Solid Oxide Fuel Cells Rodger McKain, PhD

1. Solid Oxide Fuel Cells Rodger McKain, PhD

2. Ion transport observed by William Grove in 1839…Based on hydrogen-oxygen, sulfuric acid electrolyte, and platinum electrodes “I cannot but regard the experiment as an important one…” William Grove to Michael Faraday October 22, 1842

3. Fuel Cell • An energy conversion device that directly converts chemical energy into electrical energy (dc power). • Analogous operation to a natural gas fueled electric generator: energy in fuel and oxygen are converted to electric power as long as fuel and air are supplied. • Six types, each suited for specific applications + Heat, H2O

4. Fuel Cell Types Increasing Temperature Source: U.S. Fuel Cell Council

5. Attributes of Fuel Cells AFC PACF PEM MCFC SOFC Electrolyte KOH Phosphoric Sulfonic Molten Y2O3-ZrO2 Acid Acid Carbonate Ceramic Polymer Salt Temperature 1000C 2000C 1000C 6500C 800-10000C Fuel H2 H2 H2 H2/CO H2/CO Efficiency (H2 fuel) 60% 55% 60% 55% 55% (NG fuel) -- 40% 35% 50% 50% Pollution Very low Very low Very low Low Low Hydrocarbon No Difficult Difficult Yes Yes Fuel Use Start-Up Fast Moderate Fast Slow Slow

6. Fuel Cell Stacks • Operating voltage of a single cell is ~0.7 volts • Cells are “stacked” in series to increase voltage to useful levels: Source: U.S. Fuel Cell Council

7. Useful heat Heat Management Fuel cell Stack Sub Assembly Fuel FuelProcessor PowerConditioner 10 kW Air Controls Fuel Cell Power System

8. High Efficiency

9. High Efficiency at Part Load

10. Low Emissions

12. Solid Oxide Fuel Cells • Based upon ion conductivity of certain ceramic materials at elevated temperatures (>600 C) • First observed by Nernst in 1890’s • Fluorite Structures (e.g. yttria stabilized zirconia) • Face Centered cubic arrangement • Transport through crystal lattice vacancies and oxide ions located between crystal faces • First SOFC constructed in 1937 by Baur and Preis • Requires porous electrodes and dense electrolyte, low electronic conductivity, and high strength

13. v A Oad Products T (°C) CH4 Oad CO, H2, CO2, H2O 600-1200 CnH2n Oad CnH2nO, CO2, H2O C Oad CO2 550-950 RL A Anode catalyst layer CH4 + 3O2- CO2 + H2O + 2e- O2 + 4e- 2O2- Pt Ink O2- Effluent Pt Wire Fuel/CH4 Cathode catalyst layer CH4 + CO2 2CO + 2H2 CH4 + H2O CO2 + 3H2 CO + H2O CO2 + H2 CH4 + 0.5 O2 CO + 2H2 Electrolyte Disc Yttrium-stablized Zirconia (>950 °C) Galladium-doped Ceria (>600°C)

14. Relationship between fuel processing and fuel cells

15. Basis for Fuel Cell Operation • Electron transfer – chemical reaction • Voltage determined by difference in chemical potential of fuel and oxygen • Current determined by area of cell • Catalyzed conversion of oxygen and hydrogen into reactive species O= and H • H2 + O2 = H2O + 2 electrons + heat • Electrons are separated from reactants by circuit • Need to understand electrical circuit background as it relates to fuel cell

16. Current is the flow of electrons Fuel Cell Stack Low resistance High resistance Resistance If h is 1 volt and current is 1 amp Resistance is 1 ohm Electric terms 6,240,000,000,000,000,000 electrons / sec = 1 amp Volts Copper wire, 1/16” diameter, 10 amps, electrons travel 1 cm In 28 seconds.

17. What’s a watt? Work involves height lifted and weight of ball, ft-lbs Power = (height lifted times weight of ball) times (balls per second), or Power = voltage times current, Watts = volts times amps Work has no time limit, power does 550 ft-lbs/sec = 1 horsepower = 746 watts

18. Heat Food Heat Air Work, power All the energy in the food eventually appears as heat. Energy flow Same story for electric system Food  anode, Air  cathode Stack produces power and heat In a perfect system all the energy in the food would be converted to power. Actually, only part is converted which defines the efficiency.

19. 10 ASR is the slope of the dashed red line 8 6 Height lifted or volts (V) 4 2 0 0 10 20 30 40 50 Balls lifted per hour, or amps (I) V-I scan

20. V-I scan 10 8 6 1 of these = 2 of these! Height lifted or volts (V) 4 2 0 0 5 10 15 20 25 Balls lifted per hour, or amps (I)

21. Icon Via - Anode Bond Layer e - Fuel layer #1 Electrolyte* e - e - e - e - Cathode e - e - e - e - e - e - O= O= O= O= O= O= + H2 = 2e- +H2O Porous e - ½O2 + 2e- = O= e - e - e - Air layer #1 Bond Layer e - + Micro view - Electric *A nonmetallic electric conductor in which current is carried by the movement of ions. Fuel utilization Air Stoics

22. e - e - Complete micro view Icon Fuel Flow, H2O+CO  H2+CO2 Via - H2 Anode H2O CO Bond Layer e - Fuel layer #1 CO2 Electrolyte CO CO2 CO2 CO CO e - e - Cathode CO2 CO CO2 CO2 CO CO O= O= O= O= O= O= + H2 = 2e- +H2O Porous N2 N2 N2 N2 N2 N2 ½O2 + 2e- = O= N2 N2 N2 N2 N2 N2 e - e - e - N2 N2 N2 N2 N2 N2 N2 Air layer #1 Bond Layer e - O2 O2 + Air Flow, O2 + N2

23. Co-flow Design Concept – Unit Cell Multi-layer ceramic construction Vias carry current Cell Fuel flow Air flow

25. Interconnect Ink “bumps” printed on vias Sealant Thermocouples, Voltage taps

26. Add a cell Thermocouples, Voltage taps

27. Manifold arrangement Air inlets Fuel inlets Gasket Manifold

28. Vehicle ICE vs. Fuel Cell Direct Drive Efficiency Comparison 40 100 Energy Units IC Engine 40% Power Train 37.5% 15 60 20 Idling 5 Friction 20 40 Energy Units Fuel Cell 50% Direct Drive 75% 15 20 0 Idling 5 Friction

29. Summary • Fuel Cells have been around a long time • They present the potential to be highly efficient because of direct conversion of chemical energy to electrical energy • Solid oxide fuel cells are based upon ion conducting properties of ceramic materials like doped zirconia • Temperatures above 600 C are required for operation • To be viable fuel cells must have high power per area, and operate with low cost materials