A 250-kilowatt phosphoric acid fuel cell powers a police station and electric vehicle recharging station in New York's Central Park.

Phosphoric acid fuel cells (PAFC) were the first fuel cells to cross the commercial threshold. Around 300 of these power units were placed in operation in stationary power applications in the United States and overseas. Most are the 200-kilowatt PC25 fuel cell power plant manufactured by United Technologies Corporation (UTC) and its subsidiaries. UTC has installed over 75 MW of PAFC systems, operating for over 8 million hours, in 85 cities and 19 countries. Many applications have heralded in the press, including one that powers a police station in New York City's Central Park and another that provides supplemental power to the Conde Nast Building at 4 Times Square in New York.

As the name implies, these fuel cells use liquid phosphoric acid as the electrolyte. The electrodes are made of carbon paper coated with a finely-dispersed platinum catalyst. The catalyst strips electrons off the hydrogen-rich fuel at the anode. Positively charged hydrogen ions then migrate through the electrolyte from the anode to the cathode. Electrons generated at the anode travel through an external circuit, providing direct current electric power, and return to the cathode. There the electrons, hydrogen ions and oxygen form clean water, which is discharged from the cell.

In a phosphoric acid fuel cell, hydrogen must be extracted from fuels such as natural gas outside the fuel cell (a process called "external reforming"). Phosphoric acid fuel cells operate at around 150 to 200 degrees C (about 300 to 400 degrees F), above the boiling point of water. At a phosphoric acid fuel cell's operating temperatures, the expelled water can be converted to steam for space and water heating. In this combined heat and power application, overall efficiencies can approach 80 percent. The electricity-generating efficiency is 37 to 42 percent.

UTC continues to develop the PAFC technology introducing the PureCell Model 400 which promises to be lower in cost and higher in efficiency. New York Power Authority (NYPA) selected the company to supply 12 fuel cells totaling 4.8 megawatts of power for the Freedom Tower and three other new towers under construction at the World Trade Center site in lower Manhattan. Delivery of the PureCell Model 400 fuel cell systems will begin in January of 2009. The PureCell Model 400 power plant is one of the cleanest, quietest and most energy-efficient on-site power generating technologies available. The fuel cell’s ultra low emissions meet the strictest air emissions requirements in the United States.

UTC Power has supplied fuel cells for the U.S. space program since the 1960s, and continues to provide three 12kW fuel cells that generate all onboard electrical power for each orbiter.  Backup batteries are not needed, and the water produced by the electrochemical reaction in the fuel cells is used for crew drinking and spacecraft cooling. Thus far, shuttle fuel cells have accumulated more than 92,000 hours of operation.