Classification of fuel cells and introduction to alkaline fuel cells

1. Classification of fuel cells

The types of fuel cells can be roughly classified as follows according to different methods:

(1) According to the operating mechanism of fuel cells, there are mainly acidic fuel cells and alkaline fuel cells.

(2) According to the type of electrolyte, there are mainly alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, proton exchange membrane fuel cells, regenerative hydrogen-oxygen fuel cells, direct alcohol fuel cells, as well as new energy storage batteries, solid polymer batteries, etc.

(3) Divided by fuel type, the main fuel is hydrogen, methanol, methane, ethane, toluene, butene, butane and other organic fuel supplements or gasoline, diesel and natural gas and other gas fuels. Organic fuels and gaseous fuels must undergo chemical treatment such as reforming and cracking to produce hydrogen-rich gaseous fuels before they can be used as fuels for fuel cells.

(4) According to the working temperature of fuel cells, it is mainly divided into: low temperature type, the temperature is lower than 200℃; medium temperature type, the temperature is 200℃-750℃; high temperature type, the temperature is higher than 750 ℃, among which alkaline fuel cell (operating temperature is within 100 ℃), proton exchange membrane fuel cell (also known as solid polymer fuel cell, operating temperature is within 100 ℃) and phosphoric acid fuel cell (operating temperature is about 200 ℃) are low temperature fuel cells; molten carbonate fuel cell (operating temperature is around 650°C) and solid oxide fuel cell (operating temperature is 800°C-1000°C) are called high temperature fuel cells.

2. Alkaline fuel cells

Alkaline Fuel Cell (AFC) uses strong alkali (such as potassium hydroxide, sodium hydroxide) as electrolyte, or NH3, N2H4 hydrogen as fuel. Pure oxygen or air from which trace CO2 has been removed is used as the oxidant, and the porous gas diffusion electrode prepared by using PVC, Ag, Ag-Au, Ni, etc., which have good catalytic activity for the electrochemical reduction of oxygen, is used as the electrocatalyst as the oxygen electrode, and porous gas electrodes prepared with electrocatalysts such as P-Pd/C, PVC, Ni or nickel boride with good catalytic hydrogen electrochemical oxidation properties are used as hydrogen electrodes. The bipolar plate is made of non-porous carbon plate, nickel plate or nickel-plated or even silver-plated and gold-plated metal (such as aluminum, magnesium, iron, etc.) plate, and gas flow channels of various shapes can be processed on the plate surface to form a bipolar plate.

The working principle of the alkaline asbestos membrane type hydrogen-oxygen fuel cell is shown in Figure 1.

Classification of fuel cells and introduction to alkaline fuel cells
Figure 1 – Reaction principle of alkaline fuel cell

At the anode, hydrogen and OH in the base undergo an oxidation reaction under the action of an electrocatalyst to generate water and electrons.

Anodic reaction:           H2+2OH→2H2O+2e      (1-1)

The electrons reach the cathode through the external circuit. Under the action of the cathode electrocatalyst, it participates in the reduction reaction of oxygen.

Cathodic reaction:         1/2O2+2H2O+2e→2OH    (1-2)

The generated OH migrates to the hydrogen electrode through a porous asbestos membrane saturated with lye.

In order to keep the battery working continuously, in addition to supplying hydrogen and oxygen at the same speed as the battery consumes hydrogen and oxygen, it is also necessary to discharge the water generated by the battery reaction from the anode (hydrogen pole) continuously and at the same speed to maintain the constant alkali concentration of the electrolyte. (The basic reaction principle of fuel cell.) The working temperature of the alkaline fuel cell is about 80°C, so it is necessary to remove the waste heat of the battery reaction to maintain the constant working temperature of the battery. As a result, its start-up is also fast, but its power density is nearly 10 times lower than that of a proton exchange membrane fuel cell. However, they are the cheapest type of fuel cell to produce, so they can be used in small stationary power plants. Alkaline fuel cells are also very sensitive to carbon monoxide and other impurities that can foul the catalyst. In addition, the raw material cannot contain CO, because CO can react with the KOH electrolyte to form potassium carbonate, which reduces the performance of the battery.react with the KOH electrolyte to form potassium carbonate, which reduces the performance of the battery.