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Title Hydrogen evolution involved electrodeposition of Cu-Ni alloys and

dealloying for supercapacitor application

Author(s) Lee, Sin Ching (李倩晴)

Citation

Lee, S. C. (2016). Hydrogen evolution involved electrodeposition of Cu-Ni alloys and dealloying for supercapacitor application (Outstanding Academic Papers by Students (OAPS)). Retrieved from City University of Hong Kong, CityU Institutional Repository.

Issue Date 2016

URL http://hdl.handle.net/2031/8808

Rights This work is protected by copyright. Reproduction or distribution of the work in any format is prohibited without written permission of the copyright owner. Access is unrestricted.

CITYU UNIVERSITY OF HONG KONG

DEPARTMENT OF

PHYSICS AND MATERIALS SCIENCE

BACHELOR OF SCIENCE (HONS) IN APPLIED PHYSICS 2015-2016

DISSERTATION

Hydrogen Evolution Involved Electrodeposition of Cu-Ni Alloys and

Dealloying for Supercapacitor Application

By

LEE Sin Ching

March 2016

___________________________________________________________________________________________________________________________________

This document is downloaded from Outstanding Academic Papers by Students (OAPS), Run Run Shaw Library, City University of Hong Kong

Hydrogen Evolution Involved Electrodeposition of Cu-Ni Alloys and

Dealloying for Supercapacitor Application

By

LEE Sin Ching

Submitted in partial fulfillment of the

requirements for the degree of

BACHELOR OF SCIENCE(HONS)

IN

APPLIED PHYSICS

From

City University of Hong Kong

March 2016

Project Supervisor: LI, Yangyang

___________________________________________________________________________________________________________________________________


I

Abstract

Previous research shows that porous nickels with closely packed tiny pores can be neither fabricated

by the method of electrochemical deposition nor the method of hydrogen bubble dynamic template.

Therefore, a combined method of electrodeposition and hydrogen evolution reaction is proposed

and which will be the main focus of this study.

This project is divided into three parts. The first part is the fabrication of porous nickel by the

combined method in different conditions. Second part is the fabrication of porous nickel with nickel

hydroxide by undergoing hydrothermal treatment. For the last part, samples were characterized by

SEM, XRD, CV and charge and discharge tests.

The results indicate that using this combined method can also fabricate porous nickels which have

high surface area. Also, because the nickel oxide is already formed in electrodeposition stage, this

method is convenient for making supercapacitors by just using one step. Last but not least, porous

nickel fabricated by this method has high specific capacitance and high retention. Therefore, this

method is potentially to be applied to large-scale fabrication of supercapacitors to be used in

industry.

___________________________________________________________________________________________________________________________________


II

Acknowledgements

I would like to express my deepest appreciation to those who extended their helping hand to me to

complete this report. Especially, I would like to express my gratitude to my supervisor Dr. LI,

Yangyang for offering me an opportunity to learn from her. I would like to thank my second

assessor Dr. FAN, Jun for giving her precious advice to improve my project in different aspects. I

would also like to thank my tutor Mr. LI, Zhe for giving me a clear guidance and many supports.

Last but not least, I would like to thank Mr. Maurice Chan and Ms. Conny Yau for giving me a lot

of supports and suggestions on writing the essay.

___________________________________________________________________________________________________________________________________


III

List of Figures

Figure 1 Hive like porous metal

Figure 2 Randomly distributed pores

Figure 3 Method of inverse opal

Figure 4 Porous nickel fabricated by method of hydrogen bubble template


Figure 6 Contact Angle of Porous Nickel

Figure 7 The first supercapacitor in the world

Figure 8 SEM micrographs of

a) electrochemical dealloying b) hydrogen bubble dynamic template

Figure 9 SEM micrographs of electrodeposition at -1.4V

a) before dealloying b) after dealloying





Figure 12 XRD of different voltage of electron deposition in the range of 25°to 55°

Figure 13 XRD of different voltage of electron deposition in the range of 42.5°to 45.5°

___________________________________________________________________________________________________________________________________


IV

Figure 14 SEM micrographs of sample

a) 2-2-1 b) 2-2-2 c) 2-2-3 d) 2-2-4 e) 2-2-5

Figure 15 CV curve of sample 2-2-1





Figure 20 C-DC curve at different current densities of porous nickel with hydrothermal treatment

Figure 21 C-DC curve at different current densities of porous nickel without hydrothermal treatment

Figure 22 Specific capacitance of sample with hydrothermal treatment

Figure 23 Specific capacitance of sample without hydrothermal treatment

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V

List of Tables

Table 1 Comparison of technical characteristics of EES system

Table 2 Voltage and Time Used for Fabrication

Table 3 Formulation of Solution for HT

Table 4 Temperature and Time Duration for HT

Table 5 Conditions of C-DC for 2-2-4


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VI

Table of Contents

Abstract----------------------------------------------------------------------------------------------------------- I

Acknowledgements-------------------------------------------------------------------------------------------- II

List of Figures---------------------------------------------------------------------------------------------- III-IV

List of Tables--------------------------------------------------------------------------------------------------- V

Table of Contents------------------------------------------------------------------------------------------- VI

1 Introduction-------------------------------------------------------------------------------------------------- 1

1.1 Porous Metals---------------------------------------------------------------------------------------- 1

1.2 Porous Nickel---------------------------------------------------------------------------------------- 1

1.3 Supercapacitor--------------------------------------------------------------------------------------- 1

1.4 Research Gap----------------------------------------------------------------------------------------- 2

1.5 Objective---------------------------------------------------------------------------------------------- 2

2 Literature Review------------------------------------------------------------------------------------------- 3

2.1 Porous Metal-------------------------------------------------------------------------------------- 3-6

2.2 Porous Nickel-------------------------------------------------------------------------------------- 7-8

2.3 Supercapacitor----------------------------------------------------------------------------------- 8-10

3 Experimental Procedure-------------------------------------------------------------------------------- 11

3.1 Fabrication of Porous Nickel Thin Film--------------------------------------------------- 11-12

3.2 Hydrothermal Treatment--------------------------------------------------------------------- 12-13

3.3 Testing----------------------------------------------------------------------------------------------- 14

3.3.1 Scanning electron microscope------------------------------------------------------------- 14

3.3.2 X-ray Diffraction---------------------------------------------------------------------------- 14

3.3.3 Cyclic Voltammetric Curve------------------------------------------------------------ 14

3.3.4 Charge and Discharge------------------------------------------------------------------- 14-15

4 Results and Discussion----------------------------------------------------------------------------------- 16

4.1 Porous Nickel-----------------------------------------------------------------------------------16-19

4.2 Porous Nickel and Nickel hydroxide---------------------------------------------------------19-24

4.3 Supercapacitor----------------------------------------------------------------------------------14-27

5 Conclusion--------------------------------------------------------------------------------------------28

6 References----------------------------------------------------------------------------------------------29-32

___________________________________________________________________________________________________________________________________


1

1. Introduction

With the popularization of electronic devices, development of technology and

increasing concern of environmental protection, it is important to study and develop the

technique of energy storage and conversion.

1.1 Porous Metals

Porous metals are also called nanoporous metals [1]. It is known for its large surface area

[2], high surface reaction activity, and excellent electrical conductivity [3]. The major

methods of fabricating porous metals, or rather porous nickels, include inverse opal

template [4], hydrogen bubble dynamic template [5], electrochemical dealloying [6] and

etc.

1.2 Porous Nickel

Porous nickel is a multifunctional material which has been used in many aspects. For

example, it has been applied in optical devices [7], photonic coatings [8], biosensors [9],

catalysis [10], water proof coatings [3] and so on. Besides, one of its most important

applications is serving as a 3-D substrate for loading electro-active materials and the

current collector for supercapacitor.

1.3 Supercapacitor

Supercapacitor can be divided into two types, faradaic supercapacitor [11] and electrostatic

supercapacitor [12]. The advantages of a supercapacitor are high power density, high

charging and discharging rate, [12] and long life expectancy [13]. Therefore,

supercapacitor is an important technology of developing green energy as it can act as a key

role of energy storage and conversion along with a battery.

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2

1.4 Research Gap

From the previous researches it indicates that, the porous nickel fabricated using the method

dealloying electrochemical deposited Cu-Ni alloy, the pore size is decreased when

decreasing the voltage, also, the pores are loosely packed [5]. In another aspect, the method

of hydrogen bubble dynamic template can only form very large pores porous nickel [6] and

the lower specific surface area which is unwanted for making supercapacitors. Therefore, in

order to make a porous nickel thin film of high porosity and surface area which tiny porous

are closely packed, a combined method of electrodeposition-dealloying and hydrogen

evolution reaction is proposed which will be the main focus of this study.

1.5 Objectives

The purpose of this study is combining Cu-Ni alloy electrodeposition and hydrogen

evolution reaction, followed by dealloying, to fabricate porous nickel. We will then

characterize the material and make it into supercapacitor for test. The steps are:

1. To fabricate porous nickels by using the combined method of electrodeposition and

hydrogen evolution reaction. After the fabrication, some of the samples will be

chosen to undergo hydrothermal treatment to grow nickel hydroxide in order to

enhance the performance.

2. To characterize material by using SEM, XRD and related electrochemical test

techniques.

3. To exam the supercapacitor performance by comparing the results and data.

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3

2. Literature review

2.1 Porous metals

Porous metals have been studied since the 1920s. It is a kind of metal that has a lot of pores.

The pores not only can be observed on the surface of the porous metal but also lie inside it.

Moreover, the pores can be manipulated into different shapes, such as hive (Fig.1); they can

also distribute randomly (Fig.2). The materials for making porous metals can be pure metals,

metal alloy and metal oxide [14], [15].

Figure 1 Hive like porous metal[16] Figure 2 Randomly distributed pores

[By:P. Müllner et al., Northwestern University. ]

The porous metal is a kind of porous material. According to International Union of Pure and

Applied Chemistry guidelines, porous materials can be characterized into three types.[17]

When the pore size is larger than 50 nm, it is called macroporous. For pore size between 2

nm and 50 nm, it is called mesoporous. Lastly, when the pore size is smaller than 2 nm, it is

called microporous [18]. Porous metal is the metal that has meso or macro sized pores. On

the other hand, with the development of nanoscience, it is defined that things with size

between 1 nm to 100 nm, are measured on a nano scale. Therefore, we can also called

porous metal nanoporous metal generally.

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4

There are two ways of making porous metals. The first method involves the use of template

[19], such as Triblock-Copolymer-Assisted Hard-Template Method [20], Method of Inverse

Opal Template and Hydrogen Bubble Dynamic Template. The second one is without using

template, for instance, Chemical Dealloying and Electrochemical Dealloying. With the

development of nanotechnology, a new method of making porous nickels, the method of

Lift-off-Free Nanofabrication, was discovered by Lawrence Livermore National Laboratory

in November of 2014 [21].

For method of inverse opal template, colloidal crystal is formed by array of colloids with a

tidy arrangement. In this method, colloidal crystal acts as the template. The empty space

between the colloidal particles is filled by nanoparticles. Lastly, the template is removed

and the porous metal is formed. Using this method, the arrangement of pores and pores size

can be easily controlled by making changes on the template (Fig.3)[4], [22], [23],[24],[25]

.

Figure 3 Method of inverse opal [4]

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5

For method of hydrogen bubble dynamic template, the porous metal is made by applying a

strong current in the cathode; hydrogen bubbles are then formed, and at the same time the

metal deposition occurs rapidly. It is called dynamic template because of the decreasing of

hydrogen ions content in the solution, due to the formation of hydrogen bubbles, which

affects the electrodeposition process. The porous nickel formed by this method has large

pore size which is about 10μm (shown in figure 4) [5], [26].

Figure 4 Porous nickel fabricated by method of hydrogen bubble template [5]

The method of electrochemical dealloying consists of two parts, electrodeposition and

dealloying. Using CuNi as an example, during energy deposition, the compound of CuNi is

formed on the working electrode, which is basically a piece of Indium-Tin Oxide (ITO)

glass. In the process of dealloying, the copper of CuNi dissolves back to the solution and

forms copper on the side of the counter electrode. Therefore, only nickel with pores will

stay at the electrode and becomes porous nickel. The voltage used is usually above -1.2V

___________________________________________________________________________________________________________________________________


6

because at -1.3V hydrogen evolution reaction occurs, it becomes the combined method of

electrodeposition and hydrogen evolution reaction. For the method of electrochemical

dealloying, the pore size and density of pores decreases by decreasing the voltage of

deposition. Figure 5 shows a porous nickel with loosely packed tiny pores which fabricated

at -1.2V [6], [27], [28].


Hydrothermal treatment is conducted at high temperature and high pressure. The sample

will be put into an autoclave with set temperature and pressure for a period of time. In 2014,

Feng et al. did an experiment which showed that by using hydrothermal treatment Ni (OH)2

grow on the thin film and it turns out to be a high performance pseudocapacitor electrode

with specific capacitance 1778 F g-1 at a current density of 2.5 A g-1 [29], [30].

___________________________________________________________________________________________________________________________________


7

2.2 Porous Nickel

Nickel has high resistance to corrosion, and it is also catalysis for hydrogenation reaction

[31]. Porous nickel is a kind of porous metal which is very hard and has good resistance

[10]. Furthermore, porous nickel provides a large surface area because of its porous

structure [3].

Porous nickel is used in many applications because of its properties, for example, in optical

devices [24], waterproof coatings, sensor and supercapacitor [32], [33]. The application of

making supercapacitors will be the focus of this project.

According to Cheng at el. [2], porous nickel can be used to make optic devices. In their

research, porous nickel was made by using porous silicon as a template. With the

dissolution of silicon, part of silicon was replaced by nickel, porous nickel was formed. It is

discovered that porous nickel gives out various responses in the range of visible spectrum

and shows the ability of optical sensation. It is a good material of being optical device. Also,

it can absorb light and reduce the noises because of its rough surface.

Porous nickel has good wettability. Gu and Tu have discovered that porous nickel thin film

has a super hydrophobic surface, without any modifications, which increases the corrosion

resistance. The measurement of wettability shows the water contact angle is more than 160°,

which proves the high waterproof capability of porous nickel (Fig.6) [10]

.

Figure 6 Contact angle of porous nickel [10]

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8

A research has been done by Lu et al. that porous nickel can be used as a sensor for glucose,

which helps to detect the glucose level for diabetes patients [3]. Porous nickel has a large

electrochemical active surface and high electrocatalytic activity. You et al. have found that

using porous nickel gives better result on the stability and sensation of glucose as compared

with other metals [25], [34].

2.3 Supercapacitor

A supercapacitor is similar to a battery which acts as an energy storage. It has been widely

studied since the 1990s [12]. The earliest supercapacitor was investigated in 1957 by Becker.

(as shown in Fig. 7) [35].

Figure 7 The first supercapacitor in the world [35]

Supercapacitors can be classified into two types, faradaic supercapacitor and electrostatic

supercapacitor [12]. The main difference between these two types of superconductors is that

during the charging and discharging process, faradaic supercapacitor undergoes chemical

reactions, such as redox reaction. On the other hand, electrostatic supercapacitor does not

undergo any chemical reactions [36], [37]. A faradaic supercapacitor is also called a

pseudocapacitor. It is chemically active. Similar to normal batteries, by the time charging

and discharging, redox reaction occurs. During the process, the charges are directly stored.

___________________________________________________________________________________________________________________________________


9

Also, because of this reaction, the faradaic current is generated. Compared with an

electrostatic supercapacitor, it has higher capacitance and higher energy density, about

10-100 times greater [11], [38]. In an electrostatic supercapacitor, there is no chemical

reactions occur. During the charging process, the positive ions move to the negative side

and the negative ions move to the positive side. Opposite phenomenon occurs when it

undergoes discharging.

Compared to faradaic supercapacitor, electrostatic supercapacitor has higher power density

and stability. The higher power density is due to the relatively long duration needed for

faradaic supercapacitor to process charging and discharging. According to the equation,

power is inversely proportional to time, as the time needed for faradaic supercapacitor to

charge and discharge is longer than that for electrostatic supercapacitor.

Although faradaic supercapacitor has higher energy density, its power density is lower.

Electrostatic supercapacitor also has higher stability because its concentration of electrolyte

remains unchanged. On the contrary, the concentration of faradaic supercapacitor is not

stable because of the occurrence of redox reaction.

Low energy density is one of the disadvantages of supercapacitors as compared with

batteries. The energy density of a supercapacitor is about 5 Wh/kg and that of a battery is

more than 50 Wh/kg [12]. For supercapacitors, the energy density is proportional to the size

of the capacitor. For event that requires large storage amount of energy, a large

supercapacitor has to be built. This increases the production cost. High discharging rate is

also another problem. The daily self-discharging rate of a supercapacitor is about 20-40%

which is very high (Table 1). The disadvantage will be one of the major aspects affecting

the efficiency of the application of supercapacitor.

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10

Table 1 Comparison of technical characteristics of EES system [39]

One of the advantages of supercapacitor compares with batteries is high power

density. Compared with batteries, supercapacitors have higher power density which is

about 1000-10000 W/kg. On the other hand the power density for batteries is about 150W

[12]. The power density of supercapacitors is about 10-60 times higher than that of

batteries.

Also, the charging and discharging time is much shorter for supercapacitors comparing with

batteries. For instance, the time needed to fully charge and discharge of a supercapacitor is

about 30 seconds; however, it takes about an hour to charge a battery [12].

Furthermore, supercapacitors have longer life expectancy than batteries. A supercapacitor

can conduct over 500000 charge-discharge cycles and the life expectancy is predicted as 30

years. However, the life expectancy of a battery is about 5-10 years and can only conduct

10000 cycles [13].

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11

3. Experimental Procedures

3.1 Fabrication of Porous Nickel Thin Films

Preparation of Solution

The solution contained 0.5M of Ni (NH2SO3)2 · 4H2O, 0.05M of Cu(H2O)4SO4 · H2O,

0.6M of H3BO3 , 0.1g of saccharin and water. Saccharin was used to fabricate more uniform

porous nickel.

Electrodeposition and Dealloying

Indium-Tin Oxide (ITO) glass was used as working electrode. Platinum was used as counter

electrode. Working electrode and counter electrode were parallelly placed 2cm apart. A

reference electrode, (saturated calomel electrode) SCE was used.

In this method six samples were made. For sample 1-1 and 1-2, -1.4V of voltage with

duration of 10C of cathodic charge was used in the part of electrodeposition. For sample 2-1

and 2-2, -1.5V of voltage with duration of 10C of cathodic charge was used. For sample 3-1

and 3-2, -1.6V with duration of 10C of cathodic charge was used. After energy

electrodeposition, sample1-2, 2-2 and 3-2 were dealloyed at +0.5V for 10 minutes (or until

the voltage dropped to 10-4A). These conditions were set up to determine the influence of

voltage and time when making porous nickel (Table 2).

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12

Table 2 Voltage and Time Used for Fabrication

Electrodeposition Dealloying Sample Voltage Cathodic voltage Voltage Duration

1-1

1-2-1.4V

+0.5V 10min

2-1

2-2-1.5V

+0.5V 10min

3-1

3-2-1.6V

10C

+0.5V 10min

3.2 Hydrothermal Treatment (HT)

In this part, 2-2 was chosen to be studied. Six porous nickel thin films with same fabrication

condition as 2-2 were made. They underwent HT in different conditions, including solution

(Table 3), temperature and time duration (Table 4).2-2-6 was used as reference sample.

Preparation of solution

For HT five solutions were prepared. For sample 2-2-1, 2-2-2 and 2-2-4 , the

solution contained 0.02M of Urea, 0.005M of NiSO4．4H2O and water. For sample 2-2-3,

the solution contained 0.01M of Urea, 0.005M of NiSO4．4H2O and water. For sample

2-2-5, the solution contained 0.02M of Urea, 0.005M of NiSO4．4H2O and water; also,

NH3．H2O (28% weighted percentage) was added until it turned blue (0.1mL). Sample 2-2-6

worked as control set up, it did not undergo hydrothermal treatment. The formulation of the

solution is shown in Table 3.

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13

Table 3 Formulation of Solution for HT

Sample Chemical Concentration Amount of making

25mL

Urea 0.02M 30mg 2-2-1

NiSO4．4H2O 0.005M 40mg

Urea 0.02M 30mg 2-2-2


Urea 0.01M 15mg 2-2-3


Urea 0.02M 30mg 2-2-4


Urea 0.02M 30mg

NiSO4．4H2O 0.005M 40mg 2-2-5

NH3．H2O (28% weighted %) / 0.1mL

2-2-6

Settings of Autoclave

Different temperatures and time durations were used in the samples which shows in table 4.

Table 4 Temperature and Time Duration for HT

Sample Temperature Time Duration

2-2-1 100°C 6 hour

2-2-2 100°C 12 hour

2-2-3 100°C 12 hour

2-2-4 120°C 6 hour

2-2-5 100°C 12 hour

2-2-6

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14

3.3 Characterization

3.3.1 Scanning Electronic Microscope (SEM)

SEM had been done in order to check the geometry of porous nickel thin films. During this

process, 5kV of accelerating voltage was used. SEM micrographs of 1-1 to 3-2 and 2-2-1 to

2-2-5 of magnification of 1000X, 5000X and 10000X was taken.

3.3.2 X-ray Diffraction (XRD)

XRD had been done in order to check the chemical components contained inside the sample.

The results are important for determining whether nickel hydroxide had grown on porous

nickel at the stage of electrodeposition and indicate the changes of component before and

after dealloying.

3.3.3 Cyclic Voltammetric Curve (CV)

CV was used to check the performance of storing charges. The CV curve of supercapacitor

usually has a pair of redox reaction peak. Also, if it has larger CV curve enclosed area; it

means it has higher performance. CV had been done for both before HT and after HT of

sample 2-2-1 to 2-2-5. It was done the under highest voltage of 0.7V and scan rate of 0.1

V/s with 6 segments.

3.3.4 Charge and Discharged Curve (C-DC)

C-DC had been done in order to check the charging and discharging time and calculate the

specific capacitance of porous nickel. Sample 2-2-4 and 2-2-6 were chosen to undergo this

test with the following conditions (see Table 5 and Table 6).

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15


Sample Number of cycles Current Density

1000 cycles 5 mAcm-2


300 cycles 5 mAcm-2

5 cycles 10 mAcm-2

5 cycles 5 mAcm-2

5 cycles 2 mAcm-2

2-2-4

5 cycles 1 mAcm-2


Sample Number of cycles Current Density



300 cycles 5 mAcm-2

5 cycles 10 mAcm-2

5 cycles 5 mAcm-2

5 cycles 2 mAcm-2

2-2-6

5 cycles 1 mAcm-2

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16

4. Results and Discussion

4.1 Porous Nickel

Figure 8a) and figure 8b) show the SEM micrograph of porous nickel which fabricated by

method of electrochemical dealloying at -1.2V and hydrogen bubble dynamic template

respectively. From figure 8a) it reveals that the porous nickel fabricated by method of

electrochemical dealloying had small pore size, about 0.6μm. Besides, all the pores are

loosely packed. On the contrary, figure 8b) shows that large porous size, about 10μm,

porous nickel with a closely packed arrangement can be form by method of hydrogen

bubble dynamic template. Compare with the graphs in figure 8, figure 9 to figure 11 show

that porous nickels with small pore size and high density of pores can be fabricated by the

combined method of electrodeposition and hydrogen evolution reaction, which means that

they have higher surface area.

Figure 9 to figure 11 show the morphology of CuNi thin films and porous nickels. In figure

9a), the CuNi thin film was formed at -1.4V, large ball shaped CuNi were spread all over

the thin film. After dealloying, pores were formed on thin film. The measured pores

diameter was between 100-500nm (shown in figure 9b)). From figure 10a), the CuNi thin

film was formed at -1.5V, small grainy CuNi were uniformly distributed on the thin film.

Figure 10b) shows that after dealloying, the feature size of the pores in the porous nickel

thin film is also around 100-500nm. Compare with using -1.4V, using -1.5V for

electrodeposition could fabricate porous nickel with higher density of pores. In figure 11a)

the CuNi thin film was formed at -1.6V, it was paved with broccoli like CuNi. After

dealloying, figure 11b) indicates that very tiny pores were uniformly distributed on thin film

and formed a mesh-like porous nickel.

Due to the occurrence of hydrogen evolution reaction, porous nickel could be formed with a

lot of voids. Then, at dealloying, copper deep inside the CuNi thin film could be easily

dissolved back to the solution due to sufficient contact of electrolyte. Therefore, high

surface area porous nickel can be formed by this combined method.

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17

Figure 8 SEM micrographs of a) electrochemical dealloying b) hydrogen bubble dynamic template[5]

Figure 9 SEM micrographs of electrodeposition at -1.4V a) before dealloying b) after dealloying



a b

a

a

a

b

b

b

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18

Figure 12 shows the XRD pattern of different voltage of electrodeposition in the range of

25°to 55°. Four peaks can be found from figure 12 which representing NiOOH, NiO

, Ni and Ni respectively. It discloses that at the process of

electrodeposition, porous nickel had already undergone oxidation and formed nickel oxide

on it. Oxidation occurred because of the low voltage applied, this occurrence led to one-step

fabrication of supercapacitor.

Figure 12 XRD of different voltage of electron deposition in the range of 25°to 55°

Figure 13 shows XRD pattern of different voltage of electrodeposition in the range of 42.5°to

45.5°. From figure 13, it shows that peaks at different deposition voltage were shift to the right

after dealloying, which is nearer to the angle of Ni . This phenomenon took place because

copper dissolved back to the solution at the stage of dealloying and caused the increase

concentration of nickel.

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19

Figure 13 XRD of different voltage of electron deposition in the range of 42.5°to 45.5°

4.2 Porous Nickel with Nickel Hydroxide

Figure 14 shows that after thermal treatment, some fur liked substances were grown on the

surface of porous nickel, which is Ni(OH)2. It can help to enhance the ability of porous

nickel to store charges. It is a very important feature for being a supercapacitor. From figure

14 a), figure 14 b), figure 14 c) and figure 14 e), it is discovered that a thick layer of nickel

hydroxide was grown on the samples and block most of the pores. This blockage made the

samples cannot perform ultimately. On the contrary, for sample 2-2-4, a thin layer of nickel

hydroxide grew on the surface of the porous nickel with relatively less blockage to the pores.

Therefore, sample 2-2-4 was chosen to undergo electrical tests and had further study.

___________________________________________________________________________________________________________________________________


20

a b

c

d e

Figure 14 SEM micrographs of sample a)2-2-1 b)2-2-2 c)2-2-3 d)2-2-4 e)2-2-5

Figure15, 16, 17, 18 and 19 show the cyclic voltammetry curve of 5 samples before and

after hydrothermal treatment. Generally, the CV of a supercapacitor has a pair of redox

reaction peak which represent the voltage for molecules to undergo redox reaction. Also, the

enclosed area of the CV curve representing the charge storage capacity of the sample. From

figure 15 to figure 19, all five samples had a pair of well-defined redox peak. The shape of

the CV curves showed the typical characteristic of a supercapacitor because at the stage of

___________________________________________________________________________________________________________________________________


21

electrodeposition, nickel oxides had already formed on the porous metal and it acted as an

active material for supercapacitor. Figure 15 shows that, the reduction peak and oxidation

peak of sample 2-2-1 before hydrothermal treatment were at about 0.1V and 0.55V

respectively. After, hydrothermal treatment reduction peak and oxidation peak became about

-0.05V and about 0.6V. For sample 2-2-2, figure 16 shows that its reduction peak and

oxidation peak before hydrothermal treatment were at about 0.1V and 0.5V respectively.

And after hydrothermal treatment, the reduction peak and oxidation peak were about -0.1V

and 0.7V. Then, figure 17 shows that, the reduction peak and oxidation peak of sample

2-2-3 before hydrothermal treatment were at about 0.1V and 0.5V respectively. After,

hydrothermal treatment reduction peak and oxidation peak became about -0.2V and about

0.7V. Also, for sample 2-2-4, figure 18 shows that, its reduction peak and oxidation peak,

before hydrothermal treatment, were at about 0.05V and 0.55V respectively. After

hydrothermal treatment, the reduction peak was about -0.05V and the oxidation peak is

about 0.7V. Last but not least, figure 19 shows that, the reduction peak and oxidation peak

were about 0.05V and 0.55V respectively before hydrothermal treatment. After

hydrothermal treatment, they became -0.05V and about 0.65V respectively.

In comparison, after the hydrothermal treatment all samples had larger CV curve enclosed

area than that before the hydrothermal treatment. It means that all samples had larger charge

storage capacity after hydrothermal treatment. Among all samples, sample 2-2-4 shows

largest enhancement and it had been further studied with charge and discharge tests.

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22



___________________________________________________________________________________________________________________________________


23



___________________________________________________________________________________________________________________________________


24


4.3 Supercapacitor

Figure 20 and figure 21 show the charge and discharge curves at different current densities

of sample 2-2-4 (with hydrothermal treatment) and sample 2-2-6 (without hydrothermal

treatment), including 1 mAcm-2, 2 mAcm-2, 5 mAcm-2 and 10 mAcm-2. The first few cycles

of charge and discharge usually unstable, therefore, every sample had undergone 5 cycles.

Figure 20 and 21 indicate the results of the last loop. Comparing the discharged time of

sample with hydrothermal treatment and sample without hydrothermal treatment, it is

discovered that the discharging time of sample with hydrothermal treatment are larger than

the discharging time of sample without hydrothermal treatment. Therefore, sample with

hydrothermal treatment (average ~0.152 Fcm-2) must have higher capacitance than the

sample without hydrothermal treatment (average ~0.0944 Fcm-2). The capacitance of

sample with hydrothermal treatment is about 61.8% higher than that of sample without

hydrothermal treatment

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Figure 20 C-DC curve at different current densities of porous nickel with hydrothermal treatment

Figure 21 C-DC curve at different current densities of porous nickel without hydrothermal treatment

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26

The time needed for discharging to occur are recorded in every 100 cycles in both samples:

first 1000 cycles with current density 5 mAcm-2; then following 300 cycles with current

density 10 mAcm-2; the last one with again current density 5 mAcm-2 for 300 cycles. The

thickness of porous nickel thin film is measured in nano scale which does not contribute

effects on chemical reaction so much. Also, all samples are fragile and difficult to measure

their masses. Therefore, area is used to calculate the specific capacitance instead of using

mass. The area specific capacitance is calculated by the equation:

Where I is the current, Δt is the time needed for discharge, S is the area of porous nickel and

ΔV is the voltage range. After calculating all parameters, the areal specific capacitances

were plotted against time which shown in Figure 22 and 23 as follow.

Figure 22 Specific capacitance of porous nickel with hydrothermal treatment

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Figure 23 Specific capacitance of porous nickel without hydrothermal treatment

From figure 22, porous nickel with hydrothermal treatment first experienced a relatively

high voltage so as to charge up fast. It was discovered that the specific capacitor tended to

decrease gradually and it decreased from 0.18Fcm-2 to 0.138Fcm-2 in first 1000 cycles. It

tells that after 1000 cycles, it could still maintain its function with about 77% efficiency.

Then, porous nickel with hydrothermal treatment had experienced a higher current intensity,

10mAcm-2, for 300 cycles. It reveals that high current did not break the sample, because the

sample can still work normally at 5mAcm-2 afterwards. It tells that, porous nickel with

hydrothermal treatment not only has high specific capacitance but also has high retention.

Figure 23 shows an opposite trend compared with figure 22. The specific capacitance kept

increasing for the sample without hydrothermal treatment. It increased from 0.0920 Fcm-2 to

0.100 Fcm-2 in the first 1000 cycles and increased from 0.102 Fcm-2 to 0.106 Fcm-2 in the

last 300 cycles. This extreme difference is due to the formation of nickel oxide on the

sample because of high current.

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5. Conclusion

In this project the method combining electrodeposition-dealloying and hydrogen bubble

reaction was used to fabricate porous nickel with high surface area. The results show that by

using this combined method, the limitation of using method of electrochemical dealloying

and hydrogen bubble dynamic template can be overcome. This method uses voltage that

equal or lower than -1.3V, the critical potential for hydrogen evolution reaction to occur, to

do the electrodeposition and followed by dealloying. Because of the formation of small

hydrogen bubbles, a lot of voids can be formed between particles on the CuNi thin film.

Therefore, at dealloying, copper deep inside the CuNi thin film can be easily dissolved back

to the solution as it has sufficient contact of electrolyte. This mechanism leads to formation

of high surface area porous nickel. Moreover, this combined method is also a one-pot

fabrication method of supercapacitor, because of the occurrence of oxidation in the stage

electrodeposition-dealloying. Furthermore, by using this combined method high specific

capacitance porous nickel can be formed, which is about 0.152 Fcm-2. Also, it has high

retention as after experiencing a high current it can still work with 77% efficiency. Last but

not least, this method is potentially to be applied to large-scale fabrication of

supercapacitors to be used in industry due to cost-effectiveness, easy fabrication.

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29

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