At first I thought that would be wildly impractical but CO2 goes supercritical at just shy of 90°F and a little over 1kpsi. Those kinds of pressures are pretty well unknown in a substation, but occur all the time in refineries and the like. It's basically just a pressure vessel with a relatively mild heater requirement. Eminently doable. I think we will probably see these deployed in the next few decades. SF6 is an amazing material but really, really horrible as a greenhouse gas.