In Situ Construction of Carbon Nanotubes Encapsulating Co ₂ P/CoP Bifunctional Catalyst by Fe-Induced Transformation of ZIF-8@ZIF-67 for Efficient Overall Water Splitting

Designing nonprecious metal catalysts with outstanding bifunctional activity and stability is crucial for advancing green hydrogen production technologies. In this study, a composite structure catalyst of carbon nanotubes-encapsulated Fe-Co₂P/CoP N-doped carbon polyhedrals (Fe-Co₂P/CoP@CNT NCPs) was successfully prepared by calcination-phosphorization treatment of an Fe-doped core-shell zeolitic imidazolate framework (ZIF) precursor. Trace Fe acts as an electron mediator that effectively regulates the oxidation-reduction states of Co during the phosphorization, inducing the formation of heterogeneous CoP and Co₂P phases, optimizing the electronic environment, and promoting the exposure of abundant active sites. The in situ self-assembled composite structure creates rich heterogeneous interfaces and efficient electron transfer channels to accelerate electron transfer and provides structural protection to the active species, thereby synergistically enhancing catalytic activity and stability. The resultant Fe-Co₂P/CoP@CNT NCPs catalyst shows excellent bifunctional performance with the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of 138.0 (267.5) and 296.7 (385.6) mV at current densities of 10 (200) mA cm^-2 and a stability of up to 110 h. Density Functional Theory (DFT) calculations reveal that the Fe-Co₂P/CoP@CNT NCPs catalyst exhibits the optimal *H adsorption-desorption performance, the lowest energy barrier for the rate-determining step of the OER, and significant charge redistribution at the interface, activating more Co sites. The positive shift of the d-band center and enhanced density of states further confirm the improved intermediate adsorption behavior and increased free electron density, significantly enhancing its intrinsic catalytic activity. This study offers valuable new insights into the development of ZIF-based catalysts with high-efficiency bifunctional activity and long-term stability.