Electrolysers for electrochemical CO2 reduction are often optimized to operate with high single-pass conversion (the fraction of CO2 in the input gas stream that is converted to reduced products in a single pass through the reactor); Doing so can lower downstream gas separation costs as there is less CO2 to be removed from the product stream. However, running at high single-pass conversions can hamper electrolyser performance due to low local CO2 concentrations near the reactor outlet. Similarly, electrolyser design strategies aiming to minimise cathode-to-anode crossover of CO2 — and therefore minimise CO2/O2 separation — can have the knock-on effect of increasing the reactor’s energy requirements. Now, through electrolyser modelling and techno-economic analysis, Thomas Moore, Sarah Baker and colleagues at Lawrence Livermore National Laboratory and TotalEnergies argue that, for many scenarios, having near-optimal electrolyser performance is more of a ‘priority’ than minimising downstream gas separation.
The research team focus on a reactor generating ethylene as the only carbon-based product, with hydrogen as a side-product. They find that it is typically best to use high CO2 flow rates so that CO2 concentration is not depleted towards the end of the reactor, keeping ethylene production performance high. The ideal single-pass CO2 conversion is therefore low, at around 5–10%. Although doing so increases the cost of separation, overall costs are kept down by the more optimal performance of the electrolyser due to the relatively low energy cost of the separation compared to electrochemical conversion. Similarly, the researchers conclude that strategies to minimise CO2 generation at the anode (such as swapping out anion-exchange membranes for bipolar membranes) are only worthwhile if they have little impact on electrolyser performance; for instance, if the increase in cell potential due to the change is less than around 0.5 V; or if electricity is cheap.
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