Analysis of transient phenomena in polymer electrolyte fuel cells

by Wang, Yun.

iii Dynamic characteristics of polymer electrolyte fuel cells (PEFC) are of paramount importance for the automotive applications, given the rapid variation of loads. First, a three-dimensional single-phase transient model is developed, based on the constant-flow assumption, to study the transient dynamics of PEFC operations. To justify the validity of the constant-flow assumption, a complete single-phase model, fully coupling the flow, species transport and electrochemical kinetics, is presented to explore flow physics in a PEFC. Comparison of the numerical results demonstrates that the constant-flow assumption is valid with the maximum error of 14%. In addition, various time constants are estimated for important transient phenomena of electrochemical double-layer discharging, gas transport through the gas diffusion layer (GDL) and membrane hydration/dehydration. It is found that membrane hydration/dehydration occurs over a period of 10 seconds, the gas transport of 0.01-0.1 second, while the double-layer discharging is negligibly fast. The numerical results show that the time for fuel cells to reach the steady state is on the order of 10 seconds due to the membrane hydration/dehydration. In addition, a step increase in the current density leads to anode dryout due to electroosmotic drag during transients, while it takes several seconds for water back-diffusion and anode humidified gas to re-wet the anode side of the polymer membrane. The anode dryout results in a substantial drop in cell voltage and hence temporary power loss. Under extreme situations such as dry anode feed, large stepincrease in the current density and/or lower temperatures, the cell voltage may even reverse, resulting in not only power loss but also cell degradation. iv To extend the work to two-phase transients where liquid water appears and drastically alters the dynamic behaviors of a PEFC, a steady-state model fully coupling the two-phase flow, species transport, heat transfer, and electrochemical processes was first developed to predict the liquid water distribution and flooding under non-isothermal conditions. Water condensation or evaporation and its impact on the PEFC operation are fully accounted for. A theoretical analysis is presented to show that in the two-phase zone water transport via vapor-phase diffusion under the temperature gradient is not negligible and comparable to the water production rate in PEFCs. The ensuing heat pipe effect increases about 15% thermal conductivity in the GDL. Finally, based on the non-isothermal two-phase analysis at steady state, a transient model further considering various transient terms is developed to study the dynamics of GDL de-wetting and its impact on PEFC performance. It is found that the de-wetting of fuel cells by dry gas is characterized by several regimes of different time constants. These regimes can be defined fundamentally by through-plane drying versus in-plane drying as well as the differing water diffusivity between the anode and the cathode. The differing time constants of various de-wetting regimes also impact the evolution of cell voltage due to the Ohmic loss in the membrane.