Preparation of activated carbon decorated with carbon dots and its electrochemical performance

Journal Pre-proof Preparation of activated carbon decorated with carbon dots and its electrochemical performance Su-Jin Jang, Yun Chan Kang, Kwang Chul Roh

PII:

S1226-086X(19)30581-7

DOI:

https://doi.org/10.1016/j.jiec.2019.11.003

Reference:

JIEC 4845

To appear in:

Journal of Industrial and Engineering Chemistry

Received Date:

26 July 2019

Revised Date:

31 October 2019

Accepted Date:

1 November 2019

Please cite this article as: Jang S-Jin, Kang YC, Roh KC, Preparation of activated carbon decorated with carbon dots and its electrochemical performance, Journal of Industrial and Engineering Chemistry (2019), doi: https://doi.org/10.1016/j.jiec.2019.11.003

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Preparation of activated carbon decorated with carbon dots and its electrochemical performance

Energy and Environmental Division, Korea Institute of Ceramic Engineering and Technology,

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Su-Jin Jang1,2, Yun Chan Kang2, and Kwang Chul Roh*1

Jin-ju 52851, Republic of Korea.

Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-

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Gu, Seoul 136-713, Republic of Korea

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*Corresponding author. Tel: +82-55-792-2625, Fax: +82-55-792-2580, E-mail: [email protected]

AUTHOR INFORMATION Corresponding Author

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* Tel: +82-55-792-2625, Fax: +82-55-792-2643, E-mail: [email protected]

Graphical abstract

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ABSTRACT Activated carbon decorated with carbon dots (ACD) was synthesized using an alkali

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activation process, in which metallic potassium expands the layer stacks and scissored turbostraticstructured carbon surface into nanosized particles. In addition, we demonstrated a capacitive effect

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in supercapacitors based on ACD, which has a high electrical conductivity because of the electronic pathway between the surface defects on activated carbon. The cyclic voltammogram

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curve was less distorted when the scan rate was increased to 100 mV s-1. Improved additional volumetric capacitance of up to 123.2 F cc-1 was achieved because the carbon dots provided additional capacitance resulting from intercalation into the graphitic carbon structure. The results

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obtained herein pave new ways for application of nanocarbons in energy storage.

Keywords: carbon dot; alkali activation; activated carbon; supercapacitor

1. Introduction

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Activated carbon (AC), a complex porous structure, is extensively used as an electrode material for supercapacitors and its materials have focused on porous carbon with a high specific surface area. Activated carbon is inexpensive and has a high surface area and good electrical conductivity; however, its surface defects reduce the electrical conductivity and volumetric capacitance[1-4], the latter of which is an important criterion affecting the use of supercapacitors in many applications[4-9].

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To address this problem, several studies have examined supercapacitor electrode materials not

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only with high specific surface areas to enhance the electrochemical performance of electrical double-layer capacitor (EDLC) but also with graphitizable structure to increase electrical

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conductivity and their specific properties[10-13]. For instance, the micro–meso pore structure of

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activated carbon nanochains is advantageous because of ion transport through the mesopores and increased capacitance due to the electrical double layers formed in the micropores[14]. Moreover,

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introducing porosity to the outer shells of carbon nano-onions can effectively improve the specific surface area by exposing the inner shells to electrolytes[15]. Our group reported activated carbon

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with partially graphitizable structure in activation process as rearrange structure to form graphitic. We are not yet studied conducting carbon free electrode to increase volumetric capacitance. Carbon dots, which consist of several nanosized particles or nanocrystals of quantum-confined graphene, are one of the best materials for achieving good electrical conductivity, high carrier

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mobility, and good dispersibility in a wide range of solvents[16]. There are many strategies for synthesizing carbon dots[16-23], which include top-down and bottom-up methods, both of which are well categorized and used in chemical synthesis. Carbon dot composites have been researched in detail by using laser ablation, plasma treatment, hydrothermal synthesis, and microwave synthesis technologies.

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Owing to the above mentioned properties, carbon dots have been targeted for use in applications involving photovoltaic devices, lithium ion batteries, and fuel cells since 2010; however, there have been only few reports on supercapacitors based on carbon dots[19, 20, 24-28]. Single electrode fabrication using graphene dots was reported by Jia et al.[29], who stated that the dots’ lower capacitance made them unsuitable for use as a single electrode material relative to other carbon materials. In addition, Liu et al.[25] suggested that graphene quantum dots are suitable for

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use in micro-supercapacitors and attributed the increase in capacitance to the quantum-sized

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graphene fragments. The properties of supercapacitor electrode materials, such as chemical stability, electrical conductivity, and surface area, can be modified by applying carbon dots to

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enhance the capacitive effect[19].

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In the present study, we suggest a new strategy for synthesizing carbon dots using the activation process, which has not yet been reported. Soft-carbon-type green cokes, which easily form a

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turbostratic structure at relatively low temperatures, were used as the carbonaceous precursors and were carbonized to prepare a mixed flexible layer to facilitate metallic K penetration and

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expansion. Given the importance of maintaining the balance between the SSA and electrical conductivity, the activated carbon was produced with a low KOH content relative to the KOH contents of commercial AC. The influence of the amount of KOH on the SSA and pore size distribution in the AC was also investigated, and the AC structures were analyzed using high-

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resolution transmission electron microscopy (HR-TEM). We also explored the effects of carbon dots on the electrochemical performance for supercapacitor using a conducting carbon free electrode point of view.

2. Experimental

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2.1. Preparation of activated carbon Carbonaceous precursors (GC-C) were obtained by the calcination of green coke at 600 °C for 1 h in a nitrogen atmosphere. Then, the GC-Cs were cooled to 25 °C and subsequently mixed with KOH at KOH/C weight ratios of 1.5, and 4, and were denoted as ACD, and GCA, respectively. The solid mixture was then ground to a fine powder in a port mill and heat-treated at 900 °C for 1

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h under a constant argon flow in a nickel reactor. The obtained ACs were washed with a 0.1 M hydrochloric acid solution and distilled water until the pH was 6.5, after which they were dried at

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120 °C. Finally, activated carbon decorated with carbon dots (ACD) and porous activated carbon

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2.2. Material characterization

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was obtained.

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The activated carbon decorated with carbon dots was studied using high-resolution transmission electron microscopy (HR-TEM) (JEOL, JEM-2000EX, Japan, FEI) (FEI company, Titan G2

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ChemiSTEM Cs Probe, Netherlands). The specific surface area and pore volume were calculated using the Brunauer Emmett Teller (BET) method and non-local density functional theory (NLDFT) (Belsorp-Mini II, BEL, Japan). The crystalline properties were investigated using X-ray diffraction (XRD) (Rigaku D/Max 2500/PC, Japan) with Cu Kα radiation (0.154 nm) operating at

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40 kV and 200 mA. High-resolution X-ray photoelectron spectroscopy (XPS) observations were conducted at the 8A2 beamline in the Pohang Accelerator Laboratory. The 4-points I-V measurement was used to determine the electrical conductivity by using a controlled environment sample holder (CESH).

2.3 Electrochemical measurements

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The oxygen reduction reaction (ORR) performance of ACD-1 and ACD-2 was tested with glassy carbon, platinum wire, and Hg/HgO as the working, counter and reference electrode, respectively, in a 0.1 M KOH electrolyte. The electrode was prepared by dropping 5 μL ink derived from 5 mg of the resulting sample in a 4 mL mixture of water, ethanol, and Nafion with a volume ratio of 75:21:4. We then performed linear sweep voltammetry (LSV) and rotating disk electrode (RDE) measurements in an O2-saturated electrolyte at 1600 rpm. The electrode materials for the two-

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electrode supercapacitors were prepared from activated carbon and polytetrafluoroethylene

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(PTFE) at a weight ratio of 95:5. The mixtures were pressed, using a roller press, to form sheets that were 150 µm thick. The carbon sheets were dried at 120 °C for 2 h. The electrochemical

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parameters of the activated carbon electrodes were evaluated using a 2032-type coin cell in a 1.0

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M solution of tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (AN). The test cell was charged to 2.7 V at a current of 1.0 mA cm-2 and then discharged to 1.0 V at the same current

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for galvanostatic charge–discharge. The volumetric capacitance was calculated from the slope of the discharge curves. The CV profile was obtained at 10 mV s-1 over a potential range of 0.0–2.7

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V. Electrical impedance spectroscopy (EIS) was performed at 0.1–100 kHz.

3. Results and discussion

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A series of AC samples were prepared from green coke through chemical activation using different amounts of KOH with carbonaceous precursors calcined at 600 °C. On some areas of the ACs, the carbon dots were seen to have dispersed across the surface of the AC after the activation process; the excellent crystalline structure of the carbon dots is favorable for increasing the electrical conductivity of the AC.

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For the mechanism of carbon dot formation, a previous study described a top-down method including decomposition of bulk carbonaceous materials into small pieces. In addition, alkali

(1)

CO2 + C → 2CO

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K2CO3 + 2C → 2K + 3CO

(3)

2O → 2K + CO

(4)

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K2CO3 → K2O + CO2

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activation process occurred, as indicated by the following equations [30]:

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Upon comparing the carbon dot formation and activation mechanisms, it is determined that

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similar reactions occur when metallic K expands the graphene layer stacks and then decomposes the epoxy bond and other functional groups into nanosized particles. Intermediate potassium

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compounds, such as K2CO3 and K2O, react with carbon and/or epoxy bonds on the carbonaceous surface during the activation process. As a result, the graphene layer swells and is simultaneously

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attacked by the intermediate gas product and metallic potassium. The carbonization condition is ideal for modifying the layer structure because relatively low temperatures and small KOH quantities produce a flexible structure. As shown in figure 1(e), parts of the graphene layer and carbon pieces are cut into carbon dots by breaking carbon and oxygen bonds of epoxy group and

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the remainder part undergoes a change in its surface to produce activated carbon. As this activation process is not controlled, non-uniform carbon dots are produced. [30].

The structure of the samples was observed in detail using HR-TEM (Figure 1). Activated carbon with carbon dot (ACD) was synthesized with a low KOH content, and GCA was used to compare

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the properties with those of typical soft-carbon-based AC. In figure 1(a), the carbonaceous sample has a turbostratic structure with irregular carbon domain. Figure 1(b) is a HR-TEM image of a typical amorphous carbon structure produced using a high content of KOH reagent (GCA). ACD with a range of turbostratic structures were observed on the surface of the AC. As shown in Figure 1(c), AC was decorated with non-uniform carbon dots around 5–10 nm in size; this was particularly obvious in the case of the ACD sample. Based on the HR-TEM images, the stacking of several

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graphene layers produces carbon dots with a lattice spacing of 0.26 nm, which is regarded as being

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typical of diamond (111). The corresponding hexagonal carbon lattice patterns observed in electron diffraction reveal the presence of sp2-bonded carbon frameworks with few defects (Figure

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1(d)).

To determine the elemental composition and chemical states of the carbon dot, the binding

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properties of the samples were further verified by X-ray photoelectron spectroscopy (XPS), as shown in Figure 2(a)(b). The C1s spectrum of ACD from 282.0–295.0 eV was fitted to extract the components of the peaks (Table 1), which could be assigned to the sp2 C=C, sp3 C-C, C-O/C-OC, C=O/O-C-O, and π-π* bonds[31]. The ratio of the C=C to π-π* transition bonds is related to

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the crystallinity. This ratio was higher for sample ACD than for GCA, which is consistent with the TEM results. The portion of the C-O bond is increased with the functional group on carbon dot edges and the activation partly destroyed the structural order of the sample. In general, the conductivity increases with decreasing concentration of surface oxygen functional groups. However, the electrical conductivity of ACD was observed to be higher than that of GCA (1.6 S

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cm-1) even though they contained similar concentration of oxygen (Table 2). More recently, the excellent electrical conductivity of carbon dots was characterized by Bhattacharjee et al.[27, 32] . X-ray diffraction patterns of activated carbon samples are shown in Figure 2(c). The XRD patterns of the graphite (002) and (100) plane reflections correspond to the broad peaks detected at a 2θ angle of about 24° and 42°, respectively. The estimated interlayer distances were calculated according to the Bragg equation. As the amount of KOH increased, the (002) peak weakened in

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intensity and shifted to a smaller angle.

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Raman spectra of the samples are shown in Figure S. The two characteristic peaks correspond to the disorder-induced bands (defects and disorder; D bands, ~1350 cm-1) and graphite bands (sp2

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carbon; G bands, ~1590 cm-1) of carbon. Raman spectra of GC-C represent a turbostratic

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structure[33]. The ID/IG ratios for ACD and GCA are 0.892 and 0.934, respectively. This indicates the extent of disorder in the atomic arrangement in ACD.

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The pore structure of the sample, BET surface area, pore volume (VT) and microporous size distribution are shown in Table 2 and figure S1. The isotherms were classified as type Ι according

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to the BDDT classification, indicating that the samples had microporous carbon. The results revealed that the GCA created porous carbon due to high metallic K content at the surface, which led to the formation of defects at activation. The specific surface area of ACD was calculated as 740 m2 g-1, which is lower than that of GCA (2050 m2 g-1). The pore-size distribution, derived

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using the nonlocal density functional theory (NLDFT), is shown in Figure 2 (f). The ACD exhibited a double peak induced network between carbon dots, which produces the synergetic effect to maintain a highly porous structure even with small amounts of KOH reagent[34].

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The capacitive performance of the samples was analyzed using cyclic voltammetry (CV), galvanostatic charge–discharge, and electrical impedance spectroscopy (EIS), by using a 2032type coin cell in a 1.0 M TEABF4 solution in AN. The EIS data was analyzed using Nyquist plots, where each data point corresponds to a different frequency (Figure 1(b)). The straight line in the low-frequency region of the Nyquist plot corresponds to an ideal EDLC. In the low-frequency region, the optimized capacitive behavior of the ACD samples is observed, which has a large SSA

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and therefore exhibits an increase in the electroactive surface area and ion-transport resistance, in

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addition to good crystallinity, which presents low resistance to electron transport. In the highfrequency region of the same plot, the small semicircle implies fast charge transfer related to the

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interfacial processes. The superior rate performance of the ACD is attributed to its relatively small

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ESR, since the power output of electrochemical supercapacitors is generally highly dependent on not only the rate of ionic mass transport but also the ESR.

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Cyclic voltammograms for the two-electrode coin-type cell, measured from 0.0–2.7 V at a scan rate of 10 mV s-1, are shown in Figure 1(a). To confirm the capacitive effect of the carbon dots,

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the electrodes were fabricated without any conductive additives. Nevertheless, the ACD electrode exhibited a capacitive effect that produced the near-rectangular curves for the EDLC behavior. The unique high current peak of ACD allowed easy intercalation into the stacked layer, providing additional capacitance and ion channels to the carbon dots, whereas long graphene layers in the

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GCA make it difficult to intercalate the electrolyte ions. The capacitance retention was investigated at different potential scan rates. Figure 2(c)(d) show the CV curves for all the samples recorded at scan rates of 10–100 mV s-1. With an increased potential scan rate of up to 100 mV s-1, the shape of the ACD curve remains nearly rectangular while the large resistance of GCA produces a highly distorted trace. The activated carbon with carbon dots has a higher conductivity or lower internal

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resistance than other composites, thus contributing to the capacitive effect. These trends among the sample are reconfirmed from the frequency responses obtained from EIS analysis. The Nyquist plot for the ACD is a small semicircle, causing the CV curve maintain its rectangular shape at a 100 mV s-1 scan rate, while the GCA (with low electrical conductivity) produces a large

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semicircular plot, resulting in the distorted trace at a 100 mV s-1 scan rate.

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The electrocatalytic performance of the carbon dots was examined by conventional threeelectrode LSV in a 0.1 M KOH solution with O2-saturation for the ORR[18]. The results of LSV

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are presented in Figure 4(a) . Obvious and strong reduction peaks at −0.1 V and −0.3 V are clearly

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observed for ACD in the O2-saturated solution. As expected, featureless LSV plots were observed for ACD over a weak and positive peak in the O2-saturated solution, suggesting its poor catalytic

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activity for ORR. This is a possible evidence that ACD has more carbon dots than GCA. In contrast to the inferior conductivity of ACD, GCA exhibits a high level of conductivity with a superior

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capacitive current in an O2-saturated solution.

Figure 4(d) presents the galvanostatic charge–discharge (GCD) behavior of GCA[35]. It was clear that the volumetric capacitance of the EDLC increased significantly during the first charging, which is consistent with the results of a previous study by Takeuchi et al.[8, 36]. The appearance

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of a gentle slope indicated that the TEA+ and BF4- ions in the electrolyte solution were intercalated into the graphene layer, which is a common phenomenon for AC based on soft carbon. This phenomenon is known as electrochemical activation (EA). The volumetric capacitance of a single electrode in a symmetrical two-electrode cell was calculated from the galvanostatic chargedischarge curve according to the following equation:

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C=

4×𝐼

(5)

𝑉×(𝑑𝑉/𝑑𝑡)

where C, I, V, and dV/dt represent the volumetric capacitance for a single electrode, the constant applied current, the active material volume of both electrodes, and the slope obtained from the discharge curve, respectively. The carbon dots on amorphous carbon lead to electrochemical

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activation, such that the volumetric capacitance of the ACD is found to be 123.2 F cc-1 in spite of its limited porous structure[37]. High capacitance retentions at increasing current densities and

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scan rates are observed in this electrolyte system (Figure 4(b)). In general, capacitance drop occurs

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for high microporosity (e.g., activated carbon). In case of the ACD, the capacitance slightly drops at a high current density such as 50 mA/g, which is likely related to its more hydrophilic surface

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properties owing to the presence of oxygen functional group in carbon dots, high electrical

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conductivity, and sufficiently large micropores for efficient ion transportation and adsorption according to NLDFT. The cyclability of ACD and GCA is respectively 95% and 88% at 10,000 cycles. The carbon dots create many electronic pathways between the surface defects on activated

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carbon. It could be increasing charge electronic transport pathways for enhancing electrochemical

performance.

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4. Conclusions

Carbon dots were successfully decorated on activated carbon by an alkali activation process, and

their diameters were mostly in the range of 5–10 nm. The high level of crystallinity achieved with the carbon dots enhanced the electrical conductivity because of the intercalation reactions in the graphitic structure, which also provided additional capacitance. With sample ACD, a high volumetric capacitance of 123.2 F cc-1 was realized, and cyclic voltammograms retained a near-

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rectangular shape even at high scan rates. In addition, the capacitive effect of the carbon dots resulted in the appearance of a redox peak in the CV profile. The process demonstrated herein represents a facile synthesis method that can be applied in several research areas.

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Author Contributions The manuscript was written based on contributions of all authors. All authors have given approval

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to the final version of the manuscript.

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ACKNOWLEDGMENT

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This work was supported by Energy Efficiency & Resources program of the Korea Institute of Energy Technology Evaluation Planning (KETEP), and was granted financial resources from the

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Ministry of Trade, Industry & Energy, Republic of Korea (No. 20152020105770).

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Figure 1. HR-TEM images of (a) turbostratic carbonaceous precursor 600 oC, (b) GCA, (c) carbon dots decorated on AC; (d) Layer structure of carbon dots and inset figure for Selected-area

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diffraction patterns (SADP).

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Figure 2. C1s peak in the XPS spectra of (a) GCA and (b) ACD; (c) XRD patterns of ACD, and

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GCA; (d) Raman spectra of GC-C, ACD, and GCA; (e) Curve-fitted Raman spectra of GC-C; (f) Non-local density functional theory (NLDFT) patterns for samples for the microporous size distribution.

20

of ro -p re

Figure 3 (a) Cyclic voltammograms (CV) for the two-electrode cell were measured at a sweep

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rate of 10 mV s-1 in TEABF4/AN in the potential range 0.0-2.7 V; (b) Electrical impedance spectroscopy data of ACD and GCA; CV profile of ACD (c) and GCA(d) in the rate range of 10

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ur na

mV s-1, 30 mV s-1, 50 mV s-1, and 100 mV s-1.

21

of ro -p re

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Figure 4. (a) Rotating disk electrode (RDE) voltammograms curves for ACD and GCA recorded in O2-saturated 0.1 M KOH at 10 mV s-1 at a rotation rate of 1600 rpm; (b) The volumetric

ur na

capacitance variation at different current densities of GCA and ACD; (c) Volumetric capacitance and coulombic efficiency of ACD and GCD; (d) The galvanostatic charge–discharge behavior of

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ACD;

22

Table 1. Components of the peaks and chemical states in C1s, fitted for ACD, and GCA

GCA

ACD

bond %

B.E.

%

C=C

284.2

28.7

284.0

53.1

C-C

284.7

42.4

284.7

18.0

C-O-C

285.8

12.9

285.7

10.1

C=O

286.9

5.7

286.9

7.8

O-C=O

288.2

6.0

288.2

5.8

π-π*

290.0

4.3

290.0

5.2

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of

B.E.

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Table 2. Pore characteristics and electrical conductivity of ACD, and GCA

Sample

SSAa

VTb

Vmicro

Vmeso

Vmacro

ECc

Cd

ACD

740

0.34

84.3

13.8

2.0

2.6

123.2

GCA

2050

0.95

78.9

18.9

2.2

1.6

87.6

SSA : specific surface area [m2/g]

b c

VT : total pore volume [cm3/g]

of

a

ECP : electrical conductivity [S/cm] CV : volumetric capacitance [F/cc]

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ur na

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re

-p

ro

d

24