The electrochemical behaviors of molybdenum and its oxides, both in bulk and thin film dimensions, are critical because of their widespread applications in steels, electrocatalysts, electrochromic materials, batteries, sensors, and solar cells. An important area of current interest is electrodeposited CIGS-based solar cells where a molybdenum/glass electrode forms the back contact. Surprisingly, the basic electrochemistry of molybdenum and its oxides has not been reviewed with due attention. In this Review, we assess the scattered information. The potential and pH dependent active, passive, and transpassive behaviors of molybdenum in aqueous media are explained. The major surface oxide species observed, reversible redox transitions of the surface oxides, pseudocapacitance and catalytic reduction are discussed along with carefully conducted experimental results on a typical molybdenum glass back contact employed in CIGS-based solar cells. The applications of molybdenum oxides and the electrodeposition of molybdenum are briefly reviewed.

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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 CuInSe 2 /CuIn(Ga)Se 2 (CIS/CIGS) solar cells. Introduction to seleniumSe, [Ar] 4s 2 3d 10 4p 4 is a group 16 (6A or chalcogens) member, discovered by J. J. Berzelius 1 and chemically similar to sulphur to a large extent. The preferred coordination of the Se atom is two with an optimum bond angle of 105u. 2 The bonding configuration of Se is in fact quite flexible and can have

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