Converting NO₃^- from wastewater into NH₃ offers an effective approach of wastewater treatment and holds promise as a sustainable method for NH₃ synthesis. The diverse adsorption configurations of nitrogen-containing intermediates in the NO₃^- electroreduction process pose a challenge. Besides, Cu-based electrocatalyst are advantageous for NO₃^- adsorption, but the excessive accumulation of nitrite (NO₂^-) which would result in the rapid deactivation of catalysts and sluggish kinetics of subsequent hydrogenation steps is a problem. 

To overcome these limitations, researchers designed a tandem electrocatalyst by combining Cu single atoms anchored on N-doped carbon with adjacent Co₃O₄ nanosheets (denoted as Co₃O₄/Cu₁-N-C). This combination leverages the strengths of both the ability of Cu to adsorb NO₃^- and the ability of Co₃O₄ to adsorb NO₂^-. This dual-function catalyst aims to optimize the binding energies of intermediates, thereby facilitating the electroreduction process from NO₃^- to NH₃ more efficiently. 

Besides, researchers synthesized the Co₃O₄/Cu₁-N-C catalyst through a series of steps, including the pyrolysis of Cu-doped ZIF-8 to obtain Cu single atoms on N-doped carbon, followed by the deposition of Co₃O₄ nanosheets. They characterized the structure and composition of the catalyst using techniques such as high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray absorption near edge structure (XANES) spectroscopy. The analyses confirmed the combination of Cu single atoms and Co₃O₄ nanosheets, as well as the uniform distribution of the catalytic centers. 

In addition, performance testing of the catalysts was conducted in a three-electrode H-type cell with the concentration of NH₃ product quantified using the indophenol blue method. The test revealed that Co₃O₄/Cu₁-N-C achieved a NH₃ production rate of 114.0 mg_NH3h^-1cm^-2 in the NO₃^- electroreduction reaction, which was 2.2 times and 3.6 times as high as those of Cu₁-N-C and Co₃O_4, respectively. Mechanistic investigations showed that Co₃O₄ effectively regulated the adsorption configuration of NO₂^- and enhanced its binding, thereby accelerating the overall electroreduction process from NO₃^- to NH₃. 

This study highlights a novel approach to addressing the limitations of single catalysts in NO₃^- electroreduction by using a tandem catalyst system.