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GREEN HYDROGEN COST REDUCTION Anion Exchange Membranes (AEM): This is the latest technology with only a few companies commercialising it, with limited deployment. AEM’s potential lies in the combination of a less harsh environment from alkaline electrolysers with the simplicity and efficiency of a PEM electrolyser. It allows the use of non-noble catalysts, titanium-free components, and, as with PEM, operation under differential pressure. The reality, however, is that the AEM membrane has chemical and mechanical stability problems, leading to unstable lifetime profiles. Moreover, performance is not yet as good as expected, mostly due to low AEM conductivity, poor electrode architectures and slow catalyst kinetics. Performance enhancement is typically achieved by tuning membrane conductivity properties, or by adding a supporting electrolyte (e.g. KOH, or sodium bicarbonate [NaHCO3]). Such tuning could lead to decreased durability, however. The OH- ion is intrinsically three-fold slower (lower conductivity) than H+ protons within PEM, which forces AEM developers to either make thinner membranes, or ones with higher charge density. 2.3 SYSTEMLEVELFOREACHTYPEOFELECTROLYSER System Components Alkaline systems: As for the other types, alkaline electrolysers require recirculating the electrolyte (KOH) into and out the stack components. This creates a pressure drop that requires specific pumping characteristics and negatively affects the efficiency, which is typically less than 0.1% of the stack power consumption, but can be much higher for other manufacturers. Some alkaline systems are also operated without pumping peripherals. After leaving the stack, this alkaline solution needs to be separated from the gases produced. This is done in gas-water separators that are placed above the stack at a given height, and KOH/water flows back to the stack. The water phase can be removed at the bottom and the gas phase at the top (see Figure 7). The water column within the separator also serves as a buffer storage for changing load specifications. The water management system regulates the filling level of each gas separator, and water permeation via the diaphragm needs to be considered. Figure 7. Typical system design and balance of plant for an alkaline electrolyser. Hydrogen Oxygen Water/KOH Water 34 Note: This configuration is for a generic system and might not be representative of all existing manufacturers. Based on IRENA analysis.PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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