Deposition of Ni–Co by cyclic voltammetry method and its electrocatalytic properties for oxygen evolution reaction

International Journal of Hydrogen Energy 30 (2005) 29 – 34

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Deposition of Ni–Co by cyclic voltammetry method and its electrocatalytic properties for oxygen evolution reaction Bo Chi, Jianbao Li∗ , Xiaozhan Yang, Yanli Gong, Ning Wang State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Accepted 18 March 2004 Available online 21 July 2004

Abstract In this paper, the deposition of Ni–Co alloy coating on Cu substrate was prepared by cyclic voltammetry method. The Ni– Co coating was measured by XRD, SEM and EDS to determine the composition and morphology. The deposition of Ni–Co alloy is anomalous deposition, that is, the ratio of Ni–Co in alloy is not equal to that in the deposition bath. With the increase of cyclic times, the content of Co increases. Polarization curves and AC impedance measurement show that the alloy coating electrodes are more active than the Ni electrode. With the content of Co increase in the alloy coating, the electrocatalytic activities of the alloy coating increase. Heat treatment can also improve the properties of alloy coating for OER. ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Ni–Co alloy; Cyclic voltammetry; OER

1. Introduction Nickel, cobalt and their alloys are important engineering materials used widely in many ?elds because of their special properties such as magnetic [1,2], heat-conductive [3], and electrocatalytic properties [4]. Ni–Co alloy as cathodic electrocatalyst for the hydrogen evolution reaction (HER) has been studied by many researchers [5,6]. It shows high electrocatalytic activities for HER in alkaline solution. The oxygen evolution reaction (OER) is also an important process because of its relationship with most important electrochemical processes occurring at low and moderate temperature: hydrogen production in water electrolysis, metal electrowinning, energy storage in metal air batteries, anodic organic synthesis and metal corrosion [7]. One of the major shortages of traditional electrode materials is the high overpotential associated with the OER, which leads to low eEciency. Many researchers [8–10] have tried ∗ Corresponding author. Tel.: +86-10-62772848; fax: +86-1062782753. E-mail address: [email protected] (J. Li).

to ?nd new materials with low overpotential and high electrocatalytic activities for the OER. But the research of the electrocatalytic properties of Ni–Co alloy for OER was seldom reported. In this paper, the electrocatalytic properties of Ni–Co alloy for OER were investigated. Ni–Co alloy was prepared by cyclic voltammetry method. The eHect of cyclic times on Ni/Co ratio and the electrocatalytic properties was discussed. And the eHect of the heat treatment on the alloy was also studied. 2. Experimental details The Ni–Co deposits were plated onto 1 × 1 cm2 oxygen-free copper substrates. The Cu substrates were polished with SiC grinding paper (Buehler P2500), cleaned with acetone using ultrasonics-cleaning equipment, then rinsed with distilled water. The exposed surface area of Cu was 1 cm2 , while other surfaces were coated with epoxy resin. The deposition bath was prepared according to the metal ion Ni/Co ratio of 4:1 using NiSO4 · 7H2 O and CoSO4 · 8H2 O. The bath pH value of 2 was adjusted by

0360-3199/$ 30.00 ? 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2004.03.032

B. Chi et al. / International Journal of Hydrogen Energy 30 (2005) 29 – 34

3. Results and discussion 3.1. Cyclic voltammogram of the CV plating Typical cyclic voltammogram for the CV plating of Ni–Co coatings is shown in Fig. 1 (recorded at the third cycle). The scan rate is 0:01 V s−1 . Note that at potential of about −0:7 V, obvious current can be observed. Negative to −0:7 V, the current increases rapidly, indicating that nickel and cobalt ions were deposited onto Cu substrate in the cathodic deposition region. However, neither cathodic deposition nor anodic oxidation occurs when electrode potentials are located between −0:7 and −0:3 V, while the current is very low. The larger anodic current on the CV curve at potentials that is positive than −0:3 V indicates the anodic oxidation of Ni–Co alloy [11]. Another question should be mentioned that when the potential becomes more negative than −1:4 V, the evolution of hydrogen could be observed, which is not expected because the evolution of hydrogen will interfere the evolution of alloy and de-

0.04 0.02 -2

boric acid. And the temperature was room temperature. The potential range for cyclic voltammetry was −1:3 to −0:1 V (vs SCE). The thickness of the coating was controlled by changing the times of cyclic voltammetry. The more the cyclic times are, the thicker the coating is. In our experiments, the cyclic times are 10, 20, and 30, respectively, while the corresponding electrode was named NC1, NC2, and NC3. The heat treatment of the coatings was performed at 500◦ C in vacuum for 2 h. The pure Ni coating electrode was also prepared for comparison. Cyclic voltammetry for the deposition of Ni–Co coatings was performed at CHI604A electrochemical analyzer (CH Instruments Corp., USA) in a conventional three-electrode glass cell. The potential of the working electrode was measured against a saturated calomel electrode (SCE). All potential values for the test electrodes are given with respect to the reversible potential for this reference electrode. The reference electrode was brought into contact with the cell bath through a Luggin capillary (KCl agar-agar salt bridge). The auxiliary electrode consisted of a 4 cm2 Pt foil. The thickness and the micrographs of the coatings were performed at Olympus BX60 optical microscope (OLYMPUS, Japan) and JSM-6460LV (JEOL, Japan) scanning electron microscope. The phase compositions of the coatings were measured by X-ray diHraction (XRD) patterns using a D/max-RB X-ray diHractometer (RIGAKU, Japan) with ?ltered CuK radiation of wavelength 0:15418 nm. And the ratio of Ni/Co was measured by energy dispersive spectrum (EDS) aEliated to SEM. The polarization curves for oxygen evolution reaction and AC impedance spectroscopy of the coatings were performed at CHI604 electrochemical analyzer in three-electrode glass cell at 20◦ C. The electrolyte was 1 M KOH.

current density / Acm

30

0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12 -0.14 -1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

potential / V (vs SCE)

Fig. 1. Cyclic voltammogram during the Ni–Co deposition process in bath at scan rate of 0:01 V s−1 .

Fig. 2. XRD patterns of Ni–Co coatings obtained at diHerent cyclic times.

stroy the compactness of the alloy. So the selection of the potentials range is important. 3.2. Composition of the coating Fig. 2 shows the XRD patterns of the Ni–Co coatings according to the diHerent cyclic times. Except the diHraction peaks assigned to Cu substrate and Ni–Co, no other impurity diHraction peaks could be found in Fig. 2. Since the diHraction peaks of metal nickel and cobalt have little diHerences, they cannot be separated from each other in Fig. 2. With the increase of the cyclic times, the diHraction strength of Cu substrate decreases. When the cyclic times are 30 (NC3), the diHraction peak of Cu is almost absent, which means the coating is thick enough and X-ray cannot reach the substrate. After heat treatment, no obvious change (Fig. 3) can be found except that the diHraction strength

B. Chi et al. / International Journal of Hydrogen Energy 30 (2005) 29 – 34

31

Fig. 3. XRD patterns of Ni–Co coating (NC1) before (1) and after (2) 500◦ C vacuum heat treatment.

of the characteristic peak becomes strong, which can be attributed to the better crystallinity and compactness of the Ni–Co alloy coating. 3.3. Micrographs of the coating Fig. 4 is the cross-section of the three electrodes NC1, NC2, and NC3. With the increase of cyclic times, the thickness of the coating increases. The thickness of the coating is 17, 30, and 38 m for NC1, NC2, and NC3, respectively. The increasing rate is not uniform. When the coating thickness is thin, the increasing rate is fast. As cyclic times increase, the rate slows down. In the starting stage, the content of the metal ions is high enough. And the content of the metal ions in the solution decreases with the cyclic times. Another reason is the decrease of the active sites for metal ions to deposit. The surface micrographs of the samples are shown in Fig. 5. The grain sizes grow large with the increase of cyclic times. When cyclic times are 10 (Fig. 5A), the grains are small and could not be identi?ed clearly. When cyclic times are 30 (Fig. 5C), the grains are large and the size is about 1 m. Another obvious diHerence that could be found in Fig. 5 was the change of the corrugations on the coating surface. These corrugations occur in the Cu substrate because of the polishing by SiC grinding paper. In Fig. 5A, the corrugation could be observed clearly, which means that the coating is thin. In Fig. 5C, the corrugation is not very clear and the coating becomes thick. This result consists with the true thickness of the coating. 3.4. Ni/Co content of the coating The Ni/Co ratio of the alloy coating was measured by EDS (Fig. 6). The results show that the deposition of

Fig. 4. The cross-section of the Ni–Co coating. (A) NC1; (B) NC2; (C) NC3.

Ni–Co is an anomalous codeposition, which means that the ratio of Ni/Co in the coating is not in accordance with the starting ratio in bath. The Ni/Co ratios of the coatings are 1.91, 1.71, and 1.52 in NC1, NC2, and NC3, respectively, which is smaller than that in starting bath of 4. Co is enriched within the Ni–Co alloys. Several researchers have described the above anomalous codeposition [12]. The reason may be related with the competition adsorption of metal hydroxides, monohydroxides, or metal ions [13]. An interesting phenomenon is that with the increase of cyclic times, the content of Co in alloy increases.

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B. Chi et al. / International Journal of Hydrogen Energy 30 (2005) 29 – 34

Fig. 6. Energy dispersive spectrum of NC3 Ni–Co coating.

0.75

potential / V (vs SCE)

0.70

NC1 NC2 NC3 Ni

0.65

0.60

0.55

0.50 -3.0

-2.5

-2.0

-1.5

log i / Acm

-1.0

-0.5

-2

Fig. 7. Polarization curves of Ni–Co coating electrodes and Ni electrode.

Fig. 5. SEM micrographs of Ni–Co coating. (A) NC1; (B) NC2; (C) NC3.

4. Electrocatalytic properties for oxygen evolution reaction 4.1. Polarization curves for OER Fig. 7 shows the polarization curves of Ni–Co coating electrodes for the OER, measured in 1 M KOH at 20◦ C.

The overpotential observed on the Ni electrode was high (439 mV under i = 50 mA cm−2 ), while the lower overpotentials were observed on alloy coatings. The overpotentials at 50 mA cm−2 , compared to that on the Ni electrode, were 300, 285, and 246 mV on NC1, NC2, and NC3, respectively. Improved performance for the OER on the alloy coating electrodes was also evaluated on the apparent current densities at overpotential of 300 mV. Increases in the current densities at overpotential of 300 mV were displayed on the Ni–Co alloy coating electrodes. Table 1 shows the electrocatalytic parameters for the OER of these electrodes. The results show that the alloy coating electrodes exhibit better electrocatalytic properties for the OER than the nickel metal electrode. The more the cyclic times are, the better the electrocatalytic properties are. This may be related with the increase of the Co content in the alloy coating, though it needs further studies to con?rm this presumption. The surface roughness factors (Rf ) of the electrodes, calculated from the surface double layer capacitance, were listed in Table 1. The true current densities at overpotential of 300 mV also reveal that NC3 has the best electrocatalytic

B. Chi et al. / International Journal of Hydrogen Energy 30 (2005) 29 – 34

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Table 1 Parameters of electrodes for the OER Electrode

Ni NC1 NC2 NC3

Coating thickness (m)

Ni/Co ratio

— 18 32 40

— 1.91 1.71 1.52

Rf

50

— 10.1 10.0 13.2

(mV)

50 after heat treatment (mV)

iapp; 300 (mA cm−2 )

itrue; 300 (mA cm−2 )

439 300 285 246

— 285 280 243

14.55 50.02 56.40 95.68

— 4.95 5.64 7.25

50 : overpotential at current density of 50 mA cm−2 ; iapp; 300 : apparent current density at overpotential of 300 mV; itrue; 300 : true current density at overpotential of 300 mV.

40 Rct

NC1 NC2 NC3

35

-Z'' / Ω cm-2

30

Cpe

1

25

1 1

20

0.1

15

0.1

10

0.1

10

5 0 0

10

20

30

40

50

60

70

80

frequency of corresponding impedance was also shown in Fig. 8. The resistance of the electrolyte, Rs , between working electrode and the reference electrode, was compensated before the ?tting, and therefore is not shown in the ?gure. The model of the equivalent circuit includes the constant phase element (Cpe ) in parallel with the charge transfer resistance Rct (insertion in the Fig. 8). The values of the Rct are 80.85, 69.63, and 58:86 P cm−2 for NC1, NC2, and NC3, respectively. The lowest Rct , 58:86 P cm−2 , was observed on NC3. The decrease in the Rct from NC3 to NC1 is in agreement with the increase of the current densities at given overpotential obtained by polarization measurements (Fig. 7).

Z' / Ω cm-2 Fig. 8. AC impedance spectra in complex-plane plots, measured at potential of 500 mV at 20◦ C. (Numbers of 10, 1, 0.1 in ?gure represent the frequencies (Hz).)

properties for OER in the three electrodes. Although the large surface area may increase the electrocatalytic properties for NC3, the true electrocatalytic properties of NC3 are better than the other two electrodes. After heat treatment at 500◦ C in vacuum, the electrocatalytic properties of the three alloy electrodes are also enhanced. The overpotentials at 50 mA cm−2 of NC1, NC2, and NC3 are 285, 280, and 243 mV, respectively. Heat treatment may help to eliminate the internal defect of alloy and obtain grains with good crystallinity, thus decreases the overpotential for the OER. But higher heat treatment temperature may reduce the adhesion between the Cu substrate and the alloy since their thermal expansion coeEcients are diHerent. 4.2. AC impedance spectroscopy AC impedance measurements were carried out at potential of 500 mV. The complex plane plots, −Z  imaginary impedance against Z  real impedance, are shown in Fig. 8. The experimental data in the ?gure are presented as points and the continuous lines are obtained by curve ?tting. The

5. Conclusions Ni–Co alloy was deposited by cyclic voltammetry method. The deposition of Co is anomalous. With the increase of cyclic times, the accumulation of Co in the coating was found. The alloy coating electrodes as electrocatalyst for OER show that they have better electrocatalytic properties and lower overpotential than pure Ni. As the content of Co increases, the electrocatalytic properties of the alloy coating increase. Heat treatment can also improve the properties of alloy coating for OER. References [1] Duch M, Esteve J, Gomez E, et al. J Electrochem Soc 2002;149:C201. [2] Kuo CC, Lin WC, Minn-Tsong Lin. J Magn Magn Mater 2002;239:298. [3] Singh VB, Singh VN. Plating Surf Finish 1976;63:34. [4] Lian K, Kirk DW, Thorpe SJ. Electrochim Acta 1991;36:537. [5] Miao HJ, Piron DL. Electrochim Acta 1993;38:1079. [6] Arul Raj I, Vasu KI. J Appl Electrochem 1990;20:32. [7] Restovic A, Poillerat G, Chartier P, et al. Electrochim Acta 1994;39:1579. [8] Bocca C, Barbucci A, Delucchi M, et al. Int J Hydrogen Energy 1999;24:21. [9] SuHredini HB, Cerne JL, Crnkovic FC, et al. Int J Hydrogen Energy 2000;25:415.

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[10] Tiwari SK, Chartier P, Singh RN. J Electrochem Soc 1995;142:148. [11] Gomez E, Ramirez J, Valles E. J Appl Electrochem 1998;28:71.

[12] Brenner A. Electrodeposition of alloys: principles and practice. New York: Academic Press; 1963. [13] Bai A, Hu CC. Electrochim Acta 2002;47:3447.