Multiphase, multifunctional micro- and nano-structured electrocatalytic materials for green chemistry: applications to energy conversion and environmental protection

The development of new electrocatalytic materials for the exploitation of renewable energy sources and the treatment of wastes is a key issue in the optics of the sustainable development of human activities. In this context, the electrochemical research focuses on efficient energy conversion systems and environmental protection devices. Recently, the use of Unitised Reversible Fuel Cell (URFC) as energy conversion device, which combine water electrolysis and fuel cell systems in the same apparatus, has been proposed. URFCs have a high specific energy and allow the production and storage of H2 which can be used on demand. The core of this technology is the electrode material that has to catalyse both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Disperse phase electrode materials, based on nano-structured SnO2-IrO2 mixed oxides synthesized by sol-gel technique, have been revealed to be good electrocatalysts for oxygen reactions and they have been describe in term of synthesis procedure and physico-chemical and electrochemical characterization. The synthesized materials were initially characterized by physico-chemical techniques (XRPD, SEM, XPS and BET) and by voltamperometric investigations using different supports (C-ME, RDE and Ti-plate). The reactivity of powders toward OER and ORR was investigated recording U vs I using in-house designed electrolytic cells and a RDE system. The characterization of the materials led to the selection of the most effective synthetic route (the Impregnation method) to produce powders with very interesting characteristics. More specifically, the powder with the lowest IrO2 content exhibits the highest accessibility of active sites and the highest active area. This corresponds, in oxygen–saturated solution, to higher cathodic currents correlated to the oxygen reduction reaction. In the second part of this Thesis micro- and nano-structured electrocatalytic metals were also investigated for the electroreductive degradation of Volatile Organic Halogenated (VOH), which represent a class of highly toxic molecules. The research has focused on the electrocatalytic properties of silver-based materials. Different types of silver electrodes, obtained from powders or by electrodeposition, were synthesized and electrochemical characterized using CHCl3 as a model substrate. A first study was carried out by means of cyclic voltammetry in CH3CN on electrodeposited Ag (electrolytic baths: KAg(CN)2 and AgNO3), and commercially available silver micro-sized powder. Considering the peak multiplicity and position it is possible to affirm that between the electrolytic baths, the AgNO3 provides the more active surface. In particular the peak multiplicity suggests the presence of extremely active sites and the reduction of trichloromethane proceeds to complete hydrodehalogenation. The silver micro-sized powder was also characterized in aqueous solution and was compared with Ag nanoparticles, synthesised by the chemical-reduction method and supported on active carbon at low Ag content (10wt%). The use of powders allows the setting up of tridimensional electrodes, a feature especially interesting when treating large volume of dilute solutions. Silver nano-particles allow also to reduce the silver loading without appreciable lowering of the electrocatalytic activity of the composite electrode. In conclusion the IrO2-SnO2 powders, synthetized by sol-gel technique with low content of IrO2 exhibit promising futures for OER/ORR and silver nano-particles allow to reduce the silver loading without appreciable lowering of the electrocatalytic activity.