Browsing by Author "Caravaca, C"

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    Cyclic voltammetric experiment - simulation comparisons of the complex mechanism associated with electrochemical reduction of Zr4+ in LiCl-KCl eutectic molten salt
    (The Electrochemical Society, 2012-11-29) Fabian, CP; Luca, V; Le, TH; Bond, AM; Chamelot, P; Massot, L; Caravaca, C; Hanley, TL; Lumpkin, GR
    Nuclear energy increasingly represents an important option for generating largely clean CO2-free electricity and zirconium is a fission product that is expected to be present in irradiated fuels. The present investigation addresses the electrochemical reduction of Zr4+ to Zro in LiCl - KCl eutectic molten salt in the temperature range 425–550°C using cyclic voltammetry (CV), square-wave voltammetry (SWV) and bulk electrolysis. Simulations of the CV data indicate that the initial reduction proceeds through surface confined steps: Zr4+* + 2e− ↔ Zr2+* and Zr2+* + 2e− ↔ Zr* processes (* adsorbed species) followed by a peak-shaped complex diffusion controlled step that consists of a combination of closely spaced processes associated with the reactions Zr4+ + 4e− → Zro and Zr4+ + 3e− → Zr+*. Zr+*, probably in the form of ZrCl* is then further reduced to Zro* at even more negative potentials. The simulations provide the first quantitative analysis of the thermodynamics and kinetics of the Zr4+ reduction in the LiCl-KCl eutectic. © 2012, The Electrochemical Society.
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    Electrochemical behavior of zirconium in LiCl - KCl eutectic molten salt
    (IOP Publishing, 2010-04-25) Fabian, CP; Caravaca, C; Griffith, CS; Luca, V; Lumpkin, GR
    Nuclear fission energy increasingly represents a potentially important option for generating clean, CO2-free electricity and the nuclear community is considering advanced reactor and fuel cycle technologies. Pyroelectrochemical processes based on molten salt electrochemistry have become important for the potential recycle of metal and other advanced nuclear fuels. Zirconium is one of the troublesome fission products that is expected to be present in such used fuels and hence it is important to acquire an understanding of its electrochemistry in molten salts. The present investigation addresses the electrochemical reduction of Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), constant-current chronopotentiometry (CP) and square wave voltammetry (SWV) at 425, 475 and 525 °C. The CV data clearly indicate reduction of Zr4+ proceeds through an initial adsorption process followed by two or three reaction steps dependent on the temperature. Results show three previously unobserved reaction steps at 425 and 475 °C, possibly due to improved pretreatment of the salt. The first cathodic peak is observed at -1.18 V (vs. Ag/AgCl). The second and third peaks are observed at -1.26 and -1.33 V (vs. Ag/AgCl), respectively with a separation of 60-70 mV. The CV data at 525 °C show only two cathodic peaks at about -1.1 and -1.25 V (vs. Ag/AgCl)which may indicate the effect of temperature on the mechanism of the electrochemical process. The overall reaction mechanism assessed by these electrochemical techniques will be presented. Moreover, electrodeposition of zirconium on aluminium metal will also be presented along with XRD analysis of the salt residue and zirconium-aluminium alloy. Finally, the kinetics of the electrochemical process have been studied using Electrochemical Impedance Spectroscopy and the results analyzed using the Randles-Ershler equivalent circuit model to determine the charge-transfer resistance (Rct). © 2010 The Electrochemical Society
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    Reaction mechanisms of La3+ and Zr4+ in LiCl-KCl eutectic molten salt - cyclic and square wave voltammetry data simulations
    (IOP Publishing, 2011-05-01) Fabian, CP; Chamelot, P; Massot, L; Caravaca, C; Luca, V; Lumpkin, GR
    Nuclear energy increasingly represents an important option for generating clean CO2-free electricity. This has generated a nuclear renaissance with increased interest in both innovative nuclear reactors and fuel cycle technologies. The innovative reactor concepts proposed have mainly involved closed fuel cycles with implied recycling of irradiated fuel. Pyroelectrochemical processes based on a molten alkali chloride/fluoride electrolyte system show considerable promise, and as such have received considerable attention for fuel reprocessing. Lanthanum and zirconium are two fission products that are expected to be present in irradiated fuels. The present investigation addresses the electrochemical reduction of La3+ and Zr4+ in LiCl - KCl eutectic molten salt using transient techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV) and chronopotentiometry (CP) from 425 to 550° C, at two concentration levels. Experimental CV data from 50 to 500 mV/sec appears to indicate that the lanthanum reaction mechanism involve a single step reduction process, i.e., La3+ + 3e- ↔ La. Hence a theoretical simulation of this experimental data should be straightforward as verification of the reaction mechanism. However, zirconium experimental CV data also from 50 to 500 mV/sec appears to indicate that its reduction proceeds through an ab initio sluggish charge-transfer step coupled with an adsorption/underpotential deposition: Zr4+ + 2e- ↔ Zr2+ followed by a second and faster step where two electrons further are transferred: Zr2+ + 2e- ↔ Zr. A theoretical simulation of the experimental voltammograms has been also conducted to verify this reaction mechanism. We show the parameter Λ in their potential separation, ∆Epp and scan rate plot to indicate the reversibility of their electrochemical process, and evaluate their standard electrochemical rate constant. Deconvolution of the SWV data for lanthanum and zirconium confirms that the reduction of La3+ proceeds through three-electron one-step reaction mechanism while that of Zr4+ proceeds through two two-electron step reactions independent of temperature and concentrations levels. © 2011 The Electrochemical Society