Although Cu possesses many unique advantages for electrocatalytic CO₂ reduction reaction (CO₂RR), it is not suitable for electrosynthesis of urea from CO₂ and NO₃⁻ because of high energy barriers for the formation of *COOH and *CO intermediates and C–N bonds. Herein, Cr₂O₃ nanoparticle (NP)/Cu nanosheet (NS) heterojunction electrocatalysts are reported for highly efficient electrocatalytic co-reduction of CO₂ and NO₃⁻ toward urea production. The strongly coupled heterostructure interface between Cr₂O₃ NPs and Cu NSs exhibits synergistic effect and optimizes the adsorption of intermediates. The resultant heterojunction electrocatalysts could achieve a high urea Faradaic efficiency (FE_urea) of 62% at an ultralow applied potential of 0 V vs. reversible hydrogen electrode (RHE), which is among the best results reported to date. Moreover, the electrocatalysts showed good recycling stability. The in-situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations revealed that the Cr₂O₃ NPs/Cu NSs heterostructure could not only reduce formation energy barriers of *COOH and *CO intermediates but also promote the coupling of *CO and *NH₂ to form C–N bonds, leading to a high FE_urea. This study demonstrates a heterojunction engineering strategy for the rational design of high-performance Cu-based electrocatalysts for urea generation.

Photoelectrochemical oxygen reduction reaction (ORR) toward H₂O₂ is highly desirable because only sunlight, O₂ and water are required in the process. However, the corresponding studies are still at its infancy because of the lack of suitable photocathodes, especially inorganic semiconductor photocathodes. In this work, we report CuBi₂O₄/CuO (CBO/CuO) heterojunction submicrocrystalline film photocathodes with efficient ORR activity for H₂O₂ production. The heterojunction film photocathodes were prepared through thermal evaporation of Cu and Bi metals under vacuum and subsequent annealing treatment. Furthermore, the doping of Gd³⁺ ions into CBO/CuO could significantly enhance the yield of H₂O₂. As a result, the concentration of H₂O₂ could reach 1.3 mM within 30 min, which is 6 times higher than that obtained on the pristine CBO/CuO photocathode. The theoretical calculations suggested that the introduction of Gd could adjust the electronic structure of CBO surface and promote 2e ORR pathway for selective production of H₂O₂. Our work not only provides a new strategy for designing highly efficient photocathode for H₂O₂ production but also will evoke more interest in photoelectrocatalytic ORR through inorganic semiconductor photocathode.