While a few studies have shown promise for generating HNO3 from electrochemical NO oxidation under ambient conditions, the resulting product is obtained as a diluted stream mixed with a supporting electrolyte. This necessitates additional separation steps, which increase both the energy and cost demands. Now, Feng Jiao, Samira Siahrostami and co-workers have developed a vapor-feed strategy in which NO gas and water vapor are fed into a membrane electrode assembly electrolyzer, using Vulcan carbon as the model catalyst, to generate a commercial-grade HNO3 product (32 wt%) without electrolyte additives or the need for further downstream purification. Notably, the Faradaic efficiency of HNO3 during NO electrooxidation remains high (87.5%) in a three-compartment flow electrolyzer, with 800 ppm NO as the feedstock — a concentration comparable to real-world flue gas compositions. Online flow electrolyzer mass spectrometry combined with isotope-labeling experiments and density functional theory analysis reveal that nitrous acid (HNO2) is the critical intermediate, representing a shift from the conventional thermocatalytic NO2 reaction pathway.
Overall, this work proposes a promising reaction process to valorize NO emissions into high-purity HNO3 through a potentially sustainable and cost-effective electrochemical method. However, future work is needed to increase the operational current density and durability when using practical dilute NO feedstocks (such as flue gas) to ensure better alignment with industrial requirements.
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