In a fuel cell, a continuously supplied fuel reacts with an oxidizing agent thereby producing electricity. When hydrogen is used as the fuel in the cell, the only emissions are water and heat. Fuel cells are used in stationary and mobile energy generation and – as an alternative to conventional combustion engines and battery power – in vehicles. There are different types of fuel cells, which differ primarily in the media used and their operating temperature. Polymer electrolyte membrane fuel cells are very often used due to their simple design and scalability.