Lithium mediated ammonia electrosynthesis: electrochemical protocol optimization

Ammonia (NH3) is a fundamental chemical used in numerous applications, primarily as a fertilizer. The Haber-Bosch is currently the most widely used and efficient process for its production; unfortunately, it is energy intensive and significantly contributes to the global CO2 emissions. Consequently, alternative routes for green ammonia synthesis are being investigated to mitigate environmental impact and achieve independence from fossil fuels. Among the different alternative, the lithium-mediated electrochemical nitrogen reduction reaction (Li-NRR) is nowadays considered one of the most promising. This study focuses on the optimization of the electrochemical protocol for the Li-mediated system. A batch system was employed and two consecutive regimes of applied current were imposed: (i) a pulsed-current regime, during which a solid electrolyte interphase (SEI) layer is formed, followed by (ii) a steady-current regime, in which the nitrogen reduction reaction mainly occurs. In order to make a proper comparison, the duration of steady current step was adjusted so that the total charge delivered to the system was 20 C. The primary goal was to understand how the number pulsed-current peaks influences the SEI layer formation and composition and, consequently, the ammonia production and system efficiency. Faradic efficiency (FE) and ammonia productivity were used as the main performance descriptor. Ammonia quantification was done using Berthelot reaction and UV-Vis spectrophotometry. Moreover, the SEI-layers corresponding to the conditions that yielded the best and worst results were analyzed using air-free X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The maximum Faradic efficiency obtained was approximately 18%, while the highest productivity reached was 0.16 mg/cm2h. Both were obtained when 10 current peaks at 25 mA were applied. FESEM micrographs revealed a more homogeneous and compact SEI layer under these conditions, while XRD analysis indicated the presence of LiF. Although the obtained results are still far from the performance of Haber-Bosch process, further research is necessary to better understand SEI dynamics, improve the investigated system, and explore more innovative solutions.