Redox chemistry of polyoxometalates and applications in energy storage

Abstract

The first part of this thesis describes investigations into the multi-electron reduction of (TBA)3[PMo12O40] and (TBA)3[V13O34] and interactions between the reduced anions [PMo12O40](3+n)– and M2+ and M3+ cations in non-aqueous solvents. Systematic stepwise reduction of (TBA)3[PMo12O40] with various reducing agents (Na│Hg, Li│Hg, KC8 and K+PhCN–●) in non-aqueous solvents (MeCN, propylene carbonate and PhCN) was monitored by NMR and FTIR spectroscopy. The different reducing agents produced differing results and this is thought to be due to interactions with solvent. Evidence for the formation of “super-reduced” (TBA)3[PMo12O40M24] (where M = Na or Li) was obtained for the first time (via chemical reduction and not in a Liion battery set-up), broad resonances were present in both solid state and solution 31P NMR spectra. Systematic stepwise reduction of (TBA)3[V13O34] with M│Hg amalgam (M = Na or Li) was also monitored by NMR and FTIR spectroscopy. Products from these reactions have been isolated but not all of them can be confidently assigned structures. The capped reduced compounds; (TBA)3[PMo12O40Sb2] and (TBA)3[PMo12O40{M(dmso)x}n] (M = Bi3+, n = 2 and x = 4, M = Zn2+, n = 1 and x = 2) were successfully prepared, isolated and characterised by NMR spectroscopy, FTIR spectroscopy, cyclic voltammetry and crystallography. In the second part of this thesis, investigations into the use of the polyoxovanadate Na6V10O28.nH2O as a supercapacitor cathode material are described. The decavanadate was synthesised by three different methods and was characterised by NMR and FTIR spectroscopy, TGA, CV and SEM. The electrode performance was optimised using powder processing, the results of which were monitored by SEM imaging, and by varying the electrode slurry composition. Decavanadate-containing electrodes were electrochemically characterised by three-electrode experiments, with an optimised electrode achieving a specific capacitance of 225 F.g-1. An asymmetric hybrid supercapacitor using activated carbon electrical double layer electrode and decavanadate faradaic electrode was also investigated and it exhibited an energy density of 14 W.h.kg-1 with a power density of 356 W.kg-1.