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  • Final Report Summary - RPSII (Re-wiring of photosystem II enzymes to metal-oxide electrodes in artificial photosynthetic devices for enhanced photocatalytic water splitting performance)

Final Report Summary - RPSII (Re-wiring of photosystem II enzymes to metal-oxide electrodes in artificial photosynthetic devices for enhanced photocatalytic water splitting performance)

The over-arching goal of this project is to further the basic science behind light-driven water oxidation systems, which play an important role in renewable solar fuel generation. Specifically, a number of strategies were investigated for accelerating the water oxidation rates of artificial photosynthetic systems that ‘wire’ the benchmark photocatalyst, Photosystem II (PSII), to a range of emerging electrode materials. As a result of ‘rewiring’ the PSII to the electrodes, surprising alternative energy/charge transfer pathways between the PSII and the electrode were uncovered. These pathways may contribute significantly to the dampened light-to-product conversion efficiency of the hybrid system.
PSII is a photo-enzyme crucial to the photosynthetic pathway of autotrophic organisms. It orchestrates light-absorption, charge separation and the catalysis of water oxidation to perform overall photocatalysis at impressive rates in vivo, and serves as an inspiration in synthetic photocatalyst design. The incorporation of isolated PSII into conductive electrode materials is necessary for both the study of the enzyme using electrochemical methods and the use this enzyme in proof-of-principle water-splitting devices. However, the reaction rates of the isolated PSII in these in vitro settings are always much lower than those of the PSII in its native in vivo environment. This has mainly been attributed to the poor interfacing of the PSII to the electrode material. The aim of this work is to understand and ultimately improve the electronic communication between the PSII and the electrode surface by finding more suitable electron acceptor electrode materials and rewiring the electron transfer pathway of the PSII-electrode.
A range of earth-abundant metal oxide materials were initially investigated as replacement electrode materials to the more expensive and commonly used gold and indium tin-oxide (ITO). These included emerging materials such as tungsten trioxide, iron oxide, iron sulfide and tin oxide. However, these materials exhibited poor electron mobility and gave rise to overall lower photoelectrochemical outputs. Their electronic properties must be further improved (through doping or use in composite materials) before they can serve as replacements for the more conductive ITO. Inexpensive carbon materials, such as carbon nanotubes, are highly conductive and are excellent electrode materials; however, their opaqueness limits their usefulness as a photoelectrode material. As such, this study concentrated on studying the PSII-ITO interface.
One strategy to improve the interface between the PSII and the ITO involved the use of linkers to form self-assembled-monolayers on the electrode surface. Linkers with different ionic charges were self-assembled onto the surface of ITO and carbon nanotube electrodes to control the alignment of the PSII on the electrode. In one promising study, the use of linkers containing negatively charged carboxylate functionalities gave rise to more than double the photocurrents generated by the PSII immobilised onto bare electrodes. This was attributed to more favourable interactions between the stromal side of the PSII enzyme (where the terminal plastoquinone electron donors are located) and the electrode, since the stromal side of the enzyme has a more positive polarity. Linkers with nickel-nitrilotriacetic acid groups were also investigated for specific binding to the poly-histadine residues located on the stromal side of the PSII enzyme with some success.

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