In contrast to combustion engines, chemical energy is converted into electrical energy in an electrochemical process in fuel cells. The electrochemical conversion is associated with low losses, which is why the efficiency of fuel cells for generating electricity is significantly higher than that of combustion engines. Internal combustion engines use Carnot cycles and are therefore physically limited in their electrical efficiency.
Electrochemically, the process in fuel cells corresponds to the reversal of water electrolysis. The basic structure of a fuel cell consists of electrodes that are separated from each other by an ion conductor. This ion conductor is either a semi-permeable membrane or an electrolyte or both.
The fuel cell systems are classified according to the membrane/electrolyte composition:
Alkaline fuel cell (AFC), electrical efficiency 45-60%.
Polymer electrolyte fuel cell (PEMFC), electrical efficiency 35-60%.
Phosphoric acid fuel cell (PAFC), electrical efficiency 38%.
Melted carbonate fuel cell (MCFC), electrical efficiency 48%.
Solid oxide fuel cell (SOFC), electrical efficiency 47%.
Each individual cell supplies a voltage of 0.5 - 1 V, whereby the voltage level depends on the cell type and fuel. In practice, many cells are connected in series to form a stack.
The operating temperatures of 800 - 1000°C in SOFCs are very interesting for practical applications, as process heat can be extracted at high temperatures during operation of these cells.PEC and PAFC are used for mobile applications.