  While low-cost natural gas remains abundant, the energy content of this fuel
can be utilized without greenhouse gas emissions through the production of
molecular hydrogen and solid carbon via methane pyrolysis. In the absence of a
carbon tax, methane pyrolysis is not economically competitive with current
hydrogen production methods unless the carbon byproducts can be valorized. In
this work, we assess the viability of the carbon byproduct produced from
methane pyrolysis in molten salts as high-value-added anode or conductive
additive for secondary Li-ion and Na-ion batteries. Raman characterization and
electrochemical differential capacity analysis demonstrate that the use of
molten salt mixtures with catalytically-active FeCl3- or MnCl2 result in more
graphitic carbon co-products. These graphitic carbons exhibit the best
electrochemical performance (up to 272 mAh/g of reversible capacity) when used
as Li-ion anodes. For all carbon samples studied here, disordered carbon
domains and retained salt species trapped and/or intercalated into the carbon
structure were identified by X-ray photoelectron and multinuclear solid-state
nuclear magnetic resonance spectroscopy. The latter lead to reduced
electrochemical activity and reversibility, and poorer rate performance
compared to commercial carbon anodes. The electronic conductivity of the
pyrolyzed carbons is found to be highly dependent on their purity, with the
purest carbon exhibiting an electronic conductivity nearly on par with that of
commercial carbon additives. These findings suggest that more effective removal
of the salt catalyst could enable applications of these carbons in secondary
batteries, providing a financial incentive for the large-scale implementation
of methane pyrolysis for low-carbon hydrogen production.
