Investigation of the Effects of Glycine in Ni Electrodeposition: Local pH Measurement and Its Mathematical Modelling
Authors: Takahashi Yukihiro , Sunde Svein , Seland Frode 
Published: December 2024

Ni is an important material in surface finishing for both decorative and functional purposes [1]. In order to change the properties of plating baths and to improve the quality of products, various kinds of additives are typically employed in Ni electroplating. However, a comprehensive understanding of the roles of additives is still lacking due to the complexity of the systems, in which multiple chemical species are involved in parallel chemical/electrochemical reactions.

Glycine (Gly) is widely utilized both in electro- and electroless plating, and it is believed to work as a complexing agent and as a buffer. However, studies on this species are still limited [2-3], and an even deeper understanding of its effects is desired to enable us to tune and optimize plating processes. In a system containing glycine as well as metal species such as Ni, the situation is complicated by the fact that the electrodeposition of metal almost always accompanies the hydrogen evolution reaction (HER). As protons near the cathode surface are consumed by the production of hydrogen, the near-surface pH increases during the electrodeposition process, which results in shifting the equilibria of glycine species and promoting the formation of metal-glycine complexes. To elucidate this kind of complicated system and to be able to predict its behavior, it is necessary to combine various electrochemical methods and theoretical approaches such as mathematical modelling.

In this study, the effects of glycine in Ni electrodeposition were investigated both theoretically and experimentally. First, thermodynamic calculations were carried out to predict the stabilities of Ni-glycine species at different pH and potentials. It was illustrated that glycine indeed stabilizes Ni species in an aqueous solution, as has been pointed out by many studies, preventing the formation of Ni(OH)2 even at high pH. Electrochemical measurements such as linear sweep voltammetry (LSV) were performed to investigate the electrochemical behavior of the Ni-glycine species. Due to the involvement of glycine in the equilibria as mentioned above, the local pH near the cathode surface is particularly important. Therefore, we employed a rotating ring-disk electrode (RRDE) with a metal oxide-based pH sensor coated on the ring electrode. The local pH was successfully measured along with the electrochemical responses at the disk electrode. Furthermore, a mathematical model of the Ni-glycine system was developed taking into account the chemical equilibria and the mass transfer of species as well as the electrochemical reactions at the cathode surface. The experimental results will be discussed in connection with the theoretical predictions made by the mathematical model.

References:

[1] M. Schlesinger and M. Paunovic, “Modern Electroplating (Fifth edition)”, Wiley (2014).

[2] R.A.J. Critelli, P.T.A. Sumodjo, M. Bertotti and R.M. Torresi, “Influence of Glycine on Co Electrodeposition: IR Spectroscopy and Near-Surface pH Investigations”, Electrochimica Acta,
260,762 (2018)

[3] N.V. Sotsukaya and O.V. Dolgikh, “Nickel Electroplating from Glycine Containing Baths with Different pH”, Protection of Metals, 44, 479 (2008)