This fuel cell uses ceramic material made of zirconium oxide (ZrO2) enriched with yttrium (Y) as an electrolyte that at the temperature of 1000degC is a perfect oxygen anion conductor. Owing to applying solid electrolyte almost no corrosion of electrodes occurs and there is no need to use a porous material to keep the electrolyte at the proper place. Moreover, there is no loss of electrolyte by evaporation or diffusion through the electrodes. The anode is made of porous nickel or zirconium sinter and the cathode is magnesium mixed with lanthanum manganese. The Westinghouse company made the SOFC's cathode from a small pipe made of zirconium sinter covered with zirconium oxide (ZrO2) lagged with anode. The oxidizer (air or O2) flows through the cathode-pipe and all the system is rinsed with fuel. The fuel may be hydrogen, carbon monoxide or methane. During the operation the following anodic reactions for specific fuels occur:The reaction for hydrogen fuel: H2 + O2- = H2O + 2e- The reaction for carbon monoxide fuel: CO + O2- = CO2 + 2e- The reaction for methane as a fuel: CH4 + 4O2- -> 2H2O + CO2 + 8e-On the cathode the following reaction occurs: O2 + 4e- ->2O2.Data from 06.2001 says that SOFC has the power density of 1.5 kW/m2 of active fuel cell surface at the voltage of 0.6V and works with the efficiency of 40-50%. However, the increase of the fuel pressure may lift it up to 60%. Due to high working temperature, the evolved heat may be used to transform water into steam that rotates turbines producing mechanical energy transformed into electricity later on. Moreover, this kind of fuel cell is more resistive towards sulfur impurities of the fuel than other fuel cells that make it possible to apply easily in houses using the gas from domestic cookers. SOFC is sensitive to temperature changes. The decrease of 100oC diminishes the electrical efficiency by 12%. Very high working temperature that demands usage of very specific materials that are resistive to such severe conditions. That makes the system expensive and unable to miniaturize. Because of the difficulties in maintenance of SOFC it is applied mainly in the high power stationary systems and seems to be very succesful on this field.
PEMFC,SPFC, PEM or PEFC -Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Fuel Cell.
This fuel cell uses a solid polymer, mainly Nafion by DuPont (http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.dupont.com), as an electrolyte. The working temperature of the PEMFC is 80oC and its other great advantages are no possibility of electrolyte leakage as well as lack of corrosion of electrodes.
The electrolyte has a very similar composition to Teflon with sulfonic acid binded inside. The basic structural unit formula for Nafion 117 is shown below:
Nafion 117 is a transparent polymer film about 175 microns (0.007 inches) thick. Nafion 117 contains fluorine, carbon, oxygen, sulfur, and hydrogen arranged in repeating polymer molecules. The hydrogen atom on the SO3 part of the molecule can detach from one SO3 site. The free H+ proton can hop from SO3 site to SO3 site through the material, to emerge on the other side of the membrane. This is the reason it is called a proton exchange membrane.
Anode and cathode are made from a graphite cloth or graphite sheet, protected from the water activity by Teflon enriched with platinum that plays a role of catalyst. Electrolyte is placed between cathode and anode and later on is pressed under elevated temperature forming MEA (Membrane/Electrode Assembly). The series of connected MEA, placed between plates, forms a fuel cell stack. MEA has more or less 1mm of thickness and is placed between plates full of ducts that are supplying fuel or oxidizer (O2 or air) and discharge heat from the system. The other role the plates is to humidify the membrane, otherwise it may be completely dried out and the same useless.
The reaction that undergoes on the electrodes is the same as in the case of PAFC. The components of PEMFC are presented on the picture made by Ballard ( http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.ballard.com).
The power density of PEMFC is equal to 6.4 kW/m2 of fuel cell's active surface that corresponds to 9.15 kA/m2 with the voltage of 0.7 V using compressed oxygen as an oxidizer. When instead of O2 compressed air is supplied these values are as follows: 3.78 kW/m2 and 5.4 kA/m2 with the same voltage (the data are dated on February 2000). However, the Ballard Power Systems with usage of a special membrane supplied by Dow Chemical achieved the power density of 21.5 kW/m2 that corresponds to 43 kA/m2 with the voltage of 0.5 V using compressed oxygen as an electrolyte.
According to NASA ( http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.nasa.gov), the amount of platinum needed for each of the electrodes is equal to 0.3 g/m2. However, the Los Alamos National Laboratory presents techniques that allow using 0.021 g/m2 that decreases the price of catalyst and whole fuel cell. We may foresee that this amount will be decreasing or the platinum will be replaced with other catalyst. For instance, on the Biohydrogen2002 conference in the Netherlands, the first attempts of replacing Pt with Hydrogenase enzyme were presented.
Nowadays, it is the most promising fuel cell for mobile and medium power stationary applications, because of its very high power density that together with a very low operating temperature and great properties of an electrolyte makes the PEMFC completely safe and extremely easy in maintenance.
There is a variety of applications for PEMFC, for instance starting from a very low power fuel cells used in cell phones, notebooks, and then cars, buses, planes and houses. The fuel cells for the last application are sold by Power Systems and Plug Power and have a power of 7-35 kW. With a 7 kW fuel cell, a house of two engineers from Latham company in New York City was powered and it was a first example of powering a house with a fuel cell. Another, interesting application of PEMFC is a project of Transportation Department from New Jersey designed by H Power, which aim was to modernize 65 road informative signs to be powered by this fuel cell.
The greatest disadvantage of PMEFC in the case of mobile applications, especially cars, is that it produces great amounts of water and actually is full of water itself. That may cause freezing and damage of fuel cell stack and other components like pipes and valves, when the temperature falls below 0oC (273.16K) and fuel cell is not working. In my opinion, freezing of fuel cell stack may be overcame by running fuel cell periodically, providing small amount of hydrogen and the same make it self-heating up apparatus.
AFC -Alkaline Fuel Cell.
High efficient. The drawback of the AFC is that the fuel as well as the oxidizer must be completely free of CO2 because it may react with KOH producing carbonates and the same removing the hydroxide anion that is a carrier of charge. This feature almost completely excludes AFC from commercial usage, since complete removal of CO2 from the system is difficult and expensive. Widly used by NASA (www.nasa.gov).
DMFC -Direct Methanol Fuel Cell.
This kind of fuel cells is powered with methanol that is supplied directly to a fuel cell and the oxidizer is air. It is very similar in construction to PEMFC because it uses the same electrodes and a polymer electrolyte as well. Its working temperature is equal to 50-90oC that makes it safe and easy in maintenance.
DMFC is the newest developed fuel cell but is very promising in low power-mobile applications. For instance to power a notebook and in this case it might be even superior than PEMFC because the storage of hydrogen is much more complicated nowadays than storage of methanol. The technology of notebooks' batteries enables to use these computers only for 3-4.5 hours (May 2002), since mainly Lithium Ion batteries are used and polymer ion ones are still too expensive. The operation time of a notebook powered with DMFC is limited only by the amount of methanol that we are able to carry, that means it allows using notebooks in long journeys or in places where electricity is not available. The picture of similar system powered by PEMFC is presented in Gallery section.
A huge drawback of DMFC is that it produces carbon dioxide together with water that means it is not totally ecologically friendly and I would not recommend applying it in transportation sector or any other that demands big scale production.
The anodic reactios is as follows:
CH3OH + H2O -> CO2 + 6H+ + 6e-; Eo=0.016V/SHE (Standard Hydrogen Electrode)
The cathodic reaction:
O2 + 4H+ + 4e- -> 2H2O; Eo=1.229V/SHE
The overall reaction:
CH3OH + 3/2O2 -> CO2 + 2H2O
ΔG= -702kJ/mol and H°= -726kJ/mol
(Source: Orfeo Zerbinati, A Direct Methanol Fuel Cell. Journal of Chemical Education, Vol. 79 No. 7; July 2002)
However, DaimlerChrysler presented at The Fuel Cell World conference this year in Switzerland, data on NECAR 5 powered by 75kW DMFC car that drove all the way from San Francisco to Washington D.C. 5250km (3262 miles) at average speed of 62 km/h (38 mph). They have not noticed any damages of fuel cell system. Find out more about this car in Applications section or follow the link:http://www.fcv.biz/CoastToCoastNECAR5.htm
There exist such a rumor that corrosion of electrodes, especially anode, may be observed, however, Mr. Shimshon Gottesfeld - who is believed to be one of the greatest experts on DMFC - replies: there is no such problem, of course if proper materials are used.
We can not forget about the greatest advantage of DMFC that makes it much superior over any other fuel cell. That is a relatively cheap and easy way of gaining methanol.
RFC -Reversible Fuel Cell.
This fuel cell is very similar to the PEMFC and actually it is a PEM cell that works both as a fuel cell and electrolyzer. This is the newest existing system that is composed mainly of photovoltaic cell, fuel cell and two cylinders. The cylinders are filled with distilled water, that is in contact with fuel cell electrodes. Water is split into hydrogen, that bubbles through the H2 cylinder, and oxygen, that evolves through O2 cylinder.
The table below lists "overview-properties" of types of fuel cells described above.
Type of Fuel Cell
PAFC
MCFC
SOFC
AFC
PEMFC
DMFC
Electrolyte
phosphoric acid
molten carbonates
ceramic material - Yttria-stabilized zirconia
KOH
polymer ion exchange film
polymer ion exchange film
Working temperature
190-200oC
650-700oC
1000oC
65-220oC
80oC
50-90oC
Fuel
H2, LNG, methanol
H2, CO, coal gasfied gas, LNG, methanol
H2, CO, CH4, coal gasfied gas, LNG, methanol
H2
H2, LNG, methanol
methanol
Oxidizer
O2
O2 + CO2
O2
O2
O2
O2
Charge carrier
H+
CO32-
O2-
OH-
H+
H+
Catalyst
platinum
nickel
nickel
platinum
platinum
platinum
Efficiency
40-50%
45-60%
45-65%
40-89%
40-50%
-
Power density
1.7-1.9kW/m2
-
1.5kW/m2
-
6.4(pure oxygen), 3.7(air) kW/m^2
-
Features/commnets
Close to commercialization. In use. Doubts over cost.
Can use unreformed fuel, slow start up & response
High generation efficiency Wide use of Fuel
Low corrosivity. Required pure reactants.
Compact, possibility of rapid start. Significant investment, close to market.
CO2 emmisions, small scale use only.
Use
Medium stationary.
Large utility
Large utility, small APU, residential
First use by NASA. Military, aerospace, automotive, submarine
Transport, small/ medium stationary, portable.
Battery replacement, portable.Fuell cells comparsion (www.h2net.com):
PEMFC,SPFC, PEM or PEFC -Proton Exchange Membrane Fuel Cell or Polymer Electrolyte Fuel Cell.
This fuel cell uses a solid polymer, mainly Nafion by DuPont (http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.dupont.com), as an electrolyte. The working temperature of the PEMFC is 80oC and its other great advantages are no possibility of electrolyte leakage as well as lack of corrosion of electrodes.
The electrolyte has a very similar composition to Teflon with sulfonic acid binded inside. The basic structural unit formula for Nafion 117 is shown below:
Nafion 117 is a transparent polymer film about 175 microns (0.007 inches) thick. Nafion 117 contains fluorine, carbon, oxygen, sulfur, and hydrogen arranged in repeating polymer molecules. The hydrogen atom on the SO3 part of the molecule can detach from one SO3 site. The free H+ proton can hop from SO3 site to SO3 site through the material, to emerge on the other side of the membrane. This is the reason it is called a proton exchange membrane.
Anode and cathode are made from a graphite cloth or graphite sheet, protected from the water activity by Teflon enriched with platinum that plays a role of catalyst. Electrolyte is placed between cathode and anode and later on is pressed under elevated temperature forming MEA (Membrane/Electrode Assembly). The series of connected MEA, placed between plates, forms a fuel cell stack. MEA has more or less 1mm of thickness and is placed between plates full of ducts that are supplying fuel or oxidizer (O2 or air) and discharge heat from the system. The other role the plates is to humidify the membrane, otherwise it may be completely dried out and the same useless.
The reaction that undergoes on the electrodes is the same as in the case of PAFC. The components of PEMFC are presented on the picture made by Ballard ( http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.ballard.com).
The power density of PEMFC is equal to 6.4 kW/m2 of fuel cell's active surface that corresponds to 9.15 kA/m2 with the voltage of 0.7 V using compressed oxygen as an oxidizer. When instead of O2 compressed air is supplied these values are as follows: 3.78 kW/m2 and 5.4 kA/m2 with the same voltage (the data are dated on February 2000). However, the Ballard Power Systems with usage of a special membrane supplied by Dow Chemical achieved the power density of 21.5 kW/m2 that corresponds to 43 kA/m2 with the voltage of 0.5 V using compressed oxygen as an electrolyte.
According to NASA ( http://www.pg.gda.pl/chem/Katedry/Maszyny/FC/FUEL_CELLS/PEMFC/www.nasa.gov), the amount of platinum needed for each of the electrodes is equal to 0.3 g/m2. However, the Los Alamos National Laboratory presents techniques that allow using 0.021 g/m2 that decreases the price of catalyst and whole fuel cell. We may foresee that this amount will be decreasing or the platinum will be replaced with other catalyst. For instance, on the Biohydrogen2002 conference in the Netherlands, the first attempts of replacing Pt with Hydrogenase enzyme were presented.
Nowadays, it is the most promising fuel cell for mobile and medium power stationary applications, because of its very high power density that together with a very low operating temperature and great properties of an electrolyte makes the PEMFC completely safe and extremely easy in maintenance.
There is a variety of applications for PEMFC, for instance starting from a very low power fuel cells used in cell phones, notebooks, and then cars, buses, planes and houses. The fuel cells for the last application are sold by Power Systems and Plug Power and have a power of 7-35 kW. With a 7 kW fuel cell, a house of two engineers from Latham company in New York City was powered and it was a first example of powering a house with a fuel cell. Another, interesting application of PEMFC is a project of Transportation Department from New Jersey designed by H Power, which aim was to modernize 65 road informative signs to be powered by this fuel cell.
The greatest disadvantage of PMEFC in the case of mobile applications, especially cars, is that it produces great amounts of water and actually is full of water itself. That may cause freezing and damage of fuel cell stack and other components like pipes and valves, when the temperature falls below 0oC (273.16K) and fuel cell is not working. In my opinion, freezing of fuel cell stack may be overcame by running fuel cell periodically, providing small amount of hydrogen and the same make it self-heating up apparatus.
AFC -Alkaline Fuel Cell.
High efficient. The drawback of the AFC is that the fuel as well as the oxidizer must be completely free of CO2 because it may react with KOH producing carbonates and the same removing the hydroxide anion that is a carrier of charge. This feature almost completely excludes AFC from commercial usage, since complete removal of CO2 from the system is difficult and expensive. Widly used by NASA (www.nasa.gov).
DMFC -Direct Methanol Fuel Cell.
This kind of fuel cells is powered with methanol that is supplied directly to a fuel cell and the oxidizer is air. It is very similar in construction to PEMFC because it uses the same electrodes and a polymer electrolyte as well. Its working temperature is equal to 50-90oC that makes it safe and easy in maintenance.
DMFC is the newest developed fuel cell but is very promising in low power-mobile applications. For instance to power a notebook and in this case it might be even superior than PEMFC because the storage of hydrogen is much more complicated nowadays than storage of methanol. The technology of notebooks' batteries enables to use these computers only for 3-4.5 hours (May 2002), since mainly Lithium Ion batteries are used and polymer ion ones are still too expensive. The operation time of a notebook powered with DMFC is limited only by the amount of methanol that we are able to carry, that means it allows using notebooks in long journeys or in places where electricity is not available. The picture of similar system powered by PEMFC is presented in Gallery section.
A huge drawback of DMFC is that it produces carbon dioxide together with water that means it is not totally ecologically friendly and I would not recommend applying it in transportation sector or any other that demands big scale production.
The anodic reactios is as follows:
CH3OH + H2O -> CO2 + 6H+ + 6e-; Eo=0.016V/SHE (Standard Hydrogen Electrode)
The cathodic reaction:
O2 + 4H+ + 4e- -> 2H2O; Eo=1.229V/SHE
The overall reaction:
CH3OH + 3/2O2 -> CO2 + 2H2O
ΔG= -702kJ/mol and H°= -726kJ/mol
(Source: Orfeo Zerbinati, A Direct Methanol Fuel Cell. Journal of Chemical Education, Vol. 79 No. 7; July 2002)
However, DaimlerChrysler presented at The Fuel Cell World conference this year in Switzerland, data on NECAR 5 powered by 75kW DMFC car that drove all the way from San Francisco to Washington D.C. 5250km (3262 miles) at average speed of 62 km/h (38 mph). They have not noticed any damages of fuel cell system. Find out more about this car in Applications section or follow the link:http://www.fcv.biz/CoastToCoastNECAR5.htm
There exist such a rumor that corrosion of electrodes, especially anode, may be observed, however, Mr. Shimshon Gottesfeld - who is believed to be one of the greatest experts on DMFC - replies: there is no such problem, of course if proper materials are used.
We can not forget about the greatest advantage of DMFC that makes it much superior over any other fuel cell. That is a relatively cheap and easy way of gaining methanol.
RFC -Reversible Fuel Cell.
This fuel cell is very similar to the PEMFC and actually it is a PEM cell that works both as a fuel cell and electrolyzer. This is the newest existing system that is composed mainly of photovoltaic cell, fuel cell and two cylinders. The cylinders are filled with distilled water, that is in contact with fuel cell electrodes. Water is split into hydrogen, that bubbles through the H2 cylinder, and oxygen, that evolves through O2 cylinder.
The table below lists "overview-properties" of types of fuel cells described above.
Type of Fuel Cell
PAFC
MCFC
SOFC
AFC
PEMFC
DMFC
Electrolyte
phosphoric acid
molten carbonates
ceramic material - Yttria-stabilized zirconia
KOH
polymer ion exchange film
polymer ion exchange film
Working temperature
190-200oC
650-700oC
1000oC
65-220oC
80oC
50-90oC
Fuel
H2, LNG, methanol
H2, CO, coal gasfied gas, LNG, methanol
H2, CO, CH4, coal gasfied gas, LNG, methanol
H2
H2, LNG, methanol
methanol
Oxidizer
O2
O2 + CO2
O2
O2
O2
O2
Charge carrier
H+
CO32-
O2-
OH-
H+
H+
Catalyst
platinum
nickel
nickel
platinum
platinum
platinum
Efficiency
40-50%
45-60%
45-65%
40-89%
40-50%
-
Power density
1.7-1.9kW/m2
-
1.5kW/m2
-
6.4(pure oxygen), 3.7(air) kW/m^2
-
Features/commnets
Close to commercialization. In use. Doubts over cost.
Can use unreformed fuel, slow start up & response
High generation efficiency Wide use of Fuel
Low corrosivity. Required pure reactants.
Compact, possibility of rapid start. Significant investment, close to market.
CO2 emmisions, small scale use only.
Use
Medium stationary.
Large utility
Large utility, small APU, residential
First use by NASA. Military, aerospace, automotive, submarine
Transport, small/ medium stationary, portable.
Battery replacement, portable.Fuell cells comparsion (www.h2net.com):
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