The biggest difference between PEM electrolyzer and alkaline electrolyzer is the difference in current density. The current density of alkaline electrolyzer is generally 3000-6000 A/m2, while the current density of PEM electrolyzer is generally 10000-20000 A/m2 or even higher. The difference in current density leads to:
(1) PEM electrolyzer is smaller in size and occupies less space under the same hydrogen production;
(2) PEM electrolyzer has a shorter cold start time, because the electrolyzer generates heat faster after the current density increases, the internal temperature of the electrolyzer rises faster, and the operating temperature of PEM electrolyzer (50-80℃) is slightly lower than that of alkaline electrolyzer (60-90℃).
The difference in current density between alkaline electrolyzer and PEM electrolyzer mainly comes from the difference in catalyst and the difference in pH in the reaction environment.
First of all, in terms of catalyst, the catalyst used in alkaline electrolyzer is Ni-based non-precious metal catalyst, and the catalyst used in PEM electrolyzer is Pt (cathode), Ir (anode) and other precious metal catalysts.
As can be seen from the figure, for the HER reaction, Pt metal is at the top of the volcano curve. Compared with metal Ni, when metal Pt catalyzes the HER reaction, the exchange current density is larger. When the overpotential is the same, the reaction rate (current density) of Pt catalyzing HER will be one order of magnitude higher than the current density when Ni is used as a catalyst. Similarly, from the OER volcano curve, it can be seen that the Ir-based catalyst is closer to the top of the volcano curve than the Ni-based catalyst, and the overpotential required for the Ir-based catalyst is lower under the same current density.
In terms of the pH environment under electrochemical reaction conditions, the electrolyte of alkaline water electrolysis is 30% KOH solution, and the entire reaction environment is full of OH-, which is a water electrolysis reaction occurring under strong alkaline conditions; while in PEM water electrolysis, pure water is added to the anode as a reactant, a large number of protons are generated at the anode, and then pass through the proton exchange membrane to the cathode, so PEM water electrolysis is a water electrolysis reaction occurring under strong acid conditions. Take the HER reaction as an example. This reaction is a very obvious pH-dependent electrochemical reaction, that is, the kinetics of the HER reaction is related to the pH environment in addition to the catalyst.
As shown in the figure above, in a strongly acidic environment (pH=1), the HER current density under the same applied voltage is more than 10 times the HER current density under a strongly alkaline environment (pH=13) under the same applied voltage. Therefore, for Pt, the reaction kinetics of catalyzing HER under strongly acidic conditions is much faster than that under a strongly alkaline environment.
In general, the intrinsic catalytic activity of precious metal catalysts such as Pt and Ir/Ru is better than that of Ni-based catalysts. At the same time, for pH-sensitive reactions such as HER under acidic conditions, the kinetics of Pt catalyzing HER under acidic conditions is much better than that under alkaline conditions. Therefore, the current density of the PEM electrolyzer is greater than that of the alkaline electrolyzer. In addition, the proton exchange membrane thickness of the PEM electrolyzer is small and the cell structure is relatively compact, which makes the ohmic impedance inside the cell small, which also contributes to a part of the high current density.
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Nicole
Company: Baoji Jimiyun Dynamic Co., Ltd
Country:China
Add:Baoti road,Jintai,Baoji city,Shaanxi,China
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