In low-humidity PEFCs where reactio

In low-humidity PEFCs where reaction water is used to hydrate membranes, there exist profound interactions between water transport and transient cell behavior. First, there is atime scale for membrane hydration by reaction water. Furthermore, multiple mechanisms of water transport through the membrane, such as electroosmotic drag and back-diffusion, create a complex transient response involving several time scales. For example, during a step change in the current density, the electroosmotic drag will immediately remove water from the anode side of the membrane before back-diffusion of water from the cathode to anode takes effect. This can cause a temporary dry out on the anode side of the membrane and hence a jump in membrane resistance or a sharp drop in cell voltage. This voltage drop is, however, recoverable within a period of time constant characteristic of water back-diffusion through the membrane, which is dependent on the water diffusion coefficient and membrane thickness. Understanding transient behaviors of this kind is tremendously important for successful deployment of PEFC technology in vehicles.