Supercapacitive Properties of Nanocarbons Modified with Manganese Oxide Nanoparticles and Metallotetrapyrazinoporphyrazines

Abstract

This dissertation describes the electrochemical properties of nanocarbons (multiwalled carbon nanotubes (MWCNTs) and graphene oxide (GO)) incorporated with manganese dioxide (MnO2) and metallotetrapyrazinoporphyrazines (MTPyzPz, where M = Co, Fe) as electrochemical capacitors. Supercapacitive properties of the composites formed were investigated using cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) in both threeand two-electrode systems using 1.0 M H2SO4 and 1.0 M Na2SO4 as electrolytes. This study showed that acid functionalized MWCNTs (MWCNTaf) enormously increased the specific capacitance of MnO2 from 45 F.g-1 to 1209 F.g-1. Supercapacitive properties of nanocarbons modified with cobalt(II)tetrapyrazinoporphyrazine( CoTPyzPz) and Iron(II)tetrapyrazino-porphyrazine (FeTPyzPz) were explored. MWCNTaf modified with CoTPyzPz and FeTPyzPz gave higher capacitances than phenylamine-functionalised MWCNTs (MWCNTPhNH2) modified MTPyzPz nanocomposites in acidic electrolyte. The specific capacitance obtained for MWCNTaf-CoTPyzPz at 1 A.g-1 was 1642 F.g-1 in 1.0 M H2SO4 while 410 F.g-1 was obtained for MWCNTaf-FeTPyzPz at 3 A.g-1. This large specific capacitance was attributed to the protonation and deprotonation of the porphyrazine macromolecule. FeTPyzPz nanocomposites gave higher specific capacitance in acid electrolyte than in a neutral electrolyte. An asymmetric capacitor cell (coin cell) was made to further investigate the suitability of MTPyzPz compounds as electrode capacitor material. A novel v asymmetric electrochemical capacitor (AEC) using functionalised MWCNTs and MTPyzPz nanocomposite as the positive electrode and functionalised MWCNTs and carbon black (CB) as the negative electrode in a neutral aqueous Na2SO4 electrolyte was investigated. CoTPyzPz//CB-MWCNTaf was the best AEC with specific capacitance of 70 F.g-1. This AEC cell exhibited energy density and power density of 8.5 kW.kg-1 and 4.3 KW.kg-1 respectively. A novel AEC with high energy and power densities was successfully developed using GO/CoTPyzPz as the positive electrode and GO/CB as the negative electrode in 1.0 M Na2SO4. The excellent energy and power densities (44 Wh.kg−1 and 31 kW.kg−1 obtained at high current density, 2.4 A.g-1) coupled with excellent long cycle life, short response time, and low equivalent series resistance (ESR) clearly indicate that this asymmetric electrochemical capacitor has a considerable potential in developing low-cost and green energy storage devices that operate at high energy and power densities. Interestingly, the energy density of the GO/CoTPyzPz//GOCB based AEC falls within the range usually seen for nickel metal hydride (NiMH) batteries (30 – 100 Wh.kg-1), but, more importantly, has a much better power performance than NiMH batteries (0.25 – 1 kW.kg-1) widely used in hybrid vehicles such as Toyota Prius and Honda Insight. In summary, the findings in this work clearly indicate that CoTPyzPz and FeTPyzPz are potentially useful in developing low-cost and ‘green’ electrochemical energy storage devices with high capacitance, energy and power densities. The work has indeed opened a door of opportunity that will permit the study of other MTPyzPz complexes for the development of electrochemical capacitors.