Energy storage properties of carbon onion-carbon nanofibre composites containing transition metal compounds

Abstract

The quest for electrical energy storage has been a key driver for researchers to come up with more effective means of storing this form of energy due to the intermittent nature of renewable energy sources. Several countries have swiftly adopted the transformative potential of renewables, in particular solar energy, while others have delayed the implementation due to complex policies surrounding renewable energy projects. A way forward would be innovative regulatory approaches that encourage the pairing of solar systems with other generation technologies, and with storage, to offer a “round the clock” supply. Rechargeable batteries and supercapacitors are widely employed energy storage systems. A rechargeable battery system offers high energy density, with lithium-ion batteries (LIBs) being the most widely used. For some applications, it is imperative that energy is delivered at a much faster rate. This characteristic feature is known as power density, and supercapacitors have proven to be much better than batteries in this case. The large-scale commercialization and adoption of a supercapacitor are hindered by its low energy density. The electrode material is a major determinant of the success of supercapacitors. Generally, these are supported on high surface area carbon materials. This study focused on the development of electrospun polyacrylonitrile (PAN) fibres embedded with onion- like carbon (OLC) and iron (II) phthalocyanine (FePc) particles, and encapsulation of the fibres with Molybdenum disulphide (MoS2). Furthermore, composite fibres were either integrated with manganese (III) oxide (Mn2O3) or engineered with defects for enhanced performance in symmetric supercapacitors. The synthesis of electrode materials was divided into four phases; In the first phase (1), OLC nanoparticles were embedded in electrospun PAN fibres and decorated with the Mn2O3 and evaluated as supercapacitor electrode materials. For enhanced interfacial electrochemistry and overall capacitance, the electrode material in (1) was encapsulated with MoS2 in phase (2). In phase (3) FePc embedded in the PAN electrospun fibres were evaluated for supercapacitor applications. Limited specific capacitance and poor cycling stability were observed, thus suggesting integrating OLC and further encapsulation with MoS2, in phase (4). The morphology of the fibres was vii engineered with defects in the form of Fe2+ vacancies to maximize the electrochemical reactions of the OLC/MoS2 fibre composite. The electrochemical properties of the fibre composite materials were investigated and OLC/Mn2O3-CNF exhibited a specific capacitance, energy and power density of electrodes were 200 F g-1, 44.63 Wh kg-1 and 3 235 W kg-1, respectively with excellent capacitance retention. While the MoS2 encapsulated and Mn2O3 decorated fibre composite, OLC/MoS2@Mn2O3 displayed a specific capacitance, energy and power density of 348 Fg-1 18.42 Wh kg-1 and 5 095 W kg-1, respectively. It is pertinent to note that the capacitance of the electrodes was retained throughout the 5 000 cycles of the charge-discharge test. Upon thermal treatment at 600 °C, FePc-PAN transformed into FeN4-CMF and exhibited a specific capacitance, energy and power density of 147 F g-1, 12.48 Wh kg-1 and 4 320 W kg-1, respectively. The vacancy-rich (FeN4)d-OLC- CNF@MoS2 composite obtained by the removal of Fe2+ atoms, showed a specific capacitance, energy density and power density of 481 F g-1, 76 Wh kg-1 5833 W kg-1, respectively. This study underscores strategic processes that can be adapted in the design, synthesis and optimization of supercapacitors-based electrodes for enhanced performance.