In the dry gas operation a certain amount of product water gets transported from cathode to anode, which is actually called back diffusion or back transport. When current is drawn from the cell, protons migrate from the anode to cathode and carry with them water molecules and this process is called electro-osmotic drag. The average number of water molecules transported per proton is, 1 for a vapour equilibrated and 2.5 for a liquid equilibrated Nafion membrane [14,15]. Hence, there is always a little osmotic drag, irrespective of the humidification of the membrane. Thus, the amount of water collected at the anode is not equal to the amount of water back transported from the cathode but the difference between the total amount of water back transported and the amount of water transported again to the cathode due to osmotic drag.