Near-Unity Electrochemical Conversion of Nitrate to Ammonia on Crystalline Nickel Porphyrin-based Covalent Organic Frameworks
Electrochemical nitrate reduction can potentially enable the indirect fixation of atmospheric N2, as well as the efficient nitrate removal from industrial wastewater, which has attracted rapidly increasing attention over recent years. 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 to ammonia is realized with great selectivity of ~90% under mild overpotential, remarkable production rates up to 2.5 mg h-1 cm-2, turnover frequency up to 3.5 s-1, and intrinsic stability that is best delivered under pulse electrolysis. This cathodic reaction can also be coupled with the oxygen evolution reaction to permit 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.