[Hypx_]

As I understand, the byproduct of a carbon-air cell is carbon dioxide. This doesn't mean such a cell can't make sense, but it has some fundamental downsides that are hard to mitigate. It will requirement a very specific form of carbon capture where the result is hard carbon for such cells to make sense.

That might be partially true. Especially in the case of hydrogen-to-ammonia where the process of conversion is much more straightforward. However, storing hydrogen in salt caverns for years at a time is already proven. So further conversion steps might not prove any real value except where volume of storage is important.