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Science in China Series E: Technological Sciences © 2007

Science in China Press Springer-Verlag

Electrochemical studies of nickel deposition from aqueous solution in super-gravity field GUO ZhanCheng1,2†, GONG YingPeng2 & LU WeiChang2 1 2

Metallurgical ＆ Ecological School, University of Science and Technology, Beijing 100083, China; Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China

The effect of super-gravity on electrochemical deposition of nickel from aqueous solution was studied. The SEM pictures show that the microstructure of nickel film deposited under the super-gravity condition is finer and more uniform compared with that obtained in normal gravity condition, and the crystal grains diminish with the increase of super-gravity coefficient. The XRD patterns indicate that the arrangement of crystalline grains of nickel film deposited under the super-gravity field is more regular, and the crystalline grain sizes decrease with the increase of super-gravity coefficient. Toughness, tensile stress and hardness of the nickel film are markedly raised with the increase of super-gravity coefficient, and hydrogen content in the nickel film decreases with the increase of super-gravity coefficient. From the polarization curves of hydrogen evolution reaction under the super-gravity condition, a significant reduction of over-potential on electrode was found when current density increased. The process of hydrogen evolution reaction was enhanced under the super-gravity condition. The electro-deposition rate, the microstructure and properties of deposited nickel film under super-gravity condition were still affected by the relative orientation between inertia force and depositing surface. It is favorable to gain the nickel film with better mechanic properties when inertia force orientates vertically towards depositing surface. super-gravity, electrochemistry, electro-deposition, nickel film

1 Introduction Electrochemistry of aqueous solution has been widely applied in the preparation of metal materials, surface treatment, production of chemicals in chlorine-alkali industry and generation of hydrogen by electrolysis of water. Significant progress has been achieved in recent years in the domain of electrochemical technology, such as pulse deposition, ultrasonic deposition, spray deposition and magneto electrolysis[1], and preparation method of active electrode material has been improved in Received June 13, 2005; accepted September 27, 2006 doi: 10.1007/s11431-007-0001-9 † Corresponding author (email: [email protected]) Supported by the Notional Natural Science Foundation of China (Grant No. 50674011)

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Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

chlorine-alkali and aqueous solution electrolysis[2]. In summery, the basic route of these technologies for improving electrochemical processes is to enhance the mass transfer between electrode and electrolyte, which may result in improvement in the microstructure of deposited materials or decrease of the over-potential of electrode reaction. As we know, super-gravity can enhance the mass transfer in liquid phase as well. It may take effect on electrochemical process. Atobe et al.[3] reported obvious effects of super-gravity field on formation speed and performance of polymeric aromatic compounds by electro-deposition. Eftekhari[4] revealed that polypyrrole films prepared electrochemically in the presence of centrifugal forces had good stability and conductivity. Recently, we studied the electroless plating of copper coating on nickel substrate in super-gravity field, and found that the growth rate of coating increased with augmentation of super-gravity coefficient[5]. Whether super-gravity can be utilized to electrodepositing technology and improve the properties of metal coating or not is still necessary to be discovered. Furthermore, with the development of outer space technology, it is possible to manufacture some materials by electro-deposition in the space in the future. But now, it is difficult to do a lot of experiments of electro-deposition in the space. Before testing in the space, doing experiments on the earth is necessary to predict the consequence in micro-gravity condition from the experimental results in super-gravity condition. In the present paper, electro-deposition for nickel film under super-gravity of centrifugal field and electrode reactions in aqueous solution were studied. The goal is to explore the effects of various super-gravity coefficients and relative orientation between inertia force and depositing surface on nickel film’s performance and hydrogen evolution reaction, so as to promote the advance of water electrolyte for hydrogen, chlorine-alkali industry and metallic electroplating.

2 Experimental 2.1 Nickel electro-deposition in the super-gravity field Figure 1 shows the experimental centrifugal facilities equipped with a cylindrical electrolytic cell made with plastic and a 100 ml volume. Pure titanium plate as cathode with 5.5 cm2 area and pure nickel plate as anode are electrically contacted with a constant current source via rotating rings and carbon brushes. Nickel was electrodeposited in the experiments from the following solution: 300 g/dm3 of NiSO4·6H2O, 30 g/dm3 of NiCl2·6H2O, 40 g/dm3 of H3BO3, 0.25 g/dm3 of benzosulfimide and 0.50 g/dm3 of sodium dodecyl sulfate. The solution was prepared with addition of triply distilled water. The experiments were performed at cathode current density (CD) 70–122 mA/cm2, pH 2.5 and temperature about 40℃. The different super-gravity coefficient values were realized by changing the rotational speeds of the centrifugal electrolytic cell. The super-gravity coefficient which represents the action of super-gravity field can be calculated by the following equation: G=

rω 2 N 2 × π 2 × r = , g 900 g

(1)

where G is the super-gravity coefficient, N is the rotational speed (rpm, revolutions per minute), and r is the distance from rotary center to electrode center, being 0.11 m in the experiments. 2.2 Characterization of deposited Ni film After deposition under the above conditions, nickel film stripped from substrate was measured in 40

GUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

weight and thickness, and estimated in its toughness by 180° refolding number before broken. The more refolding number means the higher toughness value. Its morphology was analyzed using a scanning electronic microscope (JEOL, JSM6700F), hydrogen content of nickel film was tested using a hydrogen analyzer (LECO, RH-402), and crystal analysis was conducted by X-ray diffractometer (RIGAKU D/max-RB, Japan) with filtered CuKα radiation of wavelength 0.15418 nm.

Figure 1 Centrifugal facilities equipped with an electrolytic cell.

2.3 Electrochemical reaction testing Hydrogen evolution reaction experiment was carried out in a cylindrical electrolytic cell connected with CHI604A Electrochemical Analyzer (CH Instrument Corp., USA). A nickel film of 0.02 cm2 area was used as research electrode. A platinum foil of 6.6 cm2 area was used as a counter electrode as well as a reference electrode as its surface area was far larger than that of the research electrode. The solution of 0.5 mol/dm3 H2SO4 was used as electrolyte. The experiments were performed in the potential range from –1.4 to –2.1V at a scanning rate of 5 mV/s. Similarly, the test of electro-deposition reaction of Ni2+ was carried out by the above procedures. The solution of 0.05 mol/dm3 H2SO4 with 0.0454 mol/dm3 NiSO4 was used as electrolyte. Voltammograms were measured in the potential range from 0.7 to –1.0 V under scanning rates from 48 to 360 mV/s and different super-gravity coefficient values.

3 Results and Discussion 3.1 Properties of Ni films electro-deposited in the super-gravity field During the course of an experiment, super-gravity coefficient was changed with rotational speed of the centrifugal electrolytic cell according to eq. (1). Because nickel film was deposited under an interacting field of electric field and super-gravity field caused by centrifugal force, the rotational direction face or back to electrode surface was considered in this work. Under the conditions of current density 0.1 A/cm2, rotational direction back to nickel depositing surface and depositing time 15 min for each experiment, the observed results of the deposited nickel film are listed in Table 1. It shows that weight and thickness of the deposited nickel film at high super-gravity coefficient are significantly lower than those at low super-gravity coefficient, i.e., electric efficiency of nickel deposition decreases with the increase of super-gravity coefficient, hydrogen content of the deposited nickel film decreases with the increase of super-gravity coeffiGUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

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cient, and toughness of the deposited nickel film increases with the increase of super-gravity coefficient. Table 1 Effect of super-gravity on performance of Ni film electro-deposited by rotational direction back to depositing surface No. 0 1 2 3 4

Experimental condition Rotating Super-gravity speed coefficient (r/min) 0 0 500 31 1000 123 1500 276 2000 492

Observed results of the deposited nickel film Weight (g) 0.146 0.140 0.132 0.129 0.128

Thickness (μm)

Electric efficiency (%)

28.0 23.2 21.8 22.6 21.2

99.2 95.2 89.7 87.7 87.0

Hydrogen conToughness tent (ppm) (refolding number) 63 57 50 45 41

1 4 7 10 12

Furthermore, changing the rotational direction facing the nickel-depositing surface, but other conditions same as the above, the observed results of the deposited nickel film are listed in Table 2. It shows that toughness of the deposited nickel film is raised markedly when rotating direction is set in facing depositing surface rather than back to depositing surface. Hydrogen of the deposited nickel film further decreases, but electric efficiency also decreases, compared with that when the rotational direction is back to the nickel depositing surface. Table 2 Effect of super-gravity on performance of Ni film electro-deposited by rotational direction facing depositing surface No. 0 1 2 3 4

Experimental condition Rotating Super-gravity speed coefficient (r/min) 0 0 500 31 1000 123 1500 276 2000 492

Observed results of the deposited nickel film Weight (g) 0.145 0.136 0.130 0.130 0.128

Thickness (μm)

Electric efficiency (%)

28.3 24.0 22.7 22.8 21.7

98.5 92.7 88.3 88.3 86.8

Hydrogen Toughness content (ppm) (refolding number) 65 51 42 37 34

2 7 20 29 38

From the experimental data, the regularity of the effect of super-gravity on mechanic property of toughness and impurity element content of hydrogen of the deposited nickel film is shown in Figures 2 and 3. The hydrogen content is reduced markedly with the increase of super-gravity coefficient, which may be one of the main reasons to increase toughness of nickel film deposited under super-gravity field. The lower the hydrogen content of the deposited nickel, the higher the toughness of the deposited nickel film. Because the hydrogen content of the deposited nickel film under the condition of rotational direction facing the depositing nickel surface is lower than that under the condition of rotational direction back to the depositing nickel surface, the deposited nickel film under the condition of rotational direction facing the depositing nickel surface gains better toughness property. For comparison with the traditional milled nickel film, we measured the toughness of traditional milled nickel film with the nearly same thickness 25 µm. The milled nickel film is only 3 to 4 in toughness. Figure 4 shows the effect of super-gravity on tensile strength and hardness of the nickel film deposited under the conditions of current density 0.10 A/cm2, depositing time 30 min, rotating direction towards the depositing surface, and under different super-gravity coefficient. It is obvious that the stronger the super-gravity, the higher the tensile strength and harness of the deposited film. For comparison with the traditional milled nickel film, we measured the tensile strength of traditional milled nickel film with nearly the same thickness 50 µm. The milled nickel film is 539 MPa 42


in tensile strength. In our previous work[6] for improving the mechanical properties of electrodeposited nickel film by the addition of ultrasonic wave and spraying respectively under current density from 0.1 to 0.9 A/cm2, the best tensile strength of the electrodeposited nickel film was less than 1000 MPa. Therefore, it indicates that the super-gravity may greatly improve the properties of electro-deposited metal materials.

Figure 2 Effects of super-gravity coefficient and direction of rotating to depositing surface on toughness of Ni films.

Figure 3 Effects of super-gravity coefficient and direction of rotating to depositing surface on hydrogen content of Ni film.

Figure 5 shows the surface morphology of the nickel films deposited under different super-gravity conditions. The surface structure of the film under super-gravity condition seems to be uniform, finely networked and with rare flaws compared with that obtained in normal gravity condition. Crystalline grains diminish with super-gravity coefficient increasing and refines further GUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

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when rotating direction faces the depositing surface. Grain refinement and dense structure may be another main reason to increase toughness, tensile strength and hardness of nickel film deposited under super-gravity field.

Figure 4 The effect of super-gravity on tensile strength and hardness of deposited film.

Figure 5 SEM micrographs of nickel film electro-deposited under different super-gravity conditions.

The crystal structure of the nickel deposited under super-gravity field was studied by X-ray 44


diffraction (XRD) analysis. Figure 6 shows respectively the XRD patterns of the nickel film electro-deposited in the presence of a centrifugal field of super-gravity coefficient at the values of 0, 31, 123, 276 and 492. The XRD patterns clearly show that the crystal of the deposited nickel has a face centered cubic (fcc) structure. The effect of the super-gravity coefficient on the deposition could also be observed in XRD results. The diffraction angles of (111), (200) and (220) crystal planes significantly increase with the supper-gravity coefficient. In normal gravity condition, the diffraction angles of (111), (200) and (220) crystal planes are 43.83, 51.08 and 75.20 degree respectively. While in the super-gravity coefficient 492 condition, the diffraction angles of (111), (200) and (220) crystal planes are 44.57, 51.84 and 76.61 degree respectively, which are close to the diffraction angles of standard nickel crystal that are 44.5, 51.8 and 76.4 degree on (111), (200) and (220) crystal planes [7]. This indicates that the arrangement of crystal grains of nickel film deposited under super-gravity field is more regular and orderly. An increase in the full width of diffraction peak at half maximum shows the diminution of crystal grains in super-gravity electro-deposition. The size of crystal grains can be estimated according to Scherrer equation. The dimensional length across (200) is 19, 15, 13, 9 and 8 nm respectively corresponding to super-gravity coefficient 0, 31, 123, 276 and 492. These estimated values indicates that the crystal size decreases with the increase of super-gravity coefficient. 3.2 Electrochemical characterization of reaction in the super-gravity condition Some elementary analysis was carried out for explaining those experimental results. Firstly, lowering of electric efficiency in deposition of nickel metal is probably caused by side reactions. The solutions were prepared with triply distilled water and all chemicals of analytically pure grade. Therefore the only reason leading to lowering of electric efficiency is participation of hydrogen evolution side reaction. Secondly, toughness of nickel film increases and hydrogen content of the deposited nickel film decreases with the increase of super-gravity coefficient. This indicates that super-gravity is probably beneficial to rapid removal of gas from reaction interface during deposition. Finally, rotating direction facing depositing Figure 6 X-ray diffraction spectra of Ni film electro-deposited surface rather than back to deposit surface under super-gravity field. leads to better results, which may be caused by different transfer process of hydrogen bubbles generated by side reaction. In order to study the hydrogen evolution behavior on electrode, the polarization curves for hydrogen evolution under normal gravity condition and super-gravity condition were measured, in which a platinum foil of 6.6 cm2 area was used as a counter electrode as well as a reference electrode, a platinum wire of 0.02 cm2 area was used as research electrode and 0.5 mol/dm3 H2SO4 solution was used as electrolyte. Figure 7 shows the hydrogen evolution polarization curves under normal gravity condition (i.e., G = 0) and super-gravity condition with G value of 123 respectively in rotating direction facing or back to the research electrode, and at the potential scanning rate of 5 mV/s. At the electrode potential of –2.0 V, hydrogen evolution polarization current density under super-gravity coefficient G value of 123 is 194.0 mA/cm2 for rotating direction facing the research GUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

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electrode and 171.2 mA/cm2 for rotating direction back to the research electrode respectively, while under normal gravity (i.e., G = 0) it is only 86.5 mA/cm2. Polarization current density under super-gravity shows 2 times higher than that under normal gravity condition. The over-potential for hydrogen evolution at given current density of 100 mA/cm2 under G value of 123 is 70 mV less than that obtained under normal gravity, which means that super-gravity can promote the evolution reaction of the hydrogen and reduce its evolution over-potential. The reaction resistance (Rr) of the linearity polarization zone is 4.617 Ω for G = 0, 4.063 and 4.024 Ω for G = 123 respectively in rotating direction back or facing the research electrode. And corresponding exchange current density (i0) is 5.63, 6.40 and 6.46 mA/cm2 respectively in the above three cases. These differences indicate that super-gravity obviously decreases the polarization potential and increases mass transfer, and the direction of inertia forcing the surface of hydrogen formation also has little effect on hydrogen evolution.

Figure 7 Comparison of polarization curves of hydrogen evolution reaction on Pt electrode under different super-gravity conditions.

Figures 8 and 9 show the cyclic voltammograms in 0.05 mol/dm3 H2SO4 and 0.0454 mol/dm3 NiSO4 solution, with Pt wire of 0.02 cm2 area as research electrode under normal gravity and super-gravity (G=123) at various potential scanning rates. The selected potential cyclic range is +0.7—−1.0 V. The voltammograms under normal gravity show obvious reduction peaks located at the potential of about –0.45 V, which could be assigned to the reduction of the Ni2+ ions. The peak current increases with the increase of the scanning rate. The voltammograms under super-gravity (G = 123) also show the same change trend. However the reduction peaks move to the potential after –0.6V. And the increase of the scanning rate leads to the motion of the peak positions to more negative range. This is quite different from that under normal gravity. Compared with the voltammograms under normal gravity, the peak current under super-gravity is much higher at the same potential scanning rate. Randles-Sevcik equation [8] gives the relationship between the scanning rate and the peak current density for a reversible electrode reaction as follows:

46


Figure 8 Cyclic voltammograms in H2SO4-NiSO4 solution under normal gravity field with different scanning rates.

Figure 9 Cyclic voltammograms in H2SO4-NiSO4 solution under super-gravity field with different scanning rates (G = 123).

ip = 0.4463nFAC (nFν D / RT )1/ 2 ,

(2)

2

where ip is the peak current density (A/cm ), n is the electron transfer number, F is Faraday constant (96485 C/mol), R is gas constant (8.314 J/(mol K)), D is diffusion coefficient (cm2/s), v is the scanning rate (V/s), C is the concentration of the ions (mol/dm3), and A is the electrode area (cm2). Eq. (2) implies that ip and v1/2 have a linearity relationship. The peak current density for a no-reversible electrode reaction is less than that of the same electrode reaction if it is reversible. For the electro-deposition of Ni2+ to Ni, although it is not a strictly reversible electrode reaction, we can still use the eq. (2) to show the effect of super-gravity on its peak current density. Figure 10 shows the relationship between the peak current density (ip) and the square root of the scanning rate (v) under both normal gravity and super-gravity (G = 123). It could be found that the curves fit the linearity well. When taking the diffusion coefficient into account under the two gravity conditions, we could find that the diffusion coefficient of Ni2+ ions under super-gravity (G = 123) is about ten times higher than that under normal gravity. This indiGUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

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cates super-gravity could promote the diffusion of the Ni2+ ions. Figure 11 shows the cyclic voltammograms in the above H2SO4-NiSO4 solution at scanning rate 60 mV/s under different super-gravity coefficient. It can be seen that current density changes largely when the potential is located at Ni2+ depositing region with the increase of super-gravity coefficient. This indicates that Ni2+ depositing reaction is enhanced by super-gravity. The waves may be caused by hydrogen bubbles evolution, which suggests the reason of decrease in Ni2+ electro-deposition efficiency. 3.3 Analysis of formation and outflow process of bubbles under super-gravity The over-potential of electrode reaction in aqueous solution in an electrochemical process with evolution of gas is related to the formation and outflow of bubble from electrode surface. The formation energy of a bubble on electrode surface is showed as follows: P + ρ (1 + G ) gh 4 4 ΔG = 4πr 2σ + πr 3 ΔGV* = 4πr 2σ + πr 3 0 ΔGV , (3) P0 3 3

where r is radius of the bubble, σ is boundary tension of gas and solution, P0 is pressure of environment, ρ is density of solution, g is gravity constant, G is super-gravity coefficient, and ΔGV is energy variation when hydrogen atoms are adsorbed on electrode forming one unit normal volume hydrogen gas.

Figure 10 Relationship between the peak current density and the square root of the scanning rate under normal gravity and super-gravity.

Figure 11 Cyclic voltammograms in H2SO4-NiSO4 solution at scanning rate 60 mV/s under different super-gravity coefficient values.

The critical radius r* for forming a bubble can be derived by derivation of eq. (3) and assumption of ∂ΔG / ∂r = 0 . P0 −2σ . ⋅ (4) r* = ΔGV P0 + ρ (1 + G ) gh

When super-gravity coefficient G = 0, namely in normal gravity condition, the critical radius for forming a bubble is calculated as −2σ r0* ≈ . (5) ΔGV When super-gravity coefficient G > 0, namely in super-gravity condition, the critical radius for 48


forming a bubble in low concentration aqueous solution is rG* ≈

r0* −2σ = . (1 + 0.1Gh)ΔGV 1 + 0.1Gh

(6)

The critical radius for forming a bubble is reduced significantly under super-gravity field. If solution depth h is 0.1 m, and super-gravity coefficient G is 1000, and then rG* ≈0.1 r0* . It means that the super-gravity favors the process of gas evolution reaction. Moreover, bubbles with the same size suffer G times buoyancy under super-gravity over that in normal gravity field, and escape more easily from electrode. On the other hand under super-gravity condition, gas volume of the same amount is only 1/G of gas volume in normal gravity condition. These explain the decrease of over-potential of hydrogen evolution, the decrease of current efficiency of Ni2+ electro-deposition, the decrease of hydrogen content in nickel film, and little crystal defect of nickel film with the increase of super-gravity coefficient. Figure 12(a) schematically shows the velocity of a bubble in solution under centrifugal super-gravity field. The bubble in the solution moves not only along circular direction, but also towards the center of circle because of buoyancy. Therefore the moving velocity direction of the bubble is not tangent, but secant to the circle; the acceleration direction of the bubble is not faced to the center, but deviated from the center. The bubble is easy to disengage from its forming surface when rotating direction faces its forming surface, as shown in Figure 12(b), and contacting time of the bubble with its forming surface is short. But when the rotating direction is back to the surface of the bubble forming, as shown in Figure 12(c), the bubble disengages from its forming surface only when it rises beyond the edge of its forming surface. This indicates that contacting time of the bubble with its forming surface is longer than that in the former case. Of course, in any case, rising velocity of a bubble is much faster under super-gravity field, and then contacting time of the bubble with its forming surface is much shorter than that under normal gravity field. Therefore the content of hydrogen in metal film electro-deposited under centrifugal condition is lower than that in normal gravity condition and decreases with the increase of super-gravity coefficient. Particularly when rotating direction faces depositing surface, the content of hydrogen in metal film is lower than that when rotating direction is back to depositing surface.

Figure 12 Moving locus of bubbles on electrode under centrifugal super-gravity field. (a) Velocity of bubbles in solution; (b) surface of generating bubble facing with the direction of rotational speed; (c) surface of generating bubble diverging to the direction of rotational speed.

4 Conclusions The nickel film deposited under super-gravity in an aqueous solution has high mechanic properties. GUO ZhanCheng et al. Sci China Ser E-Tech Sci | February 2007 | vol. 50 | no. 1 | 39-50

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Toughness, tensile strength and hardness of nickel film increase with the increase of super-gravity coefficient. Hydrogen content of nickel film decreases with the increase of super-gravity coefficient. Surface structure seems to be more uniform and crystal grains diminish compared with those obtained under normal gravity condition. But current efficiency for Ni2+ electro-deposition decreases with the increase of super-gravity. From the polarization curves of hydrogen evolution reaction under super-gravity, a significant decreasse of over-potential in nickel electrode is observed when current density increases. The effect of the super-gravity on the evolution of the hydrogen was discussed compared with the normal gravity. It was found that super-gravity is helpful to the hydrogen evolution reaction. The over-potential for hydrogen evolution reaction on Pt electrode under super-gravity field is much lower than that under normal gravity, and the over-potential for hydrogen evolution reaction is lower when rotational direction faces hydrogen evolution surface in centrifugal super-gravity field than that when rotational direction is back to hydrogen evolution surface. Furthermore, super-gravity can also greatly promote the transfer of Ni2+ ions in H2SO4-NiSO4 solution to its electro-depositing surface.

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