This is interesting:
https://www.nbcnews.com/mach/science/sc ... ncna930676
TL;DR researchers are using an unstable compound and its more stable isomer to store solar power, and release it as heat on exposure to a catalyst. Might as well mention that the unstable isomer, quadricyclane, has been used already in solid rocket propellants.
(Makes me wonder what other excited/unstable molecules might work for this.)