This might be a dumb question: what if you could build a reactor that electrolyzes water but then also converts it into methane? If the electricity used for the process was derived from renewable sources, the process would still be carbon neutral, yeah?

I'm assuming the carbon itself could be harvest through non-fossil fuel means (biological perhaps? burn cellulose?).

Pilot “methanation” plants are already producing synthetic methane from hydrogen and CO2:

https://www.uniper.energy/news/methanation-plant-in-falkenha...