Polythiophene and dye blends for the photo-electrocatalytic production of hydrogen
thesisposted on 01.03.2017, 23:53 by Ng, Chun Hin
The main finding of this thesis is the discovery of poly(2,2'-bithiophene) (PBTh) as a novel photo-electrocatalyst for the hydrogen evolution reaction (HER). This finding for PBTh is supplemented by studies showing remarkable long term stability, high Faradaic efficiencies, and successful operation over a wide pH range. In-depth mechanistic studies were also conducted and reveal significant insight into the thermodynamic and kinetic mechanism of the PBTh catalyst. The challenge of tackling climate change requires the rapid development of new technologies for sustainable energy use. To this end, hydrogen shows much promise as an alternative for fossil fuels, however, the absence of an efficient H₂ generation method has thus far limited commercial applications. Significant research has been directed towards the development of new catalysts for the HER though none have yet achieved the desired performance requirements. In this study, we utilise the inherent electrochemical and photo-active properties of conducting polymers to tackle this long-standing problem. Initial studies on conducting polymer:dye blends were inspired by earlier research but revealed little catalytic activity. Subsequent in-depth characterisation studies allowed the identification of key issues, whilst revealing promising alternative conducting polymer:dye combinations for further investigation; namely, PBTh and Cresol Red. It was eventually discovered that PBTh displayed photo-electrocatalytic activity towards the HER even without the dye component. As a result, research was redirected to focus on pure PBTh to better understand this novel behaviour. The investigations on PBTh films revealed a range of desirable catalytic properties such as the successful operation in neutral aqueous environments, good stability over 12 days and an onset potential of 0.3 V below E0. The PBTh system also showed significant scope for further improvements which included the threefold increase in the catalytic activity when changing film thickness. This potential for enhancement, together with the inherent desirable catalytic properties, presents PBTh as a particularly exciting photo-electrocatalyst. In the final part of this study, in-depth mechanistic studies were undertaken to elucidate the chemistry behind the catalytic behaviour. Key findings included the exclusion of iron as a possible contaminant, confirmation of the photo-dependence to the optical properties of PBTh and assignment of an equivalent circuit to yield insight into the electrical properties of the PBTh electrode. Most importantly, in-situ Raman spectroscopy analysis was able to reveal the formation of an S-H def. band (at 985 cm-1) which confirmed the proposed intermediate state of a PBTh chain protonated at a sulphur atom (PBTh(S-H)+). Subsequent computational studies supported these findings and revealed promising charge transfer states of the excited PBTh(S-H)+ species. A tentative reaction mechanism is thus put forward, though further experimental data is required to confirm the scheme. The studies presented herein represent the beginnings of the development of PBTh as a photo-electrocatalyst for the HER. These promising initial results have led to the publication of three papers and generated considerable scope for future studies. As it stands, the full potential of the PBTh system has yet to be realised but given the promising performance observed thus far, it is hoped that the eventual optimised system would yield a low-cost and efficient catalyst for the commercial production of H₂.