Phosphoric Acid Fuel Cells (PAFC)

Phosphoric Acid Fuel Cells (PAFC) were the first fuel cells to be commercialized. Developed in the mid-1960s and field-tested since the 1970s, they have improved significantly in stability, performance, and cost. Such characteristics have made the PAFC a good candidate for early stationary applications.

The PAFC uses an electrolyte that is phosphoric acid (H₃PO₄) that can approach 100% concentration. The ionic conductivity of phosphoric acid is low at low temperatures, so PAFCs are operated at the upper end of the range 150ºC–220ºC.

The charge carrier in this type of fuel cell is the hydrogen ion (H+, proton). This is similar to the PEFC where the hydrogen introduced at the anode is split into its protons and electrons. The protons migrate through the electrolyte and combine with the oxygen, usually from air, at the cathode to form water. The electrons are routed through an external circuit where they can perform useful work. This set of reactions in the fuel cell produces electricity and by-product heat.

Anode Reaction:	2 H2 => 4 H+ + 4 e-
Cathode Reaction:	O2(g) + 4 H+ + 4 e- => 2 H2O
Overall Cell Reaction:	2 H2 + O2 => 2 H2O

The PAFC operates at greater than 40% efficiency in generating electricity. When operating in cogeneration applications, the overall efficiency is approximately 85%. Furthermore, at the operating temperature of PAFCs, the waste heat is capable of heating hot water or generating steam at atmospheric pressure.

The high efficiency of the PAFC when operated in cogeneration mode is one advantage of this fuel cell type. In addition, CO₂ does not affect the electrolyte or cell performance and can therefore be easily operated with reformed fossil fuel. Simple construction, low electrolyte volatility and long-term stability are additional advantages.

Approximately 75 MW of PAFC generating capacity has been installed and is operating. Typical installations include buildings, hotels, hospitals, and electric utilities in Japan, Europe and the United States. One particular program sponsored by CERL was to demonstrate 30 PAFCs generating 200 kW apiece at military installations. The objective of this program was to gain experience with PAFC operation for maintenance, reliability and performance.