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- Pseudocapacitive behaviour of FeSx grown on stainless steel up to 1.8 V in aqueous electrolytePublication . Upadhyay, Kush; Tuyen, Nguyen; Moura E Silva, Teresa; Carmezim, Maria; MONTEMOR, MARIAIron sulfide was synthesized for 4 h, 7 h and 12 h by a hydrothermal process directly on stainless steel current collectors. The synthesis time determined the material morphology and electrochemical response. The shortest synthesis time promoted the formation of randomly oriented nanowires that evolved to nanosheets decorated with nanoflakes, organized in a cuboidal-like morphology upon longer synthesis times. XRD, Raman, FTIR and XPS investigations confirmed the formation of FeSx. The electrochemical activity was studied in a potential window ranging from - 0.95 to 0 V and the material obtained after 7 h of synthesis stored the maximum specific capacitance of 730 mF cm(-2) at the current density of 1 mA cm(-2). This material also retained approximately 34% of its initial capacitance at 10 mA cm(-2) and showed very good cycling stability, keeping around 95% of the specific capacitance after 2000 galvanostatic charge-discharge (GCD) cycles. The kinetic analysis of the electrochemical results revealed the predominance of diffusional controlled processes. An asymmetric cell was assembled using FeS,, as negative electrode and carbon nanofoam (CNF) as positive electrode. The FeSx parallel to CNF cell showed enhanced capacitive response in a potential window of 1.8 V in 1 M Na2SO4 electrolyte and delivered specific capacitance of 236 mF cm(-2) at 0.5 mA cm(-2) with good rate capability. The FeSx parallel to CNF cell stored maximum energy density of 0.11 mW h cm(-2) at the power density of 0.45 mW cm(-2). The cell showed very good stability by retaining 83% of the initial capacitance after 2000 cycles of consecutive charge discharge.
- Electrochemical performance of MnOx·center dot nH(2)O@Ni composite foam electrodes for energy storage in KOH mediaPublication . Siwek, Katarzyna; Eugénio, S.; Moura E Silva, Teresa; MONTEMOR, FATIMANanostructured porous MnO2, especially its hydrated amorphous and low crystalline form (MnO2·nH2O), has been one of the most promising material considered for charge storage applications, due to electrochemical similarities with RuO2 and its relative low cost. However, the intrinsic poor conductivity of MnO2 combined with the presence of structural water, which provides high ionic but low electronic conductivity, is a great hindrance for wider application. An effective approach to overcome this drawback involves the deposition of thin MnO2 layers on porous, high surface area metallic scaffolds. The present work addresses this route and provides novel insights thanks to the combination of MnOx·nH2O with custom-made Ni foams, fabricated via one-step electrodeposition using the dynamic hydrogen bubble template (DHBT). The porous Ni foams provide a scaffold with a 3D architecture with optimized pore size and surface. The composite electrode was fabricated by anodic deposition of MnOx·nH2O on the 3D Ni foams. The electrochemical behaviour was tested in 1 M KOH, since there are very few studies addressing the electrochemical behaviour of MnOx·nH2O in alkaline media for electrochemical supercapacitors applications. In addition, thermal treatment (150–250 °C) was performed to evaluate the effect of hydration on the material properties. The results revealed that the as-obtained composites are highly stable, displaying much higher specific capacitances with 73–90% (depending on the mass load) capacitance retention compared to their de-hydrated counterparts. The charge-discharge processes were found to be highly reversible throughout 5000 cycles, maintaining almost 100% columbic efficiency. In conclusion, the MnOx·nH2O@Ni composite electrodes showed a very stable pseudocapacitive behaviour and exceptional cycling performance in 1 M KOH, being therefore a promising alternative charge storage electrode for electrochemical supercapacitors.
- Electrodeposited manganese oxide on tailored 3D bimetallic nanofoams for energy storage applicationsPublication . Siwek, Katarzyna; Eugénio, Sónia; Moura E Silva, Teresa; M.F. MontemorThree-dimensional (3D) electrode design has great advantages over its two-dimensional (2D) counterparts, including higher mass loading of active material, enhanced ion diffusion, and electron charge transfer. Commercial 3D porous structures (i.e., Ni foams) do not fit the purpose of the ideal 3D electrode for supercapacitors, in which surface area (per cm(2)) is more important than large pore volume. These characteristics, however, can be tuned by the dynamic hydrogen bubble template (DHBT) electrodeposition, a route that is used to tailor 3D nanostructured (multi-) metallic porous surfaces. In addition to the higher surface area and tailored porosity, these 3D nanostructures can be subsequently functionalized with different species such as metal oxides or other compounds. Therefore, a facile two-step electrochemical fabrication of 3D composite electrode composed of a bimetallic foam functionalized with manganese (Mn) oxide is proposed. The effect of applied current densities on the distribution and structure of Mn oxide (MnOx) electrodeposited over the bare foam is discussed. The results demonstrate that this route paves the way to design high-surface-area architectures for charge storage electrodes with enhanced electrochemical performance (194Fg(-1) mg(-1) of electrodeposited MnOx at 0.5Ag(-1)) and high charge-discharge rate capabilities (91% capacitance retention at 20Ag(-1)) for supercapacitor applications.
- Capacitance response in an aqueous electrolyte of Nb2O5 nanochannel layers anodically grown in pure molten o-H3PO4Publication . Upadhyay, Kush; Cha, Gihoon; Hildebrand, Helga; Schmuki, Patrik; Moura E Silva, Teresa; MONTEMOR, FATIMA; Altomare, MarcoVertically aligned Nb2O5 nanochannel layers are grown on Nb metal substrates by self-organizing electrochemical anodization in a pure molten o-H3PO4 electrolyte. The capacitive behavior of these structures when used as negative electrodes is investigated in aqueous 1 M Na2SO4 electrolyte, in a potential range from −0.2 to −1.25 V vs. SCE. Surface chemistry, morphology and crystallographic features of the Nb2O5 nanochannel electrodes are tailored by adjusting the synthesis parameters, namely anodization time and crystallization temperature, which have a significant effect on the electrode performance. 8 μm thick Nb2O5 nanochannel layers that are converted into orthorhombic phase by crystallization at 450 °C, display a maximized areal capacitance of ∼100 mF cm-2 at a current density of 1 mA cm−2. These electrodes retain 63% of the initial capacitance at 10 mA cm−2 and 81% after 1500 charge-discharge cycles at a current density of 1.3 mA cm−2. Kinetic analysis of the electrochemical results reveals the occurrence of pseudocapacitive and diffusion-controlled processes. Electrochemical impedance spectroscopy evidences for these structures a low resistance across the electrode and at the electrode/substrate interface. These results are associated with the nanochannel morphology (high active area) of the Nb2O5 layers, and are ascribed to their crystalline nature, which provides an “oriented porosity” for ion diffusion and directional pathways for charge transport and collection.
- On the growth and mechanical properties of nanostructured cobalt foams by dynamic hydrogen bubble template electrodepositionPublication . Arévalo-Cid, Pablo; Adan-Mas, Alberto; Moura E Silva, Teresa; Rodrigues, José Alberto; Maçôas, Ermelinda Maria Sengo; Vaz; Montemor, M.F.The growth mechanism of nanostructured cobalt foams prepared by hydrogen bubble template electrodeposition has been studied. To that end, cobalt foams have been synthesized under different electrodeposition times and studied with various characterization techniques, namely scanning electron microscopy (SEM) and confocal microscopy in reflection mode. In addition, the mechanical properties of the resulting foams have been tested by bending and tensile tests. To study the influence of chemical additives in the growth mechanism and final properties of the foams, agar-agar has been added to the electrolytic bath during foam preparation. Results evidence that the addition of agar has successfully modified the microstructure of the final foams, creating a denser porous structure with smaller pore area and reduced growth rate that translates into better mechanical properties. By studying the evolution of the microstructure under different deposition conditions, a growth model of cobalt metallic foams by dynamic hydrogen bubble template is proposed. This easy and scalable route paves the way to produce tailored foams for numerous applications that include, for instance, energy storage and energy conversion.
- Tailored 3D foams decorated with nanostructured manganese oxide for asymmetric electrochemical capacitorsPublication . Siwek, K. I.; Eugenio, S.; Aldama, I.; Rojo, J. M.; Amarilla, J. M.; Da Costa Ribeiro, Ana Paula; Moura E Silva, Teresa; Montemor, M.F.Tailored 3D (Ni and NiCo) metallic foam architectures were produced by electrodeposition and decorated via electrochemical routes with manganese oxide (MnOx) to serve as positive electrodes for supercapacitors. For comparative purposes, an electrode made of commercial Ni foam was also prepared. The foam-based electrodes were paired with a carbon cloth electrode and used to assemble asymmetric electrochemical cells. The electrochemical response of these cells was studied by applying different electrochemical techniques. In addition, two different protocols (cycling and floating) were applied to assess cells durability and fade. Despite the significant differences in the decorated foams morphology and structure their electrochemical responses revealed similar trends. The electrodes made of tailored foams showed higher specific capacitance, better capacitance retention at high current load and enhanced cycling stability compared to the electrodes made of commercial foam. The asymmetric cells made with the tailored foams revealed higher (maximum) specific energy (11-14 Wh kg(-1)) and specific power (1.3-1.4 x 10(4 )W kg(-1)) compared to cells assembled with commercial foams (8.4 Wh kg(-1) and 6.3 x 10(3) W kg(-1)). The durability tests evidenced that corrosion of the NiCo electrodeposited foams and electrochemical dissolution of MnOx are possible causes of cells degradation.
- Pseudocapacitive response of hydrothermally grown MoS2 crumpled nanosheet on carbon fiberPublication . Upadhyay, Kush; Nguyen, Tuyen; Moura E Silva, Teresa; Carmezim, Maria; MONTEMOR, FATIMACrumpled MoS2 nanosheets were synthesized directly on carbon fiber paper (CFP) through hydrothermal procedure. Molybdenum sulfide precursor was first produced in the solution and then introduced into the autoclave. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images confirmed the uniform growth of crumpled nanosheets on the CFP that were assigned to MoS2 according to X-ray photo electron spectroscopy (XPS) and Raman spectroscopy results. Electrochemical measurements of the as deposited MoS2 crumpled nanosheets performed in 1 M Na2SO4 evidenced a specific capacitance of 249 F g−1 at 2 A g−1 and the good rate capability by retaining 41.3% of initial capacitance at 10 A g−1. Electrochemical Impedance spectroscopy measurements showed very low charge transfer resistance and very short relaxation time accounting for the pseudocapacitive rectangular cyclic voltammetry (CV) and high rate capability.
- Convergence rates for sequences of bifurcation parameters of nonautonomous dynamical systems generated by flat top tent mapsPublication . Moura E Silva, Teresa; Silva, Luis; Fernandes, SaraIn this paper we study a 2-parameter family of 2-periodic nonautonomous systems generated by the alternate iteration of two stunted tent maps. Using symbolic dynamics, renormalization and star product in the nonautonomous setting, we compute the convergence rates of sequences of parameters obtained through consecutive star products/renormalizations, extending in this way Feigenbaum's convergence rates. We also define sequences in the parameter space corresponding to anharmonic period doubling bifurcations and compute their convergence rates. In both cases we show that the convergence rates are independent of the initial point, concluding that the nonautonomous setting has universal properties of the type found by Feigenbaum in families of autonomous systems.
- In-situ localized pH, pNa and dissolved O2 measurements during charge-discharge of mixed Ni–Co hydroxide electrodesPublication . Adán-Más, Alberto; Taryba, Maryna; Moura E Silva, Teresa; Guerlou-Demourgues, Liliane; Montemor, M.F.This work reports, for the first time, the use of ion-selective localized electrochemical techniques to elucidate the charge-discharge mechanism of nickel-cobalt hydroxide electrodes for electrochemical energy storage. The charge-discharge mechanism of electrodeposited nickel-cobalt hydroxide electrodes was studied in Na2SO4 0.05 M by localized in situ measurements of pH, pNa and dissolved O2 during cyclic voltammetry. Local pH and pNa distributions were recorded using micro-potentiometric sensors with liquid membrane, while dissolved O2 was monitored using a fiber-optic microsensor. These original results highlight how localized potentiometry can provide new insights to better understand the charge mechanism of metal (hydr)oxide electrodes by directly measuring the concentrations/activities of relevant species at the electrode-electrolyte interface during charge-discharge.
- Nickel-cobalt oxide modified with reduced graphene oxide: Performance and degradation for energy storage applicationsPublication . Adan-Mas, Alberto; Moura E Silva, Teresa; Guerlou-Demourgues, Liliane; Bourgeois, L.; MONTEMOR, FATIMANickel-cobalt oxide is synthesized in combination with electrochemically reduced graphene oxide (Er-GO) by one-step electrodeposition on stainless steel followed by thermal treatment. The presence of reduced graphene oxide leads to enhanced electrochemical response, with a capacity increase from 113 mA h g(-1) to 180 mA h g(-1), and to increased faradaic efficiency and rate capability. Compared to Ni-Co oxide, the addition of reduced graphene oxide increases capacity retention from 58% to 83% after 5000 cycles. The material fade during cycling is studied by means of electrochemical impedance spectroscopy, electron diffraction spectroscopy and scanning electron microscopy. As a result, different degradation mechanisms are identified as source of the capacity decay, such as microstructural cracking, phase transformation and parasitic reactions.