Optimization of a flow-cell reactor operating parameters for the electrochemical ammonia production from wastewater

Although its high carbon footprint, the Haber-Bosch (HB) process mainly dominates the NH3 production market. The catalytic reaction between N2 and H2 requires high energy input, being carried out at high temperatures and pressures. HB plants are not evenly distributed in the world, thus NH3 and its derivates transportation to the final user is a large portion of the overall environmental impact. For these reasons, in the last years, the research has focused on the possibility of producing ammonia from direct nitrogen electrochemical reduction (NRR) in aqueous electrolytes under ambient conditions exploiting renewable energy. This process is limited by low selectivity at high current densities and low yield, due to the high dissociation energy of the 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, which makes the reaction thermodynamically favoured compared to N2 reduction. Due to the massive agriculture, NO3- is one of the most abundant contaminants of underground waters. Thus, the use of NO3- to produce NH3 under ambient conditions can be carried out with lower energy consumption, but can also address the water pollution issue. The aim of our work is to assess the catalytic activity of commercial MoS2 and a synthesised Bi-based catalyst in a gas-diffusion electrode flow cell of 10 cm2 geometrical area. Design of experiments and surface response methodology (DoE/RSM) were employed to gain further insight into the influence of operational conditions (potential, catalyst loading and salt concentration) on the Faradaic efficiency and NH3 yield. MoS2 catalyst showed good catalytic activity towards NH3, with a FE between 62%-77% and yields between 2.9-13 mmol g-1 h-1. On the other hand, Bi material showed 21% FE and 0.41 mmol h-1 g-1, probably due to Bi final oxidation state, which is not so active towards NH3 production. NH3 quantification has been carried out through UV-vis colorimetric method, using both Nessler’s and Berthelot’s reagents.