Solution-Processable Polymer Photocatalysts for Hydrogen Evolution from Water

Abstract

Methods of storing renewable energy are urgently required to meet future energy demands. Photocatalytic hydrogen production from water represents an attractive method of storing solar energy for a diverse range of end-use applications. Semiconducting polymers are an emerging class of photocatalysts with eminently tunable structures and properties. However, the insolubility of most polymer photocatalysts limits processability and, therefore, opportunities to optimise the morphology of these materials for photocatalytic applications. Processability was achieved with the introduction of solubilising side-chains, which were systematically varied in order to study their influence on the photocatalytic performance of polymers. It was found that high hydrogen evolution rates could be achieved by incorporating oligo(ethylene glycol) side-chains, which seem to promote interaction with water during photocatalysis as well as affording solubility in common organic solvents. The polymer backbone was also varied to further improve the performance of solution-processable polymer photocatalysts. A fluorene-based polymer, FS-TEG, was prepared that displayed high activity under visible light, with an external quantum efficiency of 10.0% at 420 nm. Polymers were processed into a variety of forms, including photocatalytic films, both free-standing and cast on substrates. The substrate was varied to improve performance, with roughened glass slides found to achieve the highest areal hydrogen evolution rates. Photocatalytic polymers were also cast on planar substrates, which enabled precise control over film formation. Important parameters such as optimum film thicknesses for hydrogen evolution performance were subsequently established. Processability also enabled facile preparation of composites and blends. Incorporation of a narrow band gap dye was shown to triple the hydrogen evolution rate of FS-TEG films while the formation of heterojunctions with inorganic photocatalysts also enhanced performance. The scope for fabricating composites of this kind is boundless and, in the long term, devices capable of overall water splitting that utilise these materials are envisaged.