In the state-of-the-art modeling of electrochemical system (e.g. fuel cell), the volume-specific and area-specific exchange current densities are linearly linked by a geometric factor, which is a kind of empirical approaches and lacking of theoretical fundamental. Based on the analytical approximate solution of charge transfer model in electrode, a novel expression is obtained for nonlinear relationship between the two exchange current densities. Along different mathematical paths, the nonlinear relationship is obtained equivalently in both Cartesian and polar coordinates. In addition to the independence on geometric configuration and operating current density, the nonlinear relationship further shows the influence of temperature and species partial pressures, which can not be explained by the empirically linear one. (C) 2014 Elsevier Ltd. All rights reserved.