"Operando interlayer expansion of multiscale curved graphene for volumetrically-efficient supercapacitors" (2025) https://www.nature.com/articles/s41467-025-63485-0

These would be the best thing ever for small cheap IoT devices, assuming they can make them cheaply and they can be assembled the same way you assemble normal SMD capacitors.

I don't think energy density is that much of a limitation, because current batteries have to be over provisioned to avoid deep cycling, and then over provisioned some more to account for calendar aging, unless it's a nearly disposable product.