Phosphoric acid fuel cells (PAFC) are a type of fuel cell that uses liquid phosphoric acid as an electrolyte. They 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. [1]
Electrolyte is highly concentrated or pure liquid phosphoric acid (H3PO4) saturated in a silicon carbide (SiC) matrix. Operating range is about 150 to 210 °C. The electrodes are made of carbon paper coated with a finely dispersed platinum catalyst.
Anode reaction: 2H2(g) → 4H+ + 4e
Cathode reaction: O2(g) + 4H+ + 4e‾ → 2H2O
Overall cell reaction: 2 H2 + O2 → 2H2O
At an operating range of 150 to 200 °C, the expelled water can be converted to steam for air and water heating (combined heat and power). This potentially allows efficiency increases of up to 70%. [2] PAFCs are CO2-tolerant and can tolerate a CO concentration of about 1.5%, which broadens the choice of fuels they can use. If gasoline is used, the sulfur must be removed. [3] At lower temperatures phosphoric acid is a poor ionic conductor, and CO poisoning of the platinum electro-catalyst in the anode becomes severe. [4] However, they are much less sensitive to CO than proton-exchange membrane fuel cells (PEMFC) and alkaline fuel cells (AFC).
Disadvantages include rather low power density and chemically aggressive electrolyte.[ clarification needed ]
PAFC have been used for stationary power generators with output in the 100 kW to 400 kW range and are also finding application in large vehicles such as buses. [5]
Major manufacturers of PAFC technology include Doosan Fuel Cell America Inc. [6] (formerly ClearEdge Power & UTC Power [7] ) and Fuji Electric.
India's DRDO has developed PAFC based air-independent propulsion for integration into their Kalvari-class submarines. [8] [9]