The Kinetics and Mass Transfer of Zinc Electrode in Acidic Zinc‐Chloride Solution

Abstract

The kinetics, mass transfer, and surface roughness of zinc electrode in acidic zinc chloride solutions have been investigated in order to understand and optimize its performance in the zinc‐chlorine hydrate battery. Polarization and limiting currents were measured using a rotating hemispherical zinc electrode. Exchange current densities and Tafel slopes are reported and the cathodic zinc deposition is confirmed to be first order. A cathodic mechanism of two consecutive one‐electron transfer steps are postulated where the first step is the rate‐determining step. A diffusion coefficient of zinc ion in 0.05MZnCl2+1.0MnormalKCl , DZn2+=0.89×10−5 cm2/sec , is obtained from the kinetics‐diffusion mixed behavior. The surface roughness during the deposition depends strongly on the zinc‐chloride concentration, the fraction of the limiting current i/il , and the deposition time. Two regions are observed: (i) at appreciable fraction of the limiting current where the roughness is determined by the hydrodynamic conditions, and (ii) at well below the limiting current where the roughness is governed by the microthrowing power of the zinc deposit. The growth of zinc dendrites at the edges of a rotating disk electrode was investigated potentiostatically. After an initiation period the current increased sharply and dendrites were detected. The growth direction of the dendrites is explained in terms of the hydrodynamic flow and the level of zinc ion concentration.

Kinetic measurements during transient film growth on zinc

“…Zinc passivation is due to the formation of a ZnO layer on the electrode surface. Higher concentrations of ClO 4 ions (>0.005 M) break down the passive layer and cause pitting corrosion at a certain anodic potential. The rate of pit nucleation increases with increasing both ClO 4 concentration and applied anodic potential.…”

“…The solutions were deaerated with N 2 before each experiment, and during the experiments N 2 was passed over the non-stirred solutions. Tests for Clions in the ClO 4 solutions revealed the absence of chloride ions after each experimental run.…”

“…These properties include the relatively active position in the electromotive force series, fast and reversible dissolution/deposition kinetics, high overpotential for hydrogen evolution, and formation of passive film in slightly alkaline solutions. 2 The electrochemical behaviour of zinc has been studied in alkaline media 3 due to the need of developing primary cells with high dissolution electrode activity and without passivation; it has also been studied in acid media 4 and slightly acid solutions 5 due to its applications in the Cl 2 /Zn and Leclanché's batteries. Despite its extensive use in galvanised steels, 6 few studies have been reported in neutral solutions, particularly in deaerated media.…”

Micro-scale mass-transfer variations during electrodeposition

“…Potential step experiments by Jorne [74,75] and Anderson [14] obtained values of the zinc exchange current in acidic electrolytes between 1-2 mA/cm 2, which are consistent with work by Hauser on zinc dissolution [21 ], but considerably lower than other reported measurements [48,49]. Yamashita reported that in a solution that was oxygen-free, he obtained an exchange-current density well below lmA/cm 2, when measuring small currents near the zinc equilibrium potential.…”

Fast electrodeposition of zinc onto single zinc nanoparticles

“…The major problems yet to be solved involving the metallic zinc electrode are related to the hydrogen evolution reaction occurring in parallel to the Zn deposition reaction [9][10][11], and the non-uniform deposition of metallic Zn leading to the formation of dendrites, and consequently to short-circuit of the electrodes [11]. Despite the fact that the zinc deposition and dissolution reaction has been studied for decades, mainly for corrosion and electrodeposition applications [12][13][14][15][16][17], further investigations are yet needed regarding the Zn reaction mechanisms in the solutions employed within the Zn-ion batteries. Several studies have shown that the reaction mechanism of the zinc deposition and dissolution reaction differs strongly in the dependence on the electrolyte in terms of the type of solvent (i.e., organic or water based), pH, anions, cations, complexing…”