Fuel cells are devices that convert the chemical energy of a fuel directly into electricity and heat without combustion, through an electro-chemical reaction process, that is, oxidation-reduction. Due to the nature of such direct energy conversion, fuel cells are not limited by Carnot cycle efficiency, and are therefore, in principle, capable of realizing high electric power generation efficiency. Fuel cells differ from conventional batteries in that the active materials taking part in the electro-chemical reaction are not contained within the cell, but are rather supplied from outside.
Fuel cells have been known for over 150 years. William Grove, British physicist and justice of British high court, first devised a type of fuel cell, and demonstrated its principle with a famous experimental demonstration in 1838. Grove constructed the first fuel cell by immersing two platinum electrodes in a container of sulfuric acid. A constant electric current would flow through an external connector between two separately sealed electrodes, one electrode being in an oxygen chamber and the other electrode in a hydrogen chamber. Therefore, fuel cells show another advantage of having no intermediate mechanical moving part. Since the initial demonstration by Grove, fuel cells have come a long way and are on the verge of commercial application such as in road vehicle, stationary and portable power devices.
The basic fuel cell unit consists of three major components, the cathode, anode, and electrolyte. In order to extract electric power, an external circuit connecting two electrodes and intermediate external loads must be added. In the case of hydrogen fuel cells in which the fuel is hydrogen gas, oxygen molecules combine with hydrogen ions and electrons at the cathode to produce water or steam, and hydrogen molecules split into hydrogen ions and electrons at the anode. The hydrogen ions transport from the anode to the cathode through the electrolyte in order to compensate for an electric current in the external circuit. Since the voltage between the two electrodes in a single cell- in other words the electromotive force- is ordinarily less than 1 V, a large number of cells need to be piled up in series for practical use. This assembly of cells is called a fuel cell stack.
Proton Exchange Membrane Fuel Cells (PEMFC) or Polymer Electrolyte Fuel Cell (PEFC) have an electrolyte that consists of a layer of solid polymer and have the characteristic of being able to operate at much lower temperatures than other fuel cell types, of around 90℃. This is expected to be most suitable for use in vehicle engines or domestic co-generation systems, and is especially on the verge of commercialization for domestic applications in the 1kW output power class. PAFC uses concentrated phosphoric acid (H3PO4) as the electrolyte and the operating temperature is in the range of 150~220℃. This type of fuel cell is an established type of fuel cell technology in the 50~200kW output range. MCFC uses a molten carbonate salt mixture of typically lithium carbonate (Li2CO3) and sodium carbonate (Na2CO3) as the electrolyte. At an operating temperature of about 650℃, the salt mixture is liquid and a good ionic conductor. SOFC is a type of fuel cell in which the electrolyte is a solid, nonporous metal oxide, typically yttria stabilized zirconia (YSZ). Operating temperatures are in the range of 700~1000℃, which is the highest among fuel cells. SOFC and MCFC can be combined with a conventional power generation process in hybrid cycles resulting in a power generation efficiency as high as 60%. AFC operates at a relatively low temperature and its electrolyte is made of an aqueous solution of potassium hydroxide retained in a porous stabilized matrix. AFCs have been used in spacecraft but not on the ground because they are highly sensitive to CO2.
A fuel cell system is typically assembled with fuel cell stacks, fuel management facilities (reformers), an inverter, heat recovery equipment, measurement and regulation instruments and compressors, although such arrangements differ depending on the type of fuel cells and its application scheme. A reformer is a device that converts a hydrocarbon fuel into a hydrogen-rich gaseous stream and removes impurities.