MULTIFUNCTIONAL NANOSTRUCTURED MATERIALS FOR THE DEVELOPMENT OF ELECTROCHEMICAL TECHNOLOGIES FOR THE ENERGY AND THE ENVIRONMENT

The environmental protection / remediation and the rational use of energy resources are fundamental topics for the balanced development of civil and industrial activities. In this context, the electrochemical technologies can offer many solutions: from accumulation / generation of energy to the design / implementation of processes with low environmental impact and reduced energy consumption. Core of a large part of the modern electrochemical systems are nanostructured multifunctional materials; In this context the PhD Thesis is focused on to the development of these materials: their electrocatalytic and functional properties can be modulated through an appropriate design, synthesis and application to the wide areas of green chemistry and energy conversion The Thesis work was articulated into two main parts, dedicated to silver-based and iridium-based materials, respectively. Silver-based nanostructured materials have been developed for environmental applications. The challenge represented by the degradation of organic halides (in the various soil, liquid and gaseous environments) have been highlighted and discussed and the electrocatalytic properties of silver as a cathode material for the dehalogenation of several classes of organic halides has been studied. In this work the research interests focus on the preparation methodologies of the Ag-electrode material, both to improve the catalyst performance and reduce the silver content. There is evidence that nano-structured particles exhibit better behaviour than massive silver while allowing a substantial reduction of Ag loading. As for the experimental conditions, the characterization of all prepared materials has been conducted both in aqueous and in organic media. The tests in aqueous media was conducted using a Cavity MicroElectrode (C-ME): this particular electrode is an innovative tool for the study of finely dispersed materials to be adopted in several electrochemical systems. The C-ME allows to (i) minimize the ohmic drop effect thanks to the micrometric size and therefore to the low associated current intensities; (ii) rule out both the contribution of a gluing agent on the electrochemical response and (iii) any contribution from the current collector, i.e. the micro-disk at the base of the cavity, since its surface area is negligible in comparison with the one of the hosted material. For the tests performed in aqueous media, tri-chloromethane was chosen as model substrate used. Analogously, benzylchloride (BzCl) was chosen for the tests in organic media, and specifically in acetonitrile (ACN). The particular attention for BzCl reduction is due to the recent new proposal for the reaction pathway that implies the interaction between the catalytic surface and the organic moieties of the substrate and the reaction intermediates is crucial and explains the extraordinary activity of Ag. Different strategies were adopted for the Ag-nanoparticle syntheses: (i) a polymer-mediated polyol process, that allows for the preparation of silver nanostructures with a number of different well-controlled morphologies (e.g., cubes, rods, wires, and spheres); (ii) the electrochemical synthesis, a facile route that leads to particles of controlled size by the easy adjusting of the current density; (iii) wet synthesis, a chemical reduction from aqueous solutions, this is an effective method for obtaining nano-sized powders and colloidal dispersion of silver. It is common knowledge that the chemical reduction method involves reduction of metal salt in the presence of a suitable protecting agent (organic stabilizer), which is necessary for controlling the growth of metal colloid. The fully characterised Ag-NP’s were supported on carbon matrices and used for the electrochemical dehalogenation of chlorinated organic compounds. In fact, the supporting matrix can play an important role in terms of stability, durability and accessibility of the electrocatalytic sites of the composite powder. The straightforward choice for supporting metal nanoparticles was represented by carbon-based materials. Interestingly, the carbon surface does not always behave as an inert electrode, but can play a role in the kinetics of the electroreduction reaction. For all these reasons, part of scientific activity was focused on the study of the most used support, that is Vulcan® XC72-R (Cabot). In particular, silver nanoparticles are supported on both as received (R-C) and HNO3-pretreated (P-C) Vulcan®. This oxidative pretreatment was made to increase the surface oxo-groups and both the electrochemical behaviour of the mere carbon and the possible synergistic effects on composite materials were analysed. The study of Iridium-based materials is devoted to the exploitation of iridium oxide as component of single and/or mixed metal oxide multifunctional nanostructured materials. Iridium oxide is well-known for its large number of applications including sensors, electrical neural stimulation, electrochromic systems, energy conversion and storage devices, and organic pollutants degradation[ ]. A particular interest is devoted toward IrO2 for its high activity as catalyst for the oxygen evolution reaction (OER, i.e. water oxidation) in acidic media. OER is the anode reaction usually coupled with most electrochemical processes in aqueous media, notwithstanding the rather high overvoltages required for OER to occur. Several oxides have been proposed as electrocatalysts for OER in acid media: IrO2, RuO2, PtO2, MnO2. Among them, RuO2 is the most active one, followed by IrO2, which, in turn, is the most stable. Consequently, IrO2 represents one of the most promising materials in the preparation and development of new catalysts for energy conversion devices. In particular in this Thesis the role of iridium oxide for the water oxidation will be discussed and analysed, using a new in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials while varying at will the electrode potential by voltammetric analysis. The preparation, characterization and electrocatalytic properties of different classes of disperse phase materials for environmental and energy-oriented applications of the future emerging technologies, are described and discussed. The key materials were developed together with key investigation tools and techniques like the Cavity-MicroElectrode hosting device and the FEXRAV combined electrochemical and XAS analysis. In particular, the electrocatalytic activity of silver-based composite materials is well proved by the electrochemical characterization. Experimental data highlight the better performance of the silver nanoparticles in comparison with a commercial silver catalyst. Among the tested nanoparticles, those prepared by electrosynthesis show the best activity probably due to synergistic effects of hydrophilic surface close to small electrocatalytic particle size. Carbon support HNO3 pretreatment ensures not only a better affinity between Ag and the carbon matrix, allowing an effective Ag clamping with homogeneous dispersion, but also the presence of oxo-groups, which impart hydrophilicity to the carbon surface. In addition the cavity microelectrode was further developed and exploited as a tool for qualitative and quantitative screening of materials. In particular, the use of C-MEs as hosting tools for Ag powders to be investigated as catalysts for the electroreduction of trichloromethane evidenced how the filled micrometric cavities may behave either as microdisks of the filling material or as 3-D electrodes whose response quantitatively depends on the amount of inserted powder. The reductive dehalogenation of organic chlorides in acetonitrile+water media on silver electrodes was also investigated: the presence of a proton donor like water favors the benzyl chloride reduction in CH3CN, as evidenced by the progressive positive shift of the reduction peak potential. The water content also influences the reaction mechanism, as evidenced by the change of peak shape and of the corresponding α values. As useful side effect, the background current increases because of the onset of the hydrogen evolution reaction. In all cases, Ep, α and j (at −2.5V) tend to stabilise for xH2O ≥ 0.06. Preliminary results of an innovative technique are also presented: the scanning electrochemical microscopy (SECM) using the micropipette delivery-substrate collection (MD-SC) was studied. In this case the technique involved the use of glass micropipettes instead on metal tips for achieving a controlled delivery of CHCl3 in the solution, since CHCl3 is not an easy reagent for electrochemical generation. In this way, it is possible having a fast screening on the activity of different silver nanoparticles for the CHCl3 reduction. Moreover the new FEXRAV technique, designed, implemented and applied by this group, allows to rapidly study any species that can be immobilized onto a conductive substrate, in terms of its oxidation state transitions (or any other property that causes a change in the X-Ray absorption coefficient) in dependence on the applied potential and extract important information on the reaction mechanisms. This is important especially if the voltammetric signal is not suitable for a “classic” treatment in a complex system like OER in acid media.