The Haber-Bosch (HB) process continues to dominate the ammonia (NH₃) production market despite its significant carbon footprint. This process heavily relies on the steam reforming of methane (CH₄) to generate hydrogen (H₂) and requires substantial energy to allow the reaction between nitrogen (N₂) and H₂. Additionally, HB plants are unevenly distributed globally, meaning the transportation of NH₃ and its derivatives to end users constitutes a considerable share of the overall environmental impact. [1]

In recent years, research has increasingly explored alternative methods for NH₃ production. One approach is the direct electrochemical reduction of N₂ (NRR) in aqueous electrolytes under ambient conditions, utilizing renewable energy. However, this method faces challenges such as low selectivity at high current densities and limited productivity. These issues stem from the high dissociation energy of the N₂ triple bond and the competing H₂ evolution reaction (HER). [2] Instead, the possibility to exploit NO₃⁻ present in groundwaters and wastewaters and electrochemically convert them into NH₃ results more feasible. [3] Indeed, this process has lower activation energy, which makes the reaction thermodynamically favoured compared to NRR. [4]

Our work aims to find the best operational parameters using a commercial MoS₂ catalyst deposited on a gas-diffusion electrode. The tests are carried out in a 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. 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. To assess wastewater treatment, it has been decided to maintain NO₃⁻ concentration at 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. Additionally, other parameters such as carbon-based electrode support, flow rate, cations in the electrolyte and type of binder have been explored, at the optimal conditions obtained in the DoE, to determine if further maximization of the response is possible. Moreover, the reaction mechanism is elucidated using pulsed chronoamperometry.