Image credit: Jake Gard on Unsplash

A recent report described an innovative rechargeable metal-nitrogen battery based on a graphene-supported palladium (Pd) cathode and a polished aluminum (Al) anode interfaced with an ionic liquid electrolyte comprised of aluminum chloride/1-butyl-3-methylimidazolium chloride.

Remarkably, the Al-N₂ battery was shown to serve the dual purpose of not only storing and retrieving energy, but also being able to fix the nitrogen (N₂) stream as ammonia (NH₃).

To amplify, the working principles of this novel Al-N₂ battery are shown in the figure, and the balanced electrochemical reactions operating in the battery are presented below:

Anode: 2Al + 14AlCl₄^– ↔ 8Al₂Cl₇^– + 6e^–

Cathode: 8Al₂Cl₇^– + N₂ + 6e^– ↔ 2AlN + 14AlCl₄^–

Overall: 2Al + N₂ ↔ 2AlN

The Al-N₂ battery demonstrated a faradaic efficiency of 51.2%, beyond the 5% of earlier reported M-N₂ batteries based on lithium (Li) and sodium (Na) alkali metal anodes.

In comparing the Al-N₂ to the prior art Li-N₂ battery, it is noteworthy that the free energy of formation of ΔG_f(AlN) = -287 kJ/mol is more negative than for ΔG_f(Li₃N) = -154 kJ/mol. Thermodynamically, this indicates the spontaneity of the reaction Al(s) + N₂(g) –> 2AlN(s) is greater than 6Li(s) + N₂(g) –> 2Li₃N(s), and the open circuit voltage for Al-N₂ (0.99 V) proves to be larger than Li-N₂ (0.53 V). Furthermore, the standard reduction potential of Al being less than Li provides greater chemical stability for Al compared to Li. In addition, the natural Al₂O₃ passivation layer on Al confers upon it greater safety in its handling compared to Li.

Aside from the ability of Al-N₂ to function as a rechargeable battery, it “kills two birds with one stone” by also functioning as a N₂ fixation device by producing an aqueous NH₃ solution from the electrochemically generated AlN according to the balanced alkali-base dissolution equation:

AlN + NaOH + 2H₂O –> NaAlO₂ + NH₃•H₂O

The N₂ fixation performance of the Al-N₂ battery is impressive, enabling an NH₃ production rate of 27.1 mg/g_cat.h.

Overall, the invention of the dual purpose energy storage-nitrogen fixation Al-N₂ device speaks well for a distributed and sustainable future form of agriculture — a vision which portends an interesting eco-friendly alternative to the energy and greenhouse gas intensive Haber–Bosch process, and emerging green ammonia aqueous electrochemical N₂ reduction alternative technologies.

Written by: Geoffrey Ozin

Solar Fuels Group, University of Toronto Ontario, Canada, Email: [email protected], Web sites: www.nanowizard.info, www.solarfuels.utoronto.ca, www.artnanoinnovations.com.

References:

Guo, Ying, et al., A Rechargeable Al-N₂ Battery for Energy Storage and Highly Efficient N₂ Fixation, Energy & Environmental Science (2020). DOI: 10.1039/D0EE01241F

Wang, Lu, et al., Greening ammonia toward the solar ammonia refinery, Joule 2.6 (2018). DOI: 10.1016/j.joule.2018.04.017