Near-unity electrochemical conversion of nitrate to ammonia on crystalline nickel porphyrin-based covalent organic frameworks

Abstract

Electrochemical nitrate reduction, which has attracted rapidly increasing attention over recent years, can potentially enable the indirect fixation of atmospheric N₂ as well as the efficient removal of nitrate from industrial wastewater. It is, however, limited by the lack of efficient and low-cost electrocatalysts available so far. To address this challenge, we here demonstrate a two-dimensional nickel porphyrin-based covalent organic framework (COF) as a potential candidate for the first time. The product has a highly ordered molecular structure with abundant square-shaped nanopores. In neutral solution, the reduction of nitrate ions at different concentrations from ammonia is realized with a great selectivity of ∼90% under a mild overpotential, a remarkable production rate of up to 2.5 mg h⁻¹ cm⁻², a turnover frequency of up to 3.5 s⁻¹, and an intrinsic stability that is best delivered under pulse electrolysis. This cathodic reaction can also be coupled with the oxygen evolution reaction to enable full-cell electrolysis at high efficiency. Theoretical computations indicate that nickel centers can stably adsorb nitrate, and facilitate its subsequent reduction by lowering the energy barrier of the rate-determining step.