Cathodic Behaviour of Nickel in Alcohol Solutions of Electrolytes Magdalena Bisztyga1,a, Urszula Lelek-Borkowska1, Edyta Proniewicz1, Jacek Banaś1 1
AGH University of Science and Technology, Reymonta 23 St., 30-059 Krakow, Poland
ABSTRACT Cathodic behavior of nickel was studied in anhydrous methanol containing sodium methylate. The behaviour of nickel at low overpotentials is determined by competitive processes, reduction of methanol and formation of surface product. The formation of surface layer is the result of a chemical oxidation of intermediate product (NiOCH3 ad). This reaction in the presence of CH3O- anions causes the precipitatation of the Ni(OCH3)2 film. The process is accompanied by parallel electrochemical reduction of Ni2+ evidenced by a “limiting” current in the cathodic area. The methoxy film formation during cathodic polarization of the metal surface was confirmed by surface morphology examinations.
Key words: methanol, nickel, cathodic reduction
a
Corresponding author: M. Bisztyga (
[email protected]), tel.: +486173047
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1. Introduction Knowledge about the electrochemical and corrosion behavior of metals in anhydrous alcoholic solution of electrolytes is important in scientific and application aspects. These solutions are used for the surface treatment of metals, metal coatings and the preparation of the composites, the preparation of metallic nanoparticles. Pure alcohols are used as fuel or fuels additives. Despite the relatively rich bibliography devoted to the anodic properties of the metals in alcoholic solutions of electrolytes [1]–[14], only little attention was paid to the cathodic reactions occurring on the metal’s surface in anhydrous alcoholic media. These reactions can substantially affect the formation of the alkoxy product on the metal surface, a phenomenon similar to the alkalization of metal surface in an aqueous media during the cathodic polarization. Such alcoxylation of metal surface promotes the formation of surface barrier layer at low cathodic and anodic overvoltage and leads to pseudopassivation of metal surface [8]. So far only few studies focused on mechanism of formation and structure these surface compounds generated on the metal surface [8]. The aim of the presented work was to explain the mechanism of cathodic reduction of the methanol on nickel electrode, especially at low overvoltages, when surface product can be formed on the metals surface. The nickel/methanol system was selected due to its simplicity: nickel is the metal with relatively simple mechanism of anodic and cathodic reactions (divalent compound Ni(II)) and methanol is the simplest alcohol .
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2. Experimental 2.1. Electrochemical investigations Electrochemical measurements (CV) were performed with use of the electrochemical workstations AutoLab PGStat 303 in three-electrode system (working electrode – Ni, counter electrode – Pt, reference electrode – Ag/AgCl/3M KCl). Every potential value has been recalculated vs. standard hydrogen electrode. Investigations were performed in anhydrous CH3OH – 0.1M CH3ONa and CH3OH –xLiCl-yCH3ONa (x+y = 0.1M) solutions deaerated by argon purging (99,997%) at the temperature 25 ºC [15] (water content in alcohol was below 0.02 %). Cyclic polarization curves were performed in the potential range -1.2 V ÷ 0.4V thereby avoiding the dissolution of nickel at the anodic range. Three cycles were recorded at each experiment. The course of the curves is very similar, indicating good reproducibility of the polarization curves. The charts presented in the paper shows the second cycle of each measurement. It is assumed that the second curve presents the processes occurring on the surface free from the natural metal oxide layer, removed during the first potential scan. Polarization curves was corrected for IR drop. The R value between the working electrode and the reference electrode was obtained with impedance measurements [6].
2.3. Surface analysis Morphology of surface products was studied using an optical microscope Nikon type ECLIPSE LV100, laser profilometer XRF SYSTEMS. After analyzing the CV curves, there were determined ranges of potentials corresponding to the cathodic processes. The samples surfaces polarized at a chosen potential were then analyzed by 3
scanning electron microscopy (SEM) with electron diffraction X-ray (EDX) using JEOL scanning electron microscope JSM 5500 LV. An atomic force microscope (AFM) images of the surface layer were measured with a MFP-3D-BIO (Asylum Research). The results were analyzed by the computer program Asylum ver. 12 Igor Pro. Topography of the surface layer was investigated using confocal microscope LEXT OLS4100 OLYMPUS. The surface product composition was examined by the FT-Raman and FTIR-ATR spectroscopy. The FT-Raman spectra were collected using an InVia spectrometer (Renishaw) equipped with an air-cooled charge-coupled device (CCD) detector. The spectra resolution was set at 4 cm-1. The 785.0 nm line of an diode laser was used as the excitation source. The laser power at the output was set at 40 mW. The FTIR-ATR measurements were recorded using FT-IR Thermo Scientific Nicolet 6700 Spectrometer with a Segull attachment adapted for an electrochemical cell. The working electrode was a Ni layer of thickness approximately 500 Å sputtered onto a ZnSe hemisphere – a material transparent to the IR beam, with Pt as the counter electrode and an Ag wire covered with AgCl serving as the reference electrode [16].
3. Results 3.1. Polarisation measurements Figure
1
presents
a
voltammetric
polarization
curve
of
nickel
in
CH3OH - 0.1M CH3ONa solution obtained with a scan rate of 1 V min-1.
4
Figure 1. Cyclic voltammogram of nickel in CH3OH – 0.1M CH3ONa
Figure 2 shows the impact of the scan rate on the course of polarization curves.
a)
5
b)
c) Figure 2. Effect of scan rate on polarization of nickel in CH3OH – 0.1M CH3ONa (a) logarithmic plot (b) forwards scan – linear plot (c) reverse scan – linear plot
The shape of the anodic part of the curves indicates the blocking of the nickel electrode by the surface product. Anodic curve exhibits the limiting current. This current is a linear function of the scan rate what is the proof that process of film formation is a surface process (Figure 3).
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Figure 3. Effect of scan rate on cathodic limiting current at constant potential of E = -0.8 V
The explanation of the shape of polarization curves in cathodic domain is complicated. At high cathodic overvoltage (< 1V) (nie powinno być -1V?), the course of polarization curves does not depend on the scan rate and corresponds to the reduction of methanol with the evolution of hydrogen. The curve from the potential of -1 V in the anodic direction
illustrates firstly the methanol reduction and then the “limiting”
cathodic current in the potential range from -0.8 V to -0.4 V (Fig.2b). This current is not associated with the anodic product reduction, as evidenced by the cyclic hysteresis of the polarization curves in cathodic domain (value of the reduction current is greater in the reverse scan - in positive direction, Fig. 1). Relationship of the “limiting” cathodic current from the square root of the scanning rate and the square root of the rotation rate of the electrode is illustrated on Fig. 4 and 5. At the high speeds of the as well scan rate as the rotation of disk electrode the cathodic current does not changes much. The polarisation curve conducted from the potential of 0.4 V in the cathodic direction (Fig. 2c) exhibits the cathodic peak at the potential about -0.8 V. This peak is probably
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associated with the reduction of the surface product formed in the anodic domain (lub potential range).
Figure 4. Effect of scan rate on cathodic limiting current at constant potential of E = -0.8 V
Figure 5. Effect of rotation rate of disc electrode on cathodic limiting current at constant potential of -1 V
The “limiting” cathodic current depends on the concentration of methoxy ion in the CH3OH – xM LiCl – yM CH3ONa solutions as shown in polarisation curves (Fig. 6 8
and 7). Even a small amount of the CH3O- anions causes the appearance of limiting current on the reverse curve.
Figure 6. Effect of concentration of CH3ONa on cathodic polarization of nickel in CH3OH – xM LiCl – yM CH3ONa (x + y = 0.1M)
Figure 7. Effect of CH3ONa concentration in CH3OH – xLiCl- yCH3ONa (x+y = 0.1M) on cathodic current at -0.7 V
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Fig. 8 compares the image of the nickel electrode surface etched at the potential of -0.6 V (under limiting current) in the CH3OH - xM LiCl - yM CH3ONa solutions with different concentration of methoxy anions. The presence of the CH3O- anions causes the formation of weakly adhesive layer. In a pure solution of LiCl, free of methoxy anions, formation of the deposit is not observed.
Figure 8. The microscopic image of nickel surface after potentiostatic etching in CH3OH-LiCl-CH3ONa solutions at potential E = -0.6V in (a) CH3OH – 0.1M LiCl, magnification: 100x; (b) CH3OH – 0.05M LiCl – 0.05M CH3ONa; magnification: 100x; (c) CH3OH – 0.1M CH3ONa; magnification: 200x.
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3.2. Surface analysis The structure and morphology of the surface layer forming on the Ni-electrode were examined with microscopic methods and "ex situ" as well as "in situ" spectral methods. Figure 9 shows image obtained with an optical microscope of the surface layer after 30 min immersion in CH3OH - 0.1 M CH3ONa solution at a potential of E = -0.2 V. The layer covering the nickel surface has a poor adhesion to the metal surface and after emerging from the solution undergoes rapid detachment (Figure 9).
Figure 9. The optical image of the surface product on Ni-electrode (CH3OH - 0.1M CH3ONa, EH = -0.2 V, immersion time: 30 min, magnification: 100x)
Figure 10 presents the topography of the surface layer obtained under the same conditions using AFM techniques. The thickness of the surface film is difficult to determine because of surface product is "wrinkled".
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Figure 10. The AFM image of the surface product on Ni-electrode in CH3OH - 0.1M CH3ONa shows not clearly identifiable forms in the 300 nm diameters (EH = -0.2 V, immersion time: 30 min)
The picture of the surface film adjacent to the nickel surface was obtained using confocal microscopy (Figure 11). As it can be seen, the layer is not uniform. The resulting surface pseudo-passive layer at its thickest point had a thickness of approx. 1 mm (these were only a single tiny pieces). Unfortunately it exhibited very poor adhesion properties and in contact with air become detached. Therefore, further analysis of the products was performed on a layer that is left on the nickel electrode. Its height, which can be seen in the picture obtained with a confocal microscope (Figure 11) was a few micrometers. Precise image analysis of this film allows to observe that the layer on the surface grows in vertical direction.
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Figure 11. The surface product obtained by confocal microscopy (CH 3OH - 0.1M CH3ONa, EH = -0.4 V,
immersion time: 60 min)
Figure 12. SEM Picture of Ni surface polarized in CH3OH-0,1M CH3ONa at potential (a) -0,2 V for 30 min (b) -0,4 V for 30 min
Figure 12 a and b presents the images of the surface topography of nickel sample 13
obtained by scanning electron microscopy with EDX analysis for two different cathodic potentials (-0.2 V and -0.4 V). SEM images and EDX analysis (Figure 12a) showed that the various parts of the surface film (area 1 and 3) exhibit relatively good homogeneity. Large surface enrichment in carbon and adequate oxygen content suggests that this layer is not a typical oxide film as which can be found in aqueous solutions, but it has the organic nature. Pseudopassive film formed on the nickel surface is most likely an amorphous layer of nickel with a large proportion of the alkoxy derivatives of this element Ni(OCH3)2 – nickel methoxylate. Figure 12 a and b show that both tested surface were uneven coated with the layer. EDX analysis revealed that the surface among the parts of the layer are rich of nickel (area 2). The presence of methanolate groups was confirmed by the FT-Raman spectroscopy. Figure 13 presents the FT-Raman spectrum of the film obtained at the surface of Ni-electrode polarized in a solution of CH3OH - 0.1M CH3ONa at a constant potential (E = -0.4 V). The presence of carbon in these bonds (Table 1) indicates the participation of OCH3- groups in the structure of the surface film.
Figure 13. FT-Raman spectra of surface product adsorbed onto the Ni-electrode in CH3OH – 0.1M CH3ONa solution
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In order to analyze the influence of the potential for the composition of the surface layer and the identification of the product on the Ni-electrode there were performed in situ measurements (FTIR-ATR).
Figure 14. In situ FTIR spectra of surface product adsorbed onto the Ni-electrode in CH3OH – 0.1M CH3ONa solution
The spectra were collected during an experiment in which the potential was stepped up from 0.0 V to -0.8 V in 100 mV increments. Spectra were collected at each step and normalised to the reference spectrum taken at 0.0 V, hence the low intensity of individual spectrum bands. Spectra obtained at various cathodic overpotentials are similar, they differ only in intensity of the signal origin from the Ni-OCH3 group. Figure 14 illustrates the in situ FTIR spectra of the surface product adsorbed onto the Ni-electrode obtained at -0.2 V in anhydrous deaerated methanol solution of 0.1M CH3ONa. The interpretation of the FT-Raman and FTIR-ATR individual signals is given in Table 1. 15
FT-Raman
FTIR-ATR
Table 1. Interpretation of band positions in FTIR-ATR and FT-Raman spectrum of nickel surface in CH3OH - 0.1M CH3ONa. Wave number [cm-1] 2819 - 3005 1329 - 1450 1055
Oscillation type
Type of bounding
Literature
CH3, O-CH3 O-CH3 alcohols
[18], [19] [18], [19] [18], [19]
O-CH3, (Ni-OCH3)
[17]
665 - 680 ~2970, ~2850 ~1450, ~1350
C-H stretching C-H scissoring C-O stretching C-O stretching (deformatiion) C-H wagging C-H stretching C-H scissoring
CH3 CH3, O-CH3 CH3
[18], [19] [18], [19] [18], [19]
~1080
C-O stretching
Ni-OCH3
[20], [21]
933
C-H stretching (deformatiion)
O-CH3 (Ni-OCH3)
[17]
1006 - 1029
Figure 15 presents the analysis of the relative intensity of bands derived from the Ni-OCH3 group compared with the polarisation curve of Ni on ZnSe. The number of OCH3 groups on the nickel surface increases in the cathodic direction. The increasing concentration of groups OCH3 causing the methoxy pseudopassive layer formation arises from the methanol reduction according to the reaction 1. 2CH3OH + 2e
2CH3O- + H2
(1)
The study as well FTIR-ATR as FTIR-Raman measurements confirmed the presence the Ni-OCH3 bands on the surface of Ni-electrode in anhydrous methanol solution of sodium methoxide.
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Figure 15. Intensity of FTIR-ATR signal from OCH3 group at various potentials signal in comparison with polarization curve
4. Discussion Studies of electrochemical properties of nickel in anhydrous methanol solution in the cathodic range of potential are summarized in Figure 16. The potential range presented in the Fig. 16 can be divided into three areas. Area A corresponds to the methanol reduction according the reaction (1). In this area rapid evolution of hydrogen is observed.
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Figure 16. Diagram illustrating the cathodic properties of nickel in anhydrous methanol containing methoxy anions (OCH3-)
In the area C nickel undergoes the “methoxy-pseudopassivation”. This film is composed of Ni-OCH3 compounds. Mechanism of this pseudopassivation can be described by scheme [8]: Ni + CH3O- ↔ NiOCH3 ad + e
(2)
NiOCH3 ad + CH3O- ⟶ Ni(OCH3)2 ad + e
(3)
The oxidation of nickel, especially the creation of a monovalent NiOCH3 product, takes place already in the cathodic potential range. In an alkaline environment, in the presence of CH3O- ions the cathodic area is interesting because of the presence of atypical appearance of hysteresis and “limiting” current, visible on polarization curves recorded from potential of -1V towards the positive direction (Fig. 16, area B). This current is not associated with the methanol reduction. It 18
also cannot be attributed to the methoxy surface layer reduction, formed in the anodic range. The explanation for the limiting current presence on the cathodic curve may be a reduction of Ni2+ ions resulting from the chemical oxidation of the NiOCH3 by the solvent - molecules of methanol. This phenomenon is similar to the anomalous dissolution of metals in aqueous environments [22]. The mechanism is presented on the scheme in Fig. 16 (area B). Reaction of chemical oxidation of NiOCH3
ad
is probably
faster than parallel reduction of Ni2+ ions. Thus, after longer polarization of nickel in cathodic area there is observed increase of the Ni(OCH3)2 film, what was evidenced by surface morphology examinations. Area C corresponds to the formation of an anodic surface film according to the reactions 2 and 3. In this area the methanol oxidation is also possible. This process is blocked by a surface layer of nickel methoxylate.
5. Conclusions The study has shown that the polarization of nickel in anhydrous methanol solution containing CH3O- ions lead to coverage of the metal surface with the methoxy film. This process is the result of two paths of reactions. The first is a classical consecutive sequence of anodic reactions leading to the formation of a thin film of Ni(OCH3)2 according to reactions (2) and (3). The presence of the anodic film of NiOCH3 on the nickel surface in anhydrous methanol solutions of electrolytes has been confirmed in our previous studies [8], [16]. The second reaction pathway leads to creations of a thicker film which is the result of a chemical oxidation of intermediate product (NiOCH3 ad). This reaction in the presence of CH3O- anions causes the precipitate of the Ni(OCH3)2 film. This process is accompanied by parallel electrochemical reduction 19
of Ni2+ evidenced by a “limiting” current in the cathodic area. The methoxy film formation during cathodic polarization of the metal surface was confirmed by surface structure investigations. References: [1]
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F. Bellucci, C. A. Farina, G. Faita, Effect of water content on the corrosion behaviour of Ni in methanol (MeOH) solutions, Electrochim. Acta, 26, (1981) 731–733
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M. Nicola, M. Constantinescu, T. Badea, Rev. Chim., 29, (1978) 156
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J. Światowska-Mrowiecka, J. Banas, Anodic behaviour of zinc in methanol solutions of lithium perchlorate, Electrochim. Acta, 50 (2005) 1829–1840
[8]
J. Banaś, B. Stypuła, K. Banaś, J. Światowska-Mrowiecka, M. Starowicz, U. Lelek-Borkowska, Corrosion and passivity of metals in methanol solutions of electrolytes, J. Solid State Electrochem., 13, (2009) 1669–1679
[9]
U. Lelek-Borkowska, J. Banas, Anodic dissolution of silicon monocrystals in anhydrous organic solutions of chlorides, 47 (2002) 1121–1128
[10] B. Stypuła, J. Banaś, Modification of Passive Film. The Institute of Materials, (1994) [11] J. Banaś, Passivity of metals In anhydrous solutions of oxy acids, Mater. Sci., (1995) 185–188 [12] J. Banaś, B. Mazurkiewicz, W. Solarski, K. Banaś, Mat Sci Forum, 871 (1995) 185–188 [13] S. P. Trassati, E. Silvieri, Electrochemical and stress corrosion cracking behaviour of titanium in n-propanol and iso-propanol solutions., Mater. Chem. Phys., 83 (2004) 367–372 [14] E. Heitz, Corrosion of Metals in Organc Solvents, Adv. Corr. Sci. Technol., 4 (1974), R.W. Staehle, and M.G. Fontana, New York: Plenum Press, pp. 149-243, 20
(1974) [15] A. I. Vogel, Preparatyka organiczna. Warszawa: WNT (1984) [16] U. Lelek-Borkowska, J. Banaś, Badanie produktów procesów elektro-chemicznych zachodzących na powierzchni niklu w roztworze CH3OH-LiClO4 metodą spektroskopii FTIR-ATR, Ochr. przed Korozją, 54 (2011) 91–93 [17] D. Lin-Vien, N. B. Colthup, W. G. Fateley, J. G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, San Diego CA: Academic Press (1991) [18] “NIST Standard Reference Database 69.” [Online]. Available: http://webbook.nist.gov/chemistry/. [19] B. P. Brandwal, R. C. Methrota, Synthesis and characterization of some alkoxide derivatives of nickel (II), Aust. J. Chem., 33 (1980) 37–43 [20] A. Kakos, G. Winter, Aust. J. Chem., 27, (1968) 793 [21] I. S. Ignatyev, M. Montejo, J. J. Lopez Gonzalez, Vib. Spectrosc., 51 (2001) 218 [22] D. M. Drazić, J. P. Popiś, Anomalous dissolution of metals and chemical corrosion, J. Serb. Chem. Soc., 70 (2005) 489–511
Tables and figures captions Tables 1. Interpretation of band positions in FTIR-ATR and FT-Raman spectrum of nickel surface in CH3OH - 0.1M CH3ONa. Figure 1. Cyclic voltammogram of nickel in CH3OH – 0.1M CH3ONa Figure 2. Effect of scan rate on polarization of nickel in CH3OH – 0.1M CH3ONa (a) logarithmic plot (b) forwards scan – linear plot (c) reverse scan – linear plot Figure 3. Effect of scan rate on cathodic limiting current at constant potential of E = 0.2 V Figure 4. Effect of scan rate on cathodic limiting current at constant potential of E = -0.8 V Figure 5. Effect of rotation rate of disc electrode on cathodic limiting current at constant potential of -1 V 21
Figure 6. Effect of concentration of CH3ONa on cathodic polarization of nickel in CH3OH – xM LiCl – yM CH3ONa (x + y = 0.1M) Figure 7. Effect of CH3ONa concentration in CH3OH – xLiCl- yCH3ONa (x+y = 0.1M) on cathodic current at -0.7 V Figure 8. The microscopic image of nickel surface after potentiostatic etching in CH3OH-LiCl-CH3ONa solutions at potential E = -0.6V in (a) CH3OH – 0.1M LiCl, magnification: 100x ; (b) CH3OH – 0.05M LiCl – 0.05M CH3ONa; magnification: 100x (c) CH3OH – 0.1M CH3ONa; magnification: 200x. Figure 9. The optical image of the surface product on Ni-electrode (CH3OH - 0.1M CH3ONa, EH = -0.2 V, immersion time: 30 min) Figure 10. The AFM image of the surface product on Ni-electrode in CH3OH - 0.1M CH3ONa shows not clearly identifiable forms in the 300 nm diameters (EH = -0.2 V, immersion time: 30 min) Figure 11. The surface product obtained by confocal microscopy (CH3OH - 0.1M CH3ONa, EH = -0.4 V, immersion time: 60 min) Figure 12. SEM Picture of Ni surface polarized in CH3OH-0,1M CH3ONa at potential (a) -0,2 V for 30 min (b) -0,4 V for 30 min Figure 13. FT-Raman spectra of surface product adsorbed onto the Ni-electrode in CH3OH – 0.1M CH3ONa solution Figure 14. In situ FTIR spectra of surface product adsorbed onto the Ni-electrode in CH3OH – 0.1M CH3ONa solution Figure 15. Intensity of FTIR-ATR signal from OCH3 group at various potentials signal in comparison with polarization curve 22
Figure 16. Diagram illustrating the cathodic properties of nickel in anhydrous methanol containing methoxy anions (OCH3-)
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