They probably don't. Just as there is a critical temperature, Tc, at which superconductors stop superconducting, there is a critical current density, Ic, at wh

They probably don't. Just as there is a critical temperature, Tc, at which superconductors stop superconducting, there is a critical current density, Ic, at which they stop superconducting. That places a limit on the amount of energy you can store in a superconducting loop. Then there is also the consideration that Ic scales as approximately 1/Tc, so high-temperature superconductors have a lower critical current density than low-temperature ones.