M.Sc. Thesis Defence

M.Sc. Thesis Defence: ***

Study of electro-mechanical properties of manganese-based nanocomposites for supercapacitors

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Supercapacitor
LAMMPS

The Electric Double Layer (EDL) supercapacitors can have extremely high-power densities, incomparable cycle life, stability, and reliability, as a result they are already used in uninterruptible power systems (UPS), memory back-up systems and even in the emergency doors of Airbus A380.

Graphene-based electrodes with appropriate defects (encapsulated metal or metal oxide nanocomposite) and optimized alignment display enhanced capacitive behavior and can provide good cycling performance of energy storage material. Graphene is a mono-atomic layer of Sp2 hybridized carbon with honeycomb lattice structure. When engineered with metal oxides, they show high electrochemical double layer capacitance through synergistic effect. MnO2 has been investigated extensively as a suitable material for high-performance electrochemical capacitor due to its low-cost, rich redox behavior and good environmental compatibility. In this work, the potential for rGO-MnO2 nanocomposite to be used as supercapacitor material has been examined in detail, both experimentally and via molecular dynamics simulation. To examine the performance of the nanocomposite experimentally, Cyclic Voltammetry and Galvanostatic Charging Discharging experiment (Chronopotentiometry) have been performed at several scan rates and several current densities, respectively. For active mass of 0.000405g and potential window of -100 – 800 mV, specific capacitance of 261 F/g has been obtained from Galvanostatic Charging Discharging, which is moderately good enough to use in a supercapacitor. In depth analysis using molecular dynamic (MD) simulation method has performed to understand the atomistic configuration of existing rGO-MnO2 structure and to improve the nanocomposite electrode structure for better performance.

Our study suggests an improved design of rGO-MnO2 nanocomposite electrode by adding nano slit-pore in a particular way which can be called ‘nano slit-pore’ model. Using this model electric potential as a result specific capacitance shows higher values than typical planer model. This is because adding slit-pore in the electrode surface increases the ion-electrode interaction as a result more solvent and electrolyte ions clustered near electrode surface and provides higher capacitance values. Our results clearly indicate that slit-pore in nanoscale creates opportunity to use the potential of graphene sheet edges to improve the capacitance. —Musanna Galib

Upon charging, the structure of the EDL exhibits a greater response near the edges due to the preferential accumulation of excess charge carriers toward the edges. And also the rGO-MnO2 sheets incorporate its potential which is calculated through experiment and showed qualitatively through simulation. Summation of these two effects increases the specific capacitance of electric double layer supercapacitor and we proposed the model as ‘nano slit-pore’ model. Findings from this study will enable us to enhance the energy storage capability of rGO-MnO2 nanocomposite EDL supercapacitor.

In this study, molecular dynamics (MD) simulations have been performed to explore the variation of ion density and electric potential due to electrode surface modification. Two different surface morphologies, having planer and slit pore with different conditions of surface charge, have been studied for graphene-MnO2 surface using LAMMPS. For different pore widths, the concentration of ions in the double layer is observed to be very low when the surface of the graphene-MnO2 electrode is charged. With a view to identify the optimal pore size for the simulation domain considered, three different widths for the nano-slit type pores and the corresponding ion-ion interactions are examined. Though this effect is negligible for pores with 9.23 and 3.55 Å widths, a considerable increase in the ionic concentration within the 7.10 Å pores is observed when the electrode is kept neutral. The edge region of these nano-slit pores leads to effective energy storage by promoting ion separation and a significantly higher charge accumulation is found to occur on the edges compared to the basal planes. For the simulation domain of the present study, partition coefficient is maximum for a pore size of 7.10 Å, indicating that the ions’ penetration and movement into nano-slit pores are most favorable for this optimum pore size for MnO2-graphene electrodes with aqueous NaCl electrolyte.

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References

2020