Nanostructured TiO2 Based Materials for electrocatalysis and Photoelectrocatalysis

2011/2012 Fossil energy sources are non-renewable being an irreplaceable endowment produced from millennia of biological and geological processes, which means that on the human time-scale they cannot be naturally regenerated and are only available in a finite amount on earth. Scientific and technological data concerning renewable fuels are exponentially growing and in parallel the governments are more and more attracted by these more sustainable energy sources. Overall, solar energy is the most abundant and easily available renewable resource which, however, has its own problems such as neither constantly available nor distributed equally over the surface of the globe. Hydrogen and various bio-fuels, such as bio-ethanol, biodiesel or biogas, have the potentiality to store the solar energy, playing a crucial role in the development of future renewable energy strategies. Nevertheless, as a general comment, it is very difficult and expensive to harness enough power from them to match the effectiveness of non-renewable resources. Thus, it is a big challenge to develop new and high efficient approach to improve the efficiency in production and use of these renewable resources. Nanotechnology is a key area that can help solving this issue. In fact, by using the tools offered by nanotechnology, it is possible to obtain tailored nanostructured catalytic materials that show remarkably better performance than that currently achievable even with state-of- the-art materials. The fields of catalysis, electrocatalysis, photocatalysis and photoelectron- catalysis are all examples of where nanotechnology is deeply impacting on current science, and in particular in energy related applications. The main focus of this PhD thesis is on nanotechnology applied to material science to enhance the performances of various on two important energy-related processes: namely the Fuel Cells (especially the Direct Alcohol Fuel Cell - DAFC) and the hydrogen production process. The H2 production processes include the electrochemical H2 production approach (the water electrolysis technique) and the photocatalytical H2 production approach (the photocatalytic decomposition of water into H2 technique). In the both the energy conversion processes, TiO2 nanotube arrays (TNTA) architectures were used as substrates and the Palladium (Pd) nanoparticles (NPs) were used as supported nanocatalysts. Therefore the most important results in this thesis are the design, realization, functional testing and characterization of supported Pd nanocatalysts on various TiO2 substrates with tailored and well-defined structures, in addition their use for energy-related applications, which are organized as follows: In the Chapter 1, the general principles of the fuel cells technique; the electrolysis technique; the TNTA substrate architecture and the principles of photocatalytic processes for H2 production are outlined or described in details. In addition, the development status and the preparation strategies of catalysts for the alcohol electrochemical oxidation are introduced in this chapter. In the chapter 2, an overview of the main characterization techniques is reported, all of which have been used in this thesis, in order to study the reactivity and the morphological and chemical properties of the samples. The aim of the present chapter is not that of providing exhaustive information about all the techniques. Rather, it is expected to furnish to the reader the main elements to better appreciate the results obtained and described in the following chapters of this thesis. Since the catalytic performance of the nanocatalysts can be finely turned by their shape, which determines surface atomic arrangement and coordination. In the chapter 3,we report a novel method of metal NPs modification, denoted as Electrochemical Milling and Faceting (ECMF), by which large supported Pd NPs (35 nm) of low-index facets supported on TNTA substrate can be milled into many small NPs (7 nm) with some HIF or high density of step atoms. By this approach, the catalytic activity of supported Pd NPs was enhanced by an order of magnitude to the ethanol electrooxidation, and was even 3 times higher than the highest value reported so far. This new approach to the synthesis of HIF-Pd NPs allows one to control metal loading, particle size and surface structure, independently from each other. Furthermore, in a practical catalytic system, such as the DAFC; the electrolysis system and the photocatalytical H2 production system, the electrochemical activity of the supported catalysts is not the only one parameter which needs to be concerned about, the other parameters for the whole test system’s establishment such as the selection and preparation of the substrate material also need to the carefully optimize. In the chapter 4, a new type of Ti network substrate with the TNTA on top was prepared and introduced into the DAFC test system and also used in the electrolysis and photocatalytical H2 production process. This kind of substrate solved the typical problems of the DAFC such as the fuel solution diffusion limitation and the stability of the as supported catalysts drop during the large current density discharge. It was also proved to be a good choice as the substrate for the Photocatalytic decomposition of alkaline ethanol aqueous into H2, which showed good performances of the H2 photochatalytic evolution. Chapter 5 is the conclusion of my PhD thesis. The results clearly demonstrate the novelty and the advantage of the present approach for the obtainment of active and stable electrochemical catalysts for the DAFC and the electrolysis system, and also represent an important step forward in the exploration of new active nanosystems for the conversion of solar light into storable chemical energy. All the findings greatly contributed to the development of catalytic materials for energy-related applications. XXV Ciclo 1983