Grain Refinement and Porous Architecture Boost Charge Transport and Rate Kinetics in MOF-Derived CoP@C for...

Metal phosphides have emerged as competitive candidates for supercapacitors owing to their metallic conductivity and ultrahigh theoretical capacitance. Nevertheless, their practical implementation is hindered by sluggish reaction kinetics and structural degradation during cycling. Herein, we propose a synergistic strategy integrating grain refinement and hierarchical porosity design through pyrolysis of ZIF-67, fabricating structurally optimized Co₃O₄@C and CoP@C electrode materials. The carbon skeleton inherits the hierarchical porous polyhedral structure of ZIF-67, while the confinement effect of the carbon matrix ensures the formation of ultrasmall Co₃O₄/CoP nanoparticles. This architecture offers dual advantages: (1) interconnected carbon channels facilitate rapid ion/charge transport, and (2) ultrasmall nanoparticles maximize active sites for redox reactions. Notably, the CoP@C electrode delivers remarkable areal capacitance (343.1 mC cm^-2/902.0 mF cm^-2 at 1 mA cm^-2) and rate retention (87.8% at 10 mA cm^-2), surpassing the Co₃O₄@C counterpart by 1.5-fold and 1.2-fold, respectively. Advanced Amplitude modulation-Kelvin probe force microscopy (AM-KPFM) analysis reveals that CoP@C possesses a reduced surface potential (ΔΨ = 11.6 mV) in CoP@C compared to that in Co₃O₄@C (20.5 mV), displaying faster interfacial charge transfer kinetics. Density functional theory (DFT) calculations further elucidate interfacial electron redistribution, where electron donation from carbon to Co₃O₄ or CoP. CoP@C behaves with a lower work function (4.8 eV) and closer d-band center positions (-1.47 eV vs Fermi level) than Co₃O₄@C (5.4, -1.67 eV), thereby optimizing the adsorption energetics of electrolyte ions and reducing charge transfer resistance. Assembled asymmetric supercapacitors (CoP@C//AC) achieve a high energy density of 46.8 Wh kg^-1 at 1696.7 W kg^-1, with ultralong cyclability (91.3% retention after 10,000 cycles), surpassing most reported metal phosphide-based devices. This work highlights the synergy between grain refinement and porous architecture engineering in enhancing charge transport and rate kinetics, providing a viable strategy to improve the power density and cycling stability of metal phosphide-based electrodes for advanced energy storage devices.