Urea (CO(NH₂)₂) has been applied both in agricultural and pharmaceutical field. The widely used Bosch-Meiser process has high energy consumption and CO₂ emission. Therefore, it is imperative to explore energy-saving and economical routes for urea synthesis under mild conditions.

However, the electrosynthesis of urea with CO₂ and NO₃- under ambient conditions is an efficient way, but it is far from application. This is because the key step needs an efficient electrocatalyst enabling adsorption and activation of NO₃- and CO₂ to accomplish the C-N coupling.

Researchers from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has now developed a liquid-phase laser irradiation route to fabricate symbiotic carbon encapsulated amorphous iron (Fe(a)@C) and iron oxide nanoparticles (Fe₃O₄ NPs) on carbon nanotubes (denoted as Fe(a)@C-Fe₃O₄/CNTs).

The as-fabricated Fe(a)@C-Fe₃O₄/CNTs contained two Fe-based active components, namely, Fe@C NPs with the particle sizes of 10~20 nm and Fe₃O₄ NPs with the particle sizes of 1~5 nm.

The presence of two different structural units in Fe(a)@C-Fe₃O₄/CNTs made it possible to synergistically electrocatalytic activate CO₂ and NO₃- to realize the C-N coupling for urea synthesis.

As expected, Fe(a)@C-Fe₃O₄/CNTs exhibited superior activity toward the electrocatalytic coupling of CO₂ and NO₃- for urea synthesis, affording a urea yield of 1341.3±112.6 μg h-1 mgcat-1 and a faradic efficiency of 16.5±6.1% at -0.65 V (vs. RHE) in 0.1 M KNO₃ electrolyte.

Both experimental and theoretical results unveiled that Fe(a)@C was mainly responsible for the electrocatalytic reduction of NO3- to form *NH₂ intermediates, while Fe₃O₄ was more beneficial for the electrocatalytic reduction of CO₂ to form *CO intermediates.

The synergistically catalytic effect contributes the excellent electrocatalytic performance of urea synthesis at ambient conditions.

This work was financially supported by the Natural Science Foundation of China.

(a). Schematic illustration of the synthetic process of Fe(a)@C-Fe₃O₄/CNTs; (b). Low- and (c) high-magnification TEM images of Fe(a)@C-Fe₃O₄/CNTs. HRTEM images of Fe(a)@C (d) (from the red dashed box in c) and Fe₃O₄ NPs (e) (from the yellow dashed box in c); (f), (g) HAADF-STEM images and corresponding elemental mapping images of Fe₃O₄ and Fe(a)@C. (Image by GENG Jing)