High specific capacitance of electrochemically synthesized nano MnO 2 - gold electrodes for supercapacitors

Transition metal ions like MnO2 are promising materials for electrodes in supercapacitors owing to their high capacitance for storing electrical charges and also eco-friendly with plenty of availability. We have decorated honey-bee like MnO2 nanostructure over gold coated silicon wafer by electrodeposition. The electrodeposited material was studied by scanning electron microscope (SEM), which reveals the honey-bee like structure. The thickness was found to be in the range of 30–80 nm using atomic force microscope (AFM). The specific capacitance of this electrode is found to be 1149 Fg -1 , which is very high and flexible for high power applications. Copyright © 2013 VBRI press.


Introduction
For a huge power surge or instantaneous power release in demanding application such as rocket launching, batteries become unsuitable due to their slow rate of energy release [1]. Supercapacitors are electrochemical energy storage devices that provide high power density and remarkable energy. Electrochemical supercapacitors can deliver high levels of electrical power and offer long operating lifetimes [2][3][4][5][6][7][8][9]. These characteristics are of increasing interest in energy storage applications such as: electric vehicles, backup power systems, electronic components, etc. They are based on a storage mechanism that results from the formation of an electric double layer at the interface between an electronically conductive material and an electrolyte solution [2, 10]. But their low energy storage density inhibits the use for high power applications. Basically the electrode materials may be either carbon based or metal oxide or conducting polymer. Conducting polymer may be helpful as they are less expensive and able to store energy through a redox process [11]. But its lower cycle life and the low specific capacitance value inhibit its usage for potential applications. Recently D.S. Patil et al. [12] reported silver/polyaniline based electrodes for supercapacitors with a specific capacitance of 512 Fg -1 . Carbon materials in different forms such as carbon fibres, carbon aerogels, activated carbons and carbon nanotubes are most commonly used in high specific area electrode materials. These materials exhibit various attractive physical and chemical properties such as high conductivity, high surface area, high temperature stability, good corrosion resistance, high porosity, easy processability and good compatibility with composite materials and relatively low cost [13]. In this category, CNT attracted much attention owing to its flexibility [14][15]. But the specific capacitances are lower to use it in high power applications. The usage of transition metal oxide proves to be a better idea because they are available in plenty, low cost, nontoxic, etc., [16][17] but their low conductivity does not help. In transition metal oxides, MnO 2 could be used to make electrodes in such supercapacitors [16,18], because they are predicted to have a high capacitance for storing electrical charge but the poor conductivity of MnO 2 limits the charge/discharge rate for high-power applications. To overcome this problem an electrode made of gold could be used. An electrode made of gold nanoparticle facilitates fast ion diffusion between the materials and the electrolytes which can act as double-layer capacitors or bio-sensors [19].
Electrochemical synthesis is a better technique for developing indigenous supercapacitors and also for many other applications such as gas-sensors, bio-sensors, etc. [20]. Here, we report that honey-bee structures made of MnO 2 decorated in gold coated silicon wafer using electrochemical synthesis (cyclic Voltammetry) show a high capacitance for high energy applications. We have decorated the MnO 2 structure on a gold electrode. The nanoporous gold nanoparticle and the honey-bee MnO 2 structure facilitate fast ion diffusion and serves as a better material for supercapacitor. We obtain the value of 1149Fg -1 , which is close to the theoretical value [16]. This value is very high compared to previous reports in the metal based/CNT based/polymer based. It is believed that the honey-bee structure is the reason for the high specific capacitance of our electrode. The other advantage of our report is that we have obtained this value at high scan rate and this facilitates to use this material for high density applications.

Materials and methods
Analytical grade Na 2 SO 4 (99% purity), (CH3COO) 2 Mn·4H 2 O (99.99% purity) and gold wafer were purchased from Sigma Aldrich. All electrochemical experiments were performed using CHI 600 series Electrochemical Analyzer/Workstation. Distilled water was used for the solution preparation. A conventional three-electrode system which consists of a gold wafer as working electrode, an Ag/AgCl (saturated KCl) as a reference and a platinum wire as counter electrode were used. 0.1 M Na 2 SO 4 and 0.1 M manganese acetate were dissolved in the 100ml water and the electrolyte solution was prepared. Gold wafer electrodes were immersed in deionized distilled water and kept for ultra-sonication for one hour. After this process, the electrodes were washed thoroughly with deionized distilled water and used for the electrochemical deposition process. Here, the cyclic voltammetry has been employed for the electrochemical deposition process. Pre-treated electrode has been immersed in the 0.1 M Na 2 SO 4 solution containing 0.1 M manganese acetate solution and the potential has been fixed from 0 to 0.4 V at the scan rate of 20 mV/s for three cycles and the manganese oxide materials were directly deposited on the electrode surface. Finally the manganese oxide materials was rinsed with deionized water and employed for the electrochemical analysis.

Characterization
XPERT-PRO X-ray diffractometer using Cu Kα Radiation (wavelength λ = 1.54016 A°) at 40 keV was used to confirm the formation of MnO 2 and to calculate the particle size. The morphology of nano particles were analyzed using HITACHI SUI510 scanning electron microscope operating at 10 kV. The morphology of MnO 2 decorated in gold electrode was analysed using park system XE70 Atomic force microscope. Electrochemical charaterizations were performed using CHI 600 series Electrochemical Analyzer/Workstation.

Results and discussion
The MnO 2 decorated in gold coated silicon wafer photograph was present in the Fig. 1. The schematic representation of electrochemical reaction mechanism was depicted in the Fig. 2. In this process, the electrolyte solution is the mixer of Na 2 SO 4 and (CH 3 COO) 2 Mn.4H 2 O. Here the role of Na 2 SO 4 is to increase the mobility of the Mn 2+ ions towards the gold electrode. At suitable voltage, the Mn 2+ ions forms Mn(OH) 2 and oxidises to MnO 2 . Since gold particle in the electrode are nanoporous, MnO 2 forms structure like pattern in the electrode. However, the detailed chemistry of the formation of the honey-bee like structures is not known. The colour changes from yellow to brownish black confirm the formation of MnO 2 in the gold electrode. The deposition was the result of the below mentioned sequential process. At first, water electrolysis occurred at the gold electrode surface generating OH -. This OHbonds with Mn 2+ causing nanoparticle deposition in the form of structures.
Mn(CH 3 COO) 2 Mn 2+ + (CH 3 COO) - XRD of the MnO 2 decorated in gold coated silicon wafer is presented in Fig. 3. The two sharp peaks confirm the presence of MnO 2 in pure form and no other impurities are found (JCPDS number is #895171). It also reveals the uniform particle size distribution of 30 nm using Scherrer formula. Fig. 4 shows typical scanning electron microscopy (SEM) of MnO 2 decorated in gold coated silicon wafer at different magnifications. It clearly shows that grown MnO 2 in the gold coated silicon wafer forms a neat porous network. The exact morphology was not seen clearly in 1 µm resolution, though it indicates a new structure formation. Fig. 4b shows clearly that MnO 2 was decorated similar to honey-bee structure.  Atomic force microscopy (AFM) images were presented in Fig 5. Fig. 5 shows the topography view of manganese oxide coated over the gold wafer. Left side of the image shows the manganese oxide coated region which is darkened and the right side of the shows the gold wafer without any deposition. The bottom image of Fig. 5 shows the top view topography of manganese di-oxide coated over gold wafer. Since the AFM tip is made to scan between the coated and uncoated region, the thickness of the coating is determined. The thickness is found to be in the regime of 30-80nm. It also shows the uniform deposition of MnO 2 over the gold wafer. Fig. 6 shows cyclic voltammograms (CV) for MnO 2 plated in gold substrate. The investigations were carried out for all electrodes in Li 2 SO 4 solutions at room temperature. The current increases with scan rate for all electrodes. CV shows a rectangular behaviour for all scan rates and shows a good high-rate performance. The electrode shows a high capacitance (1149 Fg -1 ) with the plating time of 10 min at the scan rate of 0.01 Vs -1 which is very high to the previous reports [21 -23].
The capacitance was determined by integrating the area of the cyclic voltammetry graph. The formula is given below, where C is Specific capacitance, ∫ i(E)dE is the total voltammetric charge obtained by integrating the positive & negative sweep in cyclic voltammetry, (E2-E1) is the difference in applied potential, m is the difference in mass which is, after and before the deposition of manganese oxide over gold wafer and V is the Potential scan rate. The specific capacitance is calculated by integrating the equation (1) as,   7 shows specific capacitance of plated MnO 2 versus scan rate. The specific capacitance was maximum at the scan rate of 0.01Vs -1 and decreases gradually up to 0.1 Vs -1 . There is sudden increase in the specific capacitance when the scan rate was as 0.2 Vs -1 . Beyond 0.2 Vs -1 , the specific capacitance decreases very slowly and shows almost constant at higher scan rate. This shows that the electrode is ideal and it can be used for high power applications.

Conclusion
Electrodeposition of MnO 2 in gold coated silicon wafer was performed and SEM shows the formation of honey-bee like structure. Thickness of the deposited structure was analysed from AFM and found to be in 30-80 nm. CV shows a good rectangular behaviour and the current increases with the scan rates. The specific capacitance was found to be 1149 Fg -1 which is very ideal value for high power applications.