Save Nic's Finger, page-51

Energy medium is something that allows transfer of energy from a to b.
Like you put kinetic energy into wave and you get kinetic energy back in form of wave.
You put electricity into wire and you get electricity back on the other end.
Or you put electricity into lithium battery and you get electricity back.
So lithium battery is just an energy medium.

Hydrogen is energy source as you have to convert potential energy into usable form of energy (electricity) through hydrogen battery cell. Just like you have to burn fuel to get usable form of energy from it.

It's not really important but interesting for discussion, no need to get offended