In the field of water electrolysis hydrogen production technology, AEM electrolyzers and PEM electrolyzers are two highly regarded technologies. What are the differences between these two types of electrolyzers?
AEM electrolyzers operate on principles similar to traditional alkaline water electrolysis. They conduct hydroxide ions through an anion exchange membrane while separating the anode and cathode to prevent the mixing of hydrogen and oxygen products. These can use either pure water or low-concentration alkaline solutions as the electrolyte. PEM electrolyzers use a proton exchange membrane. At the anode, water is oxidized into protons, electrons, and oxygen. Protons pass through the membrane to the cathode, where they are reduced to hydrogen molecules. The reaction gases precipitate at the back of the catalytic layer, and this compact design allows for high current density operation.
Powder electrocatalysts: For HER, nickel-based compounds are commonly used. N-Mo alloys are among the better-performing non-precious metal hydrogen evolution catalysts. For OER, materials based on transition metal alloys and oxides are common. Nickel-iron layered double hydroxides have high catalytic activity, but the properties and conductivity of the catalyst layer need attention. Anion exchange membranes are composed of polymer backbones and cationic groups. An ideal membrane should have high ion conductivity, appropriate size, and chemical stability. However, high ion conductivity and high stability are hard to achieve simultaneously. Several foreign companies have developed related products, such as the FAA3 series from Germany's Fumatech, though there is still room for improvement in terms of ion conductivity and stability.
Catalysts: The anode catalyst often uses Ir-based and Pt-based catalysts, which need high activity, stability, and corrosion resistance to withstand the strongly oxidative environment of the anode. The cathode catalyst usually employs Pt catalysts because of their fast hydrogen evolution reaction kinetics in acidic conditions.
The main types include fully fluorinated proton exchange membranes (such as the Nafon series), partially fluorinated proton exchange membranes, non-fluorinated proton exchange membranes, and organic/inorganic composite proton exchange membranes. Fully fluorinated proton exchange membranes are widely used but are costly and pose challenges in balancing thickness and performance.
AEM electrolyzer system design can follow the traditional alkaline water electrolyzer design or adopt a design philosophy similar to PEM water electrolysis. The traditional design is beneficial for industrialization, but some technical features might not be fully utilized. Designs similar to PEM need to solve issues related to balancing membrane ion conductivity and gas barrier properties. Currently, overall system design concepts need further refinement and upgrades. Operational characteristics: Operating voltage and current density affect energy utilization efficiency and hydrogen production. The actual operating current density should be lower than the critical current density. Elevated operating temperatures can improve reaction kinetics but also accelerate anion exchange membrane degradation. Increased system pressure can facilitate hydrogen storage and utilization but must address hydrogen crossover permeability issues. PEM electrolyzers have a mature system design and can adjust operating parameters such as voltage, current density, temperature, and pressure according to demand. Operating temperature and pressure have a significant impact on performance; higher temperatures reduce cell voltage and increase current density but require higher material standards.
They use alkaline electrolytes, retain the alkaline reaction system, and allow the use of non-precious metal catalysts and bipolar plates, reducing costs.
Good compatibility with traditional alkaline electrolyzers, beneficial for industrialization.
Similar design philosophy to PEM electrolyzers, with development potential.
Better dynamic response capability, better suited to the intermittency of renewable energy.
Higher technological maturity with existing commercial products, high current density, and high integration of electrolyzers.
Proton exchange membranes have high proton conductivity, allowing high differential pressure operation and improving gas purity. They have good dynamic response characteristics, quick startup speed, and can rapidly respond to changes in power input, making them well-suited for coupling with renewable energy.
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