Converting hazardous industrial waste into high-value energy materials represents a sustainable closed-loop strategy for environmental management. Herein, we report a “turn-waste-into-treasure” approach where spherical NiO serves as a highly efficient scavenger for toxic phosphine (PH₃) tail gas and is subsequently transformed in-situ into a robust electrocatalyst. The NiO precursor achieves a superior PH₃ removal efficiency of 99.2%. By precisely regulating the phosphidation kinetics driven by the captured PH₃, a unique Ni₂P/Ni₅P₄ heterostructure is constructed from the spent adsorbent. The resulting catalyst exhibits exceptional alkaline hydrogen evolution reaction (HER) performance, requiring an overpotential of only 158 mV to reach 10 mA·cm⁻² with a low Tafel slope of 86 mV·dec⁻¹. Density functional theory (DFT) calculations reveal that the interfacial built-in electric field and modulated electronic structure are critical: They not only optimize the Gibbs free energy of hydrogen adsorption but also thermodynamically promote water dissociation by enhancing the Lewis acidity of surface sites. This work demonstrates a scalable protocol for the dual-functional resource utilization of phosphorus waste, bridging the gap between industrial pollution control and green hydrogen production.