A Study of Nickel Electrodeposition on Paraffin-Impregnated Graphite Electrode by MagdalØna Strec ˇkovµ a ), Renµta Orin ˇµkovµ *a ), Roland Rozik b ), Libus ˇe Trnkovµ b ), and Miriam Gµlovµ a ) a ) Institute of Chemistry, Faculty of Science, P.J. S ˇ afµrik University, Moyzesova 11, 04154 Kos ˇice, Slovak Republic (phone: + 421-55-6222605; fax: + 421-55-6222124; e-mail: renata.orinakova@upjs.sk) b ) Department of Physical and Theoretical Chemistry, Faculty of Science, Masaryk University, Kotlµr ˇskµ 2, 61137 Brno, Czech Republic The aim of the present work was to elucidate the mechanism of electrolytic deposition of Ni on par- affin-impregnated graphite electrode (PIGE). This process is influenced by H 2 evolution, which occurs in the same potential region. On the basis of the results obtained by linear and cyclic voltammetry, elimi- nation voltammetry with linear scan (EVLS) was used to evaluate both processes. H 2 Evolution alone was studied in sulfate supporting electrolyte, and the previously suggested mechanism for this process according to Volmer–Heyrovsky was confirmed by EVLS. It was found that both the Ni 2+ concentration and pH affect the polarization behavior of PIGE significantly. Two separated cathodic peaks were observed at low Ni 2+ and high H + concentrations, and the separation was better at higher scan rates. EVLS confirmed the most-probable mechanism of Ni deposition as being controlled by slow transfer of the first electron under formation of [NiOH] + as an intermediate. EVLS also indicated slow reduction ofH + preceding the reduction of Ni 2+ . The same was confirmed by studying the anodic dissolution at dif- ferent switching potentials. The results were complemented by scanning electron microscopy (SEM). Introduction. – Electrodeposition of nickel (Ni) and its alloys on various less-noble surfaces from Watts electrolyte containing both sulfate and chloride ions is a widely used technique for improving the mechanical and chemical properties of plated surfa- ces. During recent years, electroplating from mixed electrolytes has become most wide- spread in metallurgy industry. However, why exactly this electrolyte composition is so convenient, remains a challenging question. Without a detailed study in the two sepa- rate electrolytes (chloride and sulfate bath), as well as in the presence and/or absence of boric acid, the elucidation of the Ni-reduction mechanism from complete Watts electro- lyte is a very difficult task. Several studies have addressed different aspects of Ni elec- trodeposition in the two above-mentioned electrolyte systems [1–5]. One of the main components of Watts bath is boric acid [6][7]. Our previous studies confirmed that the presence of boric acid alters the polarization behavior of Ni reduc- tion from the two above-mentioned electrolytes [8][9]. Moreover, higher-quality deposit has been obtained when boric acid was used in chloride solution [8–12]. The evolution of H 2 is another important effect occurring during Ni electrodeposition. The great affinity between H 2 and Ni facilitates the adsorption step on the Ni surface, probably due to the formation of the activated complex [NiH···H···OH] ¼6 . Hence, the Heyrovsky electrochemical-desorption step seems most likely to be rate determining [13]. # 2006 Verlag Helvetica Chimica Acta AG, Zürich Helvetica Chimica Acta – Vol. 89 (2006) 622