微波法制備電化學電容器用花生殼基活性炭

吳明鉑，李如春，何孝軍，張和寶，隋吳彬，譚明慧

(1.中國石油大學(華東)化學工程學院，重質油國家重點實驗室，山東 青島 266580;2.安徽工業大學 化學與化工學院，安徽省煤清潔轉化與綜合利用重點實驗室，安徽 馬鞍山 243002)

1 Introduction

China is the world’s largest peanut producer with a capacity of 5 million metric tons of peanut shells each year.As a byproduct of peanut production，peanut shell is normally used as animal feed or fuel without any value and technology-added applications.On the other hand，activated carbons (ACs)with sufficient pores and good adsorptivity have been widely used in various fields［1］，e.g.industrial purification，chemical recovery and electrode materials for electrochemical capacitors (ECs).The fact that peanut shell is cost-competitive，readily available in such large quantity，leads us to believe that converting peanut shell into useful technology-added ACs should be highly beneficial and meaningful［2］.Traditionally，ACs are usually prepared by physical or chemical activations via conventional heating.Among the chemical activation methods，KOH activation is a rather efficient activation agent for preparation of microporous carbons［3-7］.However，the biggest economic barrier to convert peanut shell to ACs is the high cost of activation，which usually requires extended activation time，high energy consumption and excessive activation agent.This being the case，it would be imperative to invent a rapid and efficient approach to make low-cost ACs.

In terms of heating means，microwave heating has lots of advantages over conventional heating means，e.g.high heating efficiency，easy control of the heating process，rapid temperature rise at low energy consumption［8，9］.For these reasons，the microwave heating has received extensive attention over the past few years for AC preparation［10-15］.To date，little has been reported on an efficient and straight-forward preparation of ACs from renewable peanut shell for use as ECs electrode materials.

Herein，the prospect in deriving ACs from peanut shell using microwave heating for ECs was investigated at lowered KOH/peanut shell mass ratios with shortened activation times.The effects of the key determining factors，such as KOH/peanut shell mass ratio and activation time，on the pore structure and electrochemical performance of ACs in ECs were studied.

2 Experimental

2.1 Preparation and characterization of ACs

Peanut shell was obtained from Huaian in Jiangsu，China.Peanut shell particles with a size of about 3 ×10 mm2were cleaned by water scrubbing and then dried at 383 K for 24 h.The analysis results of peanut shell on air dry basis by mass fraction are as follows:9.52% of moisture，1.30% of ash，68.34% of volatile matter and 20.84% of fixed carbon.

Dried peanut shell (9 g)was mixed with KOH solution at different KOH/peanut shell mass ratios.The mixture was dried at 383 K for 24 h after being impregnated for 12 h at room temperature，and then transferred to a crucible.The crucible was heated in a LWMC-205-type microwave oven with a microwave power of 600 W from 6 to 10 min of activation to prepare the ACs.The temperature of reactants in the crucible was measured by an armor-type thermocouple during microwave heating.

ACs were successively washed with 0.5 mol/L HCl solution and distilled water，until pH=7.0 was reached.ACs were then dried at 383 K for 24 h.The resultant AC is designated as ACx-y-z，where x refers to the KOH/peanut shell mass ratio，y the microwave power，and z the activation time.For example，AC1-600-8refers to the AC prepared with a KOH/peanut shell mass ratio of 1，a microwave power of 600 W and an activation time of 8 min.Characterization of the pore structure of ACs was performed on the basis of nitrogen adsorption-desorption isotherms measured on a sorptometer ASAP2010 at liquid nitrogen temperature.

2.2 Preparation and electrochemical measurements of AC electrodes

The electrode slurry was made by mixing AC，carbon black (CB)and poly (tetrafluoroethylene)(PTFE)in a mass ratio of 87∶5∶8.The slurry was coated onto nickel foam with a diameter of 12 mm.Prior to packaging and test，two disk electrodes with an active mass of about 40 mg were dried at 393 K for 2 h under vacuum.One cell was composed of two similar electrodes separated by polypropylene membrane.The cell was tested in 6 mol/L KOH solution using a symmetrical button cell configuration by cyclic voltammetry on an electrochemical workstation(CHI-760C，Chenghua，Shanghai).The electrochemical performance of AC electrodes in the cell was investigated on a land cell tester (Land，CT-2001A).

3 Results and discussion

3.1 Pore structure of ACs

The N2adsorption-desorption isotherms of the ACs are shown in Fig.1.

Fig.1 N2adsorption-desorption isotherms of ACs.

It is found that all the ACs are microporous as evidenced by the Type I isotherm.The pore structure parameters of the ACs are shown in Table 1.It can be seen that the specific surface area，total pore volume，micropore volume of the AC all exhibit maxima with activation time for 8 min or with KOH/peanut shell mass ratio for 0.8 under otherwise identical conditions investigated.At a KOH/peanut shell mass ratio of 1.0 with a microwave power of 600 W and an activation time of 8 min，the SBETof AC1-600-8reach 1277 m2/g.The SBETof ACs produced by microwave heating is larger than that from biomass using conventional heating methods even at a longer activation time［16］，which is ascribed to the efficiency of microwave heating at molecular level.The yields of AC1-600-6，AC1-600-8and AC1-600-10are 24.4%，21.8%and 18.0%，respectively，showing the same trend in the yields of ACs with activation temperature［17］.The final activation temperature of AC1-600-6，AC1-600-8and AC1-600-10are 1 083，1 113 and 1 133 K，respectively，and the average heating rate in the preparation of ACs is rather high，ranging from 84 to 132K/min.Elevated activation temperatures for AC1-600-6，AC1-600-8and AC1-600-10caused by the increasing activation time are favorable for releasing more gaseous products and thus are responsible for the decreasing AC yields.

In KOH activation at over 673 K，the reaction between KOH and carbon occurs based on the following equation［18］.

6KOH+2C=2K+3H2+2K2CO3

Table 1 Pore structure parameters of ACs.

The formed metallic potassium intercalates to the carbon matrix，resulting in a widening of the spaces between carbon atomic layers and an increase of pore volume.At over 923 K，the surface metal complexes are responsible for a further gasification，which leads to the widening of micropores.In particular，the Dapof AC1-600-10is 2.03 nm，the largest among all the ACs.

The yields of ACs are related to both on the KOH/peanut shell mass ratio and the activation time.The yield of AC0.8-600-8is 24.5%，the highest in this work.Table 1 shows that all the ACs are microporous with the SBET，Vtand Vmicof AC1-600-8being the largest among all ACs.These micropores less than 1 nm would be a positive contributor to the improved capacitance of ACs in ECs［19］.

3.2 Electrochemical performance of ACs

Cyclic voltammetry is usually used to characterize the capacitive behaviors of electrode materials in ECs.Fig.2 shows the cyclic voltammetry curves of AC1-600-8electrodes at different scan rates.The cyclic voltammetry curves of AC1-600-8electrodes retain a symmetric rectangular shape with increasing the scan rate from 2 to 50 mV/s，which suggests a quick charge propagation in AC1-600-8electrodes.

The specific capacitance of a AC electrode in ECs was calculated from the slope of the discharge curve［20］.The variation of specific capacitance of the AC electrodes with discharge current density is presented in Fig.3a，b.The inset in Fig.3a is the 1000thcharge-discharge curve of AC electrodes at a current density of 0.05A/g.A slight decrease in specific capacitance is observed with increasing discharge current density in Fig.3a，b.

Fig.2 Cyclic voltammetry curves of AC1-600-8electrode at different scan rates.

Fig.3a indicates that specific capacitance of AC electrodes increases with heating time from 6 to 10 min，and eventually reaches a plateau at 8 min.Fig.3b shows that specific capacitance of AC electrodes is associated with the KOH/peanut shell mass ratio.The specific capacitance of AC electrodes prepared with a KOH/peanut shell mass ratio of 1.0 and an activation time of 8 min is the highest among those AC electrodes with a KOH/peanut shell mass ratio from 0.6 to 2.0 and an activation time of 8 min，which is ascribed to the largest SBET，Vtand Vmicof AC1-600-8.In contrast，the specific capacitance of AC1-600-10electrode in Fig.3a is smallest among AC electrodes investigated，which is likely due to that some carbonylic functional groups in AC1-600-10were removed and/or some micropores in AC1-600-10were widen caused by an extended heating or elevated temperature［21-23］.Apparently，Table 1 shows that the Dapand Sextof AC1-600-10are the biggest among all ACs.The specific capacitance of AC1-600-8decreases from 242.8 to 228.4 F/g with current density from 0.05 to 1.20 A/g，and the capacitance retention of AC1-600-8electrode eventually reaches as high as 94.0%.The capacitance retention of other AC electrodes is also found rather high，ranging from 91.5% to 92.6%.It is noteworthy that the specific capacitances of all AC electrodes derived from peanut shells via KOH activation by microwave heating in 6 mol/L KOH electrolyte are significantly improved over those of AC electrodes from dehydrogenated chars by conventional phosphoric acid activation for 30 min［24］.

Fig.3 Specific capacitance of AC electrodes vs.current density:(a)ACs made at different activation times，(b)ACs made at different KOH/peanut shell mass ratios.

For ECs made of AC electrodes，energy density of ECs (E，in Wh/kg)is usually calculated on the basis of Eq.(1)［25］.

Where C is the capacitance of the two-electrode capacitor (F/g)，V the usable voltage (V)excluding the IR drop occurring at the discharge.

Average power density of ECs (P，in W/kg)is calculated according to Eq.(2)［7］.

Where Δtddenotes the time spent in discharge.

The variation of energy density of AC capacitors with power density is presented in Fig.4.

Fig.4 Energy density of AC capacitors vs.average power density:(a)ACs made at different activation times，(b)ACs made at different KOH/peanut shell mass ratios.

Fig.4a，b demonstrate that energy density of AC capacitors decreases with increasing the power density for all AC capacitors，which suggest that less energy are released at higher power output.At lower discharge current density of 0.05，0.1，0.2，0.4 and 0.8 A/g in Fig.4a，the energy density of AC1-600-10capacitor is nearly equal to that of AC1-600-6capacitor under the same power density.However，the energy density of AC1-600-10capacitor at higher discharge current density of 1.2，2.0 and 3.0 A/g is bigger than that of AC1-600-6capacitor under the same power density.The bigger Dapof AC1-600-10electrode is responsible for the bigger energy density of AC1-600-10capacitor at higher discharge current density due to the faster ion transport.Fig.4a shows that energy density of AC capacitors reach a maximum with increasing the activation time from 6 to 10 min at the same current density.Fig.4b also shows that energy density of AC capacitors reaches a maximum with increasing KOH/peanut shell mass ratio from 0.6 to 2.0 at the same current density from 0.05 to 2.00 A/g.Specifically，the energy density of AC1-600-8capacitor is the largest among all AC capacitors，as shown in Fig.4a，b.The energy density of AC1-600-8capacitor decreases only from 8.41 to 5.98 Wh/kg with increasing the discharge current density from 0.05 to 3.00 A/g，indicating a retention rate of the energy density of 71.1%at the highest current density.The energy density retention rate for other AC capacitors at the highest current density ranges from 62.6% to 69.8%.

4 Conclusions

ACs were derived from peanut shell by KOH activation by microwave heating for only 6-10 min.The prepared ACs were used to prepare electrodes for ECs.The SBETand Vtof ACs，specific capacitance of AC electrodes as well as energy density of AC capacitors all exhibit maxima with activation time from 6 to 10 min or with KOH/peanut shell mass ratio from 0.6 to 2.0.The SBETof AC1-600-8reaches 1 277 m2/g and AC1-600-8capacitor demonstrates a high cycle stability with an energy density of 8.38 Wh/kg even after 1000 cycles.KOH activation of peanut shell by microwave heating is found to be an efficient and straightforward approach to the production of low-cost ACs for ECs.

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