I'm designing a world in which starships have renewable energy, refueled by parking around black holes or stars and recharging batteries with solar panels, but what fuel for an engine could be extracted from these astral bodies, and what type of engine could be designed to accept that fuel?
The nearest thing to this is the Bussard ram jet. The principle is easy to understand - the ship generates a magnetic field that funnels hydrogen from the interstellar medium into the reaction chamber, where some of it is used to fuel a fusion reactor and the rest is used as reaction mass. The reactor is used to superheated the reaction mass and generate electricity to generate the magnetic fields.
The math is close enough that it remains plausible with very little handwavium. In real life, the energy losses in the system are just sufficient that it won't quite work.