Download Fuel Cells – Power Point

Sustainable Energy Workshop for Science and
Technology Teachers (SEWFSTT)
Module 2
Fuel Cells and the Hydrogen
Economy
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Fuel Cells
Discussion:
Prior Exposure with Fuel Cells
(outside of the automotive industry)
Fuel Cells come in all shapes
and sizes
8
6
4
7
3
5
7
2
Basic Definition of a Fuel Cell
Electrochemical Energy Conversion Device
Different Kinds of Fuel Cells
Energy Conversion: PEM Fuel Cell
2H 2  O2 
 2H 2O  energy
Hydrogen and oxygen are combined in a non-combustion process
Electricity, heat and water are produced
Reduction-Oxidation Rxn
(redox)
Anode Half-Reaction
Cathode Half-Reaction
Reductant 
 Product  e 
Oxidant  e  
 Product

2H 2 
 4H  4e

O2  4H   4e 
 2H 2O
Electric Potential Developed
• Redox potential is a measure of a substance’s
electronegativity (affinity for electrons)
• “Downhill” in Energy Diagram -- Free energy
• Don’t have oxidation reaction without reduction reaction
present at same time (matched set)
• Nernst Equation
• Calculation of electric potential in “non-ideal” circumstances
Oxidation
Half-Reaction
Reduction
Half-Reaction
2H 2 
 4H   4e 


O2  4H  4e 
 2H 2O
E 0  0 VSHE
E 0  1.229 VSHE
Proton Exchange Membrane Fuel Cell
Oxidation reaction facilitated by a catalyst
- typically Pt ($$$)
(Polymer Electrolyte Membrane)
2H 2 
 4H   4e 
Between the reduction and oxidation stages, the
electrons are routed through a circuit
Hydrogen ions (protons) permeate through the
electrolyte membrane
Reduction reaction
O2  4H   4e 
 2H 2O
1.23 V
Power Produced – Watts/m2
• Activation Loss
– potential difference above
the equilibrium value
required to produce a current
(depends on activation
energy of the reaction)
– energy is lost as heat
• Ohmic Loss
– voltage drop due to
resistance of the cell
components and
interconnects
• Mass Transport Loss
– depletion of reactants at
catalyst sites under high loads
PEM (Polymer Electrolyte Membrane)
• Polymers such as polyphenylenes,
Nafion are used
• Water is a crucial participant in the
process
•absorption of water increases the
proton conductivity
•membrane is confined – not free
to swell – pushes electrodes
PEM (Polymer Electrolyte Membrane)
• Thickness of the membrane and catalyst in the
PEM can vary …
• Example: catalyst layers containing about 0.15
milligrams (mg) Pt/cm2
• thickness of the catalyst layer is close to
10 micrometers
•yields a MEA with a total thickness of
about 200μm (or 0.2 mm or 20 sheets of
paper)
•generates more than half an ampere of
current per cm2 at a voltage of 0.7 volts
Platinum needs to be placed to maximize surface area
Needs to be encased in engineered components
Design Goals: Limited Overview
• Deliver Hydrogen
• Deliver Oxygen
• Chemical reaction – what can influence rate of reaction
• Water Management
• Maintain hydration levels
• Remove water by-product
• Efficient path for electrons to ‘migrate’ to
electrodes
• Thermal management
Parts of a Fuel Cell
Bipolar Plates
• Serpentine channels for
hydrogen and oxygen to flow
through device
• Acts as a current collector –
electrons enter and exit cell
through the plate
Anode
• Conducts electrons away from
catalyst to external circuit
• Channels to supply H2 evenly
to the surface of the catalyst
Cathode
• Channels to supply O2 evenly
to the surface of the catalyst
• Conducts electrons back to
catalyst for recombining
Parts of a PEM Fuel Cell
Membrane Electrode Assembly
• Anode
• Cathode
• PEM (Polymer Electrolyte Membrane)
• conducts only positively charged ions
• blocks electrons and other substances
• Catalyst
• thin coat of platinum powder applied to
carbon paper or cloth
•maximizes surface area
• Backing Layers
• porous carbon cloth conducts electrons
away from catalyst to external circuit
• allows right amount of water vapor to
enter/exit
• too much blocks the pores
• membrane needs to be humidified
Schematic
of Fuel Cell
Operation
1
H 2  O2 
 H 2O  energy
2
1.2 V = theoretical maximum voltage
generated by this reaction
Typical output = 0.7V – 0.9V ….. (1 W per cm2)
Schematic of Fuel
Cell Operation
• Anode
Hydrogen gas is
circulated through
‘serpentine’ channels
Hydrogen from channels
passes through porous
medium
(gas diffusion backing)
Electron is stripped from Hydrogen
as it makes contact with Pt catalyst
which is embedded in a carbon
nanoparticle
Electron conducted away
through circuit
Hydrogen nucleus (proton) passes
through PEM membrane to cathode
Activity: PEM Fuel Cell Car (Pairs)
• Outline:
– Produce hydrogen and oxygen
via electrolysis
– Use stored H2 and O2 to generate
electricity and drive motor
Educational Objectives for this Activity:
• Recognize H2 and O2 as portable fuels: same role as gasoline in an IC engine
• Recognize that a separate process is required to produce hydrogen
• Observation of the relationship between the volumes of displaced water in the
hydrogen and oxygen tanks (and relationship to redox equations)
• Recognize that the hydrogen and oxygen produced came from initial injection
of water
• Discussion of extension activities
Parts Identification
• Battery components
– Battery pack
– 2 AA Batteries
– Connection Cable
Add batteries to
battery pack
• Chassis
• Fuel Cell
– Identify Hydrogen and Oxygen side
Incredibly important!
• H2 and O2 Storage Tanks
Two cylinders + 2 cup-like caps w/ long
hoses attached
… but wait … there’s more …
• Hydration Components
–
–
–
–
Parts Identification
Syringe
Two short, narrow tubes with black and red caps
Short length of wide tubing
90mL of distilled water + cup
Very important - needs to
be distilled water (NOT
Purified water)
------ Why?
If you do not ALSO have a short length of wide tubing, you’re OK – just
remove the black plug and use the narrow tube
Hydrate Fuel Cell
• Fill the syringe with distilled water and (gently) inject
a small amount in to the LOWER nozzle on the
HYDROGEN side
You will see the water fill in the fuel cell – you can
go all the way until the water pours out the top
nozzle.
GENTLY
• Remove the
syringe and
insert the
tube with the
black cap in
the LOWER
nozzle
Hydrate Fuel Cell
• Fill the syringe with distilled water and (gently) inject
a small amount in to the LOWER nozzle on the
OXYGEN side
You will see the water fill in the fuel cell – you can
go all the way until the water pours out the top
nozzle.
GENTLY
O2
• Remove the syringe
and insert the tube
(red cap) in the LOWER
nozzle
Prepare to Generate Hydrogen and Oxygen
• Insert the cup-like caps in to
the Hydrogen and Oxygen
tanks
0 mL
• Align the notch in the cap
with the gap in the tank
– we want to allow trapped
air to escape when we fill
the tanks with water
• Fill the tanks to the zero (0) mL mark with distilled water
– Suggestion: Use the syringe (each will take about 30mL)
Prepare to Generate Hydrogen and Oxygen
Connect the tank hoses to the upper
nozzles on their respective sides
(i.e. Hydrogen tank to Hydrogen nozzle)
Don’t forget to connect the
Oxygen side too!
Prepare to Generate Hydrogen and Oxygen
Make sure battery pack is turned off
Connect battery pack to connector
Connect banana plugs to fuel cell
(black to black, red to red)
Don’t turn it on yet …..
Double -Check
Double check connections
(Black-to-Black, Red-to-Red)
Turn on the battery pack and observe the production of H2 and O2
Disconnect Battery Pack
You now have a full ‘gas tank’ and a flow-through battery
Need a DC motor and wheels to drive a car
Transfer Assembly to Car and Connect Motor
• As a unit,
transfer the gas
tanks and the
fuel cell to the
car chassis
• Connect the
banana plugs
from the motor
to the fuel cell
(black-to-black)
to begin
operation
Be careful moving the tanks – a leak at this stage
means you are “out of gas”!
Reflection
• Amount of hydrogen and oxygen produced during electrolysis
• Source for all this power – the original fuel?
• Moving the gas tanks to the car – Production of H2? Where?
Activity: Construct a PEM Fuel Cell
• A small, single cell, PEM
fuel cell can easily be
constructed
• Source of hydrogen needed
• Chemical reaction
• Fuel cell production + storage
• Kits:
• Helpful hint: accordion
Activity
Comments
• Fuel cells need to be hydrated in
order to run properly, if a fuel cell
has been sitting un-used for a long
time it may need a soaking rest to
re-hydrate. Try placing it in a
plastic bag with a wet towel for a
few hours
Educational Objectives – Curricular
Connections
Discussion:
Available Animations
Energy Chain for Fuel Cells
Are there associated societal
issues associated with fuel
cells (power generation &/or
propulsion?)
Something is missing here ….
Energy Chain for Fuel Cells
?!
Multiple Instructional Levels
• Exposure/Exploration
• Process understanding
• Chemical Reaction
– Rate of reaction:
dependence on pressure,
temperature, etc.
• Load Impact on Cell
Efficiency
• High voltage vs. low
voltage applications
• Activation energy
Activity: Fuel Cell Stack
• Exploration of a pressurized
fuel cell stack
Different From a Battery?
Redox (Oxidation-Reduction Reaction)
Baghdad Battery – 250 BC
Lead-Acid Batteries
(2V per cell)
• e.g. car batteries, deep-cycle batteries
Anode : Pb( s )  SO42 ( aq) 
 PbSO4 ( s )  2e 
2
4 ( aq )
Cathode : PbO2 ( s )  SO


 4 H  2e 
 PbSO4 ( s )  2 H 2O(l )
Energy-to-weight ratio very low
Energy-to-volume ratio: low
But ….Power-to-Weight ratio:
LARGE
RECHARGABLE
Similarities and Differences
Similarities
Differences
• Chemical potential
energy converted in
to Electric potential
energy
• Passage of H2 and O2
thru vs. storage of
chemicals in battery
• Cellular structure
• Redox reactions
• Flow battery
Cellular Structure of Fuel Cells
• Batteries in series
• Fuel cells are essentially flow-through batteries
• Challenge is getting H2 and O2 uniformly to all of the cells
Stationary Power Facility: Stacks
5 kW Fuel Cell System,
Manufactured by PlugPower,
Installed at a USDOD Facility
5 PC 25TM Fuel Cells sited in Anchorage,
Alaska (International Fuel Cells, LLC)
200-kW + 900,000 BTU heat
PAFC (Phosphoric Acid Fuel Cells)
Parts of a PEM Fuel Cell System
• Propulsion System
Automotive Application
Volkswagen’s HTFC
The Hydrogen Economy
• Hydrogen as a storage medium for energy
Problem: Hydrogen does not occur naturally in nature as H2
The Hydrogen Economy
• Infrastructure
How does one go about
developing a production, delivery
and use system for an energy
storage medium that is only in its
infancy
http://www.hydrogen.energy.gov/systems_integration.html
Hydrogen Production and Delivery
Advantage: Hydrogen is storage medium – Production from a variety of sources
Currently: Steam Reforming of Natural Gas
• Biological Water Splitting
• Photoelectrochemical Water Splitting
• Reforming of Biomass and Wastes
• Solar Thermal Water Splitting
• Renewable Electrolysis
Community Adoption – Priming the Pump
Hydrogen Storage
Hydrogen is not an ‘energy dense’ fuel (need lots to go anywhere)
• Pressurized Steel and Composite Tanks
• Hydrogen can cause metals to become
brittle (not good!)
• Metal Hydride
• H2 is locked in another chemical
• Chemical reaction releases that metal
• Micropore Storage
• Buckyballs & nanoscale
methods
Metal Decorated Nanostructures
The Hydrogen Safety Movie
• It’s not what you may think
Discussion:
Synthesis of Fuel Cell
Challenges for PEM Fuel Cells
• Platinum: reduction of amt of material used = reduced cost
– Wikipedia: 2002 cost was $1,000 per kW
• Water management
– Too little --- membrane dries up
– Too much --- pores blocked, efficiency drops
• Steady Fuel Supply
– Controlling amount of incoming gas + pressure
• Poisoning of the anode by carbon monoxide
• Temperature control
This technology is coming out of its infancy …..