Catalytic methane decomposition (CMD) is a promising low-carbon route for hydrogen production. However, its industrial viability is severely hindered by the co-disposal of solid carbon byproduct and spent catalysts. Herein, we propose an innovative valorization strategy that leverages the multi-dimensional in-situ forging effect inherent to the CMD process. This approach directly upcycles the spent nickel‑carbon catalyst (along with the deposited carbon) into a high-performance electrode substrate, eliminating energy-intensive separation or metal recovery processes. The synergistic interplay of high temperature, CH₄/H₂ reduction atmosphere, and nickel catalyst synchronously tailors the precursor's physicochemical properties via four critical dimensions: carbon structure evolution, defect engineering, interfacial reconstruction, and pore structure regulation. This forging process converts the waste into a graphitized 3D conductive network with abundant vacancy defects and embedded Ni active sites. Leveraging this unique substrate, a CoNi layered double hydroxide (LDH) composite electrode is fabricated via sequential electrodeposition, where strong interfacial electronic coupling between the residual Ni⁰ and the CoNi‑LDH further accelerates charge‑transfer kinetics. The electrode delivers a high specific capacity of 1253 C/g at 1 A/g, and an assembled asymmetric supercapacitor achieves an energy density of 93.5 Wh/kg at a power density of 849.4 W/kg, with 93.4% capacity retention after 5000 charge-discharge cycles. This work provides a sustainable, cost-effective route for circular utilization of CMD waste and offers new mechanistic insights into process‑intensified fabrication of advanced energy storage materials.

Methane decomposition; Spent catalyst upcycling; Supercapacitor; CoNi-layered double hydroxide; Electrodeposition