Mostrando artículos por etiqueta: 2020 publication

Abstract

In this work, it is proven that a biochar obtained from a commercial gasifier can be used as electrode material for supercapacitors (SC). This biochar was produced at 1000 °C from corn cob wastes (GAS), and was compared to an activated biochar obtained in a traditional lab pyrolysis process (LAB). Both biochars were characterized by different physicochemical techniques, observing their amorphous nature with well-developed microporosity dependent of their pretreatment and production methodology. Furthermore, a computational modeling based on Molecular Dynamics at the ReaxFF level was also performed to elucidate the geometry of the resulting microporous structure after simulated pyrolysis. X-ray structure and pore size distribution are in agreement with those results obtained via computational simulation. Both carbon materials were electrochemically evaluated in acidic electrolyte using 3 and 2 electrode systems, obtaining capacitances of 130 F g−1 (20 mV s−1), and excellent performance compared to commercial activated carbons, with only about 10% of capacitance loss after 5000 cycles. However, GAS performance in SC was higher than activated biochar due to its higher micropore volume. This study provides a novel useful application to use gasifier residues from agricultural biomass waste for energy storage devices.

Abstract

This work aims to propose a sustainable green process to obtain bio-derived carbons (BDCs) for utilization in supercapacitors. The process consists in carrying out solar pyrolysis to produce BDCs from abundant lignocellulosic wastes, Agave Angustifolia leaves and pruned tomato plant. Concentrated solar radiation from a high flux solar furnace was utilized to reach sample temperatures between 450 and 1564 °C in a spherical reactor. Before pyrolysis, both wastes were characterized by thermogravimetric analysis to semi-quantify cellulose and hemicellulose as well as ash content. XRD was used to determine the ash composition in both wastes, and the effect of solar pyrolysis temperature on the obtained BDCs. Additional structural properties of BDCs were analyzed by SEM, Raman spectroscopy, and physisorption.

Elemental analysis and EDAX were used to determine the chemical composition of wastes, and the effect of this on BDCs. Electrochemical properties of BDCs were analyzed by cyclic voltammetry in half cells, and those showing better performance were also tested in supercapacitor cells. Results show that BDCs from tomato plant waste have higher surface areas, with well-developed microporosity, without needing an additional activation process. This is attributed to self-activation during pyrolysis, produced by the high K and Ca content of the tomato plant pruning. Ragone plots indicate that the assembled supercapacitor cells employing the best BDCs from solar pyrolysis have specific energies and power values similar to a commercial carbon designed for supercapacitors. These results indicate that the proposed green procedure is suitable for obtaining BDCs with properties suitable for supercapacitors.

Abstract

As a semiconductor of “earth-abundant” elements, Sn2S3 with a bandgap (Eg) close to 1 eV merits attention, but a method to prepare phase-pure thin film remains elusive. We report the formation of Sn2S3 thin film of 360 nm in thickness by heating chemically deposited tin sulfide thin films at 450 °C during 30–45 min in presence of sulfur at a pressure, 75 Torr of nitrogen. Energy dispersive x-ray emission spectra and grazing incidence x-ray diffraction established a reaction route for this conversion of SnS completely to Sn2S3 via an intermediate phase, SnS2. The optical bandgap of the material is 1.25 eV (indirect) and 1.75 eV (direct, forbidden). The optical absorption suggests a light-generated current density of 30 mA/cm2 for the Sn2S3 film (360 nm) as a solar cell absorber. Thin film Sn2S3 formed in 30 min heating has a p-type electrical conductivity in the dark of 1 × 10−4 Ω−1 cm−1, which increases to 3 × 10−4 Ω−1 cm−1 in 0.2 s under 800 W/m2 tungsten-halogen illumination. An estimate made for its mobility-lifetime product is, 6 × 10−6 cm2 V−1. We discuss the prospects of this material for solar cells.

Abstract

Hybrid perovskite films are prepared via two-step spin coating. The impact of magnetic fields during the spin coating of the PbI2 precursor solution is assessed with atomic force microscopy and scanning electron microscopy of the obtained layers. Deeper and narrower peak–valley–peak formations are obtained in PbI2 films when the magnetic field applied and the spinning direction result in a Lorentz force that pushes the [PbI6]4− ions towards the inner area of the substrate. This produces rougher and more porous PbI2 films with an increased surface area that facilitates the infiltration of the methylammonium iodide and chloride precursor solution, thereby enhancing the formation of perovskite. Increased cell performance and more repeatable results are obtained when the PbI2 film is spin-coated under the influence of a negative magnetic field. Opposite effects are obtained when the direction of the magnetic field, and therefore the Lorentz force, is inverted. This demonstrates that a magnetic field can be used to modify the surface morphology of spin-coated thin films prepared from ionic precursor solutions.

Abstract

The use of controlled inert ambient to produce hybrid perovskite solar cells (PSCs) makes them less competitive towards the commercialization. Herein, progress is made with the preparation of hybrid perovskite (CH3NH3PbI3) in a high relative humidity (RH ~ 60%) ambient condition by using a mixture of ethyl acetate (EA) and 4-Tertbutyl-Pyridine (tBP) as anti-solvent. A small amount of tBP helps to form a more homogeneous perovskite surface with a higher hydrophobicity. The mixture of EA and tBP is found to be an efficient anti-solvent to extract the primary solvent and moisture in the precursor solution and form mirror-like perovskite thin films.

More than 200 perovskite solar cells, with active area of 0.1 cm2, were prepared with the perovskite thin films prepared under ambient conditions. It is showed that a small amount of tBP in EA improves consistently all the photovoltaic parameters of our PSCS, from an average efficiency of 8.64% over 113 cells with only EA as anti-solvent to 13.64% over 131 cells with EA + tBP as the anti-solvent. More than 55% of the last ones show efficiencies higher than 16%. The champion cell, recording an efficiency of 17.41% in the beginning and reaching the maximum efficiency of 18.04%, remains more than 80% of the initial efficiency after more than 180 days of storage in ambient conditions without encapsulation. The proposed method opens the possibility to fabricate highly efficient and stable perovskite solar cells under ambient condition without glove box.

Abstract

In this work, an experimental and theoretical study on CH3NH3PbI3 perovskite solar cells was performed. A theoretical validation of experimental results in perovskite solar cells with efficiencies of 13.32% is presented. An optimization study which involves the spiro-OMeTAD and perovskite thickness’ influence on electrical output parameters (Voc, Jsc, FF and PCE) showed a promotion of solar cell efficiency to 15.50% under 100 nm and 400 nm for hole transport material and absorber, respectively. The importance of the diffusion length of the absorber is discussed. In order to enhance the efficiency, a study of defect density (NT) was applied at the range of 1016 cm−3 (experimental) to 1010 cm−3 (theoretical) where we achieved an efficiency of 20.26%. The present work illustrates the importance of thickness optimization and the reduction of defect density (by the improvement of the quality of processed film) to obtain a better performance of this type of solar cell. Furthermore, the relevance of the implementation of a back contact with higher work function was studied.

Abstract

Interfacial charge transfer is a determining factor for the development of highly efficient solar cells. In antimony-based solar cells, interfacial engineering at the absorber/electron transport material interface relies on the use of a toxic CdS interlayer. In this work, an environmental-friendly interfacial engineering approach for planar Sb2S3 solar cells was implemented by using graphene-based nanoribbons. Graphene nanoribbons (GNR) and sulfur-doped graphene nanoribbons (S-GNR) sheets were incorporated as interlayer between TiO2 and Sb2S3 films in planar Sb2S3 solar cells, resulting in an enhanced photovoltaic performance up to 4.1%. Kelvin probe and C-V measurements revealed that the improvement was related to the superior built-in voltage due to the lower work function of the graphene-based interlayers along with a suitable cascade interfacial charge transfer. More importantly, surface photovoltage transient and intensity-modulated photocurrent and photovoltage spectroscopies also demonstrated that the presence of these interlayers decreased the electron transport time and suppressed the formation of interfacial states, which in turns reduced the interfacial recombination pathways boosting the performance of the devices fabricated. Although S-doped GNR provides the best results, more work is in progress to determine the cause of increased efficiency and the lack of significant Voc increase.

Abstract

The use of concentrated solar energy in processing ceramic materials is an attractive route to obtain these materials with low CO2 emissions. In this work, nanostructures of monoclinic zirconia (m-ZrO2) were obtained using concentrated solar energy provided by the IER-UNAM solar furnace as a heat source. In the first stage of the process, a Zr/O/C complex was obtained by sol-gel method at a temperature of 120 °C using zirconium n-propoxide and sorbitol as precursors reagents. This complex was used in a second stage to obtain m-ZrO2 by heating it at a temperature of 1200 °C for one hour in air atmosphere. This last stage was performed in a solar furnace. Samples were analyzed by characterization techniques: FT-IR, TGA/DSC, XRD, TEM, and SEM confirming the formation of nanostructures of zirconia in monoclinic phase.