Can you make ethanol from thin air, such as with the Sabatier reaction? I like the fact that you can take electrical energy input, atmosphere, and get methane. This means I could convert excess solar into a mass and volume dense combustible, and then retrieve some of the store energy back later.

Personally I'm principally nerding out about energy storage for sailing cruising yachts. But I have by no means done an exhaustive search of the space.