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Evaluation of zinc electrodeposition kinetics from acidic zinc sulfate solutions using a UPD modified platinum substrate Guerra, Eduard

Abstract

Zinc is produced mainly by electrowinning from acidic zinc sulphate electrolytes. Electrowinning performance is often reported in terms of the specific electrical energy consumption, SEEC, which is a function of cell voltage, U. There is a fundamental relationship between cell voltage and current density. Establishing the parameters governing this relationship, as a function of solution composition and temperature, is critical for the design and optimisation of zinc electrowinning reactors. Zinc is a soft and reactive metal, which makes preparation of an electrode of well-defined and reproducible surface area, on which to conduct electrochemical polarization experiments, difficult. In addition, bulk zinc deposition occurring during cathodic polarization of the electrode causes irreversible morphology changes that alter the real surface are of the electrode. In general, underpotential deposition, UPD, describes the formation of a two-dimensional layer of metal onto a foreign substrate at a potential more positive than that for overpotential deposition, OPD, of the metal. Use of this phenomenon is proposed as a novel technique for generating smooth and reproducible electrode surfaces of reactive metals, using zinc UPD on platinum as a model case. The technique involves polarization of a polished platinum electrode to cause zinc UPD followed by a pulsed polarization step to grow a bulk zinc metal deposit on the electrode. The steady-state zinc deposition rate is recorded as a function of the applied potential. Mass transfer effects are controlled by the use of a rotating disc electrode. After each potential step, the electrode is polarized to a potential near the UPD potential, which dissolves the bulk zinc and regenerates the original smooth electrode. In this manner the voltage - current density relationship for the zinc deposition reaction may be mapped for a particular solution composition. Experiments were conducted to characterize UPD of zinc on platinum in magnesium sulphate and sulphuric acid supporting electrolytes. UPD of zinc on platinum occurs at a voltage approximately 1 V more positive than that of bulk zinc deposition with an estimated charge density of 260 ±30 μC cm⁻², which is in the order of a monolayer of zinc. The UPD layer was determined to evolve into a Pt-Zn alloy which further inhibited hydrogen evolution, relative to the freshly deposited UPD layer. Bulk zinc deposition experiments were carried out in pure zinc sulphate solutions at 25 °C, using the developed technique, and kinetic parameters were evaluated and compared to previously reported values. The Tafel slope for zinc deposition from pH neutral electrolytes was determined to be ca. 60 mV dec⁻¹, while in highly acid electrolytes was ca. 30 mV dec⁻¹, due to the inhibiting effect of hydrogen adsorption. The transition of zinc deposit morphology from a relatively smooth deposit to a dendritic deposit was confirmed to occur at ca. 1000 A m⁻² in 1.0 mol dm⁻³ ZnSO₄. By virtue of the low value of the Tafel slope, the current density for zinc deposition is highly sensitive to overpotential (increasing tenfold for every 30 mV increase). Dendritic growth in industrial zinc electrowinning at a conventional current density of ca. 500 A m⁻² was attributed to the effects on the local current distribution from the vertical distribution of ohmic drop in the electrolyte and screening of the cathode by attached bubbles. The use of forced convection to mitigate these effects is proposed as a means of extending the current density range of zinc electrowinning.

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