Determining the thermodynamic electrode potential in the presence of a mixed potential using EQCM technique

We give the theory explaining how to use Electrochemical Quartz Crystal Microbalance (EQCM) to determine the thermodynamic equilibrium electrode potential in a system where the open circuit potential is a mixed potential. The approach is applicable to electrodes of the first or the second kind in the presence of one or more soluble redox couples, at least one of which is irreversible. The key insight to be gained is that if the mass of an electrode of the first or the second kind does not change, its electrode potential is the thermodynamic equilibrium potential. This is true regardless of any other redox processes involving only soluble species that may be occurring at the electrode/solution interface. The model system used to test and confirm the theory was an electrode of the first kind, NiH_{x, solid}/Ni(II) undergoing active corrosion caused by hydrogen peroxide. The electrochemical quartz crystal microbalance method can be applied in fundamental and applied studies, e.g., of electroless deposition and corrosion systems containing multiple redox systems.