My understanding is that there is not a single confirmed case of this, at least since capital punishment was reinstated.
If you have evidence to the contrary, I'd very much like to see it.
Would also like to point out something you're probably well aware of. Most of those claimed as "innocent" are more accurately described as having gotten a court to agree their trial was conducted improperly in some way.
To my mind, at least, there's a really big jump from "His trial didn't meet standards," to "He actually didn't commit the crime."
Hurricane Carter, guilty as Hell and free as a bird until prostate cancer claimed him.
http://www.graphicwitness.com/carter/