Coordination Symmetry Breaking of Single-Atom Catalysts for Robust and Efficient Nitrate Electroreduction to Ammonia.

Nitrate electrocatalytic reduction (NO3 RR) for ammonia production is a promising strategy to close the N-cycle from nitration contamination, as well as an alternative to the Haber-Bosch process with less energy consumption and carbon dioxide release. However, current long-term stability of NO3 RR catalysts is usually tens of hours, far from the requirements for industrialization. Here, symmetry-broken Cusingle-atom catalysts are designed, and the catalytic activity is retained after operation for more than 2000 h, while an average ammonia production rate of 27.84 mg h-1 cm-2 at an industrial level current density of 366 mA cm-2 is achieved, obtaining a good balance between catalytic activity and long-term stability. Coordination symmetry breaking is achieved by embedding one Cu atom in graphene nanosheets with two N and two O atoms in the cis-configuration, effectively lowering the coordination symmetry, rendering the active site more polar, and accumulating more NO3 - near the electrocatalyst surface. Additionally, the cis-coordination splits the Cu 3d orbitals, which generates an orbital-symmetry-matched π-complex of the key intermediate *ONH and reduces the energy barrier, compared with the σ-complex generated with other catalysts. These results reveal the critical role of coordination symmetry in single-atom catalysts, prompting the design of more coordination-symmetry-broken electrocatalysts toward possible industrialization.

Built-in Electric Field Triggered Interfacial Accumulation Effect for Efficient Nitrate Removal at Ultra-Low Concentration and Electroreduction to Ammonia.

A built-in electric field in electrocatalyst can significantly accumulate higher concentration of NO3 - ions near electrocatalyst surface region, thus facilitating mass transfer for efficient nitrate removal at ultra-low concentration and electroreduction reaction (NO3 RR). A model electrocatalyst is created by stacking CuCl (111) and rutile TiO2 (110) layers together, in which a built-in electric field induced from the electron transfer from TiO2 to CuCl (CuCl_BEF) is successfully formed . This built-in electric field effectively triggers interfacial accumulation of NO3 - ions around the electrocatalyst. The electric field also raises the energy of key reaction intermediate *NO to lower the energy barrier of the rate determining step. A NH3 product selectivity of 98.6 %, a low NO2 - production of <0.6 %, and mass-specific ammonia production rate of 64.4 h-1 is achieved, which are all the best among studies reported at 100 mg L-1 of nitrate concentration to date.