Porous Two-dimensional Iron-Cyano Nanosheets for High-rate Electrochemical Nitrate Reduction.

Ammonia (NH3) is an essential ingredient in agriculture and a promising source of clean energy as a hydrogen carrier. The current major method for ammonia production, however, is the Haber-Bosch process that leads to massive energy consumption and severe environmental issues. Compared with nitrogen (N2) reduction, electrochemical nitrate reduction reaction (NO3RR), with a higher NH3 yield rate and Faradaic efficiency, holds promise for efficient NH3 production under ambient conditions. To achieve efficient NO3RR, electrocatalysts should exhibit high selectivity and Faradaic efficiency with a high NH3 yield rate. In this work, we developed two-dimensional (2D) iron-based cyano-coordination polymer nanosheets (Fe-cyano NSs) following in situ electrochemical treatment for high-rate NO3RR. Owing to the strong adsorption of nitrate on Fe0 active sites generated via topotactic conversion and in situ electroreduction, 2D Fe-cyano electrocatalyst exhibits high catalytic activity with a yield rate of 42.1 mg h-1 mgcat-1 and a Faradaic efficiency of over 90% toward NH3 production at -0.5 V (vs reversible hydrogen electrode, RHE). Further electrochemical characterizations revealed that superhydrophilic surface and enhanced electrochemical surface area of the 2D porous nanostructures also contributed to the high-rate NO3RR activity. An electrolyzer toward NO3RR and oxygen evolution reaction (OER) in a two-electrode configuration is constructed based on 2D Fe-cyano, achieving an energy efficiency of 26.2%. This work provides an alternative methodology toward topotactic conversion of transition metal nanosheets for NO3RR and reveals the often-overlooked contribution of hydrophilicity of the catalysts for high-rate electrocatalysis.