A Flow Cell Reactor Exceeding 100 h Electrochemical Nitrate Reduction to Ammonia

Despite its high carbon footprint, the Haber-Bosh (HB) process mainly dominates the NH3 production market. Such a process is still strictly dependent on the steam reforming of CH4 to produce H2 and high energy demand is requested to run the reaction between N2 and H2. Moreover, 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 this reason, in the last years the research has focused on other possibilities for NH3 production. On one hand, there is the direct N2 electrochemical reduction (NRR) in aqueous electrolytes under ambient conditions exploiting renewable energy, which is limited by low selectivity at high current densities and low productivities, due to the high dissociation energy of the N2 triple bond and the unavoidable hydrogen evolution reaction (HER). On the other hand, there is the possibility to exploit NO3− present in groundwaters and wastewaters and electrochemically convert them into NH3. This process has lower activation energy, which makes the reaction thermodynamically favoured compared to NRR. Our work aims to assess the catalytic activity of commercial MoS2 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 electrochemically active surface as a result of its high porosity. Design of experiments and surface response methodology (DoE/RSM) have been chosen to gain further insight into the influence of some operating conditions (i.e., potential, catalyst loading, and supporting salt concentration) on the Faradaic efficiency (FE) and NH3 production rate. Even being the catalytic activity higher at a high concentration of NO3−, it has been decided to carry out the test in a concentration of NO3− similar to that found in polluted waters (500 mg L¬−1). The model suggests the presence of two optimal conditions: one for the FE (76.9%) at -1.2 V vs Ag/AgCl potential and K2SO4 0.3 M as supporting salt and one for the NH3 production rate at -1.6 V vs Ag/AgCl and K2SO4 0.36 M (77.67 µg h-1 cm−2). Catalyst loading did not show any effect on the system responses. Setup stability has been tested for over 100 h. FE and productivity rate for NH3 were stable at values around 60% and 19.6 µg h-1 cm−2, respectively, for all the first 100 h, during which electrolyte was changed every 8 or 16 h. In the last 50 h of electroreduction, it has been decided to maintain the same electrolyte to understand the kinetic of NO3− reduction over time and the decrease in FE due to the loss of reactive species.