Development and Characterization of Electro- and Photo-Electrochemical Systems for High-Added-Value Products

Compendi d'articles, Doctorat internacionalThe main challenge in the development of electrochemical and photoelectrochemical systems is the kinetics of the anodic oxygen evolution half-reaction (OER), which is intrinsically slow. For this reason, in this thesis all efforts have been devoted to elaborate strategies that allow a better exploitation of the oxidative process in (photo)-electrochemical cells. Among these strategies, on the one hand, porous electrodes based on pencil graphite rods and doped with NiFe have provided a large surface area and have been used for the electrochemical oxidation of water to oxygen. On the other hand, new oxidation systems have been employed, such as the photoelectrochemical oxidation of primary alcohols, which have allowed the selective synthesis of organic chemical products with high added value. Finally, a set of characterization techniques including structural and morphological techniques, as well as analytical and electrochemical methods, have allowed us to differentiate and understand the processes that take place in the (photo)-electrochemical cells studied. Among the different techniques used, impedance spectroscopy has proved to be fundamental for the determination of surface states related to these processes.Programa de Doctorat en Cièncie

Light intensity influence on strontium Titanate based photo-electrochemical cells

The influence of light intensity on photo-electrochemical cells (PECs) consisting of an n-type strontium titanate (SrTiO₃) photoanode and nickel cathode in potassium hydroxide electrolyte is studied. The band levels of an electrolyte metalsemiconductor-electrolyte system are presented and the effect of different light intensities on the energy levels is investigated. Photocurrent density, quantum efficiency, and open circuit potential measurements are performed on the processed PECs under different light intensities (375 nm). It is demonstrated that a threshold value of the light intensity has to be reached in order to obtain positive photo activity and that beyond this value the performance remains nearly constant

Photoluminescence-based measurements of the energy gap and diffusion length of Zn_3P_2

The steady-state photoluminescence spectra of zinc phosphide (Zn_3P_2) wafers have revealed a fundamental indirect band gap at 1.38 eV, in close proximity to the direct band gap at 1.50 eV. These values are consistent with the values for the indirect and direct band gaps obtained from analysis of the complex dielectric function deduced from spectroscopic ellipsometric measurements. Bulk minority carrier lifetimes of 20 ns were observed by time-resolved photoluminescence decay measurements, implying minority-carrier diffusion lengths of ≥ 7 µm

Electrochemical Investigation of High-Performance Dye-Sensitized Solar Cells Based on Molybdenum for Preparation of Counter Electrode

In order to improve the photocurrent conversion efficiency of dye-sensitized solar cells (DSSCs), we studied an alternative conductor for the counter electrode and focused on molybdenum (Mo) instead of conventional fluorine-doped tin oxide (FTO). Because Mo has a similar work function to FTO for band alignment, better formability of platinum (Pt), and a low electric resistance, using a counter electrode made of Mo instead of FTO lead to the enhancement of the catalytic reaction of the redox couple, reduce the interior resistance of the DSSCs, and prevent energy-barrier formation. Using electrical measurements under a 1-sun condition (100 mW/cm(2), AM 1.5), we determined that the fill factor (FF) and photocurrent conversion efficiency (eta) of DSSCs with a Mo electrode were respectively improved by 7.75% and 5.59% with respect to those of DSSCs with an FTO electrode. Moreover, we have investigated the origin of the improved performance through surface morphology analyses such as scanning electron microscopy and electrochemical analyses including cyclic voltammetry and impedance spectroscopy

Synthesis of modified fullerenes for oxygen reduction reactions

In the search for carbon based catalysts for Oxygen Reduction Reactions (ORR), two different classes of fullerene hybrids and metal free fullerene derivatives have been prepared by highly selective metal- and organo-catalyzed synthetic methods. They were included as both electron acceptors and catalysts in polymer-based photo-electrochemical cell,fully demonstrating electrocatalytic activity. Remarkably, the activity of the metal free fullerenes proved to be as high as that observed for metallofullerenes based on noble metals, and up to ten-fold higher than by using PCBM

Autocatalytic and cooperatively-stabilized dissociation of water on a stepped platinum surface

Water-metal interfaces are ubiquitous and play a key role in many chemical processes, from catalysis to corrosion. Whereas water adlayers on atomically flat transition metal surfaces have been investigated in depth, little is known about the chemistry of water on stepped surfaces, commonly occurring in realistic situations. Using first-principles simulations we study the adsorption of water on a stepped platinum surface. We find that water adsorbs preferentially at the step edge, forming linear clusters or chains, stabilized by the cooperative effect of chemical bonds with the substrate and hydrogen bonds. In contrast with flat Pt, at steps water molecules dissociate forming mixed hydroxyl/water structures, through an autocatalytic mechanism promoted by hydrogen bonding. Nuclear quantum effects contribute to stabilize partially dissociated cluster and chains. Together with the recently demonstrated attitude of water chains adsorbed on stepped Pt surfaces to transfer protons via thermally activated hopping, these findings candidate these systems as viable proton wires.Comment: 19 pages, 4 figure