The Chemiosmotic Hypothesis is the proposal in 1961, by Peter D. Mitchell, that the mitochondrion functioned as a kind of electrochemical capacitor, using the energy of NADH and FADH2 to create a proton gradient across the mitochondrial membrane and that this energy was used by a reversible proton pump, the ATP synthase, to create ATP. This was a radical proposal at the time, and not well accepted. The prevailing view was that the energy of electron transfer was stored as a stable high potential intermediate, a chemically more conservative concept.
The problem with the older paradigm is that no high energy intermediate was ever found, and the evidence for proton pumping by the complexes of the electron transfer chain grew too great to be ignored. Eventually the weight of evidence began to favor the chemiosmotic hypothesis, and in 1978, Peter Mitchell was awarded the Nobel Prize in Chemistry.