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SCALING UP ELECTROLYSERS TO MEET THE 1.5°C CLIMATE GOAL 2.2 CELL LEVEL FOR EACH TYPE OF ELECTROLYSER Alkaline electrolysers: These have a simple stack and system design and are relatively easy to manufacture. Currently, they have electrode areas as high as 3 square metres (m2). They operate with high concentrate KOH (typically 57 moles of solute per litre of solution [mol*L-1]) as electrolyte, robust ZrO2 based diaphragms and nickel (Ni) coated stainless-steel for the electrodes. The ionic charge carrier is the hydroxyl ion OH-, with KOH and water permeating through the porous structure of the diaphragm to provide functionality for the electrochemical reaction. This allows the intermixing of the produced gases (hydrogen and oxygen – H2 and O2) that are dissolved in the electrolyte, limiting lower power-operating range and the ability to operate at higher pressure levels. To prevent this, thicker (0.252 millimetre [mm]) diaphragms are used, but this creates a higher resistance and lower efficiencies. Spacers are sometimes included by some manufacturers between electrodes and diaphragms to further avoid the intermixing of gases. These thick diaphragms and added spacers result into high ohmic resistances across the two electrodes, drastically reducing current density at a given voltage. Today ́s advanced designs, using zero- gap electrodes, thinner diaphragms and different electrocatalyst concepts to increase current density, have already reduced their performance gap in comparison to PEM technology. On the other hand, classic and sturdy alkaline designs are known to behave very reliably, reaching lifetimes above 30 years. Polymer Electrolyte Membrane (PEM) electrolysers: These use a thin (0.2 mm) PFSA membrane and electrodes with advanced architecture that allows achieving higher efficiencies (i.e. less resistance). The perfluorosulfonic acid (PFSA) membrane is also chemically and mechanically robust, which allows for high pressure differentials. Thus, the PEM cells can operate at up to 70 bar with the oxygen side at atmospheric pressure. The acidic environment provided by the PFSA membrane, high voltages, and oxygen evolution in the anode creates a harsh oxidative environment, demanding the use of materials that can withstand these conditions. Titanium-based materials, noble metal catalysts and protective coatings are necessary, not only to provide long-term stability to cell components, but also to provide optimal electron conductivity and cell efficiency. These requirements have caused PEM stacks to be more expensive than alkaline electrolysers. PEMs have one of the most compact and simplest system designs, yet they are sensitive to water impurities such as iron, copper, chromium and sodium and can suffer from calcination. Today, electrode areas are quickly approaching 2 000 square centimetres (cm2), yet this is still far from future concepts of large MW stack units using single stack concepts. Last but not least, the reliability and lifetime characteristics of large-scale, MW PEM stacks still have to be validated. Each technology has its own challenges, from critical materials to performance, durability and maturity; there is no clear winner across all applications, which leaves the door open for competition and innovation driving costs down Solid oxide electrolysers (SOEC): These operate at high(700-850°C)temperatures.Thisenables:the favourable kinetics that allow the use of relatively cheap nickel electrodes; electricity demand decreases and part of the energy for separation is provided through heat (waste heat can be used and apparent efficiencies based on electricity can be higher than 100%); the potential for reversibility (operating as fuel cell and electrolyser)8; co- electrolysis of CO2 and water to produce syngas (which is the basic building block for the chemical industry). On the downside, thermo-chemical cycling, especially under shutdown/ramping periods, leads to faster degradation and shorter lifetimes. Other issues related to stack degradation include: challenges related to sealing at higher differential pressure; electrode contamination by silica used as sealants; and other additional contaminant sources from piping, interconnects and sealing. SOECs are today only deployed at the kW-scale, although some current demonstration projects have already reached 1 MW. 8 Reversible PEM or Alkaline technologies exist, but are much less efficient and more complex, and have not being commercially demonstrated yet. Reversible operation also compromises durability 33PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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