Study Of Current Oscillations Of Nickel In H₃PO₄/SCN^-Solutions

Chemical oscillation, especially electrochemical oscillation which is therepresentative of non-equilibrium and non-linear in chemistry phenomenon gotincreasing attention. Further investigation of electrochemical oscillation not only canenrich and improve the nonlinear science and its theoretical research also can promotethe progress of the nonlinear dynamics.This paper studies the current oscillation of Ni dissolution in H₃PO₄solutioncontaining SCN^-. The effect on the oscillation behavior of applied potential, theconcentration of SCN^-and H₃PO₄are separately investigated by constant potential,cyclic Voltammetry （CV） and linear Voltammetry methods. Besides, the current-time（i-t） series is analysed by phase space reconstruction and power spectrum methods.Current oscillatory behavior of Ni in H₃PO₄solution containing SCN^-main presentsingle periodic oscillation, and with potential, SCN^-and H₃PO₄concentration change,also can present mixed mode oscillation （MMO）. Research shows that there generate alayer of black film on Ni electrode surface in the cyclic voltammetry, and black filmgradually rupture and fall off between oscillation region and passive region. Inductiontime of oscillation is much shorter and the average period of oscillation can be dividedinto active period （tactive） and passive period （tpassive）. The influence of applied potentialand SCN^-concentration on maximum current （imax）, amplitude （Δi）, tactive, tpaaiveandreconstruct phase space are different, so electrode potential and SCN^-concentration canbe thought of as a pair of mutual constraints factors.And with the increase of H₃PO₄concentration can accelerate the active and passive process, the oscillatory frequency（1/tp）, tactiveand tpassivedecrease, above characteristics is different at applied potential andSCN^-in oscillatory nature.On the basis of the above study, the effect of stirring on oscillation behavior isstudied and results show that oscillatory behavior of Ni/H₃PO₄/SCN^-system can beinfluenced by interface solution concentration between electrode and solution, which isrelated with diffusion steps. Ultraviolet visible spectrophotometer （UV-Vis） have beenemployed to test Ni²⁺concenctration and then estimate the apparent rate constant （Kb）of electrode reaction. Above results also show that the Ni²⁺concentration increases withincrease of oscillation time, the reaction that Ni generated Ni²⁺can be regarded as thefirst order reaction and the Kbis1.43×10-4s-1. X-ray photoelectron spectroscopy （XPS） technique and XPS etching technology are used to identify composition and content ofblack film on Ni surface, and observe the color variations of electrode surface in i-texperiment during preparation process of XPS sample. XPS results show that thesurface active ingredient of black film main contains Ni（OH）₂, S, S^2-, Ni_xS_y and S_xO_y^2-.XPS etching analysis reveal that Ni（OH）₂mainly exist in the film surface, which maybe associated with pH value near Ni electrode surface. Furthermore, content of18s and20s samples in S, NiS, Ni_xS_y, S^2-, Sn^2-and S_xO_y^2- are compared. SEM shows that thequantity of black attachments increase with oscillation time increasing, and themorphology change of oscillatory activation and passivation is not obvious, which canbe thought of as attachments are no significant influence on oscillatory nature.Finally, reasonable explanation to the formation of black film and peeling process,then current oscillation mechanism are discussed, simplify the reaction process.A threemolecular mode of oscillation mechanism is proposed to account for the periodiccurrent oscillation in Ni/H₃PO₄/SCN^-system. Above simulation results are consistentwith experimental results.