Article pubs.acs.org/JPCC
Water as an Inhibitor of Metal Corrosion in Hydrophobic Ionic Liquids Olga Lebedeva,*,† Giljana Jungurova,† Alexandre Zakharov,† Dmitry Kultin,† Elena Chernikova,‡ and Leonid Kustov*,†,‡ †
Department of Chemistry, Lomonosov Moscow State University, 119991, Moscow, Russia Zelinsky Institute of Organic Chemistry, Leninsky Prospect, 47, 119991, Moscow, Russia
‡
ABSTRACT: Water traces in hydrophobic ionic liquids surprisingly proved to exhibit unusual corrosion inhibiting behavior by protecting metal copper and nickel from electrochemical corrosion under aerobic conditions. The anodic dissolution of a copper electrode results in the formation of Cu (I) species. The simultaneous re-electrodeposition of nanocrystalline copper on the cathode occurred without additives to the resulting electrolyte.
1. INTRODUCTION The use of ionic liquids as green solvents is well-documented in open literature. The recent interest in their application definitely shifts toward the electrochemical processes. Indeed, quite a number of ionic liquids do behave not only as benign solvents suitable for the electrochemical devices and methods but also as unique and robust electrolytes with high stability (both electrochemical and thermal as well as hydrolytic) and outstanding conductivities and electrochemical windows. One of the advanced electrochemical applications is electroplating and electropolishing of metal surfaces. Diverse metals have been studied in this application. Metal copper is known to be extremely sensitive to electrochemical corrosion in aqueous solutions. Aerobic conditions favor copper corrosion in water to give hydroxo carbonates that exhibit no protective effect with respect to further oxidation processes. In this case, the inhibition of the copper corrosion may be efficient in the presence of organic compounds specifically adsorbed on the surface of the metal to exclude the contact with the aggressive medium. Copper is of intensive use as a current collector in lithium-ion secondary batteries.1−6 So, the problem of the metal release to the environment by electrochemical corrosion keeps on drawing a great attention. Water is known to be the agent causing the corrosion of copper.7−10 To the best of our knowledge, there is no mention in the open literature about the inhibition of corrosion due to the presence of water in the solution. Replacement of water solutions with organic media, for example, hydrophobic ionic liquids is a commonly accepted way to develop green chemical processes.11 The anodic behavior of copper in nonaqueous solutions is yet poorly studied. There are limited studies dedicated to the electrochemical corrosion of copper in alcohols,12,13 acetonitrile,14 DMF,15 and other organic solvents16 with the focus on the impact of water upon the anodic behavior of copper.12,13,15,16 The high-temperature corrosion behavior of several metals (Ni, © 2012 American Chemical Society
Cu, and alloys) in bmimTf2N (IL) under aerobic conditions has been investigated by electrochemical methods.17 The anodic behavior of nickel was previously investigated.18 It was reported19 that dry ionic liquid such as IL did not contain any readily available proton source. Water addition is a source of protons available in ionic liquids. However, there are no data with respect to the water-inhibited anodic dissolution of copper and nickel in hydrophobic ionic liquids.
2. EXPERIMENTAL DETAILS 2.1. Materials. Materials used: copper foil, purity 99.99%, nickel foil, purity 99.9%. Benzotriazol (BTA) and 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide (bmimNTf2) were obtained from Merck, Germany. 1-Butyl-3methylimidazolium chloride (bmimCl) prepared by the authors was a metastable overcooled liquid containing less than 600 ppm of water. 2.2. Electrochemical Measurements. The two- and three-electrode cells with separated and unseparated compartments, respectively, were used. The volume of ionic liquid was 3 to 4 mL. The working electrode was a Cu or Ni foil. The working surface in contact with the electrolyte was S = 1 cm2. The auxiliary electrodes were copper or nickel foil. The quasireference electrode was a silver wire (diameter 0.5 mm). All experiments were carried out at a constant temperature T = 25 C in air. Before electrochemical measurements, the naturally surfaceoxidized (NSO) electrode was degreased with acetone and accurately dried. The etched Cu and Ni samples were preliminarily pretreated with a mixture of 30% nitric and sulfuric acids (1:1 volume ratio), then were degreased with acetone and accurately dried. Received: June 6, 2012 Revised: October 3, 2012 Published: October 4, 2012 22526
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and to follow the Faraday’s law in all the cases. The choice of the current density was based on the data of cyclic voltammetry (CVA). On the one hand, it should exclude the anodic destruction of IL and Joule’s thermal losses. On the other hand, the value of the current density should be close to the values of the current density in the range of passivation. The increase in the concentration of water in the electrolyte surprisingly results in the reproducible decrease in the rate of copper dissolution. So, water acts as some inhibitor lowering the rate of anodic dissolution of the metal by a factor of 2 for “dry” and water-saturated IL, respectively (Table 1). “Dry” IL is not absolutely water-free.19,22,24 The concentration of the residual water in “dry” IL was 3.8 × 10−2 M (Table 1).
The water-saturated samples of IL were prepared as follows: 2 mL of ionic liquid and 2 mL of deionized water were added to a small glass tube, shaken, and allowed to stand for ∼2 h under stirring. After water layer being removed, IL was titrated by the K. Fischer method. Cyclic voltammograms of Cu and Ni were obtained in ionic liquids at room temperature. The potential scanning rate was 10 or 1 mV·s−1. The measurements were carried out using the AUTOLAB PGSTAT 302N potentiostat/galvanostat. The current density (for the mass loss of the anode upon galvanostatic polarization) was varied within 2−6 mA•cm−2. The dry electrodes were weighed with an analytical balance KERN ABT 220-4M. 2.3. Scanning Electron Microscopy. The surfaces of the samples were studied by SEM (EVO-50 “Zeiss”, EDX analyzer, magnification 1000, 5000, 10 000, 20 000, and 100 000). 2.4. XPS Measurements. The XPS measurements were carried out at room temperature with an XSAM-800 spectrometer with a Mg Kα (1253.6 eV) X-ray source.
Table 1. Values of kefF and the Molar Water Concentration (Cw) for “Dry” IL, Water-Saturated IL, and 1:1 and 1:2 Volume Mixtures of “Dry” and Water-Saturated IL, Respectively
3. RESULTS AND DISCUSSION 3.1. Anodic Behavior of Copper and Nickel in Ionic Liquids in the Presence of Water Traces. This work reports the electrochemical behavior of copper and nickel in IL and bmimCl under aerobic conditions in the presence of water traces. Up to now, water had to be removed to prevent the copper corrosion in ionic liquids because all ionic liquids were considered to be hygroscopic to a certain extent and rapidly adsorb water from the ambient air.19−24 However, we found that water affects the corrosion of copper and nickel in hydrophobic ionic liquids in the same way as usual inhibitors of corrosion. Figure 1 shows the data on water-dependent corrosion of copper in IL under aerobic conditions at room temperature.
a
b
dry
1:1
1:2
water-saturated
kefF, g·mol−1a Cw·102, mol·l−1
53.8 3.8
41.4 17.8
35.0 22.4
27.6 31.75
F is the Faraday’s constant.
The experimental plots (Figure 1) are described by eq 1 mCu = kef ·Q
(1)
where mCu is the mass loss of copper, Q is the quantity of electricity, and kef is the effective constant. The effective constant kef (g·C1−) was found to depend on the water concentration according to eq 2 kef = 6.2 × 10−4 − 4.5 × 10−4 ·Cw /Cs
(2)
where Cw and Cs are the molar concentrations of water in IL used and in water-saturated IL, respectively. The insertion in Figure 1 shows the plot of mCu versus the Cw/Cs ratio for 12 C, as an example. The mass of the dissolved metal seems to be somewhat lower for “dry” IL than it can be anticipated on the basis of the Faraday’s law describing the removal of Cu+ ions from the electrode (Figure 1). The decrease in the mass loss of the copper anode with the increase in the water concentration in IL is likely to result from a competitive reaction according to eq 3 that is accompanied by gas evolution. So, the decrease in kef is also partially connected to that reaction (Table 1). 2H 2O − 4e = O2 + 4H+
Figure 1. Plots of the mass loss of copper (mCu) versus the quantity of electricity (Q, C) for electrochemical corrosion in IL under aerobic conditions for “dry” IL (ILd) (1), water-saturated IL (ILs) (4), 1:1 (2) and 1:2 (3) volume mixtures ILd and ILs (ILd:ILs), and “dry” IL in the presence of 1 wt % of BTA (5). Insertion: Plot of mCu versus Cw/Cs (Cw and Cs are the molar concentrations of water in IL used and water-saturated IL, respectively) for electrochemical copper dissolution in IL.
(3)
The cyclic voltammograms for the copper electrode (the surface was 0.05 cm2) in IL are presented in Figure 2. The anodic curves for the first anodic cycles are depicted in Figure 2 from the steady-state potential up to +1.4 V. It is obvious that the values of the steady-state potentials in “dry” IL both for NSO copper (1) and for preliminarily etched copper (2) are higher than those for etched copper pretreated with watersaturated IL (3). It is of interest that the steady-state potential for etched copper in “dry” IL in the presence of 1 wt % of BTA is the same as that for etched copper pretreated with watersaturated IL (Table 2). Moreover, the inhibition efficiency of copper corrosion by water in water-saturated IL is similar to that produced by a 1 wt % solution of BTA in “dry” IL (Figure 1).
Copper foil was preliminarily pretreated with a mixture of nitric and sulfuric acids (etched copper) to remove surface metal oxides.25 The mass loss of the sacrificial copper anode upon galvanostatic polarization (the current density was 4 mA•cm−2) is evident to be dependent on the water concentration in IL 22527
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depicted in Figure 3B. In the presence of water-saturated IL (Figure 3C), the surface of the sample is seen to be partially covered with a copper oxide film protecting the metal from electrochemical corrosion. According to the data of elemental analysis, this film may be conventionally described as Cu2O (65.8 and 31.8 at % of Cu and O, respectively). The pitting of the electrode with copper oxide of different width and composition is also supported by XPS measurements. The Cu/O atomic ratio according to the XPS data varied from 0.117 to 0.042 for the copper foil electrode after the anodic polarization in “dry” and water-saturated IL, respectively. In the spectra of Cu 2p photoelectrons, two components of the Cu 2p3/2 and Cu 2p1/2 duplet and intensive satellites are observed, which indicate the presence of Cu2+ ions on the surface of the copper electrode after electrochemical treatment in “dry” and water-saturated IL. At the same time, the energetic position and the peak/satellite intensity ratios reveal the presence of Cu+ and Cu0 in both samples. Moreover, it may be concluded that the surface of the sample corresponding to Figure 3B is substantially enriched with the lower oxidation state copper atoms, in agreement with the elemental analysis and SEM data. The copper oxide films inhibiting the anodic dissolution of the copper electrode in ethylene carbonate were also previously observed.5 The anodic polarization of the copper electrode in the “dry” IL in the presence of 1 wt % BTA results in the pitting of the surface of the sample (Figure 3D) as well. The electropolishing occurs in this case only on the BTA-free sites in contrast with the sample shown in Figure 3B. The similar electropolishing action of IL also takes place for the sample containing electrodeposited copper (Figure 4A). After being current-exposed in “dry” IL, the sample of electrodeposited copper is substantially changed (Figure 4B). The electropolishing behavior of IL was observed also for the Ni electrode. The anodic dissolution of Ni foil was carried out under the same conditions as those for the copper electrode. The water inhibition effect was also found in the case of the nickel electrode. Figure 5 shows the data on water-dependent electrocorrosion of nickel in IL under the same conditions, as those used for copper. Nickel foil was pretreated with a mixture of nitric and sulfuric acids to remove surface metal oxides.25 The mass loss of the sacrificial nickel anode upon galvanostatic polarization (the current density is 4 mA•cm−2) is evidently dependent on the water concentration in IL and follows the Faraday’s law in all cases. Water acts as an inhibitor lowering the rate of anodic dissolution of the metal by a factor of 3 for “dry” and water-saturated IL, respectively. The mass of the dissolved metal for “dry” IL corresponds to the Faraday’s law describing the removal of Ni2+ ions from the electrode (Figure 5). Figure 6 depicts some polarization curves for the etched copper and nickel electrodes in the “dry” and water-saturated ionic liquid. The polarization curve for Cu in the watersaturated ionic liquid is similar to the curves observed in aqueous solutions.26,27 The only difference is the value of the corrosion current that is significantly higher in aqueous solutions (Table 3). The discrepancy between our data and data of ref 17 can be related to the different pretreatment of the electrode surface as well as different residual contents of water in the presumably “dry” ionic liquid. The region of active dissolution for copper in ILd ranges from −0.4 to +0.5 V. The region of active dissolution for copper in ILs extends from −0.8 to −0.6 V. The further increase in the voltage (from −0.6 to
Figure 2. Anodic curves of the first cycles of voltammograms for: the NSO copper electrode in “dry” IL (1), the etched copper electrode in “dry” IL (2), the etched copper electrode in water-saturated IL (3), and the etched copper electrode in “dry” IL in the presence of 1 wt % BTA (4). Insertion: anodic curves of the first cycle for the NSO copper electrode in “dry” IL (1) and the sixth cycle for the NSO copper electrode in “dry” IL (5). The potential scan rate is 10 mV·s−1. The counter electrode is the Cu foil. The reference electrode is the Ag wire.
Table 2. State of IL, State of the Sacrificial Copper Electrode, and Steady-State Potentials (E) IL “dry” “dry” water-saturated “dry” + 1 wt % BTA
electrode a
NSO etched etched etched
E (V)b −0.2 −0.2 −0.8 −0.8
a
NSO is naturally surface-oxidized.5 bReference electrode is the Ag wire.
The first and sixth anodic CVA curves for NSO copper corrosion in “dry” IL are shown in the insertion (Figure 2). The shape of the anodic curve in this case is dramatically changed. The two small peaks (30 pA) at the first cycle disappear after the subsequent six cycles and then the only diffused peak is registered. This peak increases with the number of cycles. The shape of the voltammogram with the increasing number of cycles becomes similar to that obtained for the etched sample of copper in “dry” IL (Figure 2). So, it can be assumed that the surface oxides are removed from the copper surface after cycling in “dry” IL. The increase in the potential from the steady-state value up to +0.1 V results in the change of the cathodic current from −30 pA to 0. At 0.8 V, a diffused irreversible peak appears; it is attributed to the oxidation of Cu to Cu+. It can be concluded from the mass loss of the copper electrode (Figure 1, insertion) that soluble Cu+ compounds are formed. The CVA curve is changed by the addition of both water and BTA. The current is close to zero from the steady-state potential up to +0.2 V and then increases. Passivation of the copper electrode surface occurs in this potential range. The anodic curve of the CVA in the water-saturated IL is similar to that obtained for ethylene carbonate, 5 alcohols, 12 and acetonitrile.14 Two current peaks are related to the consecutive oxidation of Cu to Cu+ and Cu+ to Cu2+. Figure 3A shows the images of the surface fragments of the initial sample of the copper foil preliminarily etched with sulfuric acid to remove copper oxide films. The result of the anodic polarization (12 C) of the initial sample in “dry” IL is 22528
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Figure 3. SEM micrographs of the surface fragment of the preliminarily etched copper electrode: Initial sample (A), the sample after anodic polarization in “dry” IL (12 C) (B), the sample after anodic polarization in water-saturated IL (12 C) (C), and the sample after anodic polarization in “dry” IL in the presence of 1 wt % BTA (12 C) (D).
Figure 4. SEM micrographs of the surface of copper electrodeposited from the aqueous solution of CuSO4: (A) initial sample and (B) after anodic polarization in “dry” IL (4 C).
ent of the water concentration in ionic liquids. The mass/ electrical charge ratio (F(Δm/ΔQ)) for Cu in bmimCl is 63.5 g•mol−1. It is possible to say that the metallic copper dissolution takes place only by means of a one-electron transfer. The mass/electrical charge ratio (F(Δm/ΔQ)) for Ni in bmimCl is 28.5 g•mol−1. The metallic nickel dissolution takes place by means of a two-electron transfer. 3.2. Redeposition of Dissolved Copper. Ionic liquids offer advantages for the electrodeposition of several met-
+1.0 V) results in a decrease in the current density by one order of magnitude due to the passivation tendency.17 The region of active dissolution for nickel in ILd ranges from −0.8 to +1.2 V. The region of active dissolution for Ni in ILs extends from −0.2 to +0.1 V. The increase in the voltage leads to a decrease in the current density. The water inhibition effect is not found in the case of Cu and Ni electrodes in bmimCl. The mass loss of sacrificial copper and nickel anodes upon galvanostatic polarization is independ22529
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Table 3. Comparison of Electrochemical Corrosion Data at Room Temperature for Copper and Nickel in Aerated Electrolytes
Figure 5. Plots of the mass loss of nickel (mNi) versus the quantity of electricity (Q, C) for electrochemical corrosion in IL under aerobic conditions for “dry” IL (1), water-saturated IL (3), and 1:1 (2) volume mixtures ILd and ILs (ILd:ILs).
electrolyte
metal
βa (V·dec−1)
icorr (μA•cm−2)
reference
IL IL ILd ILs ILd ILs H2O + HCl + bmimCl
Cu Ni Cu Cu Ni Ni Cu
0.227−0.282 0.275 0.08 0.07 0.12 0.06 0.130
0.97−1.20 0.396 2.12 2.10 0.41 0.40 18
17 17 this this this this 26
work work work work
peaks of copper are clearly visible and are similar to those obtained in [Cu(CH3CN)4][Tf2N].29 The SEM micrograph of Figure 7 shows the surface morphology of the electrodeposited copper layer on the nickel cathode obtained during galvanostatic polarization of the copper anode (the current density was 4 mA•cm−2, Q = 12 C) in ILd at 25 °C. It is seen that the deposit is dense and contains fine crystallites with average sizes of about 100−250 nm. The cathodic electrodeposition of nanocrystalline copper proceeds in bmimCl and IL without additives in the resulting electrolyte. According to the elemental analysis, the sample of electrodeposited Cu contains O − 8.7%, Cu − 89.8%.
als.28−31 Most of copper compounds have limited solubility in the ionic liquids. The best methods for introducing copper cations into ionic liquids are the synthesis of new ionic liquids with a metal-containing cation29 or anodic dissolution of a copper anode.31 The voltammogram (Figure 2, curve 2) shows no area of zero current as it would be expected for a classic ionic liquid. On the contrary, the characteristic reduction and oxidation
Figure 6. Potentiodynamic polarization curves for the metals in IL: (A) Cu in ILd, (B) Cu in ILs, (C) Ni in ILd, and (D) Ni in ILs. Scan rate is 1 mV•s−1. The reference electrode is the Ag wire. 22530
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Figure 7. SEM micrographs of the surface of copper electrodeposited from the resulting electrolyte ILd: (A) ×20 000 and (B) ×100 000. (12) Stypila, B.; Banas, J.; Starowicz, M.; Krawec, H.; Bernasik, A.; Janas, A. J. Appl. Electrochem. 2006, 36, 1407−1414. (13) Starowicz, M.; Sojka, A.; Stypula, B. Arch. Mater. Sci. Eng. 2007, 2, 609−612. (14) Klunker, J.; Schafer, W. J. Electroanal. Chem. 1999, 466, 107− 116. (15) Gonçalves, R. S.; Luchob, A. M. S. J. Braz. Chem. Soc. 2000, 11, 486−490. (16) Rao, N. N.; Singh, V. B. Corros. Sci. 1985, 25, 471−482. (17) Perissi, I.; Bardi, U.; Caporali, S.; Lavacchi, A. Corros. Sci. 2006, 48, 2349−2362. (18) Bozzini, B.; Gianoncelli, A.; Kaulich, B.; Kiskinova, M.; Mele, C.; Prasciolu, M. Phys. Chem. Chem. Phys. 2011, 13, 7968−7974. (19) Zhao, S.-F.; Lu, J.-X.; Bond, A. M.; Zhang, J. Chem.Eur. J. 2012, 18, 5290−5301. (20) Di Francesco, F.; Calisi, N.; Creatini, M.; Melai, B.; Salvo, P.; Chiappe, C. Green Chem. 2011, 13, 1712−1717. (21) Masaki, T.; Nishikawa, K.; Shirota, H. J. Phys. Chem. B 2010, 114, 6323−6331. (22) Seddon, K. R.; Stark, A.; Torres, M.-J. Pure Appl. Chem. 2000, 72, 2275−2287. (23) Maiti, A.; Kumar, A.; Rogers, R. D. Phys. Chem. Chem. Phys. 2012, 14, 5139−5142. (24) Miran, M. S.; Kinoshita, H.; Yasuda, T.; Susanzand, A. B. H.; Watanabe, M. Chem. Commun. 2011, 12676−1279. (25) Ein-Eli, Y.; Starosvetsky, D. Electrochim. Acta 2007, 52, 1825− 1838. (26) Scendo, M.; Uznanska, J. Int. J. Corros. 2011, 718626. (27) Sherif, E. M.; Park, S.-M. Electrochim. Acta 2006, 51, 6556− 6562. (28) Schubert, T.; Zein el Abedin, S.; Abbott, A.; McKenzie, K.; Ryder, K.; Endres, F. In Electrodeposition from Ionic Liquids; Endres, F., MacFarlane, D., Abbott, A., Eds.; Wiley-VCH: Weinheim, Germany, 2008; pp 83−124. (29) Brooks, N. R.; Schaltin, S.; Van Hecke, K.; Van Meervelt, L.; Binnemans, K.; Fransaer, J. Chem.Eur. J. 2011, 17, 5054−5059. (30) Abbott, A. P.; El Ttaib, K.; Frisch, G.; McKenzie, K. J.; Ryder, K. S. Phys. Chem. Chem. Phys. 2009, 11, 4269−4277. (31) Zein el Abedin, S.; Saad, A.; Farag, H.; Borisenko, N.; Liu, Q.; Enders, F. Electrochim. Acta 2007, 52, 2746−2754.
4. CONCLUSIONS On the basis of the data for mass changes of copper and nickel electrodes, CVA, and SEM, it could be concluded that at least three anodic processes occur on the metal electrode in IL: (i) Anodic dissolution of copper and nickel to give soluble products (decrease in the mass of the electrode). (ii) Anodic dissolution of metal to form insoluble products (SEM, voltammograms, potentiodynamic polarization curves). The insoluble products inhibit the anodic dissolution of the metal. (iii) Anodic decomposition of water with oxygen evolution. The results of the competition of these processes depend on the water content in IL. The increase in the water content in IL results in the inhibition of metal corrosion. (iv) The simultaneous cathode electrodeposition of copper nanoparticles occurs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (O.L.), lmkustov@ mail.ru (L.K.). Notes
The authors declare no competing financial interest.
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REFERENCES
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