Copper confined in vesicle-like BCN cavity promotes electrochemical reduction of nitrate to ammonia in water

Electrochemical methods to convert high-concentration nitrates present in sewage into high-value-added ammonia do not just alleviate the problem of environmental pollution but provide less energy-intensive alternatives to the Haber–Bosch process. In this work, a metal–boron organic polymer precursor was annealed at high temperature to obtain copper nanoparticles encapsulated in a vesicle-like BCN matrix (BCN@Cu). In the electrochemical reduction of nitrate (E-NIRR), this species exhibited excellent catalytic activity. Specifically, the ammonia yields of BCN@Cu under applied potentials of −0.3 V, −0.4 V, −0.5 V, and −0.6 V versus the reversible hydrogen electrode were 271.1 μmol h−1 mgcat.−1, 354.8 μmol h−1 mgcat.−1, 435.6 μmol h−1 mgcat.−1, and 576.2 μmol h−1 mgcat.−1, respectively, and the corresponding Faraday efficiencies were 86.3%, 88.0%, 89.3%, and 88.9%. Isotope labeling experiments with 15NO3− confirmed that the detected ammonia had originated from the electrochemical reduction of NO3− on the catalyst surface. Moreover, the E-NIRR activity of BCN@Cu remained high even after ten consecutive uses or 20 h of continuous operation, suggesting the practicality of the industrial application of BCN@Cu. The presence of copper was a key in determining BCN@Cu’s E-NIRR activity, while the presence of boron greatly improved its catalytic performance. Furthermore, density functional theory calculations indicated that BCN does not itself promote a reaction but rather assists the dispersion of Cu nanoparticles, thereby expanding the catalyst’s active surface area.