Fuel cells are customarily classified according to the electrolyte employed. The five most common technologies are:
- Proton Exchange Membrane (PEMFC)
- Alkaline Fuel Cells (AFC)
- Phosphoric Acid Fuel Cells (PAFC)
- Molten Carbonate Fuel Cells (MCFC)
- Solid Oxide Fuel Cells (SOFC)
PEMFCs employ an ion-conducting polymer membrane as an electrolyte and operate in temperatures where water is in liquid form. Low temperature, however, requires the use of expensive platinum catalysts. PEMFCs are intended for use in electric utilities, portable power applications, and transportation.
The electrolyte in an AFC is an aqueous solution on potassium hydroxide. AFCs performance is high due to the fast cathodic reactions in alkaline electrolyte. The electrolyte is very sensitive to carbon dioxide and requires expensive removal of CO2 from fuel and air streams. AFCs are utilized in military and space applications, where cost is not an issue, but high performance is required.
The electrolyte of PAFCs is phosphoric acid. Operation temperature of about 200°C allows very high efficiency electricity and heat co-generation, so a PAFC is able to use impure hydrogen as fuel. PAFCs need platinum catalysts and their low current and power density limit the range of applications. PAFCs have been used in electric utilities and in some transportation applications.
MCFCs and SOFCs share many characteristics. Their operation temperature between 600C and 1000C allows many advantages: high efficiency through electricity and heat co-generation, fuel flexibility, and the use of inexpensive catalysts as the reactions occur much faster as the temperature is increased. MCFCs evolved from the work aimed at producing a fuel cell that would operate directly on coal. MCFC uses a molten carbonate mixture, usually lithium, sodium, and/or potassium carbonates, soaked in a matrix as the electrolyte. SOFC is based on the use of solid ceramic electrolyte-zirconium oxide stabilized with small amounts of yttrium. Unlike MCFCs, SOFCs are safe from corrosion due to the liquid electrolyte. MC and SO fuel cells are intended for use in electric utilities, but are still in the early field test phase.
Another type of fuel cell is the Direct Methanol Fuel Cell (DMFC). As the name implies, DMFC uses methanol as fuel. Cathode reaction is similar to PEMFC cathode reaction but anode reaction is different: methanol molecules are broken apart when methanol-water solution is introduced to the negatively charged electrode. Carbon atoms combine with oxygen atoms from methanol and water to form carbon dioxide. Hydrogen is oxidized on the anode, and protons pass through the electrolyte to the cathode. As reaction byproducts, water is produced on the cathode and carbon dioxide on the anode.
A single fuel cell produces a limited voltage, usually less than 1 volt. In order to produce a useful voltage, a number of fuel cells are connected in series. Series-connected fuel cells form a fuel cell stack. The number of unit cells in a stack depends on the desired voltage.
Aside from military and space flight use, less specialized fuel cell applications can be categorized in three groups: stationary power generation, portable power generation, and transportation.
Stationary power generation applications include both large-scale utility plants and smaller scale systems for distributed electricity and heat generation in buildings and individual homes. Fuel cells are already an alternative for power generation in areas where there is no existing power grid or the power supply is often unreliable.
New applications are emerging in the field of portable power generation, where fuel cell systems are expected to replace primary and rechargeable batteries in portable electronics. Major drawbacks of batteries are limited capacity and slow recharging. With a suitable hydrogen storage method, a fuel cell system can achieve higher power and energy capacity. In addition, battery performance deteriorates when the charge level drops, whereas a fuel cell operates on constant level as long as fuel is supplied.
In the area of transportation, striving for zero-emission vehicles catalyzes the use of fuel cell engines. Typical systems use high pressure fuel storage. This will lower pollution emissions due to higher energy conversion efficiency. Direct hydrogen engines produce no pollutant emissions.
From CIE