(249d) Electrodeposition of Chromium from Chromium(III)-Ionic Liquid Solutions

Liyuan Sun^* and Joan F. Brennecke

Department of Chemical and Biomolecular
Engineering, University of Notre Dame

Notre Dame, IN 46556, USA

lsun4@nd.edu

Electrodeposition
of chromium (Cr) is indispensable for various industries, such as automotive,
aerospace, electronics, optics, and sensors.  The most common industrial
chromium coatings are usually electrodeposited from hexavalent chromium
(Cr(VI)) aqueous solutions, which possesses severe concerns because Cr(VI) is
highly toxic.  A straightforward alternative is less toxic trivalent
chromium (Cr(III)) based solutions.  However, the high stability of the
hexaaquochromium(III) complex formed in aqueous electrolyte solutions may
obstruct or even prevent the electrodeposition of chromium from aqueous
solutions.  Another considerable drawback of electrodeposition of chromium
from aqueous solutions is hydrogen evolution, which results in safety issues
and reduced current efficiency.  Using ionic liquids (ILs) for Cr(III)
electrodeposition is a promising alternative that could address the above
issues. So far, only a few limited examples of chromium electrodeposition from
ILs have been reported (e.g., using a chromium chloride hexahydrate/choline
chloride solution,¹ or chromium chloride
hexahydrate/1-buthyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄])
solution²).

In this work, we
investigate new Cr(III)-IL solutions as potential candidates for chromium
electrodeposition. To optimize the transport properties of the mixtures,
different amounts of ultrapure water is added to the Cr(III) salt ¨C IL
mixtures. The conductivity, viscosity, and 'ionicity' of the solutions are
characterized over the temperature range from 283.15 K to 353.15K.
Electrochemical reduction of Cr(III) is demonstrated by cyclic voltammetry and
preliminary electrodeposition tests.  The physicochemical and
electrochemical properties of the solutions depend heavily on the water content
and the ion type.  The new Cr(III) salt ¨C IL mixtures are excellent
candidates for chromium electrodeposition under carefully selected conditions
where hydrogen evolution can be avoided or inhibited.

Reference:

1.
Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K. Ionic liquid analogues
formed from hydrated metal salts. Chemistry-a European Journal 2004, 10,
3769-3774.

2.
Eugenio, S.; Rangel, C. M.; Vilar, R.; do Rego, A. M. B. Electrodeposition of
black chromium spectrally selective coatings from a Cr(III)-ionic liquid
solution. Thin Solid Films 2011, 519, 1845-1850.