Development and testing of electrocatalysts for water splitting and production of green H2

Nowadays the development of low-cost, earth-abundant, non-toxic, noble metal-free, and highly efficient electrocatalysts for the sluggish oxygen evolution reaction (OER) remains one of the most difficult challenges to ultimately be faced to achieve efficient water splitting for the hydrogen production (energy carrier and clean chemical fuel). OER occurs at the anode in water-splitting electrochemical cells, it is more complicated than the hydrogen evolution reaction (HER). It involves the four electrons transfer coupled with O-O bond formation, and cleavage of the O-H bond compared to HER. Thus, this reaction needs higher overpotentials to overcome high kinetic energy barriers and, for this reason, the use of electrocatalysts is envisaged to accelerate the reaction kinetics and reduce the energy barrier. Metal oxide electrocatalysts containing transition metals are developed to facilitate the reaction and to replace the expensive and low reserves of noble metal-based electrocatalysts that are present at an industrial level. This thesis work shows the efforts made to address this challenge, developing an electrocatalyst based on manganese oxobromide (Mn7.5O10Br3), which appears to be an attractive and valid alternative to electrocatalysts based on manganese oxides. Initially, the work was focused on the development of a Mn7.5O10Br3 electrocatalyst, optimizing all three steps of the synthesis, and subjecting the electrocatalysts to characterization using XRD, SEM-EDX, and BET techniques. Subsequently, the samples were subjected to electrochemical tests for electrochemical performance evaluation. Furthermore, the performance of the electrocatalyst is examined through electrocatalytic kinetic parameters (such as overpotential (η, mV) and Tafel Slope (b, mV dec-1)), which give detailed information regarding the reaction mechanism. These parameters in electrocatalysis are influenced by many factors such as, for example, pH of the electrolyte (acid or basic), catalyst, working electrode, ink deposition method, presence of a halide (Br) and others. Therefore, it emerged that Mn7.5O10Br3, obtained with a US-assisted synthesis and drop-deposited on Ti-mesh FTO-coated, is the catalyst with the best performance, leading to an overpotential of 153 mV at a current density of 10 mA cm-2, showing good stability for 1 hour, and Tafel Slope values of 103 mV dec-1 at low current densities and 160 mV dec-1 at high current densities. However, this thesis work also shows many bottlenecks of these electrocatalysts, such as, for example, the inhomogeneity of the electrocatalyst powder with consequent lack of reproducibility of the tests, offering ideas on which to work in the future both to improve the development and the electrocatalytic performances to make it an effective substitute for manganese oxide-based electrocatalysts.