Use of COMSOL Multiphysics for Optimization of an All Liquid PEM Fuel Cell MEA

1. Use of COMSOL Multiphysics for Optimization of an All Liquid PEM Fuel Cell MEA George H. Miley (Speaker), Nuclear, Plasma and Radiological Engineering E. D. Byrd Electrical & Computer Engineering University of Illinois at Urbana-Champaign Urbana, IL 61801 USA COMSOL USERS CONF. 2006 BOSTON, MA

2. Outline • NaBH4/H2O2 Fuel Cell • Description of Model • Physical Layout • Electrical Considerations • Mass/Momentum Balance Considerations • COMSOL Application Mode coupling • Pressure Differential Simulations and Results • Land Area vs. Permeability and Conductivity Simulation and Results • Conclusions COMSOL USERS CONF. 2006 BOSTON, MA

3. NaBH4/H2O2 Fuel Cell • Use in fuel cells is a relatively new development • H2/H2O2 and NaBH4/H2O2 cells were investigated at NPL Associates, Inc., the University of Illinois (UIUC), and elsewhere • Have shown great results, demonstrating the general feasibility of a peroxide based cell • Excellent potential for space applications due to high power density and air (oxygen) independence. COMSOL USERS CONF. 2006 BOSTON, MA

4. UIUC/NPL Direct Peroxide Fuel Cells • The sodium borohydride/hydrogen peroxide reactions. Anode: Cathode: COMSOL USERS CONF. 2006 BOSTON, MA

5. Test Cells - Compact 1-30 W Power Units The 15-W cell shown here uses an integrated cooling channel to dissipate the waste heat generated in the relative small 25-cm2 active area. An optimized version of this small cell generated 36-W at ~ 60ºC, representing the highest power density reported to date for a small fuel cell working at sub-100C. 15-W NaBH4/H2O2 Test Fuel Cell as assembled. • Flow rate of approximately 200 cm3/min • Minimal pressure drop even with parallel flow due to low flow rate • Temperature rise of approximately 15°C • Heat flux is approximately equal to electrical power (500-W) COMSOL USERS CONF. 2006 BOSTON, MA

6. The 500-W UIUC/NPL NaBH4/H2O2 Fuel Cell Stack The active area per cell was 144 cm2 and 15 cells were employed to provide a total stack active area of 2160 cm2. COMSOL USERS CONF. 2006 BOSTON, MA

7. UIUC/NPL Direct Peroxide Fuel Cells COMSOL USERS CONF. 2006 BOSTON, MA

8. Objectives for COMSOL modeling • Gain insight into behaviors governing flow and current distributions • Determine space (diffusion layer parameters, conductivity effects, flow channel and land dimensions) for detailed optimization physics • Guide future design improvements COMSOL USERS CONF. 2006 BOSTON, MA

9. Model Description- geometry Physical Layout • Based on repetitive cross section of MEA and flow channels. • Outlined area represents the physical model. • Portion of graphite plates included to see the current density in the plate and to be able to vary their conductivity. COMSOL USERS CONF. 2006 BOSTON, MA

10. Model Description - electrical • Standard Electrical Model • DC current conduction - applies to each section with different conductivity (graphite, diffusion layers, membrane) • Butler-Volmer Equations Anode: Cathode: COMSOL USERS CONF. 2006 BOSTON, MA

11. Modified Bulter-Volmer • The Butler-Volmer equation was modified to obtain an alternative version that is more robust when solving numerically in Comsol. In this version, the hyperbolic identity of Eq. 2-5 is used to form Eq. 2-6. • (2-5) • (2-6) COMSOL USERS CONF. 2006 BOSTON, MA

12. Model Description – conservation equations • Mass Balance • Momentum Balance – Darcy’s Law COMSOL USERS CONF. 2006 BOSTON, MA

13. COMSOL Application Mode Coupling COMSOL USERS CONF. 2006 BOSTON, MA

14. Parameters used • Necessary parameters (other than exchange current and equilibrium potentials, discussed next) were acquired through experimental means or published values These include the conductivities, permeabilities, diffusion coefficients, and viscosities given in the following table. COMSOL USERS CONF. 2006 BOSTON, MA

15. Parameter set 1 COMSOL USERS CONF. 2006 BOSTON, MA

16. Parameter set 2 - determination of the exchange current density and reversible potential • A Hydrogen half-cell was constructed and used to determine the exchange current density and additional parameters such as the Tafel slope in the Butler-Volmer eqns.. The reversible potential of each cell half was determined using the Gibb’s Free Energies applied to the reactants and products in each reaction. COMSOL USERS CONF. 2006 BOSTON, MA

17. Model verification: I-V Curve calculated for the reference case agrees well with corresponding experiment – model next used to explore design changes COMSOL USERS CONF. 2006 BOSTON, MA

18. Simulations – Pressure Differential • Vary the pressure differential between the two flow channels. • Reasons • Different flow velocities create different pressure differences • Different locations have different pressure drops COMSOL USERS CONF. 2006 BOSTON, MA

19. Simulations – Pressure Differential- Higher values optimal • Results • Low pressure drops cause less permeation in the diffusion layer, causing mass transport losses. • High pressure drops allow reactants to easily reach under the land area. • Reactant permeation under flow channel depends on fluid velocity and location along channel. COMSOL USERS CONF. 2006 BOSTON, MA

20. Simulations – Land Area selection • Current collector land area width to flow channel width ratio is varied (collector + channel widths = constant). • Land Area Width varied while also varying diffusion layer permeability. • Land Area Width varied while also varying diffusion layer conductivity. COMSOL USERS CONF. 2006 BOSTON, MA

21. Simulations – Land Area – high permeability give flexibility in width Permeability • Max Power at different Permeability with varying land areas. • Low Permeability diffusion layers have optimum current collector land area to flow channel ratio. • High Permeability diffusion layers function well with wide current collector widths. Maximum Power vs. Land Area Width COMSOL USERS CONF. 2006 BOSTON, MA

22. Simulations – Land Area – optimum with high conductivity and equal width design Conductivity • Max Power at different Conductivity with varying land areas. • High conductivity diffusion layers have are optimum with equal width current collectors and flow channels. • Low conductivity diffusion layers function better with wider land areas and narrower flow channels. Maximum Power vs. Land Area Width COMSOL USERS CONF. 2006 BOSTON, MA

23. Conclusions • Simulations performed of all-liquid PEM fuel cell using COMSOL Multiphysics. • Normalization uses data from half cell for io and Vrev. • Pressure differentials, conductivities, permeabilities, and current collector widths varied in the simulations. • Cell performance varies with different flow velocities and along the flow channels. • Optimum current collector widths predicted for diffusion layers with known conductivities and permeabilities. • Model is very useful for optimization in region around normalization. • Simulations narrow region for experimental studies to zone near optimum performance. Greatly reduces time and expense of experimental studies. COMSOL USERS CONF. 2006 BOSTON, MA

24. Acknowledgement We would like to thank: • NPL Associates, Inc. for their support with starting the project. • E. Byrd wishes to acknowledge fellow researchers N. Luo, J. Mather, G. Hawkins, and L. Guo for their help. • This research was supported by DARPA SB04-032. • Continuing studies are supported by DARPA/AFRL. COMSOL USERS CONF. 2006 BOSTON, MA

25. Dr. George. H. Miley UIUC Phone: (217) 333-3772 email: ghmiley@uiuc.edu Ethan D. Byrd UIUC email: ebyrd@uiuc.edu Thank You For more information please contact: COMSOL USERS CONF. 2006 BOSTON, MA