Transcript Hydrogen Fuel Cells - University of Massachusetts Boston

Hydrogen Fuel Cells
Basic electrochem
• Galvantic cell
• 2H2 + O2 → 2H2O
• Anode (oxidation)
H2 → 2H+ + 2e• Cathode (reduction)
O2 + 4e- → 2O2-
Typical electrochemical cell/battery
Flow of electrons
Anode
oxidation
potentiometer
Cathode
reduction
Salt bridge
Junction potentials
Fuel cell
• The fuel is the anode
• The oxidant is the cathode
• The fuel and oxidant continuously flow
through the cell
• An electrolyte separates the fuel and
oxidant channels
• Solid or liquid electrolyte that conducts
protons
• Need catalyst at low temp
Hydrogen Fuel Cell
Potential Advantages
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Clean; only product is H2O and heat.
More efficient than heat engine.
Higher part load efficiency
Excellent response time
Co-generation
No tuning required
No recharging required
Disadvantages
• H2 is difficult/expensive to produce, store
and transport.
• Fuels cells require pure fuel.
• Platinum catalysts are expensive and rare
• Proton exchange membranes must be
kept moist
• Hydrogen fuel cell stacks are heavy
uses
• Stationary power plants; small, power to
neighborhoods, hospitals, ect.
• Submarines
• Buses
• Cars
Proton Exchange Membrane
(PEM) Fuel Cells
• Electrolyte is a thin solid polymer film (acidified
Teflon)
• Conducts H+ from the anode to the cathode
• Low temp (160-195 C)
• 15-30 psi
• 1.1 V
H2 → 2H+ + 2e1/2O2 + 2e- + 2H+ → H2O
Direct Methanol Fuel Cells
CH3OH + H2O → 6H+ + CO2 + 6e3/2O2 + 6e- + 6H+ → 3H2O
• Still pretty new technology, uses a different
catalyst at a higher temp.
• Not as efficient
Several Advantages
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Tolerant to CO2 in oxidant
Low temp
Dry electrolyte
Non-corrosive electrolyte
High current, voltage and power density
Tolerant to differential pressures
Disadvantages
• Anode and cathode needs platinum
catalysts
• Tolerates only about 50 ppm of CO and a
few ppm of Sulfur compounds in fuel
• Gas humidification required
• Expensive membrane
Alkaline Fuel Cells
• Molten KOH as electrolyte
• Conducts OH- from cathode to anode
• Circulating electrolye, removes heat and
water/or a stationary paste needs
H2 + 2OH- → 2H2O + 2e1/2O2 + H2O + 2e- → 2OHRemoval of water is critical
Advantages
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Low temp
Fast start up
High efficiency
Little or no platinum catalyst needed
Minimal corrosion
Disadvantages
• Extremely intolerant of CO2 (350 ppm) and
somewhat intolerant of CO
• Liquid electrolyte handling
• Complex water management
• Short lifetime
Other barriers to the Realization of
a Hydrogen Economy
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Platinum catalyst
Humidification
Needs pure fuel
Fuel cell stacks are heavy
Hydrogen production is expensive
Hydrogen transportation is expensive
Hydrogen storage is vehicles is a bit
impratical
Hydrogen Production
• Hydrogen is an energy carrier, not an energy
source
• Fossil fuels especially, coal and natural gas
– methane reforming and partial oxidation (burning)
– High temperatures and steam/more efficient than
combustion
• Renewable electrolysis (wind, solar, geothermal,
hydroelectric)
• Nuclear
• Biomass
• Photo-electrochemical using algae
– Consume water and solar energy and produce H2
Hydrogen Transport
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Pipelines
High pressure tubes
Cryogenic tankers
Chemical carriers
Research
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Less costly materials for pipelines
Less expensive compression technology
Less costly liquefaction processes
More cost effective bulk storage strategies
Hydrogen Storage in vehicle
• Hydrogen has a low energy to volume
ratio.
• The hydrogen fuel tank takes up a lot of
space.
• Can we store hydrogen in a different form
(metal hydride)?
• This is why methanol as a fuel is an
attractive option.