Experimental and theoretical reactivity studies of molecular metal oxides for energy conversion and storage

Abstract

Nowadays, polyoxometalate attracting much attention in many applications such as energy conversion and storage, molecular magnetism and oxidative (photo-) catalysis. The incorporation of reactive metal sites into the polyoxometalate framework can lead to novel cluster properties, tuning the electronic structures and thus affects the reactivity in redox-mediated processes. To this end, the present thesis combined the experimental investigations with theoretical calculations to achieve a better understanding of the electronic structure, redox and catalytic activity of metal-functionalized polyoxometalates. The coordination of CoII to cyclic metavanadate [V4O12]4- via three terminal V=O oxo ligands results in the formation of a CoII-based coordination site for a series of anions, which suitable for both homogeneous and heterogeneous colorimetric anion sensing – even under harsh conditions – based on spectral response upon anion binding. Furthermore, tunable electrochemical properties of dodecavanadates via incorporation of redox active centers such as FeIII. Experimental and theoretical studies including electron paramagnetic resonance spectroscopy and density functional theory verify the formation of the low-valent iron (I) species in the high-valent vanadium (V) based molecular metal oxide cluster {FeV12}. Tuning of the electronic structure and catalytic activity of the Anderson anion was also achieved by incorporation of a series transition metals (e.g. Mn3+, Fe3+, Co3+) into the POM framework. Experimental investigations are combined with theoretical calculations to gain insight the electronic structure of the cluster framework and the role of the central transition metals for electrochemical and photocatalytic properties. Beyond the metal-functionalized POMs, organo-functionalization of POMs with phosphonates anchoring group as well as a reactive azide unit via covalent attachment was achieved. Subsequent “CLICK” chemistry via Cu(I)-catalyzed alkyne-azide cycloaddition was investigated to verify accessibility and reactivity of the azide. In the field of energy conversion and storage, colloidal manganese vanadium oxide particles with catalytic water oxidation activity were generated by the conversion of the manganese-based water oxidation catalyst {Mn4V4}. Recently, molecular vanadium oxide clusters mainly decavanadate have received enormous interest as molecular alternatives for solid-state vanadium oxide active materials in batteries. Here we investigated the molecular structural stability of decavanadate under typical electrode fabrication processes and pre-heat treatment for the removal of crystal water. XRD and Raman spectroscopy together with wet-chemical studies showed that the molecular vanadium oxide clusters are unstable under these conditions and undergo irreversible structural degradation and conversion to solid-state vanadium oxides. Furthermore, new strategy towards the fabrication of carbon conductive additives - and binder- free POMs-based electrode material for Li-ion batteries and post-Li-ion batteries was designed and successfully employed. To this end, the integration of POMs in conducting organic polymers (CPs) was achieved by in-situ chemical or electropolymerization of EDOT in the presence of POM