If there is a good, easy way to generate methane using atmospheric CO2, then, we could have a chemical battery in the form of methane, which is far easier to handle than Hydrogen.
But I wonder what the round trip efficiency of such a system would be. Current lithium batteries have it at around 80%
The key feature of hydrogen as energy storage wouldn't necessarily be round trip efficiency but cost effectiveness (compared to batteries) of long-term storage over months.
Think about transporting peaks of renewables electricity generation that are not economically usable at the time when they're produced to times when renewables produce too little to meet demand. (Mostly in regions where generation depends significantly on seasons.)
But I wonder what the round trip efficiency of such a system would be. Current lithium batteries have it at around 80%