Electrochemical impedance spectroscopy and first-principle investigations on the oxidation mechanism of hypophosphite anion in the electroless deposition system of nickel

In this work, the oxidation mechanism of hypophosphite anion (H ₂PO₂^-) in acidic and alkaline media in the presence of Ni(II) specie was investigated by using electrochemical impedance spectroscopy (EIS) and density functional theory (DFT). In EIS, three major electrochemical processes in the electroless deposition process were found, when the solution pH ranged from 5.5 to 9.5. To understand the microscopic mechanisms involved, all participate species in the reaction pathways were calculated by the DFT method along with a natural bond orbital (NBO) analysis. Two emulating reactions were demonstrated: Path (I) passes through a primary dehydrogenation (D-RP), and Path (II) includes a primary addition of OH ^- (A-RP) on the hypophosphite anion. By comparison of the energy levels of all species, it can be concluded that Path (II) is energetically favorable under both acidic and alkaline conditions. The DFT and NBO analysis can provide strong evidence for the loops detected in the EIS, especially especially for the inductive loop (IL-M) in the medium-frequency domain that is caused by the formation of [Ni¹-H₃PO₂(OH)] and the capacitive loop (CL-L) in the low-frequency domain by [H₂PO ₂(OH)]. The combination of electrochemical analysis (EIS) and first principle theory (DFT) analysis proves that it is helpful to explore the nature of the interaction between anodic and cathodic reactions in the electroless deposition process.