Synthesis, characterization and thermoelectric properties of conducting metallopolymers

Abstract

In this thesis, four series of metallopolymer-based OTE molecules were synthesized. Upon doping with single-walled carbon nanotubes (SWCNTs), the thermoelectric (TE) properties of the as-formed composite films were tested, the electrical conducting mechanism was proposed and the effects exerted by metal centers were discussed in detail. In Chapter 1, the differences in phenomenon and mechanism between the Joule heating and thermoelectric effect were compared at the very beginning, and the discovery of thermoelectric effect was also introduced. Then, several thermoelectric-related parameters were demonstrated to evaluate the thermoelectric properties of a thermoelectric generator. According to the molecular structures, typical examples of inorganic, organic and coordinated thermoelectric materials were described. Besides, practical optimization strategies on improving thermoelectric performance from various aspects were illustrated in detail. At the end of this section, current research gaps and the significance of my research work were illustrated. Chapter 2 listed the chemicals and the instrumentation methods employed in this thesis. In Chapter 3, the effects of linking patterns of conducting polymers exerted on the TE properties were investigated. Two series of poly-Schiff bases were designed, synthesized and the thermoelectric properties of SWCNT-blended films were studied to compare the differences induced by C=C and C≡C. SEM revealed the morphology evolution from severe phase separation to well-dispersed microstructures with the differences that the double bond-featured series always exhibited better dispersed morphology. Raman spectra indicated the existence of the interfacial interactions between the poly-Schiff bases and SWCNTs. TE tests indicated that C=C-featured composites showed an overall higher performances. Spectroscopic studies and DFT calculations indicated that such an advantage stemmed from the planarized molecular conformation that the carbon-carbon double bonded poly-Schiff base adopted and the resulting enhanced conjugation. Upon metallation, the TE properties of the composite films could be feasibly tuned. The performance was affected by the redox-matching mechanism at low doping ratios and was governed by the morphology when a larger amount of SWCNT was added. This work may provide a valuable reference for the design of metallopolymer-based OTE materials. To increase the solubility and promote the phonon scattering of the OTE materials, a collection of soluble metallopolymers were synthesized in Chapter 4. CV diagrams suggested the formation of phenoxy radicals, a highly conductive species, during oxidation. TE tests indicated that the electrical conductivity was significantly improved by doping the metallopolymers with SWCNTs, and that nickel-containing composites showed much higher conductivity at lower doping levels, which can be benefitted from the redox nature of Ni(II). In the meanwhile, Seebeck coefficient declined, possibly due to the passage of the low-energy carriers which was induced by the increased CNT content. In this composite system, the highest PF was achieved at around 170 μW/m·K2 by the zinc-containing composite at a doping ratio of 60%. In Chapter 5, several terpyridine-based, Wolf type III conducting metallopolymers were synthesized. Band structure analysis suggested all three neat metallopolymers possessed nearly the same HOMO energy level, but the copper-containing one showed the narrowest band gap. Raman spectra indicated the interactions between the metallopolymer chains and SWCNT networks. TE property assessments suggested that Cu coordinated polymer yielded overwhelming electrical conductivity as well as power factor, which could be resulted from the reduced band gap, lowered activation energy, the diversity of electronic states brought about by Cu2+ and a stronger interaction with SWCNTs. The final chapter showed the synthesis of a series of novel, fluorinated (metallo)porphyrin derivatives and the exploration of the spectroscopic and TE properties. Morphology studies revealed the evolution from aggregations to a uniform surface. Raman spectra confirmed the interactions between (metallo)porphyrin and SWCNT networks. Band structure discussion showed that the HOMO level was slightly elevated upon metallation, and that the incorporation of Zn2+ helped to maintain the narrow band gap as the free base does. TE tests of the (metallo)porphyrin-SWCNT composites revealed that the zinc-containing composites afforded high electrical conductivity and PF at low doping ratios, while the performance was predominantly affected by the morphology at high CNT contents.