Hydrogen Fuel Cells. Basic electrochem Galvantic cell 2H 2 + O 2 → 2H 2 O Anode (oxidation) H 2...

Hydrogen Fuel Cells

Basic electrochem

• Galvantic cell

• 2H2 + O2 → 2H2O

• Anode (oxidation)

H2 → 2H+ + 2e-

• Cathode (reduction)

O2 + 4e- → 2O2-

Typical electrochemical cell/batterypotentiometer

Salt bridge

Anodeoxidation

Cathodereduction

Flow of electrons

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+ + 2e-

1/2O2 + 2e- + 2H+ → H2O

Direct Methanol Fuel Cells

CH3OH + H2O → 6H+ + CO2 + 6e-

3/2O2 + 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 + 2e-

1/2O2 + 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.