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44 GREEN HYDROGEN COST REDUCTION Nickel alloys: Highly caustic KOH at high concentrations requires inorganic ZrO2 diaphragms, nickel- and zincbased materials. Nickel alloys need to be free from chromium and iron, which could leach and end up contaminating the electrodes, which reduces efficiency and durability. Water impurities: Higher degradation due to low-quality water circulation has been observed, so the lifetime of the plant is affected as a function of operating hours. Many elements, including the diaphragm, catalysts, and other components, can be adversely affected by water impurities such as iron (Fe), chromium (Cr), copper (Cu), silicon (Si), aluminium (Al) and boron (B). PEM electrolysers have reported lifetimes of more than 50 000 hours. Some of the factors affecting their lifetime are: Operating conditions: Higher temperature, pressure and current density can have a negative impact on lifetime. Mild conditions are 50-60°C, 10 bar and 2 ampere per square centimetre (A/cm2) respectively, while the next generation PEM is expected to run at more demanding conditions (80°C, 70bar and 5 A/cm2). Some solutions to handle these conditions are overdesigned stacks with thick membranes, high catalyst loadings and protective coatings over porous transport layers (PTLs) and bipolar plates. Variable load: Electrolysers were previously operated with almost constant power supply to satisfy a fixed demand. The coupling with variable renewable electricity will lead to a variable load, which results in voltage fluctuations that can potentially trigger additional corrosion of stack components and reduce durability. Though very true in PEM fuel cells, there is little evidence of this in PEM electrolysers. Gas permeation: The membrane is subject to a large differential pressure (if it is operated under this) that negatively affects membrane mechanical stability. This also increases gas permeation, which can potentially lead to further degradation issues. A measure to tackle this is to use an additional catalyst to reconvert the permeated hydrogen (to the oxygen side) back to water. Anode dissolution: Iridium oxide on the anode can be prone to dissolution depending on the temperature, voltage and electrode architecture. One solution is to use a larger amount of catalyst (> 5 milligrammes per square centimetre [mg/cm2] or 2.5 grammes per kilowatt [g/kW]) and additional high loadings of precious metals in protective layers over the stack components. The anodic PTL uses porous titanium with a thickness above 1 mm to support the membrane, especially under differential pressure. This PTL is typically coated with platinum (> 1 mg/cm2 or 0.5g/kW) to minimise or mask the titanium oxidation. Water impurities: Poor water quality is one of the main reasons for stack failure for PEM electrolysers. Higher degradation due to water circulation is seen at partial load, so the lifetime of the plant is affected as a function of operating hours. Many elements are quickly affected due to impurities such as membrane, ionomer in the catalyst layer, catalysts, and PTLs. The water purification unit, responsible for providing American Society for Testing and Materials (ASTM) type II water, contributes to lower efficiency. SOEC electrolysers can achieve lifetimes of 20 000 hours, but under constant power and well-defined operating conditions (i.e. not coupled to variable renewable energy [VRE]). The main degradation mechanism is the thermal cycling, due to the high operating temperatures and need to cool down in case of dynamic operation. Reversible operation of solid oxide cells (electrolysis + fuel cell) could help increase the hours of operation and thus keep the system at operating temperature. Deploying SOEC at large scale would require larger cells than currently used (up from 300cm2 to more than 1 000 cm2), which renders them more prone to failure. Another important aspect is silica contamination and the instabilities of sealing concepts.PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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