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Abstract

As an alternative to high-temperature methods like thermal and plasma spraying, electrodeposition (ED) of molybdenum (Mo) is a relatively simple, low-cost, low-temperature method that has potential for mass production of thin coatings. However, ED of Mo from aqueous solutions has so far yielded unsatisfactory coatings due to high hydrogen (H₂) evolution rates. Among the five studies to date on aqueous ED of Mo, none have evaluated the corrosion and catalytic properties of the coatings, despite the enormous potential to expand their future applications. Therefore, this work aimed to characterize the corrosion and catalytic behavior of electrodeposited Mo coatings. Coatings were deposited by direct current ED from highly concentrated aqueous-acetate solutions containing molybdate ions (MoO₄²⁻). Temperature, deposition time, pH, and current density were varied to quantify their effects on coating composition and mechanical properties. Compact (~20 μm thick) and hard (5,550 MPa hardness) Mo coatings were successfully deposited over an 8-h period from a neutral electrolyte at 30 °C deposition temperature and 0.4 A/cm² current density. The corrosion and catalytic properties of 6 μm thick coatings were evaluated in 3.5 wt.% NaCl (pH 5.5) and 0.5 M H₂SO₄ (pH 0.5). Compared to coarser grained (bulk) Mo, the coatings had adequate corrosion resistance and higher catalytic activity. The enhanced electrochemical activity was attributed to a nanocrystalline (6.5 nm) structure, not intrinsic properties. The smaller crystallite size provided more active sites for electrochemical reactions, thus enhanced electrochemical behavior. The current density was stable when subjected to continuous H2 evolution for 48 h in 0.5 M H₂SO₄. The Mo coatings also required a lower activation energy (12.7 kJ/mol at −0.7 VSHE) for H₂ evolution, making them suitable for H₂ production.