Controllable Synthesis of Porous Core–Shell NiCo Bimetallic Selenides for High-Performance Supercapacitors

Metal–organic framework (MOF)-derived electrode materials suffer from poor conductivity and structural instability. Herein, a porous core–shell NiCo bimetallic selenide (NiCo PBA–Se) is synthesized via a multistep liquid-phase transformation followed by low-temperature selenization. A Zeolitic Imidazolate Framework-67 (ZIF-67) shell is first epitaxially grown on a Ni-MOF core to construct a core–shell MOF hybrid as the precursor for the preparation of subsequent electrode materials. Unlike conventional methods that convert only the outer MOF layer, the precisely controlled ion-exchange process converts both the inner Ni-MOF and outer ZIF-67 into a NiCo-PBA precursor while preserving the core–shell spherical morphology. Gas-phase selenization at 350 °C yields the target product, which retains a three-dimensional porous spherical architecture composed of stacked ultrathin nanosheets. Ni and Co are uniformly incorporated into the selenide lattice, forming a single-phase (Ni, Co)Se2 solid solution that shortens ion transport paths and increases active sites. Kinetic analysis reveals a synergistic combination of diffusion-controlled and capacitance-controlled charge storage, enabling rapid charge/discharge. In three-electrode tests, NiCo PBA–Se delivers a specific capacitance of 1642 F·g–1 (738.9 C·g–1) at 1 A·g–1. The assembled asymmetric supercapacitor achieves an energy density of 42.52 Wh·kg–1 at 775 W·kg–1 and powers an electronic timer for 20 min. This work demonstrates the effectiveness of the MOF-on-MOF strategy with full conversion of both core and shell and simultaneous maintenance of the core–shell structure for constructing high-performance multimetal selenides.