Basal-Plane Orientation of Zn Electrodeposits Induced by Loss of Free Water in Concentrated Aqueous Solutions

Abstract

Concentrated aqueous solutions attract considerable attention because water electrolysis can be suppressed due to a decrease in the amount of free water. The present study focuses on electrodeposition behaviors of metallic zinc (Zn) using concentrated aqueous solutions containing bis(trifluoromethylsulfonyl)amide (Tf₂N⁻) anions. An increase in Tf₂N⁻ concentration significantly enhances water-anion interactions, giving characteristic infrared spectra for the breakdown of the hydrogen-bonding networks of water clusters, i.e. loss of free water. For the Tf₂N⁻ system Zn electrodeposits with the preferred orientation of hcp basal plane was observed, while, for the SO₄²⁻ system with the presence of the hydrogen-bonding networks, preferred orientation of basal plane was not observed. The preferred orientation of basal plane is not attributed to the adsorption of Tf₂N⁻ anions on the electrode, proved by the use of mixed Zn(Tf₂N)₂-ZnSO₄ concentrated solutions. The loss of free water in the concentrated Zn(Tf₂N)₂ solutions will suppress hydrogen adsorption at the cathode to promote surface diffusion of intermediate Zn⁺ adions and growth of Zn crystals. Consequently, the promotions and the easier growth of Zn basal planes with the lowest interfacial free energy will enhance the horizontal growth of Zn basal planes.