p-d orbital hybridization enhanced MOF-on-MOF derived CoP/Fe2P bimetallic phosphide electrocatalyst for stable water electrolysis

The design and fabrication of ordered epitaxial MOF-on-MOF heterostructures as highly efficient electrocatalysts for water splitting is crucial but still challenging. In this study, a simple coordination-driven self-assembly method is used to fabricate controllable MOF-on-MOF multiscale heterostructures, where triangular host MOF (ZIF-67) nanosheets undergo in situ epitaxial growth to form uniform orthogonal guest MOF (CoFe PBA) nanosheets. Phosphorus (P) is further introduced in situ to fabricate CoP and Fe₂P heterostructured nanosheets (CoFe-P-NS), which exhibit excellent bifunctional electrocatalytic performance due to the enhancement of intrinsic electrocatalytic activity by p-d orbital hybridization. Specifically, the CoFe-P-NS requires low overpotential of 259 and 307 mV to reach 500 mA cm⁻² for HER and OER, respectively. Remarkably, the assembled electrolysis cell maintained a large current density of 300 mA cm⁻² for over 360 h with negligible voltage increase during alkaline seawater electrolysis. Experiments and theoretical calculations show that the synergistic catalytic activity of bimetallic phosphides arises from p-d orbital hybridization, where the CoP-P sites enhance HER by optimizing H* adsorption in the Volmer-Heyrovsky steps, while the Fe₂P-Fe sites accelerate OER by lowering the energy barrier of the rate-determining step from O* to OOH*. This study provides valuable insights into the design of a controllable MOF-on-MOF-based electrocatalyst toward alkaline seawater splitting.