1. Intro to PEM Fuel Cells

2. What is a Fuel Cell? A fuel cell is an energy conversion device that reacts a fuel and oxygen to produce electricity. The most common fuel is hydrogen. -+ e-e- e-e-

3. Is a Fuel Cell a Type of Battery? No. Though both produce electricity, they operate in different ways. The reactants that a battery converts into electricity are stored within itself (e.g. battery acid) while the reactants for a fuel cell (e.g. H 2, O 2 ) are supplied externally.

4. Batteries and Fuel Cells H 2 in A battery’s reactants are self-contained A fuel cell’s reactants are supplied externally H 2 out Air in Air out -+ - +

5. Fuel Cell Applications Each type of fuel cell is particularly suited to certain applications: PEM: most versatile, used for portable power, transportation, and stationary power SOFC: primarily used for stationary power, in development for transportation (e.g. semi trucks) MCFC: power plants AFC: power and water production for space vehicles (e.g. Apollo and Space Shuttle spacecrafts), in development for more general use due to breakthroughs in alkaline media PAFC: stationary power, power plants

6. Why Use a Fuel Cell? A fuel cell provides very clean energy virtually emissions-free. In addition to electricity, it produces water and heat. Because the fuel cell’s reactants are supplied externally, there is no charge or discharge period as with a battery. Additionally, very high energy density can be achieved because the fuel cell design is not dependent on reactant storage. (Energy density = lifetime between recharges) A fuel cell can be used repeatedly - there is no package to throw away.

7. Why are People interested in Fuel Cells: DMFCs vs. batteries…. Energy Density Li-ion batt: 150-200 Wh/kg 20 W, 2 day fuel cell system: easily exceed 1000 Wh/kg TODAY

8. Many Types of Fuel Cells The main fuel cell technologies today are: PEM (polymer electrolyte membrane fuel cell) SOFC (solid oxide fuel cell) MCFC (molten carbonate fuel cell) AFC (alkaline fuel cell) PAFC (phosphoric acid fuel cell)

9. How are They Different? All fuel cells react a fuel and oxygen to produce electricity, but differ in the medium or “electrolyte” in which these reactions occur. The nature of the electrolyte determines all of the important characteristics of the fuel cell such as its operating temperature, materials of construction and the variety of fuels with which it can be used. PEM (proton-conducting polymer) SOFC (oxide ion-conducting ceramic) MCFC (molten carbonate salt in a ceramic matrix) AFC (aqueous potassium hydroxide in a matrix) PAFC (phosphoric acid in a matrix)

10. Why Use so Many Types? Each type of fuel cell has particular advantages and disadvantages PEM: solid construction, low temperature, sensitive to impurities, can only be used with hydrogen or methanol. SOFC: can be used with many fuels, doesn’t require precious metal catalysts, solid, rugged, very high temperature, expensive materials. MCFC: can be used with many fuels, efficient, doesn’t require precious metal catalysts, high temperature, very corrosive electrolyte. AFC: most efficient medium for oxygen reaction - high performance, doesn’t require precious metal catalysts, sensitive to carbon dioxide, caustic medium. PAFC (phosphoric acid fuel cell): same electrochemical reactions as PEM, but not as sensitive, very corrosive.

11. What Types of Fuel are Used? Typical fuel cells run on hydrogen and oxygen, but are “fuel flexible” because many types of fuel (e.g. methane, gasoline) can be reformed to make hydrogen or be used in its place (e.g. methanol in PEM fuel cells). Pure oxygen is rarely used except for special applications. Air is used instead, and is supplied from a pressurized gas cylinder or from the room or outside air via diffusion or a device such as a blower.

12. About Hydrogen Hydrogen can be produced in a number of ways, either as a direct byproduct of a reaction or by desorption from a material. In the latter case, the material also acts as a means of storage: Byproduct: steam reforming of fossil fuels, anaerobic oxidation of bacteria, reaction of chemical hydrides and water Desorption: metal hydrides, carbon nanotubes Storage: gas cylinders, tanks, bladders, metal hydrides, carbon nanotubes

13. PEM FC - The Whole Picture of a Single Cell EERE

14. How Does a PEM FC Work? Anode: 2H 2(g) -----> 4H + (aq) + 4e - Cathode: O 2(g) + 4H + (aq) + 4e - ----> 2H 2 O (l) PEM Cathode (Positive)Anode (Negative) H2H2 H+ + e-H+ + e- O2 + H+ + e-O2 + H+ + e- H2OH2O Hydrogen reacts with the Pt catalyst on the PEM to form protons and release electrons The protons travel across the PEM H+H+ The protons combine with O 2 and electrons to form water This reaction is also catalyzed by Pt

15. Direct Methanol PEM FC Anode: CH 3 OH (aq) + H 2 O (l) -----> CO 2(g) + 6H + (aq) + 6e - Cathode: 3/2O 2(g) + 6H + (aq) + 6e - ----> 3H 2 O (l) PEM Cathode (Positive)Anode (Negative) CH 3 OH H+ + e-H+ + e- O2 + H+ + e-O2 + H+ + e- H2OH2O Methanol reacts with the Pt/Ru catalyst on the PEM to form protons and release electrons The protons travel across the PEM H+H+ The protons combine with O 2 and electrons to form water This reaction is catalyzed by Pt

16. Current Flow in a Fuel Cell -+ e-e- e-e-

17. So what’s happening during operation? We’re feeding in gases (say, hydrogen and air) at some flow rate Fuel or oxygen utilization {= 1/stoichiometric ratio (usually called ‘stoich’)}: ratio of the moles used per unit time (related to current density by some conversion factors) to the incoming flow rate Gases may be humidified (esp. for single cells)

18. So what’s happening during operation? II We’re generating current and product water (that we may have to get rid of!!!) at cathode We’re generating heat (that we may have to get rid of!!!) Fuel converted to protons (go through membranes) and electrons (go through external circuit) at anode Possibly also produce gas (e.g. DMFC)

19. What Does a PEM FC Look Like? Anode FlowfieldCathode Flowfield Anode BackingCathode Backing CCM

20. Fuel Cell Components CCM (Catalyst Coated Membrane): Proton-conducting membrane plus 2 electrodes GDL (Gas Diffusion Layer): Carbon cloth or paper with carbon particle filler and Teflon Bipolar plate: Graphite, carbon composite or metal with machined or stamped ‘flow field’ Gaskets and seals: seals around edge of structure

21. Fuel Cell Power A single fuel cell doesn’t produce enough power for most applications; single cells are “stacked” together to meet power needs. The voltage of the application will determine the number of cells in a stack; the size of the plates will affect the overall power output Fuel cells stacks are part of systems with other parts to deliver gases, manage electrical output etc. Fuel cell systems can operate in the W - MW range.

22. A Portable PEM FC Stack LANL

23. Major System Components Water Air Fuel/air module Power module Electronics module Blower Water treatment Steam generator Heat exchanger Fuel cell stacks Fuel processor Power inverter Controls DC power AC power Fuel Air Steam Warm exhaust Heat to customer (optional) Exhaust

24. DMFC Stacks & Systems Condenser and Fan DMFC-BB Support Module Electronics DMFC-BB Mother Board Electronics Flex Circuit Sensor Board LANL MeOH Sensor Fluidics Module: Feedstock Res H 2 O Res Filters Heater Mixing Tube, etc. Aluminum Brassboard CO 2 Exhaust Port Air Exhaust Port Air, Feedstock, H 2 O and MeOH Pumps Under Stack LANL DMFC STACK BRASS-BOARD SYSTEM PACKAGED SYSTEM

25. PREFERENTIAL OXIDIZER AUTOTHERMAL REFOR MER 10% CO 2,000 ppm CO WATER GAS SHIFT REACTOR Gasoline Ethanol Methanol Nat. Gas <100 ppm CO O2O2 H2OH2O H2H2 FUEL CELL STACK Gasoline to Electricity for Autos The DOE/OAAT-PNGV Program

26. Gaskets and Seals Critical component! Adhesive is good if available Frames may be combined with gaskets Very important that this component be gas tight and not leach anything… May require high temperature stability Should be a good electrical insulator

27. Small ‘Battery’ - Fuel Cell Fuel Cells for Personal Electronics (“Micro” FCs) Substantial interest in < 2 W systems. Higher current densities, but higher A/V ratios. Maximizing active area is key. –Different designs than the larger stacks. 1 W Air-Breather

28. Efficient Fuel Cell Systems: 1.5 kW Adiabatic Stack

29. Where Can You Buy a Fuel Cell Today? Bad news: Not too many places to buy a fuel cell….. The Good News: Lots of Opportunity!

30. What Can You Operate with a Fuel Cell? Small-scale systems: Portable Power

31. What Can You Operate with a Fuel Cell? Large-scale systems: Stationary Power and Transportation