Loading...
5 results
Search Results
Now showing 1 - 5 of 5
- Convergence time to equilibrium distributions of autonomous and periodic non-autonomous graphsPublication . Morais Silva, Teresa; Silva, Luís; Fernandes, SaraWe present some estimates of the time of convergence to the equilibrium distribution in autonomous and periodic non-autonomous graphs, with ergodic stochastic adjacency matrices, using the eigenvalues of these matrices. On this way we generalize previous results from several authors, that only considered reversible matrices.
- Equilibrium distributions of discrete non-autonomous graphsPublication . Morais Silva, Teresa; Silva, Luis; Fernandes, SaraWe introduce the notions of equilibrium distribution and time of convergence in discrete non-autonomous graphs. Under some conditions we give an estimate to the convergence time to the equilibrium distribution using the second largest eigenvalue of some matrices associated with the system.
- Enhancement of the Ni-Co hydroxide response as energy storage material by electrochemically reduced graphene oxidePublication . Adán-Más, Alberto; Duarte, Raquel G.; Morais Silva, Teresa; Guerlou-Demourgues, Liliane; MONTEMOR, MARIANickel-Cobalt double hydroxide materials are combined with Electrochemically Reduced Graphene Oxide (Er-GO) on top of Stainless Steel collectors by means of a one-step pulsed electrodeposition technique. Results show that, when Er-GO is integrated into the Nickel-Cobalt hydroxide matrix, there is an improvement of the electrochemical performance of the material, which shows increased capacity, stability and rate capability. As seen by Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), Grazing-Incidence X-Ray Diffraction (GIXD) and Raman Spectroscopy, there is an excellent integration between the materials that leads to the enhanced electrochemical response. Capacity values up to 96 mA h/g and a 62% capacity retention after 5000 cycles were achieved. Moreover, scan rates up to 2000 mV/s without loss in electrochemical response were possible proving its good rate capabilities. A detailed study of the cycling degradation phenomena was also done by means of electrochemical impedance spectroscopy (EIS). Thus, this novel electrodeposited material serves as an excellent material for energy storage applications.
- Hydrothermally grown Ni0.7Zn0.3O directly on carbon fiber paper substrate as an electrode material for energy storage applicationsPublication . Upadhyay, Kush; Eugénio, Sónia; Della Noce, Rodrigo; Morais Silva, Teresa; M.J. Carmezim; M.F. MontemorNickel oxides because of their excellent electrochemical performance have been considered attractive materials for electrochemical energy storage. However, their application as active material for redox supercapacitor electrodes has been limited by poor electrical conductivity. In order to improve this property, herein we synthesized a nanonet of Ni
Zn-mixed oxide, by facile hydrothermal route, directly on the substrate. The Zn-modified oxide material showed good electrochemical performance, displaying specific capacitance of 770 F g−1 at 1 A g−1 and almost 120% capacitance retention after 2000 cycles of charge discharge at 2 A g−1 in 2 M KOH. Electrochemical impedance results revealed that the Ni0.7Zn0.3O mixed oxide displayed increased conductivity compared to the single NiO material.
- Highly porous NiFe nanofoams synthesized by dynamic hydrogen bubble template for hydrogen evolution in alkaline mediaPublication . Carvalho, Gabriel Garcia; do Nascimento, Ricardo Espingardas; Morais Silva, Teresa; Montemor, Maria de FátimaWhen coupled with renewable energy sources, alkaline electrolysis (AEL) is a clean technology to produce hydrogen. The conventional electrodes that have been established as the commercial standard for AEL are perforated nickel plates, which have low surface area and high mass loads of active material. The use of Ni has been associated with the adsorption of protons and their recombination into H2 molecules, assisting and enabling the hydrogen evolution reactions. Due to the rising Ni price, there is a demand for less expensive electrode materials with identical, or better, performance. Thus, this work explores new alternative electrode materials and combines Ni with a cheaper and also electroactive metal, iron. For this purpose, highly porous bimetallic nanofoams are produced through the dynamic hydrogen bubble template. NiFe electrodes of different compositions are characterized by scanning electron microscopy, energy-dispersive spectroscopy, linear sweep voltammetry, and chronopotentiometry analysis. The results allow obtaining key parameters that are essential for the optimization of the electrode's response considering overpotentials, Tafel slopes, and other electrochemically relevant parameters.