It's SAD Citroen had a Air Pressure Low pressure in the boot high pressure under the seat, when you brake it moves from low to high, when you drive it reverses, just 2 chambers and a dual direction pump, no chemicals or batterys to wear out, sadly couldn't get the backing to make the production line so went batterymuch smarter energy regen system!!
It's funny you mentioned that in this thread about super-caps....
I brought this up years ago. Compressing air underground via excess power from windmills, to later be released through turbines for power generation. https://io9.gizmodo.com/how-to-turn-caves-into-giant-batteries-5893480
Efficient? I dunno.
But there's lots of ways to skin a cat.
Those old Citroens, aside from being frog cars, were actually pretty cool.
Remember the rear suspension?
Same air.
.