Ammonia from nitrates (feat. bipolar membranes)
It has been known for quite some time that electrolysis of nitrates (usually saltpeter) can produce NH3 by reduction of
NO3- at the cathode. However, undesirable reactions at the anode reduce the usefulness of this approach. Fortunately, there is a
solution: bipolar membranes.
A bipolar membrane (BPM) consists of a cation-exchange layer and an anion-exchange layer, separated by an interfacial layer. The idea is that the BPM
facilitates the disproportionation of water into H+ and OH- ions, which leave the BPM on opposite sides. This allows a current
to flow through the BPM while blocking ion transport across it.
Figure 1 from this article (open access) illustrates the approach:
Xu, Z., Wan, L., Liao, Y. et al. Continuous ammonia electrosynthesis using physically interlocked bipolar membrane at
1000 mA cm−2. Nat Commun 14, 1619 (2023). https://doi.org/10.1038/s41467-023-37273-7.
Unfortunately, the article shows that commercially available BPMs are not suitable for NH3 electrosynthesis, unlike the authors' proposed
design. This makes the approach less accessible to amateurs, and personally I don't have the knowledge or safety equipment to use reagents such as
trifluoroacetic acid (which is required to make the authors' BPM). Nonetheless, I thought that this was interesting enough to share anyway.
P.S. You can also use BPMs for splitting NaCl into NaOH and HCl (rather than NaOH and Cl2), and for other purposes.
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