Ionic liquids as additives on the electrodeposition of cobalt, nickel and its alloy onto copper in acidic sulfate bath

Abstract

The electrocatalytic properties of nickel (Ni), cobalt (Co) and nickel-cobalt (Ni-Co) alloy coating qualified them to be utilized in industrial applications. Traditional organic additives have been used to enhance the deposits properties but some of them are not eco-friendly and obtained low deposits quality. Green ionic liquids (ILs) have become an alternative additive to be used in the electrodeposition due to their excellent properties. In the present study, the influence of two new ILs, namely, 1-methyl-3-((2- oxo-2-(2,4,5 trifluorophenyl) amino)ethyl)-1H-imidazol-3-ium iodide ([MOFIM]I) and 1-(4-fluorobenzyl)-3-(4- phenoxybutyl)imidazol-3-ium bromide ([FPIM]Br) were investigated as green additives for Ni, Co and Ni-Co alloy electrodeposition from acidic sulfate bath on a copper substrate. The resultant surface morphologies demonstrated that both studied ILs served as effective leveling agents but [MOFIM]I was more effective than [FPIM]Br owing to their molecular structures. Both studied ILs led to the formation finer-grained, more ordinated crystals, compact, free-cracked and highly uniform Ni, Co and Ni-Co alloy deposits compared to that obtained from free ILs bath as shown by scanning electron microscopic (SEM) studies. Atomic force microscopic (AFM) analyses exhibited that the roughness of all films deposited with [MOFIM]I were lower than that with [FPIM]Br. Both ILs led to a homogeneous distribution of the Ni and Co elements and confirmed the Ni-Co formation, as shown by the EDXmapping. The X-ray diffraction (XRD) patterns exhibited the fcc crystal structures with (2 2 0) was preferred growth orientation of the Ni, Co and Ni-Co alloys crystallites without and with both studied ILs. The average crystallite size of the Ni, Co and Ni-Co alloy films decreased by 30%, 12% and 27% respectively with [MOFIM]I and by 25%, 5% and 18% respectively with [FPIM]Br. The microhardness of the Ni, Co and Ni-Co alloys increased in the presence of both ILs. All voltametric measurements indicated that the inhibition of Co2+ and Ni2+ reduction in the presence of both ILs occurred via their adsorption on the cathode surface, which obeyed the Langmuir adsorption isotherm. The optimal bath conditions that led to the highest CCE% values involved a current of 20 mA cm-2, deposit potential of 6.5 V, pH of 4.5, temperature of 25°C and deposit time of 10 min. The percentage current efficiency (CCE%) values of Ni, Co and Ni-Co alloy electrodeposition were very high (nearly 100%) in the presence of both studied ILs. The highest corrosion resistance was for Ni deposit in the NaCl solution with [MOFIM]I, compared to that with [FPIM]Br. However, Co deposit exhibited lowest corrosion resistance with [MOFIM]I and [FPIM]Br respectively. Ni-Co2 and Ni-Co3 alloys deposited at the optimal conditions were the two best alloys to resist corrosion among all the Ni-Co alloys examined in the current study. The co-deposition of Ni-Co alloy obeyed the anomalous type. This anomalous behavior was alleviated after [MOFIM]I and [FPIM]Br were introduced in the Ni-Co deposition baths. Quantum chemical calculations were performed at the B3LYP/6-311++G(d,p) level of the density functional theory (DFT). Several quantum parameters and natural atomic charges were calculated. The results showed that the calculated values of the quantum parameters and natural atomic charges were consistent with the experimental findings.