Carbon-nanofiber electrodes directly grown on a nickel foam current collector for electrochemical energy storage devices

Supercapacitors are gaining interest in the energy storage sector and are working their way to replace batteries or to provide a high-power supplement to the battery energy. Despite their high-power density and long cycle life, their energy density is low presently. Even with the use of high surface area materials like graphene, activated carbon, and metal oxides, there has not been a considerable increase in the energy density, mostly due to the electrode preparation methods. This thesis has focused on preparing carbon nano fibers (CNF) that are directly grown on nickel foam current collectors, resulting in electrodes that are not only mechanically stable but also have an extended micro-porous structure on the macro-porous nickel foam. This leads to a electrochemical active surface area ca. 400 m2/g and thus provides a good architecture for enhanced performance of surface-limited electrochemical reactions. The electrode production process does not require an additional catalyst coating step, like other processes for production of carbon nano tubes (CNT) and CNF. Also, the CNF growth process yielding the dense CNF forest is performed at a very low temperature of 400 oC when compared to typical CNT growth processes. This largely simplifies fabrication of the overall structure.The unique structure of the CNF with diameter ca. 50 nm on nickel foam was amalgamated with hydrous ruthenium oxide to achieve higher specific energy and power. Ruthenium oxide was coated on the directly grown CNF on nickel foam using a thermal decomposition technique. The underlying enhanced surface area allowed the coated ruthenium oxide to have a higher surface area, which in turn resulted in large pseudocapacitance. It was also observed that the preparation temperature greatly affected the capacitance.iiiThe prepared materials were characterized using a Scanning electron microscope (SEM), X-ray photon spectroscopy (XPS) and Energy-dispersive X-ray spectroscopy (EDS) to investigate the surface morphology and elemental composition. A thorough electrochemical characterization using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy was done to investigate the material performance as a supercapacitor electrode.The as-grown CNF on nickel foam showed steady retention of capacitance of ca. 140 ± 7 mF/cm2 even when current density increased from 3 to 20 mA/cm2. This is unique to this material as for most electrodes, capacitance significantly drops with increasing current. These electrodes also showed a 100 % retention in capacitance even after 10000 cycles at 10 mA/cm2.For the hydrous ruthenium oxide coated electrodes, a capacitance of 822 ± 4 mF/cm2 at a current density 20 mA/cm2 was achieved, which makes them among the best-performing electrodes reported in the literature. These electrodes also exhibited excellent cycle life with 94 % initial capacitance retention after 5000 charge-discharge cycles. A unique capacitance trend with oxide formation temperature was observed for the ruthenium oxide-CNF electrodes when compared to the trend for ruthenium oxide alone.Overall, this PhD project clearly shows that the hierarchical structures of CNF directly grown on nickel foam provide excellent electrode materials for high specific power supercapacitors, and it also acts as a good support material to form metal-oxide coating to yield high supercapacitor performance metrics. The overall electrochemical behavior of these materials also makes them an ideal candidate for other applications which require high surface area, like hydrogen production

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