Some Basic Definitions in Electrochemistry

Electrochemical cell consisting of two electrodes (Fe/Fe and H2/H+ electrode), separation of a chemical process via a diaphragm in two partial processes with electronic exchange via external circuit. 

One distinguishes metal/metal ion electrodes, which involve the transfer of metal cations across the interface, or redox electrodes with a transfer of electrons across the interface. For electrodes consisting of compounds, as for most semiconductors, cations or anions may be transferred besides electrons thus decomposing the electrode surface. For example, the anodic decomposition of CdS involves the transfer of Cd ions to the electrolyte, leaving elemental S at the surface of the CdS electrode. 

The above discussed electrochemical equilibrium and the related Nernst potential E with the same processes in opposite direction has to be distinguished from the situation when two different processes compensate each other at the rest or mixed or corrosion potential Er. Spontaneous iron dissolution with hydrogen evolution in an acidic electrolyte is an example. If no external current is applied, Fe + dissolution is compensated by H+ reduction, i.e., the anodic and cathodic processes compensate each other. A deviation from the rest potential E - Er = jr is called polarization. For jt 0, the anodic Fe dissolution is faster and requires an external current. For ji 0, the hydrogen evolution exceeds iron dissolution with a cathodic overall current. 

---