Transition metal oxide and phosphate-based/carbon composites as supercapacitor electrodes

Ho, Mui Yen (2017) Transition metal oxide and phosphate-based/carbon composites as supercapacitor electrodes. PhD thesis, University of Nottingham.

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Abstract

Electrochemical capacitors, also known as supercapacitors, have attracted considerable attention over the past decades owing to their higher power density, long cycle life and moderate energy density compared. A high-performance supercapacitor integrates innovative electrode materials with desirable properties coupled with low cost and sustainability. In this thesis, a series of low cost transition metal oxide-activated carbon composite materials, lithium iron phosphate-activated carbon composite materials as well as metal oxide-graphene composite materials were prepared, characterized and evaluated as supercapacitor electrodes.

Iron oxide (Fe3O4) – activated carbon (AC), zinc oxide (ZnO) – AC and titanium oxide (TiO2) – AC nanocomposites were prepared by using simple mechanical mixing method. The charge storage capabilities of these metal oxide-based composites with different loading ratios were evaluated in both mild aqueous 1 M Na2SO3 and 1 M Na2SO4 electrolytes. The incorporation of small amount of metal oxides onto AC could effectively enhance the capacitive performance of pure AC electrodes. It is believed that the presence of metal oxide nanoparticles can provide favourable surface adsorption sites for sulphite anions (SO32-). Nevertheless, bulk increasing of the metal oxide content is found to distort the capacitive performance and deteriorate the specific surface area of the electrode, mainly due to the aggregation of the metal oxide particles within the composite. On the other hand, composite materials consisting of lithium iron phosphate (LiFePO4) and AC exhibit high specific capacitance of 112.41 F/g in 1 M Na2SO3 with the incorporation of 40 wt % of LiFePO4. The synergistic effect between the faradaic battery type materials and the EDLC-based materials is greatly demonstrated. The intercalation and extraction of Li+ ions in LiFePO4 lattices are responsible for the reversible Faradaic reaction on top of the adsorption and de-adsorption of SO32- anions from Na2SO3 electrolyte.

In the preparation of SnO2-graphene and MoO3-graphene nanocomposites, low-temperature solvothermal method using mild reducing agents was adopted. The preparation steps do not require high pressure or extreme synthetic condition and do not involve the usage of hazardous reactants. The electrochemical results of SnO2-graphene composite electrodes demonstrate that the composite electrodes possess a high specific energy (14 Wh/kg) with 93 % capacitive retention after 1500 cycles while MoO3-graphene composite electrodes yield an enhanced specific energy (16.3 Wh/kg) which is 28 % higher than that of pure MoO3 (11.8 Wh/kg). A maximum specific capacitance of 99 F/g was obtained from the optimized SnO2-graphene composite electrodes while a high average specific capacitance of 148 F/g was achieved for MoO3-graphene composites at a scan rate of 5mV/s in neural 1 M Na2SO3 electrolyte. The incorporation of graphene onto both SnO2 and MoO3 respectively, can promote the electrochemical utilization of metal oxides as well as the electrical conductivity of the electrodes. The graphene sheet serves as a good support in promoting effective charge transfer for redox reactions of MoO3. Additionally, deposition of metal oxides on graphene sheets prevents the graphene sheets from agglomeration, resulting in facile ion transportation pathway for electrolyte to access the surface of active material.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Khiew, Poi Sim
Keywords: supercapacitors, graphene, nanocomposite, neutral electrolyte
Subjects: T Technology > TP Chemical technology
Faculties/Schools: UNMC Malaysia Campus > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 40274
Depositing User: HO, MUI YEN
Date Deposited: 17 Nov 2017 04:00
Last Modified: 06 Dec 2017 01:56
URI: http://eprints.nottingham.ac.uk/id/eprint/40274

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