Not an expert in the field but the source below already claims 80% efficiency in the large scale electrolysis of water, so with little room for improvement.
This research is to find cheaper catalyst for electrolysis. The efficiency is good with expensive catalysts, but the cost of the catalysts makes scaling up hard. They want to find cheaper catalysts so that the use of electrolysis to produce hydrogen can be increased.
Even better would be to use this to efficiently produce CH4. Now the entire natural gas power infrastructure becomes a giant battery.
You'd still need to capture/sequester the CO2, which to me sounds much harder (or else you're only solving the "renewable" part rather than the global warming part). Water is nice because it naturally precipitates out of the atmosphere and returns to oceans/lakes/etc.
CO2 + 2H2O + energy -> CH4 + 2O2
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.
Right. My point is that, as with a battery, you'd need to store the product so you can reverse the reaction. Practically none of our current infrastructure for using CH4 is set up to store the output CO2 + H20; it just dumps it into the atmosphere.
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.
> My point is that, as with a battery, you'd need to store the product so you can reverse the reaction.
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.
It's not necessary, just very advisable to do it. The lower the concentration is, the harder it gets to extract something. Ideally CO_2 is captured where it is concentrated the most, i.e. right after generating it.
Most of the concentrated sources of CO2 are used to produce energy. The equation is like
We want energy. The problem is that we want energy.
something + O2 --> CO2 + energy
If you try to reverse it with
the problem is that you need more energy to reverse the reaction. If you stick one 100% carbon capture magical device to a power plant, you will not produce any energy in the power plant. Moreover you will need to buy a lot of energy.
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.
What i understood that methane production needs concentrated CO2. It is possible to get that from air, but it is energy inefficient.
For comparison, the efficiency of photosynthesis is something like 1% (probably 2% in C4) but the result is glucose instead of methane. Most organic reactions have a very low result, I'd be surprised if your reaction has more than a 10%. Most of the time the molecules have another opinion about how to combine and they don't produce what you want.
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.
If we want to do combustion in a gas turbine as a "battery" for the grid, then electrolysis -> hydrogen -> hydrogen burning gas turbine has a ridiculously superior efficiency and CAPEX compared to going via methane. All the big players for stationary gas turbines are bringing hydrogen as a fuel forward at a rapid rate.
I believe the current plan for the first full-scale hydrogen gas turbine (400+ MW) is the Magnum powerplant in 2025-ish.
> Most of the time the molecules have another opinion about how to combine and they don't produce what you want.
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.
My handwave chemistry is not strong enough to be sure, but I expect that this will form also a small amount of ethane (that is not a problem) and also some soot like goo that will clog the battery and be difficult to burn cleanly anyway.
Here is a 30MW project in Germany, aiming for 700MW in 2030 to make hydrogen > methane > kerosene from renewable surplus electric and captured CO2 
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.
Unfortunately, CH4 is a potent greenhouse gas, and developing that industry further wouldn't help with the leaks (around 10% of production, most during transport, IIRC).
Totally solvable. We should have cracked down on leaks already, and have satellites coming on line to do so.
It seems like they "just" created better tooling for searching for reaction catalysts, rather than actually found a good new catalyst.
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.
This century will be defined by flipping competition upside down to a race for efficiency and not power increase.
They could call this new efficiency savings negawatts ? maybe write a book about it called "Reinventing Fire" .. could be big
would be nice to have a link to the paper. Anyone?
My understanding is that a key reason cheap electrolysis is important is that the hydrogen it produces can be substituted for fossil fuels in many industrial processes, thereby eliminating an important source of co2 emissions.