Fig. 6: Mechanism of nano-confinement effect in urea electrosynthesis.

a Urea yield rate in 0.1 M KNO₃ with catholyte media of H₂O and D₂O, respectively. The data are presented as mean values ± s.d. (n ≥ 3). b A schematic diagram of proton sources and destinations in urea electrosynthesis with KNO₃ and CO₂ is presented, where pore size determines urea selectivity by modulating the binding pathways of *H. c, EPR spectra of three catalysts with different nano-confinement scales using DMPO as the radical trapping reagent in 0.1 M KNO₃ under CO₂, and of the CuRu/MCHS-7 catalyst in 0.1 M KNO₃ under Ar. d Computed concentration and distribution of species: CO₂, NO₃^–, *H and urea concentration. Three-dimensional MCHS with precisely tuned pore diameters (4, 7, and 11 nm) and a 2 nm catalytic layer were constructed to match with the structural features of CuRu/MCHS catalysts (Figs. S34, S35 and Table S13). The diffusion coefficients of solution species are provided in the Fig. S36 and Table S14. The computational model was fully immersed in the initial reactant environment (CO₂/NO₃^–), maintaining consistent local concentrations across simulations. Colour scale, in mol L^–1. e The production of *H, *NO, H₂ and urea as a function of pore size. f Calculated C_Interior/C_Exterior of CO₂, NO₃^–, *H, *NO and urea in pore channel of the three catalysts. g Schematic diagram of the diffusion mechanism of C–N coupling on CuRu/MCHS catalyst and diffusion variations across three pore sizes.