lanthanum oxide nanocomposites as electrode materials of supercapacitors

lanthanum oxide nanocomposites as electrode materials of supercapacitors

Journal of Power Sources 419 (2019) 99–105 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loc...

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Journal of Power Sources 419 (2019) 99–105

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

The graphene/lanthanum oxide nanocomposites as electrode materials of supercapacitors

T

Jiaoxia Zhanga,∗, Zhuangzhuang Zhanga, Yueting Jiaoa, Hongxun Yangb, Yuqing Lic,∗∗, Jing Zhanga, Peng Gaod,∗∗∗ a

School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, 212003, China School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang, 212003, China c Testing Center, Yangzhou University, Yangzhou, 225009, China d Laboratory of Advanced Functional Materials, Xiamen Institute of Rare-earth Materials, Chinese Academy of Science, 1, Xiamen, 36102, China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

RGO/La O composites were prepared • by a simple heating reflux reaction. composites are fabricated • RGO/La2O3 as the electrode material in a super2

3

capacitor device.

combines high specific ca• Composite pacitance and high cycle stability.

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene Lanthanum oxide Nanocomposites Electrode materials Supercapacitors

In this paper, the nanocomposites of reduced graphene oxide/lanthanum oxide are prepared by a simple reflux process. The reduced graphene oxide with a large specific surface area is successfully decorated with lanthanum oxide. The reduced graphene oxide/lanthanum oxide composites are fabricated as the electrode material in a supercapacitor device which displays a high specific capacitance of 156.25 F/g at a current density of 0.1 A/g and excellent cycle stability. The material keeps 78% of its initial charge-discharge efficiency after 500 cycles. The excellent electrochemical performance of composite material may be attributed to the deposited lanthanum oxide nanoparticle on the surface of reduced graphene oxide, which increase the effective conductive area of reduced graphene oxide and the contact area between electrolyte and graphene. The graphene-lanthanum oxide composites can significantly improve the stability and electrical performance of supercapacitors and has great potential in chemical sensors, microelectronics, energy storage and conversion applications.

1. Introduction As an efficient energy storage devices, supercapacitors have attracted significant attention due to the excellent performances such as

high power density, fast charging/discharging rates, and long life-cycles, which make them one of the most promising candidates for nextgeneration power devices [1–5]. However, the energy density of existing supercapacitors is one order of magnitude lower than that of



Corresponding author. Corresponding author. ∗∗∗ Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (Y. Li), [email protected] (P. Gao). ∗∗

https://doi.org/10.1016/j.jpowsour.2019.02.059 Received 19 December 2018; Received in revised form 16 February 2019; Accepted 18 February 2019 0378-7753/ © 2019 Elsevier B.V. All rights reserved.

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(25%–28%, AR) and Ethyl alcohol absolute were purchased from Shanghai Richjoint Chemical Reagents Co., Ltd. Potassium hydroxide (KOH, AR) was purchased from Jiangsu Qiang Sheng Functional Chemical Co., Ltd. Acetylene black (Battery level) and PTFE (60%, AR) were purchased from Shijiazhuang Aokai Zhuote Instrument Technology Co., Ltd.

batteries [6]. It is known that the electrode materials play a vital role in the development of high-performance supercapacitors [7–10]. A lowcost and stable electrode material with right electrochemical property and high specific power [5,11–13] is essential for the application of supercapacitors. Graphene with outstanding electrical conductivity and a large specific surface area exhibits excellent energy storage ability and attracts much attention in the field of the supercapacitor research [14–19]. In 2008, Stoller et al. [20] reported supercapacitor with graphene as electrode showing higher specific capacitance than carbon nanotubes. The specific capacitance of graphene-based supercapacitor was 135 F/g (in water electrolytes) and 99 F/g (in organic electrolytes) respectively. However, there are drawbacks with the supercapacitors based on single component electrodes, such as low capacitance of carbon materials [21], instability of conductive polymers [22], and high electrical resistance of transition metal oxides [23]. Therefore, composite electrode materials composed of carbon (e.g., carbon nanotube or graphene), conductive polymers (e.g., polyaniline (PANI) and polypyrrole (PPy)), or transition metal oxides (e.g., Co2O3, MnO2, and Fe3O4) have attracted tremendous attention [24–28]. In recent years, many graphene/metal oxide composites emerged as potential supercapacitor electrode materials with very promising electrochemical property [25,29–31]. For example, Cui and colleagues [32] fabricated a composite electrode with Mn3O4 and graphene by a two-step solvothermal method showing improved specific capacity and cycling performance. As one of the most abundant products of light rare earth, lanthanum oxide (La2O3) has excellent physical and chemical properties, it can be used to manufacture ceramic capacitor, high precision optical glass and high refraction optical fiberboard for camera, microscope lens and prism of advanced optical instrument, etc [33–35]. There have many reports about used transition metal oxide as electrodes for supercapacitor [36–39]. However, it is difficult to find about lanthanum oxide in energy storage areas. So in this work, we report a simple approach to prepare the reduced graphene oxide/lanthanum oxide (RGO/La2O3) composite, in which La2O3 nanoparticles are homogeneously loaded onto the RGO sheets during the process of forming reduced graphene oxide. The composite was evenly mixed with conductive carbon black and the binder to obtain the mud sheet. Finally, the electrode was prepared by pressing the composite sheet onto the nickel foam. A potentiostat was used to test the AC impedance and cyclic voltammetry curve of the composite electrode. The circular electrode sheet was assembled into a button type supercapacitor according to Fig. 1 to evaluate the charge/discharge efficiency and cyclic stability.

2.2. Synthesis of RGO/La2O3 nanocomposites 2.2.1. Synthesis of GO GO was synthesized from natural graphite flakes using a modified Hummers method [40]. Briefly, graphite flakes (5 g) were mixed with concentrated H2SO4 (7.5 mL, 98%), K2S2O8 (0.5 g) and P2O5 (0.5 g). The resulting mixture was stirred continuously and heated to 80 °C for 6 h, then gradually cool down to room temperature. The pre-oxidized graphite was obtained after dilution, filtration. Then the raw product was washed thoroughly until neutral and naturally dried at room temperature. Then the pre-oxidized graphite (1 g) and NaNO3 (0.5 g) were mixed with H2SO4 (23 mL, 98%) and stirred for 15 min in a 500 mL beaker immersed in an ice bath. KMnO4 (3 g) was slowly added to the above mixture in six portions, and the ice water bath was maintained at 15 °C, and the reaction was continued for 2.5 h with a dark green color. Then, warm deionized water (45 mL) was slowly added to the reaction liquid, and the temperature was maintained at about 98 °C for 15 min 35% H2O2 (8 mL) was then added to consume the unreacted permanganate. The resulting reaction solution was washed with HCl (5%) and copious deionized water to remove residual salts. After three days of dialysis, the resulting brownish yellow precipitate was ultrasonically dispersed in an appropriate amount of deionized water for 1 h. The solution was centrifuged at a speed of 5000 rpm for 15 min. Finally, the upper layer solution was freeze-dried to obtain GO. 2.3. Synthesis of reduced graphene oxide (RGO)/La2O3 nanocomposites A 100 mL aqueous dispersion of GO (0.5 mg/mL) and a 20 mL aqueous solution of La(NO3)3 (50 mg/mL) were mixed with 100 mL of hydrazine monohydrate solution by sonication for 30 min [41]. Then the resulting mixture was sealed in a three-necked flask and refluxed at 100 °C for 12 h. Finally, the as-prepared graphene/La2O3 nanocomposite was obtained after filtration and vacuum drying at 100 °C for 24 h. The reaction scheme is described in Fig. 2. For comparison, unmodified RGO was synthesized by the same procedure in the absence of La (NO3)3.

2. Experimental section 2.4. Characterization 2.1. Materials The structure of the samples was examined by X-ray diffraction (XRD-6000, Japan Shimadzu Corporation) with Cu Kα radiation (λ = 1.54 Å), scanning from 10° to 80° and using an operating voltage

Graphene oxide (GO) was made in-house. Lanthanum nitrate (La (NO3)3, AR), Hydrated hydrazine (80%, AR), Ammonium hydroxide

Fig. 1. Diagram showing the assembly of button type supercapacitor. 100

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Fig. 3. The SEM and TEM images of (a), (b) RGO and (c), (d) RGO/La2O3.

3. Results and discussions Fig. 2. Synthetic schematic of RGO/La2O3 composites.

Fig. 3 shows the SEM and TEM images of RGO and RGO/La2O3. As shown in Fig. 3a, RGO has an apparently layered structure and surface folds. The corresponding TEM images in Fig. 3b shows highly transparent folded and wrinkled RGO sheets implying a low number of layers. The pure La2O3 exhibits a diameter of about 19 nm as shown in Fig. 3c–d. The La2O3 nanoparticles in RGO/La2O3 are homogeneously and densely wrapped on the RGO surface (see Fig. 3c). TEM image in Fig. 3d disclose that the wrapped La2O3 are rod-like nanoparticles with a diameter of around 19 nm and decorate homogeneously on the surface of graphene. It is evident that the wrinkle texture becomes less prominent after the loading of lanthanum oxide, which can retard the wrinkle and agglomeration of RGO and increase its specific area [42]. Raman Spectroscopy is a convenient and effective tool for characterizing the structure of carbon-containing materials. The Raman spectra of GO (i), RGO (ii) and RGO/La2O3 (iii) are shown in Fig. 4a. The structural regularity of carbon nanomaterials is usually expressed by the intensity ratio of the D band versus the G band (ID/IG) [43–45]. The lower the ID/IG value, the more regular the carbon nanomaterials. For example, graphite has an intense G band at 1570 cm−1, indicating an intact structure of graphite. For GO in Fig. 4a(i), the D band which is caused by C-C disordered vibration in the sp3 hybrid structure locates at 1350 cm−1. The G band at 1593 cm−1 is caused by the stretching vibration of C=C characterizing sp2 hybridized carbon structure. RGO also has D and G bands as shown in Fig. 4a(ii), the intensity ratio (ID/ IG = 0.67) of which is lower than that of GO (ID/IG = 1.24), indicating higher content of sp2 hybridized C than that of sp3 hybridized case [46,47]. This illustrates that some the sp3 hybridized C atoms evolved into sp2 hybridized C atoms during the reduction process. The ID/IG value of RGO/La2O3 (calculated from Fig. 4a(iii)) is 1.33, which is much higher than that of RGO (ID/IG = 0.67). This is mainly due to the decreased regularity of RGO after the loading of lanthanum oxide, which is another indication that the lanthanum-metal oxide has been successfully loaded on graphene surface and crystalline of graphene is decreased. From Fig. 4b(i), the characteristic (001) diffraction peak of GO appears at around 10° [44]. The broad characteristic peak in Fig. 4b(ii) at 24.8° belongs to (002) plane of to RGO, and the diffraction peak from GO at 11° disappeared. For the RGO/La2O3 nanocomposite in Fig. 4b (iii), the diffraction peaks appearing at 28.6°, 39.5°, 48.1°, and 55.4° are

and current of 40 kV and 30 mA, respectively. The surface groups on the RGO/La2O3 were investigated by Fourier transform infrared (FTIR) spectrometer (FTS2000) using KBr pellets in the range 400–4000 cm−1. X-ray photoelectron spectroscopy (XPS) investigation was performed with the ESCALAB 250Xi (Thermo Fisher Scientific) using Al K Alpha (1361 eV) as an excitation source. The products were characterized by transmission electron microscope (TEM) (JEM-2001F, Holland Philips Company).

2.5. Electrochemical measurement The working electrodes were prepared by pressing mixtures of the as-prepared powder samples, acetylene black and polytetrafluoroethylene (PTFE) binder (weight ratio of 80:10:10) onto a nickel foam current collector. The effective mass of electroactive materials on the working electrode was obtained through substracting the weight of bare nickel foam from the weight of nickel foam pressed with the mixtures above. A typical three-electrode experimental cell was assembled to measure the electrochemical properties of the working electrode. A platinum foil and a saturated calomel electrode were used as the counter electrode and reference electrode respectively. All electrochemical measurements were carried out in 3 M KOH aqueous solution as an electrolyte on a CHI660D potentiostat. The cyclic voltammetry (CV) tests were performed at a potential window of -1 - 0V with different scan rates. The galvanostatic charge/discharge curves were also measured from -1 - 0V. The specific capacitance Cm (F/g) was calculated by means of galvanostatic charge-discharge cycles following the equation:Cm=It/(mΔU'Cm=It/(mΔU′)Cm = It/(m ΔU ′) , where I is the charge/discharge current, t is the discharge time, m is the mass of the active materials on each electrode, ΔU′ is potential window deducting the internal resistance (IR) drop. The energy density and the power density were calculated using the equations: E= Cm ΔU ′ 2/7.2 for energy density and P= 3.6E/t for power density. EIS measurements were done in a frequency range from 105 Hz to 0.1 Hz at open circuit potential with an alternating current perturbation of 5 mV.

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Fig. 4. (a) The Raman spectra of (i) GO, (ii) RGO and (iii) RGO/La2O3; (b) XRD patterns of (i) GO, (ii) RGO, (iii) La2O3 and (iv) RGO/La2O3; (c) FTIR spectra of (i) GO, (ii) RGO and (iii) RGO/La2O3.

RGO. The C/O ratio of GO (8.48) is significantly higher than that of GO (2.08), which indicates that the amount of oxygen-containing functional groups in graphene oxide is decreased. The XPS spectrum of the RGO/La2O3 nanocomposites (Fig. 5a(iii)) shows the signal of La 3d and the rebounded oxygen content (C/O = 1.35), suggesting that the lanthanum-metal oxide was successfully loaded onto the surface of graphene. The increase in the oxygen content agrees with previous experiments. Enlarged regions are shown in Fig. 5b–d. For GO (Fig. 5b), the fitted positions of the C1s binding energies are 286.52 eV, 285.34 eV, 284.48 eV, and 283.4 eV, which represent -COO-, C=O, CO/-OH, C-C, respectively [41,48]. As shown in Fig. 5c, four C1s binding energy peaks of RGO locate at 287.5 eV (-COO-), 285.34 eV (C=O), 284.6 eV (C-O/-OH) and 284 eV (C-C), respectively. It is evident that the oxygen-containing groups (-COO-) in RGO are significantly decreased. However, in the C1s spectra of RGO/La2O3 (Fig. 5d), only two peaks corresponding to C-O/-OH, C-C appear at the binding energies of 284.01 eV and 283.15 eV, while these peaks representing -COO- and -C=O- disappear. This may be because the GO in the composite RGO/ La2O3 was largely reduced, and the lanthanide oxide is successfully loaded onto the graphene surface. The AC impedance measurement is a fundamental method to study the electrochemical behavior of a device by the Nyquist charts. The

consistent with the characteristic peaks of La2O3 (JCPDS 5–602) as shown in Fig. 4b(iv) corresponding to the (101), (102), (110) and (112) planes of loaded lanthanum-metal oxide. In the FTIR spectrum of GO (Fig. 4c(i)), there is a broad -OH absorption peak at near 3400 cm−1 and a stretching vibration characteristic absorption peak of C=O at 1644 cm−1. The peak at 1402 cm−1 is the stretching vibration absorption of the conjugated C=C, and the absorption peak at 1107 cm−1 corresponds to the deformation vibration of the C-O single bond [41,48]. The peak near 3400 cm−1 is reduced in RGO (Fig. 4c(ii)) compared with GO, and the same trend is observed in the stretching vibration peak of the C=O double bond at 1644 cm−1, which indicates the successful reduction of graphene oxide. At the same time, the deformation vibration of the C-O located at 1107 cm−1 also decrease significantly, further confirming the decrease of oxygen content in graphene. After the loading of La2O3 on RGO, the characteristic absorption peaks at 3400 cm−1, 1644 cm−1, and 1107 cm−1 are all weakened (Fig. 4c(iii)). However, the residual oxygen-containing groups make these peaks stronger than the case of RGO. It could be due to the loaded lanthanum oxide on the surface of the RGO and the GO is only partly reduced. XPS was also performed on GO, RGO, and RGO/La2O3 samples. Fig. 5a (i) and (ii) show the O1s and C1s XPS signal peaks of GO and

Fig. 5. The XPS spectra of (a) O1s, C1s, and La3d for i. GO, ii. RGO, and iii. RGO/La2O3; (b) C1s XPS spectra of GO, (c) RGO and (d) RGO/La2O3. 102

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electrode material. The composite electrode RGO/La2O3 exhibits minimal internal resistance (0.5Ω), indicating that the electron transfer resistance in the electrode material is small [51]. It also shows that the RGO/La2O3 composite has a small internal resistance. The electrode based on RGO also has a low internal resistance (1.5 Ω), but La2O3 does not form a semicircle in the high-frequency region which indicates that its transfer resistance is relatively large and the electron transfer/ion transport is retarded. In addition, the curve of the low-frequency region implies the capacitive behavior of the electron materials. The linear parts are nearly perpendicular to the X-axis for the RGO/La2O3 and RGO materials indicating the excellent capacitive behavior, representing fast ion diffusion and adsorption in/on the electrode material. However, the capacitance of the RGO/La2O3 composite is higher than that of individual La2O3 and RGO. The appropriate amount of La2O3 loaded on the surface of the RGO reduces not only the agglomeration phenomenon of RGO but also defects on RGO leading to lower internal resistance for the RGO/La2O3materials. Fig. 6b–c shows the cyclic voltammetry behavior of La2O3, RGO, and RGO/La2O3 electrodes respectively at a scan-speed of 100 mv/s. It can be seen from Fig. 6b that the curves do not have redox peaks, which are typical electrical double layer capacitances [52]. Moreover, the CV curve of the RGO/La2O3 composite electrode displays a rectangle with minimal curve deformation. Fig. 6c shows the CV curve of the RGO/ La2O3 materials at different scan speeds. Throughout the scanning rate range, the CV curve presents a symmetrical rectangular-like shape. The corresponding current increases firstly, but begins to decrease when the scanning rate reach more than 100mv/s and the CV curve has little change in shape. The CV curve is still similar to the rectangle when the scanning speed is increased to 200 mv/s, indicating that RGO/La2O3 nanocomposites electrode has good reversibility and excellent charge transport and ion transport. At the same time, the CV curve test also shows that the composites electrode prepared by loading lanthanum metal oxide onto the surface of graphene has good rate performance and low internal resistance. Fig. 7a shows the result of charging and discharging curves of RGO and the RGO/La2O3 composite electrodes at a current density of 0.1 A/g using a three-electrode system with a potentiostat. The charge-discharge window is selected from −1 V to 0 V. The triangle shape of the charge-discharge curve exhibits the typical double layer capacitor behavior. Fig. 7b is the charge-discharge curve of RGO/La2O3 composites electrode under different current densities. Fig. 7c is the Ragone diagram depicting the relationship between the energy density (E, Wh/kg) and the power density (P, kW/kg) obtained by charging and discharging the composite electrodes at constant current density. The nanocomposites electrode revealed different mass-specific capacitance values, power densities, and energy densities, as shown in Table 1.

Fig. 6. (a) Nyquist plot of La2O3, RGO/La2O3, RGO (the insert is a magnified high-frequency area; (b) Overlapped CV curves of RGO, RGO/La2O3, La2O3 at 100 mv/s; (c) Overlapped CV curves of RGO/La2O3 at different scanning speeds.

Nyquist charts of La2O3, RGO, and RGO/La2O3 nanocomposites are shown in Fig. 6a. The Nyquist plots are obtained by plotting the imaginary part (reactance) against the real part (resistance) of the impedance at different frequencies. The Nyquist plots are divided into four sections: X-axis intercept, the high-frequency region (lower left corner), intermediate frequency area and the low-frequency region (upper right corner). The X-axis intercept represents the equivalent series resistance (ESR) of the electrode, including the electrode, electrolyte, internal resistance of the collector and interface contact resistance [49,50]. The equivalent series resistance of the composite RGO/La2O3 and the RGO electrode is 0.45 Ω as shown in the intercept in Fig. 6a inset. The diameter of the semicircle corresponds to the internal resistance of the

Fig. 7. (a) Galvanostatic charge/discharge curves of RGO, RGO/La2O3 composites at a current density of 0.1 A/g. (b) Galvanostatic charge/discharge curves of RGO/ La2O3 composites at different current density. (c) Ragone plots of the button-type capacitor of RGO/La2O3. 103

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which affects the charging and discharging process. Fortunately, when the charge and discharge times are 500 times, the charge and discharge efficiency is still more than 78%, indicating that the composite material has excellent capacitance efficiency as a supercapacitor electrode.

Table 1a Specific capacitance of different composite materials at 0.1 A/g current density. Different electrode materials (0.1A/g) Mass-specific capacitance (F/g)

RGO 138.89

La2O3 low

RGO/La2O3 156.25

4. Conclusion Table 1b specific capacitance of RGO/La2O3 at a different current density. Different current density(A/g) Mass-specific capacitance (F/g) Power density (kW/kg) Energy density (Wh/kg)

0.1 156.25 1.56 21.7

0.5 109.38 7.81 15.19

1 93.75 15.62 13.02

2 62.50 31.25 8.68

We present a simple and convenient method for preparing a nanoscale RGO/La2O3 composite. In the condensation reflux process, graphene oxide is reduced while lanthanum oxide is loaded onto its surface. In a button-shaped supercapacitor device, the cyclic voltammogram tests indicate that the composite material has excellent rate performance and low resistance. At the same time, the RGO/La2O3 composite electrode has the most substantial mass-specific capacitance at a charge/discharge current density of 0.1 A/g and a specific capacitance of 156.25 F/g, which is larger than the specific capacitance of pure RGO (138.90 F/g). The loading of lanthanum prevent the aggregation of graphene sheets and lead to a more extensively conductive surface, which benefits the process of ion adsorption/desorption and improves the capacitive property of the composite electrode. Moreover, when the power density is increased to 15.62 kW/kg, the energy density value reached 13.02 Wh/kg, indicating the potential of this composite material in small-current double-layer capacitors. Stability test of the RGO/La2O3 composite electrode shows more than 95% retention of the initial charge-discharge efficiency after 100 recycles and after 500 cycles, the material still maintain 78% of the charge-discharge efficiency. We believe that the facile synthesis approach presented here may pave the way for further application of graphene-lanthanum oxide composites in chemical sensors, microelectronics, energy storage and conversion applications.

3 43.20 43.2 6

Fig. 8. (a) The discharge capacity and capacitance efficiency in the first 100 cycles (0.1 A/g), and (b) the cycle stability of RGO/La2O3 at a current density of 0.1 A/g (Cycles 500).

5. Notes

As can be seen from Table 1a, the mass-specific capacitance of the RGO/La2O3 composite electrode is 156.25 F/g, which is larger than that of RGO (138.89 F/g). In order to evaluate the stability of supercapacitor, the voltage drop in the charge-discharge process was discussed. As Fig. 7a showed, the IR drop of the RGO/La2O3 composite electrode is small and negligible indicating excellent rate capability and low resistance. In Table 1b, the RGO/La2O3 electrode has the most substantial mass-specific capacitance at the current density of 0.1 A/g, reaching 156.25 F/g. At the same time, the mass-specific capacitance of RGO/La2O3 composites at the current density of 0.5 A/g is dropped to 104.17 F/g. At a current density of 0.1 A/g, the button-type supercapacitor shows a high energy density of 21.7 Wh/kg. When the current density increases to 1 A/g, the power density increases from 1.56 to 15.62 kW/kg, and the corresponding energy density is 13.02 Wh/kg. It can be concluded that the energy density retention rate is 60%. These results indicate that the composite material obtained by loading metal lanthanum oxide on the graphene surface has excellent conductivity and electrochemical stability. The assembled button cell based on composite RGO/La2O3 electrode is schematically shown in Fig. 1, which was tested in a blue electric test system while recording discharge diagram for 100 times at a current density of 0.1 A/g. As shown in Fig. 8a, with the increase of the chargedischarge cycles to 100 times, the discharge capacity decrease to 75 mAh/g. This is because the internal resistance of the material increases with the increase of charge-discharge cycles. Meanwhile, in Table 1b, the RGO/La2O3 supercapacitor has an energy density of 21.7 Wh/kg and a power density of 1.56 kW/kg at the current density of 0.1 A/g, indicating excellent Power characteristics. Fig. 8b is cycling stability of RGO/La2O3 at the current density of 0.1A/g from 80 to 500 cycles. The results show that the charge-discharge efficiency is always more than 95% at the first 300 times of constant current charge and discharge. With the further increase of charge and discharge times, the efficiency will decrease, which is due to the increase of charge and discharge times leads to a certain degree of shedding of active electrode materials,

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