Discharging the battery is this reaction in reverse. It's net zero carbon. It's basically a rechargeable fuel air battery using incredibly abundant materials so it can scale.
If you want to reuse existing CH4 distribution pipelines, that's fine, but now you need to pipe back the CO2 to the plant that makes the CH4 in order to complete the cycle. That's much harder than pumping CH4 and would require a whole new parallel pipeline infrastructure that can transport highly pressurized CO2 back to the source plant. Alternatively, you could regenerate the CH4 on location, but then you need to pipe the energy in; in that case, you're not using CH4 to deliver energy at all.
You could make the CH4 from CO2 in coal or something, but then you're no longer carbon neutral (since that CO2 will end up in the atmosphere rather than the battery). You could also try carbon capture, but honestly thermodynamics suggests that that will never be a viable solution; far easier to order your coffee black than to unmix the cream from it.
My point is that, unless I'm misunderstanding something, existing distribution systems for natural gas are not useful for CH4-based battery storage.
No, you wouldn't, because CO2 is everywhere in the atmosphere. The methane making plant draws CO2 from the air around it; discharging batteries emit CO2 into the air around them. It's no different from plants taking in CO2 and animals breathing it out.
something + O2 --> CO2 + energy
If you try to reverse it with
CO2 + more energy --> something + O2
There are some process that emit a lot of CO2, but use it mostly for the chemical reactions and not to extract energy like in cement production  and somewhat in blast furnaces . You can capture the CO2 produced by them to get guilty free cement or steel, but is much much much more efficient to use the energy to replace the energy of a power plant that burns coal / oil / whatever. (Call it carbon emission trading.)
Once we close all the power plants that burn coal / oil / whatever, it can be useful to add carbon capture.
And there is also a problem with the combustion, in a gas turbine you have only an efficiency up to 65%. So it's a very ineffective battery.
I believe the current plan for the first full-scale hydrogen gas turbine (400+ MW) is the Magnum powerplant in 2025-ish.
For energy storage, this is, by definition, all of the time. The fact that the molecules have a different opinion of how they should combine is what gives potential energy to the form we produce in the first place.
This articles language is almost absurd. As though economic power-to-fuel technology is some futuristic dream - its treated more mysteriously than fusion research and deep AI.
> One beacon of hope is the idea that we could use renewable electricity to split water to produce energy-rich hydrogen, which could then be stored and used in fuel cells.
The storage technology which 100% renewable power requires is already available, and economic - there just hasn't been much value in storage yet, since there hasn't been much surplus. All research discoveries and "strides" are showing that its going to get very cheap - pretty much as soon as there is policy to build it, or simply enough wind turbines and solar to employ it.
Maybe this is a great stride; but it's difficult to tell from the article whether that's really the case or whether it's mostly fluff in a grad student's project. We'll know if/when they actually find something with it.