Development of Catalytic Electrodes and Cell Design for Solar Fuel Generation

The possibility of exploiting solar energy for the direct production of fuels and chemicals (e.g. hydrogen, hydrocarbons, alcohols) represents a future challenge to move towards a new green economy, recently defined as “solar-driven chemistry”. In this view, this PhD thesis focuses on the development of a new approach to convert solar energy, through the synthesis of innovative photoactive materials/electrodes for the production of solar fuels. By assembling these photo-electrodes in a photo-electrochemical (PEC) cell, designed on purpose to mimic what nature does with the leaves, solar energy can be captured and used to produce hydrogen from water (by water photo-electrolysis) or to generate value-added carbon products (by reducing atmospheric CO2) in a one-step process, like an “artificial leaf”. Thus, the main objective of the present PhD work was to develop photocatalytic thin films able to work as photoanodes in efficient PEC devices, especially for the production of hydrogen. The PhD activities were carried out at the laboratory CASPE/INSTM (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. During the three years of activity, all the aspects concerning the performances of the photocatalysts and the related PEC electrodes and cell, have been carefully evaluated. Initially, the research activity focused on the preparation of titania (TiO2) nanotubes synthesized by controlled anodic oxidation technique. The peculiarity of this method is the possibility to “tailor” the morphology and the nanostructure of the catalyst by modulating some parameters during the synthesis (such as the electrolyte composition, the pH, the applied voltage, the anodization time). In general, the use of titanium dioxide as a photocatalyst, despite many advantages (low cost, non-toxicity, resistance to photocorrosion, high quantum yield), has two main drawbacks: i) the low absorption of light in the visible region, due to the high band gap (in the range of 3.0-3.2 eV) and ii) the fast charge recombination, which usually occurs at the grain boundaries of the particles. The latter can be mitigated by the realization of nanostructures such as nanotubes or nanorods, which may improve the vectorial transport of electrons to the collector layer. VI Different characterization techniques (SEM-EDX, TEM, XRD, UV-vis Diffuse Reflectance Spectroscopy) were used to investigate the properties of the as-prepared TiO2 nanotube arrays, as well as to evaluate their electrochemical behaviour (Cyclic voltammetry, Chronoamperometry). Part of the characterization by electron microscopy was carried out in collaboration with the Department of Chemical Sciences of the University of Padua. The main aim was to obtain a correlation between synthesis parameters, nanostructure properties and photo-catalytic performances. Moreover, particular attention was given to the evaluation of the efficiency of the PEC cell. To pursue this aim, titania nanotubes of different lengths (from 0.5 to 6 m) were synthesized by varying the anodization time from 30 min to 5 h. A monochromator and a spectroradiometer were used to evaluate the light irradiance at different wavelengths directly inside the PEC device. These measurements allowed for the calculation of different kinds of efficiencies: i) the photoconversion efficiency, also called solar-to-hydrogen efficiency (STH), which takes into account the amount of energy supplied in terms of light and the products obtained (i.e. hydrogen); ii) the Faradaic efficiency (η), which relates the photo-generated current to the produced hydrogen; iii) the quantum efficiency, expressed as IPCE (incident photon to current efficiency) and APCE (absorbed photon to current efficiency). The most important results (reported in detail in Chapter 3) showed that, for use in a PEC cell, the 45- min-anodized nanotube arrays (tube length of about 1 μm) provided the best performance, with a H2 production of 22.4 mol h-1 cm-2 and a STH efficiency as high as 2.5%. These values are among the best ever reported insofar as undoped TiO2 photoanodes are used and in absence of external bias or sacrificial agents. The final part of Chapter 3 was dedicated to the preparation of 3D-type meso/macro porous structured photoanodes based on Ti mesh, working as a hierarchical structure (consisting of Ti mesh macropores and TiO2 nanotube mesopores) to improve the mass and charge transport within the PEC cell. In order to improve the light absorption in the visible region, it is necessary to dope the nanostructured TiO2 materials with heteroatoms or decorate their surface with metal nanoparticles. In this direction, nanoparticles of gold (Au) were deposited on the surface of TiO2 nanotubes by optimizing three different techniques (wet impregnation, photo-reduction and electrodeposition) and their performances were studied by using a gas photo-reactor (GP) VII and a photo-electrochemical (PEC) cell, both homemade. Furthermore, with the aim of exploiting earth-abundant and low-cost materials, photocatalysts based on Cu-doped TiO2 nanotubes were also synthesized and successfully tested in the PEC cell for H2 production in water-photo-electrolysis and ethanol photo-reforming. This part of the work was carried out in collaboration with the Institute of Chemistry in Araraquara (Brazil). The CuO nanoparticles were deposited by adopting two different techniques, dip-coating and electrodeposition. The results (reported in detail in Chapter 4) showed that the presence of small metal (Au and Cu) nanoparticles strongly increased H2 production rate in a gas photo-reactor, with a maximum of about 190 mol in 5 h of light irradiation obtained for Au-doped TiO2 nanotubes prepared by electrodeposition. However, in the PEC cell (with oxidation/reduction half reactions separated in two different chambers of the cell) it was observed that the presence of metal nanoparticles on TiO2 surface at the photo-anode can create a counter-circuited current, diminishing the H2 production at the cathode side. However, this phenomenon was successfully minimized by preparing very small CuO nanoparticles (lower than 2 nm) decorating the internal walls of the TiO2 nanotubes by controlled dip-coating technique. Finally, nanostructured tantalum oxynitride (Ta-oxy-N) electrodes were synthesized through controlled anodic oxidation technique, by adapting the synthesis conditions previously optimized for TiO2. The advantages of these tantalum-based materials refer to their lower band-gap (1.9-2.5 eV) with respect to titania (3.0-3.2 eV), thus improving light absorption in the visible region. After the anodization, a high temperature nitridation process (600-900 °C) was needed to replace partially oxygen with nitrogen in the Ta2O5 lattice. The results (reported in detail in Chapter 5) allowed to obtain a clear correlation between the parameters using during the synthesis (i.e. applied voltage, anodization time) and the Ta-oxy- N nanostructures (nanotube diameter and length, wall thickness and grade of voids). The best photocurrent response was obtained for the Ta-oxy-N sample anodized at 40 V for 1 min and then thermally treated with ammonia at 800°C. However, further investigation is needed to improve the mechanical resistance of these photo-catalysts.