Fabrication and the electrochemical activation of network-like MnO2 nanoflakes as a flexible and large-area supercapacitor electrode

Porous network-like MnO2 thick films are successfully synthesized on a flexible stainless steel (SS) mesh using a simple and low-cost electrodeposition method followed by an electrochemical activation process. Morphology, chemical composition, and crystal structure of the prepared electrodes before and after the activation process are determined and compared by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) analyses. The results show that the implementation of the electrochemical activation process does not change the chemical composition and crystal structure of the films, but it influences the surface morphology of the MnO2 thick layer to a flaky nanostructure. Based on the electrochemical data analysis, the maximum specific capacitance of 1400 mF (381 F g−1) and 3700 mF (352 F g−1) are measured for small (2.6 cm2) and large (10 cm2) surface area electrodes, respectively. In addition, a flexible symmetric MnO2//MnO2 solid-state supercapacitor shows a capacitance of 0.3 F with about 98% retention at different bending angles from 0 to 360°.


Introduction
In recent years, great efforts have been devoted for exploring the flexible and wearable energy storage devices, such as flexible displays, artificial skins, and portable electronic equipments [1][2][3]. Among the energy storage devices, supercapacitors (SC) which fill the gap between batteries and conventional capacitors have attracted attention due to their ability to supply more power, long cycle life, and fast dynamic of charge propagation. There are two basic mechanisms for storing energy in supercapacitors including electric double layer capacitance (EDLC) originating from pure electrostatic charge accumulation at the interface between the electrolyte and the electrode e.g. carbon-based materials [2], and the pseudo-capacitance due to the fast and reversible surface redox reactions of various electroactive materials, e.g. metal oxides and conductive polymers [4][5][6][7].
MnO2 is one of the most attractive candidates because of its plausible advantages such as having a high theoretical specific capacitance ( =1370 F g -1 ) combined with natural abundance, low-cost, and being environmentally benign [17][18][19][20][21][22][23]. However, the poor electrical conductivity and low ion diffusivity of MnO2 has remained a major challenge that limits the rate capabilities for high power performances [24]. Many strategies have been proposed to mitigate these limitations including the increase of the specific surface area of the electrode material [25], using conductive additives, and the deposition of manganese oxide nanostructures on a highly conductive porous substrate [26,27].
In general, the overall activation processes for both EDLCs and faradic-based supercapacitor electrodes influenced the morphology, chemical composition or crystallinity of the active materials ( Fig. 1). In MnO2-based SC electrodes, it seems that the electrochemical activation process could affect the nature of the porous MnO2 active material.

Fig. 1
In this work, to address the above mentioned issues, a porous layer of MnO2 was synthesized on a flexible stainless steel substrate to fabricate SS/MnO2 electrode. Then, the electrochemical activation process was conducted on the fabricated electrode to investigate the effects of activation process on the chemical composition, crystal structure and the surface morphology of the MnO2 thick layer in detail. Finally, the electrochemical and 3 supercapacitive behavior of the activated SS/MnO2 electrode was studied as well. The maximum electrochemical capacitance of 381 F g -1 was obtained at constant discharge current density of 0.1 A g -1 for MnO2-based electrode. Moreover, the synthesis process as well as the scalability and bendability of the prepared MnO2 electrodes show the potential applications in future flexible energy storage devices.

Experimental Preparation of manganese dioxide film
Manganese oxide film was prepared by using the electrochemical deposition in a three- After the heat treatment, the prepared electrodes were electrochemically activated by potential cycling at a scan rate of 50 mV s -1 in a potential window from 0 to +1 V for 300 cycles in 1.0 M Na2SO4 aqueous electrolyte (activation process in Scheme 1).
The mass loading was determined separately by measuring the mass of the electrodes before and after electrodeposition/activation process by using semi-micro analytical balances (GR-202) with the accuracy of 0.02 mg.

Electrochemical analysis
The electrochemical properties of the samples were analyzed by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD using an electrochemical analyzer (AUTOLAB PGSTAT 302N) in 1.0 M Na2SO4 aqueous solution. CV measurements were carried out at different scanning rates in the potential range of 0 to +1.0 V. The GCD tests were conducted within the same potential window at various current densities from 0.10 to 2.0 A g -1 .

Results and Discussion
Fabrication and characterization  XRD analysis indicates that the activation process had no significant effect on the crystal structure of the electrodeposited film (Fig. 2b) (111), (311), (400) and (440) crystal planes of the MnO2, respectively (JCPDS card no. 00-042-1169). According to FTIR spectra analysis (Fig. 2c), it further supports our XRD results, i.e. activation process causes no significant change in chemical composition. In addition, the peaks at around 3400 and The morphology of the MnO2 films was investigated by FE-SEM (Fig. 2d). It is obvious that the MnO2 layer was uniformly grown on the SS mesh (see the cross-sectional images, Fig. S3). It was found that the morphology of the smooth surface was changed to a porous network-like structure (nanoflakes) during the activation process (Fig. 2e). The thickness of nanoflakes and spaces between them were measured about 30±5 and ~100 nm [36,14].
Therefore, the electrolyte can also cause wetting of the porous layer during the activation process. The crystalline structure and the chemical composition of MnO2 nanostructure have not been changed during the activation process. So, the main reason for the improved capacitance of activated SS/MnO2 electrode is the change in the morphology of the electrode surface, as confirmed by FE-SEM images. On one hand, the morphology change can cause the capacitance of the electrode, and on the other hand, besides the morphology change, the wetting of the electrode surface can also affect the capacitance of the electrode.

Fig. 2
The electrochemical properties of the MnO2 electrodes for different electrodeposition time were also investigated using CV measurement. Fig. 3a and b show the normalized CV curves to the active mass and the geometric surface area of the MnO2 electrodes, respectively. The sample prepared with deposition time at 1000 s was chosen as the optimized electrode for further studies. to 100 mV s -1 in the potential range from 0 to +1.0 V. CV curves show a good rectangular shape which is due to redox reactions. The GCD curves of the electrode were obtained at various discharge currents (Fig. 4b). The GCD curves show nearly triangular shape with low IR drop at various current densities confirming the ideal capacitive properties with good electrochemical reversibility.
Calculated specific and real capacitances as a function of current density are shown in Fig.   4c. The specific capacitance was measured to be 381 F g -1 and 176 F g -1 at the current density of 0.1 and 2.0 A g -1 , respectively.
To further study the application potential of MnO2/SS electrode, an electrode with larger area (at about 10 cm 2 ) was also prepared, which demonstrates the scalability of the prepared samples. The electrochemical results showed that the specific (and real) capacitance of the fabricated electrode was about 353 F g -1 (and 3.7 F) at the current density of 0.10 A g -1 (Table S2).

Fig. 4
In continuing the work, a flexible (Fig. S4) and symmetric cell was prepared to investigate and evaluate the actual performance of the MnO2 as an electrode material for supercapacitor device. Symmetric and relatively packed capacitor, MnO2//MnO2, was assembled with a filter film as a separator. Fig. 5a and b represents the CV curves of the solid state MnO2//MnO2 device at different cell voltages ranging from +0.4 V to +1.0 V at different scan rates. The specific (and real) capacitance of the symmetric system was calculated of about 43 F g -1 (and 0.3 F) according to the GCD curves based on the total active mass in both electrodes at the different current densities as shown in Fig. 5c and d. As illustrated in Fig. 5d, the symmetric system presents rate capability retention of about 63%.
After 1000 cycles, specific capacitance of the device still remains 80% of its initial specific capacitance. These results suggested that the structure exhibited an outstanding flexibility,