A fuel cell is an electrochemical cell that converts the chemical energy of a fuel ,hydrogen or methane, and an oxidizing agent, usually oxygen, into electricity through a pair of reduction reactions. Fuel cells have an efficiency of about 40%.
Fuel cells could be used on Mars for vehicles or for emergency use. They could also serve as long term back up source of electricity if efficient methane based fuel cells became available. Large quantities of methane will be required for a martian settlement to provide propellant for the return to Earth of space vehicles.