Rectification, the tendency of bidirectional ionic conductors to favor ion flow in a specific direction, is an intrinsic property of many ion channels and synthetic nanopores. Despite its frequent occurrence in ion channels and its phenomenological explanation using Eyring's rate theory, a quantitative relationship between the rectified current and the underlying ion-specific and voltage-dependent free energy profile has been lacking. In this study, we designed nanopores in which potassium and chloride current rectification can be manipulated by altering the electrostatic pore polarity. Using molecular dynamics-based free energy simulations, we quantified voltage-dependent changes of free energy barriers in six ion-nanopore systems. Our results illustrate how the energy barriers for inward and outward fluxes become unequal in the presence of an electromotive driving force, leading to varying degrees of rectification for cation and anion currents. By establishing a direct link between potential of mean force and current rectification rate, we demonstrate that rectification caused by energy barrier asymmetry depends on the nature of the permeating ion, can be tuned by pore polarity, does not require ion binding sites, conformational flexibility, or specific pore geometry, and, as such, may be widespread among ion channels.