The origin of this asymmetry lies i

The origin of this asymmetry lies in the formation of water at the cathode of PEM fuel cell that constantly maintains the cathode at an adequately hydrated state. Since water alsoarrives from anode by electro-osmotic drag, and water desorption from membrane for removal through cathode GDL is an inherently slow process, water tends to accumulate at the cathode region near the interface between membrane and CL. By means of back-diffusion and even pressure-driven permeation, the high water content at the cathode region then acts as a concentration barrier to prevent further incoming water from anode by electro-osmotic drag. In this way, any incoming water due to inlet anode humidification will tend to be circulated and confined within the membrane by the counteracting actions of electro-osmotic drag and back diffusion, aided by the blocking action of the high water concentration at cathode, and pressure-driven permeation.