WHAT ARE FUEL CELLS.....

fuels like methane are commonly burned to provide thermal energy at high temperatures for use in heat engines. however a comparison of the reversible works obtained in the last two examples reveals that the exergy of the reactants(818 MJ/kmol CH4) decreases by 288 MJ/kmol as a result of the irreversible adiabatic combustion process alone. that is , the exergy of the hot combustion gases at the end of the adiabatic combustion process is 818-288=530 MJ/kmol methane. in other words, the work potential of the hot combustion gases is about 65 percent of the wok potential of the reactants. it seems that when methane is burned, 355 of the work potential is lost before we even start using the thermal energy.

thus, the second law of thermodynamics suggests that there should he a better way of converting the chemical energy to work . the better way is ,of course, the less irreversible way, the best being the reversible case.

in chemical reactions, the irreversibility  is due to uncontrolled electron exchange between the reacting components. the electron exchange can be controlled by replacing the combustion chamber by electrolytic cells.like car batteries. (this is analogous to replacing unrestrained expansion of a gas in mechanical system by restrained expansion) 

in the electrolytic cells, the electrons are exchanged through conductor wires connected to a load, and the chemical energy is directly converted to electric energy. 
the energy conversions devices that work on this principle are called FUEL CELLS.

fuel cells are not heat engines, and thus their efficiencies are not limited by the carnot efficiency . they convert chemical energy to electrical energy essentially in an isothermal manner.
a fuel cell functions like a battery,except that it produces its own electricity by combining a fuel with oxygen in a cell electro chemically without combustion. and discards the waste heat. a fuel cell consists of two electrodes separated by an electrolyte such as a solid oxide, phosphoric acid, or molten carbonate. the electric power generated by a single fuel cell is usually too small to be of any practical use therefore  fuel cells are usually stacked in practical applications. this molecularity gives the fuel cells considerable flexibility in application:the same design can be used to generate a small amount of power for a remote switching station of a large amount of power to supply electricity to entire town. therefore . fuel cells are termed the "microchip of the energy industry"


the operation of a hydrogen -oxygen fuel cell

hydrogen is ionized at the surface of the anode,and hydrogen ions flow through the electrolyte to the cathode. there is a potential difference between the anode and  the cathode, and free electrons flow from the anode to the cathode through an external circuit (such as a motor and a generator). hydrogen ions combine with oxygen and the free electrons at the surface of the cathode, forming water. therefore , the fuel cell operates like an electrolysis system working in reverse.in steady operation ,hydrogen and oxygen continuously enter the fuel cell as reactants and water leaves as the product. therefore ,the exhaust of the fuel cell is drinkable quality water.


this was the brief overview and there is a lot on this topic...and also a major future hope .....of energy sectors......of the world



for more information ref: Yunus A Cenegal , Michael A Boles THERMODYNAMICS....8TH EDITION.




i know it was a bit heavy to take it a once ......but am sure here after u aren't new to this topic..


any kind of queries and suggestions are most welcomed ...



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