1. Water Formation and Flooding Phenomena in Proton Exchange Membrane Fuel Cells Yi-Shen Chen a, Chin-Hsiang Cheng a,*, Chun-I Lee b, Shiauh-Ping Jung b, Chi-Chang Chen b, Ozhgibesov Mikhail a a Dept. of Aeronautics and Astronautics, National Cheng Kung University, Tainan, Taiwan b Green Energy and Environment Research Lab., Industrial Technology Research Institute, Hsinchu, Taiwan

2. Fuel Cell 1/50 A Fuel Cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent; Fuel Cell can produce electricity continually for as long as fuel and oxygen are supplied; As the main difference among fuel cell types is the electrolyte, fuel cells are classified by the type of electrolyte they use. The energy efficiency of a fuel cell is generally between 40-60%, or up to 85% efficient if waste heat is captured for use. Proton exchange membrane Fuel Cell (PEMFC) is a type of fuel cell being developed for a wide range of applications, including stationary and portable fuel cell based devices;

3. Proton Exchange Membrane Fuel Cell (PEMFC) 2/50

4. PEMFC Design Issues 3/50 Water and air management: The membrane must be hydrated; requiring water to be evaporated at precisely the same rate that it is produced; Quick evaporation -the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell; Slow evaporation- the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction; Temperature management: The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading. This is particularly challenging as the 2H 2 + O 2 -> 2H 2 O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell. Durability, service life, and special requirements for some type of cells: Stationary fuel cell applications typically require >40,000 hours of reliable operation at a temperature of -35 °C to 40 °C; Automotive fuel cells require a 5,000 hour lifespan under extreme temperatures.

5. Highlights 4/50 This work deals with the numerical simulations of liquid water formation and migration in PEMFC; All calculations have been carried out by using the CFD- ACE+ software; Distributions of the flow field, concentration, electric field and pressure have been calculated;

6. Studied Model parallel flow channel of fuel cell Solution Domain end plate of anode; end plate of cathode; flow channel of anode; flow channel of cathode; GDL; Cathode GDL Anode GDL Cathode Catalyst Membrane Anode Catalyst 5/50

7. Results (Current Density) 6/50 Average current density [A/m 2 )]versus time [s] The distribution of current density 10s 20s 30s 45s 60s 65s 70s 75s 80s 85s

8. Results (Water Distribution) 7/50 10s 20s 30s 45s 60s 65s 70s 75s 80s 85s water formation does not take place in first 60 sec; Fig. (g) to Fig. (j), the liquid water is produced at the region of rib; water diffuses to the region of flow channel.

9. Results (Water Vapor Distribution) 8/50 10s 20s 30s 45s 60s 65s 70s 75s 80s 85s the water vapor distribution of the interface between the catalyst layer and cathode GDL; the water vapor concentration decreases at the region of liquid water formation (Fig.(e)- Fig.(f)); (Fig.(h)-Fig.(i)) the water vapor concentration increases, due to the decay on previous step.

10. Conclusion Numerical calculation of transient mode in PEMFC requires consideration of the time term effect of electrochemical reaction, diffusion of fuel and energy transmission. The system reaches steady state regime in 50s from initial state ; In transient mode, one should consider that the water vapor concentration in gas increases or decreases by the water condensation or evaporation, so adds the source of the mass flow rate per unit volume from phase change in species diffusion equation. 9/50

11. Thanks for your attention!

12. 統御方程式 質量守恆方程式： 動量守恆方程式： 能量守恆方程式： 物種擴散方程式： 電流守恆方程式： VOF 方程式： 水凝結 / 蒸發方程式： 源項計算： ； 1/50

13. 模型建立 測試例的參數設定 Boundary conditions ANODECATHODE Inlet temperature 70 ℃ Outlet temperature 70 ℃ Wall temperature 70 ℃ relative humidity 100% stoichiometry1.52.5 Inlet velocity (m/s) 0.291.26 Operation voltage0.6 V 42/50