Concentrated phosphoric acid in a silicon carbide (SiC) teflon matrix is used as the electrolyte in this fuel cell type. Due to the aggressive medium, only diluted acid was used some years ago. Newly developed corrosion-resistant materials have made it possible to operate with concentrated acid, which increases the electrolyte conductivity. The electrodes are based on the same material as in PEMFC, carbon-supported platinum (Pt), and here the cathode also requires a higher catalyst loading than the anode. For operation with reformed hydrogen, ruthenium (Ru) is used in addition to Pt on the anode. With Ru as a binary catalyst, carbon monoxide in the fuel is oxidised more easily. Since hydrogen (or reformed hydrocarbons) and air are used as the fuel, the electrochemical reactions that take place on the electrodes are the same as in the PEMFC:

Anode oxidation of hydrogen: 

H₂ 2H⁺ + 2e^-

Cathode reduction of oxygen:

½O₂ + 2H⁺ + 2e^- H₂O

Total PAFC reaction:

H₂ + ½O₂ H₂O 

The PAFC operates at around 200°C, which means that it is less sensitive to impurities like CO than the PEMFC. However, it cannot be fed directly with fossil fuels and thus requires an external reforming unit. CO also has to be shifted by a water gas shift reaction to below 3 to 5 vol% or it will affect the catalyst. Due to the higher operating temperature, the excess heat is of higher value than in the other low-temperature fuel cells. A combined heat and power supply is therefore possible. The many basic similarities between the PAFC and the PEMFC leads to several of their components being the same.

Fuji-Electric 100 kW.

UTC Fuel Cell Onsi PC25.

Toshiba-UTC PC25.

Water management is an even more important issue than in PEMFC, as a liquid electrolyte is used. At these operating temperatures it is difficult to keep the water in the cell. With concentrated acid, this problem has been reduced considerably. The high freezing temperature of phosphoric acid, 40-50°C, leads to a longer start-up time than the PEMFC. In addition, phosphoric acid is a poor ionic conductor at lower temperatures and CO poisoning of the Pt electrocatalyst in the anode becomes severe. The main advantages are simple construction, thermal, chemical and electrochemical stability, and a low volatility of the electrolyte (i.e. no problems with CO₂ in the fuel). Compared to the other fuel cell systems it has a lower power density and the PAFC systems achieve only 37 to 42 % electrical efficiency (based on the LHV of natural gas). This is at the low end of the efficiency goal for fuel cell power plants.

Of all the fuel cell types, the PAFC systems are closest to becoming commercialised. They are mainly used in stationary power plants, but are also integrated into large vehicles such as city buses. World-wide, many systems in the MW range have already been installed to supply electricity, heat and hot water. Best known are the ONSI PC25 systems from UTC Fuel Cells with 200 kW electril power and 220 kW heat. Over 200 systems have been installed all over the world. UTC Fuel Cells co-operates with both Toshiba and Ansaldo in distribution and development. One of the largest fuel cell power plants installed to date is an 11 MW PAFC system in Japan.

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