3D-honeycomb carbons for high performance electrical double layer capacitors electrodes

QIAO Zhijun, RUAN Dianbo, YUAN Jun, FU Guansheng, YANG Bin



carbons for high performance electrical double layer capacitors electrodes

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(Institute of Supercapacitors, Ningbo CRRCNew Energy Technology Co., Ltd., Ningbo 315112, Zhejiang, China)

Sodium fulvic acid based hierarchical porous carbons (SFA-HPCs) with a specific surface area of 1919 m2·g–1and total volume of 1.7 cm3·g–1has been synthesized by a simple self-template method. The carbon skeleton can be formatted by the decomposition process of sodium fulvic acid (SFA) in a N2atmosphere. The sodium compund in SFA is used as a self-template to create the hierarchical porous structure. The unique hierarchical structure of SFA-HPCs provides an efficient pathway for electrolyte ions to be diffused into the internal surfaces of bulk electrode particles. It results in a high charge storage capacitance of 186 F·g–1at current load of 40 A·g–1. The capacitance of 230 F·g–1at 0.05 A·g–1and 186 F·g–1at 40 A·g–1show its good rate capability. Besides, it also achieves desirable cycling stability, 99.4% capacitance remained after 10000 cycles at 40 A·g–1.

sodium fulvic acid; hierarchical porous carbons; self-template method; high rate capability

The topic of hierarchical porous carbons (HPCs) being used as electrical double layer capacitors (EDLC) electrodes has recently attracted the attention of many scientists[1-5]. These HPCs materials have a unique pore structure withmicropores, mesopores and macropores, facilitating the diffusion of the electrolyte ions into the effective internal surfaces of bulk electrode particles to obtain high capacitance. However, the most common methods to synthesize HPCs,activation method and template method, generally require reagents to develop hierarchical structures. Furthermore, the preparation processes of bothmethods are

CHENl[6]successfully synthesized afish scale-based 3D hierarchical lamellar porous carbon () by a simpletemplate process. A mineral phase from the fish scale was adopted to act as a natural template for the formation of the hierarchical porous structure, avoiding the using oftemplate reagent. But the specific capacitance of FHLC was only 168 F·g–1at 0.05 A·g–1. GAO[7] reported a process which was a combination of self-template method and chemical activation method from enteromorpha prolifera (). Similarly, LV et al[8] reported the preparation process of banana peel based hierarchical porous carbons (BP-HPCs). Although the as-prepared EP based HPCs and BP-HPCs electrodes both displayed an excellent electrochemical performance,would still cause environmental issues.

,for theof HPCs based on sodium fulvic acidSFA-HPCs). As a natural polymer polysaccharide compound, sodium fulvic acid (SFA) extensively exists in all kinds of, which are low-cost, sustainable and environmental friendly. The polymercomponent of SFA provides carbon source for HPCs, while the sodium salt acts as a, leading to.

1 Experimental

1.1 Raw materials and the preparation of SA-HPCs

Sodium fulvic acid (SFA) was provided by Tianjin Guangfu Co., Ltd. It was placed in a tube furnace under N2gas flow at 800 ℃ for 2 h with a heating rate of 3 ℃·min–1. The obtained black solid was named as SA-800 and then thoroughly washed by distilled water at 80 ℃ for 6 h. After dried at 80 ℃ for 24 h, the obtained sample was labeled.

1.2 Characterizations

The morphology of SFA-HPCs was observed by using scanning electron microscopy (SEM, Hitachi, S4800), transmission electron microscopy (TEM, Philips Tecnai G2 F20). X-ray diffraction (XRD) patterns were recorded in reflection mode (Curadiation) on a Bruker D8 diffractometer at the 2range of 10°—90°. Thermogravimetric analysis (TGA) was carried out on a NET-ZSCH TG 209 apparatus with a heating rate of 10 ℃·min–1 in the atmosphere. The surface area and the porosity of SFA-HPCs were analyzed by N2sorption at 77 K (ASAP2020). The specific surface area was determined by. The microporous suface area (mic) and micropore volume (mic) were analyzed by the-plot method. The mesopore volume (meso) was calculated using the Barrett-Joyner- Halenda method. The total pore volume (tot) was calculated from the amount of N2adsorbed at a relative pressure (/0) of 0.99.

1.3 EDLC fabrication processes and electrochemical measurements

All electrochemical experiments were conducted by using symmetric capacitors at room temperature. The symmetric capacitors were made from SFA-HPCs/graphene/polytetrafluoroethylene (PTFE) (a weight ratio of 8∶1∶1) as electrodes, a polypropylene membrane as the separator and 6 mol·L–1KOH aqueous solution as the electrolyte. The mass of SFA-HPCs contained in each electrode was 4 mg. Cyclic voltammetry (CV) test was analyzed on a PARSTAT 2273 electrochemical workstation. Galvanostatic charge/discharge cycle tests were measured on an Arbin MSTAT instrument. The specific capacitance of EDLC was calculated based on charger/discharger according to the formula:C= 2Δ/ (Δm), whereCis the gravimetric specific capacitance (F·g–1),is the current (A), Δis the discharge time (s), m is the(g) of the HPCs in the signal electrode and Δis the potential change during the discharging process (excluding the IR drop).

2 Results and discussion

2.1 TG/DTA and XRD patterns of SFA and SFA-HPCs

The TGA curve (Fig.1) showed that SFA contained 46.82% (weight ratio) of organic matter, demonstrating that the carbon skeleton of SFA-HPCs was derived from organic matter of SFA. Fig.2 showed the XRD pattern of SFA and SFA-HPCs powder. The diffraction peaks of SFA indicated that NaCl existed in SFA, while the XRD pattern of SFA-HPCs exhibited no residual salts in the SFA-HPCs. It can be estimated that the salts could be removed easily by water washing, which meant no additional chemical reagents removal process was required.

The SEM and TEM micrographs of the SFA-HPCs are shown in Fig.3. From the SEM images at different levels of magnification [Fig. 3(a, b)], it can be observed that the sample had 3D honeycomb structures

with open macropores of ~300 nm diameter, also proved by TEM image Fig. 3(c). The high-resolution TEM image indicated that the SA-HPCs sample had a disordered carbon structure with a large number of micropores. The results revealed that the sodium salt acted as self-templates or space fillers tothe hierarchical porous structure. Besides, no distinct domains of graphite structure were observed in the SEM and TEM images (Fig.3). In the XRD pattern of SA-HPCs (Fig.2), no obvious peak was detected at the position of graphite peak, (002) (about 26°), which indicated the amorphous carbon structure of SFA-HPCs.

2.2 Porous texture characterization

Further information of the porosity ofwas obtained from the N2adsorption/desorption isotherm and DFT pore size distribution (PSD) (Fig.4). The N2isotherm of SFA-HPCs exhibited a characteristic of type IV, with a BET surface area of 1919 m2·g–1, atotof 1.7 cm3·g–1, amicof 0.27 cm3·g–1and amesoof 1.29 cm3·g–1. It showed the diversity of pores and the presence of micropores and mesopores in SFA-HPCs. The PSD of FSA-HPCs as provided in Fig.4(b) can be divided into three regions: (1) ultrafine micropores (0.5­—1 nm) and micropores (1—2 nm); (2) mesopores (2—50 nm); and (3) macropores (>50 nm). From the PSD results, it can be observed clearly that SA-HPCs had a well-developed hierarchical structure containing micropores, mesopores and macropores, which are well consistent with the data of SEM and TEM analysis.

2.3 Electrochemical characterization

To investigate electrochemical performances of the SFA-HPCs, the CV and galvanostatic charge/discharge measurements were conducted in a 6 mol·L–1KOH aqueous solution. It can be observed that the SFA-HPCs electrode presented a nearly perfect quasi-rectangular voltammogram shape at scanning rates of 50, 100, 200 300, 400, 500, 600 mV·s–1, proving that the sample was suitable for rapid charging/discharging [Fig.5(a)]. Fig.5(b) illustrated the galvanostatic charge/discharge curves of the EDLC at 0—1 V, measured at current load of 5, 10, 20, and 40 A·g–1. The charge/discharge curves were close to symmetrical isosceles lines, demonstrating that the SFA-HPCs electrode had a typical porous carbon supercapacitive behavior and electrochemical stability.

The calculated specific capacitances of the SFA-HPCs electrode at a series of charge/discharge rates are shown in Fig.5(c). SFA-HPCs electrode had a supercapacitive behavior with high specific capacitance of 230 F·g–1at current load of 0.05 A·g–1and 186 F·g–1at 40 A·g–1[Fig.5(c)]. The high BET surface area, as well as the well-developedsystem,to the enhanced specific capacitance. In comparison, the specific capacitances of fish scale-based FHLC were only 168 F·g–1at 0.05 A·g–1and 130 F·g–1at 40 A·g–1[6]，indicating that the hierarchical pore structure of SFA-HPCs performed better on the ions transferring and energy storage. In addition, the SFA-HPCs electrode achieved a stable cycling performance with capacitance retention of 99.4% after 10000 cycles [Fig.5(d)]. It was believed that these distinguished electrochemical performances were attributed to the hierarchical pore structure of SFA-HPCs, because macropores could act as electrolyte solution buffering reservoirs while mesopores significantly facilitated the penetration of the electrolyte into the micropores.

3 Conclusions

A simple self-template method was investigated to prepare sodium fulvic acid based hierarchical porous carbons (SFA-HPCs). The obtained SFA-HPCs possessed a 3D hierarchical pore structure and exhibited an excellent electrochemical performance in 6 mol·L–1KOH aqueous solution. It indicated that sodium fulvic acid was a promising precursor for fabricating high-performance hierarchical porous carbons based EDLC.

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Foundation: This study was supported by the Ningbo’s Industrial Technology Innovation and Industrialization of Scientific and Technological Achievements Program (2013B6003)

TQ 424.1 Document code：A Article ID：2095-4239（2016）04-527-05

date: 2016-01-28.

Qiao Zhijun (1985—), PhD, fields of research: carbon material and super capacitor, E-mail：zjqiao@csrcap.com; Corresponding author：Ruan Dianbo, PhD, Senior Engineer, fields of research: development, manufacturing and application of the supercapacitor, E-mail：ruandianbo@csrcap.com.

10.12028/j.issn.2095-4239.2016.04.019