Feasibility and challenges of the electrochemical ammonia production from nitrogen and nitrate in a flow cell reactor

The Haber-Bosh process is the most optimized and only known way to produce the quantities of NH3 needed to satisfy the overall world demand. Despite that, the urge to reduce as much as possible greenhouse gases emission paved the way for the discovery of alternative and cleaner paths towards the production of such a fundamental molecule indispensable to sustain agriculture in this period of demographic growth. The research has focused on the possibility of producing NH3 from direct N2 electrochemical reduction (NRR) in aqueous electrolytes under ambient conditions, the limitations of which are the low selectivity at high current densities and low yield, due to the high dissociation energy of N2 triple bond and the unavoidable hydrogen evolution reaction (HER). On the other hand, NO3− can be easily converted into NH3 thanks to the lower activation energy and it is one of the most abundant contaminants of underground waters. Thus, the use of NO3− to produce NH3 under ambient conditions can also address the water pollution issue. The research on such topics is still in the first stages and very few literature reports reliable and consistent results. Our work is divided into different lines: i) the NO3− electrochemical reduction using commercial MoS2 in a gas-diffusion electrode flow cell of 10 cm2, ii) NRR tests using different catalysts in different conditions, iii) the study of different sources of contamination, among all that coming from Nafion membrane. Regarding the first line, the design of experiments and surface response methodology (DoE/RSM) revealed the possibility of having stable operations for over 100 h with low NO3− concentration (500 mg L−1), with FE and productivity rate for NH3 stable at values around 60% and 19.6 µg h−1 cm−2, respectively. Unfortunately, even with the hardest effort, we were not able to obtain reliable and good results for NRR in all the explored conditions. False positives have been demonstrated to be related to the presence of contaminants, especially the one coming from the Nafion membrane, which can absorb NH3 in a magnitude dependent on the electrolyte composition and on the cell conditions.