Intro to PEM Fuel Cells

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Intro to PEM Fuel Cells

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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.

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Batteries and Fuel Cells
A batterys reactants are self-contained
A fuel cells reactants are supplied externally
-

H2 in
Air in

-
H2 out
Air out
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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

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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 cells 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.

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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

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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)

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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)

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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, doesnt
  require precious metal catalysts, solid, rugged,
  very high temperature, expensive materials.
• MCFC can be used with many fuels, efficient,
  doesnt require precious metal catalysts, high
  temperature, very corrosive electrolyte.
• AFC most efficient medium for oxygen reaction -
  high performance, doesnt 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.

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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.

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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

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PEM FC - The Whole Picture of a Single Cell
EERE
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How Does a PEM FC Work?

• Anode 2H2(g) -----gt 4H(aq) 4e-
• Cathode O2(g) 4H(aq) 4e- ----gt 2H2O(l)

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Direct Methanol PEM FC

• Anode CH3OH(aq) H2O(l) -----gt CO2(g)
  6H(aq) 6e-
• Cathode 3/2O2(g) 6H(aq) 6e- ----gt 3H2O(l)

Cathode (Positive)
Anode (Negative)
PEM
Methanol reacts with the Pt/Ru catalyst on the
PEM to form protons and release electrons
CH3OH
O2 H e-
The protons combine with O2 and electrons to
form water
H e-
H2O
The protons travel across the PEM
This reaction is catalyzed by Pt
H
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Current Flow in a Fuel Cell
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So whats happening during operation?

• Were 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)

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So whats happening during operation? II

• Were generating current and product water (that
  we may have to get rid of!!!) at cathode
• Were 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)

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What Does a PEM FC Look Like?

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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

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Fuel Cell Power

• A single fuel cell doesnt 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.

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A Portable PEM FC Stack

LANL
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Major System Components
Heat to customer (optional)
Exhaust
Heat exchanger Fuel cell stacks Fuel processor
Blower Water treatment Steam generator
Warm exhaust
Power inverter Controls
DC power
AC power
Air
Air
Water
Steam
Fuel
Fuel
Power module
Electronics module
Fuel/air module
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DMFC Stacks Systems
CO2 Exhaust Port
Fluidics Module Feedstock Res H2O Res
Filters Heater Mixing Tube, etc.
Air Exhaust Port
LANL DMFC STACK
LANL MeOH Sensor
Air, Feedstock, H2O and MeOH Pumps Under Stack
Flex Circuit Sensor Board
DMFC-BB Mother Board Electronics
Condenser and Fan
DMFC-BB Support Module Electronics
Aluminum Brassboard
BRASS-BOARD SYSTEM
PACKAGED SYSTEM
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Gasoline to Electricity for Autos The
DOE/OAAT-PNGV Program
H2O
lt100 ppm CO
2,000 ppm CO
10 CO
H2
O2
PREFERENTIAL OXIDIZER
FUEL CELL STACK
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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

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Small Battery - Fuel Cell
1 W Air-Breather

• Fuel Cells for Personal Electronics (Micro FCs)
• Substantial interest in lt 2 W systems.
• Higher current densities, but higher A/V ratios.
• Maximizing active area is key.
• Different designs than the larger stacks.

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Efficient Fuel Cell Systems1.5 kW Adiabatic
Stack
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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!

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

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What Can You Operate with a Fuel Cell?

• Large-scale systems Stationary Power and
  Transportation