Periodic surface defects on ultrathin Cu nanodisks for nitrate electroreduction to produce ammonia efficiently

Abstract

Ammonia has wide applications in agriculture and chemistry industry, it is also seeing important use in the future as a carbon-free energy carrier to decarbonize the global economy.[1] While presently, most ammonia is manufactured by the Habor-Bosch process which uses ultrapure nitrogen and hydrogen gas as the feedstock. Both gases are produced with high cost, especially the hydrogen production that emits a large amount of greenhouse gas and inputs high energy. [2] With the effort made by researchers, ammonia can be synthesized via many clean and feasible routes, with the electrochemical nitrate reduction (ENR) more interesting that transforms the toxic nitrate ions to value-added ammonia rather than useless N₂. Efficient ENR with high ammonia yield depends on catalysts. Recent studies have shown that some metal oxides like TiO₂, and metals like Pd, Cu are active to the ENR to produce ammonia. [3-5] Among them, Cu shows great potential due to high intrinsic activity, selectivity, and low cost. However, the development of nanostructured Cu has been challenged, especially for those with well-defined crystalline faces. [6] Cu triangle sheets have been reported to catalyse CO₂ to hydrocarbons, while the dimension of the Cu triangles lie in the microscale. Uniform and stable Cu nanodisks with the size down to 100 nm is still not available.

Here we develop a facile method for the synthesis of stable and uniform freestanding Cu(111) nanodisks with the size less than 80 nm. The thickness of the Cu(111) nanodisks is 7 nm. The as-synthesized Cu nanodisks electrocatalytically transform nitrates to ammonia with the Faradaic efficiency as high as 78.9%. The highest ammonia yield rate reaches 2161 μg mg^-1_cat . The stability is ascribed to the ultrathin oxidized surface. The high activity originates from the reconstructed periodic Cu atoms after the removal of surface oxygen, demonstrated by theoretical calculations.