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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.
- 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.
- 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.
- Electrodeposited MoOx films as negative electrode materials for redox supercapacitorsPublication . Upadhyay, Kush; Nguyen, Tuyen; Moura E Silva, Teresa; Carmezim, Maria; MONTEMOR, FATIMAMoOx films composed of agglomerated nanoparticles were synthesized by potentiostatic electrodeposi-tion on stainless steel collectors for charge storage electrodes working in aqueous electrolyte (1 M H2SO4) and negative potential window. The agglomerated nanoparticles were amorphous in nature and possess mixed valance state. By optimizing the charge density applied during electrodeposition, the electrodes delivered maximum specific capacity of 228 C g_1 (507 F g_1) at 1 A g_1 for the sample electrodeposited at _0.3 C cm_2. This film showed very good rate capability and retained around 48% of specific capacity at 10 A g_1. The galvanostatic charge discharge cycling stability test showed 87% of initial capacity retained after 900 cycles, suggesting good stability behaviour. Electrochemical impedance spectroscopy (EIS) measurements evidenced lower equivalent series resistance for the sample electrodeposited at _0.3 C cm_2 among the tested samples, revealing its better electrochemical performance. Aging of the electrode with higher specific capacity was also investigated by performing EIS after different cycles; the results revealed an increment on the overall resistance, thus clarifying the capacity degradation.
- On the supercapacitive behaviour of anodic porous WO3-based negative electrodesPublication . Upadhyay, Kush; Altomare, Marco; Eugénio, Sónia; Schmuki, Patrik; Moura E Silva, Teresa; MONTEMOR, MARIAHerein we illustrate the functionality as pseudocapacitive material of tungsten trioxide (WO3) nanochannel layers fabricated by electrochemical anodization of W metal in pure hot ortho-phosphoric acid (o-H3PO4). These layers are characterized by a defined nanochannel morphology and show remarkable pseudocapacitive behaviour in the negative potential (−0.8–0.5 V) in neutral aqueous electrolyte (1 M Na2SO4). The maximum volumetric capacitance of 397 F cm−3 is obtained at 2 A cm−3. The WO3 nanochannel layers display full capacitance retention (up to 114%) after 3500 charge-discharge cycles performed at 10 A cm−3. The relatively high capacitance and retention ability are attributed to the high surface area provided by the regular and defined nanochannel morphology. Kinetic analysis of the electrochemical results for the best performing WO3 structures, i.e., grown by 2 h-long anodization, reveals the occurrence of pseudocapacitance and diffusional controlled processes. Electrochemical impedance spectroscopy measurements show for the same structures a relatively low electrical resistance, which is the plausible cause for the superior electrochemical behaviour compared to the other structures.
- 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.