That's how nuclear thermal rockets work. Boiling and spraying is going to be much, much higher thrust than what you're getting out of the electric drive but the spray will be going much slower so the amount of boilable propellant you carry with you will end up being the limiting factor in how fast you can do. And at that point you might as well forget about the electric part.

Now, since you you can be boiling pure hydrogen rather than the water that comes out of a chemical rocket you can make the propellant faster. But you're limited in temperature by what your reactor can take without melting so that only buys you a power of 2 or so in efficiency.

I wrote a series about how the different types of rockets compare starting here: http://hopefullyintersting.blogspot.com/2015/03/rockets-some...

That's just reducing the effective exhaust velocity of your engine. Rocket equation says you will run out of propellent [your "something"] faster than you would like.

Yes, but what? You have to lift that mass to orbit. What mass boils off with greater energy than, say, mass that combusts?