Why voltammetric current is scan rate dependent? Tangible representation of a mathematically dense concept using conceptual thinking.

Cyclic voltammetry (CV) is the most popular electrochemical technique for the study of the electrode processes. CV experiment is a rapid way providing information on the thermodynamics of redox processes. The time scale of a voltammetric experiment can vary several orders of magnitude by varying the scan rate, the rate at which the potential is scanned. The characteristic shapes and diagnostic criteria at various scan rates make CV a powerful technique for study of the kinetics of heterogeneous electron-transfer reactions and their coupled chemical reactions. However, understanding the scan rate dependence of voltammetric currents is a complicated concept and requires several mathematical equations. One of the basic questions that newcomers face in electrochemistry is: why CV peak current increases proportional to square root of scan rate? Plotting the current as a function of the time for a CV experiment is the key point in understanding the current dependence of scan rate. Among the possible mass transfer regimes, CV relies only on diffusion. The voltammetric experiment take longer and the thickness of the diffusion layer expands further from the electrode surface. Consequently, the flux of electroactive species to the electrode surface becomes smaller for expanded diffusion layer at slower scan rates.¹
Herein, an open-source simulation package has been developed using Excel and Python software’s that simulates the potential-time, potential-current-, time-current, and the dynamic change in concentration profile of electroactive species at the electrode surface. All these plots being animated in real time of CV experiment and the user-friendly interface of the simulation package enables the students to change the scan rate and observe the changes in all the plots simultaneously. Comparing these dynamical plots and concentration profiles help the students to understand the relation between currents and evolving concentration profiles, and the relations between peak currents and scan rates of voltammetric experimnts.