Imidazole-based Ionic Liquid In The Mechanism Of Zinc Electrodeposition

Electrodeposition is a kind of important methods for metal extraction in metallurgical industry. It is found that the using of additives is absolutely necessary for obtaining fine quality metal during metal electrodeposition. Although colloidal and organic additives have been widely used in industrial production and achieved good additive effect, they are readily decomposed and environmental unfriendly for their shortcomings, such as bad chemical and thermal stability, high toxicity, etc. As a kind of novel green solvents, ionic liquids have many attractive properties, for instance, high chemical and thermal stability, very low vapor pressure, high ionic conductivity, low toxicity and a wide electrochemical potential window due to their unique structural characteristics. Therefore, it is hopeful to overcome these defects of traditional colloidal and organic additives and achieve the green production process for metal electrodeposition.A series novel 1-alkyl-3-methylimidazolium hydrosulfate ionic liquid, such as [BMIM]HSO4, [HMIM]HSO4, [OMIM]HSO4, etc are synthesized and used as the additives for zinc electrodeposition from acidic sulfate electrolyte for the first time in this paper. The effect of them on the zinc electrocrystallization has been investigated by using many electrochemical measurements, including cyclic voltammetry, cathodic polarization, electrochemical impedance, chronoamperometry, chronopotentiometry, etc. Moreover, the adsorption behaviors of these additives on the metal surface are theoretically discussed by the quantum chemical calculations and molecular mechanics simulation method, which are based on their structural characteristics. And then the corresponding adsorption model is established to analysis their adsorption mechanism. The major research results are drawn as follows:(1) [BMIM]HSO4 is a good levelling agent for zinc electrodeposition. Addition of 5 mg/L [BMIM]HS04 is found to increase current efficiency by ca.3.5 percent point, reduce specific electric energy consumption ca.142 kWh/t-Zn, decrease corrosion current density of deposit zinc ca.48% and produce smooth, leveled and compact cathodic deposits. [BMIM]HSO4 is observed to have a pronounced inhibition effect on Zn2+ electroreduction and hydrogen evolution similar to that achieved with the traditional.colloidal additives, which does not change the structure of the electrodeposited zinc but affects the crystallographic orientation of the crystal planes. It is also found that increasing the carbon chain length of the 1-alkyl-3-methylimidazolium hydrosulfate ionic liquids increases their surface adsorbability, leading to reduce their addition dosage. Similar levelling performance can be obtained when 1-2 mg/L additives are added.(2) [BMIM]HS04 is found to markedly reduce the oxygen evolution charge transfer resistant and oxgen evolution potential. Addition of 5 mg/L [BMIM]HSO4 can obviously decrease the resistant value at least 50% in the potential range from 1.85to2.10V and reduces oxgen evolution potential ca.50 mV. The results indicate that the oxygen evolution is catalyzed by [BMIM]HSO4 appearing as effective increase in the reaction rate. The analysis results obtained from XRD and SEM show that the presence of [BMIM]HSO4 inhibits the generation of the intermediate products,β-PbO2 and promotes the generation of the larger in size, loose and porous, a-PbO2, which is favoured to the progression of oxgen evolution reaction.(3) [BMIM]HSO4 is observed to relieve the harm effect of several common impurities such as Cu, Fe, Co, Ni, Cd, Pb, Sb, Mn, etc, to some extent and improve the quality of zinc deposit. The addition of 5 mg/L [BMIM]HSO4 is found to increase current efficiency ca.1-11 percent point, inhibit cathodic zinc contamination of metal impurities and obtain smooth and compact deposits in comparison to the solution with different concentrations of metal impurities. The results show that the influence of these impurities on decrease in current efficiency is in the order:Sb3+>Ni2+>Co2+>Cd2+>Cu2+> Fe2+> Pb2+>Mn2+, and the extent of contamination appears to be in the order:Cu+>Pb2+>Cd+>Fe2+>Ni2+ > Co2+>Sb3+>Mn2+. Sb3+is found to catalyze hydrogen evolution strongly. A rapid decrease in current efficiency by ca.4 percent point is observed when its concentration exceeds 0.02 mg/L. However, it is noteworthy that the combination of 0.02 mg/L Sb3+and 5 mg/L [BMIM]HSO4 results a high current efficiency,94.7%, which is ca.5 percent point more by comparing with additive-free solution. Electrochemical analysis indicates that the effects of impurities on electrode reaction are quite different. Co2+ is found to depolarize the cathode and is similar to that of Cu2+. Ni2+, Fe2+ and Pb2+are observed to polarize the cathode. The effect of Cd2+is more special than other impurities. Depolarization of the cathode is noted when concentrations of Cd2+are less than 10 mg/L whereas polarization of the cathode is observed when Cd2+concentration exceeds 10 mg/L. In comparison to the solution with 10 mg/L different impurities, the reduction potential of zinc is found to negatively move at different degree with addition of [BMIM]HSO4, which indicates the additive inhibit the electroreduction of these impurities.(4) Temperature and current density have little effect on the levelling performance of [BMIM]HSO4, however, they have obvious effect on current efficiency and specific electric energy consumption in the range from 303 to 318 K and 300 to 600 A/m2. Addition of 5 mg/L [BMIM]HSO4 with temperature elevating by 10 K is observed to increase current efficiency by 1-2 percent point and bring along the decrease in specific electric energy consumption ca.100-250 kWh/t-Zn; In comparison to the additive-free solution, a rise of 0.5-2 percent point on current efficiency and a decrease in specific electric energy consumption about 50-100 kWh/t-Zn is obtained at lower current density (<500 A/m2) in the presence of 5 mg/L [BMIM]HSO4. The current efficiency is found to decrease by 0.5-2 percent point along with the growth of specific electric energy consumption ca.70-180 kWh/t-Zn when the current density exceeds 500 A/m2.(5) Electrochemical analysis shows that [BMIM]HSO4 has little effect on transfer coefficient, but has obvious effect on exchange current density. The transfer coefficient values (a) for the Zn2+reduction is practically constant over the studied temperature range and slightly decrease with an increase in the [BMIM]HSO4 concentration (c) and its values for aluminum and zinc electrodes are: a= 0.485-0.003c±0.005 (Aluminum electrode) a= 0.500-0.004c±0.004 (Zinc electrode) respectively and the exchange current density (io) for the Zn2+reduction depends on temperature (T) and the additive concentration (c) as ln(i0)=-(14570-139.35c)/T-0.476c+37.038 (Aluminum electrode) ln(i0)=-(11760-160.98c)/T-0.577c+33.664 (Zinc electrode) for aluminum and zinc electrodes, respectively.The results indicate the electrode reaction process obeys mixed control relation. Changes in electrochemical kinetic parameters, specially in exchange current density caused by the adsorption of [BMIM]HSO4 on the electrode are the main reason for achieving the excellent levelling performance of additive. The researches on the adsorption behavior of additive on the electrode surface demonstrate that this adsorption is a spontaneous process and obeys the Langmuir's adsorption isotherm. [BMIM]HSO4 is chemisorption on both aluminum and zinc electrodes and of more excellent chemical and thermal stability in comparison to these traditional colloidal additives, such as gelatine and gum arabic.(6) The presence of [BMIM]HSO4 is found to inhibit the zinc electrcrystallization process and decrease the diffusion coefficient of zinc ions without changing the electro-crystallization mode involving three dimensional instantaneous nucleation and growth. Nucleation rate constant and crystal growth rate constant progressively increase with increase in the applied potential, whereas both of them are inhibited with the addition of [BMIM]HSO4. However, the inhibition effect of [BMIM]HSO4 is more concentrated on the crystallite growth so that the crystal nucleus density is observed to relatively increase, and thus leads to obtain more fine-gained cathodic deposits. When the applied potential is large enough, the crystal nucleus density will tend to be a saturated value.(7) The-C=N-group and imidazolium ring in the structure of the imidazolium compounds show an outstanding contribution to their absorption through the frontier orbital theory, from which the imidazolium cation-electrode adsorbed model is proposed by molecular mechanics method using multilaminate metal atoms based on the adsorption characteristics of imidazolium compounds. The results suggest that the adsorption of imidazolium compounds on the electrode surface should be chemisorption that some kinds of "ionic bonds" are formed between these imidazolium cations and negatively charged electrode surface. The strong adsorption of the imidazolium compounds on the electrode surface inhibits the electroreduction of zinc ions and their asymmetrical growth, and then leads grain refining. It is also found that increasing the carbon chain length of the alkyl increases the charge density of imidazolium ring, resulting in high surface adsorbability. The imidazolium compounds can selective adsorption on the different crystal planes of zinc electrode. Under the same condition, crystal planes such as (002) (100) (201) (112) are preferential adsorbed. Obtained results from adsorption simulation, quantum chemical calculations, electrochemical researches and XRD and SEM measurements are in good agreement with each other.The study of using a series novel imidazolium ionic liquids as additives during the zinc electrodeposition in this paper not only provides a kinds of high efficient, low toxicity and good chemical and thermal stability novel additives, but more importantly, the utility of these methods involved and the electrode surface model in simulating the adsorption of additives provide certain scientific basis for the screening, designing and synthesis new additives and promote the development and improvement of the theory of additives for metal electrodeposition.