Chengdu electrolytic water hydrogen production is characterized by green environmental protection, flexible production, high purity (up to 99.9%) and by-product high-value oxygen. Its unit energy consumption is about 4-5 kWh/m3 hydrogen. The cost of hydrogen production is greatly affected by electricity price, which accounts for more than 70% of the total cost. It is generally believed that when the electricity price is lower than 0.3 yuan/kwh, the cost of hydrogen production from electrolytic water will be close to that from traditional fossil energy.

Hydrogen production by electrolysis of water is to dissociate water molecules into hydrogen and oxygen through electrochemical process under the action of direct current, which are separated at the cathode and anode respectively. At present, the main hydrogen production units of electrolytic water are alkaline water electrolysis tank (AE) and proton exchange membrane water electrolysis tank (PEM).

The chemical reaction formula of hydrogen production by electrolysis of water is as follows:

Cathode: 4H2O+4e -=2H2 ↑+4OH-

Anode: 4OH -- 4e -=2H2O+O2 ↑

Total reaction formula: 2H2O=2H2 ↑+O2 ↑

1. Hydrogen production by electrolyzing water in alkaline liquid electrolyzer

Alkaline electrolyzer technology In order to be mature, the production cost is low, the capacity range of gas production is large, the process is simple, the cost is low, and it is easy to maintain. At present, it is widely used. Generally, the working current density of alkaline liquid electrolyte electrolyzer is about 0.25A/cm2, the energy efficiency is usually about 60% - 70%, and the gas output of a single unit can reach 1000Nm3/h.

Alkaline liquid water electrolysis technology uses KOH and NaOH aqueous solutions as electrolytes, asbestos cloth, polyphenylene sulfide fiber diaphragm, etc. as diaphragms, and electrolyzes water under the effect of DC to generate hydrogen and oxygen. The produced gas needs to be dealkalized. Alkaline liquid water electrolysis was industrialized in the middle of the 20th century. The technology is relatively mature, and the service life can reach 15 years.

The alkaline liquid electrolyte electrolyzer is difficult to shut down or start quickly, and the speed of hydrogen production is difficult to adjust quickly. Moreover, the pressure on both sides of the anode and cathode of the electrolytic cell must be kept balanced at all times to prevent hydrogen and oxygen gas from mixing through the porous asbestos film and causing explosion.

Therefore, it is difficult for alkaline liquid electrolyzer to cooperate with renewable energy with rapid fluctuation characteristics.

2. Hydrogen production by proton exchange membrane (PEM) electrolysis of water

PEM electrolysis uses proton exchange membrane instead of asbestos membrane to conduct protons and isolate the gas on both sides of the electrode, which avoids the disadvantage of using strong alkaline liquid electrolyte in alkaline liquid electrolyte electrolyzer.

The operating current density of PEM electrolytic cell is usually higher than 1A/cm2, at least four times that of alkaline water electrolytic cell. It has the advantages of high efficiency, high gas purity, green environment, low energy consumption, no alkaline solution, small size, reliability, and higher gas production pressure. At present, this technology has not been commercialized on a large scale in China. The gas production of a single set in China is small and its service life is less than 10000 hours.

The main components of a typical PEM hydrolysis tank include the cathode and anode plates, the cathode and anode gas diffusion layer, the cathode and anode catalytic layer and the proton exchange membrane. Compared with alkaline water electrolysis, PEM water electrolysis system does not need dealkalization and has greater pressure regulation margin. In the early stage of commercialization, the cost of PEM was mainly concentrated in the PEM cell itself. Increasing the running current density can reduce the equipment investment of electrolysis. Moreover, the wide range of operating current density is more conducive to matching the volatility of renewable energy. It is considered as a promising water electrolysis technology.