(277c) Electro-Osmotic Drag in Nafion

The impact of the electro-osmotic drag phenomenon on fuel cell operation was investigated. If the electro-osmotic coefficient, á is 0.5 or greater, it would not be possible to operate the fuel cell with dry feeds, necessitating humidification equipment and control system. Experimental measurements were done with the Nafion 115 membrane using a fuel cell and using a permeation cell. In the fuel cell experiments, dry reactant feeds were used and the anode relative humidity was measured as a function of fuel cell current and temperature. At all the temperatures examined, as current increased, the anode relative humidity increased. This indicated that electro-osmotic drag was insignificant compared to the back-diffusion of water from the cathode to the anode. The permeation cell was used to measure the flux of water across the membrane under an applied potential. When liquid water was present on one side of the membrane, the water flux would increase when a potential is applied, independent of the direction of the potential, due to the influence of an osmotic pressure flow. When humidified hydrogen is present on one side of the membrane, the protonic flux does result in an increase in water flux across the membrane. However, at all the temperatures and wet side water activities probed, the electro-osmotic drag coefficient has been determined to be less than 0.2. Overall, this study has widespread implications for the operation and design of fuel cell systems.