Tafel-Butler constant: An overpotential of 0.300 V for platinum-doped graphene indicates moderate catalytic efficiency. Overpotential is the extra voltage required for a reaction to proceed at a certain rate beyond its equilibrium potential. In the case of electrocatalytic reactions such as or oxygen evolution (OER), a lower overpotential suggests better performance. The platinum sites in the graphene matrix provide active sites that reduce the energy barrier for these reactions.