Energy to hydrogen back to energy, so electrolysis to a hydrogen fuel cell. I think burning hydrogen directly is even less efficient.

directly storing electricity as a chemical battery system is likely going to be more efficient (way more optimized and generally a lot simpler) and something like thermal energy storage (really, really simple, and very, very effective, plus pretty cheap, there just isn’t much accessible tech out there at the moment, though it suffers from the same conversion problem, it’s certainly a lot simpler than hydrogen.)