Electrochemical Ammonia Production from Wastewater in a Flow Cell Reactor

Abstract

Although its high carbon footprint, the Haber-Bosh (HB) process mainly dominates the NH3 production market. Indeed, H2 is mainly produced by fossil fuels steam reforming processes and the catalytic reaction between N2 and H2 requires high energy input, being carried out at high temperatures and pressures. Moreover, it should be considered that 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.1 Moreover, as the population is growing, the NH3 demand is increasing fast to sustain the massive fertilizer utilization.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).2 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.3 Due to the massive agriculture, NO3 - is one of the most abundant contaminants of underground waters and high levels in the human body can cause many diseases. 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.4 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. Such a setup has the advantage of guaranteeing a better mass transport of the active species and of being scaled up. The electrodes are mainly made by airbrush deposition of a catalyst ink on a carbon paper support, which allows obtaining a high electrochemical active surface as a result of its high porosity. Catalyst loading and NO3 - concentration was varied to find the optimal condition in terms of Faraday efficiency (FE) and yield. MoS2 catalyst showed good catalytic activity towards NH3, with a FE between 62%-77% and yield between 2.9-13 mmol g-1 h-1. On the other hand, synthesised 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 colourimetric method, using both Nessler’s and Berthelot’s reagents. Even if the concentration of NH3 is high enough to exclude that it can derive from contamination, more sensitive quantification is needed for these experiments, together with the quantification of side products, such as NO2 -, H2 and N2.