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Solid Oxide Fuel Cells Rodger McKain, PhD 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 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 Increasing Temperature Fuel Cell Types Source: U.S. Fuel Cell Council Attributes of Fuel Cells AFC PACF PEM Electrolyte KOH Phosphoric Acid Temperature 1000C H2 Fuel Efficiency (H2 fuel) (NG fuel) Pollution Hydrocarbon Fuel Use Start-Up 60% -Very low No Fast MCFC SOFC Sulfonic Acid Polymer Molten Carbonate Salt Y2O3-ZrO2 Ceramic 2000C 1000C 6500C H2 H2 H2/CO H2/CO 55% 40% Very low Difficult 60% 35% Very low Difficult 55% 50% Low Yes 55% 50% Low Yes Moderate Fast Slow Slow 800-10000C 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 Fuel Cell Power System Useful heat Heat Management Fuel Air Fuel Processor Fuel cell Stack Sub Assembly Controls Power Conditioner 10 kW High Efficiency High Efficiency at Part Load Low Emissions Average U.S. Utility Emissions ONSI PC25 200 kW NG Fuel Cell (lbs per megawatt-hour) (lbs per megawatt-hour) Nitrogen Oxides 7.65 0.016 Carbon monoxide 0.34 0.023 Reactive organic gases 0.34 0.0004 Sulfur oxides 16.1 0 Particulates (PM10) 0.46 0 Contaminant 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 RL O2 + 4e- 2O2Pt Ink v A Anode catalyst layer CH4 + 3O2- O2- CO2 + H2O + 2e- Effluent Pt Wire Fuel/CH4 Cathode catalyst layer Electrolyte Disc Yttrium-stablized Zirconia (>950 °C) Galladium-doped Ceria (>600°C) CH4 + CO2 CH4 + H2O CO + H2O CH4 + 0.5 O2 A Oad Products CH4 Oad CO, H2, CO2, H2O CnH2n Oad CnH2nO, CO2, H2O C Oad CO2 2CO + 2H2 CO2 + 3H2 CO2 + H2 CO + 2H2 T (°C) 600-1200 550-950 Relationship between fuel processing and fuel cells Under development Fuel Cell Types Solid Fuel Coal, Pet Coke n easi Incr Coal-based SOFC 500 to 1000 oC Gasification g co y exit mp l HC-based SOFC S-removal s roce el p Natural Gas of f u Liquid Fuel sin crea ; de sing Conversion to H2/CO g ef ncy ficie Hydrogen 500 to 1000 oC Gas Cleaning Shift Reaction H2/CO CO Selective Oxidation SOFC Thermal integrated Reformer 500 to 1000 oC MCFC Thermal Intergrated Reformer 600 oC PAFC (CO<5 %) 200 oC PEMFC (CO<10 ppm) 80 oC Ref: N.F. Brandon, S. Skinner, B.C.H. Steele, Annu. Rev. Mater. Res. 2003. 33:183-213 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 Electric terms 6,240,000,000,000,000,000 electrons / sec = 1 amp Current is the flow of electrons Fuel Cell Stack Low resistance Volts Copper wire, 1/16” diameter, 10 amps, electrons travel 1 cm In 28 seconds. High resistance Resistance If h is 1 volt and current is 1 amp Resistance is 1 ohm What’s a watt? Work involves height lifted and weight of ball, ft-lbs Work has no time limit, power does 550 ft-lbs/sec = 1 horsepower = 746 watts Power = (height lifted times weight of ball) times (balls per second), or Power = voltage times current, Watts = volts times amps Energy flow Heat Food Air Work, power Same story for electric system Food anode, Air cathode Stack produces power and heat 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. All the energy in the food eventually appears as heat. V-I scan 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 Balls lifted per hour, or amps (I) 50 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 Balls lifted per hour, or amps (I) 25 Micro view - Electric Icon Via - Anode Bond Layer Porous Bond Layer e- e- e- e- O= Fuel layer #1 e- e- e- e- O= O= O= e- e- e- e- e- e- Air layer #1 O= Electrolyte* Cathode O= + H2 = 2e- +H2O e- ½ O2 + 2e- = O= e- + Fuel utilization Air Stoics *A nonmetallic electric conductor in which current is carried by the movement of ions. Complete micro view Icon Fuel Flow, H2O+CO H2+CO2 H2 H2O Bond Layer CO CO 2 CO Porous O= N2 Bond Layer N2 N2 N2 N2 N2 N2 O2 e- e- N2 N2 N2 N2 O2 Air Flow, O2 + N2 N2 N2 N2 e- N2 CO2 O= O= O= e- Fuel layer #1 e- CO CO2 CO CO2 Via Anode CO CO2 CO2 CO e- eCO - N2 N2 e- Air layer #1 O= N2 N2 e- + Electrolyte Cathode O= + H2 = 2e- +H2O ½ O2 + 2e- = O= Co-flow Design Concept – Unit Cell Multi-layer ceramic construction Vias carry current Cell Air flow Fuel flow Interconnect Sealant Ink “bumps” printed on vias Thermocouples, Voltage taps Add a cell Thermocouples, Voltage taps Manifold arrangement Fuel inlets Manifold Air inlets Gasket Vehicle ICE vs. Fuel Cell Direct Drive Efficiency Comparison 40 100 Energy Units IC Engine 40% Power Train 37.5% 20 Idling 60 15 5 Friction 20 40 Energy Units Fuel Cell 50% 20 Direct Drive 75% 0 Idling 5 Friction 15 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