Upcycling waste phosphate into CoP/FeCo2O4 heterostructured electrodes: a closed-loop strategy via in-situ phosphorus speciation.

The redox asymmetry conversion mechanism of adsorbed phosphate during thermal reconstruction remains elusive, limiting the exploitation of spent adsorbents' untapped potential. Here, we report a counterintuitive phenomenon: upon annealing phosphate-saturated CoFe layered double hydroxide (CoFe-LDH) in a reducing atmosphere, the phosphate does not decompose completely but selectively co-generates conductive cobalt phosphide (CoP) and active PO species, forming a self-supported CoP/FeCo2O4 heterostructured electrode. Specifically, the metallic CoP facilitates rapid electron transport, while the PO species synergistically enhance redox kinetics and interfacial compatibility. The pivotal finding is that this in-situ phosphorus speciation challenges the homogeneity assumption inherent to conventional doping paradigms. X-ray photoelectron spectroscopy (P 2p bimodal coexistence) and transmission electron microscopy reveal that the "partial reduction-partial retention" behavior of phosphate arises from synergistic regulation by interlayer confinement effects and metal-phosphate coordination structures within the LDH precursor. Benefiting from this unique configuration, the electrode achieves an exceptional areal capacitance of 3.71 F/cm2 at 1 mA/cm2 in 3 M KOH, with 69.74% capacitance retention after 5000 cycles. Density functional theory calculations demonstrate that the built-in electric field at the CoP/FeCo2O4 interface facilitates charge redistribution, reducing the interfacial charge-transfer resistance to 0.48 Ω·cm2. This study establishes "phosphorus speciation" as a new principle for concurrently optimizing electronic conductivity and redox activity, offering a paradigm for the function-oriented repurposing of phosphate-laden remediation materials into high-performance energy storage electrodes.