Intro to PEM Fuel Cells. What is a Fuel Cell? A fuel cell is an energy conversion device that reacts...
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Transcript of Intro to PEM Fuel Cells. What is a Fuel Cell? A fuel cell is an energy conversion device that reacts...
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.
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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. H2, O2) are supplied externally.
Batteries and Fuel Cells
H2 in
A battery’s reactantsare self-contained
A fuel cell’s reactants aresupplied externally
H2 out
Air in
Air out
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Fuel Cell ApplicationsEach 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
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.
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
Many Types of Fuel CellsThe 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)
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)
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.
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.
About HydrogenHydrogen 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
How Does a PEM FC Work?Anode: 2H2(g) -----> 4H+
(aq) + 4e-
Cathode: O2(g) + 4H+(aq)
+ 4e- ----> 2H2O(l)
PEMCathode (Positive)Anode (Negative)
H2
H+ + e-
O2 + H+ + e-
H2O
Hydrogen reacts with the Pt catalyst on the PEMto form protons andrelease electrons
The protons travel across the PEM H+
The protons combine with O2 and electrons to form water
This reaction is also catalyzed by Pt
Direct Methanol PEM FCAnode: CH3OH(aq) + H2O(l) -----> CO2(g) + 6H+
(aq) + 6e-
Cathode: 3/2O2(g) + 6H+(aq)
+ 6e- ----> 3H2O(l)
PEMCathode (Positive)Anode (Negative)
CH3OH
H+ + e-
O2 + H+ + e-
H2O
Methanol reacts withthe Pt/Ru catalyst onthe PEM to formprotons and release electrons
The protons travel across the PEM H+
The protons combine with O2 and electrons to form water
This reaction iscatalyzed by Pt
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)
So what’s happening during operation? IIWe’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)
Fuel Cell ComponentsCCM (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
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.
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 Fuel
Air
Steam
Warm exhaust
Heat tocustomer(optional) Exhaust
DMFC Stacks & Systems
Condenserand Fan DMFC-BB
Support ModuleElectronics
DMFC-BBMother BoardElectronics
Flex Circuit Sensor Board
LANL MeOH Sensor
Fluidics Module: Feedstock Res
H2O Res Filters Heater
Mixing Tube, etc.
AluminumBrassboard
CO2 ExhaustPort
Air ExhaustPort
Air, Feedstock,H2O and MeOHPumps Under
Stack
LANL DMFC STACK
BRASS-BOARD SYSTEM
PACKAGED SYSTEM
PREFERENTIAL OXIDIZER
AUTOTHERMAL REFOR MER
10% CO
2,000 ppmCO
WATER GAS SHIFT REACTOR
GasolineEthanol
MethanolNat. Gas
<100 ppm CO
O2
H2O
H2FUEL CELL
STACK
Gasoline to Electricity for AutosThe DOE/OAAT-PNGV Program
Gaskets and SealsCritical component!Adhesive is good if availableFrames may be combined with gasketsVery important that this component be gas
tight and not leach anything…May require high temperature stabilityShould be a good electrical insulator
Small ‘Battery’ - Fuel Cell
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.
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
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!