Phosphoric acid fuel cells

Phosphoric Acid Fuel Cell (PAFC) uses phosphoric acid as electrolyte, oxygen as oxidant, hydrogen-rich gas (such as natural gas reformed gas) as fuel, and platinum alloy as catalyst at both anode and cathode. The electrochemical reaction takes place on the surface of catalyst particles supported by carbon black, and the working temperature is around 200°C. The working principle of phosphoric acid fuel cell is shown in Figure 1.

Phosphoric acid fuel cells
Figure 1 – The working principle of phosphoric acid fuel cells

The reaction when hydrogen is used as fuel and oxygen is used as catalyst is as follows.

Anode reaction:             H2→2H++2e

Cathodic reaction:          1/2O2 +2H++2e→H2O

Due to the easy availability of phosphoric acid and the mild reaction, phosphoric acid fuel cells have become the fuel cells with the fastest development, the most mature research and the most applications among all fuel cells., which is currently the fastest commercialized natural material battery, which uses liquid phosphoric acid as the electrolyte, usually in a silicon carbide matrix. The working temperature of phosphoric acid fuel cell is slightly higher than that of proton exchange membrane fuel cell and alkaline fuel cell, which is about 150 ℃ ~ 200 ℃, but still needs platinum catalyst on the electrode to accelerate the reaction. The reactions at the anode and cathode are the same as in the PEM fuel cell, but because of its higher operating temperature, the reaction rate at the cathode is faster than that of the PEM fuel cell cathode. At the same time, the higher working temperature also makes it more resistant to impurities. When the reactant contains 1%-2% CO and several parts per million sulfur, the phosphoric acid fuel cell can still work.

At present, the power generation efficiency of phosphoric acid fuel cells is 40%~45%, and the thermal efficiency of cogeneration is 80%. Due to the special adsorption of acid anions, the electrochemical reaction rate of oxygen is much slower than that in alkaline solution, so noble metals need to be used as catalysts, and the reaction temperature needs to be increased.

The advantage of phosphoric acid fuel cell is to overcome the disadvantage that alkaline fuel cell is intolerant of CO2, and the maximum allowable mass fraction is 30%, thus reducing the requirement for oxidant. The operating temperature is low, which is easy to operate, and can be combined with other power generation methods to form a combined heat and power generation. The disadvantage is that due to the use of phosphoric acid as the electrolyte, the plates and containers must be made of materials that are not prone to corrosion, and the precious metal platinum is required as a catalyst, which is expensive and prone to CO and H2S poisoning. Sintering and corrosion may occur during operation, resulting in performance degradation and shortened life. Preheating is required during startup and cannot be started quickly.

At present, the United States, Japan, Canada, South Korea and many European countries have developed phosphoric acid fuel cell power plants. Practice has proved that phosphoric acid fuel cell is a reliable power supply, which can provide uninterrupted power supply for military bases, hospitals, and computer stations. Due to its operating temperature of about 200 ℃, low utilization rate and long startup time, it is not suitable as a mobile power supply.