Hydrogen-bond mediated electrocatalytic nitrate reduction to ammonia over metal–organic frameworks with industrial current density

Abstract

Electrocatalytic reduction of the pollutant nitrate to ammonia (NO₃RR) using clean energy is being considered as a viable alternative to the Haber–Bosch process for producing industrially valuable ammonia. However, the multi-electron–proton transfer process of the NO₃RR to ammonia usually leads to poor selectivity and low current density, which still cannot meet the industrial requirements. Stabilizing the key intermediates during the reaction is particularly important for achieving high selectivity in the NO₃RR towards the production of NH₃. Herein, we develop a hydrogen bonding strategy to stabilize the key intermediates of the NO₃RR, which involves the design and synthesis of trinuclear copper(I) cluster-based metal–organic frameworks (MOFs). The methyl groups in the copper-based MOFs (DiMe-Cu₃-MOF) can regulate the electron density around the Cu₃ site and stabilize the key intermediates, *NO₂, through hydrogen bonding interaction with methyl groups. Thus, the DiMe-Cu₃-MOF electrocatalyst delivers a high NH₃ faradaic efficiency (95%) for the NO₃RR with a high ammonia production of 401 μg cm⁻² h⁻¹, and the partial current density of ammonia reaches an industrial level value of −950.6 mA cm⁻². Control experiments and theoretical studies demonstrated that the introduction of methyl groups into the DiMe-Cu₃-MOF can facilitate atypical hydrogen bonding with the intermediates of the NO₃RR and thus enhance the adsorption of intermediates and reduce the energy barrier of the conversion of NO₃⁻ to NH₃. This work highlights the vital importance of adjusting the microenvironment through hydrogen bonding for enhancing the NO₃RR performance.