Hydrogen (H₂) has been proposed to be an energy carrier that is free of greenhouse gas (GHG) emissions. Though currently used primarily for chemical synthesis (e.g. ammonia, petroleum reforming), GHG-free H₂ is a versatile chemical that has the potential to reduce or eliminate carbon dioxide (CO₂) emissions from power generation, manufacturing, and transportation. However, the current dominant method for H₂ production, steam reforming of natural gas, is CO₂-intensive, producing ~10 kg of CO₂ per kg of H₂. Thus, the development of efficient, scalable, and cost-effective technologies that decarbonize H₂ production will significantly reduce the greenhouse gas impact of multiple sectors. Methane pyrolysis is a promising method that produces H₂ and solid carbon, emitting <1 kg CO₂ per kg H₂ produced. Since there is no or minimal formation of carbon oxides, downstream product separation is simplified. Further, the solid carbon is a commercial byproduct that will reduce or offset the cost of GHG-free H₂ production, with the potential to achieve costs of ~$1/kg-H₂, which is the DOE target for the Hydrogen Earthshot.

One of the main challenges in catalytic methane pyrolysis is catalyst regeneration for long-term sustained activity and selectivity. The deactivation is mainly from the covering of the active catalyst sites by deposited solid carbon. To address this, we have developed a thermochemical cycle to weaken the bond between the catalyst particle and the pyrolytic carbon produced, so that the carbon can be dislodged from the catalyst surface. Specifically, we have investigated multiple gas-solid chemistries to maintain stable methane conversion over 10 cycles. The H₂ selectivity was sustained above 99% for all pyrolysis cycles as we avoided hydrocarbon biproduct. To understand regeneration mechanism, we characterized quality of the carbon nanotube and state of the catalyst by Raman spectroscopy, x-ray fluorescence spectroscopy, thermogravimetric analysis, electron microscopy, and x-ray diffraction.

We have conducted an initial technoeconomic analysis of our catalytic pyrolysis process. Even when assuming zero value from the solid carbon product, the H₂ production cost can be <$2/kg-H₂. When assuming 50% of the carbon produced can be sold at $1/kg-C, the H₂ production cost is completely offset, i.e. $0/kg H₂. This system level analysis indicates the technology is promising and provides a roadmap to further improve the catalytic methane pyrolysis process.

Edward Sun

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