In aerobic respiration, what is the direct source of energy that atp synthase uses to synthesize atp?

For the answer to the question above asking In aerobic respiration, what is the direct source of energy that ATP synthase uses to synthesize ATP? I think this is Proton Gradient. It is t he product of the electron transport chain. A higher concentration of protons outside the inner membrane of the mitochondria than inside the membrane is the driving force behind ATP synthesis.