The best option is to use a similar process to HB, but using “green” H2, so yet more inefficiencies from “excessive” renewable electricity generation.
https://royalsociety.org/topics-policy/projects/low-carbon-e....
https://royalsociety.org/topics-policy/projects/low-carbon-e....
What about the energy to run the HB reaction? It's super high temp and high pressure. Is there a zero carbon way to make that?