Computational Chemistry and Electrochemical Mechanism Studies of Auxiliary Complexing Agents Used for Zn-Ni Electroplating in the 5-5'-Diethylhydantoin Electrolyte

An effective approach is developed to investigate potential complexing agents in Zn-Ni bath by using density functional theory (DFT), molecular dynamic (MD) simulations and electrochemical experiments. Pyrophosphate is predicted as the most promising auxiliary complexing agent in introduced DMH based bath compared with other complexing agents owing to its weaker adsorption capacity. The DFT calculations show that the most possible and stable structures for zinc (II) and nickel (II) in the electrolyte are the complexes that with one pyrophosphate and two 5-5'-diethylhydantoins (DMH) as the complexing agents. The electrodeposition mechanism of Zn-Ni alloys in selected DMH-pyrophosphate electrolyte was studied by electrochemical measurements. The effects of Zn content, Ni content and bath temperature in the bath on CV curves were also investigated. The rate controlling rate is determined based on Koutecky-Levich equation and a mixed controlled process is observed. The nucleation mechanism of Zn-Ni alloys is a 3D instantaneous nucleation and the bath temperature has no effect on the nucleation of Zn-Ni alloys. The combined theoretical and experimental tests indicate that pyrophosphate is an excellent auxiliary complexing agent in DMH-based Zn-Ni bath and can also provide an efficient method to study or design complexing agents for other metal electroplating.