钒(Ⅳ／Ⅴ)电对在碳纸电极上的反应机理研究

The Mechanism of Vanadium(Ⅳ/Ⅴ) Couple Redox Reaction at Carbon Paper Electrode

采用循环伏安、极化曲线和交流阻抗技术研究了在0.8mol·L-1VOSO4+3.0mol·L-1H2SO4中,V(Ⅳ/Ⅴ)电对在碳纸电极上的反应机理及可能的速度控制步骤。研究结果表明:V(Ⅳ/Ⅴ)电对在碳纸电极上的反应属准可逆过程,且氧化过程包含有后置化学转化步骤,计算得到VO2+的扩散系数为4.5×10-5cm2·s-1。理论计算得到了电化学步骤和后置化学转化过程分别为控制步骤时的Tafel斜率值为0.12和0.06,实验得到的Tafel斜率值为0.127,交换电流密度为6.7×10-4mA·cm-2。表明电极氧化过程受电化学过程控制,不同极化电位下的交流阻抗图谱拟合结果表明,电化学反应阻抗值远大于其他阻抗值,说明电化学反应可能是电极反应的控制步骤,与实验得到的极化曲线分析结果相一致。

The V（Ⅳ/Ⅴ） reaction mechanism was investigated on a carbon paper（CP） electrode by means of cyclic vohammetry, polarization curve and impedance spectroscopies. Cyclic vohammetry analysis indicated that V（Ⅳ/Ⅴ） redox reaction at CP electrode was quasi-reversible. A certain chemical reaction step was suspected to follow the electron transfer step of this reaction. The diffusion coefficient for V（Ⅳ） was 4.5×10^-5cm^2·s^-1 and exchange current density was 6.7 ×10^-4 mA·cm^-2. The Tafel slope value（0.127） obtained from the polarization curve agrees well with theoretical value（0.12） when the electron transfer step was supposed to be the rate-determining step. Impedance spectroscopies at various potentials were studied and simulated by equivalent circuits. The impedance value of the electron transfer step modified was greatly higher than that of the others, which revealed that the electron transfer step was the rate-determining step. This further confirms the conclusion obtained by polarization curve test.