Breaking the continuous hydrogen adsorption active sites for boosted urea electrosynthesis

Abstract The efficient electrosynthesis of urea from CO2 and NO3− relies on the suppression of the competitive hydrogen evolution reaction (HER), especially the Tafel and Heyrovsky steps. We designed CuNi alloyed structure encapsulated into nitrogen doped carbon nanotubes (CuNi-NCNT) for superior urea electrosynthesis. The interstitial Cu atom breaks the successive hydrogen binding active sites, Ni atoms, as NO3− and CO2 strongly adsorb on Cu sites, hindering the Tafel step of HER. Besides, due to the steric effect, the Heyrovsky step is also restrained. Thus, the HER catalysis is well suppressed, leading to a high Faradaic efficiency (33.6%) of urea electrochemical synthesis for CuNi-NCNT at −0.4 V vs. reversible hydrogen electrode (RHE) associated with yielding rate of 5.3 mmol·h−1·gcat−1, boosted by factors of 6.7 and 4.4 with relative to Ni-NCNT and metallic Cu, respectively. The in-situ Raman spectroscopy and integrated crystal orbital Hamilton population (ICOHP) theoretical calculation indicate that Ni sites strongly adsorb hydrogen atom for hydrogeneration, while CO2 and NO3− are electrochemically bound with Cu atoms, synergistically contributing to the efficient urea formation. Moreover, the in-situ Raman spectroscopy suggests the reaction pathway of urea electrosynthesis via the formation of *NH2 and *CO species from NO3− and CO2. Consequently, a lowered energy barrier for C–N coupling (0.55 eV) can be achieved on CuNi compared to Cu(111) and Ni(111). This work offers significant insights into the critical roles of Cu and Ni in urea electrosynthesis and promotion in catalytic activity in Cu-system.