What are the main mechanisms by which mass transport occurs in electrochemical cells?

• A. Migration, diffusion, and convection
• B. Diffusion only
• C. Migration and diffusion only
• D. Convection only

Answer: (C)

Mass transport in electrochemical cells arises from how ions move under concentration differences and electric fields, with an additional mechanism if the liquid itself is moving. The two essential processes here are diffusion and migration. Diffusion follows Fick’s principle: ions spread from regions of high concentration to low concentration, smoothing out gradients over time. Migration is driven by the applied electric field: charged species drift toward electrodes of opposite charge, with the flux depending on the ion’s charge and mobility. In many well-controlled, quiescent cells, there’s little bulk fluid motion, so convection isn’t a dominant transport driver and can be neglected in basic analyses. The comprehensive description that includes all three mechanisms is the Nernst-Planck framework, where diffusion and migration are the primary terms under common experimental conditions. Convection becomes important only when stirring, gas evolution, or buoyancy effects introduce fluid flow. That’s why diffusion and migration are considered the main mechanisms in this context.