Why is an overpotential required in water electrolysis for hydrogen and oxygen evolution?

• A. Overpotentials account for kinetic barriers (activation energy), mass transport, and surface phenomena; realistic electrode potentials exceed the thermodynamic minimum.
• B. Overpotentials are purely thermodynamic corrections to the standard potential.
• C. Overpotentials only affect noble-metal electrodes and can be ignored otherwise.
• D. Overpotentials lower the energy required for water splitting.

Answer: (A)

Overpotential is the extra potential you must apply to drive an electrochemical reaction at a practical rate beyond its thermodynamic minimum. In water electrolysis, the theoretical energy you’d need is set by the thermodynamics of splitting water, but real electrodes require more because of kinetic and transport barriers.

The hydrogen and oxygen evolution steps involve multiple electron transfers and surface intermediates, which means there are activation barriers to overcome. The rate at which these steps proceed (activation overpotential) grows with current, so you must push the electrode potential higher to achieve useful current densities. At the same time, mass transport effects come into play: as gas bubbles form and diffuse away, or as reactants/products migrate to and from the electrode, concentration polarization raises the needed potential. Surface phenomena, such as adsorption of intermediates, changes in surface state, or roughness and bubble coverage, also alter the number of active sites and effective reaction kinetics, contributing to the overpotential. Ohmic losses in the electrolyte add another layer of extra potential required to overcome resistance.

All of these factors mean the realistic electrode potentials exceed the thermodynamic minimum by an amount known as the overpotential. The statement correctly captures that overpotentials account for kinetic barriers, mass transport, and surface-related effects, and that they cause the applied potential to be higher than the ideal minimum.

The other views are off because overpotential is not a purely thermodynamic correction, it is not exclusive to noble-metal electrodes, and it does not lower the energy required—it raises the practical energy needed to drive the reaction.