| Summary: | 博士 === 國立成功大學 === 化學工程學系碩博士班 === 100 === Carbonaceous nanomaterials, inclusive of single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT), carbon nanocapsules (CNC) and graphenes (Gr) appear to be focus of attention for their high aspect ratio, excellent electronic, mechanical, optical, and chemical characteristics. Carbonaceous nanomaterials provides a broad survey in the context of commercially available nanomaterials as well as emerging technologies and future applications in the fields of molecular electronics, sensoring, nano- and micro- electromechanic devices, field-emission displays, energy storage, and composite materials. Photoanode in dye-sensitized solar cell (DSSC) is a primary decisive component controlling its light-to-electric conversion efficiency. The mesoporous TiO2 film commonly constructed based on high-surface area nanoparticles, is accompanied with uncountable crystal defects and grain boundaries that seemingly retard electron transport rate, as well as promoting recombination of electrons and holes.
In the first part of this study, we examined the electrical properties of the nanocomposite for TiO2 nanoparticles mixed with CNCs and MWCNTs pre-treated with plasma modification process followed by grafting with maleic anhydride (MA). The conductive carbon nanomaterials are expected to provide pathway for electron transfer to achieve better photovoltaic performance. The TiO2 nanoparticles were prepared by sol-gel and autoclaving technique. The carbon nanomaterials were dispersed to achieve homogenous suspension prior to mixing of TiO2 nanoparticles. The series of anatase TiO2-based nanocomposite incorporated with carbon nanomaterials-MA prepared by physical blending shows its capability for efficient electron transport when used as photoanode in DSSC. Compared to the conventional DSSCs, the TiO2 film with 1-D MWCNTs-MA possesses more outstanding ability to transport electrons injected from the excited dye within the device under illumination. As a result, at an optimum addition of 0.3wt% MWCNTs-MA in TiO2 matrix, the photocurrent voltage (J-V) characteristics showed a significant increase in the short-circuit photocurrent (Jsc) of 50%, leading to an increase in overall solar conversion efficiency by a factor of 1.5. Electrochemical impedance spectroscopy analyses revealed that the MWCNTs-MA/TiO2 incur smaller resistances at the photoanode in assembled DSSC when compared to those in the anatase titania DSSCs. On the other hand, DSSC based on CNCs-MA/TiO2 achieved about 30% improvement in efficiency. The results suggest that both the conducting properties and 1-D microstructure of the MWCNTs-MA within the anodes are crucial for achieving a higher transport rate for photoinduced electrons in TiO2 layer by serving as charge conduits with specific direction for better electron percolation. The best cell efficiency for MWCNTs-MA composite was 9.03% at 0.3wt.%, whilst 6.76% for 0.5wt% CNCs-MA photoanode system.
Along with these finding, the TiO2 nanoparticulate system was further replaced by nanofibers prepared using electrospinning technique to provide a more continuous conduction path with minimal amount of crystal defects among randomly distributed nanoparticles. The MWCNTs-MA were added into the TiO2 sol-gel suspension in polymeric (polymethyl methacrylate) solution before electrospinning. The results demonstrated results in good consistency with the previous part, wherein the incorporation of highly conductive MWCNTs-MA provided improvement in the cell performance. The highest conversion efficiency was found to be 4.88%, with about 47% improvement when compared to pristine TiO2 nanofibrous photoanode. This optimal efficiency is apparently lower than the one of nanoparticulate, attributed mainly to the relatively smaller surface area available for dye adsorption.
In the second part, quasi-state DSSC was prepared to overcome the solvent volatility and leakage problems accompanied with the liquid electrolyte conventionally used. Graphene (Gr), the 2-D carbon nanomaterial was used in the polymer gel-state electrolyte (PGE) based on polyacrylonitrile (PAN) to compensate for its weaker ion conductivity due to poorer mobility. The DSSCs fabricated with optimized composite electrolyte, 0.2wt% PAN-Gr gel electrolyte, showed significantly improved conversion efficiency (4.43%) in comparison to the DSSC fabricated with pristine PAN PGE, implying better ion conductivity and Lil dissociation contributed by addition of Gr. Moreover, the cell efficiency achieved 7.93% when the cell was assembled using the 0.3wt% MWCNTs-MA/TiO2 composite.
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