Comparison of Methane Cracking by Pulse Dielectric Barrier Discharge and Spark Discharge: Interaction between Plasma Energy Density, Hydrogen Production Rate, and Carbon Morphology

To overcome the efficiency and coking bottlenecks of conventional methane-to-hydrogen technologies, this study systematically compares pulsed Dielectric Barrier Discharge (DBD) and Spark Discharge (SD) plasma configurations. We quantified the impacts of electrical modulation and reactor geometry on methane cracking efficiency and product selectivity via in situ optical emission spectroscopy (OES), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Optimal methane conversion (82%) and ECE (60%) were observed in the DBD system at 25 kV and 500–700 Hz. In comparison, the SD configuration leverages higher energy density at 150 kV to drive deep cracking pathways, yielding a hydrogen selectivity of nearly 98%. The distinct performance stems from divergent radical evolution: OES spectra revealed that SD promotes Hα and C 2 emissions ─ indicators of extensive C–H bond cleavage─whereas DBD was restricted to CH-dominated primary activation. This shift in cracking depth directly dictates the solid-phase morphology, where SD facilitates the growth of graphitic fibrous or tubular carbon, contrasting with the defective amorphous carbon deposited in DBD. These findings offer a mechanistic basis for tailoring plasma modes to specific hydrogen and carbon product requirements.