Adélio Mendes

Dye-sensitized solar cells: A safe bet for the future.

This review describes the main features of dye-sensitized solar cells (DSCs) and highlights recent breakthroughs in this promising thin-film photovoltaic (PV) technology. After a brief presentation of the commercially available technologies, the general operation principles and the most relevant characteristics of DSCs are summarized. Recent major advances in high efficiency sensitizers, nanostructured semiconductors and robust electrolytes offer an opportunity for DSCs integration into the marketplace. With attractive features, like low-cost potential, simple processing, wide range of applicability – from low-power electronics to semi-transparent windowpanes for electricity generation – and good performance under typical operating conditions, these cells are one step from large-scale commercialization. We describe major strategies that are under way to make DSCs a key technology in the future PV paradigm.

An overview of photocatalysis phenomena applied to NOx abatement.

This review provides a short introduction to photocatalysis technology in terms of the present environmental remediation paradigm and, in particular, NOx photoabatement. The fundamentals of photoelectrochemical devices and the photocatalysis phenomena are reviewed, highlighting the main reaction mechanisms. The critical historical developments on heterogeneous photocatalysis are briefly discussed, giving particular emphasis to the pioneer works in this field. The third part of this work focus mainly on NOx removal technology considering topics such as: TiO2 photochemistry; effect of the operating conditions on the photocatalysis process; Langmuir-Hinshelwood modeling; TiO2 photocatalytic immobilization approaches; and their applications. The last section of the paper presents the main conclusions and perspectives on the opportunities related to this technology.

Biocompatibility of poly(lactic acid) with incorporated graphene-based materials

The incorporation of graphene-based materials has been shown to improve mechanical properties of poly(lactic acid) (PLA). In this work, PLA films and composite PLA films incorporating two graphene-based materials - graphene oxide (GO) and graphene nanoplatelets (GNP) - were prepared and characterized regarding not only biocompatibility, but also surface topography, chemistry and wettability. The presence of both fillers changed the films surface topography, increasing the roughness, and modified the wettability - the polar component of surface free energy increased 59% with GO and decreased 56% with GNP. Mouse embryo fibroblasts incubated with both fillers exceeded the IC(50) in both cases with a concentration of 10 μg mL(-1). No variations in cell proliferation at the surface of the composite films were observed, except for those containing GO after 24 h incubation, which presented higher cell proliferation than pristine PLA films. Platelet adhesion to PLA and PLA/GNP films was lower in the presence of plasma proteins than when no proteins were present. Furthermore, incorporation of GNP into PLA reduced platelet activation in the presence of plasma proteins. The results indicated that low concentrations of GO and GNP may be incorporated safely in PLA to improve aspects relevant for biomedical applications, such as mechanical properties.

On the stability enhancement of cuprous oxide water splitting photocathodes by low temperature steam annealing

Given the intermittent nature of solar radiation, the large-scale use of solar energy requires an efficient energy storage solution. So far, the only practical way to store such large amounts of energy is in the form of a chemical energy carrier, i.e., a fuel. Photoelectrochemical (PEC) cells offer the ability to convert solar energy directly into chemical energy in the form of hydrogen. Cuprous oxide (Cu2O) is being investigated for photoelectrochemical solar water splitting since it has a band gap of 2.0 eV with favorable energy band positions for water cleavage; it is abundant and environmentally friendly. A major challenge with Cu2O is its limited chemical stability in aqueous environments. We present a simple and low-cost treatment to create a highly stable photocathode configuration for H2 production, consisting of steam treatment of the multilayer structures. The role of this treatment was investigated and the optimized electrodes have shown photocurrents over −5 mA cm−2 with 90% stability over more than 50 h of light chopping (biased at 0 VRHE in pH 5 electrolyte).

Treatment of azo dye-containing wastewater by a Fenton-like process in a continuous packed-bed reactor filled with activated carbon

In this work, oxidation with a Fenton-like process of a dye solution was carried out in a packed-bed reactor. Activated carbon Norit RX 3 Extra was impregnated with ferrous sulfate and used as catalyst (7 wt.% of iron). The effect of the main operating conditions in the Chicago Sky Blue (CSB) degradation was analyzed. It was found that the increase in temperature leads to a higher removal of the dye and an increased mineralization. However, it also increases the iron leaching, but the values observed were below 0.4 ppm (thus, far below European Union limits). It was possible to reach, at steady-state, a dye conversion of 88%, with a total organic carbon (TOC) removal of ca. 47%, being the reactor operated at 50°C, pH 3, W(cat)/Q=4.1 g min mL(-1) (W(cat) is the mass of catalyst and Q the total feed flow rate) and a H(2)O(2) feed concentration of 2.25 mM (for a CSB feed concentration of 0.012 mM). The same performance was reached in three consecutive cycles.

Cyclic adsorption separation processes: analysis strategy and optimization procedure

Abstract An innovative analysis strategy and an optimization procedure have been developed with the purpose of design, evaluation and optimization of small- and large-scale units of cyclic adsorption processes using the classical Skarstroms cycle: pressure swing adsorption (PSA) and vacuum swing adsorption (VSA). The system of partial differential equations of the dynamic simulator model was solved using a recent numerical technique developed within our group, based on an adaptive multiresolution approach, thus ensuring stability and accuracy. The simulator provides models for the multiple phenomena involved in fixed-bed adsorption: pressure drop, mass transfer resistance and energy balance. An extended parametric analysis is presented for the particular case of oxygen production from air by PSA and VSA: influence of the normalized purge flow rate, the high and low pressure values, dimensionless pressurization time, dimensionless production time, pressure drop and temperature in the bed.

Removal of acetone, ethyl acetate and ethanol vapors from air using a hollow fiber PDMS membrane module

Abstract Polyetherimide (PEI) membranes coated with polydimethylsiloxane (PDMS) are frequently used for removing volatile organic compounds (VOCs) from air. An experimental and theoretical study was performed to evaluate the effects of the feed flow rate, feed VOC composition, feed and permeate total pressure using a hollow fiber membrane module for separating acetone, ethyl acetate and ethanol from air. The results are discussed on a basis of permeate flux and permeate VOC concentration. It was found that the hollow fiber module provides good separation capabilities, in spite of the observed PEI sub-layer mass transport resistance, and the proposed model fits the experimental results fairly well.

On the determination of diffusivity and sorption coefficients using different time-lag models

Abstract Different models have been proposed to fit experimental permeation data in order to obtain membrane transport parameters (sorption capacity and solute diffusivity) when using the time-lag method. Depending on the assumptions, their complexity can vary from a simple linear equation to a more complex non-linear partial differential equation. The applicability, limitations and inaccuracies of these different models are discussed. A numerical procedure to solve the time-lag equation in a membrane slab considering permeate pressure build up is presented. This procedure involves the numerical computation of an inverse Laplace transform using a fast Fourier transform. This is applicable to the full range of the experimental permeation data, provided that both sorption and diffusivity coefficients are constant within the experimental pressure range. Due to its simplicity, the linear time-lag equation is the most commonly used model. However, it does not account for permeate pressure build up. It is shown that this leads to errors in the determination of the permeation parameters which increase with the capacitance parameter (related with the volume available for permeation). For values above a certain threshold ( η >0.05) these errors are above 5%. Emphasis is put on providing a simple and straight forward way to minimise them.

Influence of sodium cations of N3 dye on the photovoltaic performance and stability of dye-sensitized solar cells.

We report on the effect of substituting the two tetrabutyl ammonium counter ions of the standard N719 dye by sodium ions on the performance and stability of dye-sensitized solar cells (DSCs). The disodium analogue of N719 in conjunction with a non-volatile electrolyte gives a conversion efficiency of 7.6% under standard global AM 1.5 sunlight. Devices maintain 99% of their initial performance after 1000 h under full sunlight aging at 50 degrees C. Electrochemical impedance spectroscopy and photovoltage transient decay studies reveal the evolution of the solar cell parameters during aging. Remarkably, upon aging a decrease in the rate of electron back reaction with the triiodide ions across the TiO(2)/electrolyte interface appears as well as enhanced electronic conduction in the TiO(2) film.

Peptide-based solids: porosity and zeolitic behavior

Among the greatest challenges in the field of microporous solids is the development of “smart” materials, displaying environment-triggered property tuning. These could be used both in traditional and new applications of microporous materials. In this context, supramolecular peptide-based solids have recently emerged as interesting alternatives to standard microporous solids, such as zeolites and carbon molecular sieves. They possess framework and conformational flexibility, are kinetically stable and reasonably thermally resistant. Important properties such as pore size and inner wall chemistry can be controlled through appropriate chemical modification of the peptide molecules. Peptide-based porous solids have permanent microporosity, often with molecularly sized cavities created by removal of co-crystallised solvent. Some have already been successfully tested as adsorbents and permselective materials, confirming their potential. This review covers the identification, synthesis, characterization techniques and properties of peptide-based microporous solids, discussing their most unique functionalities.