Undergraduate Research Opportunities Programme (UROP) Report Student Name: Chen Yu Supervisor: Dr Palani Balaya Mentor: Dr. S. Devaraj Capacitive characteristics of nanostructured mesoporous MnO2 INTRODUCTION Fossil fuels have been used as energy source for a long time. In 21 st century, global warming is becoming a major issue, and emission of CO 2 due to combustion of fossil fuels is a significant factor. The world now is looking for alternative energy sources that are cleaner and renewable. But many energy sources that generate electricity such as solar panel and wind turbine often require energy storage device. Moreover, the popularization of electric mobile devices and electric cars also make energy storage a hot topic. Among all the energy storage devices, capacitors can provide higher power density. However, the energy that common electrostatic and electrolytic capacitors can store is very limited. Supercapacitors (Ultracapacitors in the following Ragone Plot [1]) have a relatively high energy density without so much compromise of power density. Compared with fuel cell and conventional batteries, Supercapacitors are very suitable for high power applications such as electric vehicles and portable electronic devices that require fast charging. Figure 1: Ragone Plot (Comparison of performance of different energy storing devices) [1]
Capacitor Electrostatic Capacitor Electrolytic Capacitor Electrochemical Capacitor Figure 2: Categorization of capacitors Electrochemical Double Layer Capacitor (EDLC) Pseudocapacitor (Supercapacitor) According to the Ragone Plot [1], EDLCs and Pseudocapacitors (Supercapacitors) usually have a larger energy density compared to electrostatic and electrolytic capacitors. The working principle of EDLCs is charging and discharging at the interface of two layers of a same substrate. The difference between supercapacitors and the rest of the capacitors is that the charge storage mechanism is not electrostatic, but faradaic in nature[2]. The difference between supercapacitors and batteries is that the potential is approximately linear in charge. Metal oxides are considered to be promising materials for promising supercapacitor materials [3]. RuO 2 is a conventional material for high capacitance, and was reported to have a capacitance of over 72F/g[4]. However, the high cost and toxicity of the material make it difficult to find commercial applications. There are several Metal oxide alternatives for RuO 2. One of them-mno 2 is a readily available chemical and relatively safe to work with. It also has high performance as a material for capacitors [3]. Graphene materials are commonly studied as electrodes for electrochemical double-layer capacitors (EDLCs) and have a good electrochemical performance (135 F/g) [5]. The combination of Graphene and MnO 2 as electrodes will provide both pseudocapacitance and electrochemical double-layer capacitance [6], and the performance of the supercapacitor may be enhanced. In this project, the main purpose is to use MnO 2 -Graphene composite as electrode material for supercapacitor and investigate the performance. A simple and fast synthesis method is applied, and several tests are performed.
EXPERIMENTAL Synthesis of MnO 2 MnO 2 was synthesized by a redox reaction between Potassium Permanganate ( KMnO 4 ) and Hydrazine (N 2 H 4 ). In this experiment, 2.37g KMnO 4 was added to 15mL MiliQ water, and 1mL of N 2 H 4 was added and stirred continuously for 1h. The precipitate was washed 4 times with DI water, centrifuged and dried at 7 C in vacuum for 12h. The resultant crystals were then ground into powder form, and stored for further testing. Synthesis of MnO 2 -Graphene Composite MnO 2 -Graphene was synthesized using similar method as above, which is to reduce Graphene Oxide (solution) and KMnO 4 (solid) simultaneously using Hydrazine (N 2 H 4 ) as a reducing agent. The resultant mixture of MnO2-Graphene was then used for testing. Instead of 15mL MiliQ water, 2.37g KMnO 4 was added to 15ml of Graphene Oxide solution and reduced by 1mL of N 2 H 4 and stirred for 1h. A dark brown precipitate was thus formed. After being centrifuged and washed by the same process as previous, the resultant was also grounded into powder form and used as comparison. Electrochemical Testing Electrochemical Fabrication 1cm x 15cm strips of High-purity stainless steel-foil (SS) with average thickness.2 mm were used as the current collector. Both sides of the strips was polished with successive grades of emery, cleaned with detergent, washed copiously with DI water, rinsed with ethanol and dried before use. Active material (Graphene-MnO 2 Composite), acetylene carbon black and polyvinylidene difluoride (PVDF) in mass ratio of 7%:2%:1% were ground in a mortar; a few drops of 1- methyl-2-pyrrolidinone (NMP) were added to form slurry. It was coated on pre-treated SS-foil over an area of 1 cm2 on each side and dried at 8 C under reduced pressure. Finally, the electrodes were dried at 8 C in vacuum for 12 h. Cell Assembly A three electrode system is used for the electrochemical tests. The counter electrode used is Platinum which is used to measure current. The reference electrode used is Ag/AgCl which is used to measure and control potential of the working electrode. The coated strips were soaked for at least 3 minutes before beginning the experiment and were used as working electrode. Testing Cyclic voltammetry method and Galvanostatic Charge-Discharge Cycling method was applied to characterize the electrochemical properties of Graphene-MnO 2. Cyclic voltammetry is done by increasing potential of the working electrode with time and measuring current. The range of the potential is from to.8v, and five cycles for each test are
recorded. Scan rates of 2, 5, 1, 2, 5 and 1 mv per second were applied to the working electrode. The shape of the curve can be used to determine the property of the electrode (either it performs like batteries or capacitors). The area below the curve also provides a estimation of the capacitance of the electrode. Galvanostatic Charge-Discharge tests can provide precise calculation of the value can be done. Currents of 2, 1, 5, 2, 1 and.5ma were applied on the working electrode, with a potential range from to.8v, and putting it through five charge-discharge cycles. The specific mdv capacitance of the material can be determined via the equation: C I /,where I is the dt current, m is the active material mass, and dv is the scan rate of the experiment. dt
Potential/ V Current/mA RESULT & DISCUSSION Cyclic Voltammetry (CV) of MnO 2 and MnO 2 -Graphene Composite in.1m Na 2 SO 4 1.5 1.5 -.1.1.3.5.7.9 -.5 Potential/ V -1-1.5 -Graphene Composite Figure 3: Cyclic voltammetry of MnO 2 and MnO 2 -Graphene Composite in.1m Na 2 SO 4 at scan rate of 5mVs -1 Charge/Discharge (CD) Cycles of MnO 2 and MnO 2 -Graphene Composite in.1m Na 2 SO 4.8.6.4.2 5 1 Time/ s 15 -Graphene Composite Figure 4: Charge/Discharge(CD) Cycles of MnO 2 and MnO 2 -Graphene Composite in.1m Na 2 SO 4 at current of 1mA
Potential/ V Current/mA Cyclic Voltammetry (CV) of MnO 2 and MnO 2 -Graphene Composite in.1m Mg(ClO 4 ) 2 2 1.5 1.5 -.1 -.5.1.3.5.7.9-1 -1.5 Potential/ V -2-2.5-3 -Graphene Composite Figure 5: Cyclic voltammetry of MnO 2 and MnO 2 -Graphene Composite in.1m Mg(ClO 4 ) 2 at scan rate of 5mVs -1 Charge/Discharge (CD) Cycles of MnO 2 and MnO 2 -Graphene Composite in.1m Mg(ClO 4 ) 2.8.6.4.2 Time/ s 5 1 15 2 25 -Graphene Composite Figure 6: Charge/Discharge (CD) Cycles of MnO 2 and MnO 2 -Graphene Composite in.1m Mg(ClO 4 ) 2 at current of 1mA
MnO 2 : Specific Capacitance vs. Current Measured by Charge/Discharge (CD) Cycles of MnO 2 electrode in.1m Mg(ClO 4 ) 2 applying current of 2,1, 5, 2, 1 and.5ma 12 Specific Capacitance/Fg -1 1 8 6 4 2 5 1 15 2 25 Current/mA Figure 7: Specific Capacitance vs. Current: Measured by Charge/Discharge (CD) Cycles of MnO 2 Composite electrode in.1m Mg(ClO 4 ) 2 applying current of 2,1, 5, 2, 1 and.5ma MnO 2 -Graphene Composite: Specific Capacitance vs. Current Measured by Charge/Discharge (CD) Cycles of MnO 2 -Graphene Composite electrode in.1m Mg(ClO 4 ) 2 applying current of 2,1, 5, 2, 1 and.5ma 14 Specific Capacitance/Fg -1 12 1 8 6 4 2 5 1 15 2 25 Current/mA Figure 8: Specific Capacitance vs. Current: Measured by Charge/Discharge (CD) Cycles of MnO 2 - Graphene Composite electrode in.1m Mg(ClO 4 ) 2 applying current of 2,1, 5, 2, 1 and.5ma
Potential/ V The result shows that MnO 2 -Graphene composite have a capacitor performance and the capacitance is larger than MnO 2. They both have a better performance in Mg(ClO 4 ) 2 solution. The maximum specific capacitance attained was 18.9F/g for MnO 2 and 122.7F/g for MnO 2 - Graphene composite Graphene composite, for currents of.5ma. Effect of Graphene Concentration In order to know the effect of Graphene concentration, another synthesis was done by reducing the amount of KMnO 4 to 1.185g (half of the amount mentioned above). And a Charge/Discharge test in.1m Na 2 SO 4 was performed to compare the results..8.6.4 -Graphene Composite.2 5 1 15 Time/ s -Graphene Composite with higher Graphene concentration Figure 9: Charge/Discharge (CD) Cycles of MnO 2 and two types of MnO 2 -Graphene Composite in.1m Mg(ClO 4 ) 2 at current of 1mA The figure shows that a higher concentration of Graphene will perform not as good as the previous ratio, and further research can be done to find the MnO2-Graphene ratio that will lead to maximum capacity. CONCLUSION From the results it can be concluded that MnO 2 -Graphene can be a potential electrode material for supercapacitors. Compared to MnO 2, the specific capacitance of MnO 2 -Graphene is larger. Both material performs better in Mg(ClO 4 ) 2 solution. Finally, further investigation on concentration of the optimized ratio of the two materials can be performed. ACKNOWLEDGEMENT I would like to thank Dr. Palani Balaya for providing me the chance to do this project and appreciate the supervision and guidance from him and Dr. S. Devaraj for throughout the time, as well as the help and advice from Gabriel Gay during the experiment process.
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