Electrodeposition of manganese metal: role of quaternary ammonium salts on current efficiency, morphology and polarisation behaviour

The electrodeposition of manganese metal from aqueous solutions is problematic due to its high electroreduction potential demanding unique process challenges. In the present work, electrolytic manganese metals (EMMs) of high quality are deposited from sulphate solutions in the presence of quaternary amines: tetra ethyl ammonium bromide (TEABr), tetra propyl ammonium bromide and tetra butyl ammonium bromide. The concentrations of these additives were varied over a relatively broad range to evaluate their effect on current efficiency (CE), specific energy consumption, polarisation behaviour of the cathode and deposit morphology of the electrodeposited manganese metal. A maximum CE of more than 68% was achieved in the presence of 100 mg.L⁻¹ TEABr in the electrolytic bath at a current density of 500 A.m⁻². X-ray diffraction studies revealed that (330, 411) planes were the most preferred planes of crystal growth during manganese electrodeposition resembling alpha-manganese regardless of the type of quaternary amines used. A smooth and compact deposit was obtained with TEABr that was also supported by the scanning electron micrographs showing uniform deposit morphologies. The presence of these quaternary amines in the electrolyte causes polarisation of the cathode during electrodeposition. The degree of polarisation increases with an increase in the bulkiness of the alkyl group on the quaternary nitrogen atom of the additive. The quaternary amine TEABr was found to be the most suitable additive for producing the desired quality of EMM with better current yield.