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A Study on the Effect of Electrodeposition Parameters on the Morphology of Porous Nickel Electrodeposits SRIJAN SENGUPTA, ARGHYA PATRA, SAMBEDAN JENA, KARABI DAS, and SIDDHARTHA DAS In this study, the electrodeposition of nickel foam by dynamic hydrogen bubble-template method is optimized, and the effects of key deposition parameters (applied voltage and deposition time) and bath composition (concentration of Ni2+, pH of the bath, and roles of Cl and SO42 ions) on pore size, distribution, and morphology and crystal structure are studied. Nickel deposit from 0.1 M NiCl2 bath concentration is able to produce the honeycomb-like structure with regular-sized holes. Honeycomb-like structure with cauliflower morphology is deposited at higher applied voltages of 7, 8, and 9 V; and a critical time (>3 minutes) is required for the development of the foamy structure. Compressive residual stresses are developed in the porous electrodeposits after 30 seconds of deposition time (189.0 MPa), and the nature of the residual stress remains compressive upto 10 minutes of deposition time (1098.6 MPa). Effect of pH is more pronounced in a chloride bath compared with a sulfate bath. The increasing nature of pore size in nickel electrodeposits plated from a chloride bath (varying from 21 to 48 lm), and the constant pore size (in the range of 22 to 24 lm) in deposits plated from a sulfate bath, can be ascribed to the striking difference in the magnitude of the corresponding current–time profiles. https://doi.org/10.1007/s11661-017-4452-8 The Minerals, Metals & Materials Society and ASM International 2018

I.

INTRODUCTION

NICKEL foam has attracted increasing academic interest as electrodes for electrocatalysis,[1,2] supercapacitors,[3] sensor,[4] and modified current collector for Li-ion battery[5–8] due to high surface area, high electrical conductivity, and ability to withstand compressive stress. Higher surface area leads to enhanced kinetics and accessibility of reactive species, higher electrical conductivity results in rapid electrochemical reactions, and stress-absorbing ability plays an important role in alleviating the cyclic strain-controlled fatigue

SRIJAN SENGUPTA and ARGHYA PATRA are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, WB 721302, India. SAMBEDAN JENA is with the School of Nano Science and Technology, Indian Institute of Technology Kharagpur, Kharagpur, WB 721302, India. KARABI DAS and SIDDHARTHA DAS are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur and also with School of Nano Science and Technology, Indian Institute of Technology Kharagpur. Contact e-mail: [email protected] Srijan Sengupta and Arghya Patra have contributed equally to this study. Manuscript submitted June 7, 2017.

METALLURGICAL AND MATERIALS TRANSACTIONS A

stress developed in the conversion-type electrode due to repeated lithiation and delithiation. Porous nickel has been mainly synthesized through self-assembly[5] followed by electrodeposition, directional freeze casting,[9] and hydrogen bubble-templated electrodeposition.[10] The method of synthesis of the porous scaffold affects the pore volume, pore size distribution, and interconnectivity, and all the factors synergistically affect the electrochemical properties. Of all the methods, dynamic bubble hydrogen-templated (DBHT) electrodeposition is an inexpensive, room-temperature process with an intricate control of the microstructure. The role of key deposition parameters (applied voltage and deposition time) and bath composition (pH, role of anion, and concentration of Ni2+ ions) for electrodeposition of nickel foam by DBHT electrodeposition have not been studied extensively. In case of DBHT, at high overvoltage and low metal-ion concentrations, deposition of metal around hydrogen bubbles leads to the development of a 3D interconnected structure with open porosity at the surface due to coalesced bubbles and closed porosity beneath the surface due to entrapped small bubbles. The first study on hydrogen bubble-template-assisted electrodepositions of tin and copper foam were performed by Liu et al.,[11] and the thickness and average

diameter of the pores were measured over time. The mechanism of formation of copper electrodeposits with dendritic, honeycomb-like, and dish-like morphologies has been extensively studied by Popov et al.[12–14] That author group laid out a foundation for the principle of electrodeposition of porous copper and linked the morphology with the effective overpotential and volume of H2 evolution.[12] The transformation from dendritic morphology at medium effective overpotential to globular morphology at high effective overpotential in copper electrodeposits has been established.[15] Of all the metal/alloys foams synthesized by hydrogen bubble-template-assisted electrodeposition, copper is the most studied system along with some studies on tin,[11] lead,[16] ruthenium,[17] silver,[18,19] and gold.[20] Past studies on electrochemical synthesis of nickel foam have focussed primarily on the effect of NH4Cl, deposition time and applied current density/voltage on the morphology. Marozzi and Chialvo developed highly porous nickel electrodeposits with appropriate mechanical resistance as electrocatalysts for HER (hydrogen evolution reaction) from a similar bath (consisting of nickel chloride and ammonium chloride) and studied the effects of NH4Cl concentration in the electrolyte and applied current density. Honeycomb-like structures were obtained from electrodeposits from 0.2 M NiCl2 and 2 M NH4Cl at a current density of 0.3 to 0.7 Acm2 for a deposition time of 1 hour.[21] Eugeńio et al. electrodeposited Ni-Cu alloy foams from a sulfate bath in a typical two-electrode configuration and studied the roles of the applied current density and the deposition time on morphology, pore size distribution, and surface area. Honeycomb-like structures with a regular pore size were obtained at 1.0 to 1.8 Acm2 for a deposition time >30 seconds.[22] Yu et al. studied the effect of NH4Cl concentration on the mechanism of structural evolution in porous nickel electrodeposits and noted that NH4Cl concentrations in the ranges from 0.4 to 0.75 and 0.85 to 3.0 M produced porous nickel films with dish-shaped structures with irregular-sized pores and honeycomb structures with regular-sized pores, respectively.[23] Wang et al. developed 3D hierarchically porous nickel electrodeposits from aqueous solution containing 0.1/ 0.2 M NiCl2 and 2 M NH4Cl with 2.5/5 Acm2 current density resulting in a pore size distribution ranging from 100 nm to 20 lm.[24,25] Multistructural porous NiAg films with nanoarchitecture walls were electrodeposited by Yu et al.[26] at a cathode current density of 1.0 Acm2 for 30 seconds with a pore size of ~5 lm in the case of nickel foam and in the range from11 to 27 lm in the case of NiAg alloy foams. Zhuo et al., Rafailovic et al., and Mattarozzi et al. reported fabrication of nickel-based alloy foams (Ni-Co, Ni-Cu, Ni-Sn)[27–29] using DBHT electrodeposition. Normally, the ridges in the honeycomb structure are made up of circular grains in case of nickel or nickel-major electrodeposits but dendritic morphology is observed in nickel-minor (especially in the case of Ni-Cu and Ni-Sn) electrodeposits. Most studies on the nickel foams for electrodes for electrocatalysis, double-layer supercapacitors, and sensor applications have focused on the porous nature

facilitating the diffusion and kinetics improvement by increase in surface area and have rarely connected the morphology to the mechanical properties of the foam. In the case of application of nickel foams as modified current collectors for conversion anodes for Lithium-ion battery,[5–8] the state of stress in the foam plays an important role in stress alleviation, and a prior knowledge of the state of stress in the nickel foam can help in better stabilization strategies for high capacity anodes for Li-ion battery. In the present study, the deposition time has been optimized for the generation of necessary compressive stress and morphology. As pointed out by Bhargav et al.,[10] the key parameters for morphology control are (a) H+ concentration and source (controlled by pH of the bath and molar concentration of NH4Cl), (b) applied potential, and (c) metal-ion concentration (controlled by change of NiCl2 concentration). In this study, the electrodeposition of nickel foam by dynamic hydrogen bubble-template method is optimized, and the effects of key deposition parameters (applied voltage and deposition time) and bath composition (concentration of Ni2+, pH of the bath, roles of Cl and SO42 ions) on pore size, distribution, morphology, and crystal shapes are studied. Residual stress evolution in nickel foam with deposition time is also observed.

II.

EXPERIMENTAL

Cu strips (99.99 pct pure, Alfa Aesar Inc.) with an area of 20 mm X 20 mm are cut and polished using standard metallographic techniques. Prior to deposition the substrates (Cu) are cleaned with dilute sulfuric acid solution to remove any surface oxide. Then, the prepared Cu strips are electroplated with nickel using compositions and deposition parameters as mentioned in Table I. Pt electrode with a surface area of 2 cm2 is used as the counter electrode in a standard two-electrode set up. Electrodeposition is carried out potentiostatically with a potentiostat/galvanostat (Autolab PGSTAT 302N). The electrolyte pH is adjusted with the addition of hydrochloric acid and sulfuric acid in the case of chloride-based bath and sulfate-based bath, respectively. A Sartorius Professional meter PP-50 is used to measure the pH of the bath. Electrodeposition is carried out using an electrolyte of 100 mL volume at room temperature. The anode and the cathode are placed parallel at a distance of 20 mm during electrodeposition. Deposition is performed in a stationery electrolyte without stirring. The samples are washed and dried immediately after the experiment to remove any electrolyte. The prepared electrodeposits were structurally characterized using X-ray diffraction (Bruker D8 Discover, Germany) using a Co target, and FESEM (FEI Quanta FEG 250, Netherlands) operated at 20 kV. The XRD measurements for the samples were carried out using Bruker D8 Discover (Co Ka radiation = 0.1789 nm at 0.02 deg s1 step size with 40 kV/40 mA operating parameter) in a two-theta mode from 20 to 80 deg with the source angle kept fixed at 5 deg. This was done to reduce the intensity of the METALLURGICAL AND MATERIALS TRANSACTIONS A


Effect of 0.01 0.05 0.10 0.50 1.00 Effect of 0.01 0.05 0.10 0.50 1.00 Effect of 0.01 0.05 0.10 0.50 1.00 Effect of 0.10 0.10 0.10 0.10 0.10 0.10 0.10 Effect of 0 0 0 0 0 0 0 Effect of 0.10 0.10 0.10 0.10 0.10 0.10 0.10

NH4Cl (M)

NiSO4 (M)

(NH4)2SO4 (M)

8 8 8 8 8

3 3 3 3 3 3 3 3 3 3 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

6 6 6 6 6 1 2 3 4 5 6 7 1 2 3 4 5 6 7 6 6 6 6 6 6 6

3 4 5 6 7 8 9

8 8 8 8 8 8 8

8 8 8 8 8 8 8

8 8 8 8 8

8 8 8 8 8

Voltage (V)

3 3 3 3 3

Time (min)

Blank Blank Blank Blank Blank 1) 1 1 1 1 1

pH

planar rough with no distinct pore formation rough with no distinct pore formation dish-shaped pores honeycomb structure honeycomb structure honeycomb structure

planar honeycomb structure honeycomb structure honeycomb structure honeycomb structure dish-shaped pores rough with no distinct pore formation

planar honeycomb structure honeycomb structure honeycomb structure honeycomb structure dish-shaped pores rough with no distinct pore formation

planar honeycomb structure honeycomb structure insoluble (nickel hydroxide) insoluble (nickel hydroxide)

planar planar planar planar (de-cohesion) planar (de-cohesion)

planar honeycomb structure honeycomb structure rough with no distinct pore formation rough with no distinct pore formation

Morphology

circular circular circular circular circular

grains grains grains grains grains

compact circular grains dendritic dendritic cauliflower cauliflower cauliflower cauliflower

compact circular grains cauliflower cauliflower cauliflower cauliflower cauliflower dendritic

compact circular grains cauliflower cauliflower cauliflower cauliflower cauliflower dendritic

compact circular grains cauliflower cauliflower — —

compact compact compact compact compact

compact circular grains cauliflower cauliflower compact circular grains compact circular grains

Microarchitecture

silvery silvery silvery silvery silvery

bright silvery gray black black black black black

gray black black black black black black

gray black black black black black black

bright silvery black black — —

bright bright bright bright bright

bright silvery black black black black

Appearance

The Resulting Morphologies, Microarchitectures, and Appearances of the Various Porous Nickel Deposits Obtained with Different Bath Compositions, pH values, and Different Electrodeposition Parameters

Concentration @ Intermediate pH (pH 3 to 4) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 Concentration @ Highly Acidic Condition (pH 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 Concentration @ Basic Condition (pH 6) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 pH (Chloride Bath) 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 pH (Sulfate Bath) 0 0.10 1 0 0.10 1 0 0.10 1 0 0.10 1 0 0.10 1 0 0.10 1 0 0.10 1 Voltage 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0 1 0 0

NiCl2 (M)

Table I.

bright silvery bright silvery gray black black black black compact circular grains compact circular grains compact circular grains cauliflower cauliflower cauliflower cauliflower planar planar isolated pore formation dish-shaped pores dish-shaped pores honeycomb structure honeycomb structure 2/60 10/60 30/60 1 3 5 10

III.

RESULTS AND DISCUSSION

The effect of different operating bath parameters on the morphology of the nickel electrodeposits is investigated and reported in this section. The resulting morphologies, microarchitectures, and appearances of the various porous nickel deposits obtained with different parameters and bath compositions are presented in Table I. A. Effect of Metal Ion Concentration at Different pH SEM micrographs presented in Figure 1 show the change in morphologies of the electrodeposited Ni-foam for different NiCl2 concentrations at three pH levels (1, 3 to 4, and 6). This is done to coarsely optimize the concentration and pH for the development of honeycomb structure. Figures 1(a1) through (a5) show nickel electrodeposits at pH 1 for 0.01, 0.05, 0.1, 0.5, and 1 M NiCl2. Figures 1(a1) through (a3) show a planar and compact deposit. Figures 1(a4) and (a5) also show planar deposition with severe delamination. Figure 1(b1) does not show any sign of pore formation. Figures 1(b2) through (b5) show pore formation in which (Figures 1(b2) and (b3)) the foamy natures of the deposits are very prominent. For pH 6, the foamy nature is found even at low concentration (Figure 1(c1)). Figures 1(c2) and (c3) show development of a honeycomb-like structure with interconnected porosities. The three primary reactions governing the behavior of the porous nickel deposits at different pH ranges are: Ni2þ þ 2e ! Ni

½1a

Ni2þ þ 2OH ! Ni(OH)2 #

½1b

Ni2þ þ nðNH3 Þ ! NiðNH3 Þ2þ n

½2

Effect of Time 0.10 0.10 0.10 0.10 0.10 0.10 0.10

1 1 1 1 1 1 1

0 0 0 0 0 0 0

0 0 0 0 0 0 0

6 6 6 6 6 6 6

8 8 8 8 8 8 8

Appearance Microarchitecture Morphology Voltage (V) Time (min) pH (NH4)2SO4 (M) NiSO4 (M) NH4Cl (M) NiCl2 (M)

continued Table I.

reflections from the thin (100 lm) copper substrate. The crystallite sizes were estimated by analyzing the peaks using the Williamson–Hall method.[30] The residual stress measurements of the as-prepared porous deposits were performed by XRD in Bragg Brentano configuration by the Sin2W technique following the methodology provided by Fitzpatrick et al.[31]

The pH of the bath plays an important role in complexation of the metal ions which in turn affects the structure and morphology of the deposited films. At a low pH, Reactions [1b] and [2] are suppressed due to high H+ concentration in the bath, and nickel deposition takes place primarily from Ni2+ ions in the electrolyte (from 1a) resulting in planar films which are adherent at low concentration and gets delaminated with the increasing concentration.[32] The planarity of the deposits (Figures 1(a1) through (a3)) can be attributed to very high efficiency of nickel deposition at low pH.[33] Delamination is observed for samples with METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 1—SEM micrographs (at 5009) showing different morphologies evolved due to different concentrations of NiCl2 at pH 1, 3 to 4, and 6. (a), (b), and (c) indicate depositions done at pH 1, 3 to 4, and 6, respectively. 1, 2, 3, 4, and 5 indicate depositions done with concentrations of 0.01, 0.05, 0.1, 0.5, and 1 M of NiCl2, respectively. The electrodeposition applied potential is fixed at 8 V with a plating time of 3 min.

high concentration (Figures 1(a4) and (a5)), since the deposits became thick, less adherent, and then are delaminated due to development of tensile stresses.[32] Thick electrodeposited nickel films are known to develop tensile residual stress after a critical thickness. It may be generally perceived that higher the extent of hydrogen production at the cathode along with nickel, the greater will be the porosity generation. Interestingly, at pH 1, all the electrodeposits failed to form a foamy network; where severe hydrogen evolution is observed during deposition for all the concentrations. Therefore, it is inferred that it is not the extent of hydrogen evolution, but the time of attachment of the hydrogen bubble that decides the foamy morphology. As the pH of the bath increases, chances of hydrolysis increase, and reaction 1b becomes predominant. However, the increase in pH increases the basicity of the bath discharging the ammonia from NH4Cl and helps in the complexation of Ni(OH)2 (reaction 2). In order to increase the residence time of the bubble stay at the surface, the rate of formation of hydrogen bubbles is controlled by increasing the pH. Reduction potential increases, and efficiency of H2 evolution decreases upon increasing the pH of the bath.[34] This is validated from Figures 1(b2), (b3) and (c2), (c3) as the deposits are porous due to simultaneous hydrogen evolution. The efficiencies of hydrogen evolution and deposition of nickel are optimal at 0.05 and 0.1 M, which allow for high rates of transfer of Ni2+ ions from the double layers and simultaneous evolution of H2 providing a pseudo template for foamy structure development. Foamy structure (with dish-shaped pores/honeycomb structure) is obtained from baths containing 0.05 M and 0.1 M NiCl2. On increasing the concentration of Ni2+, efficiency of hydrogen evolution again decreases[35] producing compact deposits with poorly developed pores from baths containing 0.5 and 1 M NiCl2


(Figures 1(b4) and (b5)). Dendritic/Cauliflower morphology is not obtained from the concentrated bath as the deposition progresses via activation controlled process as there are no diffusion limitation due to high concentration of metal ions in the bath. Due to high concentration of Ni2+ at 0.5 and 1 M, precipitate of Ni(OH)2 is obtained at pH 6 which could not be complexed by 1 M of NH4Cl due to insufficient amount of NH4Cl. Deposition was not performed from both these baths. From these sets of results, it is clearly seen that the deposit with 0.1 M NiCl2 is able to produce the honeycomb-like structure with uniform/regular-sized pore. Therefore, for further investigation of the foamy deposits, the bath composition of 0.1 M NiCl2 with 1 M NH4Cl is chosen. B. Effect of Time on Morphology and Structure of Electrodeposited Nickel Foam 1. Morphology evolution with time The mechanism of formation of honeycomb structure over time is traced with investigating the SEM micrographs of porous nickel structures formed after different deposition times. Figure 2 shows the dependence of morphologies of the porous nickel electrodeposits over time electrodeposited from the previously optimized bath at 8 V. The morphologies have been presented in the form of 3 9 7 matrix in which micrographs a-g indicate nickel deposits obtained after electrolysis time of 2, 10, 30, 60, 180, 300, and 600 seconds, respectively, and 1, 2, and 3 denote magnifications of 100, 500, and 2500 9 , respectively. Figures 2(a1) through (a3) show deposition of compact nickel film; which is also verified by the change in OCP from 0.4 to 1.1 V after deposition (not in figure). Figures 2(b1) through (b3) and (c1) through (c3) show

Fig. 2—SEM micrographs showing different morphologies evolved due to variation of electrodeposition times at different magnifications (a1 through a3) 2 s, (b1 through b3) 10 s, (c1 through c3) 30 s, (d1 through d3) 1 min, (e1 through e3) 3 min, (f1 through f3) 5 min and (g1 through g3) 10 min. The applied deposition potential is fixed at 8 V in a chloride bath (0.1 M NiCl2 + 1 M NH4Cl) at pH 6. 1, 2, and 3 denote magnifications of 100, 500, and 25009, respectively.


planar deposition with no pore formation. After 10 seconds, many nucleation centers can be seen and small spherical compact structures fill the entire surface. As the deposition time increases (in the interval of 10 to 30 seconds), the fraction of available copper (substrate) sites decreases. The surface now gets covered with a thin layer of nickel. This in turn, results in the decrease in the hydrogen evolution (as H2 has overpotential of 0.28 V over nickel surface which is more than that of copper), and the hydrogen bubbles can be more efficiently entrapped by nickel due to the higher residence time. Figure 2(d1) appears to be planar at low magnification, but at higher magnification, it shows initiation of the foamy structure at random places. These are the places of irregularity (due to the presence of surface-related defects) specifically near the edge of the copper strip. With further increase in the deposition time, the first few disjointed cells of the honeycomb structure form scattered uniformly over the surface. Figure 2(e1) shows two cells where one of the cells developed at an earlier time and acted as a ‘‘nucleation site’’ for the next cell, thus producing conjoined cells. The cells are formed by the growth of nickel cauliflowers beside entrapped hydrogen bubbles.[19] Figure 2(e1) shows disorganized pores, and no obvious pore walls formed due to the lack of nickel deposits. New pores develop around the previously developed pores. Figure 2(f1) shows uniformly distributed microsized pores all over the surface. Thus, the critical time required for the development of the structure is 5 minutes for nickel foam. Figure 2(g1) also shows evenly distributed microsized pores after 10 minutes of deposition but the pore size is bigger than that of Figure 2(f1). Figure 2(g3) is presented at 10009 rather than 25009. This can be attributed to the fact that the hydrogen bubbles when coming out of the nickel scaffold get more time to coalesce in order to reduce surface energy leading to formation of bigger pores. The increase in pore size with time is associated with the decrease in the number of pores. The main features which are observed with increase in time are: compact deposit with circular grains (2, and 10 seconds), isolated pores at irregularities especially at edges (30 seconds), pore coalescence (1 and 3 minutes), development of honeycomb structure (5 minutes), and honeycomb structure with de-cohesion at parts due to highly stressed sample (10 minutes). Nickel electrodeposited around the H2 bubble acts as the key factor for the development of the honeycomb structure with regular/irregular-sized pores. As the deposition time progresses, the pores get deeper and the diameter increases, leading to a decreasing pore density (Figures 2(d2) through (g2)). The enlargement of the pores is indicative of the growth or coalescence of hydrogen bubbles. It’s these expanded gas bubbles that led to the increased pore diameter. The effect of time is primarily two fold : a) increase in amount of nickel deposit on the ridges resulting in thickening of the ridges b) there exists a critical residence time for the bubbles to coalesce and leave the surface depending on the orientation of the electrode. The critical residence time also dictates the nickel deposition around the H2 bubble. Point b is corroborated from the METALLURGICAL AND MATERIALS TRANSACTIONS A

micrographs of 2 and 10 seconds (less than the critical residence time) which show no sign of pores. 2. Evolution of the 111 nickel peak of the electrodeposits with time Figure 3 shows the shift in peak positions for Ni (111) peaks with change in deposition time. Initially, the 2h position shifts towards left for 10 seconds deposit. This indicates the development of a tensile stress in the sample. Then the peaks shift towards right from 10 seconds up to 10 m deposit. This indicates the development of a compressive stress in the sample. This is similar to the results reported by Abermann[36] for metals with high adatom mobility. The first compressive stress generates because of island film growth. The tensile stress appears because of coalescence of crystallites. Then again as the film grows, compressive stress redevelops into the system. Moreover as the reaction progresses, the FWHM of the peaks decreases indicating increase in crystallite size with time. The crystallite sizes, as calculated by Scherrer equation considering machine broadening, are shown in Table II. 3. Variation of residual stress over time (by XRD) Figure 4 shows the change in the nature of residual stress with the time of deposition. It shows that the stress just after 2 seconds of deposition is highly compressive (807.8 MPa) which becomes tensile (65.6 MPa) after 10 seconds of deposition. After 10 seconds, stress reversal occurs at 30 seconds and continues till 10 minutes. The electrodeposits after 30 seconds, 1, 3, 5, and 10 minutes exhibit compressive stress of magnitude 189.0, 575.2, 621.2, 1056.6 and 1098.6 MPa, respectively. This trend is similar to that of the variation of stress observed in thin films by Abermann[37] for elements with high atomic mobility. According to Zangari,[38] the kinetics of film growth in case of electrodeposition is analogous to physical vapor deposition, and hence, the mechanism of stress development in films synthesized by electrodeposition is expected to be similar to that of physical vapor deposition. The compressive stress at the beginning (2 seconds) is generally associated with island film growth during the early stages of nucleation. This has been attributed to the effect of surface capillary forces on the isolated cluster.[39,40] The tensile stress which develops after 10 seconds (65.6 MPa) is associated with the elastic strain at the time of coalescence of crystallites[37] and formation of grain boundaries.[41] With increase in time, as the film thickens, the tensile stresses generated during crystallite coalescence are relaxed, as exhibited by electrodeposits with a compressive residual stress after 30 seconds of deposition (189.0 MPa). Relaxation processes associated with high adatom mobility have been traditionally ascribed as the root cause for the decrease in the tensile stress.[42] The compressive stress ultimately reaches a steady-state value after 5 minutes (1056.6 MPa) and no significant increase in magnitude is observed till 10 minutes (1098.6 MPa). It is interesting to note that the residual stresses with planar nickel electrodeposits from a sulfate

Fig. 3—XRD diffractogram for nickel electrodeposits at 2, 10, 30, 60, 180, 300, and 600 s from a chloride-based bath (0.1 M NiCl2 + 1 M NH4Cl) with pH 6 at an applied voltage of 8 V.

Table II. Deposition Time (s) 2 10 30 60 180 300 600

Shift in Ni 111 Peaks with Variation in Crystallite Size with Time Ni 111 Peak Position (deg)

Crystallite Size of Nickel (nm)

51.63 51.09 51.19 51.41 51.64 51.78 51.88

18.44 24.62 26.82 30.02 31.76 38.23 55.28

bath are tensile at high current densities and also at low current densities in the range 1.5 to 5 mA cm2.[43] However, in this case, at a very high current density of 0.8 Acm2 at pH 6 (Figure 11(a)) compressive residual stresses are getting developed in the porous electrodeposits. Compressive residual stresses are important for modified current collectors for Li-ion battery as it helps to sustain high capacity anodes like Sn, Si which undergoes ~300 pct volume change on lithiation. C. Effect of Applied Voltage 1. Morphology evolution with applied voltage Figure 5 shows the change in morphologies of the electrodeposited Ni-foam obtained at 3, 4, 5, 6, 7, 8, and 9 V potential applied for 5 minutes. The morphologies are presented in the form of 4 9 7 matrix format in which morphologies at voltage 3 to 9 are shown for four different magnification (at 100, 500, 2500, and 100009). Figures 5(a1) through (g1) (at 1009) give a qualitative idea that the porous network has been formed. Figures 5(a2) through (g2) (at 5009) give an overall idea about the number of pores along with their distribution. Figures 5(a3) through (g3) (25009) focus on individual pores from which pore diameters have been calculated. A gradual increment in pore diameter is

Fig. 4—Evolution of residual stress over time (calculated employing Sin2w method) from a chloride-based bath (0.1 M NiCl2 + 1 M NH4Cl) with pH 6 at an applied voltage of 8 V.

observed. From Figures 5(a4) through (g4) (at 100009) the shape of the crystals and the nature of growth can be identified. Deposit at 3 V (Figure 5(a1)) shows planar morphology with spiral flower-like building blocks (Figure 5(a4)). Deposits at both 4 V (Figure 5(b1)) and 5 V (Figure 5(c1)) show discontinuous nickel network made of dendrites (Figures 5(b4) through (c4)). Electrodeposit at 6 V shows initiation of pore formation in random places (Figure 5(d4)). From 7 V onward the honeycomb structure starts appearing. From 7 to 9 V, the pore size increases gradually (Figures 5(e2), (f2), and (g2). At 9 V, the pore diameter is so large that Figure 5(g3) is shown at 10009 rather than at 25009. The morphologies are linked to the role of the applied voltage in increasing the hydrogen evolution efficiency producing a mechanical mixing, which decreases the diffusion layer thickness and aids the deposition of nickel ions.[44] The nanoscale morphologies of the building blocks of the porous nickel electrodeposits at different voltages are controlled by the nucleation and growth mechanism and can be divided into three major types, namely, carrot-like elongated structures (at 3 V), dendrites (at 4 and 5 V), and cauliflower structure comprising spherical agglomerates of homogeneous nickel particles at high overvoltages (at 6, 7, 8, and 9 V). Such morphological evolution is also observed in the case of porous copper electrodeposits[15] where carrot-like structures are obtained in the activation diffusion mixed control layer, followed by dendritic structures at the plateau of the limiting diffusion, and finally decomposition of water took place beyond the diffusion-controlled plateau. The applied overpotential at 3 V lies in the activation–diffusion-controlled electrodeposition as verified by the elongated carrot-like structures indicating activation only around the tip of the protrusion. Such type of growth has been observed and analyzed by Popov et al. for porous copper electrodeposits.[15] At 4 and 5 V, electrodeposition is


Fig. 5—SEM micrographs showing different morphologies evolved due to different applied voltages at different magnifications (a1 through a4) 3, (b1 through b4) 4, (c1 through c4) 5, (d1 through d4) 6, and (e1 through e4) 7, (f1 through f4) 8, and (g1 through g4) 9 V. The electrodeposition is carried out from a chloride-based bath (0.1 M NiCl2 + 1 M NH4Cl) with pH 6 for a plating time of 5 min.

controlled by the limiting diffusion current density resulting in the formation of nickel dendrites. The deposition of nickel in this regime is primarily controlled by diffusion-controlled growth aided by low solution METALLURGICAL AND MATERIALS TRANSACTIONS A

mixing due to moderate-to-low hydrogen evolution. The hydrogen evolution is not intensive enough to create enough mechanical mixing to aid the transport of nickel ions to the deposit. The average current efficiency of

Fig. 6—Current vs time profiles for electrodeposition at applied voltages of 3, 4, 5, 6, 7, 8, and 9 V from a chloride-based bath (0.1 M NiCl2 + 1 M NH4Cl) with pH 6 for plating time of 5 min.

hydrogen evolution increases with increase in applied voltage for electrodeposition of nickel resulting in increasing amounts of hydrogen evolution. Beyond 7 V, at higher overvoltages (Figures 5(d) through (f)), H2 evolution rate increases with the decreasing diffusion layer thickness, which assists in the faster mass transfer of Ni2+ ions resulting in the formation of compact cauliflower-like structures due to a spherical diffusion layer.[15] 2. Variation of current–time profile with applied voltage Figure 6 shows the current–time profiles for the applied voltages 3, 4, 5, 6, 7, 8, and 9 V. The quick decrease (in magnitude) in current in the first 10 seconds indicates the coating of the copper strip with a thin layer of nickel which increases the hydrogen overvoltage and decreases the current density. The low current at lower potentials (3, 4, 5 V) verifies an activation-controlled or a mixed activation–diffusion-controlled electrodeposition with predominantly nickel deposition with zero or minor hydrogen evolution. The steady current–time profile indicates a steady–state growth of metallic film with a stable diffusion layer. The magnitude of current increases when the applied voltage is increased from 3 to 5 V as, comparatively, more hydrogen is getting evolved since the increasing overvoltage increases hydrogen evolution efficiency.[45] At 6 and 7 V, the current increases linearly and minute serrations are observed in the curve due to evolution of H2. The minute serrations are associated with the development of isolated pores in the microstructure. The current transients in this zone are

typical of a diffusion-controlled growth with a linear increase in current density over time. The rapid increase in the current density value can be ascribed to the increase of hydrogen evolution reaction and faster nucleation rate at high overvoltage. Honeycomb-like structures (Figures 5(e2) and (g2)) are observed only at 8 and 9 V where serrations are more prominent due to more severe mechanical agitation by bubble evolution. As a result of intensive stirring due to bubble evolution, serrations occur in the current–time profiles due to local rapid changes in the hydrodynamic conditions near the electrode. The serrations in the curve can also be explained as: when the bubble coalesce and leave the surface, the current density falls due to increase in effective surface area and current density increases during bubble coalescence due to decrease in effective surface area. According to Popov et al.,[12] the volume of hydrogen evolved has little effect on ‘‘bubble breakoff diameter’’ and the pore diameter should decrease with increase in current density. However, in this case, pore size increases when the voltage increases from 7 V to 9 V and the crystallite size of the building structures decreases with increasing potential. This can be clearly observed from Figure 5(d4) through (g4). Similar observations were also reported for hydrogen bubble-templated electrodeposition of Ag foams[18,19] where pore size increased with increasing current density. D. Morphology Evolution with pH Figure 7 shows the change in morphologies of the electrodeposited Ni-foam at different pH. The METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 7—SEM micrographs showing different morphologies evolved due to different pH values in a chloride bath (0.1 M NiCl2 + 1 M NH4Cl) [(a1 through a4) for 1, (b1 through b4) for 2, (c1 through c4) for 3, (d1 through d4) for 4, (e1 through e4) for 5, (f1 through f4) for 6, and (g1 through g4) for pH 7, respectively]. The electrodeposition applied potential is fixed at 8 V. The electrodeposition applied potential is fixed at 8 V, and the plating time is 5 min.

morphologies have been presented in the form of 4 9 7 matrix format in which morphologies at pH 1 to 7 are shown for four different magnifications (at 100, 500, 2500, and 100009). Figures 7(a1) through (g1) (at 1009) give a qualitative idea about the formation of a METALLURGICAL AND MATERIALS TRANSACTIONS A

porous network. Figures 7(a2) through (g2) (at 5009) give an overall idea about the number of pores along with their distribution. Figures 7(a3) through (g3) (25009) focus on individual pores from which pore diameters have been calculated. It shows a gradual

Fig. 8—Histograms of pore diameter distribution of nickel electrodeposited from Chloride bath (0.1 M NiCl2 + 1 M NH4Cl) at pH 2, 3, 4, 5, and 6, and their corresponding distribution curves and variations of number of pores (at 5009) and pore diameter (lm) with the change of pH, as shown in the inset. The electrodeposition applied potential is fixed at 8 V, and the plating time is 5 min.

increment in pore diameter. From Figures 7(a4) through (g4) (at 100009) shape of the crystals and the nature of growth can be identified. Figures 7(a1) through (a4) show planar nickel deposition at pH 1. Even at a very high magnification (Figure 7(a4)) planar nature is visible with no clear building blocks. Figure 7(b1) (pH 2) shows evenly distributed pores. This porous feature continued till pH 6 (Figures 7(b1) through (f1)). Figures 7(b2) through (f2) and (b3) through (f3) (pH 2 to 6) show gradual increment in pore diameter as pH is increased. Figures 7(g1) through (g4) show no sign of porosity. Figure 7(a4) shows that the nature of growth is planar. Figures 7(b4) through (f4) show that the nature of crystal growth is of cauliflower type. From Figures 7(b2) through (f2) (at 5009), the number of pores and the pore diameter have been calculated which are shown in Figure 8. It is found that with the increment in pH, the pore diameter increases, and the number of pores decreases. As shown by the histograms of pore diameter distribution and their corresponding distribution curves (Figure 8), the pore diameters of nickel prepared at pH 2, 3, 4, 5, and 6 are 20.79 ± 6.11, 24.80 ± 7.33, 28.66 ± 10.60, 33.29 ± 8.12, and 48.48 ± 13.03 lm, correspondingly. The number of pores (observed at a magnification of 5009) falls sharply from 45 to 7 as the pH is increased from 2 to 6. The porosity in the electrodeposits generates due of simultaneous reduction of H+ ions along with the Ni2+ ions. The hydrogen gas generated, acts as a dynamic template cutting off the electrical contact between the substrate and the electrolyte. Thus, the spherical or

hemispherical bubbles get arrested within the nickel deposit until the bubbles are released. This creates the foamy network as a whole. The surface pore created after bubble coalescence mimics the bubble shape and size just before release. Hence, bigger the bubble after coalescence, bigger will be the pore size. The concept of a transition concentration for bubble coalescence has been proposed by Craig et al.[46] Transition concentration of a salt is defined as the temperature at which 50 pct coalescence has taken place. NH4Cl has a transition concentration value of 0.1 M and is known to resist bubble coalescence. In this study, the amount of NH4Cl added is 1 M, which is well above the transition concentration. However, the electrodeposition process itself alters this concentration leading to bubble coalescence as pH is increased. The possible mechanism has been discussed below. During electrodeposition from a chloride-based bath, initially both H+ and NH4+ ions are present apart from Ni2+. The reduction potential of hydrogen from hydrogen ion depends on the pH of the bath. As pH increases, the reduction potential of hydrogen increases, and hydrogen evolution by reduction of H+ becomes more and more difficult. Hence, the following alternate cathodic reaction starts taking place. NHþ 4 þ e ¼ NH3 þ Hadsorbed

½3

As the above reaction proceeds, the concentration of NH4+ species starts diminishing. Similarly in the anode it is possible to have the following reactions 2Cl 2e ¼ Cl2 ð1:36 VÞ

½4


2H2 O 4e ¼ O2 þ 4Hþ ð1:23 V at pH 1; 0:87 V at pH 6Þ

½5

It is clear from the reduction potential data that reaction 5 is thermodynamically easy to take place. However, O2 has a very high overpotential on Pt (counter electrode), and thus Reaction [4] takes place kinetically leading to a continuous decrement of Cl ions from the bath.[47] Thus, the concentration of NH4Cl falls gradually over time as the reaction progresses. The depletion of concentration of NH4Cl decreases its capability to resist bubble coalescence. At higher pH, hydrogen production by reduction of H+ becomes more difficult, and Reaction [3] becomes more predominant. Therefore, at higher pH, the extent of depletion of NH4+ is faster as both the reduction potentials of Ni2+ and H+ increase with the increasing pH. So, the increment in pH leads to faster depletion in concentration of NH4Cl, which further leads to loss of coalescence capability. This is the reason why bigger pores are observed as pH is increased. The increase in pore diameter can also be explained from the current–time profile and has been analyzed in Section III–E–2. E. Effect of pH (in a Sulfate Bath) 1. Morphology evolution with pH Figure 9 shows the change in morphologies of the electrodeposited Ni-foam at different pH values. The morphologies have been presented in the form of 4 9 7 matrix format in which morphologies at pH from 1 to 7 are shown for four different magnifications (at 100, 500, 2500, and 100009). Figures 9(a1) through (g1) (at 1009) give a qualitative idea about the formation of a porous network. Figures 9(a2) through (g2) (at 5009) give an overall idea about the number of pores along with their distribution. Figures 9(a3) through (g3) (25009) focus on individual pores from which pore diameters have been calculated. It shows a gradual increment in pore diameter. From Figures 9(a4) through (g4) (at 100009), the shape of the crystals and the nature of growth can be identified. Figures 9(a1) through (a4) show planar nickel deposition at pH 1. Even at a very high magnification (Figure 9) planar nature is visible with no clear building blocks. Figure 9(b1) (pH 2) shows evenly distributed pores. This porous feature continues till pH 6 (Figures 9(b1) through (f1)). Figures 9(b2) through (f2) (pH 2 to 6) and Figures 9(b3) through (f3) (pH 2 to 6) show gradual increment in pore diameter as pH is increased. Figures 9(g1) through (g4) shows no sign of porosity. Figure 9(a4) shows that the nature of growth is planar. Figures 9(b4) through (f4) show cauliflower type growth in the ridges of the honeycomb structure developed at pH 2 to 6. Figures 9(g4) shows dendritic nature of growth in the electrodeposit at pH 7. The same sets (varying pH) of experiments were performed with (NH4)2SO4 as was done with ammonium chloride. In both the cases, similar trend in pore METALLURGICAL AND MATERIALS TRANSACTIONS A

diameter is observed i.e., with increase in pH the pore diameter increases. However, it is found that the overall pore sizes in the Ni electrodeposited from sulfate bath are smaller than that of the same deposited from chloride bath. This can be explained by difference in bubble coalescing power of (NH4)2SO4 and NH4Cl. The surface pore sizes are a result of the coalescence of bubbles, the more the coalescence the bigger the pore size and vice-versa. From Figures 9(b2) through (f2) (at 5009), the number of pores and the pore diameter have been calculated and shown in Figure 10. It is found that with the increment in pH, the pore diameter increases with the decrement in number of pores. As shown by the histograms of pore diameter distribution and their corresponding distribution curves, the pore diameters of nickel prepared at pH 2, 3, 4, 5, and 6 are 18.60 ± 4.01, 22.41 ± 3.97, 23.38 ± 4.27, 23.43 ± 4.66, 24.29 ± 4.79 lm, correspondingly. The pore size does not change appreciably with the increasing pH from 2 to 6 in the case of sulfate bath. The number of pores (observed at a magnification of 5009) decreases from 75 to 60 as the pH is increased from 2 to 6. The number of pores observed in case of electrodeposits from a sulfate bath is significantly higher than that of a chloride bath. The concept of transition concentration, which deals with the concentration of inorganic salt in the electrolyte which can prevent coalescence of bubbles in an aqueous medium, was put forward by Craig et al.[46] It is found that (NH4)2SO4 has a transition concentration of 0.031 M. Therefore, during deposition from a sulfate bath, the depletion of ammonium ions is not sufficiently high, (NH4)2SO4 concentration does not drop to less than 0.031 M, and a concentration of > 0.031 M is retained. Hence, the power of resisting bubble coalescence is higher for (NH4)2SO4 than that for NH4Cl, if they are used in the same initial concentration, i.e., 1 M.[46] Furthermore, in the case of (NH4)2SO4, the anions present in the bath are SO42 and OH. SO42 has more oxidative potential than OH. This means that in an electrolytic cell, it is easier to oxidize OH over SO42. Therefore, when a SO42 bath is chosen, oxidation of OH takes place at the anode with enrichment of H+ into the bath. This H+ decreases the pH of the bath, and the activation polarization of H+ reduction decreases. Hence, the H+ reduction occurs preferentially to NH4+. Thus, both NH4+ and SO42 stay in the bath almost in the pristine concentrations. The retention of resistance to bubble coalescence by (NH4)2SO4 is the primary reason for the production of nickel foam with smaller pore diameter from a sulfate bath. 2. Effects of current–time profile on pore size and pore density The relative role of pH in pore formation in chloride-based electrolyte and sulfate-based electrolyte can be traced to the current–time profiles. In case of deposition from both types of baths, current density decreases with increase in pH as evident from Figure 11(a). This is explained in the previous

Fig. 9—SEM micrographs showing different morphologies evolved due to different pH values from a sulfate bath (0.1 M NiSO4 + 1 M (NH4)2SO4) at different magnifications (a1 through a4) 1, (b1 through b4) 2, (c1 through c4) 3, (d1 through d4) 4, and (e1 through e4) 5, (f1 through f4) 6, and (g1 through g4) 7. The electrodeposition applied potential is fixed at 8 V for 5 min.

section. The increasing nature of pore size in a chloride bath from 20.79 to 48.48 lm and the almost constant pore size of around 22 to 24 lm (Figure 11(b)) in a sulfate bath can be ascribed to the striking difference in the magnitude of the current profiles. In both the cases, current density decreases with increase in pH from 2 to 6. This can be attributed to primarily three reasons: the

decomposition potential of water increases by 0.059 V per unit increase in pH,[34] efficiency of electrodeposition of metallic nickel decreases with increase in pH[33] and also the discharge potential required for nickel deposition from Ni(NH3)n (2 < n < 6)[23] formed at higher pH becomes higher. These synergistically cause the current density to decrease with increase in pH in both the cases. METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 10—Histograms of pore diameter distribution of nickel electrodeposited from sulfate bath (0.1 M NiSO4 + 1 M (NH4)2SO4) at pH 2, 3, 4, 5, and 6 for a plating time of 5 min, and their corresponding distribution curves and variations of number of pores (at 5009) and pore diameter (lm) with the change of pH, shown in the inset.

The decrease in the bubble breakoff diameter with the increasing current density can be attributed to the change of surface tension (wettability) with varying electrode potentials (electrocapillary effect). The bubble breakoff diameter, d is presented by the following equation[13,48]: d ¼ d0

I 0:45 1 þ 0:2 ; S

where d is the bubble breakup diameter; I/S is the current density, and d0 is a constant. d0 is given from the equation qproposed by Frietz and Stephan: ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d0 ¼ 1:20# gðq cq Þ where 0 is the contact angle, c is L

G

the surface tension at the interface, and qL and qG indicate the densities of the liquid phase and gaseous phase, respectively.[48] However, in this context, d0 has been treated as a constant, and the qualitative inverse variation of d and I/S has been emphasized. Therefore, in both the cases (of deposition from chloride bath, or a sulfate bath), pore size increases with the increasing pH due to increase in the breakoff diameter caused by decreasing current density as seen from Figure 11(b). However, in the case of chloride bath, the current–time profiles are widely spaced, whereas in case of the sulfate bath, the current time profiles are tightly spaced. There is no appreciable increase in the current density on increasing pH in case of sulfate bath as can be seen from Figure 11(a). Hence, the pore size remains almost constant in the pH range from 3 to 6. However, in the case of chloride bath, the METALLURGICAL AND MATERIALS TRANSACTIONS A

pore size increases linearly due to the gradually decreasing current density with the increasing pH from 2 to 6. F. Possible Applications of Porous Nickel Eletrodeposits Table III illustrates selected examples where porous nickel nanostructures have been used for specific mechanical, thermal, and electrochemical applications depending on the porosity (pore diameter and distribution) and where the optimized electrodeposited nickel electrodeposits stand in this regard. The correlation between surface morphology generated and applications have been summarized in Table IV for nickel electrodeposits synthesized through electrodeposition. Both the tables corroborate the fact that the versatility of morphology generated (carrot-like, dendritic and cauliflower) coupled with the porosity distribution in the optimized porous nickel electrodeposits can offer potential applications in diverse fields.

IV.

CONCLUSIONS

In this study, the electrodeposition of nickel foam by dynamic hydrogen bubble-template method is optimized, and the effects of key deposition parameters (applied voltage, and deposition time) and bath composition (concentration of Ni2+, pH of the bath, role of Cl and SO42 ions) on pore size, distribution, and morphology and crystal shapes were studied.

Fig. 11—(a) Current–time profile in chloride bath (0.1 M NiCl2 + 1 M NH4Cl) and sulfate (0.1 M NiSO4 + 1 M (NH4)2SO4)) at different pH values for a plating time of 5 min. (b) Variations of pore size and pore density with pH for sulfate bath (shown in square box, red in case of pore diameter and black in case of number of pores) and chloride bath (shown in circles, red in case of pore diameter and black in case of number of pores) (Color figure online).


Table III.

Studies on Application Nickel Foam and Correlation Between Application, Type of Nickel Foam Used, and Porosity (Pore Diameter and Distribution)

References

Type of Nickel Foam Used

Application

Porosity (Pore Diameter and Distribution)

49

commercial 3D nickel foam

50

commercial 3D nickel foam

sound absorption performance (200 to 2000 Hz) pseudocapacitor based on MnO2

51 6 52

commercial 3D nickel foam electrodeposited film commercial 3D nickel foam

cathode current collector anode current collector alkaline fuel cells

53

commercial nickel foam

novel oil–water separation material

54


55


flow, thermal, and structural applications microchannel heat exchangers

This study

electroplated

porosity: 89 pct; average pore diameter: 0.57 mm) open porous 300 to 500 lm, pores/ mm2 = 4 open porous 200 lm, pores/mm2 = 4 ~8 lm pore diameter open porous 300 to 500 lm, pores/ mm2 = 4 open porous 100 to 200 lm, pores/ mm2 = 10 open porous 50 lm, pores/mm2 = 25 PPI 17 to 23 dp (mm) 0.65 porosity (e) 0.961 pore diameter: 20.79 lm to 48.48 lm (chloride bath); 22 to 24 lm (sulfate bath) number of pores @5009: 45 to 7 (chloride bath); 75 to 60 (sulfate bath)

Table IV. Studies on Electrodeposited Nickel and Correlation Between Applications, Type of Nickel Electrodeposit Used, and Surface Morphology Reference

Synthesis Route

Application

Morphology epitaxial Ni films on GaAs (001) (planar epitaxial growth) faceted, chips-like

56

electrodeposition

structural and magnetic applications

57

electrodeposition

58

electrodeposition and laser cladding electrodeposition

morphology, structure, and magnetic properties wear and corrosion protection

This Work

Optimization based on Ni2+ concentration: Nickel

deposit with 0.1 M NiCl2 bath concentration is able to produce the honeycomb-like structure with regular-sized pores at moderately acidic–basic range among concentrations of 0.01, 0.05, 0.1, 0.5, and 1 M NiCl2 solution. Optimization-based on time of electroplating: A critical time (>3 minutes) is required for the development of the honeycomb structure. Compressive residual stresses are developed after 3 minutes. Pore size increases with the increase in deposition time. Optimization based on applied voltage: Honeycomb-like structure can be deposited only at higher applied voltages of 7, 8, and 9 V. At lower applied voltages, planar films are formed. Cauliflower-like morphology is observed at high voltages with dendritic morphology at intermediate, and carrot-like structures at low voltages (from a chloride bath). Effect of pH is more pronounced in a chloride bath compared with a sulfate bath. The increasing nature of pore size in nickel electrodeposits plated


cauliflower planar; carrot-like; dendritic; cauliflower; honeycomb

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