Download Document

Information of Fuel Cells
Fuel Cells Basic
A fuel cell is a device that generates electricity without combustion. In a fuel cell,
chemical energy is converted directly into electrical energy.
Fuel cell is similar to battery. There are two electrodes in a fuel cell. These electrodes
are separated by an electrolyte. Fuel is supplied to the anode and oxidant is supplied
to the cathode. As long as fuel and oxidant are supplied to the fuel cell, the fuel cell
will continue to operate.
The most common fuel for fuel cell is hydrogen. The oxidant is usually oxygen in air.
The common types of fuel cell:
Type
Alkaline (AFC)
Operating Temperature
Applications
50C -250C
U.S. Space Program
Polymer Electrolyte (PEFC) 50C -100C
Battery replacement,
Transportation
Phosphoric Acid (PAFC)
200C
Airports, School, Hospitals
Direct Alcohol (DAFC)
50C -100C
Battery replacement
Molten Carbonate (MCFC) 600C
Cogeneration
Solid Oxide (SOFC)
Cogeneration
500C -1000C
Different types of fuel cells have different reactions at the electrodes.
Alkaline Fuel Cell (AFC)
At the anode:
2 H2(g) + 4OH-(aq)  4H2O(l) + 4eAt the cathode:
O2(g) + 2H2O(l) + 4e-  4OH-(aq)
Polymer Electrolyte (PEFC), also called Proton Exchange Membrane Fuel Cell
(PEMFC)
At the anode:
2H2(g)  4H+ + 4eAt the cathode:
4H+ + 4e- + O2  2H2O
Typical Fuel Cell Operation
Basic terms related to Fuel Cell
Anode – the negative electrode
Cathode – the positive electrode
Electrochemical – convert chemical energy to electrical energy
Electricity –flow of electrons in a circuit
Electrolysis – a process of chemical changes by passing an electric current through a
solution resulting in a transfer of electrons to the electrode
Electrolyte – a chemical means in the transfer of ions from one medium to another
Fuel cell – an electrochemical/self generating power source
Hydrogen – chemical fuel of a fuel cell flowing through the anode
Membrane – substance allowing hydrogen and oxygen to pass through
Oxygen – chemical fuel flowing through the cathode
PEM – proton exchange membrane
Stacking – method of adding fuel cells to increase voltage output
Voltage – a measure of electrical potential of an electrical circuit or device
ZEV – zero emission vehicle
Comparison between Battery and Fuel Cell
Battery
Fuel Cell
Energy derived from chemical energy.
Energy derived from chemical energy.
Consist of two electrodes separated by an
electrolyte.
Consist of two electrodes separated by an
electrolyte.
At least one of the electrodes is solid. It is the No electrode is consumed by the reaction
major reactant in the reaction to generate
electricity. The electrode is consumed during
the reaction.
All reactants are stored inside the battery.
Reactants are supplied continuously from
outside sources.
All reaction products are stored inside the
battery.
Reaction products are removed from the fuel
cell continuously.
Contain large amount of heavy metal. Spent
batteries cause pollution.
Precious metal is used as catalyst. Pollution
should not be a problem.
Low power density.
High power density.
Applications of fuel cells
1.
Space program
2.
Power Generation
3.
Transportation
4.
Landfill and Waste Water Treatment
5.
Portable Power
Problems with the Fuel Cell Technologies
In general, the development cycle of products from scientific discovery to
commercialization usually takes about one to two decades. For example, the first
ENIAC computer appeared in 1950, and the mass-market adoption of computer began
in 1960 and 1970.
It took the Otto internal combustion engine three years from invention to mass
adoption in automobile. But for fuel cells, it has been over one hundred and sixty
years since William Grove developed the basic theory and we still do not see any
prospect of commercialization. Major factors that hinder the commercialization of
fuel cells are:
1.
The basic operating theory of fuel cells may be simple, but the related
technologies, especially the technologies related to material processing had not
been developed; therefore no fuel cell has been in use before 1960.
(a) Fuel cells require catalyst to operate. The common catalysts are platinum
and palladium, which are very expensive.
(b)
(c)
To reduce the problem of catalyst poisoning, the fuel and oxidant must be
very pure. Techniques in producing pure hydrogen and pure oxygen are
difficult.
To withstand the highly corrosive environment of alkaline solution at high
temperature, specialty materials must be used in the construction of fuel
cells. Progress in material technology is slow and proper material cannot
be found.
2.
Hydrogen is highly flammable and explosive; hydrogen must be handled with
care. Distribution of hydrogen fuel is a major obstacle in the commercialization
of fuel cells. To build the hydrogen distribution system is highly expensive.