I just realized that heating and expelling something electrically vs just burning in a rocket engine makes absolutely no difference from an ISP perspective. All that matters is exhaust velocity, and that basically only scales with temperature and molecule size (lighter being better), so steam is no good anyway.

Hypothetically though, if you would just pressurize e.g. H2, then cool it, then pressurize it further-- should that not get you higher exhaust velocity that just heating/burning? why does that scheme not work?