Materials Science Forum Vol. 1012

Abstract: The storage of energy generated by photovoltaic system is one problem of it. In this aspect, integrated energy conversion and storage systems, IECSS, using supercapacitors are presented as a solution. Dye sensitized solar cell becomes a main candidate for use in IECSS due its variety of applications. Recent studies shown that zinc oxide (ZnO) is a natural candidate for use in solar cells and supercapacitor due to its high energy density of the order 650 A g^-1. The aims of this paper were: i) the study of the influence of the morfology of nanostructured ZnO nanoparticles obtained by the hydrothermal method using distincts complexing agents: etylenediaminetetraacetic acid (EDTA); hexamethyltetramine (HMT) and diaminometanal (urea), besides commercial ZnO; ii) study of the ZnO and activated carbon at ratio X:Y of 10:90, 20:80 and 30:70 in proportion of mass (%) in the preparation of electrodes. The commercial ZnO, which presents particles with spherical and porous morphology, presented the best capacitance result 8.38 Fg^-1 at 10:90 ratio, that demonstrates the ZnO is an excellent candidate for material for supercapacitor coupled with dye solar cell.

Abstract: This work investigates the microstructural and morphological changes on CIGS thin films submitted to a post-deposition heat treatment. The CIGS 1000 nm-thick films were deposited at room temperature by RF magnetron sputtering onto glass substrates covered with molybdenum films. After deposition, the samples were submitted to a heat treatment, with temperatures ranging from 450 to 575 oC. The treatment was also carried out under a selenium atmosphere (selenization), from 400 to 500 oC. Morphological analyzes showed that the as-deposited film was uniform and amorphous. When the treatment was carried out without selenization, the crystallization occurred at or above 450 oC, and the grains remained nanosized. However, high temperatures led to the formation of discontinuities on the film surface and the formation of extra phases, as confirmed by X-ray diffraction data. The crystallization of the films treated under selenium atmosphere took place at lower temperatures. However, above 450 °C the film surface was discontinuous, with a lot of holes, whose amount increased with the temperature, showing that the selenization process was very aggressive. X-ray diffraction analyses showed that the extra phases were eliminated during selenization and the films had a preferential orientation along [112] direction. The results indicate that in the manufacturing process of solar cells, CIGS films deposited at room temperature should be submitted to a heat treatment carried out at 450 °C (without selenization) or 400 °C (with selenization).

Abstract: Electric double-layer capacitors prepared using activated carbons have been subjected to vacuum heat treatments at low and high temperatures (200, 400, 600, 800 and 1000°C). The electrodes have been tested at a potential of 1.1 V employing a KOH electrolyte (1.0and 6.0 mol.L^-1). The effect of or HDDR upon the electrical properties has been investigated by cyclic voltammetry. It has been shown that the specific capacitance at 5 msV^-1 increases from 50 Fg^-1 to 130 Fg^-1 after a heat treatment at 400°C for 1 hour under back pump vacuum. At 400°C the diminution in the specific capacitance with higher scanning rate (10 msV^-1) was much less pronounced (from 130 Fg^-1 to 100 Fg^-1). Equivalent series resistance (ESR) and equivalent parallel resistance of supercapacitors electrodes have also been investigated. Internal resistances of the supercapacitors were calculated using the galvanostatic curves at current densities (100 mAg^-1).A compositional and morphological evaluation of these electrodes showed no significant change on the activated carbon structure.

Abstract: The microstructure, chemical composition, equivalent series resistance (ESR) and specific capacitance (C_s) of supercapacitors electrodes have been investigated. Commercial activated carbon electrodes employing organic electrolyte have been tested at a potential window of 1.1 and 2.7 V. Specific capacitances were calculated from cyclic voltammetry curves at room temperature employing various scan rates (2-70 mVs^-1). Internal resistances of the supercapacitors were calculated using the galvanostatic cycling curves at several current densities (10-175 mAg^-1). A maximum specific capacity of 58 Fg^-1 has been achieved with the organic electrolyte at a current density of 30 mAg^-1 and a potential window of 2.7V. After this initial study, the organic electrolyte was removed from the electrodes by back pumping vacuum. Two new aqueous electrolytes have been substituted in the commercial electrodes for a comparison: Na₂SO₄ and KOH (1.0 mol.L^-1). At a discharge density of 75 mAg^-1, the electrodes with KOH showed a maximum specific capacitance of 53 Fg^-1 whereas the Na₂SO₄ showed only 6 Fg^-1. ESR of the electrodes with organic electrolyte and KOH were in the range of 20 Ωcm² whereas with Na₂SO₄ of 14 Ωcm². The microstructures of the electrode material have been investigated using scanning electron microscopy (SEM) and chemical microanalyses employing energy dispersive X-ray analysis (EDX). A compositional and morphological evaluation of these electrodes showed a very homogeneous structure.

Abstract: We present here a study on the electrical and structural properties of p-type PbTe films doped with CaF₂. The layers were grown by molecular beam epitaxy on freshly cleaved (111) BaF₂ substrates. The doping level was monitored by the CaF₂ solid source cell temperature (T_CaF2), which varied from 500 to 1150 °C. The films with low doping level, T_CaF2 ≤ 1010 °C, exhibited flat surfaces with crystalline quality close to the undoped PbTe sample. In contrast, samples with high levels of doping (T_CaF2 > 1010 °C) presented CaF₂ agglomerates on the surface and a worse crystal quality. The hole density at 77 K versus T_CaF2 oscillated between 1.3 × 10¹⁷ and 3.6 × 10¹⁷ cm^-3 and did not exhibit a systematic behavior as the fluoride supply is raised. The results indicate that CaF₂ is not an effective p-type dopant for PbTe, due to the abscence of a resonant level close to the valence band or to compensation of extrinsic dopant levels.

Abstract: In the present work, attempts of reducing a graphene oxide powder using a low temperature hydrogenation disproportionation desorption and the recombination process (L-HDDR) has been carried out. A lower processing temperature in large scale production is significant when costs are concerned. Graphite oxide was prepared using a modified Hummers’ method dispersed in ethanol and exfoliated using ultrasonication to produce Graphene Oxide (GO). Investigations have been carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experimental results of L-HDDR processing graphene oxide powder, using unmixed hydrogen at 400°C and relatively low pressures (<2 bars) have been reported. X-ray diffraction patterns showed a reduction of graphene oxide with the L-HDDR process. The results showed that both processes, the L-HDDR as well as the standard HDDR, may be applied to the reduction of graphene oxide in order to produce supercapacitor materials. The advantage of employing the L-HDDR process is a relatively low temperature reducing the cost of treatment, what is a very important factor for producing a large amount of material. Thus, the L-HDDR process has been considered a promising alternative method of reducing graphene oxide with efficiency, with the possibility of large scale production.

Abstract: The equivalent series resistances (ESR) and specific capacitances (C_s) of supercapacitors carbon electrodes have been investigated using cyclic voltammetry and electrochemical impedance spectroscopy. Commercial activated carbon electrodes employing organic electrolyte have been tested using a potential window in the range of 2.7–3.8 V. Specific capacitances were calculated from cyclic voltammetry curves at room temperature employing various scan rates (30 and 100mVs^-1). Internal series resistances of the supercapacitors were measured using the galvanostatic curves at room temperature and above (25 and 50°C). The ESR increase to 9.8 Ω at 25° and 2.7V to 78.8 Ω at with operating temperature raise and also with overpotential. A compositional and morphological evaluation of these electrodes showed a very homogeneous structure. It has been shown that the specific capacitance decreased considerably with scan rate, current density, electrochemical potential window and working temperature.

Abstract: In this article the results of the negative electrode performance produced by La_0.7Mg_0.3Al_0.3Mn_0.4Co_0.5Ni_3.8 as-cast alloy adding 1 to 10% of carbon nanotube (CNT) or reduced graphene oxide (rGO) were investigated as Ni-MH batteries. The as cast alloy were investigated with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The CNT and rGO were characterized by high resolution SEM-FEG. The discharge capacity obtained during the electrochemical characterization showed that in the addition of 1% rGO the discharge capacity was 332 mAh and 1% CNT 364 mAh , being that the rGO batteries maintaining better cyclic stability during the electrochemical test.

Abstract: Nowadays, humanity has become aware of the consequences that the use of fossil fuels entails, and the latest developments in the energy sector are leading to a diversification of energy resources. In this context, researching on alternative forms of producing electric energy is being conducted. At the transportation level, a possible solution for this matter may lie in hydrogen fuel cells. The electrolysis of water is one of the possible processes for hydrogen production, but the reaction to break the water molecule requires a great amount of energy and this is precisely the biggest issue involving this process. In this work, low cost electrodes of 254 stainless steel and electrolytic graphite were used for hydrogen production, allowing high efficiency and reduced oxidation during the process. The selection of these materials allows to obtain a high corrosion resistance electrolytic pair, by replacing the high cost platinum electrode usually employed in the alkaline electrolysis process. The formic acid of biomass origin was used as an electrolyte. It was observed that the developed reactor have no energy losses through heat and it was possible to obtain approximately 82% conversion efficiency in the gas production process.

Abstract: The stability of nanoparticles in natural aquatic systems is of great interest to the environmental risk assessment. The relevance of this study lies in the fact that nanoparticles are being produced and used in commercial products on a large scale, which makes the need to study its transport through the environment, especially in soil and water important due to their potential interactions with the ecosystems. In this research, the effects of nanoparticles of zinc oxide (NPZnO) in the behavior of nanoparticles of titanium dioxide (NPTiO₂) was investigated. The influence of pH, ionic strength and zeta potential of the hazardous nanoparticles into soil landfills are studied using experimental procedures. Leaching experiments were prepared within soil column simulating landfills layers. Leaching experiments were carried out to simulate the capture and attenuation of these nanomaterials in municipal waste landfills. The results found that the presence of NPTiO₂ in suspensions increases the stability of the suspensions keeping higher nanoparticles concentrations, while NPZnO promotes rapid sedimentation with lower equilibrium concentration of nanoparticles.