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