Formation of needle-like and honeycomb-like magnesium oxide/hydroxide structures by electrodeposition from magnesium nitrate melts

Abstract

The processes of electrochemical deposition of magnesium oxide/hydroxide on glassy carbon (GC) electrode from magnesium nitrate hexahydrate melt have been investigated. A novel procedure predicting a possibility of direct formation of magnesium oxide during electrodeposition from the nitrate melt used is reported. XRD analysis of the obtained deposits showed the formation of magnesium oxide along with magnesium hydroxide. The electrodeposition of magnesium oxide/hydroxide commences in magnesium underpotential (UPD) and continues through the magnesium overpotential (OPD) region. Network of individual or intertwined very thin needles as well as those grouped in flower-like aggregates or honeycomb-like structures were formed in both magnesium UPD and OPD regions. Formation of the long needles was explained through theories of mechanisms of dendrite formation. Hydrogen evolution commences in the magnesium OPD region and increases with the applied overpotential. Holes observed in the deposit originated from the detached hydrogen bubbles. The number, shape and size of the hole strongly depended on both the applied cathodic potential and the hold time of electrodeposition. Magnesium oxides/hydroxides syntheses taking part simultaneously at various applied potentials are a result of reactions between magnesium cations and products of water and nitrate anions reduction processes. Chemical reactions responsible for direct formation of magnesium oxide observed are those of magnesium ions and oxygen ions, formed by nitrate reduction taking part in the close vicinity of the working electrode.