• Energy Research

Abstract A new-designed transparent, conductive and porous electrode was developed for application in a compact laboratory-scale proton exchange membrane (PEM) photo-electrolyzer. The electrode is made of a thin transparent quartz sheet covered with fluorine-doped tin oxide (FTO), in which an array of holes is laser-drilled to allow water and gas permeation. The electrical, morphological, optical and electrochemical characterization of the drilled electrodes is presented in comparison with a non-drilled one. The drilled electrode exhibits, in the visible region, a good transmittance (average value of 62%), a noticeable reflectance due to the light scattering effect of the hole-drilled internal region, and a higher effective surface area than the non-drilled electrode. The proof-of-concept of the applicability of the drilled electrode was achieved by using it as a support for a traditional photocatalyst (i.e. commercial TiO 2 nanoparticles). The latter, coupled with a polymeric electrolyte membrane (i.e.Nafion 117) and a Pt counter electrode, forms a transparent membrane electrode assembly (MEA), with a good conductivity, wettability and porosity. Electrochemical impedance spectroscopy (EIS) was used as a very powerful tool to gain information on the real active surface of the new drilled electrode and the main electrochemical parameters driving the charge transfer reactions on it. This new electrode architecture is demonstrated to be an ideal support for testing new anodic and cathodic photoactive materials working in tandem configuration for solar fuels production by water photo-electrolysis.

Metal-free dye molecules for dye-sensitized solar cells application can avoid some of the typical drawbacks of common metal-based sensitizers, that are high production costs, relatively low molar extinction coefficient in the visible region, limited availability of precursors, and waste disposal issues. Recently we have proposed an innovative organic dye based on a simple hemi-squaraine molecule (CT1). In the present work, the effect of the sensitization time of the TiO2photoelectrode in the dye solution is studied with the aim of optimizing the performance of CT1-based DSCs. Moreover, the addition of the chenodeoxycholic acid (CDCA) as coadsorbent in the dye solution at different concentrations is investigated. Both CT1-sensitized mesoporous TiO2photoanodes and complete solar cells have been fully characterized in their electrical and absorption properties. We have found that the best photoconversion performances are obtained with 1 hour of impregnation time and a 1 mM CDCA concentration. The very fast kinetics in dye adsorption, with optimal sensitization steps almost 15 times faster than conventional Ru-based sensitizers, confirms the theoretical predictions and indicates a strong interaction of the semisquaric acid group with the anatase surface. This result suggests that this small molecule can be a promising sensitizer even in a continuous industrial process.

A highly efficient ZnO photoanode for dye-sensitized solar cells was successfully grown by a simple, low cost, and scalable method. A nanostructured coral-shaped Zn layer was deposited by sputtering onto fluorine-doped tin oxide/glass slices at room temperature and then thermally oxidized in ambient atmosphere. Stoichiometry, crystalline phase, quality, and morphology of the film were investigated, evidencing the formation of a highly porous branched nanostructure, with a pure wurtzite crystalline structure. ZnO-based dye-sensitized solar cells were fabricated with customized microfluidic architecture. Dye loading on the oxide surface was analyzed with ultraviolet-visible spectroscopy, and the dependence of the cell efficiency on sensitizer incubation time and film thickness was studied by current-voltage electrical characterization, incident photon-to-electron conversion efficiency, and impedance spectroscopy measurements, showing the promising properties of this material for the fabrication of dye-sensitized solar cell photoanodes with a solar conversion efficiency up to 4.58%. Copyright © 2012 John Wiley & Sons, Ltd.

Abstract Carbon nanotubes (CNTs) exhibit extraordinary mechanical, electronic and thermal properties, because of which they have been used in several applications like mechanical reinforcement of polymers, electrocatalysis, sensors, electronics, batteries, etc. Nafion is a sulfonated fluoropolymer that has become standard material in fuel cell applications for its excellent proton conducting property, and for its thermal and mechanical stability. Recently, Nafion/CNTs composites with CNT amounts less than the percolation threshold have been used to increase the mechanical stability and decrease methanol permeation of nafion, with very little effect on proton conductivity and avoiding short circuit risks. Above the percolation threshold, such membranes show the potentialities to allow a separate proton/electron conduction paths within the membrane. This aspect opens new ways for applications in selective membranes for artificial innovative devices capable of using sunlight to produce hydrogen from water splitting. Nafion + multi-walled CNTs (MWCNTs) nanocomposite films with varying amounts of filler were fabricated and their proton/electron transport behaviors were characterized. Samples were prepared by recasting Nafion with MWCNTs and conditions were optimized to obtain freestanding films of thickness around 100 μm. After ultrasonication, we observed a uniform dispersion of the carbon fillers inside the matrix, with a good affinity between the two materials without the need of CNT surface functionalization. Membranes were characterized by Scanning Electron Microscopy, Electrochemical Impedance Spectroscopy and Electronic Conductivity tests.

The electric response to an external periodic voltage of small amplitude of dye-sensitized solar cells (DSCs) made up with an alternative architecture has been investigated. DSCs have been fabricated with a reversible sealing structure, based on microfluidic concepts, with a precise control on the geometric parameters of the active chamber. Cells with different electrolyte thicknesses have been characterized, without varying the thickness of the TiO2layer, both under illumination and in dark conditions. Measurements of the electric impedance have been performed in the presence of an external bias ranging from 0 V to 0.8 V. The experimental data have been analyzed in terms of a transmission line model, with two transport channels. The results show that the photovoltaic performances of the microfluidic cell are comparable with those obtained in irreversibly sealed structures, actually demonstrating the reliability of the proposed device.

Abstract The interest in clean energy is of increasing importance nowadays. In this regard, hydrogen production from sun-assisted water splitting is a field of intense research in the scientific community. In the design of artificial innovative devices devoted to this task, the simultaneous transport of protons and electrons from an anode (where they are generated by water oxidation) to a cathode (where they have to recombine in order to form molecular hydrogen) is essential. Nafion, a sulfonated fluoropolymer, is of great interest because of its proven ability as a proton-conducting membrane in fuel cells, and also for its good thermal and mechanical stability. Carbon nanotubes (CNTs), rolls of graphene sheets, possess a high thermal and mechanical stability together with excellent electron conductivity. Nafion/CNTs nanocomposites with CNT amounts above the percolation threshold have been recently obtained which are capable of simultaneously conducting protons and electrons. Here we report on the fabrication of nafion/CNTs membranes in an alternative configuration, namely vertically-aligned carbon nanotubes columns embedded in a nafion matrix. Nanocomposite films of nafion embedded with an array of CNT columns were prepared by solvent casting. Films of thickness around 100 μm were obtained. They were characterized by scanning electron microscopy, electrochemical impedance spectroscopy, electronic conductivity tests, water uptake and water contact angle. Electronic and protonic conductivities were measured in ambient and wet conditions and turned out to be higher than those previously reported for nafion/CNTs nanocomposites in the conditions where such devices are expected to operate.

Here we report on a novel polymer electrolyte membrane for quasi-solid dye-sensitized solar cells (DSSCs) with excellent efficiency and extended durability. The electrolyte is prepared by an elegant, rapid and cheap UV-induced polymerization method and the chemometric approach is used for the first time, to the best of our knowledge, for the optimization and the fine tuning of the experimental conditions.