Split the water to make hydrogen, capture the carbon from atmospheric Co2.
Not real efficient, but energy once stored this way is good for almost forever.
If the energy used, was going to waste, I am not sure the efficiency matters that much.
Compared to batteries, Hydrocarbon storage is decades ahead.
if the storage method was carbon neutral, it may help sell the idea to the AGW crowd.
Once you can make methane, you can make almost any hydrocarbon, Gasoline, Diesel, Jet Fuel, ect..
So the answer to the question about what we do when we run out of oil, simple we make more!