How Does Hydrogen Generation By Water Electrolysis Work?

May 30, 2020
How Does Hydrogen Generation By Water Electrolysis Work?

At present, there are various known methods for producing hydrogen, includingtimg2

ammonia decomposition hydrogen production, natural gas hydrogen production, methanol cracking hydrogen production, water electrolysis hydrogen production, etc. The most widely used method is water electrolysis hydrogen production.


hydrogen-generation-methods

Hydrogen production from water electrolysis

Among them, hydrogen production by water electrolysis can be divided into two types: alkaline hydrogen electrolysis and PEM pure water electrolysis. The advantages and disadvantages of these two hydrogen production methods are:

hydrogen-production

(1) Hydrogen production by alkaline water electrolysis: The hydrogen production technology has a long history, first learned from the Soviet Union, and later applied to domestic power plants, polysilicon, chemical companies, small laboratory chromatographic matching gas sources and other places, reliable equipment operation, low cost. The purity of hydrogen is generally around 99.8%. However, hydrogen contains corrosiveness, and it will corrode various parts after long-term operation for more than 5 years. It needs a major repair every five years, and the maintenance cost is relatively high. Abandoned lye needs to be treated in an environmentally friendly manner, and special personnel are required to be trained to work. Special attention should be paid to the regulation of hydrogen and oxygen.

Alkaline electrolyte for hydrogen generation

(2) PEM pure water electrolysis to produce hydrogen: This technology was first derived from the principle of the oxygen generator of the US nuclear submarine, because it uses a proton membrane electrolyzer, which can physically separate high-purity hydrogen and oxygen by electrolyzing pure water. It is widely used, including power plants, chemical industry, metallurgy, glass, medical treatment, high-end laboratories, etc., and there is a tendency to replace alkaline 

electrolyse-graphic-en


liquid electrolytic hydrogen production. Moreover, the technical equipment is small in volume, and the purity of hydrogen produced is high, ≥99.9995%, the whole operation process is pollution-free, the operation is simple, and remote unattended operation can be achieved. The safety performance is high, the running time is long, and the life of the electrolytic cell is generally about 15 years.

The biggest feature of this type of device is the use of pure water electrolysis, no pollution, no corrosion, and higher purity of hydrogen. Compared with the traditional alkaline hydrogen production device, its advantages include the ability to meet the large current density operation, the current density can reach 1A / cm2 or more; low energy consumption and high efficiency, the electrolytic hydrogen production efficiency can reach more than 85%; 3. Light weight. It has good adaptability to wide power fluctuation power supply, and can realize intelligent regulation of hydrogen production from 0 to 100%. The device has a high degree of integration and can realize long-term stable operation.

The disadvantage is that the cost of equipment is more expensive than that of lye electrolysis for hydrogen production, mainly from platinum catalysts. At present, various countries are researching alternative materials for breakthrough platinum catalysts on proton membranes. Future development is expected.


The principle of alkaline water electrolysis hydrogen production

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Water electrolysis hydrogen production equipment has been more and more widely used, including new energy, power, petrochemical, pharmaceutical, metallurgy, polycrystalline silicon, meteorology, aerospace and other fields and major gas companies. The purity of hydrogen produced by the electrolytic water method can be as high as 99% or more, which is an important method for industrial production of hydrogen. When electrolyzing sodium hydroxide (potassium) solution, oxygen is released from the anode and hydrogen is released from the cathode.


The principle of hydrogen production from electrolyzed water is the process of electrolysis. With the help of direct current, the electrolyte dissolved in water is decomposed into new substances. The principle of electrolyzed water When direct current is applied to some aqueous electrolyte solutions, the decomposed substances have nothing to do with the original electrolyte. What is decomposed is water as a solvent, and the original electrolyte remains in the water. For example, sulfuric acid, sodium hydroxide, potassium hydroxide, etc. belong to this type of electrolyte. When electrolyzing water, since the ionization degree of pure water is very small, the conductivity is low, it is a typical weak electrolyte, so the aforementioned electrolyte needs to be added to increase the conductivity of the solution, so that the water can be smoothly electrolyzed into hydrogen and oxygen. Potassium hydroxide and other electrolytes will not be electrolyzed. Now take potassium hydroxide as an example:

 (1) Potassium hydroxide is a strong electrolyte. The following ionization process occurs after dissolving in water:

Therefore, a large amount of K + and OH- are produced in the aqueous solution.

 (2) The activity of the metal ions in the aqueous solution is different, and they can be arranged in the  order of activity: K> Na> Mg> Al> Mn> Zn> Fe> Ni> Sn> Pb> H> Cu> Hg> Ag> Au, in In the above arrangement, the front metal is more lively than the rear.

(3) In the active sequence of metals, the more active the metal is, the easier it is to lose electrons, otherwise the opposite is true. From the point of view of electrochemical theory, metal ions that are easy to obtain electrons have a high electrode potential, while metal ions ranked in the order of activity are difficult to obtain electrons and become atoms because of their low electrode potential. The electrode potential of H + = -1.71V, and the electrode potential of K + = -2.66V, so when H + and K + are present in the aqueous solution at the same time, H + will first get electrons on the cathode and become hydrogen, while K + will remain In solution.

(4) Water is a weak electrolyte and difficult to ionize. When KOH is dissolved in water, polarized water molecules surround the ionized K + and become hydrated potassium ions, and the water molecules have a polar direction due to the action of K +. Under the action of direct current, K + moves to the cathode with water molecules with polar orientation. At this time, H + will first obtain electrons and become hydrogen.

 

The electrolysis equation of water: When direct current is applied to the aqueous solution of potassium hydroxide, the following discharge reactions occur on the cathode and anode respectively, as shown in the figure below.

water electrolysis               

   Electrolysis of alkaline aqueous solution

(1) Cathode reaction. The H + (produced after water ionization) in the electrolyte is attracted by the cathode and moves toward the cathode, accepting electrons to precipitate hydrogen, and its discharge reaction is:

图片1

 (2)Anode reaction. The OH- in the electrolyte is attracted by the anode and moves to the anode, and finally emits electrons to become water and oxygen. Its discharge reaction is:

图片2

The total reaction formula of anode and cathode together is:

图片3

Therefore, in the electrolysis process using KOH as the electrolyte, water is actually electrolyzed to produce hydrogen and oxygen, and KOH only plays a role of carrying charges.


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