Investigation of Wet/Dry Cycling of Polymer Electrolyte Membrane By Full-Cell Scale Numerical Simulation with Transient Load Profiles
Wet/dry cycling in membrane of a PEFC is simulated under time-dependent load profiles which imitate actual driving conditions. Wet/dry cycling is an important issue to extend durability of PEFCs, since mechanical degradation is more likely to take place where gradation of water content distribution in membrane is large and wet/dry cycling is frequent. Performances of a 30cm times 10cm full cell is simulated, calculating transport of gas species and liquid water coupled with electrochemical reactions and current balances. Load profile is given by time-dependent total current conditions. Based on simulated results, dependence of wet/dry cycling on channel configurations and operating conditions are discussed.
Numerical Modeling and Results
Simulation is performed by our own simulation software for PEFCs, which is capable of simulating full-stack fuel cell system for fuel cell vehicles (up to 400 cells) under transient operating conditions. An important feature of this software is that macroscopic models are applied for microscopic phenomena in the MEA such as electrochemical reactions and proton/water transport. Heat transfer and fluid of liquid-gas two phase fluid with phase changing are also taken into account. These models are coupled with multi-dimensional heat and mass transport equations, including effective parameters for gas diffusivity and permeability in the GDL and equivalent hydraulic diameter in the flow channel. This software can also simulate cathodic degradation by carbon corrosion reactions.
Performance of cell is simulated under assumed load profiles, modeled by transient output current boundary conditions. Typical time-scale and range of supposed current conditions is 1[sec] and0.1-1.0[A/cm2], respectively, which imitates actual driving conditions on urban streets. Stoichiometric ratio and humidity of reactant gas are also changed simultaneously with the load profile.
Simulated results show transient behavior of wet/dry cycling. For example, soon after the output current is increased, large difference in reaction current density can be created, depending on the distribution of reactant gas flow. This results in large gradient of water content in membrane, where mechanical degradation is expected depending on flow distributions and operating conditions. In the same way, water content distribution between stacked cells can be simulated. In addition, effect of different time-scales of load profiles and response of water distribution can be discussed. These results can be useful information to consider the possibility of mechanical degradation.