Electrochemistry Under Microscope: Measuring the Thickness of Diffusion Layer in Thin Layer Electrochemical Cells with Optical Microscope

Abstract

Electrochemical reactions are heterogeneous in nature, and an electrochemical reaction involves mass transfer from the bulk solution to the electrode surface, heterogeneous electron transfer at the electrode surface, and mass transfer back to the bulk solution. There are three modes of mass transport to the working electrode surface: diffusion, migration, and convection. Among these modes, diffusion is the one that involved in all electrochemical processes and sometimes is the sole mode of mass transfer. Thus, the key concept underpinning electrochemical science are that of the diffusion and diffusing layer, the region in the vicinity of an electrode layer that affected by electrode reaction. The thickness of diffusion layer is an important parameter in understanding chronoamperometry, cyclic and hydrodynamic voltammetry. The thickness of this zone can be measured either by simulation or analysis of the electrochemical responses. But interpretation of the concentration profiles and related current time responses require understanding the Fick’s law and its related mathematical representations. Although these basic are crucial in understanding electrochemistry but their mathematical representation is challenging specially for the students or scientists who are new to the field. The thickness of diffusion layer for a typical electroanalytical experiment varies between 20 to 200 micrometers, depends on the time of experiment. The size is not visible to the human eye easily but doesn’t required expensive research-grade microscopes and can be seen using even kids grade microscopes with 100 times magnification. Here we have developed a thin-layer electrochemical cell compatible with low-cost microscopes to demonstrate a microscopy activity in which students measure the thickness of micrometer-sized diffusion layer. The electrode processes involve the reactions of redox indicators with sharp color changes or acid base indicators with sharp color change in response to electrochemically generated proton or hydroxide, from water electrolysis. This Imaging tools advance electrochemistry education by enabling students to see the electrode processes that are not visible to the human eye without any research grade instrumentation. This device also aids in the explicit visualization and interpretation of a typical concentration profiles near the working electrode surface for chronoamperometry, cyclic and hydrodynamic (e.g., rotated disk) voltammetry experiments.