Where is electrostatic potential energy stored in a capacitor system?
I believe it would be a &quot;volume&quot; thing based on the wording of the question.   In parallel plate capacitor, you can easily find the '1/2 energy' point just by thinking about it.  It'll be midway between the plates.   So half the volume on either side.
In a cylinder, that point won't be 1/2 way between the 'plates'.  It'll be much closer to the outside shell.   You'll have to do some math to figure out where.  (geometry and/or calculus, not circuit analysis).  There will be some gradient in the 'energy per unit volume', which is where the calculus comes in.