Year

2016

Degree Name

Doctor of Philosophy

Department

Australian Institute for Innovative Materials

Abstract

In this study the biomimetic reproduction of the Oxygen Evolving Centre (OEC) of PSII, consisting of a CaMn4O4 cluster held in place by the surrounding protein scaffold, was attempted in a simplified manner. In the OEC the water oxidation reaction takes place on one binuclear Mn-O2-Mn site in a repetitive manner. A mimic of this feature was attempted by using Mn porphyrins in close proximity held in place by a conducting poly(3,4-ethylenedioxythiophene) (PEDOT) matrix. In another approach, an inorganic, crystalline MnxOy deposited on a conducting graphene substrate was used for the same purpose.

Films of Mn porphyrin / PEDOT (PEDOT:PSS in the case of electrochemical polymerisation) were fabricated by embedding the porphyrin in PEDOT during vapour phase polymerisation and electrochemical polymerisation of the conducting polymer. The Mn porphyrin species studied were 5,10,15,20-tetraphenylporphyrinato manganese(III) chloride (MnTPP), 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato manganese(III) chloride sodium salt (MnTPPS), 5,10,15,20-tetrakis(4- methylpyridinium)porphyrinato manganese(III) chloride tetraiodide (MnTMPyP), poly(5-(4-vinylphenyl)10,15,20-tris(4-sulfonatophenyl) porphyrinato manganese(III) chloride sodium salt (MnPVTPPS). The films were tested by linear sweep voltammetry (LSV) and chronoamperometry (CA) under illumination to analyse photocurrent activity in an aqueous electrolyte. The samples were further studied by UV-Vis spectroscopy and elemental analysis to confirm the exact status of the porphyrin - complexed Mn ion in the film corresponding to observed levels of photocurrent activity. The MnTPP/PEDOT composite was singled out for detailed analysis to confirm, by gas chromatography, the evolution of O2 and H2 at a constant potential of 0.7 V (vs Ag/AgCl) under illumination. This material was then further studied by elemental analysis and UV-Vis spectroscopy to reveal that the Mn ion was lost, from the porphyrin centre, during the polymerisation step, leaving a free base porphyrin in the film. The gas evolution was therefore linked to decomposition processes rather than the interaction between Mn centres.

Using a different approach, MnxOy - birnessite was electrodeposited on conductive FTO glass as well as graphene - coated substrates. To mimic reaction centre composition, Ca ions were incorporate into manganese oxides. This was achieved by adding Ca ions during an electrodeposition step or embedded into the graphene substrate prior to electrochemical process. These materials underwent a study by linear sweep voltammetry and chronoamperometry to ascertain the most productive combination of the catalytic species and variations of the graphene substrate. While the Ca ion incorporation did not lead to an appreciable increase in water oxidation, the MnxOy/RLCGO composite featured a low onset of water oxidation at 1.1 V (vs Ag/AgCl) with electrocatalytic performance surpassing that of Pt in the range 1.1 – 1.3 V (vs Ag/AgCl) in an aqueous electrolyte.

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