Electrochemical investigation of the corrosion properties of three-dimensional nickel electrodes on silicon microchannel plates

Corrosion Science 100 (2015) 113–120

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Electrochemical investigation of the corrosion properties of three-dimensional nickel electrodes on silicon microchannel plates Shaohui Xu a,c,∗ , Yiping Zhu a , Dayuan Xiong a , Wenchao Zhang b , Lianwei Wang a,c , Pingxiong Yang a , Paul K. Chu c a Key Laboratory of Polar Materials and Devices, Ministry of Education, and Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Minhang District, Shanghai 200241, China b School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 24 September 2014 Received in revised form 24 July 2015 Accepted 27 July 2015 Available online 5 August 2015 Keywords: A. Nickel B. Polarization C. Kinetic parameters

a b s t r a c t The corrosion properties of three-dimensional nickel electrodes deposited on silicon microchannel plates by electroless and electrodeposition processes are studied by electrochemical methods. The nickel thin film deposited on the silicon microchannel plate protects the silicon skeleton in the aqueous electrolytes containing alkalis and salts and the electrolyte containing salts may be a good choice due to the low corrosion rate and no damage to silicon. The nickel-coated silicon microchannel plate electrodes exhibit low resistance and high surface defect concentration after the electrodeposition process and p- and n-type semiconductor properties are observed in the alkali and salt media, respectively. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Three-dimensional (3D) architectures enable better exploitation of space in energy storage devices with a smaller footprint while retaining the high power and energy density due to their high surface-to-volume ratio. In this respect, it is a desirable strategy to fabricate rechargeable energy devices such as batteries and supercapacitors using a 3D architecture utilizing the microelectromechanical (MEMs) technique [1–10]. At the same time, the drive to further miniaturize existing on-chip systems makes it possible to develop energy devices integrated with other elements. The 3D electrode is the basic and important component in 3D energy devices and there are different kinds of 3D electrodes such as interdigital structures [5–10], nanometer or micrometer-size wire arrays [1,3,4], and microchannel plates (MCP) composed of glass or silicon [2,11–19]. The silicon microchannel plate (Si–MCP) consists of a perforated “through-hole” substrate formed by photo-assisted electro-chemical etching and delamination from the Si substrate using a modified electrochemical procedure. The Si–MCP has a large

∗ Corresponding author at: Key Laboratory of Polar Materials and Devices, Ministry of Education, and Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Minhang District, Shanghai 200241, China Fax: +86 21 54345119. E-mail address: [email protected] (P.K. Chu). http://dx.doi.org/10.1016/j.corsci.2015.07.015 0010-938X/© 2015 Elsevier Ltd. All rights reserved.

area ratio (>70%) and aspect ratio (length/diameter of hole >40 or more) [11]. Surface area gains (ratio of the surface area of the 3D structure to that of a similar planar electrode) of 50 or 100 times can be achieved. For 3D Li-ion batteries based on the MCP structure composed of glass, the reversible 2.0 mAh cm−2 battery capacity is about 15 times (1500%) larger than the 0.133 mAh cm−2 capacity reported for the best commercial 2-D thin-film batteries [2]. The half-cell 3D lithium-ion micro-battery comprising a lithium foil as the counter electrode and Ni-coated Si–MCP as the anode exhibits a charge capacity of 3520 mAh g−1 with a coulombic efficiency close to 95.8% in the initial cycle [18]. With regard to the 3D supercapacitor based on the interdigital structure, a large capacitance of 90.7 mF cm−2 and fast power of 51.5 mW cm−2 are obtained in conjunction with good stability and high charge/discharge efficiency [10]. As for the 3D electrode based on Si–MCP, a high specific capacitance of 6.90 F cm−2 is achieved at a discharge current density of 10 mA cm−2 for Co(OH)2 nano-sized flakes prepared in acetone [16] and the specific capacitance is 7.8 F cm−2 at a discharge current density of 20 mA cm−2 for heterostructured Ni(OH)2 –Co(OH)2 composites [17]. These results indicate that the 3D design offers remarkable improvements in the energy and power densities especially when considering the geometric footprint of the device. However, the stability of the energy devices is a practical issue because that the 3D framework is normally quite thin in order to obtain a large surface-to-volume ratio and decrease the mass of the energy device. In addition, the device must work under dif-

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ferent conditions such as non-aqueous and aqueous electrolytes containing acids, alkalis, and salts and damage to the 3D framework may compromise the performance. The largest contributor to the degradation of the Ni/Si–MCP electrode is loss of Ni and silicon consequently affecting the cyclic performance of the energy devices such as 3D Li-ion batteries consisting of the Si–MCP skeleton [19]. In the fabrication process of energy devices based on Si–MCP, the Ni coating on both the outer surface and inner side walls of the Si–MCP can keep the silicon framework away from the electrolytes thereby protecting the 3D skeleton. On another hand, the plated Ni layer can also work as a current collect due to its high conductivity. However, the Ni layer may suffer corrosion in non-aqueous or aqueous electrolytes and the corrosion process of the Ni layer in different electrolytes should be studied. In fact, the corrosion properties of nickel and their alloys have already been studied in different electrolytes [20–31] and the corrosion resistance of nickel has been modified by co-deposition of SiC particles [32]. Nevertheless, it is necessary to understand the corrosion properties of the 3D nickel electrode in order to provide new information and insights pertaining to the framework in order to fabricate high-performance 3D energy devices.

2. Experimental details Commercial 100 mm, p-type, (100) silicon wafers with a thickness of 525 ␮m were used in our experiments. The standard microelectronics fabrication steps were carried out to prepare the Si–MCP with a large aspect ratio. The steps included thermal oxidation to produce a masking layer and 3 ␮m × 3 ␮m squares were patterned by lithography and wet etching. The patterned wafer was pre-etched in a tetramethyl ammonium hydroxide (TMAH) solution (25% at 85 ◦ C) and anodized on a computer-controlled photo-assisted electrochemical system. More details about the process can be found in Refs. [11,12]. The Si–MCPs were arranged in a square array with 5 × 5 ␮m2 pores and 1 ␮m thick wall. The Si–MCP had a resistivity on the order of k×cm measured by a multimeter on the surface at a distance of about 1 cm. Electroless plating was conducted in our experiments [18]. The silicon MCP was dipped in diluted HF for 30 s to remove the native oxide and then soaked in a buffer solution (0.1% Triton X-100 solution) for 30 s to decrease the inner stress and enhance wetting prior to immersion in a plating bath for 20 min. Afterwards, electroless deposition of Ni was carried out in the mixture containing 0.3 mol L−1 NiCl2 ·6H2 O (nickel chloride hexahydrate), 0.1 mol L−1 NH4 Cl (ammonium chloride), and 0.1 mol L−1 NaH2 PO2 ·H2 O (sodium hypophosphite monohydrate) at 85 ◦ C for 15 min. The pH was adjusted to 8.5–9.5 by addition of ammonia. NiCl2 ·6H2 O was used as the Ni ion source, NH4 Cl and ammonia were used to adjust the pH as complexing agents for Ni2+ , and NaH2 PO2 ·H2 O served as the reducing agent. Electroless nickel plating was an auto-catalytic chemical technique which only deposited a limited nickel content on both the outer surface and inner side walls of Si–MCP. In order to decrease the resistance and improve the surface area, another Ni film was electrodeposited in a standard two-electrode glass cell at room temperature using an electrolyte consisting of 0.5 mol L−1 H3 BO3 and 0.1 mol L−1 NiCl2 .6H2 O at a pH value of 3.5. A clean electroless plated Ni/Si–MCPs served as the working electrode and a Pt foil formed the counter electrode. The distance between the two electrodes was 1 cm and electrodeposition (galvanic deposition) was carried out at a constant current density of 200 mA cm−2 for 300 s. All the chemical reagents were AnalaR (AR) grade and used without further purification. The aqueous solutions were made from 18 M cm de-ionized water and for comparison, a high purity nickel sheet (>99.9%) was prepared. The same electroless

conditions were adopted to prepare the nickel-coated Si wafer. The samples were designated as Ni (nickel sheet), SiNi (nickel coated Si wafer), MCPNi (nickel coated Si–MCP by electroless plating), and MCPNN (nickel coated MCPNi by electrodepositing). In these samples, the materials influence was assessed based on the experimental data acquired from Ni and SiNi revealing compact and porous nickel layers on the two-dimensional structure (plate), respectively. The structure was investigated based on the experimental data obtained from SiNi and MCPNi indicating the plated and 3D structure with the same porous nickel layer on the surface, respectively. The morphology of the Si–MCP and nickel electrodes was examined by scanning electron microscopy (SEM, Hitachi S-4800, Japan). The electrochemical measurement was performed immediately after electroless and electrdeposition to avoid Ni oxidation. In the electrochemical analysis, the samples were exposed to a 2 mol L−1 KOH or 1 mol L−1 Na2 SO4 aqueous solution at room temperature (25 ◦ C) and served as the working electrode. A platinum electrode was used as the counter electrode and a mercury oxide electrode (Hg–HgO) served as the reference electrode. The electrochemical tests were performed on a three-electrode electrochemical workstation (CHI660D, Chenhua, Shanghai). Cyclic voltammograms (CV) were acquired in the potential from −1.2 to 0.7 V/Hg–HgO at scanning rates from 10 to 200 mV s−1 . A moderate scanning rate such as 50 mV s−1 was selected to show the C–V properties for different electrodes. The Tafel plots were obtained from the potentiodynamic polarization tests conducted using sweeping rates of 1 mV s−1 towards the anodic direction. In the analysis, the data in the Tafel plots in the range of ± 200 mV from the open circuit potential were selected to evaluate the corrosion behavior. In the Mott–Schottky analysis, the impedance-potential curves were measured by sweeping the potential in the positive direction at a potential step of 10 mV, excitation voltage of 5 mV, and frequency of 100 Hz. Here, the electrodes were first polarized for several minutes at a potential E of −1.2 V/Hg–HgO until producible impedance diagrams were obtained. The data were normalized by the footprint area.

3. Results and discussion After electrochemical etching, the Si–MCP is well aligned and the Si–MCP has square holes 5 ␮m in size. The depth of the Si–MCP can be varied from 100 to 250 ␮m by changing the etching conditions and big surface area gain is achieved [34]. However, as shown in Fig. 1(a) depicting the top-view SEM images of the Si–MCP fabricated by electrochemical etching, the width of the side wall is less than 1 ␮m. Owing to etching in an aggressive electrolyte such as an alkaline one, the skeleton may collapse after some time. As shown in Fig. 2(a), the CV curve of the Si–MCP in the 2 mol L−1 KOH aqueous solution shows a large current density of about 0.02 A cm−2 at −1 V/Hg–HgO at a scanning rate of 50 mV s−1 and it is almost 300 times larger than that of the Si wafer. As for the structure of Si–MCP with a depth of 200 ␮m, period width of 6 ␮m, and wall thickness of 1 ␮m, the surface gain can reach about 60 [33]. It indicates that the higher current density of the Si–MCP electrode in KOH solution stems from the 3D architecture of the Si–MCP (60 times) and surface structure of the silicon inner side walls (almost 5 times). To characterize the corrosion behavior quantitatively, the Tafel plots obtained at a scanning rate of 1 mV s−1 from the 2 M KOH aqueous electrolyte are shown in Fig. 2(b). The anodic and cathodic polarization curves exhibit a linear behavior. The corrosion potentials Ecorr (V/Hg–HgO), polarization resistance Rct ( cm2 ), cathodic Tafel slope bC (mV dec−1 ), and corrosion current density Icorr (A cm−2 ) can be obtained by extrapolation of the anodic and cathodic branches [34–37] and the data are listed in Table 1. For

S. Xu et al. / Corrosion Science 100 (2015) 113–120

115

Fig. 1. (a) Top-view SEM images of the SiMCP fabricated by electrochemical etching; (b) morphology of the side wall of the electroless plating nickel coated SiMCP; (c) Top-view and (d) cross-sectional SEM view of the Ni coated SiMCP formed by electrodeposition.

Si–MCP, lower corrosion potential Ecorr (−0.25 V/ Hg–HgO) and polarization resistance Rct (2.2 × 106  cm2 ) are observed due to the larger surface area of the 3D architecture. A large corrosion current density of Icorr (4.6 × 10−7 A cm−2 ) is observed from Si–MCP and the corrosion current density Icorr is only 7.9 × 10−9 A cm−2 after removing the architecture effect. The corrosion current density decreases for Si–MCP due to the low carrier concentration in the depleted wall structure indicating that the corrosion susceptibility of the Si–MCP is better than that of the silicon wafer due to high resistivity for the depleted wall structure and insulating effects of the Si–MCP preventing corrosion. However, the 3D Si–MCP architecture is still damaged after a long operating time due to the thin wall thickness, large surface area, and rough surface of the Si–MCP prepared electrochemically. After electroless nickel plating, nickel nano-grains several to hundreds of nanometers can be found from the inner walls, as shown by the morphology of the side wall of the electroless plating nickel coated Si–MCP in Fig. 1(b). Even nano-particles appear on the surface and the relatively thick Ni surface film (almost 500 nm) can protect Si. Owing to the nano-size of the nickel grains deposited on the outer and inner surfaces of the Si–MCP, the surface area of the 3D nickel electrode can increase by almost 100 times due to the 3D architecture of the Si–MCP and nanoporous structure of the Ni layer [33]. In this case, the Ni coated Si–MCP can be used to fabricate supercapacitors and Li-ion batteries in different electrolytes. In this study, 2 M KOH and 1 M Na2 SO4 are chosen to study the corrosion properties of the Ni coated Si–MCP. The cyclic voltammograms (CV) acquired from Ni, Si–Ni, MCP–Ni, and MCPNN in the potential range of −1.5–0.8 V/Hg–HgO at a scanning rate of 50 mV s−1 are shown in Fig. 3. In order to show the small peaks, the double axes are used in Fig. 3a. The same electrochemical behavior is observed and it is typical of oxidation–reduction reaction of Ni in KOH [39,40]. During the increasing sweep, the passive layer which consists of a NiO layer is formed, as indicated by the current peak P1 . In the potential domain of the plateau (B), the amount of nickel oxide increases

and there is simultaneous transformation of the ␣-Ni(OH)2 amorphous layer into less hydrated and crystallized ␤-Ni(OH)2 . At a potential higher than that of the oxidation peak P2 , ␤-Ni(OH)2 is oxidized to ␤-NiOOH and the transformation is complete when the water oxidation current is observed. During the decreasing potential sweep, the reverse reduction reaction from ␤-NiOOH to ␤-Ni(OH)2 is observed corresponding to the cathodic peak P3 . The similar cyclic voltammogram and same oxidation–reduction process indicate that nickel is deposited on the Si wafer and Si–MCP and it is also confirmed by the XRD data in Ref. [15]. However the different current densities are shown, such as the typical oxidation peak P2 , due to the influence by the surface structure. Compared to the Ni sample (7.6 × 10−5 A cm−2 ), the current density of the SiNi (1.2 × 10−2 A cm−2 ) increases by almost 15 times and MCPNi sample (3.2 × 10−2 A cm−2 ) increases by almost 40 times. The results suggest that the larger current density of the MCPNi electrode stems from the porous structure of the nickel electrode (almost 15 times) and 3D architecture of Si–MCP (almost 2.5 times). The Tafel plots obtained in 2 M KOH are shown in Fig. 4(a). Generally, the cathodic polarization curve is assumed to represent the cathodic hydrogen evolution via water reduction and the corrosion potential (Ecorr ) and corrosion current density (icorr ) can be derived directly from the region in the cathodic polarization curves by Tafel region extrapolarization [41,42]. The obtained corrosion data are listed in Table 1. The corrosion potential decreases from −0.45 for Ni to −0.51 for SiNi and to −1.18 V/Hg–HgO for MCPNi. However the polarization resistance increases from 15,072 for Ni to 21,137 for SiNi and decreases to 39  cm2 for MCPNi. Owing to the limited Ni amount and passivation effects of the porous Ni metal on the silicon wafer, the polarization resistance increases and corrosion density current decreases for SiNi (6.8 × 10−6 A cm−2 ) compared to the compact Ni sheet (1.6 × 10−5 A cm−2 ). On account of the 3D architecture of Si–MCP, the polarization resistance decreases to 39  cm2 and corrosion density current increases to 3.4 × 10−3 A cm−2 (5.9 × 10−5 A cm−2 for material corrosion current density) for MCPNi. With regard to the compact surface on the

−0.25 ± 0.18 2.2 ± 0.14 × 106 −208 ± 17 4.6 ± 0.2 × 10−7 (7.9 ± 0.2 × 10−9 ) – –

−0.20 ± 0.11 1.2 ± 0.14 × 107 −271 ± 14 2.4 ± 0.1 × 10−8

– –

Ecorr Rct bc Icorr

EFB NA (ND )

−1.18 ± 0.19 39 ± 51 −138 ± 12 3.4 ± 0.2 × 10−3 (5.9 ± 0.2 × 10−5 ) −0.32 ± 0.14 8.7 ± 0.2 × 1024

−0.51 ± 0.13 21137 ± 159 −142 ± 13 6.8 ± 0.3 × 10−6

−0.45 ± 0.12 15072 ± 110 −189 ± 16 1.6 ± 0.1 × 10−5 −0.29 ± 0.10 −0.25 ± 0.12 1.2 ± 0.1 × 1021 1.7 ± 0.1 × 1022

MCPNi

SiNi

Ni −0.97 ± 0.18 109 ± 58 −108 ± 10 2.6 ± 0.2 × 10−3 (4.5 ± 0.2 × 10−5 ) −0.25 ± 0.11 4.1 ± 0.3 × 1025

MCPNN −0.89 ± 0.19 1381 ± 21 −101 ± 11 3.2 ± 0.1 × 10−4 (5.5 ± 0.1 × 10−6 ) −0.51 ± 0.12 3.0 ± 0.2 × 1024

−0.85 ± 0.14 2261 ± 88 −137 ± 18 1.2 ± 0.1 × 10−5

−0.83 ± 0.10 25253 ± 155 −131 ± 16 3.8 ± 0.1 × 10−6 −0.43 ± 0.10 −0.53 ± 0.13 1.8 ± 0.1 × 1021 2.3 ± 0.1 × 1021

MCPNi

SiNi

Ni

1 M Na2 SO4

−0.82 ± 0.12 92 ± 14 −53 ± 9 2.3 ± 0.1 × 10−3 (4.0 ± 0.1 × 10−5 ) −0.52 ± 0.14 2.6 ± 0.4 × 1026

MCPNN

-2

Log (Current density (A cm ))

-2

Current density (A cm )

a In the table, Ecorr and EFB are the corrosion and flat-band potential, respectively, in units of V/Hg–HgO. Rct is the polarization resistances in unit of  cm2 . bc is the cathodic Tafel slope in unit of mV dec−1 . Icorr is the corrosion current density in unit of A cm−2 . The data in brackets is the material corrosion current density through removing architecture effect. NA (or ND ) is the surface defect concentration in units of cm−3 . The statistical analysis was done based on 3 or 5 samples for each kind of electrode.

Si–MCP

Si

Samples

Electrolyte 2 M KOH

Table 1 Data obtained from the chemical measurement for all samples in two kinds of aqueous electrolytea .

116 S. Xu et al. / Corrosion Science 100 (2015) 113–120

0.06

a

0.04

0.02

-5

-0.6

Si-MCP Si

0.00

-0.02

-0.04

x 300

-0.06 -1.5 -1.0

Potential E (V / Hg-HgO)

-0.5

-0.4

0.0

-0.2

0.5

b

-6

-7

-8

Si SiMCP

-9

-10

Potential E (V / Hg-HgO) 0.0

Fig. 2. (a) Cyclic voltammograms (scanning rate of 50 mV s−1 ) and (b) Tafel plots (scanning rate of 10 mV s−1 ) of Si wafer (dotted lines) and SiMCP (solid lines) in 2 M KOH.

pure Ni sheet, the porous Ni layer and 3D architecture of Si–MCP decreases the corrosion potential of SiNi and MCPNi. At the same time, the porous layer is formed on the rough inner wall surface of Si–MCP (as shown in Fig. 1b), and the corrosion current density increases by about 4 times compared to the compact Ni sheet and 9 times relative to the porous Ni layer on Si (SiNi). The electroless plated Ni layer can protect the Si skeleton and 3D silicon framework from the KOH solution. However, the limited thickness and large corrosion rate of Ni layer limit the life time of the 3D electrode, as demonstrated by 3D Li-ion batteries with the Si–MCP skeleton [19]. In this case, electrodopostion of Ni on MCPNi can increase the content of Ni and decrease the resistance of the current collection layer. After electrodeposition of the Ni layer (galvanic deposition), the current density of the oxidation peak P2 in the CV curve can reach 0.17 A cm−2 as shown in Fig. 3a (D) and this is almost 6 times larger than that of MCPNi due to the larger effective nickel content and surface area caused by the nano-

S. Xu et al. / Corrosion Science 100 (2015) 113–120

a B

0

-0.2 0.03

0.00

0.00 -0.03

-0.01 0.000 2 B (SiNi)

0.01

0.0000

0.00

-0.0002 0.0005

-0.01 A (Ni)

0.0005

0.0000

0.0000

-0.0005

-0.0005 -1.2

-0.8

-0.4

0.0

-2

P3 C (MCPNi)

-2 -3 -4 -5 -6 -7

0.4

-8

Potential E (V / Hg-HgO)

a

Ni SiNi MCPNi MCPNN

-1

Log (Current density (A cm ))

-2

Current density (A cm )

0.0

-2

0.00 -0.05 0.01

a

0.2

P2 P1

Current density (A cm )

0.05 D (MCPNN)

-1.2

0.1

D (MCPNN)

b

-0.6

-0.3

0.0

-1

0.1 -0.1

C (MCPNi)

-2

0.0 -2

Log (Current density (A cm ))

-2

-0.9

Potential E (V / Hg-HgO)

0.0

Current density (A cm )

117

-0.1 0.000

B (SiNi)

-0.001 0.0005

A (Ni) 0.0000

-0.0005 -1.0

-0.5

0.0

0.5

Potential E (V / Hg-HgO) Fig. 3. Cyclic voltammograms of (A) Ni, (B) SiNi, (C) MCPNi and (D) MCPNN at a scanning rate 50 mV s−1 in the potential range −1.3−0.6 V/Hg–HgO in 2 M aqueous KOH (a) and 1 M aqueous Na2 SO4 (b).

flake structure on the surface and side wall of Si–MCP, as shown in Figs. 1 (c) and (d). The intensity of the passive peaks P1 can also provide information about the Ni content. On account of the limited Ni concentration on the Si surface, the passive peak P1 for SiNi is small (1.1 × 10−4 A cm−2 ) and it is almost not observed from the CV curve, whereas the passive peak P1 is clearly observed from the compact Ni sheet (2.2 × 10−4 A cm−2 ). At the same time, a larger current density of the passive peaks P1 appears from MCPNN (2.2 × 10−2 A cm−2 ) and it is almost 100 times that of the compact Ni sheet and 3 times that of the porous Ni layer on Si–MCP (MCPNi, 6.7 × 10−3 A cm−2 ). Additionally, the electrodeposition process improves the compactness and surface properties of the Ni layer. The corrosion potential decreases from −1.18 for MCPNi to −0.97 V/Hg–HgO for MCPNN and the polarization resistance increases from 39  cm2 for MCPNi to 108  cm2 for MCPNN. The corrosion current density decreases from 5.9 × 10−5 A cm−2 for MCPNi to 4.5 × 10−5 A cm−2 for MCPNN. The cathodic Tafel slope bc shown in Table 1 can be obtained from the Tafel plots for the four electrodes in KOH. The cathodic Tafel slopes bc decrease steadily from −189 for the Ni electrode to −142 mV dec−1 for SiNi, to −138 mV dec−1 for the MCPNi electrode and −108 mV dec−1 for the MCPNN electrode. It indicates that the larger surface area rendered by the porous Ni and 3D architecture of the Si–MCP for SiNi and MCPNi can facilitate electron transfer from the electrode to protons in the solution resulting in the larger current density and smaller Tafel slopes for the cathodic current

-3

-4

-5

Ni SiNi MCPNi MCPNN

-6

-7

-8 -1.2

-0.9

-0.6

Potential E ( V/ Hg-HgO) Fig. 4. Tafel plots of Ni, SiNi, MCPNi and MCPNN in aqueous solutions: (a) 2 M KOH and (b) 1 M Na2 SO4 .

flow. After electrodeposition of the Ni layer on MCPNi, smaller Tafel slopes are shown for the MCPNN electrode due to the larger surface area resulting from the nano-flake structure on the surface and side walls of Si–MCP. While the KOH aqueous solution is often used in pseudo- or Faraday supercapacitors, the aqueous Na2 SO4 solution is another common electrolyte in electrical double layer supercapacitors. In this case, the electrical charges are held in the double layer at the electrode/electrolyte interface. The CV curves in Fig. 3b only show cubic-similar curves and not the clear peaks for the electrochemical passive reaction because of the electrical double layer structure at the interface of Ni/Na2 SO4 . Compared to the Ni sheet (9.6 × 10−5 A cm−2 at potential −0.4 V/Hg–HgO), the current density increases clearly for SiNi (1.8 × 10−4 A cm−2 ) and MCPNi (2.4 × 10−2 A cm−2 ) due to the porous Ni layer and 3D architecture. Electrodepostion of Ni increases the current density to almost 0.04 A cm−2 at the potential −0.4 V/Hg–HgO for MCPNN. By analyzing the Tafel plots, the corrosion current density (icorr ) and corrosion potential (Ecorr ) are deduced from the extrapolation of the cathodic branch of the Tafel plots to the corrosion potential, as shown in Table 1. The corrosion potential is about −0.85 V/Hg–HgO for the four electrodes. However, the corrosion potential decreases

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S. Xu et al. / Corrosion Science 100 (2015) 113–120

from −0.83 V/Hg–HgO for the Ni sheet to −0.85 V/Hg–HgO for SiNi due to the porous surface structure, and to −0.89 V/Hg–HgO for MCPNi due to the 3D architecture of Si–MCP. Electrodepostion of Ni on MCPNi can improve the stability of the Ni layer. The corrosion potential increases to −0.82 V for MCPNN and approaches that of the Ni sheet (−0.83 V/Hg–HgO). Because of the porous structure of the Ni layer and 3D architecture of Si–MCP, the polarization resistance decreases from 25,253 for Ni to 2261 for SiNi and 1381  cm2 for MCPNi. After electrodepostion of Ni on MCPNi, the polarization resistance decreases to 92  cm2 for MCPNN. It indicates that the passivation effect for Ni does not appear in Na2 SO4 . In this case, the corrosion density current of SiNi (1.2 × 10−5 A cm−2 ) increases by almost 3 times compared to the compact Ni sheet (3.8 × 10−6 A cm−2 ) due to the porous Ni structure. However, the corrosion density current of MCPNi (5.5 × 10−6 A cm−2 ) deceases by almost 2 times compared to SiNi which is similar to that of the compact Ni layer. After electrodepostion of Ni on MCPNi, the corrosion density current of MCPNN (4.0 × 10−5 A cm−2 ) increases by almost 7 times comparing to MCPNi. According to Faraday’s law, there is a linear relationship between the metal dissolution or corrosion rate (VCR , in unit of cm s−1 ) and the corrosion current density Icorr . The corrosion current density is obtained by the Tafel extrapolation method and the corrosion rate is given by [34]: VCR =

M Icorr , nF

a

b

(1)

where M is the atomic weight of the metal,  is the density of the corroding species, n is the charge number which indicates the number of electrons exchanged in the dissolution reaction, and F is Faraday’s constant. The corrosion rate can be calculated without knowledge of the electrode-kinetic parameters. Although the accuracy of this approximation may not be always sufficient [38], Eq. (1) provides a unique basis for rapidly measuring the relative corrosion rates. From the corrosion current density of Ni sheet, the corrosion rate of Ni in Na2 SO4 can be estimated as 1.5 × 10−10 cm s−1 (or 4.7 × 10−2 mm per year), which is similar to that in Ref. [34]. The corrosion rate of SiNi (4.5 × 10−10 cm s−1 ) is almost 3 times that of the Ni electrode due to the porous surface of the Ni layer on the Si wafer. The corrosion rates of MCPNi and MCPNN are 2.2 × 10−10 and 1.5 × 10−9 cm s−1 , respectively. That is to say, compared to the semiconductor properties of silicon, the high resistivity of the Si–MCP due to the low carrier concentration in the thin depleted wall can limit the corrosion process of nickel and decrease the corrosion current density and corrosion rates. In this case, Na2 SO4 is a better electrolyte to fabricate the 3D energy device based on the Si–MCP due to the smaller corrosion current density of the Ni layer (almost smaller by one order of magnitude) compared to that in KOH. Additional, it cannot damage the Si–MCP skeleton even if the Ni layer of surface is corroded completely due to the stability of Si in Na2 SO4 . Our experiments show that the supercapacitor performance can be improved by electrodepositing active materials such as MnO2 onto the MCPNi in the Na2 SO4 electrolyte [43]. After electrodepostion of Ni on MCPNi, the corrosion current density and corrosion rate both increase due to the larger effective nickel content and surface area caused by the nano-flake structure on the surface and side wall of Si–MCP. Owing to the fast adsorption-desorption process at the surface of electrodes in Na2 SO4 , the larger surface area of the electrode results in increasing cathodic current flow and small Tafel slopes, for example, 53 mV dec−1 for MCPNN electrode. Electron transfer through the thin film is crucial to the corrosion behavior of a metal and the electronic properties can be investigated by semiconductor electrochemistry [44]. A Schottky barrier is present at the semiconductor electrolyte interface with a poten-

Fig. 5. Mott–Schottky plots of Ni, SiNi, MCPNi and MCPNN at 100 Hz in aqueous solutions: (a) 2 M KOH and (b) 1 M Na2 SO4 .

tial drop and the differential space charge capacitance (CSC ) can be fitted to the Mott–Schottky equation [44]: −2 CSC =

2

0 r eNA

[−(E − EFB ) −

kB T ], e

(2)

where 0 (r ) is the vacuum (relative) dielectric constant, e is electron charge, NA is volumetric dropping concentration, kB is Boltzmann’s constant, T is absolute temperature, E is the applied potential, and EFB is the flat band potential, and the other coefficients have the usual meaning. The space charge capacitance CSC can be obtained by the alternating voltage impedance techniques by the following equation [39]: CSC =

1 2fIm(Z)A

(3)

where A is the apparent electrode area, Im(Z) is the imaginary part of impedance data, and f the frequency (100 Hz in our case). The Mott–Schottky plots are shown in Fig. 5 for KOH (a) and Na2 SO4 (b). The flat-band potential can be obtained at the intersection between the lines of the Mott–Schottky plots and x-axis. The volumetric concentration NA (or ND ) can be estimated from the slope of the Mott–Schottky plots (assuming r = 12 [39]) and the data are listed in Table 1. The negative slope in the Mott–Schottky plots implies that the passive film on Ni in KOH is a p-type semiconductor and the flat-band potential is about −0.3 V Hg/HgO. At the poten-

S. Xu et al. / Corrosion Science 100 (2015) 113–120

tial, complete reduction of NiOOH into Ni(OH)2 leads to saturation of proton sites but incomplete occupation of electron sites due to the point defects in the oxide lattice. Hence, the reduced materials act as a p-type semiconductor [39]. Point defects exist in the oxide lattice and electronic acceptors, and the acceptor density is about 1.2 × 1021 cm−3 in the Ni sheet in 2 M KOH similar to previous results [39]. On account of the nano-size electroless plated Ni, the acceptor density increases to 1.7 × 1022 cm−3 in SiNi. Because of the 3D architecture of the Si–MCP, the acceptor density increases to 8.7 × 1024 cm−3 in MCPNi. After electrodepostion of the Ni layer, the acceptor density reaches 4.1 × 1025 cm−3 for MCPNN due to the larger Ni content and surface area. Owing to the large acceptor density of the Ni-coated Si–MCP, the 3D electrode structure improves adsorption of ions [45]. Contrary to the observation in KOH, the positive slope in the Mott–Schottky plots implies that the surface film on Ni in Na2 SO4 is characteristic of an n-type semiconductor. Ni can only be oxidized to Ni2+ in NaSO4 (pH 7) as shown by the Pourbaix diagram for the Ni-water system [34]. At this potential, Ni2+ is converted into Ni(OH)2 by insertion of hydrate. Because of the limited hydrate concentration in Na2 SO4 solution, incomplete occupation of the proton sites (point defects in oxide lattice) can act as electronic donors thereby producing the n-type semiconductor NiO2 H1+x [39,40,46]. Compared to the flat-band potential of the Ni sheet (−0.43 V Hg/HgO), the more negative flat-band potential is shown (about −0.52 V Hg/HgO) for other three samples. The donor density is about 1.8 × 1021 cm−3 for the Ni sheet in 1 M Na2 SO4 . Due to the nano-size electroless plated Ni, the donor density increases to 2.3 × 1022 cm−3 in SiNi and because of the 3D architecture of the Si–MCP, the donor density increases to 3.0 × 1024 cm−3 in MCPNi. After electrodeposition, the acceptor density reaches 2.6 × 1026 cm−3 in MCPNN because of the larger Ni concentration and surface area. The higher acceptor density indicates that ion transport is easier in the film and this is the reason why the 3D electrode based on Si–MCP delivers excellent supercapacitor performance [16]. 4. Conclusion Three-dimensional (3D) nickel electrode structures based on silicon microchannel plates are prepared by electroless plating and electrodeposition (galvanic deposition). The corrosion properties of the nickel-coated Si–MCP electrode are evaluated by cyclic voltammetry, Tafel plots, and electrochemical impedance spectroscopy. The results show that the nickel layer on the silicon surface can protect the Si–MCP skeleton effectively in aqueous electrolytes consisting of alkalis and salts. Electrodeposition can decrease the corrosion susceptibility and improve the performance of the electrode due to the low resistance and high surface defect concentration. Because of the different chemical reactions, the nickel-coated Si–MCP electrodes show p-type and n-type semiconductor properties in the alkaline and salt media, respectively. The results suggest that nickel-coated Si–MCP electrodes are promising in the fabrication of miniaturized 3D Li-ion batteries or supercapacitors. Acknowledgements This work was jointly supported by Shanghai Foundamental Key Project No. 11JC1403700, 10JC1404600, PCSIRT, Shanghai Pujiang Program (14PJ1403600) and China NSFC Grant number 61176108, 60990312 and 61076060. The work is also supported by Guangdong – Hong Kong Technology Cooperation Funding Scheme (TCFS) GHP/015/12SZ and City University of Hong Kong Applied Research Grant (ARG) No. 9667104.

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