Selenium (Se) is an important element belonging to the chalcogen family. Electrochemistry is of significant importance in various applications of Se and its compounds, ranging from non-vacuum processed solar cells, direct methanol/polymer electrolyte membrane fuel cells, electrocatalysts, batteries, and metallic alloys. However, rich information on Se electrochemistry remains unexplored in the last few decades. Here we made an attempt to assess the reported studies on electrochemical behaviours of Se. Fundamental concepts of Se electrochemistry in aqueous media and ionic liquids at various electrodes, polarographic and cyclic voltammetric analysis, cathodic and anodic stripping analysis, electrodeposition mechanism and electrocatalysis are discussed. Electrochemical atomic layer epitaxy (ECALE) in engineering Se atomic layers and fabrication of Se nanostructures by electrochemical methods are highlighted. Se electrochemistries in application areas are reviewed. Voltammetric studies of Se electrodeposition on molybdenum (Mo) electrode are presented with a view of its applicability in electrodeposited CuInSe₂/CuIn(Ga)Se₂ (CIS/CIGS) solar cells.

A model is proposed on the origin of the reversible redox transitions of molybdenum (Mo) and its oxides. The major and minor redox transitions are proposed to be associated with two thermodynamically non-equivalent adsorption sites at the structured surface oxides. Anion interaction/adsorption was found to be a decisive factor. The criterion is based on the combination of evidences and assumptions and is derived from the variations observed in the two redox transitions during a series of cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and open circuit potential (OCP) transient studies. The experiments were conducted using different electrolytes with dissimilar anions (Cl⁻, SO₄²⁻ and ClO₄⁻) and spanned a range of different pH values. The results described herein constitute an important step towards understanding the interfacial electrochemical processes of Mo and its oxides, and will be of use for a wide range of their applications.