Define Interfacial potential differences?

What is the source of an open-circuit, zero-current cell potential? When no electric current passes through the cell, the electric potential must be uniform within each bulk phase that is an electrical conductor, because otherwise there would be a spontaneous movement of charged particles (electrons or ions) through the phase. Electric potential differences in a cell without current therefore exist only at phase boundaries. The equilibrium cell potential is the cumulative result of these potential differences at interfaces between different conducting phases within the cell. An interfacial potential difference appears as a vertical step in a profile of the Galvani potential, as shown schematically in Fig.(a). The zero-current cell potential, E_{cell, eq}, is the algebraic sum of the interfacial potential differences within the cell.

When an external resistor is connected to the terminals to form a circuit, current passes through the cell and the cell performs electrical work on the surroundings. Figure (b) shows what happens to the potential profile in this case: the interfacial potential differences are still present within the cell, and the internal resistance of the electrical conductors causes E_cell to be reduced in magnitude compared to E_{cell, eq}. We shall next look briefly at the origin and consequences of potential differences at interfaces between (1) two different metals, (2) a metal and an electrolyte solution, and (3) two different electrolyte solutions. Keep in mind that these potential differences are theoretical concepts whose values cannot be measured experimentally.