Passivation of Zinc Anodes in Alkaline Electrolyte: Part I. Determination of the Starting Point of Passive Film Formation

Abstract

In the present contribution we demonstrate a new testing cell, which allows simultaneous in situ optical and electrochemical investigations of passivation processes on zinc anodes in quiescent alkaline electrolytes. By combination of microscopy and galvanostatic impedance spectroscopy it was possible to detect the starting point of passive film formation, which has not been achieved so far. We found that formation of the so-called type 1 passive film is not dependent on the anodic current density. This passive film appears at electrode overpotentials of <0.15 V after an amount of zincate ions of approx. 8.2 • 10 -4 mol cm -2 has accumulated at the anode in a 30 wt% KOH electrolyte with 2 wt% ZnO at room temperature. Consequently, the passivation of zinc in quiescent electrolyte cannot be avoided even at very low dissolution currents. On the other hand, the so-called type 2 passive film appears below the first film due to direct oxidation of zinc. At overpotentials of ≥0.15 V the direct oxidation of zinc is favored and the potential-dependent type 2 passive film appears before type 1 film is formed.

Analysis of Electrochemical Impedance Spectroscopy on Zinc-Air Batteries Using the Distribution of Relaxation Times

“…In operating phase II, an equilibrium of the presented reactions ( 16)-( 18) has already been reached. A constant deposition of zinc oxide follows, as long as no passivation occurs within the porous anode [15] or secondary processes occur in the cathode, e.g., flooding [74] or pore clogging [75]. The DRT in Figure 7 shows no significant variation between 20 and 70% discharge capacity and currents up to a maximum of 4 mA, which is a characteristic of stable and constant reactions.…”

“…Two theories have been proposed for this behavior. The first has discussed two different oxidation states of zinc, i.e., "solid-state" and "dissolutionprecipitation" [15]. In this case, the "solid-state" oxidation was observable only at higher overpotentials, which could be caused by higher discharge currents or by depletion of hydroxide ions in the anode.…”

“…A suitable electrocatalyst for both an efficient oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) cycle has not yet been found [3][4][5], nor has a filter that regulates the oxygen and the humidity from the ambient air been developed [6,7]. The dendrite growth [8][9][10][11], the shape change [12][13][14], the passivation of the anode [15][16][17][18] and the zincate transport into the gas diffusion layer (GDL) during charging [19] are also critical parameters for rechargeability. In order to prevent such processes, it is necessary to gain a fundamental understanding of the reactions and their interactions [20].…”

Compositional Effects of Gel Polymer Electrolyte and Battery Design for Zinc‐Air Batteries

“…ZnO þ H 2 O þ 2OH À . [50][51] Figure 3b shows the combined effects of discharging and different concentrations of ZnO added to GPEs on charging profiles at 20 mA cm À 2 . For this experiment, cut-off voltages for discharge and charge were adjusted to 0.9 V and 2.2 V, respectively, because the thickening passive film could hamper the charging reaction.…”

Effect of a Fast Potential Change on the Early Stage of Zinc Passivation in a Saturated Calcium Hydroxide Solution

“…Most of the literature on zinc passivation in alkaline media details the zinc active-passive transition under galvanostatic or potentiodynamic conditions (Bockelmann et al, 2017), which is different from spontaneous passivation. Two main mechanisms of passive film formation have been postulated: the "dissolution-precipitation" mechanism used for the interpretation of galvanostatic measurements and the "solidstate" reaction mechanism used for potentiodynamic measurements (Bockelmann et al, 2018). According to previous studies on the behavior of zinc and galvanized steel in fresh concrete, saturated calcium hydroxide solution, and simulated pore solutions, during first hours of immersion or embedding, occurs a spontaneous sudden change of potential from an active state to a passive state (Macías and Andrade, 1987a;Macías and Andrade, 1987b;Macías and Andrade, 1990;Zhang, 1996;Yeomans, 2004;Ghosh and Singh, 2007;Videm, 2007;Tittarelli and Bellezze, 2010;Dobias et al, 2017;Zheng et al, 2018b).…”