There's also thermal (non-ionized) electric propulsion, e.g., arcjet (which heats the propellant with an electric arc) and resistojet (which heats the propellan

There's also thermal (non-ionized) electric propulsion, e.g., arcjet (which heats the propellant with an electric arc) and resistojet (which heats the propellant with a resistance heater). Both arcjet and resistojet have been used on some satellites. The nice thing about those is that they don't rely as much on the physical or chemical properties of the propellant. Just about anything that can be vaporized by an arc or heating element will work. https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Arcjet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket https://en.wikipedia.org/wiki/Resistojet_rocket