Evaluating the Thermodynamics of Electrocatalytic N₂ Reduction in Acetonitrile

The development of a sustainable ammonia synthesis by proton-coupled electroreduction of dinitrogen (N₂) requires knowledge of the thermodynamics described by standard reduction potentials. The first collection of N₂ reduction standard potentials in an organic solvent are reported here. The potentials for reduction of N₂ to ammonia (NH₃), hydrazine (N₂H₄), and diazene (N₂H₂) in acetonitrile (MeCN) solution are derived using thermochemical cycles. Ammonia is thermodynamically favored, with a 0.43 V difference between NH₃ and N₂H₄ and a 1.26 V difference between NH₃ and N₂H₂. The thermodynamics for reduction of N₂ to the protonated products ammonium (NH₄⁺) and hydrazinium (N₂H₅⁺) under acidic conditions are also presented. Comparison with the H⁺/H₂ potential in MeCN reveals a 63 mV thermodynamic preference for N₂ reduction to NH₃ over H₂ production. Combined with knowledge of the kinetics of electrode-catalyzed H₂ evolution, a wide working region is identified to guide future electrocatalytic studies.