Electrochemical analysis of nickel electrode deposited on silicon microchannel plate

Electrochimica Acta 90 (2013) 344–349

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Electrochemical analysis of nickel electrode deposited on silicon microchannel plate Shaohui Xu a,∗ , Fei Wang a , Li Mai a , Lianwei Wang a , Paul K. Chu b 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 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 19 October 2012 Received in revised form 11 December 2012 Accepted 12 December 2012 Available online xxx Keywords: Three-dimensional electrode Nickel Silicon microchannel plate

a b s t r a c t Three-dimensional (3D) nickel electrode structures are prepared by electroless plating of nickel on silicon microchannel plates. The material is characterized by cyclic voltammograms and electrochemical impedance spectra. Compared to the nickel sheet and nickel deposited on the silicon substrate, the effective nickel content in the oxide layer per unit area increases almost 100 times due to the porous nickel structure and 3D architectures of silicon microchannel plates. The larger surface area of the 3D electrode structures exchange the more electrons (one electron) per nickel atom in the redox reaction. However, the redox reaction rate determined by the mass transfer due to the slow ion transfer speed. The results suggest that the electrode structures based on silicon microchannel plates is a promising choice for fabricating 3D Li-ion battery or supercapacitors in a miniature size after the suitable structure design. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Due to the high surface-to-volume ratio, the three-dimensional (3D) architectures enable better exploitation of space in the energy storage device enabling a smaller footprint while retaining high power and energy density. In this case, the combination of rechargeable devices such as batteries and supercapacitors are fabricated in 3D structure based on microelectromechanical (MEMs) technique is a good direction, and the main goal of which has been to improve their performance in a limited space by using highcapacity active materials deposited on large-area ratio structures [1–11]. At the same time, the desire to further miniaturize the existing on-chip systems makes it possible to develop energy devices integrated with other elements. Li-ion batteries and supercapcitances based on 3D structures have attracted a lot of attention due to their high energy and power density at a smaller foorprint. For the 3D Li-ion batteries based on the MCP structure, the reversible 2.0 mAh/cm2 battery capacity about 15 times (1500%) higher than the 0.133 mAh/cm2 capacity reported for the best commercial 2D thin-film batteries [2,11]. For the 3D supercapacitor based on interdigital structure, a large capacitance of 90.7 mF/cm2 and a fast power of 51.5 mW/cm2 are obtained, and robust stability and high charge/discharge efficiency are also observed [10]. It

∗ Corresponding author. Tel.: +86 21 54342501; fax: +86 21 54345119. E-mail address: [email protected] (S. Xu). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.12.044

indicates that the 3D design offers remarkable improvements in energy and power density especially with respect to the geometric footprint of device. To form the 3D devices, the three-dimensional electrode structure is the basic and important component. It determined how to fabricate the 3D architecture and always limit the performance of energy device. There are different kinds of 3D electrode structures, such as interdigital structures [5–10], nanometer or micrometersized wire arrays [1,3,4] and the microchannel plate structure (MCP) [2,11–16]. The Si MCP is the perforated “through-hole” substrate, which can be formed by photo-assisted electro-chemical etching and peel off from the Si substrate at the modified electrochemical procedure, and form the free-standing stabling layer structure. For the MCP, it shows the large area ratio (>70%) and high aspect ratio (length/diameter of hole >40 or more). In this case the surface area gain of MCP structure can be larger than 50. It gives the largest surface area gain in the 3D architecture. Additionally, due to the stabling structure of Si MCP in most of acid, and even can be annealed in the high temperature condition, e.g. more than 1000 ◦ C. In this case, the Si-MCP can be selected to fabricate the 3D Li-ion battery and supercapacitance and get the better performances, such as the high capacity of Li-ion battery and supercapacitor. More recently [17], Ni/Si-MCP was studied to serve as an anode of lithium ion battery and a lithium foil acted as the counter electrode. The electrolyte was 1.0 mol L−1 LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC):dimethyl carbonate (DMC) (1:1:1, W:W:W), and the Ni/Si-MCP anode exhibits better cycle performance in the galvanostatic half-cell measurements.

S. Xu et al. / Electrochimica Acta 90 (2013) 344–349

However, it is seldom to study the properties of 3D electrode structure itself base on Si MCP [18]. In this paper, 3D Nickel electrode based on silicon microchannel plate are studied systemically by electrochemical methods, the characterization of 3D structure can give a illumination and framework for fabricating the highperformance 3D 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 form the silicon MCP with a high aspect ratio. The steps include thermal oxidation as a masking layer and 3 ␮m × 3 ␮m squares patterned by lithography and wet etching. The patterned wafer was pre-etched in a tetramethyl ammonium hydroxide (TMAH) aqueous solution and then anodized in a computer-controlled photo-assisted electrochemical system [described in Refs. 12,13]. The obtained silicon MCP was then cut into 1 cm × 1 cm squares for further fabrication. The electroless plating is chosen in our experiments because the channels in the silicon MCP could be coated with metal uniformly and the thickness of the metal film could be easily controlled by means of the bath composition and bath temperature [14]. The silicon MCP are put into a dilute HF solution 30 second to remove

345

Table 1 Chemicals and the conditions used in the electroless deposition of nickel. Chemicals

Concentration (M)

Main function

Nickel chloride hexahydrate (NiCl2 ·6H2 O) ammonium chloride (NH4 Cl) Sodium hypophophite monohydrate (NaH2 PO2 ·H2 O) Ammonia

0.3

Ni source

0.1

Reductant

0.1

Stabilizing agent

×

Adjust pH 8.5–9.5

the native oxide, then put into a buffer solution (0.1% Triton X-100 solution) for 30 s to decrease the inner stress and enhance wetting and then immersed in the plating bath for 20 min. Afterwards, they were taken out and rinsed with water. The materials and the conditions used in the electroless plating nickel process are listed in Table 1. As for comparison, a high purity nickel sheet (>99.9%) and a nickel coated Si wafer are also prepared. The same experimental condition is selected to form the nickel-coated Si wafer. In order to simplify, the three samples are marked as Ni (nickel sheet), Si + Ni (nickel coated Si wafer) and MCP + Ni (nickel coated Si microchannel plate).

Fig. 1. The top-view (a) and cross-sectional (b) SEM images of the silicon MCP fabricated by electrochemical etching are depicted, (c) the cross-sectional SEM image of the Ni/Si MCP formed by electroless deposition, and (d) the schematic illustration of the unit cell of MCP.

S. Xu et al. / Electrochimica Acta 90 (2013) 344–349

The morphologies of the silicon MCP and nickel electrode layer are observed by scanning electron microscopy (SEM). In the electrochemical analysis, the samples are exposed to 2 M KOH aqueous solution at the room temperature (25 ◦ C) and served as the working electrode. A platinum electrode is used as the counter electrode and a mercury oxide electrode (Hg–HgO) served as the reference electrode. The electrochemical tests are performed on a CHI660D electrochemical workstation to determine the electrochemical properties. The current and impedance data are normalized by the footprint area.

a

0.003

A2 0.002

Current density J ( A/cm 2 )

346

0.001

B

A1 0.000

-0.001

C2

C1

-0.002

3. Results and discussion -0.003 -0.8

-0.4

0.0

0.4

E / ( V / Hg-HgO )

b

0.004

A2 A1

0.002

B 0.000

-0.002

C1

C2

(1) -0.004

-1.2

-0.8

-0.4

0.0

0.4

E ( V / Hg-HgO )

c 0.15

A2 A1

0.10

Current density J ( A/cm 2 )

In this case, the surface area gain can reach 42 (as shown in Fig. 1(b), the depth of Si MCP is almost H = 150 ␮m) when WA = 1 ␮m, WB = 5 ␮m and WC = 6 ␮m. The more surface area gain can obtain as the depth of MCP increase, such as the surface gain can reach 58 when the depth of MCP is 200 ␮m. Fig. 1(c) shows the cross-sectional SEM image of the Ni/Silicon MCP formed by electroless deposition. The surface and side-wall of the silicon MCP were coated by the disordered nickel. The porosity Nickel layer is consist of micrometer-sized or nanometer-sized sphere structure and the thickness of nickel layer is about 0.5 ␮m. The cyclic voltammograms (CV) of three samples are shown in Fig. 2(a)–(c) in the potential range of −1.3 to 0.6 V Hg–HgO at a scan rate of 50 mV/s. They illustrate the same electrochemical behavior of these oxides with the potential region. They are the typical oxidation–reduction reaction curves of Ni in KOH solution. During the first increasing sweep from low potential, the passive layer, which consist of ␣-Ni(OH)2 amorphous layer, is formed corresponding to the current peak A1 . When the second potential sweep is reversed at −0.35 V Hg–HgO, the reduced process can appear corresponding to the cathodic peak C1 . In the potential domain of the plateau (B), the amount of nickel oxide increase and the simultaneous transformation of ␣-Ni(OH)2 into a less hydrated and crystallized ␤-Ni(OH)2 . At potential higher than that of the oxidation peak A2 , ␤-Ni(OH)2 is oxidized into ␤-NiOOH and the transformation is complete when the water oxidation current is observed. During the decreasing potential sweep, the reverse reaction, which reduction of ␤-NiOOH into ␤-Ni(OH)2 is observed, corresponding to the cathodic peak C2 . The ratios of anodic (A2 ) and cathodic (C2 ) peak current density are 1.01 (Ni), 1.08 (Si + Ni) and 1.10 (MCP + Ni) [19]. The potential differences of anodic (EA ) and cathodic (EC ) peak are the 0.093 (Ni), 0.195 (Si + Ni) and 0.214 V (MCP + Ni). The almost same current density of anodic and cathodic peak and the larger potential differences (comparing to the 0.059 V/n, where n is the number of electron transferred) between anodic and cathodic peak position determine the quasireversible redox reaction between ␤-Ni(OH)2 and ␤-NiOOH. In this case, the dependence of the EA and EA/2 can be expressed using

B

0.05 0.00 -0.05 -0.10

C2

C1

-0.15 -1.2

-0.8

-0.4

0.0

0.4

E ( V / Hg-HgO )

d

Current density J ( A/cm 2 )

[4 × WB × H + 2 × (2 × WA × WC − WA × WA )] AG = 2 × WC × WC

-1.2

Current density J ( A/cm 2 )

The top-view and cross-sectional SEM images of the silicon MCP fabricated by electrochemical etching are shown in Fig. 1(a) and (b). It can be observed that the silicon MCP is well aligned, and the MCP has square holes 5 ␮m long. Electrochemical etching ends when the silicon MCP is separated from the substrate. The depth of the MCP can be varied from 100 to 200 ␮m by changing the etching time, current density, bias, and temperature. In Fig. 1(d) shown the schematic illustration of the unit cell of Si MCP, the width of side-well and hole are WA and WB , and the length of period is WC (=WA + WB ), the depth of Si MCP is H, then the surface area gain (AG) can be calculated by Eq. (1) (consider of the upper and down surfaces):

0.00

-0.05

MCP Ni Si Ni Ni

-0.10

X 50 -0.15

X 50 -0.20 -0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E / ( V / Hg-HgO )

Fig. 2. The cyclic voltammograms of Ni (a), Si + Ni (b) and MCP + Ni (c) in the potential range of −1.3 to 0.6 V Hg–HgO at a scan rate of 50 mV/s, and (d) the voltammeteric reduction peaks of three samples in the potential range of −0.6 to 0.6 V Hg–HgO at a scan rate of 10 mV/s.

S. Xu et al. / Electrochimica Acta 90 (2013) 344–349

Nicholson and Shain equation [19], which can be used to calculate the number of electron participating in the rate determining stages EA − EA/2 =

1.857RT ˛nF

area and the nickel content in the oxide layer [20,21]. The integration results show that the integration area value of Si + Ni increases almost 2 times comparing to that of Ni sheet and the integration area value of MCP + Ni increases almost 100 times. It gives a suggestion that the increasing integration value of 3D nickel electrode cased by the porosity structure of nickel electrode (almost 2 times) and the 3D architectures of MCP (almost 50 times). The increasing area value caused by the 3D architectures is almost equal to the surface area gain of MCP. It indicates that the 3D architecture of Si-MCP can increase the performance of the Ni electrode efficiently. The cyclic voltammograms with different sweep speed can give a more information of electrochemical reaction. As shown in Fig. 3(a)–(c), the CV results of three samples are obtained in a 2 M KOH solution using the different scan rates from 5 to 100 mV/s in the potential range 0–0.6 V Hg–HgO. In order to show the stability of electrode, the two period CV curves of Ni sheet is shown in Fig. 3(a), the almost overlap of first and second period indicates the good stability. It shows that the anodic (cathodic) peaks shift to the high (low) potential when the scanning rate increases, and the currents densities of anodic and cathodic peaks increase continually. The shift of anodic and cathodic peaks potential position with sweep rate indicated also quasi-reversible redox reaction of ␤-Ni(OH)2 and ␤-NiOOH. Due to the high surface area at a footprint, the redox process of MCP + Ni shows the very high current density, which can reach current density 0.3 A/cm2 at a scan rate 100 mV/s. As shown in the inset in Fig. 3(a)–(c), it can be found that the anodic peak current densities of Ni sheet are proportional to the scanning rates, indicates the adsorption-controlled redox process on the surface of Ni sheet. However, the anodic peak current densities of Si + Ni and MCP + Ni are proportional to the square root of

(2)

where EA/2 is the potential of half peak height, R is the universal gas constant, T is the absolute temperature, and F is the Faradic constant (96,487 C) and ˛ is the charge transfer coefficient. It can be calculated that the number of electron involving redox reaction are 0.4 (Ni), 0.67 (Si + Ni) and 1.0 (MCP + Ni), indicating that the more electrons can involve the redox reaction with the surface enlargement due to 3D architectures of electrode. The similar cyclic voltammograms spectra and the same oxidation–reduction process indicated that the nickel element deposited on the Si wafer and MCP structures, and it also proved by the XRD data in Ref. [16]. However, due to the porosity structure of nickel and 3D architecture of MCP, the oxidation and reduction peak become wide and current density increase. The voltammatric reduction peaks C2 are measured with the subsequent test: a potential Emax = 0.6 V Hg–HgO is first imposed for 60 s to totally transform Ni(OH)2 into NiOOH, and a decreasing potential sweep rate at 10 mV/s was carried out from Emax to Emin = −0.6 V Hg–HgO. The voltammeteric reduction peaks of three samples are shown in Fig. 2(d). Comparing to the Ni sheet, the changes of cathodic current peak include the increasing of width (for Si + Ni) and both current density and width (for MCP + Ni). Due to the return sweep from positive to negative provides the best approximation of the steady state condition and there would be less interference from the redox reaction, the integration of the cathodic current density between these two limits made it possible to estimate the charge per unit

a

b

0.0015

1 mV/s 5 mV/s 10 mV/s 30 mV/s 50 mV/s 100 mV/s

0.002

Current density J ( A/cm2)

0.0010

Current density (A/cm 2)

347

0.0005

0.0000

-0.0005

-0.0010

0.001

0.000

-0.001

-0.002 -0.0015 0.0

0.1

0.2

0.3

0.4

0.0

0.5

0.1

0.2

0.4

Current density J ( A/cm2 )

0.3 0.2

0.5

0.6

0.7

0.1

0.577

0.1 0.0 -0.1 -0.2 -0.3 -0.4 0.0

0.4

d

5 mV/s 10 mV/s 30 mV/s 50 mV/s 100 mV/s

Current density (A/cm 2)

c

0.3

E ( V / Hg-HgO )

Potential E (V / Hg-HgO)

MCP+Ni Si+Ni Ni

0.01

0.69

1E-3

0.8

1E-4

0.1

0.2

0.3

E ( V / Hg-HgO )

0.4

0.5

0.6

0.01

0.1

Scan rate (V/s))

Fig. 3. The cyclic voltammograms of Ni (a), Si + Ni (b), and MCP + Ni (c) with the different scan rates 5–100 mV/s in the potential range 0–0.6 V Hg–gO. Insets show the relationship between anodic peak current densities and the scanning rates.

S. Xu et al. / Electrochimica Acta 90 (2013) 344–349

IA = 2.99 × 105 n(˛n)1/2 AD1/2 C0 V 1/2

(a)

1.778 HZ -6000

-2

-60

-30

-2000

0

0

30

60

90

-2

Re ( Z ) /ohm.cm

0 0

2000

4000

6000 -2

Re ( Z ) /ohm.cm (b)

(3)

where D is the proton diffusion coefficient in nickel hydroxide, C0 is the concentration of hydrogen in the solid, A is the area of electrode and the other coefficients have the usual meaning. According to the data obtained in Eq. (2), the ion diffusion coefficient of MCP + Ni is 5.5 × 10−11 cm2 /s. The lower ion diffusion coefficient of MCP + Ni, comparing to the Refs. [20,21], may caused by the 3D architecture of MCP. It indicates that it is a competing relationship between the larger surface area and improving the reaction rate. Always, larger surface area contact with solution can increase electrical double layer capacitance and the probability of redox reaction. However, the larger surface areas need more ions or electrons to join the reaction and carrier (especially ions) need transfer long road through 3D architecture. The limited ion transfer speed and number of ions will prevent the redox reaction and decrease the performance of energy storage devices, such as Li-ion batteries and pseudocapacitor. The problem is due to the counter-electrode (Pt electrode) stay outside of the MCP structure as in the test device, and it can be resolved by deposition of electrolyte and thin film counter-electrode into the holes make the reaction path short [2,4]. Three kinds of electrode are also investigated by electrochemical impedance spectra (EIS) test over a frequency range from 0.1 Hz to 100 kHz with potential amplitude of 5 mV. The impedance diagrams of three samples at the operation potential E = 0 and 0.5 V are shown in Fig. 4(a)–(c) in the plots of Nyquist and Bode planes. Fig. 4(a) inset shows the high frequency domain of three samples. It shows that the solution interface resistance RS is 14.85  cm−2 for Ni sheet, and RS decrease to 13.05  cm−2 for Si + Ni due to the porosity structure of electroless plating Ni, and the RS increase to 39.6  cm−2 for MCP + Ni due to the resistance effect of 3D architecture. For Ni sheet, the Nyquist plot in the low frequency range shows an almost vertical straight line. However, the Nyquist plot of Si + Ni shows a decrease of Im (Z) at the low frequency range, approaching the Warburg resistance [23], and the Nyquist plot of MCP + Ni show a more tend to approach the Warburg resistance at the low frequency range. As shown the Bode plot in Fig. 4(c), the phase angle of MCP + Ni is 60◦ , approach the Warburg value (45◦ ),

-4000

Im ( Z ) /ohm.cm

Im ( Z ) /ohm.cm-2

-90

Modular |Z| /ohm. cm-2

scanning rates. It indicates that the electrode reaction rate is determined by the mass transfer (ion diffusion) from the bulk solution to the electrode surface. It can be explained based on the different effective surface area. For Ni sheet, due to the less interface contact with solution, the redox reaction is mainly determined by the rate of adsorption/de-adsorption. For the Si + Ni and MCP + Ni, due to the porosity nickel structure of Si + Ni and additional 3D architecture of MCP + Ni, the larger effective surface contact with the solution make the electrode need more ions to join the reaction, and the redox reaction is mainly determined by the rate of ion diffusion from solution to electrode surface. In order to discuss the process quantificationally, the curves of log J vs. log V are shown in Fig. 3(d). J is the current density (A/cm2 ) and V is the sweep rate (V/s). It can be found that the curves show the good linear relationship between log J vs. log V. It indicates that the redox reaction process of MCP + Ni is determined by the ion diffusion due to the slope of curve is 0.577, which approaching the value 0.5 (the value indicated the fully diffusion-control process). However, redox reaction process of MCP + Ni is mainly determined by the ion diffusion due to the slope of curve is 0.8, which approaching the value 1 (the value indicated the fully adsorption-control process). For Si + Ni, the redox reaction process is controlled by the mixed processes of diffusion and adsorption, due to the slope of curve is 0.69 between 0.5 and 1.0. As a stationary electrode, a linear relationship between peak current density and the square root of potential sweep rate over the range of sweep rate can be represented as [22]

10000

1000

100

10 0.1

1

10

100

1000

10000

100000

Frequency (Hz) (c)

0

-20

Phase (degree)

348

-40

-60

-80

0.1

1

10

100

1000

10000

100000

Frequency (Hz) Fig. 4. (a) Impedance diagram in Nyquist planes for Ni (square + line), Si + Ni (circle + line) and MCP + Ni (triangle + line) at the operation potential E = 0 V Hg–HgO. Inset show impedance diagrams in high frequency domain of three samples. Impedance diagram in modulus |z| (b) and phase angle (c) in Bode planes for Ni (square + line), Si + Ni (circle + line) and MCP + Ni (triangle + line) at the operation potential E = 0 (solid symbol) and 0.5 V Hg–HgO (shallow symbol).

and the phase angles are about 80◦ for Ni sheet and Ni + Si. The EIS results indicate that the larger surface area makes the surface reaction rate fast and the total reaction rate is determined by the mass transfer, as indicated in CV test. Because that operation of positive battery electrode and of supercapcitor device is based on the protons insertion–deinsertion reaction of Ni(OH)2 and NiOOH, the EIS test at operation potential E = 0.5 V which the Ni electrode transform from Ni(OH)2 to NiOOH, can give a more information about the electrode. The Bode plots are shown in Fig. 4(b) and (c). The modulus of Z shows that resistance decrease for Si + Ni due to the porosity structure of electroless

S. Xu et al. / Electrochimica Acta 90 (2013) 344–349

plating Ni, and resistance increase for MCP + Ni due the resistance effect of 3D architecture. Later the suitable methods are necessary to decrease the resistance of 3D electrode architecture as shown in Ref. [18]. In Fig. 4(d), the phase angles show the resonant frequency at the different position for three samples, which Ni is 206 Hz, Si + Ni is 4.64 HZ and MCP + Ni is 0.97 Hz. In this case, the relaxation time constant  0 can be used to describe the reaction speed ( 0 define as the minimum time needed to discharge all the energy from the device with an efficiency of greater than 50%) [24]. Comparing to the relaxation time constant  0 of Ni sample (Ni 0.77 ms), the relaxation time constant of Si + Ni increases almost 44 times (Si + Ni 34.3 ms), and relaxation time constant of MCP + Ni increases almost 206 times (MCP + Ni 164 ms). The increasing of relaxation time constant is because that the ions need more times to transfer through the inn porous network of nickel electrode for Si + Ni sample. Additionally, the relaxation time constant  0 would increase continually due to the transfer process of the ions through the micrometersized hole in the MCP structure. From the data, it gives a suggestion that the increasing relaxation time constant of 3D nickel electrode would deduce by the porosity structure of nickel electrode (almost 44 times) and the 3D architectures of MCP (almost 4.5 times). It indicates that the 3D architecture of Si-MCP can increase delivering high power and energy through forming the smaller size of nickel particles, such as nano-size [9]. 4. Conclusions Three-dimensional (3D) nickel electrode structures based on silicon microchannel plates are prepared by electroless plating. The electrochemical properties have been evaluated by means of cyclic voltammograms and electrochemical impedance spectra. It shows that the 3D electrode structure shown the high performance including of the high effective content of Ni and more electrons join the reaction. After the suitable structure design, such as the nano-size metal particles and thin film counter-electrode into the holes, it indicates that the large aspect ratio of silicon MCPs are suitable to produce 3D electrode structure for fabricating the miniature power sources. Acknowledgements This work was jointly supported by Shanghai Natural Sciences Foundation No. 11ZR1411000, Shanghai Foundamental Key Project No. 11JC1403700, PCSIRT, and China NSFC Grant number 61176108. The work is also supported by City University of Hong Kong Strategic Research Grant (SRG) No. 7008009, and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112510. References [1] J.W. Long, B. Dunn, D.R. Rolison, S. Henry, White, three-dimensional battery architectures, Chemical Reviews 104 (2004) 4463.

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