ZHAO Guang-yao, WANG Fang-cheng, LIU Ming-jie, SUI Yi-ming, ZHANG Zhuo, KANG Fei-yu, YANG Cheng,

（1. Institute of Materials Research, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China;

2. Department of Chemistry, Oregon State University, Corvallis, 97331-4003, USA）

Abstract： The rapid development of flexible electronics has produced an enormous demand for supercapacitors. Compared to batteries, supercapacitors have great advantages in terms of power density and cycling stability. They can also respond well on a time scale of seconds, but most have a poor frequency response, and behave more like pure resistors when used at high frequencies (e.g.,above 100 Hz). It is therefore challenging to develop supercapacitors that work at a frequency of over 100 Hz. We report a high-frequency flexible symmetrical supercapacitor composed of a MnO2@carbon cloth hybrid electrode (CC@MnO2), which is synthesized by the defocused-laser ablation method. This CC@MnO2-based symmetric supercapacitor has an excellent specific areal capacitance of 1.53 mF cm?2 at a frequency of 120 Hz and has good cycling stability with over 92.10% capacitance retention after 100 000 cycles at 100 V s?1. This remarkable electrochemical performance is attributed to the combined effect of the high conductivity of the 3D structure of the carbon cloth and the exceptional pseudo-capacitance of the laser-produced MnO2 nanosheets. The defocused laser ablation method can be used for large-scale production using roll-to-roll technology, which is promising for the wide use of the supercapacitor in high-frequency electronic devices.

Key words: High-frequency supercapacitors；Defocused-laser ablation method；Flexible electrode；Manganese dioxide (MnO2)；Carbon cloth

1 Introduction

Rapid increasing demands of the portable miniaturized electronics have encouraged the development of the energy storing devices, particularly the supercapacitors, which is attractive due to the advantages of fast charging/discharging rate, high power density, and long cycling life compared with batteries.Nevertheless, the supercapacitors perform weakly as filtering capacitors due to the serious drop of supercapacitors’ capacitance using alternating current (AC).Most supercapacitors always have poor frequency response when used at a high frequency (e.g., above 100 Hertz), and behave more like pure resistors[1].Therefore, the high-frequency supercapacitors (HFSCs) which mean they can work surpass 100 Hz with almost no thermosteresis have been a challenging task.

So far various researches have been carried out to realize the HFSCs. Milleret al.firstly fabricated vertically oriented graphene sheets with open pores on the nickel for a high-frequency supercapacitor, which delivered a specific areal capacitance (CA) of 0.2 mF cm?2and retained 0.09 mF cm?2at 120 Hz. To date, serval carbon composite materials and polymers such as carbon cloth (CC), reduced graphene oxide(rGO), and conducting polymers have been proposed for HFSCs because the high phase angles are high in the frequency range from tens to hundreds Hz, such as graphene-based HFSCs (80 μF cm?2at 120 Hz), carbon nanotubes-based HFSCs (601 μF cm?2at 120 Hz),and melamine-based HFSCs (132 μF cm?2at 120 Hz).Among them, the poor capacitances seriously limit their performance[2]. The area capacitances of these works are still not comparable with that of the commercial tantalum capacitors (1.5 mF cm?2, Samsung B3528)[1]. To solve this problem, one efficient way is toin situgrow pseudocapacitive materials on the CC.

Pseudocapacitive materials with high specific capacitances are emerging as a promising alternative/complement for the conventional double-layer-type materials. Among them, transition metal oxides(TMOs) are receiving the most interest owing to their particularly high theoretical specific capacitances,such as RuO3, Co3O4, and MnO2. Besides the high capacitance, MnO2outstands in the materials because of its low-cost and environment-benign properties.However, poor electrical conductivity and high charge-transfer resistance of MnO2seriously limit the specific capacitance and power characteristics[3].

In present study, MnO2nanosheets were grown on the CC by the defocused-laser ablation method.Compared with other ways (e.g.,electro-deposition[3]and hydrothermal[4]), the defocused-laser ablation method could not only reduce Mn(AC)2to MnO2on the CC, but had a great advantage in pattern and mass production. Besides, as-prepared LCC@MnO2symmetric supercapacitor exhibited the high CA of 1.53 mF cm?2at 120 Hz and excellent cycle stability(the capacity maintained over 92.10% after 100 000 cycles at 100 V s?1). Also, the method presents potentials on preparing flexible electrodes. The device based on the LCC@MnO2electrode showed a stable capacitance performance when bent in different angles(0°-180°) and good cycle stability (104.40% capacitance retention after 10 000 cycles at 100 V s?1).

2 Experimental

2.1 Materials

All chemicals were analytical reagents and used directly. The carbon cloth (CC, wos 1009) was obtained from Taiwan Tanneng company (thickness:0.41 mm, China). Manganous acetate (Mn(AC)2) was obtained from Aladdin. Sodium sulphate (Na2(SO)4)was obtained from Alfa Aesar. Deionized (DI) water was obtained from a Milli-Q system (Millipore).

2.2 Growth of MnO2 on CC

Inspired by the previous work, defocused laser induced graphene[5]helps to make the energy distribution uniformly. By changing the distance of z-axis to the focal plane, different spot sizes and energy distribution can be acquired. Using a suitable spot size, the processing speed will be increased and the risk of sample burning due to high temperature is also reduced[6]. This work involved this method to treat CC to make MnO2nanosheets generate on the surface.The carbon cloth was cut into pieces of CC (1×1 cm2)and then treated by infrared laser first to improve surface morphology to enhance wettability. 100 μL of 0.5 mol L?1Mn(Ac)2solution was dipped and coated on the CC and then the LCC/Mn(AC)2was dried in air for 3 h. After that, the dried materials were ablated through laser processing at a power of 4.2 W, a speed of 50 mm s?1, a step size of 1 064 nm, a diameter of spot of 141.47 μm, and a defocus distance of 10 mm to form LCC@MnO2composites. Then, the electrode was dried at 60 °C overnight. The illustration of the preparation of the LCC@MnO2electrode is shown in Scheme 1.

2.3 Structure characterization

Field emission scanning electron microscopy(HITACHI SU8010) was used to analyze the morphologies of LCC@MnO2. X-ray diffraction (Bruker D8 Advance) by CuKα radiation withλ=0.154 18 nm(The diffraction angle was from 10° to 85°, and the scanning rate was 5° min?1) was applied to characterize the crystallographic information of LCC and LCC@MnO2. Laser Microscopic confocal Raman spectroscopy (Horiba LabRAM HR800) was used to obtain the Raman spectra, The transmission electron microscopy (TEM) images were recorded by the FEI Tecnai G2 spirit and the LCC@MnO2was cut into some pieces and then dispersed to the supporting carbon films. The X-ray photoelectron spectroscopy(XPS) of the materials was tested by a PHI5000VersaProbeII.

All electrochemical measurements were carried out on the electrochemical station (CHInstruments,Inc., Shanghai). Cyclic voltammetry (CV), galvanostatic charging/discharging (GCD) were tested and electrochemical impedance spectra (EIS) of the studied electrodes were carried out from 100 kHz to 0.01 Hz. The LCC@MnO2electrode was examined by a traditional three-electrode system. The symmetric supercapacitor was measured by a coin cell system.The electrolyte was 1 mol L?1Na2SO4solution. During CV and GCD tests, the potential window of the LCC@MnO2electrode was from 0 to 0.8 V, the potential window of the symmetric device was from 0 to 1.6 V.

The CA values were obtained from the data of the CV curves using the following equation[1]:

WhereAis the area of the working electrode (cm2),vis the voltage sweep rate (V s?1), ΔVis the applied potential window, and ∫I(V)dVis the integral area of the CV curve.

The specific areal capacitance (CA, μF/cm2) at different frequencies was calculated by[1]:

Wheref(Hz) is frequency,Z″ (Ω) is the imaginary impedance, andSis the area of electrode.

3 Results and discussion

SEM is applied to investigate the morphologies of the LCC@MnO2electrodes (Fig. 1). In Fig. 1a and b, the surface of CC becomes rough after laser treatment and the diameter of the carbon fibers is about 20 μm. According to Fig. 1c and d, the MnO2nanosheets can be clearly seen on the carbon fibers.The energy-dispersive spectroscopy (EDS) mapping images indicate the uniform distribution of C, Mn and O elements in the LCC@MnO2composites (Fig. 1e).HRTEM image of LCC@MnO2illustrates that MnO2is successfully anchored to CC through the defocuslaser method. The lattice fringe is 0.45 nm, which is ascribed to the (101) plane about the MnO2[7]. All of this prove that the MnO2nanosheets are successfully grown on the CC.

The XRD patterns of CC and LCC@MnO2samples can be seen in Fig. 2a, the typical C peak can be seen at 2θ= 25.5°, which proves the existence of amorphous graphite carbon of the CC. The diffraction peaks of XRD pattern of LCC@MnO2demonstrate the presence of cubic phase α-MnO2(JCPDS no. 42-1169) and orthorhombic phase β-MnO2(JCPDS no.50-0866)[7-8]. The Raman spectra of CC and LCC@MnO2are shown in Fig. 2b. The two peaks of 1 350 and 1 600 cm?1represent theDandGpeaks of carbon, respectively. The photoinduced defect density is presented byID/IGratio. The value ofID/IGfor CC and MnO2@LCC is 1.05 and 1.17, respectively, which may be due to more defects formed after laser ablation. In addition, the peak of MnO2at 646 cm?1can be observed, confirming the successful preparation of MnO2[7].

Additionally, as shown in the XPS spectra in Fig. 3a, MnO2@LCC contains C, O and Mn elements compared to CC. The spectrum of C 1s (Fig. 3b) is fitted into two peaks at 284.8 eV and 286.1 eV,which are assigned to C―C and C＝C bonds, respectively. The spectrum of O 1s can be fitted into three peaks with Mn―O―Mn (529.9 eV), Mn―O―H(531.2 eV), and H―O―H (532.6 eV) bonds, as shown in Fig. 3c. From Fig. 3d, two peaks of 641.9 eV and 653.4 eV of Mn 2p spectrum are related to Mn 2p3/2and Mn 2p1/2of MnO2, respectively. The spin energy separation between the Mn 2p3/2and Mn 2p1/2is 11.5 eV, conforming to the reported studies about MnO2[7].

The performance of LCC@MnO2composite is evaluated in 1 mol L?1Na2SO4solution using a traditional three-electrode system. The CV curves of CC,LCC, and LCC@MnO2electrodes at 50 mV s?1suggest that CC and LCC contribute negligible capacitance in the LCC@MnO2composite electrode(Fig. 4a). As shown in Fig. 4b, although MnO2prepared on the carbon fibers increases the resistance, the resistances of these electrodes are still less than 5 Ω,indicating the excellent conductivity of LCC@MnO2composite. Fig. 4c and 4d show the CV curves and specific areal capacitance with various scanning rates.The CV curves display a rectangular shape even the scan rate increases to 300 mV s?1, showing excellent capacitive behavior. The CA value is 424 mF cm?2at 2 mV s?1. The GCD curves of LCC@MnO2electrode at different current densities are shown in Fig. 4e.They keep a nice linear shape, and the charging/discharging process keeps an excellent symmetry. A high CA value of 672.5 mF cm?2is achieved at 1 mA cm?2for LCC@MnO2composite. The capacitance of LCC@MnO2composite maintains 106.4% of the origin value after 8 000 cycles (Fig. 4f) as revealed by a cycling test at 100 mV s?1.

The electrochemical performance of a LCC@MnO2symmetric supercapacitor is evaluated in a coin cell using 1 mol L?1Na2SO4solution as the electrolyte. Fig. 5a and 5b demonstrate the CV profiles and specific areal capacitance with various scanning rates, respectively. The CV curves present a rectangular shape even the scan rate is increased up to 100 V s?1, showing distinguished high-frequency capacitive behavior. The CA is 1.5 mF cm?2at 100 V s?1. From Fig. 5c, the LCC@MnO2symmetric supercapacitor shows the best specific areal capacitance among the three symmetric supercapacitors at 100 V s?1. Fig. 5d shows a good conductivity of LCC@MnO2symmetric supercapacitor. Usually, in order to compare the high-frequency performance of a device, the cross-frequency at ?45° of the impedance phase angle is used as a key indicator[9]. For LCC@MnO2//MnO2@LCC, the cross-frequency is found to be 212 Hz (Fig. 5e), indicating a good highfrequency property. Furthermore, the symmetrical capacitor could deliver a CA of 1.53 mF cm?2at 120 Hz and a good cycle stability with over 92.10% capacitance retention after 100 000 cycles at 100 V s?1(Fig. 5f). This performance of LCC@MnO2material makes it promise for high-frequency applications,where the supercapacitor is required to charge/discharge at 120 Hz.

To evaluate the mechanical flexibility of the LCC@MnO2symmetric supercapacitor, bending tests are performed (Fig. 6a) at different bending angles(0°, 45°, 90° or 180°) at 100 V s?1. Consequently, the CV curves keep the constant shape, indicating that the favorable flexibility. CV testing at 100 V s?1for 10 000 cycles is conducted to assess the electrochemical stability of the electrode (Fig. 6b). This flexible supercapacitor displays distinguished cycle stability,which maintains 104.40% capacitance retention after cycling for 10 000 times. In a nutshell, this symmetric supercapacitor based on LCC@MnO2exhibits excellent flexibility and electrochemical performance.

4 Conclusion

We have put forward a fast strategy through defocused laser ablation for supercapacitors used at high-frequency. Because of the synergistic effects of the CC and the MnO2nanosheets, the LCC@MnO2symmetric supercapacitor exhibits an excellent CA performance of 1.53 mF cm?2at 120 Hz and excellent cycle stability (92.10% capacitance retention after 100 000 cycles at 100 V s?1), which have reached the standards of commercial tantalum capacitors(1.5 mF cm?2at 120 Hz). When encapsulated in the flexible device, the device shows an excellent flexibility （0°-180°） and stable cyclic stability (104.40% capacitance retention after 10 000 cycles at 100 V s?1).In view of the high flexibility and excellent high-frequency specific area capacitance, this electrode based on LCC@MnO2is believed to have huge potential in the applications of flexible, lighter, and faster electronic devices.

Acknowledgements

The authors thank the National Natural Science Foundation of China (52061160482), the Tsinghua University Spring Breeze Fund, the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111), Guangdong Provincial Key Laboratory of Thermal Management Engineering & Materials (2020B1212060015),Shenzhen Technical Project (JSGG20191129110 201725) and Shenzhen Geim Graphene Center for financial supports..