Abstract: To construct an efficient metal-free catalytic system, a series of KOH-modified activated carbon-based composites loaded with carbon black were developed, and the effects of KOH concentration, de-potassium treatment, and carbon-black loading time on methane decomposition performance and carbon-product morphology were systematically investigated. The results show that KOH modification markedly enhances catalytic activity. Although the specific surface area increases after de-potassium treatment, the methane conversion decreases significantly, indicating that residual K species play a key promotional role in CH4 activation. Carbon-black loading slightly reduces the initial activity but still yields superior overall performance compared with the unmodified sample. In particular, the catalyst with 6 h carbon-black loading achieves a methane conversion of 30% at 50 min, higher than the 27% obtained with the 2 h loading. After reaction, carbon deposits evolve from bulk amorphous structures to tubular carbon nanomaterial–like products with diameters of approximately 50 nm. This study elucidates a synergistic mechanism of “KOH etching – K-species promotion – carbon-black-enhanced conductivity” in tuning pore structure and carbon-deposition morphology, enabling simultaneous methane-to-hydrogen conversion and co-production of high-value carbon materials.