The Interplay between Interfacial Solvation and Surface Kinetics Tunes the Selectivity between Hydrogen Evolution and...

Electrodeposition is a key technology for the fabrication of semiconductor interconnects, corrosion protection, decorative coatings, and even for chemical energy storage in batteries. Empirically discovered additives are regularly implemented in electroplating baths, but their impact on the electrodeposition mechanism, which involves the desolvation of metal ions across the electric-field-dependent double layer, has remained poorly understood. Herein, we perform overpotential-dependent Arrhenius analysis on Zn electrodeposition from strongly solvated zincate ([Zn(OH)₄]^2-)-containing alkaline electrolytes in the absence and presence of the cationic polyquaternium-2 (PQ-2) electrolyte additive. We hypothesized that the water-soluble and positively charged PQ-2 can modulate the double-layer electrostatics, leading to distinct changes in the Arrhenius activation parameters. Without PQ-2, we observe that electrodeposition competes with hydrogen evolution, which is reflected in an extended region where an increasing (apparent) activation energy is overcompensated by an increasing Arrhenius prefactor. In contrast, when we add the positively charged PQ-2, we observe that the compensation region is suppressed and the Zn deposition kinetics proceed via efficient Butler-Volmer-type kinetics. The latter might arise due to charge inversion and a closer approach of the negatively charged zincate to the electrode surface that allows for electron transfer. Quartz crystal microbalance and electron microscopy support that the Butler-Volmer kinetics arise from a surface-controlled mechanism, which is linked to distinct morphological changes on the surface. These results are important to understand how the interfacial microenvironment can tune the activity and selectivity and how electrochemical kinetics can switch between solvation and surface-controlled regimes.