Edited by: Wenyao Li, Shanghai University of Engineering Sciences, China
Reviewed by: Bo Li, Shanghai Jiao Tong University, China; Guangjin Wang, Hubei Engineering University, China
This article was submitted to Electrochemistry, a section of the journal Frontiers in Chemistry
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A large-area MnO2 stalagmite nanorod arrays (SNAs) growing vertically on flexible substrates were successfully fabricated by an easy heat-electrodeposition method. The large specific capacitance (646.4 F g−1 at 500 mA g−1) and excellent rate capability (42.3% retention with 40 times of increase) indicate that the prepared MnO2 SNAs flexible electrode has outstanding electrochemical performance. Furthermore, after 5,000 repetitions of CV tests, the overall specific capacitance could retain ~101.2% compared with the initial value meant a long cycling life. These outstanding properties could be ascribed to the effective conductive transport path between Ni substrate and MnO2 nanorods, and owing to the stalagmite like structure of MnO2 nanorods, the exposed sufficient active sites are beneficial to the electrolyte infiltration.
The expanding requirement for energy consumption has stimulated the development of electrochemical energy storage devices (Simon and Gogotsi,
There are many different SCs electrode materials, such as carbon-based material (Li G. et al.,
As we know, the effects of active materials on the performance of SCs include morphology, structure, contact with collector plate and active sites, etc. Unfortunately, during the redox reaction, the electrons transport of MnO2 electrodes often restricted by its high electrical resistivity. In addition, the defect of the conventional powder electrode preparation is that the generated 'dead volume' by the bonding process of the active material to substrate could make a deterioration in the electrochemical properties of the electrodes (Liu et al.,
In this paper, the heat-electrodeposition method was adopted to formulate a nanoarrays of stalagmite like MnO2 on the flexible substrates. The prepared electrode gets capacitance of 646.4 F g−1 (500 mA g−1) and 42.3% retention (current density increased 40 times) for a remarkable rate capability. And the total capacitance retention rate after 5,000 cycles is ~101.2%. Furthermore, to verify the generality of the synthesis method, another flexible activated carbon fiber (ACF) was also used as the substrate for the growth of MnO2 nanoarrays.
The preparing method in detail was: the heat-electrodeposition process carried on a 3D porous Ni foam. Before electrodeposition, the Ni foam was cut into ~ 3 × 1 cm2, and then immersed into a 5 mol/L HCl solutions along with supersonic wave treatment for 10 min to dissolve the NiO layer on the surface. The Ni foam obtained from the previous step was rinsed to neutral with distilled water, and then subjected to vacuum drying (60°C, 4 h). The heat-electrodeposition occurred in a cell with the water bath. The composition of the electrolyte was as follows: Mn(CH3COO)2 (0.01 M), CH3COONH4 (0.02 M) and dimethylsulfoxide (DMSO, 10 vol.%). The corresponding working electrode, the counter electrode and the reference electrode were the treated Ni foam, the Pt plate (1.5 × 1.5 cm2) and saturated calomel electrode (SCE), respectively. The heat-electrodeposition condition was applied at a constant current (0.5 mA cm−2) by the Autolab electrochemical workstation at ~ 80°C for 60 min. After that rinsed the obtained sample to neutral and placed it in a 60°C vacuum dryer for 4 h. Finally, the sample was calcined in N2 atmosphere (heating-up 0.5°C min−1, 250°C, 2 h). The weight gain of the sample after the deposition was the active matter weight.
In order to analyze the samples qualitatively, the X-ray diffractometer (XRD; Rigaku D/max-2550 PC, Cu-Kα radiation) spectrum was utilized. To observe the microstructures, scanning electron microscope (SEM, Hitachi S-4800) and transmission electron microscope (TEM, JEOL JEM-2100F) were adopted. Mass weighing (Mettler Toledo XS105DU, δ = 0.01 mg).
The Electrochemical Workstation (Autolab PGSTAT302N, electrolyte 0.5 M Na2SO4) was used to measure electrochemical performance. In the test, the obtained MnO2 electrode (~1 cm2) was used as the working electrode. The counter and reference electrode were the same as mentioned above. Specific capacitance calculation (Guan et al.,
where
The weight of the 1 cm2 MnO2 electrode was ~ 1.32 mg. The cyclic voltammetry (CV) potential window was −0.1 to 0.9 V. The scan rates increased from 1 to 100 mV s−1. The galvanostatic charge-discharge (GCD) curves were measured under current densities from 0.5 to 20 A g−1. The cycle life was obtained by CV test (50 mV s−1) with repetitions of 5,000.
Except for the two strong peaks of 3D Ni foam substrate, the XRD diffraction pattern in
XRD pattern of the prepared MnO2 sample.
An aligned and dense MnO2 nanoarray is presented in
In order to understand how the unique structure of stalagmite MnO2 nanorod arrays (MnO2 SNAs) was formed, the time-dependent electrodepositing experiments were carried out through controlling the electrodeposit reaction time.
SEM images of the stalagmite MnO2 nanorod arrays for different electrodepositing time:
Furthermore, to verify the generality of the synthesis method, another flexible ACF substrate was chosen to replace the Ni foam.
In
After 5,000 cycles, a high specific capacitance retention of 101.2% indicated that the cyclical stability of the MnO2 SNAs electrode was good. The strong contact between the active matters and the substrate facilitating the collection and enhancement of the electron participation reaction could be an important reason for the effective cycle stability. Thus, it is concluded that the electrode material of the as-synthesized MnO2 SNAs demonstrates an excellent cycle life.
In conclusion, the stalagmite MnO2 nanorod arrays successfully grew on the flexible substrate by heat-electrochemical deposition method. The prepared MnO2 SNAs electrode has the high specific capacitance, the outstanding rate capability and the long cycle life, all of which all suggest its excellent electrochemical performance. In addition, this approach could pave the way for a facile low-temperature heat synthetic route for generating a variety of metal oxides arrays flexible substrate electrode.
The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.
YG did the experiments and described the images of figures. XW helping with writing. TZ was the supervisor of this research work. All authors participated in the analysis of experimental data and manuscript preparation.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
The Supplementary Material for this article can be found online at: