Hydrogen Fuel Cells

Basic electrochem Galvantic cell 2H 2 + O 2 → 2H 2 O Anode (oxidation) H 2 → 2H + + 2e- Cathode (reduction) O 2 + 4e- → 2O 2-

Typical electrochemical cell/battery potentiometer Salt bridge Anode oxidation Cathode reduction 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 H 2 O and heat. More efficient than heat engine. Higher part load efficiency Excellent response time Co-generation No tuning required No recharging required

Disadvantages H 2 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 ( C) psi 1.1 V H 2 → 2H+ + 2e- 1/2O 2 + 2e- + 2H + → H 2 O

Direct Methanol Fuel Cells CH 3 OH + H 2 O → 6H + + CO 2 + 6e- 3/2O 2 + 6e- + 6H+ → 3H 2 O Still pretty new technology, uses a different catalyst at a higher temp. Not as efficient

Several Advantages Tolerant to CO 2 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 H 2 + 2OH - → 2H 2 O + 2e- 1/2O 2 + H 2 O + 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 CO 2 (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 H 2

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.