There's tons of hydrogen ions just floating around in space, not necessarily in deep space but near our solar system. Maybe it's just using that and a magnetic field as propulsion. Kind of funneling in ions and then accelerating them

Because the phase change from liquid to gas actually absorbs quite a lot of energy. It's called latent heat of evaporation. It's the reason why when you pour methanol or alcohol on your hands, they feel cold as it evaporates.