We already mix hydrogen and oxygen to create thrust in today's rockets...yep them boosters are full of hydrogen and the oxygen is readily available in the air.
Hydrogen Fuel cells are way different...in a hydrogen fuel cell Hydrogen acts as..well fuel...and oxygen..simply enough..as an oxidant. Fuel on the anode side and oxidant on the cathode side. Introduce an electrolyte between them like sodium hydroxide or potassium hydroxide which act as catalysts for bringing fuel (hydrogen) and oxidant (oxygen) together. The electricity results in a sort of convoluted way. As the hydrogen comes in contact with the catalyst the protons and electrons are diffused. The protons will travel through the catalyst membrane while the electrons are routed through a circuit which goes to power whatever. When the electrons return from powering whatever they meet up with the protons and the oxygen to form water vapor.
I always though H-fuel had a more nuclear nature in FL but I'm not sure. As for nanobots those are machines built out of some nano-size material...probably carbon tubes or continuous planes. These could feasibly take on the shape of some repair vessel but on a very small scale.
