Dynamic Characteristics Of The Transfer Phenomena In PEM Fuel Cell

Proton exchange membrane fuel cell (PEMFC) has a lot of virtues, such as highpower density, high-energy efficiency, low-temperature operation, fast start-up andzero-emission, so it is the most probable to apply in automobile, bus, electricalequipment, portable power and stationary product, and it has a very good foreground.But the actual fuel cell is in the research stage, and some of critical questions aren'tdissolved.The paper focuses on the transfer phenomena inside the fuel cell and studies thedynamic characteristics of PEMFC with CFD software Fluent.The transient equations of PEM fuel cell were described in this dissertation,including the fluid flowing, gas diffusion, electrochemistry reaction, water transferred,and so on. A Single straight channel model was builded and modeled in the PEMFCmodule of the Fluent software, the influence of current density and relative humidityof reactant gases on the dynamic transport of water in membrane was studied. Theresult shows that the hydration process of membrane takes about 10s to reach thesteady state; current density takes about 10s to reach steady state in the hydratedprocess of membrane, while it takes about 40s in the de-hydrated process.Then, a typical serpentine flow-field model was builded and the dynamiccharacteristics of the transfer phenomena in PEMFC were studied by consideringdifferent operating parameters and structural parameters. It indicates that for a fuelcell with 5.06cm~2 active area, time of hydrogen to reach steady state is 0.5～0.7s andthat of oxygen is 0.3～0.5s; but time of water in membrane to reach steady state ismuch longer, about 10s. When stoichiometry is greater than one and the relativehumidity of reactant gases is bigger, current density jumps, the increase of waterconcentration makes a decrease in oxygen concentration, which is just the reason ofthe overshoot behavior of current density. Parametres such as stoichiometry, operatingtemperature and the thickness of membrane all affect the overshoot of current density,the magnitude of overshoot/undershoot decreases with the increase of stoichiometry,operating temperature and thickness of membrane. The test results show that time ofcurrent density to reach steady state is three times of the modeling results; the magnitude of overshoot is only about 25％of the modeling results. But trend of thetest results and the modeling results is accordant.Based on the water dynamic transport in membrane and cell's dynamic transportcharacteristics and the correlative effect of each factor, and the test of dynamicprocess of current density or voltage, this research reveals to some extent the dynamiccharacteristics of the transfer phenomena in PEM fuel cell, the results are useful fordesign and control of the stack.