Transition time in chronopotentiometry is an important parameter, which is widely used for electrochemical characterization of various systems. Occurrence of multiple transition times is typical for multicomponent or multilayer electrode and membrane systems. In this paper we show that there may be another cause of multiple transition times. It is electrical heterogeneity of ion exchange membrane surface. It is found that there are two transition times on the chronopotentiograms of a commercial anion-exchange MA-41 (Shchekinoazot) and two specially prepared cation-exchange heterogeneous membranes in dilute electrolyte solutions (0.02 M NaCl in the experiment). It is found that both transition times are determined by diffusion limitations of ion delivery to the membrane surface in the depleted diffusion layer. The value of the first transition time depends on the dimensions of the conductive regions and their surface fraction; this transition time is determined as the time necessary for the depletion of electrolyte concentration near the conductive regions of the surface. The rate of concentration depletion depends on the electromigration through the conductive regions, on the one hand, and on the normal the tangential ion diffusion to the conductive surface regions, which mitigate the concentration decrease, on the other hand. The experimental value of the first transition time for the laboratory-made membranes is in a good agreement with simulation using a 3D electrodiffusion model. The value of the second transition time is in a good agreement with the Sand theory, hence it is conditioned by the normal diffusion delivery of electrolyte from the solution bulk to the entire membrane surface. The tangential diffusion plays a secondary role in this stage of concentration polarization since current-induced convection levels off the concentrations along the heterogeneous membrane surface.

计时电位法中的跃迁时间是一个重要参数，已广泛用于各种系统的电化学表征。对于多组分或多层电极和膜系统，通常会出现多个过渡时间。在本文中，我们证明了可能存在导致多个过渡时间的另一个原因。这是离子交换膜表面的电异质性。发现在阴离子电解质溶液（实验中为0.02 M NaCl）中，商用阴离子交换MA-41（Shchekinoazot）的计时电位图上有两个跃迁时间，还有两个特殊制备的阳离子交换非均质膜。发现两个转变时间是由离子输送到耗尽的扩散层中的膜表面的扩散限制所决定的。第一次过渡时间的值取决于导电区域的尺寸及其表面分数。该转变时间被确定为耗尽表面的导电区域附近的电解质浓度所需的时间。浓度耗尽的速度一方面取决于通过导电区域的电迁移，另一方面取决于法向到导电表面区域的切向离子扩散，这又减轻了浓度的降低。实验室制膜的第一个过渡时间的实验值与使用3D电扩散模型进行的模拟非常吻合。第二次过渡时间的值与Sand理论非常吻合，因此，它是由电解质从溶液块到整个膜表面的正常扩散输送所调节的。切向扩散在浓度极化的此阶段起次要作用，因为电流引起的对流使沿异质膜表面的浓度趋于平稳。