Towards hydrogen energy: progress on catalysts for water splitting

This article reviews some of the recent work by fellows and associates of the Australian Research Council Centre of Excellence for Electromaterials Science (ACES) at Monash University and the University of Wollongong, as well as their collaborators, in the field of water oxidation and reduction catalysts. This work is focussed on the production of hydrogen for a hydrogen-based energy technology. Topics include: (1) the role and apparent relevance of the cubane-like structure of the Photosystem II Water Oxidation Complex (PSII-WOC) in non-biological homogeneous and heterogeneous water oxidation catalysts, (2) light-activated conducting polymer catalysts for both water oxidation and reduction, and (3) porphyrin-based light harvesters and catalysts

Studies of manganese porphyrin-PEDOT and manganese oxide-graphene composites as water oxidation electrocatalysts and photocatalysts

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

Kinematic molecular manufacturing machines

The principles of kinematic manufacturing machines of the type widely used since the industrial revolution are reviewed. Consideration is then given to how the principles of kinematics (\u27the geometry of motion\u27) may manifest in molecular catalysts. Actions of this type involve synchronized, regular, repeated and rapid conformational flexing along geometrically optimum pathways that define a single degree of freedom. The proposition that many of the catalysts of biology, enzymes, may generally exploit a kinematic action is discussed. Thereafter, in the major portion of this work, representative abiological molecular catalysts whose actions display the characteristic features of kinematic manufacturing processes, are reviewed. In accordance with the principles of kinematics, molecular catalytic actions of this type are shown to be capable of transforming unremarkable chemical species into powerful catalysts with high activities, selectivities, and durabilities

Demetallatation of electrochemically polymerised Mn porphyrin anion / PEDOT composites under light-illumination

This work reports photo-demetallation studies of thin-layer, electropolymerized poly(3,4-ethylenedioxythiophene) (PEDOT) loaded with low levels of: (i) an anionic Mn porphyrin monomer (5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato manganese(III) chloride (MnTPPS)), and (ii) an anionic Mn porphyrin polymer (poly(5-(4-vinylphenyl)-10,15,20-tris(4-sulfonatophenyl)) porphyrinato manganese(III) chloride (MnPVTPPS)). UV-vis and other measurements confirm that, like previously-studied cationic Mn(III) porphyrins embedded within vapour-phase polymerized PEDOT in low loadings, de-metallation under light illumination also occurs in these systems. However, it occurs to a significantly lesser degree. It can be concluded that demetallation appears to be an inherent feature of PEDOT coatings containing low levels of Mn porphyrins when they are illuminated with light. The demetallation process does not depend on the mode of polymerisation or the nature of the Mn porphyrin dopant. These findings have potentially important implications in water-splitting photocatalysis by Mn porphyrin-doped conducting polymers

Studies of poly(3,4-ethylenedioxythiophene) (PEDOT) films containing cationic Mn porphyrins. A loading-dependent demetalation of Mn(III)TPP in PEDOT (Mn(III)TPP=5,10,15,20-tetraphenylporphyrinato manganese(III))

Thin films of vapor-phase polymerized PEDOT incorporating various cationic Mn porphyrins were assessed for water oxidation catalysis under light illumination. Only Mn(III)TPP/PEDOT displayed a notable photocurrent and this was, counter-intuitively, greatest at the lowest loading levels examined. Studies revealed that a proportion of the Mn(III)TPP within the PEDOT became demetalated during polymerization, leaving free and protonated TPP. Despite the presence of an excess of chemical oxidant during the polymerization step, the Mn(III) ion was reduced - likely under the influence of light - to Mn(II), which was labilized out of the film. Whereas PEDOT films loaded with anionic Mn porphyrins may be active and selective water oxidation photocatalysts, their analogs containing cationic Mn porphyrins, like Mn(III)TPP, are catalytically inert

A light-assisted, polymeric water oxidation catalyst that selectively oxidizes seawater with a low onset potential

Vapour phase polymerisation (vpp) of PEDOT to incorporate high levels of a sulphonated manganese porphyrin yields a vivid green conducting polymer that, under illumination, catalyzes selective oxidation of water from seawater from ca. 0.40 V (vs. NHE; Pt counter electrode) without observable chlorine formation. This onset potential is comparable to that of certain metal oxide semiconductors that achieve higher photocurrents but are not capable of selectively oxidising the water in seawater

Towards hydrogen energy: progress on catalysts for water splitting

This article reviews some of the recent work by fellows and associates of the Australian Research Council Centre of Excellence for Electromaterials Science (ACES) at Monash University and the University of Wollongong, as well as their collaborators, in the field of water oxidation and reduction catalysts. This work is focussed on the production of hydrogen for a hydrogen-based energy technology. Topics include: (1) the role and apparent relevance of the cubane-like structure of the Photosystem II Water Oxidation Complex (PSII-WOC) in non-biological homogeneous and heterogeneous water oxidation catalysts, (2) light-activated conducting polymer catalysts for both water oxidation and reduction, and (3) porphyrin-based light harvesters and catalysts