The role of ion solvation in lithium mediated nitrogen reduction

Since its verification in 2019, there have been numerous high-profile papers reporting improved efficiency of lithium-mediated electrochemical nitrogen reduction to make ammonia. However, the literature lacks any coherent investigation systematically linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. In this study, we discover that the salt concentration has a remarkable effect on electrolyte stability: at concentrations of 0.6 M LiClO4 and above the electrode potential is stable for at least 12 hours at an applied current density of -2 mA cm(-2) at ambient temperature and pressure. Conversely, at the lower concentrations explored in prior studies, the potential required to maintain a given N-2 reduction current increased by 8 V within a period of 1 hour under the same conditions. The behaviour is linked more coordination of the salt anion and cation with increasing salt concentration in the electrolyte observed via Raman spectroscopy. Time of flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy reveal a more inorganic, and therefore more stable, SEI layer is formed with increasing salt concentration. A drop in faradaic efficiency for nitrogen reduction is seen at concentrations higher than 0.6 M LiClO4, which is attributed to a combination of a decrease in nitrogen solubility and diffusivity as well as increased SEI conductivity as measured by electrochemical impedance spectroscopy.

Automated summary

Since 2019, numerous high-profile papers have reported improved efficiency of lithium-mediated electrochemical nitrogen reduction to make ammonia. However, there is a lack of systematic investigation linking bulk electrolyte properties to electrochemical performance and Solid Electrolyte Interphase (SEI) properties. This study reveals that the salt concentration has a remarkable effect on electrolyte stability and the formation of a more stable SEI layer with increasing salt concentration. Higher salt concentrations result in decreased faradaic efficiency due to decreased nitrogen solubility and diffusivity, as well as increased SEI conductivity.