• Energy Research

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.

We present results on optical, structural and electrical properties of a-Si1-xCx:H films deposited by plasma enhanced chemical vapor deposition in the low power regime, with C fraction from 0 to 0.28. The absorption coefficient has been obtained in the region 0.73-4.5 eV by means of ellipsometry, reflectance-transmittance and photothermal deflection spectroscopy. The addition of carbon in the alloy increases the disorder and the density of defects: samples deposited with high carbon content have poorer optoelectronic properties. Two conduction regimes are observed: extended state conduction and hopping conduction in the conduction band tail. A visible PL peak that widens and shifts to higher energies as the carbon content increases has been observed. (c) 2006 Elsevier B.V. All rights reserved.

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.

Carbon black based electrodes are generally recognized as state of the art for PEM fuel cell technology due to the high performance achieved with a relatively low Pt content. However, the catalyst carbon support is prone to carbon oxidation. This leads to a loss of the catalyst area and overall performance, along with a higher mass transport loss due to an increased flooding tendency. This phenomenon is particularly severe when the fuel cell experiences repetitive start-stop cycles. Therefore, specific countermeasures against catalyst layer carbon oxidation are required, especially for automotive and backup power applications, where the startup/shutdown rate is considerably high. The authors evaluated a basic design that uses a stack shunt. A properly modified control protocol, which includes the stack shunt, is able to avoid high cathode potential peaks, which are known to accelerate catalyst carbon support corrosion and its negative effects. During two separate durability tests, one adopting the shunt design and another using nonprotected shutdown, a 24-cell stack was subjected to continuous starts and stops for several months and its performance constantly monitored. The results show that when the shunt is used, there is a 37% reduction in the voltage degradation rate for each startup/shutdown cycle and a two-fold increase in the number of startup/shutdown cycles before an individual cell reached the specified “end of life” voltage criteria. Furthermore, ex situ FE-SEM analysis revealed cathode catalyst layer thinning, which is an indication that the emerging degradation mechanism is the catalyst support carbon corrosion, as expected. This provides further support that the constant voltage degradation rate typically experienced in PEMFCs can be primarily attributed to the catalyst support carbon corrosion rate. The proposed shunt protocol is very cost effective and does not require any substantial changes in the system. For this reason, its adoption is recommended as a viable method to decrease the catalyst support carbon corrosion rate and extend the operating life of the PEMFC stack.

Aim of the present work is to establish the optimum deposition conditions for a pure SiH4 + CH4 plasma in order to obtain a good quality baseline a-SiC:H material. The effects of CH4 flow, substrate temperature, and pressure on the optoelectronic properties of deposited films have been carefully studied by means of a numerical procedure which allows to study a large number of parameters with a small number of experiments. Optimized films of a-SiC:H with energy gap in the range 1.84–1.90 eV, Urbach energies below 60 meV, photoconductive gain higher than 106 cm2V−1 and ημτ product higher than 8.9 × 10−8 have been obtained.

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.

?-Al2O3 nanoparticles were prepared with a fast chemical synthesis route.o?-Al2O3 NPs were dispersed in a commercial electrolyte to increase its viscosity.oOptical measurements demonstrate a light scattering effect due to NP inclusion.oThe NP-based electrolyte exhibits faster charge diffusion.oThe presence of NPs improved the long-term stability of the DSCs.