Electrochemical characterization and mathematical modeling of zinc passivation in alkaline solutions: A review

Passivation of zinc electrodes caused by formation of oxy-products with low electrical conductivity is one of the most important phenomena which limits the current density during discharge and contributes to the poor cyclability of zinc-based batteries. The present review critically discusses the existing experimental findings related to passivation of zinc in alkaline electrolyte as well as the state of the mathematical modelling of this process. Despite numerous investigations over several decades, no general consensus about the underlying mechanisms and the strategies to prevent passivation could be achieved. Main reason for this uncertainty is that different processes can lead to passivation. On the one hand, precipitation of so-called type 1 films occurs above a critical zincate concentration while on the other hand the direct oxidation of the zinc surface takes place below a certain potential, leading to type 2 passive films. As long as formed films remain porous, passivation does not necessarily lead to complete breakdown of the electrode activity. It is therefore recommended to conduct further measurements employing a combination of techniques in which both the potential and the concentration of zincates at the electrode surface can be precisely controlled. It is also proposed to carry out systematic studies on the influence of temperature, electrolyte flow, and electrolyte composition - including additives - on the passivation time and the structure of the passive film. Finally, the purity and the structure of the zinc electrode may also have an influence on the passivation processes and should be considered during future studies.