| Abstract: | This work demonstrates that the operation of a subsaturated polymer electrolyte fuel cell in counterflow mode results in a significantly elongated relaxation time after a load change, if compared to coflow mode. This effect is investigated here by using combined dynamic locally resolved measurements of the current density, the high frequency resistance, and the relative humidity. It is shown that the elongated relaxation time is a consequence of slow membrane hydration in the region of the cell, downstream the anode flow field, where the diffusive flux of water across the membrane occurs from the anode to the cathode. Here, the anode gas stream, which is humidified upstream the anode flow field via back diffusion of water from the cathode to the anode, is the only source of water for both membrane hydration and the internal humidification of the cathode gas stream, which passes the cell in opposite direction. |
