Solid carbon co-products from hydrogen production by methane pyrolysis: Current understandings and recent progress

Methane pyrolysis is a potential low-emission hydrogen production method by splitting CH4 from natural gas or biogas into H2 gas and solid carbon (CH4 → 2H2 + C). The utilization of carbon co-products plays a critical role in the success of any commercial CH4 pyrolysis process. Several recent reviews on CH4 pyrolysis have focused on catalysts, reactor design, and chemical process development, with limited discussion on carbon co-products. This review summarizes recent research findings (i.e., from 2018 to 2023) related to solid carbon co-products from CH4 pyrolysis. The different types of CH4 pyrolysis methods are first categorized based on catalysts, heating methods, and energy sources. Next, the current understanding of the carbon material formation mechanism in CH4 pyrolysis is summarized. Afterward, carbon materials produced by various methods are compared, including non-catalytic gaseous pyrolysis and catalytic pyrolysis using metal catalysts, carbon catalysts, and molten metals/salts. Further, studies on the applications of these solid carbon co-products are discussed. Overall, diverse carbon materials are currently produced in CH4 pyrolysis with limited controls. Systematic studies of carbon materials' purification and applications are still limited. Last, we propose several future research topics critical to realizing CH4 pyrolysis as a low-emission H2 production process.