Effects of boric acid and water on the deposition of Ni/TiO2 composite coatings from deep eutectic solvent

Type III deep eutectic solvents (DES) have been extensively studied as a green electrodeposition solvent due to their wide potential window and degradability. The preparation of nickel-based coatings from DES commonly uses nickel chloride as the main precursor of Ni2+, but it generates harmful chlorides during long-term electrodeposition. This article uses nickel sulfate as the main precursor of Ni2+ to investigate the effects of boric acid and water on the deposition of Ni/TiO2 composite coatings from 1:2 choline chloride (ChCl) - ethylene glycol (EG) DES. It is found that light green Ni (OH)2 crystalline solid and solvent decomposition products such as trimethylamine appeared in the coating. Interestingly, the acidic environment created by boric acid can inhibit the formation of Ni(OH)2 and the decomposition of solvents. Moreover, adding water to DES can reduce its viscosity and increase its conductivity, which will speed up the reduction of nickel ions. Water can change the complex form of Ni2+ in DES and reduce the average crystallite size of the coating. In this paper, the mechanism of the effect of boric acid and water on the deposition behavior of Ni2+ in DES is analyzed, and the deposition model is established, which provides a theoretical and experimental reference for green preparation of composite coatings.