Researchers develop efficient tandem catalyst to enhance nitrate electroreduction to ammonia

A research team has designed a tandem catalyst to improve the electroreduction of nitrate into ammonia. By coupling Cu single atom catalysts with adjacent Co₃O₄ nanosheets, the team successfully regulated the adsorption energy of intermediates in the nitrate electroreduction process, promoting the synthesis of ammonia.

Their findings are published in Nature Communications. The team was led by Prof. Zeng Jie and Prof. Geng Zhigang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS).

Converting nitrate (NO₃^-) from wastewater into ammonia (NH₃) not only offers an effective approach to wastewater treatment but also holds promise as a sustainable method for ammonia synthesis. However, the diverse adsorption configurations of nitrogen-containing intermediates in the NO₃^- electroreduction process pose a challenge, making it difficult for a single catalyst to optimize adsorption simultaneously.

While Cu-based electrocatalysts are advantageous for NO₃^- adsorption, one key issue is the excessive accumulation of nitrite (NO₂^-) which would result in the rapid deactivation of catalysts and sluggish kinetics of subsequent hydrogenation steps.

To overcome these limitations, the 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 innovative combination leverages the strengths of both components: Cu's ability to adsorb NO₃^- and Co₃O₄'s ability 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.

Specifically, the 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. The structure and composition of the catalyst were characterized using various 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.

These analyses confirmed the successful combination of Cu single atoms and Co₃O₄ nanosheets, as well as the uniform distribution of the catalytic centers.

Finally, 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 an ammonia 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 that of Cu₁-N-C and Co₃O₄, respectively.

Mechanistic investigations showed that Co₃O₄ effectively regulates the adsorption configuration of NO₂^- and enhances its binding, thereby accelerating the overall electroreduction process from NO₃^- to NH₃.

This research highlights a novel approach to addressing the limitations of single catalysts in nitrate electroreduction by using a tandem catalyst system. It not only provides a deeper understanding of the catalytic mechanisms involved but also sets the stage for future developments in the design of advanced electrocatalysts for similar applications.