ORIGINAL PAPER

Activated carbon from the waste water purifierfor supercapacitor application

Guojin Li1 &Qiang Li1 & Jianglin Ye2 &Guangsheng Fu1& JunJian Han1

& Yanwu Zhu2

Received: 17 April 2017 /Revised: 13 May 2017 /Accepted: 16 May 2017 /Published online: 3 June 2017# Springer-Verlag Berlin Heidelberg 2017

Abstract Activated carbon has been synthesized from thewaste filter carbon in household water purifier. The originalwaste filter carbon (OFC) gives poor specific capacitance.However, the activated carbon SA-900-4 (OFCwas heat treat-ed at 900 °C to get SC-900, then SC-900 was activated byKOHwith mass ratio of SC-900:KOH = 1:4 at 800 °C) showsgreat electrochemical performance. The SA-900-4 has a spe-cific capacitance of 122.8 F g−1 at current density of 1 A g−1 in1 M H2SO4 electrolyte. Furthermore, the specific capacitanceof SA-900-4 is 93.8 F g−1 at 10 A g−1 and 78.1 F g−1 at20 A g−1, nearly 76 and 63.6% of the value at 1 A g−1, respec-tively, indicating good rate capacity of this activated carbon.In addition, the highest energy densities of 15.4 Wh kg−1 in1 M H2SO4 and 32.9 Wh kg−1 in 1 M Na2SO4 were obtainedrespectively for the two-electrode supercapacitors based onSA-900-4. It is demonstrated that the waste filter carbon hasthe potential to be recycled as electrode material for electro-chemical energy storage.

Keywords Activated carbon .Waste water purifier . KOHactivation . Supercapacitor .Waste recycling

Introduction

In modern society, the energy storage devices with fastcharge/discharge capability, high energy density, and longcycling stability are urgently needed in many areas; there-fore, supercapacitor has attracted increasing research at-tention to improve its energy density [1–3]. Many re-search focus on fabricating activated carbon (AC) to bethe electrode material for different types of supercapacitorbecause the AC is typically low cost and also has goodchemical stability, high specific surface area, well-developed hierarchical pore structure, and high electricalconductivity [4–7]. Compared to traditional fossil fuelssuch as coal and petroleum, biomass wastes have recentlybeen used as carbon precursors to prepare AC due to theirrenewable nature, abundant sources, and easy availability[8]. For example, coconut shell has been used to fabricateAC as electrode material for supercapacitor [9, 10].Others, such as waste leave [11–13], prawn shell [14],chitin shell [15], rice husk [16, 17], banana peel[18–21], and eggshell [22], have also been the precursorsto synthesize AC for electrochemical energy storage.Most of these reported AC derived from biomass wastesexhibit excellent electrochemical performance, becausethey could inherit both microstructural flexibility and po-rous diversity of their natural biomass precursors,resulting with excellent transportation and storage capac-ity of electrolyte ions in AC electrodes [23, 24]. In addi-tion, biomass-based AC could greatly reduce the depen-dence on fossil fuels as raw material, which leads to theprotection of natural resources and environment.

Besides the natural biomass wastes, human society isgenerating more and more industrial and householdwastes, and to some extent, these wastes can become re-sources, which leads to circular economy [25]. Recently,

* Qiang Liqli@hfut.edu.cn

* Yanwu Zhuzhuyanwu@ustc.edu.cn

1 School of Electronic Science and Applied Physics, Hefei Universityof Technology, Hefei, Anhui 230009, China

2 Department of Materials Science and Engineering, University ofScience and Technology of China, Hefei, Anhui 230026, China

J Solid State Electrochem (2017) 21:3169–3177DOI 10.1007/s10008-017-3653-9

industrial waste and pollutant were also used as carbonprecursors to fabricate AC for supercapacitor. For exam-ple, Rajagopal et al. prepared AC from printed circuitboard waste for supercapacitor via pyrolysis and followedby CO2 activation process [26]. Yin et al. initially adoptedmethylene blue (MB,C16H18ClN3S), a type of environ-mental pollutant (dye) widely used in industry, as precur-sor to synthesize AC by ZnCl2 activation which exhibitshigh specific capacitance and long-term cycling stability[27].

Nowadays, people pay more and more attention onsafe drinking water due to increasing health concernsand continuous improvements in lifestyle. According toThe World Water Organization, the global water purifiermarket is expected to witness a double-digit growth dur-ing 2013–2019 [28]. AC has been recognized as one ofthe most popular and widely used adsorbents in watertreatment throughout the world [29]. As we know, AC(filter carbon) in water purifier needs to be replacedwhen reaching its use limit. Not only it is a waste ofresources but also may cause environmental pollution ifthe waste filter carbon is discarded after reaching its uselimit. Based on the view of resource conservation andenvironmental protection, this work explored therecycling of waste filter carbon in a household waterpurifier as the electrode material for supercapacitor. Itwas found that, after heat treatment and KOH activation,the waste filter carbon shows outstanding electrochemicalperformance.

Experimental

Materials

The waste filter carbon from a household water purifier (brand3M DWS2500-CN (Fig. 1a) which purified about 1800 l ofcity tap water before replaced in the city of Hefei, Anhui,China) was dried in oven at 60 °C for 24 h before furthertreatment. Other reagents were used directly without furtherpurification.

Heat treatment and activation of the waste filter carbon

The original waste filter carbon (OFC) (Fig. 1b) was heated atdifferent temperatures (from 700 to 1000 °C) in a horizontaltube furnace under N2 atmosphere for 2 h. The obtained prod-ucts were named as SC-X (X is the heat treatment tempera-ture). Electrochemical test results show that 900 °C is the bestheat treatment temperature, and the sample was named as SC-900.

Typically, the obtained SC-900 was mixed with KOH(mass ratio of SC-900:KOH = 1:2, 1:4, 1:6, and 1:8, re-spectively). Then, the mixture was dissolved in deionizedwater and sonicated for a few minutes, subsequently driedat 100 °C with uninterrupted stirring until water evaporat-ed. The dried product was put into tube furnace and heat-ed at 800 °C under N2 atmosphere for 2 h. The obtainedAC was treated by 0.5 M HCl firstly, then washed withdeionized water thoroughly, and dried at 80 °C for 24 h.

Fig. 1 a–c Schematic illustrationof supercapacitor based onactivated carbon from the wastewater purifier. SEM images of dOFC. e, f SC-900. g, h SA-900-4.HRTEM images of i SC-900 and jSA-900-4

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All the ACs were named as SA-900-X (X represents themass ratio of KOH, for example SA-900-4 is sample ofSC-900:KOH = 1:4).

Characterizations

Morphologies of the obtained ACs were observed byscanning electron microscopy (SEM, JSM-6700F). High-resolution transmission electron microscopy (HRTEM,JEOL 2010) was used to observe the microstures ofACs. Structures of the ACs were examined by X-ray dif-fraction spectroscopy (XRD, D/max-TTR III with Cu Kαradiation with a scan rate of 5° min−1 from 10° to 80°)and Raman spectroscopy (Renishaw inVia RamanMicroscope, 532 nm laser with a power of 5 mW).Surface areas of the ACs were calculated by followingBrunauer-Emmett-Teller (BET) methods via N2 adsorp-tion–desorption (autosorb iQ). X-ray photoelectron spec-troscopy (XPS) examination was carried on ESCALAB250 X-ray photoelectron spectrometer.

Electrochemical measurements

The two-electrode supercapacitor cell (sandwich-typeconstruction, as shown in Fig. 1c) was prepared for elec-trochemical performance test. The mixture of obtainedsample and polytetrafluoroethylene (PTFE, 60 wt.% sus-pension in water) binder with mass ratio of 95:5 wasrolled into uniform thickness sheets (about 40 μm) andthen punched into plates. After dried under vacuum for24 h at 110 °C, two mixture plates, two Pt plates, and oneseparator (Celgard 3501) were assembled into a two-electrode supercapacitor cell (the mass of each electrodeis about 2 mg). The 1 M H2SO4 and 1 M Na2SO4 wereused as electrolyte, respectively. Electrochemical perfor-mance of the assembled supercapacitor were evaluated byPrinceton Parstat 4000. According to the galvanostaticcharge/discharge results, specific capacitance (Cm,F g−1), energy density (E, Wh kg−1), and power density(P, W kg−1) of the samples were calculated by the follow-ing equations [30]:

Cm ¼ 4IΔtmΔV

ð1Þ

E ¼ 1

8Cm ΔV2 ð2Þ

P ¼ EΔt

ð3Þ

where I is the discharge current, m is the total mass for twoelectrodes,ΔV is the voltage change (IR drop was excluded),and Δt is the discharge time.

Results and discussion

Material characterization

The surface morphologies of OFC, SC-900, and SA-900-4were depicted in Fig. 1d–h. Figure 1d shows the SEMimage of OFC; it looks like a muddy material. After heattreatment, there are many holes and cracks in SC-900, andalso some impurities in holes, as shown in Fig. 1e, f. AfterKOH activation, the SA-900-4 shows more visible holes(Fig. 1g, h). The impurities in the holes of SA-900-4 wereobviously less than those in SC-900. The HRTEM imagesof the SC-900 and SA-900-4 (Fig. 1i, j, respectively) con-firm the defective microporous morphologies and graphit-ic structures of the two samples with no observablemacro-size pores.

XRD patterns of the OFC, SC-900, and SA-900-4 wereshown in Fig. 2a. According to the standard diffractionpattern, the series of peaks of OFC and SC-900 areassigned to be the multicomponent compounds based onTi, Ca, Fe, Mn, Cu, C, and O atoms (JCPDS 39-0375, 42-1251, 21-0838, 35-0796, 21-1272, 30-0571). These com-pounds probably came from the impurities which filteredout from the city tap water, which is revealed by the SEMresults too. In contrast, for SA-900-4, there are no sharppeaks, due to KOH activation process leading to the dis-appearance of compounds. The two broad weak peaks ataround 22.3° and 43.8° of SA-900-4 correspond to the(002) and (100) diffraction peaks of activated carbon, re-spectively, which indicate the randomly disordered carbonlayer and low degree of graphitization [31]. The largeincrease in the low-angle scatter indicates a high densityof micropores in SA-900-4 [7]. Raman spectra of OFC,SC-900, and SA-900-4 were performed to further investi-gate their chemical structures, which all show the D-bandand G-band of activated carbon (Fig. 2b). D-band is lo-cated at around 1349 cm−1, referring to the disorderedcarbon [31]. The G-band induced by vibration of sp2-hy-bridized carbon locates at around 1586 cm−1 [32]. The ID/IG ratios of the three samples are about 1.00, indicatingthe high percentage of imperfect structures within them[16].

Pore structures of SC-900 and SA-900-4 were obtainedfrom N2 adsorption–desorption isotherm measurements.As shown in Fig. 2c, SC-900 shows a typical type-I iso-therm, and SA-900-4 shows a type-I and type-IV com-bined isotherm with a hysteresis loop. The sharp increaseof the two isotherms at low relative pressure (p/p0 < 0.1)demonstrates that there are large number of micropores inSC-900 and SA-900-4 [33, 34]. For isotherm of SA-900-4, the observable hysteresis loop at medium relative pres-sure range (p/p0 = 0.4–0.85) indicates the presence ofabundant mesopores [35, 36]. Figure 2d shows that the

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pore diameters of SC-900 are mainly around 0.56 and0.85 nm with a small part of 2.3 nm, and SA-900-4 hasconsiderable mesopores with diameters of about 2.8 and4.1 nm. Although both the XRD pattern and N2 adsorp-tion–desorption isotherm of SA-900-4 demonstrate the ex-istence of abundant micropores, however, unfortunately,pore size distribution curve below 2.6 nm is undetectable.In addition, the BET SSA of SA-900-4 is 1103 m2 g−1,

and it is 992 m2 g−1 for SC-900, because of KOH activa-tion process leading to a higher porosity.

The full XPS spectra of OFC, SC-900, and SA-900-4are shown in Fig. 2e; all of them exhibit a strong C 1speak at 284.8 eV and O 1 s peak at 533 eV. Compared toOFC and SC-900, SA-900-4 has no Ti 2p peak at 459 eV,due to KOH activation leading to the disappearance of Tiatom. The C content of OFC is about 78.1%, and the C

Fig. 2 aXRD and b Raman spectra of the OFC, SC-900, and SA-900-4.cNitrogen adsorption–desorption isotherm and d pore size distribution ofSC-900 and SA-900-4 (the insert is the enlarge area of the original graph).

eXPS spectra of the OFC, SC-900, and SA-900-4. fHigh resolution XPSspectra of the C1s

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contents of SC-900 and SA-900-4 increase to 84.2 and90%, respectively, because of the pyrolysis and KOH ac-tivation process. The C1s spectra of the three samples are

shown in Fig. 2f; the peaks of them are all located nearly284.8 eV, mainly representing for the C–C and C = C[37].

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Electrochemical performance

The electrochemical performances of OFC, SC-X, andSA-900-X were carried out based on two-electrodesupercapacitor in 1 M H2SO4 electrolyte firstly.Figure 3a shows the cyclic voltammetry (CV) curves ofOFC at different scan rates. The galvanostatic charge/discharge (GCD) curves of OFC at different current den-sities were shown in Fig. 3b. According to GCD curve,the specific capacitance of OFC is very low, nearly19.2 F g−1 at current density of 1 A g−1. Then, in orderto learn the influence of heat treatment on electrochemicalperformance of OFC, the OFC samples were heat treatedat 700, 800, 900, and 1000 °C, respectively. As shown inFig. 3c, the CV curves of SC-700, SC-800, SC-900, andSC-1000 at the scan rate of 10 mV s−1 from 0 to 1 V areall rectangular shape, indicating that OFC shows an idealelectrical double-layer capacitor (EDLC) nature after heattreatment. As shown in Fig. 3d, the specific capacitancesof SC-700, SC-800, SC-900, and SC-1000 at current den-sity of 1 A g−1 are calculated to be 51.2, 59.4, 70.7, and68.3 F g−1, respectively, resulting with SC-900 giving thehighest specific capacitance. Therefore, it is obvious thatheat treatment can improve the electrochemical perfor-mance of OFC.

Additionally, the SC-900 was activated by KOH at800 °C (the mass ratios of SC-900: KOH are 1:2, 1:4,1:6, and 1:8, respectively, and named as SA-900-X; X isthe mass ratio). As shown in Fig. 3e, the CV curves ofSA-900-X at scan rate of 10 mV s−1 are all rectangularshape, which indicates the good EDLC property of SA-900-X. According to the GCD curves of SA-900-X atcurrent density of 1 A g−1 (Fig. 3f), the specific capaci-tances of SA-900-2, SA-900-4, SA-900-6, and SA-900-8were calculated to be 82.6, 122.8, 104.0, and 95.1 F g−1,respectively. It is obvious that the specific capacitances ofSA-900-X are higher than that of SC-900, and the SA-900-4 has the highest specific capacitance. As shown inthe results of XRD, nitrogen adsorption–desorption, andXPS, SA-900-4 has higher micro/mesopores, BET SSA,and C content than those of SC-900; therefore, SA-900-4exhibits superior electrochemical performance than SC-900. The rectangular shapes of CV curves of SA-900-4undergo no observable change from scan rate of 10 to

100 mV s−1 (Fig. 3g). As shown in Fig. 3h, the smallIR drops of triangular GCD curves of SA-900-4 fromcurrent density of 1 A g−1 to 20 A g−1 confirm the lowinternal resistance of SA-900-4. The specific capacitanceof SA-900-4 is 93.8 F g−1 at 10 A g−1 and 78.1 F g−1 at20 A g−1, nearly 76 and 63.6% of the value at 1 A g−1,respectively. Therefore, both CV and GCD results demon-strate the good rate capacity of SA-900-4 [38].

In order to improve the energy density of SA-900-4, 1 MNa2SO4 electrolyte was used to assemble two-electrodesupercapacitor with a wider operating voltage [6, 39]. Asshown in Fig. 4a, the CV curves of SA-900-4 at scan rate of10 mV s−1 can also keep rectangular shapes with voltageranges from 1.0 to 1.8 V. Figure 4b shows the GCD curvesof SA-900-4 at current density of 1 A g−1 with voltage win-dows from 1.0 to 1.8 V. The small IR drops of GCD curvesfurther indicate low internal resistance of SA-900-4 electrodein 1 M Na2SO4. Figure 4c shows the CV curves of SA-900-4with voltage window of 1.8 V at scan rates from 10 to100 mV s−1. All the CV curves keep rectangular shape, indi-cating the EDLC feature of SA-900-4 within 1 M Na2SO4

electrolyte. According to GCD curves of SA-900-4 at variouscurrent densities with voltage window of 1.8 V (Fig. 4d), thespecific capacitance of SA-900-4 is 81.2 F g−1 at a currentdensity of 1 A g−1 and 54.9 F g−1 at 10 A g−1. Therefore,the SA-900-4 keeps good rate capability in 1 M Na2SO4 elec-trolyte too.

Figure 5a shows the correlation of specific capacitances ofSA-900-4 with current densities in 1 M H2SO4 electrolyte,which demonstrates the good rate capability SA-900-4.Figure 5b shows the cycle performance of SA-900-4 at thecurrent density of 5 A g−1. It was calculated that, from the firstcycle to the 800th cycle, the specific capacitance of SA-900-4decreases dramatically from 106.4 to 73.5 F g−1, about 69.1%retention. However, when cycled to the 3000th, its specificcapacitance is nearly 73 F g−1, just 0.5 F g−1 lower than thatof the 800th cycle, and the 6000th cycle still has 70.1 F g−1.This result shows a relatively stable cycle performance of SA-900-4 after the 800th cycle

The electrochemical impedances of SC-900 and SA-900-4 were examined from 10 mHz to 100 kHz at opencircuit voltage in 1 M H2SO4 electrolyte. The Nyquistplots of both SC-900 and SA-900-4 exhibit a nearly ver-tical line within low-frequency region, indicating theirtypical EDLC capacitive behavior (Fig. 5c). The internalequivalent series resistances (RESR, the intercept at realaxis) are just 0.34 Ω for SC-900 and 0.47 Ω for SA-900-4, respectively (the inset in Fig. 5c). The low RESR

would attribute to good electron conductivity within theelectrode, leading to a high power density [40]. In addi-tion, there is no obvious semicircle at high frequency re-gion for SA-900-4, suggesting the low interfacial chargetransferring resistance in SA-900-4 electrode [41].

�Fig. 3 Electrochemical performance of all samples in 1 M H2SO4

electrolyte. a CV curves of OFC at different scan rates. b GCD curvesof OFC at different current densities. c CV curves of the OFC heat treatedat different temperatures at scan rate of 10 mV s−1. d GCD curves of theheat-treated samples at current density of 1 A g−1. e CV curves ofdifferent mass ratios of SC-900:KOH at 10 mV s−1. f GCD curves ofthe KOH-activated samples at 1 A g−1. g CV curves of SA-900-4 atdifferent scan rates. h GCD curves of SA-900-4 at different currentdensities

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Fig. 4 Electrochemicalperformance of SA-900-4 in 1 MNa2SO4 electrolyte. a CV curvesat different voltage windows at10 mV s−1. b GCD curves atdifferent voltage windows at1 A g−1. c CV curves at differentscan rates with voltage window of1.8 V. d GCD curves at differentcurrent densities with voltagewindow of 1.8 V

Fig. 5 a Correlation of specificcapacitances with currentdensities and b cycle performanceof SA-900-4, all of them weretested in 1 M H2SO4 electrolyte. cNyquist plots of SC-900 andSA-900-4, the inset shows thehigh frequency region of theimpedance spectra. d Ragoneplots of the SA-900-4

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The Ragone plots of SA-900-4 symmetric supercapacitorin different electrolytes are presented in Fig. 5d. At currentdensity of 1 A g−1, the energy density is 15.4 Wh kg−1 in 1 MH2SO4 and 32.9 Wh kg−1 in 1 M Na2SO4 with the voltagewindow of 1.8 V. The energy density retains 19.8 Wh kg−1 in1 M Na2SO4 at 20 A g−1, revealing the good energy densityperformance of SA-900-4. Table 1 compares the electrochem-ical performance of SA-900-4 with some reported carbon-based materials. It can be observed that SA-900-4 has a supe-rior power density and also a relatively good energy density.Therefore, it is a suitable candidate for electrode material ofsupercapacitor.

Conclusion

In summary, activated carbon fabricated from the waste filtercarbon in a household water purifier was explored to be theelectrode material for supercapacitor. The OFC gives poorspecific capacitance. However, the electrochemical perfor-mance of OFC dramatically increased after heat treatmentand KOH activation. In 1 M H2SO4 electrolyte, the SA-900-4 has a specific capacitance of 122.8 F g−1 at the currentdensity of 1 A g−1, resulting with the energy density of15.4 Wh kg−1and good rate capability. Moreover, SA-900-4shows even better energy density of 32.9 Wh kg−1 in 1 MNa2SO4 electrolyte, nearly two times higher than that in 1 MH2SO4 solution. This result demonstrates that the waste filtercarbon has the potential to be reused as the electrode materialfor supercapacitor. However, the cost of waste recyclingshould be taken into account. The energy consumption perunit mass of product is comparatively higher in our laboratoryexploratory study. If it is suitable for large-scale industrial

production, further research need be carried out to reduce theprocess cost.

Acknowledgments Q. Li appreciates financial support from theFundamental Research for the Central Universities of China (Nos.2013HGXJ0199, J2014HGXJ0092) and Specialized Research Fund forthe Doctoral Program of Higher Education (No. 20120111120009). Y.Zhu appreciates financial support from the China Government 1000Plan Talent Program, China MOE NCET Program, Natural ScienceFoundation of China (51322204), and Fundamental Research Funds forthe Central Universities (WK2060140014 and WK2060140017).

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