Ultrathin nickel terephthalate nanosheet three-dimensional aggregates with disordered layers for highly efficient overall urea electrolysis

The controlled synthesis of ultrathin metal-organic framework (MOF) nanosheets and the rational design of three-dimensional (3D) aggregates of these nanosheets are vital for electrochemical applications. Herein, ultrathin nickel terephthalate nanosheet 3D aggregates were synthesized by a facile one-pot hydrothermal method and employed as electrocatalysts for overall urea electrolysis. Surprisingly, the thickness of the nanosheets could be controlled by varying the synthesis time. A low-dose high-resolution transmission electron microscopy (TEM) technique with spherical aberration corrected TEM was utilized to visualize the crystalline structures of the ultrathin MOF nanosheets. Interestingly, ~5 nm thick disordered layers were observed at the edges of the ultrathin nanosheets, plausibly induced by vacancies that could not be identified with conventional TEM. The 3D aggregates of the ultrathin nanosheets with disordered layers required an ultralow potential of 1.381 V for urea oxidation with a current density of 10 mA cm−2, which is 186 mV lower than that for the oxygen evolution reaction, and a potential of 1.52 V for overall urea electrolysis, with outstanding long-term durability. The amazing catalytic properties may be attributed to the adequate unsaturated coordination of nickel on the surface of the disordered layers, which guarantees numerous active sites; these results are consistent with the high-resolution TEM observations.