Competing Effects of pH, Cation Identity, H₂O Saturation, and N₂ Concentration on the Activity and Selectivity of Electrochemical Reduction of N₂ to NH₃ on Electrodeposited Cu at Ambient Conditions

Abstract

The electrochemical reduction of N2 to produce NH3 at ambient conditions is an effective and sustainable route to store and carry hydrogen, balance the nitrogen cycle, and provide means to produce on-demand fertilizers. The efficient electrosynthesis of NH3 is challenging because of the lower activation of N2 and higher activity toward the hydrogen evolution reaction (HER). Here, we propose theory-guided activity descriptors to identify an efficient N2 reduction reaction (NRR) catalyst, followed by its implementation in a flow-through gas diffusion electrode (GDE) to quantify the effects of pH, cation identity, H2O saturation, and N2 concentration on the kinetics of the NRR. The identified Cu catalyst with dominant (111) facets electrodeposited on a carbon paper provides optimal active sites to obtain maximum NH3 faradaic efficiency (FE) of 18 ± 3% at −0.3 V vs RHE and the maximum NH3 current density of 0.25 ± 0.03 mA cm–2 (0.86 nmol·cm–2·s–1) at −0.5 V vs RHE in alkaline medium. The electrolyte pH mostly affects the HER by pH-induced binding of *H and reorganization of H2O, which favor the NRR at an optimal pH of 13.5. Increasing the size of monovalent cations stabilizes NRR intermediates and increases the NH3 current density from Li+ to K+. However, increasing the size of the cation from K+ to Rb+ reduces the FE of NRR, which is due to a direct reduction of H2O in the solvation shell of larger cations to produce H2. Another strategy to improve NH3 FE is to reduce the H2O saturation on the catalyst, which can be achieved by sparging the reactant gas directly through the GDE. Increasing the N2(g) flow rate not only increases the gas–liquid mass transfer coefficient but also reduces the H2O saturation in the pores of the GDE, which primarily suppresses the HER. The fixed potential DFT calculations reveal an associative distal mechanism for the NRR over Cu(111), where the hydrogenation of *N2 is the rate-limiting step. This finding also corroborates with the measured reaction order with respect to N2.