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  • On the supercapacitive behaviour of anodic porous WO3-based negative electrodes
    Publication . Upadhyay, Kush; Altomare, Marco; Eugénio, Sónia; Schmuki, Patrik; Moura E Silva, Teresa; MONTEMOR, MARIA
    Herein 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.
    2017-04-01Journal article Open access Show more
  • Stochastic coordination of joint wind and photovoltaic systems with energy storage in day-ahead market
    Publication . Gomes, Isaias L. R.; Pousinho, H. M. I.; Melicio, Rui; Mendes, Victor
    This paper presents an optimal bid submission in a day-ahead electricity market for the problem of joint operation of wind with photovoltaic power systems having an energy storage device. Uncertainty not only due to the electricity market price, but also due to wind and photovoltaic powers is one of the main characteristics of this submission. The problem is formulated as a two-stage stochastic programming problem. The optimal bids and the energy flow in the batteries are the first-stage variables and the energy deviation is the second stage variable of the problem. Energy storage is a way to harness renewable energy conversion, allowing the store and discharge of energy at conveniently market prices. A case study with data from the Iberian day-ahead electricity market is presented and a comparison between joint and disjoint operations is discussed.
    2017-04-01Journal article Restricted access Show more