A fuel cell uses the chemical energy of hydrogen to cleanly and efficiently produce electricity. Fuel cells have a variety of potential applications; for example, they can provide energy for systems as large as utility power stations or as small as laptop computers. Hydrogen has unique physical and chemical properties that present benefits and challenges to its successful widespread adoption as a fuel. Polymer electrolyte membrane fuel cells—also called proton exchange membrane fuel cells—deliver high power density and offer the advantages of low weight and volume compared with other fuel cells. Alkaline fuel cells were one of the first fuel cell technologies developed, and they were the type widely used in the United States space program to produce electrical energy and water onboard spacecraft. Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte and porous carbon electrodes containing a platinum catalyst. Solid oxide fuel cells use a hard, nonporous ceramic compound as the electrolyte.