Electro-osmotic Drag of Water in Poly(perfluorosulfonic acid) Membranes

Direct methanol fuel cells (DMFCs) using poly(perfluorosulfonic acid) membranes were used to determine the water electro-osmotic drag coefficients in these ionomeric membranes. In a DMFC fed with 1.0 M aqueous methanol at the anode and dry at the cathode, the ionomeric membrane can be fully equilibrated on both sides with liquid water at a sufficiently high current density. In this special situation, the water flux across the membrane from anode to cathode is driven by protonic drag exclusively. From measurements performed on and membranes, water drag coefficients of all membranes increase with temperature. Compared with a fully hydrated membrane, a dried membrane (105°C in vacuum) known to have a lower water content has a lower water drag coefficient. For a series of Nafion membranes (including C membrane), a membrane with a higher equivalent weight has a lower water drag coefficient. The water drag coefficient of a Dow membrane is unusually low compared to that of a Nafion membrane of similar equivalent weight. These trends of water drag coefficients may correlate with the membrane structure and water content. Implications of these results on water management in polymer electrolyte fuel cells are discussed.