法拉第吸脱附偶联过程循环伏安行为的有限元分析

Cyclic Voltammetry Coupled with Faradic Adsorption/Desorption Processes:A Finite Element Simulation

法拉第吸脱附偶联过程的电化学行为较为复杂,难以定量获得其表界面反应动力学信息.本文通过COMSOL有限元软件对法拉第吸脱附偶联过程的循环伏安行为进行数值分析,研究了反应物或产物不同吸附条件下的循环伏安行为.结果表明:当反应物或产物弱吸附时,可通过阴、阳极峰电流之差实现饱和吸附量的定量表征.随着吸附平衡常数的增大,反应由弱吸附向强吸附过渡,峰电流由扩散峰与吸脱附峰相互重叠过渡到相互分离的吸脱附"前波"或"后波"特征.该吸脱附特征峰的形状和位置与电势依赖的吸附平衡常数有关.吸附平衡常数及其电势依赖程度越大,吸脱附峰偏离扩散峰越远,吸脱附峰越尖锐.该模型为法拉第吸脱附偶联过程的循环伏安研究提供了一种定量研究方法,能够帮助研究者从复杂的吸脱附伏安行为中定量获得饱和吸附量和吸附平衡常数等信息,并对涉及吸脱附的电催化研究具有一定指导意义.

Heterogeneous electron transfer(HET)coupled with Faradic adsorption/desorption is the fundamental processes involved in hydrogen evolution reaction,oxygen reduction reaction,carbon dioxide reduction reaction,and methanol oxidation reaction.However,the electrochemical behaviors of HET coupled with Faradic adsorption/desorption are complicated,and difficult to get the interface reaction kinetics quantitatively.In this paper,finite element method was adopted to simulate the cyclic voltammetric behaviors of HET coupled with Faradic adsorption/desorption processes by using the Fick’s second law and Langmuir isotherm.The cyclic voltammograms when reactant or product adsorbed weakly or strongly were simulated,and they agreed well with the classical results.In addition,taking the reactant adsorption for example,the effects of scanning rate,saturated adsorption and adsorption equilibrium constant on cyclic voltammograms were investigated.The simulation demonstrates that weak adsorption can be magnified with the increasing scanning rate,and the peak current changes gradually from being proportional to the square root of scanning rate to being proportional to the scanning rate.The difference between anodic and cathodic peak currents is linearly related to saturated adsorption.Therefore,the quantitative characterization of saturated adsorption is realized by correlating the difference between anodic and cathodic peak currents to the saturated adsorption.With the increasing adsorption equilibrium constant,the weak adsorption transfers to strong adsorption,and the adsorption/desorption peaks separate from diffusion peaks to form prepeaks or postpeaks.The potential-dependent adsorption equilibrium constant was also simulated,which demonstrates that it can further change the shape and position of adsorption/desorption peaks.The quantitative method based on this model can help researchers to obtain saturated adsorption and adsorption equilibrium constant quantitatively from the cyclic voltammograms coupled with Faradic adsorption/desorption processes.The simulation can also help researchers to understand the different cyclic voltammetric behaviors between surface processes and diffusion processes,and would be constructive for the study of electrocatalysis involving adsorption/desorption processes.