Electrodeposition of Cobalt and Cobalt‐Aluminum Alloys from a Room Temperature Chloroaluminate Molten Salt

Abstract

The electrodeposition of magnetic cobalt‐aluminum alloys was investigated in the Lewis acidic aluminum chloride‐ 1‐methyl‐3‐ethylimidazolium chloride [60.0–40.0 mole percent (m/o)] molten salt containing electrogenerated Co(II) at 25°C. Rotating disk electrode voltammetry indicated that it is possible to produce alloy deposits containing up to 62 atomic (a/o) aluminum at potentials positive of that for the bulk deposition of aluminum. The onset of the underpotential‐driven aluminum codeposition process occurred at around 0.40 V vs. the Al/Al(III) couple in a 5.00 mmol liter−1 Co(II) solution but decreased as the Co(II) concentration increased. The Co‐Al alloy composition displayed an inverse dependence on the Co(II) concentration but tended to become independent of concentration as the potential was decreased to 0 V. A rotating ring‐disk electrode voltammetry technique was developed to analyze the composition and structure of the Co‐Al alloy deposits. This technique takes advantage of the fact that the mass‐transport‐limited reduction of cobalt(II) occurs at potentials considerably more positive than that at which aluminum codeposition occurs. Scanning electron microscopy and energy dispersive x‐ray analysis of bulk electrodeposits revealed that deposit morphology depends strongly upon aluminum content/deposition potential; deposits produced at 0.40 V from 50.0 mmol liter−1 Co(II) solutions consisted of 10 to 20 μm diam multifaceted nodules of pure hcp cobalt, whereas those obtained at 0.20 V were dense and fine grained, containing about 4 a/o Al. Deposits produced at 0 V had the visual appearance of a loosely adherent black powder. X‐ray diffraction measurements revealed a lattice expansion and a decrease in grain size as the hcp cobalt was alloyed with increasing amounts of aluminum.