柠檬酸盐溶液中镍沉积的动力学机理*

摘要

应用电化学阻抗谱和动电位极化曲线方法,研究了在柠檬酸钠溶液中镍沉积的电化学行为.通过解析电化学阻抗谱,推导出镍沉积的阴极反应历程：镍沉积过程分两步得到两个电子,其中第一个得电子转移步骤为速率控制步骤,反应中生成了吸附态的电活性中间产物 Ni(OH)ads ,而且以电感的形式影响电极反应的阻抗.从理论上推导了电极反应动力学方程,通过实验验证,得出了动力学参数为：阴极过程 Tafel斜率为0．142 V/dec,表观传递系数＝0．49,交换电流密度为7．56×10-6 A/cm2,反应级数为Ni2＋＝1,在pH 值＝2~5的缓冲溶液中,OH -浓度对电极反应速度影响不大,Ni(OH)ads 覆盖率不能忽略,表观活化能平均值为50．3 kJ/mol.%Electrochemical behaviour of nickel deposition from sodium citrate solution was investigated by means of electrochemical impedance spectroscopy (EIS)and potentiodynamic polarization test.By analyzing the EIS spectroscopy,the processes of Ni electrodeposition was concluded as follows:in the acidic citrate bath,it in-volves two consecutive one-electron charge transfers reaction in which Ni(OH)ads acts as an intermediate ad-sorbed complex.The rate-determining step was the first electron transfer reaction.The low frequency inductive loop was ascribed to the relaxation of the electrode coverage by an adsorbed intermediate Ni(OH)ads .The kinetic equation of electrode reaction can be proposed from the theoretical views and verified by experiment.The results indicate as follows:the Tafel slope of the cathodic process was 0.142 V/dec,the apparent transfer coefficient was 0.49,the exchange current density was 7.56×10 -6 A/cm2 ,the order of the reaction was 1 with respect to Ni2 + .When the electrolyte pH varies between 2-5,no dependence of cathodic rate constants on pH can be found.The coverage of intermediate Ni(OH)ads can not be ignored,the average value of the apparent activation energy of electrochemical reaction was 50.3 kJ/mol.