Water balance experiments in PEM FC stacks. Measurements of water transport across the NAFION membrane

Water balance experiments are presented for PEM FC stacks with Nafion membranes. Data are obtained in various humidification conditions: cathode dry/anode dry; cathode humid/anode humid; cathode humid/anode dry; cathode dry/anode humid. The results are used to define conditions of correct hydration of the membrane as W-a/cath/(va/c)>1,where W-a/cath is the amount of water collected in the anodic/cathodic compartment and W,,,, is the theoretical value of the water removed as saturated vapor from the corresponding compartment. Based on resistance measurements, it is concluded that an optimal humidification appears when W-cath/W-vc = 1.1-1.2. Data obtained enabled also evaluating the net flux of water through the membrane and its dependence on current and humidification. The effective drag (net number of water molecules transported per proton) has values between 0.23 and -0.15 and changes with the current and type of humidification.