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 CO₂ 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 N₂ to NH₃ electrocatalytic reduction using an inorganic molecular catalyst, a tri-iron substituted polyoxotungstate, {SiFe₃W₉}, is presented. The catalyst requires the presence of Li⁺ or Na⁺ cations as promoters through their binding to {SiFe₃W₉}. 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 {SiFe₃W₉} allowing the activation of N₂. Controlled potential electrolysis with highly purified ¹⁴N₂ and ¹⁵N₂ ruled out formation of NH₃ 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 NH₃. In an undivided cell electrolyzer under 1 bar N₂, rates of NH₃ formation of 1.15 nmol sec^-1 cm^-2, faradaic efficiencies of ∼25%, 5.1 equiv of NH₃ per equivalent of {SiFe₃W₉} in 10 h, and a TOF of 64 s^-1 were obtained. The future development of suitable high surface area cathodes and well solubilized N₂ and the use of H₂O as the reducing agent are important keys to the future deployment of an electrocatalytic ammonia synthesis.