Just throwing this idea out there and wondering if there is any reason it would not work. In a parallel plate capacitor, the maximum voltage is controlled by the dielectric strength and the thickness of the dielectric. When using vacuum as a dielectric, there is a voltage dependant leakage current due to residual gas molecules and also due to thermionic emissions from both metal plates. My question is, if we could achieve a pure vacuum ( like performing this experiment in outer space ) and by cooling the plates to near absolute zero, could we reduce the leakage current between the plates to a sufficiently low value as to allow us to store a million volts or 10 million volts? in the capacitor. Are there any other factors that would limit the maximum voltage in the capacitor other than free gas and thermionic emissions? If this is the case, we could build a capacitor with a volume of a few litres, that could hold many megajoules of stored energy.