Heterogeneous and Heterogenized Catalysts for Water Oxidation Reaction as Studied by Means of Sacrificial Oxidant

Hydrogen production from solar-driven water splitting (WS) reaction is considered a promising way to store solar energy. This process can be achieved directly by means of a photo-electrochemical cell (PEC), where light absorption, charge separation and WS occur in a single device; or indirectly, by coupling a photovoltaic device to an electrolyzer. WS is a thermodynamically uphill reaction, formed by two half reactions, i.e. the reduction of protons into H2 and the oxidation of water into O2. From a kinetic point of view, the latter is the most challenging one, being generally considered as the bottleneck for a widespread use of WS for hydrogen production. Thus, regardless of whether WS is carried out in direct or indirect configurations, the efficiency of the water oxidation catalyst (WOC) is a key determinant of the overall energy storage efficiency. The present thesis fits within this context, being focused on the study of different WOCs based on earth abundant elements (i.e. Mn and Co). Different heterogeneous Mn-based WOCs were studied, including: (i) different crystal structures of Mn oxides, both commercial and lab-made (i.e. MnO2, Mn3O4 and Mn2O3); (ii) a calcium manganese oxide (containing Ca2Mn3O8 and CaMn2O4) and (iii) lanthanum manganites (LaMnO3) prepared via sol-gel (SG) and flame spray pyrolysis (FP). On the other hand, the heterogenization of a homogeneous Co based WOC, i.e. the polyoxometalate Na10[Co4(H2O)2(PW9O34)2] (CoPOM), was investigated, as well. CoPOM is known to be a highly active WOC, comprising an active {Co4O4} core stabilized by oxidatively resistant polytungstate ligands. Transferring its solution reactivity to solid substrates is a fundamental step in the realization of a PEC.