Electrocatalytic Reduction of Dinitrogen to Ammonia with Water as Proton and Electron Donor Catalyzed by a Combination of a Tri-ironoxotungstate and an Alkali Metal Cation.

The electrification of ammonia synthesis is a key target for its decentralization and lowering impact on atmospheric CO 2 concentrations. The lithium metal electrochemical reduction of nitrogen to ammonia using alcohols as proton/electron donors is an important advance, but requires rather negative potentials, and anhydrous conditions. Organometallic electrocatalysts using redox mediators have also been reported. Water as a proton and electron donor has not been demonstrated in these reactions. Here a N 2 to NH 3 electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe 3 W 9 }, is presented. The catalyst requires the presence of Li + or Na + cations as promoters through their binding to {SiFe 3 W 9 }. Experimental NMR, CV and UV-vis measurements, and MD simulations and DFT calculations show that the alkali metal cation enables the decrease of the redox potential of {SiFe 3 W 9 } allowing the activation of N 2 . Controlled potential electrolysis with highly purified 14 N 2 and 15 N 2 ruled out formation of NH 3 from contaminants. Importantly, using Na + cations and polyethylene glycol as solvent, the anodic oxidation of water can be used as a proton and electron donor for the formation of NH 3 . In an undivided cell electrolyzer under 1 bar N 2 , rates of NH 3 formation of 1.15 nmol sec -1 cm -2 , faradaic efficiencies of ∼25%, 5.1 equiv of NH 3 per equivalent of {SiFe 3 W 9 } in 10 h, and a TOF of 64 s -1 were obtained. The future development of suitable high surface area cathodes and well solubilized N 2 and the use of H 2 O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.