Electrochemical Synthezis of Co-Rh Alloys

Rhodium is a very important element due to a wide range of its applications in electronic devices, jewellery and catalysis. Rhodium like other platinum group metals (PGMs) presents a high catalytic activity for many chemical and electrochemical reactions. The position of rhodium on –logi₀ - DG_M-H plot shows its high catalytic activity for hydrogen evolution reaction (HER) [1]. The mentioned dependency may result in a conclusion that also alloys obtained through combination of elements from the transition group can demonstrate properties interesting from the point of view of catalytic activity for HER.

The present work shows results of tests over electrochemical synthesis of Co-Rh alloys from acid chloride electrolytes. Particular attention was devoted to the influence of electrolysis parameters on properties of the deposited alloys, such as the composition, structure and their morphology. The work elaborates also the mechanism responsible for formation of Co-Rh alloys through electroreduction of aquachloro- complexes of Co(II) and Rh(III) in acid environment. The identification of electrode reactions was performed based on voltammetric and electrogravimetric tests. Also, current efficiency of electrolysis was assessed. The electrolyte stability was tested with the use of UV-Vis spectrometry. The potentials range enabling the alloys synthesis was selected based on electrochemical tests. The tests included influence of concentration of Σ[CoCl_x(H₂O)_6-x]^2-x (x = 0, 1, 2) and Σ[RhCl_y(H₂O)_6-y]^3-y (y = 4, 5, 6), working electrode potential, electrolyte pH and temperature on the composition, structure and morphology of Co-Rh alloys deposited at potentiostatic conditions. The conducted tests indicate a possibility of alloys synthesis of a wide range of elemental composition and diversified morphology. The performed structural analysis showed a high influence of the synthesis conditions on the structure of the synthesized alloys.

[1] N. Pentland, J.O. Bockris, E. Sheldon, Hydrogen Evolution Reaction on Copper, Gold, Molybdenum, Palladium, Rhodium, and Iron - Mechanism and Measurement Technique under High Purity Conditions, J Electrochem Soc 104 (1957) 182-94.