Fuel cells enable highly efficient electrochemical conversion of the energy stored in a chemical energy carrier. Through direct conversion into electrical energy, the losses that occur in conventional combined heat and power machines can be avoided and the limitation imposed by the Carnot efficiency can be circumvented. In addition to hydrogen - the energy carrier of the future - fuels that are already available on a large scale, such as natural gas or other gaseous and liquid fuels, can be converted in fuel cells. Even small systems can achieve electrical net efficiencies of up to 60%, and when waste heat is used in CHP systems, the efficiencies are close to 100%. Fuel cells are currently being introduced to the market in various sectors, while today´s stationary fuel cell systems are mostly operated with natural gas, for electromobility the focus is on hydrogen-powered and thus emission-free fuel cell vehicles.