Any mention of fuel cells today usually refers to oxy-hydrogen fuel cells. However, as a galvanic element, the term principally relates to the transformation of chemical energy to electrical energy through controlled oxidation of a fuel (hydrogen in the above-mentioned case). Fuel cells all have a similar structure, consisting of two electrodes separated from each other by a gas-tight electrolyte (a porous polymer membrane where necessary). The fuel is conveyed to the anode, with the oxidation agent being directed to the cathode.

A differentiation can be made between numerous fuel cell types, depending on the electrolytes and fuel used. Liquid electrolyte fuel cells include the AFC (alkaline fuel cell), in which potash lye acts as the basis for the electrolyte, the PAFC (phosphor acid fuel cell), with phosphoric acid acting as the electrolyte, or the MCFC (molten carbonate fuel cell), where alkaline carbonate melts are employed as an electrolyte. Hydrogen is most frequently used as a fuel in the aforementioned types, but methane and coal gas are also employed in part. Solid electrolyte fuel cells include the SOFC (solid oxide fuel cell), which employs an oxygen ion-conducting ceramic electrolyte such as doped zirconium oxide or ceroxide at high temperatures (700 – 100 °C). In contrast to this, the PEMFC (proton exchange membrane fuel cell) and DMFC (direct methanol fuel cell), both of which use a semipermeable (only permeable for protons) polymer membrane as an electrolyte, function at considerably lower temperatures (60-130 °C). The fundamental difference between these two types is the fuel used. The PEMFC utilises a gaseous form (hydrogen), while liquid methanol is employed for the DMFC, making the generation of hydrogen unnecessary. The most advanced in their development are the polymer electrolyte fuel cells. The polymer membrane, which usually consists of a film with a density of 20 to 150 µm, constitutes the central element of the electrode membrane unit (MEA) in this respect. The cell is enclosed by bipolar plates that regulate the supply of fuel and oxidation gas through milled or embossed channels, blanket feeding the gas to the MEA. Bipolar plates simultaneously realise the bipolar electrical coupling of two fuel cell units of this nature. This is necessary, as the voltage generated by a single cell barely amounts to one Volt. Several individual cells are therefore connected electrically in series to form so-called stacks in order to generate higher voltages. The bipolar plate feeds fuel gas on one side and oxidation gas on the other for the adjacent cell. In order to achieve electrical contact with minimal possible losses, materials with a high electrical conductivity are used for bipolar plates, these simultaneously exhibiting temperature stability and resistance to corrosive gases. Graphite or carbonic polymers are mainly used for this reason.

The spread of this mobile energy source is in its infancy today, but the possibility of utilising regenerative energy carriers and its potentially extreme level of efficiency will lead to it growing in importance in future. This applies primarily wherever a decentralised energy supply is required, such as in notebooks, mobile phones or remotely monitored measuring stations. Fuel cells can also be employed as energy sources for the on-board power supply of yachts and campers. They are already used in drive technology on submarines. Development in the automotive sector will take some years until their serial feasibility has been achieved. Initial concepts for stationary decentralised power supplies in buildings and cogeneration power stations are currently being tested.