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Brief Introduction of Water Electrolysis

Water electrolysis technologies can be classified according to the applied electrolyte, which separates the two half reactions at the anode (oxygen evolution reaction) and cathode (hydrogen evolution reaction) of the electrolyzer. Several main types of water electrolysis systems for hydrogen production will be briefly introduced as follows:

Alkaline Water Electrolyzer

The alkaline water electrolyzer (AWE) system is usually composed of an anode and cathode, which is immersed in an alkaline solution, generally 20%-30% potassium hydroxide (KOH).18 They operate at a temperature below 100°C and atmospheric pressure. In this system, a diaphragm is needed for separating the two electrodes (anode and cathode). The diaphragm conducts hydroxide ions (OH-) through a KOH solution, which keeps hydrogen and oxygen separated to prevent their recombination. The partial reaction at the electrodes is given by19:

For hydrogen production, alkaline electrolysis represents a mature technology (up to megawatt range) at the commercial level, which has been applied for large-scale hydrogen production in the beginning of the twentieth century. However, some drawbacks still exist in the system because the diaphragm leads to high ohmic loss across it and limits the maximum current density. Moreover, the diaphragm cannot generate enough current to keep the hydrogen and oxygen from mixing. Therefore, a high pressure cannot be achieved, which would otherwise be beneficial for a bulky stack design. In addition, the permeation of gas products makes hydrogen and oxygen mix and recombine, and decreases the Faraday efficiency. Despite these disadvantages, this type of electrolyzer is the most common because it does not require expensive materials in this process. The alkaline conditions allow us to use other affordable metals such as nickel, avoiding those of the platinum group. The KOH solution for the electrolyte is also affordable.

Polymer Electrolyte Membrane Water Electrolyzer

Polymer electrolyte membrane water electrolysis (РЕМЕ) was introduced by General Electric in the 1960s.20 А РЕМЕ contains an anode and cathode that are separated by a polymer electrolyte membrane.21 They operate at approximately 100°C, which is the same as an AWE above. The following partial reactions at the electrodes take place22:

The PEM23 is often composed of Nation, which has sufficient proton conductivity to achieve electric density beyond 2 A cm-2. Moreover, the type of the PEM can separate product gases, thus enhancing its Faraday efficiency to nearly 100%. In addition, the PEM allows a high pressure operation, which is suitable for scaling-up and compressing hydrogen gas for transport. The operation pressure can be elevated to about 100 bar. However, the PEM needs to be thick, which leads to higher proton conductivity resistance. Operation at a high pressure reduces the relative volume of product gas bubbles and ohmic resistance. However, the catalyst needs stable, highly acidic conditions produced by the PEM. Therefore, an expensive platinum metal20 would be employed in the hydrogen evolution reaction (HER), while iridium oxide is used in the oxygen evolution reaction (OER) due to the high reactivity and stability. These catalysts need to be affordable as well as have the high reactivity and stability under acidic conditions.

 
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