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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.
- 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.