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At the stack level, there are mainly two strategies to achieve a lower costs: Stack design and cell composition: This includes using less critical materials, redesigning the stack to achieve a higher efficiency (i.e. lower electricity cost), higher durability (longer lifetime to distribute the investment) and increase the current density (higher production rate). Increase the module size: This can bring economies of scale to some of the balance of plant components. This strategy should consider a trade-off between a small module size that enables mass-manufacturing, standardisation and replication, and a large module size that achieves larger cost reduction in balance of plant components at the expense of fewer units deployed and less learning by deployment. 3.1 STACK DESIGN: WHAT CAN BE DONE? Alkaline electrolysers Concerning stacks for alkaline electrolysers, the key areas to focus on are the electrodes and the diaphragms. Bipolar plates and PTLs have less priority, since they are based on stainless steel plates coated with nickel, which are already significant, cost-effective components. Strategies to integrate PTLs into electrodes and consequently diaphragms can also be of key importance in reducing costs, as outlined below: Increase current densities: The current densities of the stacks can be increased, from the current, 0.5 A/cm2 to more advanced units of 2-3 A/cm2. This current density increase cannot be made, however, at the penalty of lower efficiency. Higher current densities have already been accomplished by some manufacturers, too, with electrode-separator packages that can deliver a performance range as high as 1.2 A/cm2 at 2 volts (V) now available. Power densities of 2-3 W/cm2 could be achieved by demonstrating thinner diaphragms or membranes for alkaline electrolysers. As with PEM, alkaline electrolysers also need to improve their voltage efficiency levels, reducing ohmic losses and increasing electrode kinetics. Reducing diaphragm thickness: This could improve efficiency and reduce electricity consumption. The thinner the diaphragms, the lower the resistance to transporting the OH- species from the cathode to the anode. Eventually, however, this comes at a cost of higher gas permeation, which contributes to higher safety concerns. The other downside is the lower durability, given the higher chance of pinhole formation in the diaphragm and less mechanical robustness. Overall, the diaphragm thickness should reach values that approach those of PEM and AEM. State-of-the-art membranes for PEM are about 125-175 micrometres (μm) (Babic, 2017) with a potential decrease to 20 μm or lower. Below this point (for PEM), there are limited efficiency benefits. For alkaline electrolysers, the current diaphragm thickness is about 460 μm. Decreasing this to 50μm would contribute to improving the efficiency from 53% to 75% at 1A/cm2 (see Figure 22). SCALING UP ELECTROLYSERS TO MEET THE 1.5°C CLIMATE GOAL 57PDF Image | GREEN HYDROGEN SCALING UP ELECTROLYSERS
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