This doesn't answer you question exactly but may be useful idea.
IMHO any from of chemical energy storage is not going to be compact enough to power a FTL drive.
However you could store your excess energy as cool plasma in a couple of magnetic bottles (tokamak).
Using the energy from your power source create two, balanced, streams of plasma. One highly positively charged (protons from ionized hydrogen gas), the other negatively charged (mostly the electrons from the ionized hydrogen). You feed each stream of plasma into a different magnetic bottle (or alternating bottles). Using RF generators you 'cool' the plasma, so that it has less side to side velocity and just keeps moving around the loop. You could also also add more energy by increasing the velocity of the plasma around the loop.
This is very loosely what happens in the large hadron collider. Only the LHC accelerates a small quantity of plasma very very fast.
When It comes to igniting your FTL drive you open a magnetic valve and let the two streams of plasma at each other and zoom their excessive attraction for each other powers your drive and 'blip' you are in a new location.
The limitation to the amount of energy you can store is:
1. Strength of the magnetic containment (solved problem from developing fusion generators)
2. 284
The stability of the RF cooling/acceleration system
3. Bremsstrahlung radiation (X-rays from accelerating electrons (around a loop))
Point 1 & 2 are rather obvious, nuf said.
The Bremsstrahlung radiation will cause your magnetic capacitor, to leak/loose energy over time. This is why you would want to have lots of plasma moving slowly (rather than the LHC which has a little moving fast).
Further thoughts:
Rather than hydrogen gas you could use the helium 'ash' from your fusion generator in the plasma containment rings.
You could use small streams of this plasma to meet and power 'impulse' rockets, when you want to move about in real space.
You could use bursts of plasma as a short range weapon (the plasma will dissipate rapidly in space/ when not magnetically contained. ( this could be an unconventional/ last ditch tactic; as it would bleed the FTL charge you are building up)
You could cascade several containment units, so that the discharge from one is used to accelerate the plasma in the next, which is then used to accelerate the next etc etc. Kind of like double bouncing someone on a trampoline to then double bounce on a larger trampoline. The the loss from Bremsstrahlung radiation would not be a problem as you are not trying to store high speed plasma but cascade quickly for a FTL jump. The pattern of bursts of Bremsstrahlung radiation during a cascade would be a characteristic 'jump signature' of each (class of ?) vessel.
Cool buzz words warning:
Careful engineering of the plasma containment could let you use the storage system as a synchrotron laser (Yes that is a real thing) or the Bremsstrahlung radiation from one storage ring could be used as part of a klystron (yes that is also a real thing) to power the RF generator of the next stage of the FTL cascade. 284