Colloidal synthesis of molybdenum diselenide nanomaterials for supercapacitor applications
Date
2018
Authors
Ndala, Zakhele
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Abstract
Herein we report on the synthesis of MoSe2 nanomaterials using novel colloidal synthetic methods and their application as electrode materials in supercapacitors. Supercapacitors are energy storage devices with high power density and high cycle stability that can be used in applications that require rapid charge/discharge. The drawback in supercapacitors is their low energy density. Nanomaterials with high surface area are being explored as alternatives to activated carbon which has been a commonly used electrode material in supercapacitors. This is done to increase the energy density of supercapacitors. MoSe2 has been identified as an excellent candidate for use as an electrode material in supercapacitors because of its interesting properties. MoSe2 is a layered transition metal dichalcogenide (TMD) that is similar to graphene in structure and possesses interesting structural, optical and electronic properties. MoSe2 also has a high surface to volume ratio. A colloidal synthetic process was used for the synthesis of the MoSe2 nanomaterials, after which the effect of various reaction parameters was investigated. The reaction was run at 300 °C using oleylamine (OAm) as the solvent and surfactant. A time study on the synthesis of the nanomaterials revealed MoSe2 few-layer nanosheets grow from a flocculate formed in the initial stages of the reaction (30 min). At longer reaction times (e.g. 90 min) the flocculate is consumed to form wrinkled few-layer nanosheets. A variation of the metal precursor in the reaction results in changes to the morphology of the MoSe2 nanomaterials. The formation of the flocculate in the initial stages of the reaction when molybdenum hexacarbonyl is used as the metal precursor results in the formation of wrinkled few-layer nanosheets. The use of molybdic acid as the metal precursor results in the formation of nanoparticles with a central core which leads to the formation of MoSe2 nanoflowers. The effect of adding a co-surfactant to the reaction system was also investigated. The effect of adding oleic acid as a co-surfactant in the reaction along with oleylamine resulted in changes to the thickness of the nanosheets synthesized and slight changes in the morphology. The use of 1-octadecene as a co-surfactant resulted in the increased reactivity of the selenium precursor which increased the number of nanosheets growing per nanoflower. The electrochemical properties of the MoSe2 nanomaterials were investigated to determine the suitability of the nanomaterials for use as supercapacitor electrodes. The MoSe2 nanomaterials synthesized using colloidal synthesis exhibited electric double layer capacitance behaviour. The effect of the morphology on the electrochemical performance of the MoSe2 nanomaterials was investigated using MoSe2 nanoflowers and few-layer nanosheets. The MoSe2 nanoflowers were shown to have a higher specific capacitance at 81 Fg-1 than the few-layer nanosheets at 30 Fg-1. The nanoflowers also had better capacitance retention at higher current densities in the charge-discharge analysis compared to the few-layer nanosheets. The nanoflowers had higher capacitance retention of 68% compared to 20% for the few-layer nanosheets. The nanoflowers also had a lower equivalence series resistance (ESR) of 34.0 Ω compared to that of the few-layer nanosheets at 57.1 Ω.
Description
A research report submitted in partial fulfilment of the requirement for the degree of Master of Master of Science to the Faculty of Science, School of Chemistr, University of the Witwatersrand, 2018
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Citation
Ndala, Zakhele (2018) Colloidal synthesis of molydenum diselenide nanomaterials for supercapacitor applications, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/25801