Underpotential deposition

Underpotential deposition (UPD) is an electrochemical phenomenon where a metal ion is deposited onto an electrode surface at a potential that is more positive (in a cathodic direction) than the equilibrium potential for the deposition of that metal in a bulk electrochemical process. This occurs typically for a monolayer of metal, often in the presence of another metal that can be deposited at a lower potential.