I saw something recently which is the same principle as molten salt, except the medium never melts, is stable, and… iirc is pretty much just carbon. I was trying to look for a source and all I could find was the equally-encouraging news that the humble red brick has now been turned into a capacitor (with a wall of bricks being able to perform as a supercapacitor)

https://www.sciencedaily.com/releases/2020/08/200811120154.htm