Week 2 Challenge

Fuel cells are classified on the basis of type of electrolyte used. These are as follows:

• Alkaline Fuel Cells (AFCs),
• Phosphoric Acid Fuel Cells (PAFCs),
• Proton Exchange Membrane Fuel Cells (PEMFCs),
• Molten Carbonate Fuel Cells (MCFCs) and
• Solid Oxide Fuel Cells (SOFCs).
• Direct Methanol Fuel Cells (DMFC)

Type	specifications	Application
Proton Exchange Membrane Fuel Cells (PEMFCs)	High power density. Uses solid polymer as an electrolyte and porous carbon electrodes containing a platinum or platinum alloy catalyst. Operate at relatively low temperatures around 80 degree celsius. However, it requires that a noble-metal catalyst (typically platinum) be used to separate the hydrogen's electrons and protons, adding to system cost. Typically fueled with pure hydrogen supplied from storage tanks or reformers.	Used primarily for transportation applications and some stationary applications. Particularly suitable for use in vehicle applications, such as cars, buses, and heavy-duty trucks.
Direct Methanol Fuel Cells (DMFC)	Powered by pure methanol, which is usually mixed with water and fed directly to the fuel cell anode. Higher energy density than hydrogen—though less than gasoline or diesel fuel. Direct methanol fuel cells do not have many of the fuel storage problems. Methanol is also easier to transport and supply to the public using our current infrastructure because it is a liquid, like gasoline.	DMFCs are often used to provide power for portable fuel cell applications such as cell phones or laptop computers.
Alkaline Fuel Cells (AFC)	AFCs were one of the first fuel cell technologies developed. These fuel cells use a solution of potassium hydroxide in water as the electrolyte and can use a variety of non-precious metals as a catalyst at the anode and cathode. In recent years, novel AFCs that use a polymer membrane as the electrolyte have been developed. These fuel cells are closely related to conventional PEM fuel cells, except that they use an alkaline membrane instead of an acid membrane. Achieves efficiencies above 60% in space applications. A key challenge for this fuel cell type is that it is susceptible to poisoning by carbon dioxide (CO2), so does lag behind PEMFC in performance.	AMFCs are being considered for applications in the W to kW scale
Phosphoric Acid Fuel Cells (PMFC)	Phosphoric acid fuel cells (PAFCs) use liquid phosphoric acid as an electrolyte and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially. PAFCs are more tolerant of impurities in fossil fuels that have been reformed into hydrogen than PEM cells. PAFCs are also less powerful than other fuel cells, given the same weight and volume. As a result, these fuel cells are typically large and heavy. PAFCs are also expensive. They require much higher loadings of expensive platinum catalyst than other types of fuel cells do, which raises the cost.	This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.
Solid Oxide Fuel Cells (SOFC)	(SOFCs) use a hard, non-porous ceramic compound as the electrolyte. SOFCs are around 60% efficient at converting fuel to electricity. In applications designed to capture and utilize the system's waste heat (co-generation), overall fuel use efficiencies could top 85%. SOFCs operate at very high temperatures—as high as 1000°C (1830°F). High-temperature operation removes the need for precious-metal catalyst, thereby reducing cost. They are not poisoned by carbon monoxide, which can even be used as fuel. This property allows SOFCs to use natural gas, biogas, and gases made from coal. High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation.	SOFCs can be used to produce a combination of electricity, hydrogen-rich synthesis gas, and high-temperature heat. This makes them particularly suitable both for stand-alone applications and integrated with other thermal cycles or energy conversion technologies.
Molten Carbonate Fuel Cell (MCFC)	MCFCs are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix. Improved efficiency is another reason MCFCs offer significant cost reductions over phosphoric acid fuel cells. MCFC, when coupled with a turbine, can reach efficiencies approaching 65%, considerably higher than the 37%–42% efficiencies of a phosphoric acid fuel cell plant. When the waste heat is captured and used, overall fuel efficiencies can be over 85%. Unlike alkaline, phosphoric acid, and PEM fuel cells, MCFCs do not require an external reformer to convert fuels such as natural gas and biogas to hydrogen. The high temperatures at which these cells operate and the corrosive electrolyte used accelerate component breakdown and corrosion, decreasing cell life.	MCFCs are currently being developed for natural gas and coal-based power plants for electrical utility, industrial, and military applications

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• Energy density :
  Energy density is the amount of energy stored in a given system or region of space per unit volume. Expressed in kwh/kg of the energy source used.
• Power density:
  Power density is the amount of power (time rate of energy transfer) per unit volume. Expressed in kW/kg of the energy source used.

Lithium ion batttery:

• They have one of the highest energy densities of any battery technology today (100-265 Wh/kg). In addition, Li-ion battery cells can deliver up to 3.6 Volts, 3 times higher than technologies such as Ni-Cd or Ni-MH.
• Compared to the other high-quality rechargeable battery technologies (nickel-cadmium or nickel-metal-hydride), Li-ion batteries have a number of advantages. They have one of the highest energy densities of any battery technology today (100-265 Wh/kg).

Lead Acid battery:

• The lead–acid battery has a low energy density of 25–35 Wh/kg, although it has passed through many years of development. However, the power density is as high as 150 W/kg.

Nickel Cadmium battery:

• Nickel-cadmium (NiCd) batteries compete with lead-acid batteries because they have a higher energy density (50-75 Wh/kg) and have a longer life (2000-2500 cycles).

Ultra-Capacitor:

• The fabricated supercapacitor exhibits a very high energy density of 206 Wh/kg (59.74 Wh/L)/496 W/kg at a current density of 0.25 A/g, and a high power density of 32 kW/kg (9.8 kW/L)/9.58 Wh/kg at a current density of 50 A/g

Fuel cell:

• Diesel has an energy density of 45.5 megajoules per kilogram (MJ/kg), slightly lower than gasoline, which has an energy density of 45.8 MJ/kg. By contrast, hydrogen has an energy density of approximately 120 MJ/kg, almost three times more than diesel or gasoline.

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