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Synthesis, development and investigation of conducting polymer based heterojunctions as photostimulated electrocatalysts
thesisposted on 22.02.2017, 01:54 by Gustafson, Matthew Peter
The demand for a clean, sustainable energy future has motivated the scientific community to research alternative sources of power. Energy is vital to the growth and quality of our civilization and is the origin of the nexus of major issues facing the world today. Issues such as clean water, adequate food, spread of disease, armed conflict and an increasing global population could be resolved if energy was cheap, abundant and available. The use of energy carriers such as petroleum and diesel consume a finite fuel resource while polluting the environment. Among the alternative fuels that have been proposed, hydrogen is a prominent energy carrier recognised for being clean and sustainable. Currently, the majority of the world’s hydrogen supply is provided via the steam reforming of methane. This process, like petroleum and diesel, remains limited by a finite resource while impacting on the environment. Unlocking hydrogen from water via electrolysis is seen by many as the preferred, environmentally sustainable alternative. Unfortunately, water electrolysis is an energy intensive process and remains difficult and expensive. The half-reactions that amount to the overall water electrolysis reaction are the proton reduction and water oxidation reactions. Water oxidation is identified as the main consumer of excess energy (overpotential) required for the overall reaction, due to its four-electron, stepwise process. One approach to overcome the energy required is the use of sunlight to assist or drive electrochemical water-splitting. By developing a photostimulated electrode, the sun can reduce the energy required to split water for hydrogen production. Current photostimulated electrodes or photoanodes involve the use of semiconducting inorganic compounds such as metal oxides. While these inorganic photoanodes are advancing, some require rare and expensive compounds that can impact on the environment. This work developed an organic photoelectrode based upon conjugated polymer junctions to target electrochemical reactions, primarily, the energy intensive water oxidation reaction. Selected for their semiconducting properties, the conjugated polymers were synthesised to form an interpenetrating composite material producing junctions between polymer species. Poly(thieno[3,2-b]thiophene) (PTT) and poly(dithieno[3,2-b:2’,3’-d]thiophene) (PDTT) were selected as donor materials for their adequate absorption in the visible range (~500 nm) and synthesised via vapour phase polymerisation (VPP). Both PTT and PDTT were separately blended with poly(3,4-ethylenedioxythiophene) (PEDOT) to form composite alloy materials. PEDOT was selected as a catalytic component and, due to its hole transport capabilities, provides a junction to aid in charge separation, avoiding charge recombination. A multistep VPP process was used to form the alloy materials. This approach allowed a polymer layer (e.g. PEDOT) to be oxidatively polymerised onto a substrate before being suspended in a selected monomer vapour (e.g. PTT or PDTT) chamber. This allowed the monomer to polymerise with the residual oxidant within the initial polymer for a greater interpenetrating network. This was seen as the most productive method for the manufacture of heterojunction materials. Additionally, PTT and PDTT had not previously been polymerised via a vapour phase route. Spectroscopic characterisation indicated a certain level of spontaneous interaction between the blended polymers in the alloy materials obtained, while electrochemical testing demonstrated light enhanced water electrolysis. Further investigation revealed an enhanced electrochemical performance upon annealing these alloy materials and indications of susceptibility to humidity were found. The design of the aforementioned alloy materials to achieve junctions between polymer species is based on the well-known organic photovoltaic materials. As such, it was of value to investigate a well-established bulk heterojunction material, poly(3-hexylthiophene) (P3HT) and phenyl-C₆₁-Butyric acid Methyl ester (PCBM), as an organic photo-electrode. Housing a working heterojunction relationship between P3HT:PCBM, these organic photovoltaic structures offered a suitable platform to support the proof-of-concept of a heterojunction based organic photoelectrode. Investigated in the context of the water electrolysis reaction, a photo-enhanced effect was observed confirming the participation of the active heterojunction in separating and transporting charges for use in electrochemical reactions. Overall, this study presents strong evidence that an organic photoelectrode based upon a heterojunction structure can be synthesised and applied to electrochemical reactions. Thus, a novel avenue is made available in the field of electrocatalysis for future pursuit and development.