Multivariate covalent organic frameworks with tailored electrostatic potential promote nitrate electroreduction to ammonia in acid.

Abstract

The direct synthesis of ammonia from nitrate (NO₃^-) reduction in acid is a promising approach for industrialization. However, the difficulty arises from the intense competition with the inevitable hydrogen evolution reaction, which is favoured due to the overwhelming protons (H⁺). Here, we systematically explore and rationally optimize the microenvironment using multivariate covalent organic frameworks (COFs) as catalyst adlayers to promote the nitrate-to-ammonia conversion in acid. With the application of tailored positive electrostatic potential generated over the multivariate COFs, both the mass transfer of NO₃^- and H⁺ are regulated via appropriate electrostatic interactions, thus realizing the priority of NO₃RR with respect to HER or NO₃^--to-NO₂^-. As a result, an NH₃ yield rate of 11.01 mmol h^-1 mg^-1 and a corresponding Faradaic efficiency of 91.0% are attained, and solid NH₄Cl with a high purity of 96.2% is directly collected in acid; therefore, this method provides a practical approach for economically valorising wastewater into valuable ammonia.