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Solution-Processed Nano-Structured Manganese Oxide/Carbon Cloth Composite for Flexible High-Performance Supercapacitors

Z.L. HUANG^(a,b), X. ZHAO^(a,b), Q.H. ZHANG*^(a,b)

^(a) College of Material Science and Engineering, Donghua University

Shanghai, People’s Republic of China, hfalsjt@163.com

^(b) State Key Laboratory for Modification of  Chemical Fibers and Polymer Materials,

Shanghai, People’s Republic of China, qhzhang@dhu.edu.cn, xzhao@dhu.edu.cn,

 

ABSTRACT —In this work, nano-structured manganese oxide (MnO₂) was grown directly on the treated carbon cloth (CC) through a simple hydrothermal method. The structure and thickness of MnO₂ film were controlled by the PH value of solution and reaction time. The morphology of MnO₂ deposited on CC was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the phase structure was confirmed by Raman, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Owning to the formation of MnO₂ thin film morphology, porous structure and the intimate contact between the electroactive material , the MnO₂/CC composites demonstrated a high specific capacitance of 377 mF/cm² at a current density of 1 mA/cm² , and the specific capacitance still remained about 89% of its initial value after continuous 1000 charge/discharge cycles. The excellent performance of MnO₂ is attributed to the short ion diffusion path and good electron transport environment offered by CC.

1.       Introduction

Nowadays people are facing the challege of energy and environment problems, to solve the environmental pollution problem which is brought by petrochemical energy industry, It is very emergent that to develop renewable and green energy storage devices With the raid development of electronic and energy technology, flexible and environmental friendly energy storage devices have received great interests . Among all the energy storage materials, manganese oxide (MnO₂)has been considered as one of the most promising candidates due to its low cost, facile synthesis, environmental friendly and high pseudocapacitance performance^[1]. In this work, nano-structured MnO₂ was grown directly on the treated carbon cloth (CC) through a simple hydrothermal method , and its electrochemical properties have been investigated

2.       Methods 2.1 Synthesis of the MnO₂ / CC

The carbon cloth (CC) substrate was treated bythe nitrogen plasma etching process and then immersed in concentrated sulfuric acid for 2h to improve  the surface hydrophilic properties. The hydrothermal reaction solution was obtained by dissolving 5 mM potassium permanganate (KMnO₄) in 70 mL distilled water under constant magnetic stirring and the PH value was adjusted by 5M H₂SO₄. Then the solution was transferred into Teflon-lined stainless steel autoclave. The obtained treated CC was cut into rectangular straps with length of 40 mm and width of 20 mm,  threaded by platinum wire and placed standing against the wall of Teflon-lined autoclave. The sealed autoclave was transferred into vacuum oven and kept at  140℃ for 0.1h, 0.25h, 0.5h, 1h and 2h, respectively. After the reaction was over, the CC coated with dark brown products was taken out and washed by centrifugation with deionized water in order to remove the residue acid liquor, and then dried in an oven at 70℃ for 12 h.

2.2 Characterization

Field emission scanning electron microscopy (FESEM, Hitachi S-4800) was employed to characterize the morphologies of samples. X-ray diffraction (XRD Cu Kα radiation λ=0.15406 nm) measurements were conducted on a Rigaku D-max-2500 diffractometer to analysis the crystal structures. XPS analyses were conducted with a  5000C ESCA System (Mg Ka hn=1253.6 eV) to analysis valence state of element. Raman spectra were using a LabRam-1B Raman spectroscope with He-Ne laser excitation at 632nm. The electrochemical measurements of CC/ MnO₂ were carried out in a three electrode electrochemical cell containing 0.5 M Na₂SO₄ aqueous solution as the electrolyte, a Pt foil as counter electrode and standard calmel electrode （SCE）as the reference electrode at room temperature by PGSTAT 128N AutoLab electrochemical workstation. Cyclic voltammetry (CV) measurements were performed at a scan rates of 50 mV/s across a potential range between -0.2 and 0.8 V , the galvanostatic charge/discharge were measured between 0 and 0.8 V by 0.5 mA current and the Electrochemical impedance spectra were measured at the open circuit potential in a frequency range from 10KHz to 1Hz with an AC perturbation of 5 mV.

3.       Results

Nano-structured MnO₂ were grown directly on the surface of treated CC though a simple redox reaction between KMnO₄ and CC which is described as follow^[2] :

(1)

The CC served not only as the subtract but also a regent, which can enhance the adhesion  between MnO₂ and CC. The morphologies of MnO₂ film were confirmed by FESEM. It is obvious shown that the MnO₂ film became much thicker with the reaction time increasing, and the nano-flower shaped MnO₂ particles changed to nano-rod shape. The nano-flower structured MnO₂ were porous as shown in the high magnification SEM image, which might have larger specific surface area that is beneficial to the solution diffusion and adsorption in the electrochemical reaction. The weak peaks in XRD curves of MnO₂/CC in Fugure 1(a) at  2θ=18.25°、29.66°、37.16° can be attributed to the (2 0 0)(1 3 0)(2 1 1) lattice plane, indicating the formation of alpha-MnO₂^[3]. Raman spectra of MnO₂/CC in Figure  1(b)  showed two strong peaks at 1328 cm^-1 and 1588 cm^-1, which can be attributed to CC, and at the low wave number regeion, the two peaks at 565 cm^-1and 640 cm^-1 can be attributed to the Mn-O stretching vibration and the symmetric stretching vibration Mn–O of the MnO₆ groups, respectively, confirming the existence of x-MnO2. The XPS analyses were also carried out to investigate the valence state of Mn. The two peaks at 642.8 eV and  654.8 eV shown in Figure 1(c) corresponded to the spin orbit doublet of Mn 2p 3/2 and Mn 2p 1/2 , respectively. In order to further confirm the presence of MnO₂,  the  O 1s signal of XPS spectra were deconvoluted into three peaks and the presence of characteristic peak of O-Mn bond was shown at 530 eV. From above, nano-structured MnO₂/ carbon cloth composites were successfully fabricated by a simple hydrothermal method .

 

(c)

 

 

(a)

 

In order to explore the capacitive performance of MnO₂/CC composites with the different reaction times as supercapacitor electrode, the cyclic voltammetry curves were measured at a scan rate of 50 mV/s. It is showed that polarization phenomenon occurred as the increasing mass loading of MnO₂. This may be attributed to the difficult electron and ion transmission for the thicker MnO₂ film. Thus, MnO₂/CC composites obtained from 0.5 h deposition time is better to be used as electrodes for supercapacitors.

 

(b)

 

 

(d)

 

 

Fig. 1.  (a),(b),(c) are XRD,Raman,XPS spectrogram results for structure characterization respectively and (d) is the charge-discharge results for CC/ MnO2 composites at current density of 1 mA/cm²

Fig. 1 (d) shows  the charge/discharge behavior of the MnO₂/CC composites across the voltage range of 0V to 0.8V at the current density of 1 mA/cm² . Those results showed that MnO₂/CC composites with 0.5h reaction demonstrated a high specific capacitance of 194 mF/cm² , but the value of capacity were not increase by a linear growth with time. Compared with the performance of CC, it was very easy to find the conclusion that MnO₂ film provide the majority of capacitance. Electrochemical impedance spectroscopy was applied to further investigate the kinetic feature of ion diffusion. The results suggested that the diameter of semicircle response at high frequency became bigger, indicating that the interfacial charge transfer resistance  increased by the reaction time. The cycling stability of our material was also measured at the current of 0.1 mA for 1000 cycles. MnO₂/CC composites with 0.25h reaction could remain about 89% of its initial specific capacitance. The dropping of capacitance may be attributed to the detachment of  the MnO₂ particles from the CC. However, the stability of our materials was much better than ever reported^[2]. This may be attributed to the pretreatment of CC subtrate, which might enhance the mechanical adhesion and electrical connection between two materials.

References

[1]     Ko, W.-Y.; Chen, L.-J.; Chen, Y.-H.; Lin, K.-J. Solvothermal synthesis of shape-controlled manganese oxide materials and their electrochemical capacitive performances. Journal of Materials Research 2013, 29, 107-114.

[2]     Chi, H. Z.; Tian, S.; Hu, X.; Qin, H.; Xi, J. Direct growth of MnO₂ on carbon fiber cloth for electrochemical capacitor. Journal of Alloys and Compounds 2014, 587, 354-360.

[3]     Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Manganese oxide-based materials as electrochemical supercapacitor electrodes. Chem Soc Rev 2011, 40, 1697-1721.