Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Sources of Energy
A favorite form of energy is electricity
Where does electricity come from?
•
Even though electricity is a very useful form of energy, there are very
few direct sources of electrical energy on earth. (One example is a
lightning storm.)
•
Electricity is really a secondary energy source, which we get by
converting another type of energy into it.
•
The original source of energy can be
•
Nuclear
•
Wind
•
Sun
•
Hydrodynamic
•
Chemical energy
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Current Energy System
What’s wrong with our current energy system?
•
The current world energy consumption is 13 TW, or 13 trillion watts.
•
This number is HUGE. 3000 Niagara Falls worth of energy.
•
Most (85%) of that energy is converted from chemical energy.
•
Most of the chemical energy is coming from the burning of fossil fuels: oil,
gas, and coal.
•
Burning fossil fuels generates carbon dioxide, CO2.
•
Let’s examine a gallon of gasoline.
•
Each gallon of gasoline generates over 1000 gallons of CO2 gas at
atmospheric pressure. That’s more than 17 pounds of CO2.
•
So every 100 gallons burned creates nearly TON (2000 lbs) of CO2.
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Renewable sources currently make up
a small percentage of US energy
3
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Carbon Dioxide Emissions
Why is CO2 a problem?
•
All of the fossil fuels that we are burning lead directly to carbon dioxide.
•
Most of this carbon dioxide is being poured directly into the atmosphere,
where it adds to the existing CO2 levels.
•
The CO2 concentrations in the earth’s atmosphere have already risen by
over 25% in the past century.
•
CO2 is a greenhouse gas. Increasing its concentration in the earth’s
atmosphere leads to a warming of the earth.
•
The effect is already being observed, in higher air temperatures, receding
glaciers, increase in wildfires, rising sea levels…
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Sustainable and Renewable Energy
Solutions
• To ward off significant climate change, changes will need to be made in how we
get our energy.
• Interest in sustainable, renewable, and clean sources of energy.
Sustainable energy: one that is not substantially depleted by continued use,
does not cause significant pollutant emissions or other
environmental problems, doesn’t cause substantial health
hazards or social injustices (from Boyle)
Renewable energy: energy obtained from the continuous or repetitive currents
of energy recurring in the natural environment (Twidell and
Weir, 1986)
energy flows which are replenished at the same rate as
they are “used” (Sorensen, 2000)
energy generated from natural resources (Wikipedia)
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Sustainable Energy
Some ideas that are being pursued include:
•
Wind
•
Solar cells
•
Solar thermal
•
Biofuels
•
Energy from the Ocean in the form of waves or tides
•
Geothermal energy (e.g. Iceland)
•
Clean fuels
Some forms of fuel don’t produce as much CO2. The “gold standard”
in a clean fuel is hydrogen (H2). When hydrogen is burned, it
produces no CO2 at all, only water. One of the ways of extracting
this chemical energy from hydrogen is to react it with oxygen in a
fuel cell.
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Transportable Energy
Transportable energy
•
In addition to solutions like solar cells, or wind turbines, we need a way to
store energy, and to move it around with us.
•
We need portability for many applications (e.g. driving a car)
•
We also need energy on demand (so we can have it even in the dark).
•
That’s why fuels are so desirable—they are a transportable, storable form
of energy.
•
One way to store energy is in the form of hydrogen. Remember that
hydrogen is considered the cleanest of the “clean” fuels because when it
reacts with oxygen, the only product formed is water.
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel Cells
Getting electrical energy from chemical energy
•
We could just put hydrogen and oxygen together in a reactor,
effectively burning the hydrogen, to get energy out.
hydrogen flame
•
A more efficient way of doing this is to use a fuel cell.
•
A fuel cell directly converts chemical energy (that from reacting H2 with
O2) into electrical energy.
•
It does this by only letting the oxygen contact the hydrogen in a very
controlled fashion.
•
A fuel cell is designed like a sandwich
•
Let’s delve further into fuel cells
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel Cells
•
Fuel cells are devices that convert chemical energy into electrical energy
•
Efficiencies are potentially higher than if using the fuels in an engine
•
Current efficiencies are 40-60%
•
Fuel cells are similar to batteries, but with replenishable materials (fuel)
•
Under consideration for both large scale power generation and small scale
portable applications (e.g. laptop and cell phone power)
Combustion
Engine
similarities
differences
Fuel cell
Converts fuel into
electrical energy
Unlike a combustion engine, a fuel
cell directly converts chemical
energy into electrical, without going
via heat and mechanical energy
Battery
Stores energy through an
electrochemical system
Unlike a battery, a fuel cell
is not consumed when it
produces electricity
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Pros and Cons of Fuel Cells
Advantages:
• Clean and green
• Higher potential efficiencies
• No moving parts
• Lower particulate emissions
• Silent, mechanically robust
• Scaleable, transportable
Disadvantages
• Expensive
• Fuel availability
• Power/energy density issues
(for portable applications)
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel Cell Basics: What is a Fuel Cell?
• Electrochemical energy conversion device
– directly converts chemical energy to electrical energy
– fuel can be H2 or hydrocarbon (e.g. methanol)
O2
H2O
Fuel cell
H2
Electricity
H2+½ O2  H2O
• The “combustion” reaction is split into two electrochemical
half reactions
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel Cells
•
Some fuel cell reactions:
H2+½ O2  H2O
Hydrogen
CH3OH + (3/2)O2 → CO2 + 2H2O
Methanol
•
These are basically combustion reactions.
•
As with batteries, the idea is to harness the electrons from the
“redox” reaction to produce electrical energy.
•
Fuel cells contain (1) a thin membrane that
conducts ions, (2) an anode and (3) a cathode
•
Both the anode and the cathode need to be
catalytically active or contain added catalyst
in order to break up the H2 (or hydrocarbon)
and O2.
Figure from
F. Prinz
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Oxidation and Reduction Reactions
• We are interested in a class of reactions that involve electron transfer at the
atomic scale. These are called “Redox” reactions
• The overall chemical reaction is broken up into two electrochemical half
reactions
Oxidation: Electrons are lost from a species
examples
H 2  2 H + + 2 eZn  Zn2+ + 2 e-
Reduction: Electrons are gained by a species
examples
½ O2 + 2 H + + 2 e-  H 2 O
2 e- + Cu2+  Cu
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Oxidation and Reduction Reactions
• In an electrochemical device (such as a fuel cell or battery), the electrochemical
half reactions take place at electrodes.
• The electrode is conductive, i.e. it needs to conduct charge.
Anode: the electrode where oxidation takes place
Cathode: the electrode where reduction takes place
• Whether the anode and cathode are positively or negatively charged depends
on the type of device.
• For a galvanic cell (produces electricity), the anode is negative
• For an electrolytic cell (consumes electricity), the anode is positive
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Schematic of a Fuel Cell
3
Fuel in
Air in
1
1
Flow structure
Porous
electrode
2
3
4
Anode
2
4
Electrolyte
Cathode
The steps in the fuel cell process are:
1. Deliver reactant (transport)
2. Electrochemical reaction at both anode and cathode (requires catalyst too)
3. Movement of ions through the electrolyte; movement of electrons through the
external circuit
4. Remove product (transport)
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel CellsA fuel cell is just a battery with
replenishable electrode materials
Anode
E0 = 0 (SHE)
Cathode
E0 = 1.229 V
Cell
E0 = 1.229 V
Compare with a battery (Daniel Cell)
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Membranes
Properties desired for membrane electrolyte:
• High ionic conductivity (minimizes resistive losses)
• Low electronic conductivity (minimizes current losses)
• Chemical stability in both oxidizing (anode) and reducing (cathode)
environments)
• Low fuel crossover
• Mechanical strength and manufacturability
Categories: liquid, solid, polymeric
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
State-of-Art of Fuel Cells
PEMFC
PAFC
AFC
MCFC
SOFC
Name
Proton exchange
membrane
Polymer electrolyte
membrane
Phosphoric
acid fuel cell
Alkaline fuel
cell
Molten
carbonate
fuel cell
Solid oxide
fuel cell
Electrolyte
Polymer
Membrane
Molten
Carbonate
Ceramic
Charge
Carrier
H+
H+
OH-
CO32-
O2-
Operating
Temperature
80 0C
200 0C
60-220 0C
650 0C
600-1000 0C
Catalyst
Platinum
Platinum
Platinum
Nickel
Perovskites
(Ceramic)
Cell
Components
Carbonbased
Carbonbased
Carbonbased
Stainlessbased
Ceramicbased
Fuel
Compatibility
H2,
Methanol
H2
H2
H2, CH4
H2, CH4, CO
Liquid H3PO4 Liquid KOH
(Immobilized) (Immobilized)
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
The Proton Exchange Membrane (PEMFC)
•
The membrane must conduct protons
(hydrogen ions, H+) but not electrons (otherwise
would short circuit)
•
Most common membrane for PEM fuel cells is
Nafion (Dupont), a polytetrafluoroethylene
(Teflon) with sulfonic acid (SO3-H+) functional
groups
•
Fixed charge sites (SO3-) act as temporary
centers where the moving ions can be accepted
or released. H+ ions move by detaching from
from sulfonic acid sites and forming hydronium
complexes (H3O+) with water
•
Nafion relies on liquid water humidification of
the membrane to transport protons
•
Therefore, water management (humidification)
systems are necessary.
•
Temperatures must be kept below 80-90oC so
won’t dry out.
Nafion
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Phosphoric acid fuel cell (PAFC)
• First commercial fuel cell type
• Liquid H3PO4 electrolyte in SiC matrix
• Operated at 150-200oC; expelled water used as steam for space and water heating
• Used for stationary applications with a combined heat and power efficiency of
about 80%; electrical power efficiency alone is ~40%
• PAFC’s dominate the on-site stationary fuel cell market; 200 kW and 300 kW plants
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
The Solid Oxide Fuel Cell (SOFC)
Advantages
H2
Anode
Porous
nickel/YSZ
cermet
e-
H2+O2-  H2O + 2e-
Solid ceramic
electrolyte
O2-
•
Solid electrolyte
•
Doesn’t need humidification
•
Fuel flexibility (H2 and simple
hydrocarbon)
•
Non-precious metal catalyst (at high
T, perovskites are used as catalyst )
•
Relatively high power density
YSZ
Cathode
Porous mixedconducting
oxide
½ O2 + 2e-  O2-
O2
YSZ (yttria-stabilized zirconia) cubic
fluorite structure
http://www.doitpoms.ac.uk/tlplib/fuel-cells/sofc_electrolyte.php
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
• Y3+ substitutes for Zr4+ ions
• Creates oxygen vacancies!
• For every 2 Y3+ ions substituting
for Zr4+ ions there is a O2- vacancy
created (charge neutrality)
Oxygen and vacancy exchange
• Membrane conductivity is proportional to the concentration of O2- vacancies
• But too much Y doping leads to vacancy-vacancy interactions which decreases mobility
• Maximum conductivity occurs at about 8% doping
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Current Density vs Voltage: Polarization Curve
The losses in voltage from the ideal output voltage are referred to as ‘‘polarizations’’
Activation
Losses
Ohmic
Losses
Concentration
Losses
2
Power Density(mW/cm )
Fuel Cell Voltage(volt)
Ideal output voltage
2
Current Density(mA/cm )
Energy losses associated
with the electrode
reactions
(Surface reaction kinetics)
Energy losses from electronic
impedances (electrodes,
contacts, and current collectors)
and ionic impedances (from
electrolyte)
Energy losses associated
with mass transport
limitations
(reactants and/or
products
Lecture on Sustainable Energy; Fuel Cells
Stacey Bent, Stanford University
Fuel Cells
Honda FCX Clarity zero-emissions fuel cell vehicle (shown with Jamie Lee Curtis)
Vehicle uses a PEM fuel cell stack
Will these compete with electric cars?