Double Layer Formation and Cation Pseudo-Intercalation Supercapacitor Carbon Nanotube Composite Electrodes With Enhanced Electrochemical Performances

by Yverick Rangom

Institution: University of Waterloo
Year: 2015
Keywords: electrochemical capacitor; ultra-high sweep rate; carbon nanotube; thin film; AC line filtering; composite electrode; mxene; self-standing; low current collector interface impedance
Record ID: 2058021
Full text PDF: http://hdl.handle.net/10012/9077


Among electrochemical energy storage solutions, redox-free supercapacitors exhibit the highest power densities and best cycle life, easily reaching over one million cycles. Despite these attributes, Li-ion batteries are preferred for most applications, especially in the much sought after electric vehicle applications. The reason is the lower energy density of the former compared to the latter. Nevertheless supercapacitor technology remains complementary to batteries in many ways. The excellent cycling performance of non-redox supercapacitors derives directly from their highly reversible charge storage mechanisms. Currently, efforts in the scientific community are being put into addressing i) low energy density, ii) compromised electronic and ionic charge transport, iii) small potential window for devices using aqueous electrolyte. All three research areas are fundamental to supercapacitor systems, however increased charge transport carries the greatest promises in terms of future device developments. The original research, presented in chapters 3 and 4 of this thesis, focuses on the characterization and optimization of electronic and ionic conduction phenomena in supercapacitors relying on double layer formation and pseudo-intercalation charge storage mechanisms. Chapter 3 features a novel carbon architecture that is both mesoporous and highly conductive based on naturally occurring carbon nanotube (CNT) mesh structure. The structure favors both mostly un-impaired ion movement and excellent electronic conductivity. Filtering these self-standing films from chlorosulfonic acid dispersions presents the critical advantage of not permanently functionalizing the carbon and fully preserves its electronic conductivity. The pristine condition of carbons after filtration is confirmed by fourier transform infrared (FTIR) spectroscopy based on comparison with the material before exposure to chlorosulfonic super-acid. The films were then mated to current collectors with different surface roughness and chemistries exposing the carbon/metal interface as a critical bottleneck limiting ultimate electronic conductivity. Surface morphologies were characterized at the micro- and nano-size with contact profilometer and atomic force microscopy (AFM) respectively. Surface oxidation was investigated through x-ray photoelectron spectroscopy (XPS) characterization. The study showed that surface morphology played the most important role in facilitating electron transfer between solids while the surface chemical composition was less of a factor. In Chapter 4, a 2-D Ti2C Mxene-CNT composite electrode was produced using a similar dispersion and filtration technique as in Chapter 3. The good chemical stability of Mxene allowed for dispersion in superacid with no effect on its inter-planar distances. That was verified by comparing spectra before and after exposure to the super-acid. Mxene phase electrodes have been demonstrated to have extremely high specific capacitance with regards to their specific surface area thanks to a charge storage…