Hydrogen Fuel Cells(1)

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					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
• • • • • • • 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
• • • • • • 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- → 2OH-

Removal of water is critical

Advantages
• • • • • 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
• • • • • • • 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
• • • • Pipelines High pressure tubes Cryogenic tankers Chemical carriers

Research
• • • • 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.


				
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