The disclosure describes a system for generating hydrogen gas from a hydrocarbon through pyrolysis with reduced soot formation and increased carbon loading. The system includes a pyrolysis reactor configured to generate the hydrogen gas from the hydrocarbon through pyrolysis. The pyrolysis reactor includes one or more fibrous substrates configured to provide a deposition surface for carbon generated from the pyrolysis of the hydrocarbon. Each fibrous substrate has an effective void fraction between 40% and 95%, and includes a plurality of fibers configured to maintain chemical and structural stability between about 850° C. and about 1300° C. The one or more fibrous substrates may have a relatively high surface area to fiber volume of the plurality of fibers.