This would be more compelling if there was a way to turn a cryptocurrency back into energy. And indeed, energy-backed currency seems to be a staple of science

This would be more compelling if there was a way to turn a cryptocurrency back into energy. And indeed, energy-backed currency seems to be a staple of science fiction - things like "energy credits". Cryptocurrency gets us halfway there, in the sense that you can transform energy into it, but the reverse ain't really feasible. Perhaps as (actual) batteries and supercapacitors and such continue to miniaturize we'll see something closer to a true energy credit, with financial transactions taking place through the transfer of stored energy - be it directly (people maintaining batteries as wallets) or indirectly (people exchanging currency entitling the holder to some amount of energy).