Two-phase transients of polymer electrolyte fuel cells

A three-dimensional transient model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes was developed to study the dynamics of gas-diffusion layer (GDL) dewetting and its impact on polymer electrolyte fuel cell performance. It was found that the dewetting of fuel cells by dry gas is characterized by several regimes of different time constants. These regimes can be classified by through-plane drying vs in-plane drying as well as by the differing water diffusivity in the anode and cathode. The water diffusivity in the anode GDL is several times larger than that in the cathode, therefore the anode side undergoes faster water loss to the dry gas stream. In addition, the land hampers the diffusive transport of water, therefore the liquid water tends to be trapped under the land and the water loss there starts only after through-plane drying of the GDL under the channel is completed. The different time constants of various dewetting regimes also affect the evolution of cell voltage due to the ohmic loss in the membrane. In addition, theoretical solutions are developed for the in-plane and through-plane drying regimes, and show good agreement with the numerically predicted time scales. (C) 2007 The Electrochemical Society.