Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes
Citation:Rovetta, Aurélie A.S., Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes, Trinity College Dublin.School of Chemistry.CHEMISTRY, 2018
Complete Thesis Post VIVA Aurelie.pdf (Accepted for publication (author's copy) - Peer Reviewed) 13.77Mb
In this work, MoO3 and Co(OH)2 were prepared via liquid phase exfoliation (LPE) and tested for application as supercapacitor materials and for a catalyst material for the oxygen evolution reaction (Co(OH)2 only). LPE was shown to be an effective synthetic route to prepare large quantities of high quality few layer 2-D nanosheets. When combined with liquid cascade centrifugation it allowed the size selection of the flakes within a narrow range. In chapter 4, the intercalation of lithium and potassium into MoO3 film was studied. For potassium intercalation, the influence of the support was the first parameter examined. Three carbon-based support, glassy carbon, edge and basal plane pyrolytic graphite, were drop coated with the MoO3 flake suspension and double layer capacitance values of 11 mF g-1 for glassy carbon, 20 mF g-1 for pyrolytic graphite edge plane and 43 mF g-1 for basal plane were observed. The influence of the coating technique was assessed. The suspension was sprayed and phase transferred onto an ITO support electrodes and were found to have double layer capacitance values of 68 and 26 mF g-1 respectively. Lithium intercalation, for sprayed and phase transferred films displayed double layer values of 25 and 139 mF g-1 respectively. Impedance spectroscopy investigation on the pseudocapacitive behaviour in each medium lead to respective value of 20 and 840 mF g-1 for potassium and lithium intercalations. Sprayed molybdenum oxide films were found to be comparable to the state of the art Fe2O3 anode materials, with an energy storage ability of 0.57 kJ g-1. In chapter 5, cobalt hydroxide was examined for its energy storage applications and activity for the oxygen evolution reaction. Porous glassy carbon and nickel foam support electrodes were used and compared, with Co(OH)2 coated on the latter showing to be superior for both applications. After evaluating the influence of the Co(OH)2 flake size on the performance of the catalyst prepared via spray coating, a standard size (mean size 84 nm) was chosen as it offered the best compromise between preparation time/yield and activity. Both capacitive and catalytic performances were evaluated as a function of Co(OH)2 mass loading and an optimum loading of 1 mg cm-2 and 0.89 mg cm-2 was found for supercapacitor and OER applications respectively. Both coated supports compared favourably with the current state of the regarding the supercapacitive behaviour reaching capacitance values of 400 and 2000 F g-1, GC and Ni foams. For glassy carbon, the overpotential value at 10 mA cm-2 decreased from 870 mV down to 380 mV, with a small mass loading dependency. The OER catalytic properties of Co(OH)2 flakes on Ni foam presented a mass loading goldilocks region, emphasising the compromise between amount of catalyst deposited and film conductivity/diffusion limitations. The overpotential at 10 mA cm-2 for a bare Ni foam was comparable to a coated GC foam with a value of 400 mV. This value was lowered for Co(OH)2/Ni foam, reaching an optimal value of 280 mV with a mass loading of 0.89 mg cm-2. This system had a TOF of 2.08x10-3 s-1 and a Tafel slope value of 58 mV dec-1. Moreover, all Co(OH)2 coated electrodes were shown to be stable for 24-hours at 10 mA cm-2. Electrical representation using equivalent circuits of the bare and coated foams was performed using electrochemical impedance spectroscopy. In chapter 6, the work focused on testing Co(OH)2 under conditions similar to that used in industrial electrolysers by operating at elevated temperatures, in more concentrated alkaline solution and at higher current densities. When studying the system in different alkaline solutions the influence of the viscosity of the electrolyte was shown to affect the performances of the Co(OH)2 coated nickel foams with an optimal concentration of 5 M NaOH chosen. The overpotential values of Co(OH)2/Ni foam at 50 ?C, 10 and 100 mA cm-2, were measured to be 162 mV and 201 mV respectively. When in a two-electrode cell configuration, where the Co(OH)2 coated foam is both the anode and cathode in the absence of a reference electrode which corresponds to an industrial electrolyser configuration, a cell voltage of 2.0 V was observed at 100 mA cm-2. In addition it was shown that Co(OH)2 was stable at current densities up to 500 mA cm-2 for at least 24 hours.
Author: Rovetta, Aurélie A.S.
Publisher:Trinity College Dublin. School of Chemistry. Discipline of Chemistry
Type of material:Thesis
Availability:Full text available