Enhancing Tandem Electrochemical Nitrate Reduction to Ammonia Through Cu-MOF/Co-MOF@NF Composite Nanoflower

Abstract

Electrocatalytic nitrate reduction (eNO₃RR) has gained widespread application as a green and efficient technique for ammonia synthesis. However, the accumulation of nitrite (NO₂^–) during the electrochemical reduction process significantly hampers the efficiency of converting nitrate (NO₃^–) into ammonia (NH₃). In this study, a Cu-MOF/Co-MOF@NF composite catalyst was developed on a nickel foam substrate using a hydrothermal method. The catalyst is composed of interconnected Cu-MOF@NF and Co-MOF@NF nanosheets, forming a flower-like nanostructure. X-ray photoelectron spectroscopy (XPS) reveals there is an electronic transfer between copper (Cu) and cobalt (Co) sites at the interface of Cu-MOF/Co-MOF@NF, thereby accelerating both the adsorption and reduction of NO₃^– at Cu sites and enhancing selective conversion to NH₃ at Co sites. At a potential of – 0.5 V vs RHE, the Cu-MOF/Co-MOF@NF exhibits a NO₃^– conversion to NH₃ that surpasses those of the Cu-MOF@NF and Co-MOF@NF by 8.5 and 39.6%, respectively, achieving NH₃ selectivity and yield of 93.3% and 318.5 μg·h^–1·cm^–2. In addition, Cu-MOF/Co-MOF@NF possesses exceptional catalytic activity and durability. Electron Paramagnetic Resonance (EPR) experiments reveal a significant amount of *H generated at the cathode interface. In-situ differential electrochemical mass spectrometry (DEMS) detection shows that the reduction pathway for NO₃^– over the Cu-MOF/Co-MOF@NF is as follows: *NO₃^– → *NO₂^– → *NO → *N → *NH → *NH₂ → *NH₃.