Red-Ox Reactions in Ionic Liquids and Their Impact on Electrodeposition of Metals and Alloys

Abstract

Electrodeposition from ionic liquids has some significant benefits over aqueous electrolytes in that there are no side reactions associated with the presence of water and oxygen. Metal deposits can be plated without deleterious oxides or hydrogen inclusions, which may cause a loss of mechanical and corrosion resistance properties. However, these benefits come with practical limitations and implications. In the absence of water, there are fewer restrictions on the nature of ionic metal species that can exist in the electrolyte. In some ionic liquids, multivalent metals like niobium or vanadium may have several stable oxidation states under the same conditions. Ions with high valencies can react with low valent species to form ions of intermediate oxidation states. They can also comproportionate with freshly electrodeposited metal, dissolving it back into the bath in a low valent state. In the case of alloy deposition, two or more metallic ions with different reduction potentials are dissolved in the electrolyte. The initial oxidation states of the metals in the system may change spontaneously if a red-ox reaction between different ions is possible. The resulting species may be insoluble or otherwise not suitable for the electrodeposition. The more positive ion can be reduced to the metallic state and precipitate out of solution. This makes some metals incompatible with each other in the electrolyte and limits the combinations of alloying elements. Options for anodic reaction during electrodeposition from ionic liquids are usually limited to the dissolution of sacrificial anodes. Without oxygen or water, the surface of the anode remains free of a passivating oxide layer and therefore remains very chemically active. Ions with more positive reduction potentials will attack the anode and this reaction may continue until all of the more positive ions in the bath have been reduced. Water-sensitive ionic liquids, in particular chloroaluminates, may also have H+ ions present as a result of reactions with traces of moisture. The protons can participate in the oxidation of some ions, forming hydrogen gas that eventually leaves the system. Examples of red-ox reactions in selected ionic liquid electrolytes are discussed, as well as some proposed methods of taking these processes into account.