Page 68 - MODERN ASPECTS OF ELECTROCHEMISTRY

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Direct Methanol Fuel Cells
efficiency of over 50%). These fuels (pure H , H 2 -CO 2 , H 2 -CO-CO 2 ) can
2
also be produced from renewable energy sources: biomass, solar, wind-
mills, and hydroelectric. Pure H 2 can also be generated using nuclear
power plants. Hydrogen is the most electroactive fuel for fuel cells
operating at low and intermediate temperatures. Methanol is the most
electroactive organic fuel, and when it is electrooxidized directly at the
fuel anode (instead of being transformed in a hydrogen-rich gas), the fuel
cell is called a direct methanolfuel cell (DMFC).
3. Natural Gas-Its Important Role as a Primary Fuel for All
Types of Fuel Cells
During the energy era and the energy crisis, energy conservation was one
of the main objectives. Thus there was considerable interest in producing
H , H 2 -CO 2 , or H 2 -CO 2 -CO from petroleum for utilization in fuel cells,
2
which are the only energy conversion devices that convert chemical
energy directly into electrical energy. Thus, unlike the case of thermal
engines, the second law of thermodynamics, which limits the efficiency
of energy conversion, does not apply. Hence, in principle the free energy
change in the fuel cell reaction can be converted to electrical energy, which
corresponds to a theoretical energy conversion efficiency of over 80%. In
view of the abundance of natural gas resources found since the 1980s, and
natural gas being a considerably cleaner fuel than petroleum or coal for
the fuel processors, the main goals of the major worldwide fuel cell
programs are to develop fuel cell power plants and portable power sources
using natural gas or natural gas-derived fuel cells. A chart of all types of
fuel cells using natural gas as a fuel is presented in Fig. 1; the applications
being considered for the different types of fuel cells are also indicated.
The most advanced type of fuel cell is the phosphoric acid fuel cell (PAFC)
system, which operates at about 473 K. However, in order to make this
fuel cell system reasonably efficient, it is necessary to steam reform the
fuel and use a shift converter to reduce the carbon monoxide levels to
about 1 to 2%. With natural gas as the fuel, the high temperature systems
(i.e., molten carbonate and solid oxide fuel cells, MCFC and SOFC,
respectively) are more attractive because of the following advantages: (1)
carbon monoxide is a reactant and not a poison, (2) noble metal electro-
catalysts are not required, and (3) the waste heat from these fuel cells,
which is of sufficiently high quality, can be used for cogeneration or
transferred to a bottoming cycle gas turbine to produce more electricity.