at different current densities, (d) Ragone plot and (e) cycling performance at 5 A g-1.

Ragone plot has also been used to characterize the device. As shown in Fig. 6d, the device can achieve the highest energy density of 30.4 Wh kg?1at 0.8 kW kg?1and still holds on 12.3 Wh kg?1with a power density of 18.5 kW kg?1, which is much higher than those of N-doped electrospun nanofibers[29],chemically reduced graphene[38]and chemically converted graphene sheets[39]. After 15 000 cycles at 5 A g?1, a capacitance retention rate of 73.0% is obtained, indicating a high cycling stability.

4 Conclusion

The activated carbon spheres with rich micropores and a small fraction of mesopores were fabricated via hydrothermal carbonization followed by KOH+NaOH activation using sucrose as the precursor. The mixed use of KOH and NaOH plays a vital role in maintaining the sphere shape and adjusting pore size distribution during activation. The electrode based on the activated carbon spheres exhibits intriguing electrochemical performance in terms of capacitive performance, rate performance as well as cycle stability. It can deliver the highest energy density of 30.4 Wh kg?1at 0.8 kW kg?1, which is retained at 12.3 Wh kg?1with a high power density of 18.5 kW kg?1.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51602322, 21878321),Natural Science Foundation of Shanxi Province(201801D221371) and the Outstanding Ph.D. Program of Shanxi Province (SQ2019001).