The industrial production of ammonia–based fertilizers is critical to maintaining the global food supply, with more than 100 Mtons ammonia produced per year via the energy- and resource-intensive Haber-Bosch process. One proposed alternative to this process is the electrochemical reduction of nitrogen (eN2R) to NH3, which would enable NH3 production at ambient temperatures and pressures. We use a non-aqueous, Li-mediated system to improve selectivity toward NH3 by limiting proton availability. Although recent work supports the posited mechanism of 1) Li electrodeposition onto the electrode surface, 2) reaction with N2 to form Li3N, and 3) ethanolysis of Li3N to form NH3, our understanding of these processes and their relation to Faradaic efficiency toward NH3 (FENH3) remains limited. In particular, the formation of a solid-electrolyte interphase (SEI) layer may play a significant role in eN2R, affecting Li and proton transport to the cathode as well as the stability of Li3N and other Li-containing materials at the cathode. However, the SEI layer is difficult to interrogate due to the reactivity of Li with air and the amorphous nature of the SEI. We have thus used neutron reflectometry to investigate electrode (50 nm Mo on Si) surface dynamics in operando under conditions relevant to eN2R. In electrolytes of d8-THF, 0.2 M LiClO4, +/- 1 v% EtOH, pre-saturated with N2 or He, at a current density of -0.1 mA/cm2 for 30 minutes, we observed Li plating as well as formation of two layers upon returning to open-circuit potential (OCP) that totaled approximately 20-25 nm. Interestingly, the plating rates of Li differed depending on the presence of EtOH, and we did not observe the formation of an SEI layer in the first ten minutes of applying a current density; the two-layer system only later appeared. These results suggest that the electrode surface does not return to its original state under open-circuit conditions; rather, stable layers of material remain following electrochemistry. Upon returning to OCP, the scattering length density (SLD) of observed layers increased, suggesting that the layers no longer consisted of pure Li metal and likely contained a variety of species that could include electrolyte decomposition products, as well as oxidized forms of Li. These preliminary results indicate that this reaction mechanism is indeed complex, and further in operando characterization of the cathode surface is required to develop a full understanding of this process.

In operando neutron reflectometry investigation of the electrode surface during Li-mediated electrochemical nitrogen reduction

ACS Spring 2022

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