Abstract: To develop an efficient and scalable metal-free catalytic system for methane decomposition to hydrogen with the co-production of value-added solid carbon, this work uses activated carbon （AC） as the substrate to construct KOH-modified AC catalysts and their carbon-black （CB） loaded composites. The effects of KOH concentration, de-potassium treatment, and CB loading on methane decomposition performance and carbon product structure were systematically investigated. KOH activation was employed to tailor the pore structure and defect/edge sites, while ultrasonic dispersion was used to achieve CB loading and interfacial coupling; de-potassium control samples were further prepared via two successive ultrasonic washing steps. Catalytic methane decomposition was evaluated in a fixed-bed quartz-tube reactor at 950 ℃, and the structural evolution of catalysts and the morphology of deposited carbon were characterized by XRD, FTIR, BET, and SEM/TEM. The results show that KOH modification markedly enhances catalytic activity: the etching/activation process introduces abundant defects and edge sites and reconstructs the pore architecture, providing more accessible active sites for initial CH₄ adsorption and dissociation. After de-potassium treatment, although the specific surface area increases, the CH₄ conversion decreases significantly, indicating that residual K species contribute to catalysis beyond textural effects. These K-related species act as key chemical promoters for CH₄ activation by strengthening the surface polarity/basicity and modulating the local electronic environment, thereby facilitating the initial C–H bond cleavage. CB loading slightly lowers the initial activity but still delivers overall performance superior to that of the unmodified AC. In particular, the catalyst loaded with CB for 6 h achieves a CH₄ conversion of approximately 30% at 50 min, higher than 27% for the 2 h loading counterpart. Morphological analyses reveal that, after reaction, the composite catalyst preferentially forms filamentous/tubular carbon with a diameter of 50 nm, which partially replaces bulky amorphous deposits and mitigates rapid pore-mouth blockage. Overall, this study elucidates a synergistic “KOH etching–K-species promotion–carbon-black conduction” mechanism that couples pore-structure regulation with carbon-deposition morphology control, enabling the co-production of hydrogen and high-value carbon materials from methane.