Electrodeposition of Molybdenum from Water-in-Salt Electrolytes

Abstract

Molybdenum is attractive to the manufacturing industry due to its high melting point and extremely low thermal expansion. Generally, Mo can be produced by thermal spraying, sintering, and pressing at high temperatures. Electrodeposition has been used widely as low-temperature method to prepare metals and alloys as coatings or films. However, it is well-known that metallic molybdenum is difficult to deposit from aqueous solution due to its highly negative reversible potential. More recently, a few studies have successfully deposited metallic molybdenum from highly concentrated acetate solutions1,2 as a thick coating on metallic objects. In this work, we report a systematic study on Mo deposition using this electrolyte system under the umbrella of water-in-salt concept. Aqueous electrolytes with an extremely high concentration of salt, known as water-in-salt electrolytes, offer a special electrolyte environment for electrodeposition where free water molecules are reduced by the hydration of salt.This report focuses on the effects of substrate and electrolyte chemistry on the Mo deposition process as well as the film properties. In this study, Cu electrode is fabricated by depositing Cu layer on Pt electrode. The electrochemical behavior is studied using cyclic voltammetry on Pt and Cu disc electrodes, as shown in Figure 1. Mo deposition starts at about -2.0 V on both Pt and Cu. In the anodic scan, the deposited Mo film begins to dissolve at -0.7V. X-ray fluorescence is used to measure film thickness, which is consistent with the stripping charge according to Faraday’s laws. However, the amount of Mo deposited on Cu, represented by the anodic stripping charge excluding the charge for Cu stripping, is much higher than that on Pt electrode. On the other hand, the presence of CH3COOK is found extremely critical for Mo deposition despite of a high total concentration of acetate. It is believed to relate to a suppressed proton reduction in presence of alkaline metal cations in a super highly concentrated acetate electrolyte. This correlation between hydrogen evolution reaction and Mo deposition is also consistent with the promoted deposition rate on Cu than on Pt surface. DC deposition studies are carried out to further verify these observations and to allow characterization of deposited Mo. Details will be further discussed in the poster presentation.1 Morley, T. J. et al., "The deposition of smooth metallic molybdenum from aqueous electrolytes containing molybdate ions," Electrochemistry Communications 15, 78-80 (2012).2 Syed, R. et al., "Electrodeposition of thick metallic amorphous molybdenum coating from aqueous electrolyte," Surface & Coatings Technology 261, 15-20 (2015). Figure 1