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.