Effect of boric acid on nickel electrodeposition kinetics under electroforming conditions

Nickel electroforming processes have shown a large spread in exchange current density and Tafel slopes (82–330 mV dec−1). This study investigated the effect of boric acid, temperature, and agitation on these parameters for electroforming from nickel sulfamate electrolytes, where Ni2+ reduction and hydrogen evolution proceed together. The methodology involved the determination of overall and nickel partial currents using slow linear sweep voltammetry which was then verified against results from gravimetric, electrochemical quartz crystal microbalance (EQCM), and energy-dispersive X-ray spectroscopy analyses. In quiescent solutions without boric acid, gravimetric data exhibited poor current efficiency. The deposits were cracked and contained oxides/hydroxides. EQCM measurements corroborated the formation of oxides/hydroxides in these deposits. The addition of boric acid under quiescent conditions, however, yielded deposits of near unity current efficiencies and consistently resulted in Tafel slopes between 143 and 251 mV dec−1, which again, were verified by EQCM measurements. These large Tafel slopes were attributed to the formation of an adsorbed boric acid film on the cathode surface which suppressed water reduction and facilitated Ni2+ reduction. Agitation lowered the Tafel slope, indicating removal or thinning of this film, which consequently decreased the current efficiency and exchange current densities. These findings support the model of boric acid adsorption as a factor in suppressing hydrogen evolution via water reduction, enhancing understanding of nickel electrodeposition kinetics for electroforming applications.