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Theses and Dissertations
2010
Study of fabrication of nanoporous Ni-Zr anode for
solid oxide fuel cell using electrodeposition
technique
Surya Venkata Pothula
The University of Toledo
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Recommended Citation
Pothula, Surya Venkata, "Study of fabrication of nanoporous Ni-Zr anode for solid oxide fuel cell using electrodeposition technique"
(2010). Theses and Dissertations. Paper 945.
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A Thesis
entitled
Study of Fabrication of Nanoporous Ni-Zr Anode for Solid Oxide Fuel Cell Using
Electrodeposition Technique
by
Surya Pothula
Submitted to the Graduate Faculty as partial fulfillment of the requirements for
the Master of Science Degree in Mechanical Engineering
Dr. Yong X. Gan, Committee Chair
Dr. Matthew Franchetti, Committee Member
Dr. Calvin Li, Committee Member
Dr. Patricia Komuniecki, Dean
College of Graduate Studies
The University of Toledo
May 2010
Copyright, 2010 Surya Pothula
This document is copyrighted material. Under copyright law, no parts of this document
may be reproduced without the expressed permission of the author.
An Abstract of
Study of Fabrication of Nanoporous Ni-Zr Anode for Solid Oxide Fuel Cell Using
Electrodeposition Technique
by
Surya Pothula
Submitted to the Graduate Faculty in partial fulfillment of the
requirements for the Master of Science in Mechanical Engineering
The University of Toledo
May 2010
Solid oxide fuel cells are highly efficient energy conversion devices which
convert fuels electrochemically to electricity with negligible pollution emissions. Anode
of solid oxide fuel cell plays an important role in converting hydrogen into hydrogen ions
and electrons. Many techniques like plasma spraying, tape casting, screen printing,
sintering process etc have been discovered for the fabrication of anode supported solid
oxide fuel cells. In order to meet the high demand of energy in present days a new
technique is required for increasing the efficiency. Among the various methods,
increasing porosity of anode is one. This can be achieved by fabricating the anode by
electrodeposition technique, which has been proved as an effective way of increasing
porosity. In this work, we demonstrated that nanoporous Ni can be electroplated on a Zr
substrate.
Electrochemical dealloying of copper from a Ni-Cu alloy wire is carried out first
to better understand the effect of voltage and run time on the process. As the potential
increases the amount of copper dealloyed in to the solution was also increased.
iii
Passivation of nickel occurs as the time increases, allowing the formation of NiO. The
preparation of nanoporous nickel films on zirconium by electrochemical deposition of
Ni-Cu alloy followed by selective anodic etching of the more noble metal, copper, was
studied in an aqueous solution containing Ni and Cu at room temperature. Variable
potential electrodeposition produces crystalline or grain Ni-Cu alloys on Zr substrate, in
which the Ni content increases as the deposition potential becomes more negative.
Cyclic voltammetric data indicates that the anodic dissolution of nickel is
retarded by passivation. By taking the advantage of nickel passivation, selective anodic
etching of Cu is carried out. Multicyclic electrochemical alloying/dealloying process
makes the film rich of nickel and complete dealloying of copper.
iv
To my mother, father, brother for their love, patience, support and encouragement they
have been giving me throughout my life.
Acknowledgments
I would like to thank Dr. Yong X. Gan for accepting me as his student and guiding me all
through the way to pursue my master’s degree. I would thank him for his supervision,
advice, guidance and support throughout the work. I have gained great knowledge from
him that would help me to pursue a career in the field of material science.
vi
Table of Contents
Abstract ....................................................................................................................... iii
Acknowledgments ........................................................................................................ vi
Table of Contents ........................................................................................................ vii
List of Tables ............................................................................................................... ix
List of Figures ................................................................................................................x
Introduction ...................................................................................................................1
2 Literature Review ......................................................................................................5
2.1 Introduction to Fuel Cell.........................................................................................5
2.1.1 Architecture .....................................................................................................6
2.1.2. Types of Fuel Cell...........................................................................................7
2.1.3. Advantages of Fuel Cell ..................................................................................7
2.1.4 Solid Oxide Fuel Cell.......................................................................................8
2.2 Materials Used For SOFC .................................................................................... 11
2.2.1 Electrolyte ..................................................................................................... 11
2.2.2 Cathode ......................................................................................................... 11
2.2.3 Interconnect material ..................................................................................... 12
2.2.4 Anode ............................................................................................................ 12
2.3 Nanoporous Materials .......................................................................................... 13
2.3.1 Introduction ................................................................................................... 13
2.3.2 Classification of nanoporous materials ........................................................... 14
2.3.3 Application of Nanoporous Materials:............................................................ 15
2.4 Method of Fabrication of Anode for Solid Oxide Fuel Cell ................................... 17
vii
2.4.1 Combustion synthesis .................................................................................... 17
2.4.2 Plasma Spraying ............................................................................................ 17
2.4.3 Electrodeposition ........................................................................................... 18
2.4.4 Tape Casting .................................................................................................. 18
2.4.5 Screen Printing .............................................................................................. 18
2.5 Cyclic Voltammetry.......................................................................................... 18
3 Experimental Procedure .......................................................................................... 23
3.1 Materials .............................................................................................................. 23
3.2 Electrochemical Dealloying of Copper ................................................................. 24
3.3 Preparation of Nanoporous Nickel on Zirconium Substrate ................................... 28
3.4 Morphology and Composition Analysis ................................................................ 34
4 Results and Discussion ............................................................................................. 37
4.1 Cyclic Voltammograms of Electrochemical Dealloying of Copper ....................... 37
4.2 Morphology of Nanoporous Nickel ...................................................................... 56
4.3 Electrodeposition of Ni and Selective Etching of Copper on Zirconium Substrate 59
4.4 Morphology of Ni-Zr Film ................................................................................... 69
4.5 Composition Analysis of Ni-Zr Film .................................................................... 76
5 Conclusions .............................................................................................................. 83
6 Future Work ............................................................................................................85
References .................................................................................................................... 86
viii
List of Tables
Table 2.1: Classification of Porous Materials [10].......................................................... 14
Table 3.1: Various Cases Studied for Dealloying of Copper .......................................... 26
Table 4.5.1: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 with pltainum
electrode ........................................................................................................................ 78
Table 4.5.2: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With Nickel
Electrode ....................................................................................................................... 80
Table 4.5.3: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO4 With Nickel
Electrode ....................................................................................................................... 82
ix
List of Figures
Figure 1.1: Solid Oxide Fuel Cell ....................................................................................2
Figure 2.1: Solid Oxide Fuel Cell ....................................................................................8
Figure 2.5.1: Cyclic Voltammetry .................................................................................. 21
Figure 3.1.1: Setup for Dealloying of Copper................................................................. 24
Figure 3.1.2: Dealloying of Copper at 0.2 Volts for 200 Seconds ................................... 27
Figure 3.1.3: Dealloying of Copper at 0.5 Volts for 500 Seconds ................................... 27
Figure 3.2.1: Setup for Electrodeposition of Ni and Cu and Dealloying of Cu ................ 28
Figure 3.2.2: Cyclic Voltammogram of Sample 1 for 1 Segment.................................... 30
Figure 3.2.3: Cyclic Voltammogram of Sample 1 for 60 Segments ................................ 30
Figure 3.2.3: Cyclic Voltammogram of Sample 1 for 60 Segments ................................ 31
Figure 3.2.4: Cyclic Voltammogram of Sample 2 for 60 Segments ................................ 32
Figure 3.2.5: Cyclic Voltammogram of Sample 2 for 1 Segment.................................... 33
Figure 3.2.6: Cyclic Voltammogram of Sample 3 for 60 Segments ................................ 34
Figure 3.3.7 Hitachi S-4800 High Resolution Scanning Electron Microscope ................ 35
Figure 3.3.8 (a, b) Shows the SEM Images Taken on Samples Prepared by Using Nickel
as Cathode and Zirconium as Anode .............................................................................. 36
Figure 4.1.1: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand for 200 Seconds ................................... 39
Figure 4.1.2: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand For 300 Seconds .................................. 41
x
Figure 4.1.3: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand for 500 Seconds ................................... 42
Figure 4.1.4: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.25V of Voltageand for 200 Seconds ................................... 44
Figure 4.1.5: Dealloying of Copper From Copper-Nickel Wire With an Electrolyte of
25%HNO3, 0.25V of Voltageand for 300 Seconds ......................................................... 45
Figure 4.1.6: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.25V of Voltageand for 500 Seconds ................................... 47
Figure 4.1.7: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 200 Seconds ................................... 48
Figure 4.1.8: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 300 Seconds ................................... 50
Figure 4.1.9: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 500 Seconds ................................... 51
Figure 4.1.10: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.50V of Voltageand for 200 Seconds ................................... 53
Figure 4.1.11: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.50V of Voltageand for 300 Seconds ................................... 54
Figure 4.1.12: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.50V of Voltageand for 500 Seconds ................................... 56
Figure 4.2.1: Microscopic Analysis of the Dealloyed Ni-Cu Alloy; (a) Optical
Micrograph at 50X (The left part is before dealloying; the right part is after dealloying.),
(b) Low Magnification SEM Image Showing the Global Porous Feature, (c) High
Magnification SEM Image Showing the Nanostructures of the Dealloyed Ni-Cu, (d) EDX
Spectrum ....................................................................................................................... 58
Figure 4.3.1: Graph Showing the Change of Current Density With Respect to Time Using
Platinum Electrode ........................................................................................................ 60
xi
Figure 4.3.2: Graph Showing the Change of Voltage With Respect to Time Using
Platinum Electrode ........................................................................................................ 61
Figure 4.3.3 a): Cyclic Voltammogram Showing the Changes of Current Density With
Respect to Voltage Using Platinum Electrode for 1 Segment ......................................... 61
Figure 4.3.4: Graph Showing the Change of Current Density With Respect to Time Using
Nickel Electrode ............................................................................................................ 64
Figure 4.3.4: Graph Showing the Change of Current Density With Respect to Time Using
Nickel Electrode ............................................................................................................ 64
Figure 4.3.6 a): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Elctrode for One Segment .......................................... 65
Figure 4.3.6 b): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode for 60 Segments ......................................... 65
Figure 4.3.7: Graph Showing the Change of Current Density With Respect to Time Using
Uickel Electrode and Nickel Chloride Instead of Nickel Sulphate in Electrolyte ............ 66
Figure 4.3.8: Graph Showing the Change of Voltage Density With Respect to Time
Using Nickel Electrode and Nickel Chloride Instead of Nickel Sulphate in Electrolyte .. 67
Figure 4.3.9 a): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode and Nickel Chloride Instead of Nickel
Sulphate for one Segment .............................................................................................. 67
Figure 4.3.9 b): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode and Nickel Chloride Instead of Nickel
Sulphate for 60 Segments .............................................................................................. 68
Figure 4.4.1(a,b): SEM Images at 25.0KX and 11.0KX Magnitude on a Film Prepared
From the Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using
Platinum Electrode ........................................................................................................ 70
Figure 4.4.2(a,b): SEM Images at 3.50KX and 1.20KX Magnitude on a Film Prepared
From the Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using
Platinum Electrode ........................................................................................................ 70
xii
Figure 4.4.3(a,b): SEM Images at 3.5KX and 2.2KX Magnitude on a Film Prepared From
the Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Platinum
Electrode ....................................................................................................................... 71
Figure 4.4.4(a,b): SEM Images at 45X Magnification of a Sample Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Platinum
Electrode ....................................................................................................................... 71
Figure 4.4.5(a,b): SEM Images at 450X and 600X Magnitude on a Film Prepared From
the Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Platinum
Electrode ....................................................................................................................... 72
Figure 4.4.6(a,b): SEM Images at 1.80KX Magnification of a Sample Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Platinum
Electrode ....................................................................................................................... 72
Figure 4.4.7(a,b): SEM Images at 4.0K X and 2.0K X on a Film Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Nickel
Electrode ....................................................................................................................... 73
Figure 4.4.8(a,b): SEM Images at 60X and 2.20KX on a Film Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate Using Nickel
Electrode ....................................................................................................................... 74
Figure 4.4.9(a,b): SEM Images at 30X and 250X on a Film Prepared From the Electrolyte
Solution Containing Nickel Chloride and Copper Sulphate Using Nickel Electrode ....... 75
Figure 4.4.10(a,b,c): SEM Images 1.10KX, 1.20KX and 300X Magnifications of a Film
Prepared From the Electrolyte Solution Containing Nickel Chloride and Copper Sulphate
Using Nickel Electrode .................................................................................................. 76
Figure 4.5.1: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 with Platinum Electrode
...................................................................................................................................... 77
Figure 4.5.2: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With Platinum Electrode
...................................................................................................................................... 77
xiii
Figure 4.5.3: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With Nickel Electrode 79
Figure 4.5.4: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With Nickel Electrode 79
Figure 4.5.5: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO4 With Nickel Electrode .. 81
Figure 4.5.6: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO2 With Nickel Electrode .. 81
xiv
Nomenclature
Pe…
Elemental (usually by area) Power (kW)
T….
Temperature (C)
P….
Total Pressure (kPa)
P02... Partial Pressure of Oxygen (kPa)
Ø…. Relative air humidity (%)
ie….
Elemental Current (A)
Vo...
Open circuit Voltage (mV)
b….
Empirical equation constant (mV/dec)
m…. Empirical equation constant (mV)
n…
Empirical equation constant (cm/mA)
R…
Empirical equation constant (Ohm/cm^2)
R….
Molar gas constant (8.3144 Jmol-1 K-1)
T….
Absolute temperature (K)
n….. Number of electrons transferred
F.....
Faraday constant (96485 C/equiv)
E….
Standard reduction potential for redox couple
K0… Heterogeneous rate constant
Α…
Transfer coefficient
xv
A…
Area of electrode
D0… Diffusion coefficient of Oxidant
x…
Distance from electrode surface
Cu...
Copper
Ni… Nickel
Zr…
Zirconium
xvi
Chapter 1
Introduction
With the oil and gas supply security and climate change emerging as high concern, the
need for new technologies to alternative independent of hydrocarbons and reduction of
carbon dioxide (CO2) emission become increasingly urgent. In order to achieve these
goals it is required significant change in the way the global energy system is managed
and the adoption of new technologies which produce energy from alternative fuels with
no or minimum pollution and with a high efficiency than the present techniques. Among
these alternative technologies power generation from fuel cells is one, which uses
hydrogen as fuel and contains no carbon dioxide (CO2) in its emissions.
In present days many countries put considerable effect on making commercially
available technologies to separate and store carbon dioxide from fossil fuels, produce
hydrogen from fossil, nuclear and renewable energy sources and develop fuel cell for
clean and efficient use of hydrogen. A fuel cell is defined as an electrochemical device
which converts chemical energy directly into electricity.
The basic structure of a fuel cell consists of an electrolyte layer in contact with a porous
anode and cathode on both sides. A schematic drawing of a fuel cell is shown in figure
(1.1). In a typical fuel cell, gaseous fuels are fed continuously to the anode (positive
1
electrode) compartment and an oxidant (for example, oxygen from air) to the cathode
(negative electrode) compartment. Electrochemical reaction takes place at electrodes to
produce electricity. Although this basic model resembles a typical battery structure, there
are a lot of differences between these two. A battery produces energy as long as it
contains chemical reactant and it ceases the production when chemical reactants are
consumed. We need to recharge from an external power source to reuse the battery, but
when it comes to fuel cell it produces energy as long as the gaseous hydrogen fuel and
oxygen are supplied. It is noted that in theory any substance capable of chemical
oxidation that can be supplied continuously (as a fluid) can be burned galvanically as a
fuel at anode of a fuel cell [1].
Figure 1.1: Solid Oxide Fuel Cell
Gaseous hydrogen has become the fuel of choice because of its high reactivity when
suitable catalysts are used. There are many advantages of using hydrogen as a clean fuel.
2
It can be produced from hydrocarbons from terrestrial applications and possesses a very
high energy density when stored cryogenically for environmental applications, such as in
space. A three-phase interface is established among the reactants, electrolyte and catalyst
in the region of a porous electrode. The nature of this interface plays an important role in
electrochemical performance of the fuel cell. If the porous electrode contains an
excessive amount of electrolyte, the electrode may flood and restrict the transport of
gaseous species in the electrolyte phase to the reaction site. It will reduce the
electrochemical performance of the porous electrode. Thus a balance must be maintained
between the electrode, electrolyte and gaseous phases in the porous electrode structure.
Most of recent work is concentrated on reducing the thickness of cell components while
refining and improving the electrode structure and electrolyte phase, with the aim of
obtaining a higher and more stable electrochemical performance while reducing the cost.
The electrolyte along with the transportation of dissolved reactants also conducts ionic
charge between the electrodes to complete the cell electric circuit. The main functions of
porous electrode in a fuel cell are:
1) To provide a surface site where gas/liquid ionization or de-ionization reaction
takes place
2) To conducts ions away from or into the three phase interface once they formed (so
an electrode must be made of materials with good electrical conductivity).
3) To provide a physical barrier that separates the bulk gas phase and electrolyte.
It is also corollary that the porous electrode must be permeable for both electrolyte and
gases, but it should not easily flood by electrolyte or dried by the input gases.
3
A variety of fuel cells are in different stages of development. These are classified into
different types based on the combination of electrolyte and oxidant, processing of fuel
whether inside or outside, type of electrolyte and the temperature of operation. The
common types of fuel cells based on the electrolytes used are 1) polymer electrolyte fuel
cell (PEFC), 2) alkaline fuel cell (AFC), 3) phosphoric acid fuel cell (PAFC), 4) molten
carbonate fuel cell (MCFC), and 5)solid oxide fuel cell (SOFC). The operating
temperatures for these fuel cells are 800C~PEFC, 1000C~AFC, 2000C~PAFC,
6500C~MCFC, 8000C~SOFC. Many factors like operating temperature, fuels used are to
be considered in selecting the materials for various components of fuel cells (electrodes,
electrolyte, interconnects, current collector etc). Aqueous electrolytes can be used in low
temperature fuel cells due to their high water vapor pressure and rapid degradation at
high temperatures. The operating temperature will also play a major role in selecting the
fuel. Hydrogen gas can be used as a fuel for low operating temperature fuel cells,
whereas CO and CH4(methane) can be used as fuels for fuel cells operating at high
temperatures due to the inherent electrode kinetics and due to the low use of catalytic
activity at high temperatures. In low temperature fuel cells (PEFC, AFC, PAFC) protons
or hydroxyl ions are major charge carriers in electrolytes, whereas in high temperature
fuel cells (MCFC, SOFC) carbonate ions and oxygen ions are charge carriers.
Porous electrodes are very important for good electrode performance. The main reason
for this is the current density obtained when using a porous electrode is more than a
smooth electrode with same normal size because of increase in porosity and thus
increases the number of reaction sites.
4
Chapter 2
Literature Review
2.1 Introduction to Fuel Cell
Fuel cell is a device which converts the chemical energy into electrical energy through
the electrochemical process with the input of gaseous fuels such as hydrogen, natural gas,
gasified coal and an output of heat and water. Fuel cell was first invented in 1839 by Sir
William Robert Groove, a professor of experimental philosophy at the Royal Institution
in London. Groove discovered that electricity may be generated by reversing the
electrolysis process and he demonstrated a cell with two bottles which contains hydrogen
and oxygen with platinum strips immersed in dilute sulphuric acid and named it as “gas
battery”. Later in1889 this technique was developed by Ludwig Mond and Charles
Langer and they build a device using air and industrial coal gas and named it as “fuel
cell”. In 1959 Dr. Francis Thomas Bacon replaced these platinum electrodes by nickel
and joined a number of stacks to produce a 5 KW capacity fuel cell for welding [2].
5
2.1.1 Architecture
A fuel cell mainly consists of three parts cathode, anode and electrolyte. Fuel is injected
into the anode section and air to the cathode. The cell works as long as there is fuel input.
The interconnecting material would be in contact with both electrodes and it should be
stable under both oxidizing and reducing conditions. The anode must be oxidizing
material and should be capable of holding hydrogen and cathode must be a reducing
agent. The basic architecture of a fuel cell is well described in the Figure 2.1. Two main
electrochemical reactions would occur at cathode and anode.
At the anode,
H22H+ +2eand at the cathode
2H+ + ½ O2 + 2e- H2O
The fuel which enters the anode electrode would split into hydrogen ion and
electrons. These hydrogen ions would move through the electrolyte and reaches the
cathode and electrons which move along an external circuit would be used as the power
source. At the cathode these hydrogen ions and electrons would combine with the
incoming oxygen and form water and heat as the output. The fuel cell works by catalysis,
separating the component electrons and protons of the reactant fuel, and forcing the
electrons to travel through a circuit, hence converting them to electrical power. The
catalyst is typically comprised of a platinum group metal or alloy. Another catalytic
process takes the electrons back in, combining them with the protons and the oxidant to
form waste products (typically simple compounds like water and carbon dioxide) [3].
6
2.1.2. Types of Fuel Cell
Fuel cells are mainly divided into five types depending on the type of fuel, material,
temperature and method of operation. They are
a) Alkali fuel cells
b) Molten carbonate fuel cells
c) Phosphoric acid fuel cells
d) Proton exchange membrane fuel cells
e) Solid oxide fuel cells
2.1.3. Advantages of Fuel Cell
The main advantages of a fuel cell are:
1) Efficiency of fuel conversion is very high in terms of fuel conversion.
2) Fuel cells are very friendly and pollution free for environment.
3) A fuel cell system running on hydrogen can be compact, lightweight and has no
major moving parts.
4) Fuel cells are very useful as power sources in remote locations, such as
spacecraft, remote weather stations, large parks, rural locations, and in certain
military applications.
7
2.1.4 Solid Oxide Fuel Cell
Figure 2.1: Solid Oxide Fuel Cell
Solid oxide fuel cell consists of anode which is Ni-YSZ, cathode La-manganite and
electrolyte of YSZ (yattria-stabilized zirconia). The anode should essentially be a
catalyst, which is capable of oxidizing hydrogen gas. Electrolyte must be a fast oxide ion
conducting material and cathode as a good reducing catalyst. Solid Oxide fuel cells
(SOFC) use a hard, ceramic compound of metal (like calcium or zirconium) oxides
(chemically, O2) as electrolyte. In these cells, oxygen ions are transferred through a solid
oxide electrolyte material at high temperature to react with hydrogen on the anode side.
These cells are grouped into tubular and planar designs. They have so far been operated
on methane, propane, butane, fermentation gas, gasified biomass and paint fumes.
However, sulfur components present in the fuel are disposed before entering the cell,
8
which is easily done by an activated carbon bed or zinc absorbent. Efficiency is about 60
percent. They work at very high temperatures, typically between 700 and 1,000°C. Solid
oxide fuel cells are intended mainly for stationary applications with an output from 100
W to 2 MW. At such high temperatures a reformer is not required to extract hydrogen
from the fuel, and waste heat can be recycled to make additional electricity. However, the
high temperature limits applications of SOFC units and they tend to be rather large.
While solid electrolytes cannot leak, they can crack.
Reactions are:
Anode:
H2+O2-→H2O+2eCO+O2-→CO2+2eCH4 + 4O2- → 2H2O + CO2 + 8e-
Cathode: ½ O2 + 2e- → O2The Nernst Equation (E = E° + (RT/ 2F) ln [P H2/PH2O] + (RT/ 2F) ln [PO2]) provides
a relationship between ideal standard potential E 0 for the cell reaction and ideal standard
potential at the other temperatures and partial pressures of the reactants and products.
The ideal standard potential of (E o) of a fuel cell with liquid water product is 1.229V
and 1.18V with gaseous water product. But the actual potential is decreased from its ideal
due to irreversible losses such as activation polarization, Ohmic polarization,
concentration polarization. [4]
Activation polarization is produced as a result of the energy that is used for breaking
or making of chemical bonds at anode and cathode. Ohmic polarization is caused by
electrical losses in the cell. These losses may be due to materials and contact resistance of
electrodes, electrolyte, and current collecting plates. Concentration polarization is caused
9
due to restriction in the flow of fuel gases to the reaction sites due to water generation
and excess humidification. This concentration polarization can be reduced by increasing
the gas pressure or by using thin electrodes.
When the power and physical conditions are given, current can be calculated by the
mathematical expression:
Pe(T,P,P02, Ø) = ieVo(T,P,P02, Ø)e - b(T,P,P02, Ø)ie log(ie) - R(T,P,P02, Ø) ie2 - m(T,P,PO2,
Ø) ie exp[(n(T,P,P02, Ø)ie)] - b(T,P,P02, Ø) log[P/PO2].
Voltage can be applied from:
Ve(T,P,P02, Ø) = Vo(T,P,P02, Ø)e - b(T,P,P02, Ø) log(ie) - R(T,P,P02, Ø) ie - m(T,P,P02, Ø)
exp[(n(T,P,P02, Ø)ie)] - b(T,P,P02, Ø) log[P/Po2].
Solid oxide fuel cells contain electrochemically active electrode materials and fast
oxide ion conducting materials for electrode and electrolyte. The operating temperature
of SOFC is reduced to 7000C -8000C by reducing Ohmic losses in electrolyte. SOFCs are
classified mainly into two types, anode supported electrolyte cell and self supported
electrolyte cells. Materials used for the anode are Ni-YSZ cermet, Ce0.6Gd0.4O1.8 and
La0.8Sr0.2Cr0.5Mn0.5O3 (LSCM).Among this Ni-YSZ is preferred because of its high
electronic conductivity. Cold die pressing is best for Ni-YSZ cermet anode.
Cross
sectional transmission electron microscopy of thin film samples of YSZ, (NiO)-YSZ,
YSZ/Ni-YSZ showed that both YSZ and NiO-YSZ layers were fully dense and exhibiting
10
eqiaxed grain morphologies. Selected area electron diffraction showed that YSZ crystal
structure is cubic in annealed samples and amorphous in the NiO-YSZ sample [5].
2.2 Materials Used For SOFC
2.2.1 Electrolyte
Electrolyte is a part of a fuel cell which can be used as an oxygen ion conductor and acts
as a separating material between anode and cathode. It is non-reactive with the anode and
cathode materials and it should be corrosive resistant.
High quality, low cost and more widely available fuels can be used for SOFC due to
higher operating temperatures. But these higher temperatures produce undesired reactions
and create thermal stresses during thermal cycling. Materials which can withstand these
higher temperatures are chosen and fabricated. The electrolyte must be a good ionic
conductor to minimize cell impedance and must have little or no electronic conduction.
The materials used for electrolyte in SOFCs are YSZ (yattria-stabilized zirconia), doped
ceria and doped lanthanum gallate. Out of these materials YSZ is the most commonly
used one. Ceria has highest conductivity and best stability with cathode, but has less
stability under low oxygen partials pressures. LSGM (La1-x SrxGa1-yMgyO3) also has
higher conductivity than YSZ but it is less stable with anode and preparation is difficult.
Fuel cells with layered structure are preferred but simple structures are preferred to
reduce cost and improve reliability [6]
2.2.2 Cathode
Cathode material must be a good reducing agent which reduces oxygen into oxygen ions.
Materials used for cathode are La1-xsrxMno3 (LSM), La1-xSrxFeo3 (LSF), La1-xSrxFe1yCo yO3
(LSCF). La1-xSrxFe1-yCoyO3 (LSCF) is a highly-active cathode material but it is
11
chemically incompatible with the YSZ electrolyte with solid state reactions which can
decrease the cathode performance. La1-xSrxFeo3 (LSF) is also incompatible with YSZ
electrolyte in solid oxide fuel cells [7].
2.2.3 Interconnect material
The interconnect material is in contact with both electrodes and thus must be stable in
both oxidizing and reducing atmospheres. This can be obtained by minimizing reactions
with the electrode materials and the atmosphere. Materials and fabrication costs are also
important for cost effective production of fuel cells. Nickel-based alloys, ferrite stainless
steels and chromium-based alloys have been used as interconnect materials for solid
oxide fuel cells. The coefficients of thermal expansion of nickel-based alloys are
considerably higher than those of other fuel cell components. Production cost of
chromium-based alloys is high, so ferrite stainless steels are the most attractive as this
interconnects production cost for chromium-based alloys is reduced. [8]
2.2.4 Anode
Anode material should be a good oxidizing agent, which reduces hydrogen into hydrogen
ion and should also be good redox resistant material. Materials used as anode are Ni-YSZ
cermet, Ce0.6Gd0.4O1.8, La0.8Sr0.2Cr0.5Mn0.5O3 (LSCM). When compared with the YSZ
compatibility yittrium-titania modified zirconia (YZT) is a best potential anode material
[9]. Cold die pressing is the best process for preparing Ni-YSZ cermet. Nano-porous
nickel preparation is the best way to increase the cell performance by increasing the
surface area in contact with the hydrogen gas. Although Ni-YSZ is used as anode
material for most of the SOFCs, there are some disadvantages with this material like
deposition of carbon on the electrode when used in carbon-containing fuels, increase of
12
deposition of sulfides with reduced operating temperatures and Ni-YSZ can undergo
micro structural changes during oxidation and reduction. The deposition of carbon can be
reduced by replacing nickel with copper, which does not catalyze C-C bond formation.
Due to these disadvantage alternatives for Ni-YSZ such as those containing titanium,
chromium, manganese or iron are used as anode materials [10].
2.3 Nanoporous Materials
2.3.1 Introduction
Nanoporous materials have unique surface, structural and bulk properties which lead to
application in various fields such as ion exchange, separation, catalysis, sensor, biological
molecular isolation and purification. Properties of nanoporous materials depend on the
size and shape of the pores. Pores are of two types open pore which will connect the
surface of the material and a closed pore which is isolated from the surface. Open pores
are helpful in adsorption, catalysis, sensing and filtration. Closed pores are useful in sonic
and thermal insulation and lightweight structural application. Pores are of different
shapes such as cylindrical, spherical, slit type and hexagonal. According to International
Union of Pure and Applied Chemistry, micropores are smaller than 2nm in diameter,
mesopores are 2 to 50 nm and macropores are larger than 50nm.
13
2.3.2 Classification of nanoporous materials
Table 2.1: Classification of Porous Materials [11]
Pore size
Surface
area/
Porosity
Permeability
Strength
Thermal
Stability
Polymeric Carbon
Glass
Meso-
Micro-
Meso-
macro
meso
Low
Alumino-
Oxide
Metal
Micro-
Micro-
Meso-
macro
meso
meso
macro
High
Low
High
Medium
Low
>0.6
0.3-0.6
0.3-0.6
0.3-0.7
0.3-0.6
0.1-0.7
Low-
Low-
Medium
medium
High
Low
Medium
Low
Strong
Weak
Low
Chemical
Low-
Stability
medium
High
High
Good
High
Costs
Low
High
High
Life
Short
Long
Long
14
silicate
Lowmedium
Weakmedium
Medium-
Medium-
high
high
High
Lowmedium
Mediumlong
VeryHigh
High
Strong
High
High
Medium
Medium
Long
Long
Nanoporous materials would have different set of properties based on their pore size,
pore density and pore distribution. The set of properties which makes the nanoporous
materials unique are high surface area, fluid permeability, Molecular sieving and shape
selective effects. Nanoporous nickel can be prepared by selective anodic etching of the
copper from electrodeposited Ni-Cu alloy. This Ni-Cu alloy films were electrodeposited
from a solution containing 1 M NiSO4, 0.01 M CuSO4 and 0.5 M H3BO3 at a temperature
of 250C. Three electrode-cells were used in this process. Saturated calomel electrode
(SCE) (reference electrode), platinum electrode and indium-tin oxide coated glass
(working electrode) are used as electrodes. Thin film of Ni-Cu is deposited on the
working electrode under a 1C charge, and then this Cu from the alloy is removed by
applying the anodic potential of 0.5 V. [12].
2.3.3 Application of Nanoporous Materials:
Nanoporous materials are applied in various fields such as environmental separations,
clean energy production and storage, Catalysis and photo catalysis, sensors and actuators,
electrodes for fuel cells and other various other applications.
1) Environmental separations:
Nanoporous materials can be used as environmental separators due to high
surface area and well defined pore sizes. This technique can be applied for applications
such as removal of SO2, NO2.
2) Clean energy production and storage:
Hydrogen is the future clean energy carrier and it can be produced from fossil
fuels, water electrolysis and biomass. Production of hydrogen from the coal is treated as
15
the best way because of its low cost and wide availability. Nanomaterials such as carbon
nanotubes and zirconium phosphates are proved to be promising catalyst for hydrogen
production. Nanostructure carbons prepared by templating 3-D ordered mesoporous
silicates exhibits interesting and superior performance as super capacitor and electrode
materials for Li-ion battery application.
3) Catalysis and photo-catalysis:
Efficiency of catalytic process depends on improvement in catalytic activity and
selectivity. Nanoporous materials are capable of offering such possibility with controlled
large and accessible surface area of catalyst but avoiding standalone fine particles. Photo
catalysis is the acceleration of photoreaction with the help of catalyst.
Transition metal oxides exhibit a wide range of physical, chemical and optical
properties. Titania in anatase is a best example of nanoporous material which exhibits
photo catalytic property.
4) Sensor and actuators:
Due to greater surface area and high sensitivity to slight changes in environment
such as temperature, humidity and light, nanoporous materials are widely used as
actuators and sensors. SnO2, TiO2, ZrO2 and ZnO are used as detectors of combustible
gases, humidity, ethanol and hydrocarbons. Zirconium is a good sensor of oxygen.
5) Electrode for fuel cells:
Nanoporous materials can increase diffusion of gas through the electrode and
thus increases the current density by increasing reduction at anode. Nanoporous nickel
with yttrium stabilized zirconium can be used as good electrode for solid oxide fuel cells.
16
2.4 Method of Fabrication of Anode for Solid Oxide Fuel Cell
Fabrication of solid oxide fuel cell presents many challenges mainly due to the
requirement of high porosity for the electrodes and dense coating for the electrolyte.
Many methods were widely used and are currently under research for the manufacturing
of the solid oxide fuel cell (SOFC). Combustion synthesis, plasma spraying,
electrodeposition, tape casting, foaming, screen printing etc were among the various
methods.
2.4.1 Combustion synthesis
In this technique anode materials like Ni-YSZ, (Ni, Co)-YSZ, (Ni, Fe)-YSZ, (Ni, Co)YSZ are obtained from molten nitrates and urea and pressing cermet layers on YSZ
electrolyte and co firing. This process of combustion synthesis and co firing is low cost
and simple technique providing nanometric and submicronic particle [13].
2.4.2 Plasma Spraying
Plasma spraying is a technique used to deposit metallic and ceramic coatings in the form
of powder or liquid at a temperature of order 10000k towards the substrate. This method
is widely used for depositing the metallic and ceramic coatings for thermal barrier and
wear and corrosion resistance coatings. Plasma spraying coatings typically exhibit
porosity in the range of 5-15% [14]. Use of plasma spraying has many advantages over
other processes with respect to performance and manufacturing cost. Other advantage
includes the elimination of sintering process making the plasma spraying process more
rapid.
17
2.4.3 Electrodeposition
Electrodeposition is a reverse process of galvanic cell. It is a process in which a solid
metal is deposited on a substrate by using electrolysis. In this process required metal is
made into an electrolyte solution and electroplated on the substrate by applying a
potential difference between the electrodes. This electroplating or electrodeposition
technique is widely used for the preparation of ceramic materials and alloys which are
corrosion resistance. More research still need to be done on this process for the
fabrication of electrodes, due to the advantages of low cost, corrosion resistance, easy to
fabricate and obtaining more porous electrodes of solid oxide fuel cell.
2.4.4 Tape Casting
Tape casting is one of the manufacturing methods for the manufacturing of thin ceramic
materials. This method can be used for the fabrication of both electrodes and electrolyte
of solid oxide fuel cell due to the ability of obtaining both porous and thin films. By using
water based tape casting grain size of 0.5-1µm with 98%-99% theoretical density [15].
2.4.5 Screen Printing
Screen printing method is one of the techniques used for the fabrication of solid oxide
fuel cells. Screen printing has been proved as a practical technique to elaborate at least
the electrodes for conventional devices [16].
2.5 Cyclic Voltammetry
Cyclic Voltammetry is a popular method used in many areas of Chemistry. It is rarely
used for quantitative determinations but widely used for the study of redox processes,
electrode mechanism, for the study of intermediate reactions and for obtaining stability of
18
reaction products. It was first practiced at a hanging mercury drop electrode [17]. It was
used by mainly when solid electrodes like platinum (Pt), gold (Au) and carbonaceous
were used, particularly to study anodic oxidations [18]. Several monographs [19, 20] and
texts [21, 22] offer excellent information on fundamentals of Cyclic Voltammetry.
Samples used in this technique should be dissolved in the liquid solvent and capable of
being reduced and oxidized within the potential range and should not react with electrode
materials. The technique involves application of potential (E) to an electrode and
monitoring the changes in the current (i) flowing through the cell and in some cases the
applied potential (E) is varied and the flow of current (i) is monitored over a period of
time (t). Thus this technique can be described as a function of E, i, and t. Concentrations
of redox species at electrode surface (Co and CR) and rate of reaction (KO) are controlled
by applied potential explained by Nernst equation or Butler-Volmer equations. In some
cases diffusion plays an important role for controlling the reaction, in such cases current
resulting from the redox process is related to material flux at the interface is described by
Fick’s law. For a reversible electrochemical reaction the application of potential E is
related to the concentrations of oxidizing element (Co) and reducing element (CR) by
Nernst equation as follows:
𝑅𝑇
𝐶
( 2.5.1)
𝐸 = 𝐸0 − 𝑛𝐹 𝑙𝑛 𝐶𝑅
0
If applied potential (E) to the electrodes is changed the fraction C R/C0 is to be
changed to satisfy the Equation (2.5.1), since E is directly proportional to the ln(CR/C0).
The ratio becomes larger when the applied potential becomes more negative and
accordingly ratio will become smaller when the potential becomes more positive. Butler-
19
Volmer equation can be used for some voltammetric techniques which relates current,
potential and concentration is given by
𝑖
𝑛𝐹𝐴
= 𝐾0 [𝐶0 exp −𝛼𝜃 − 𝐶𝑅 exp 1 − 𝛼 𝜃 ]
( 2.5.2)
Where, 𝜃 = 𝑛𝐹(𝐸 − 𝐸0 )/𝑅𝑇
This equation helps us to obtain two important values of i and K0. In most cases
the current flow also depends directly on the flux of material to the electrode surface. The
concentration gradient and mass transport is described by Fick’s law, which states the
flux of matter (Φ) is directly proportional to concentration gradient:
𝜕𝐶
Φ = −𝐴𝐷0 ( 𝜕𝑥0 )
( 2.5.3)
An analogous equation can be written for reducing element. The flux of oxidizing
and reducing elements at the electrode surface controls the rate of reaction, and thus the
faradic current flowing in the cell. In bulk solution, concentration gradients are generally
small and ionic migration carries most of the current. The current is the quantitative
measure of how fast a species is being reduced or oxidized at the electrode surface. The
actual value of this current is affected by many additional factors, most importantly the
concentration of the redox species, the size, shape, and material of the electrodes, the
solution resistance, the cell volume and the number of electrons transferred.
This technique is based on varying the voltage at the working electrode in both
forward and reverse direction. For example if the initial scan is in negative direction to
the switching potential, at that point the scan will be reversed and ran in positive
direction. Oxidation and reduction of compounds is based on the potential applied and
direction it is running. The response obtained from a CV can be very simple, as shown in
Fig. 2.5.1 for the reversible redox system.
20
Figure 2.5.1: Cyclic Voltammetry
From the graph we can clearly see that initially the scan starts at positive potential and
there is flat line for sometime due to no current which happened because applied potential
is far from reduction potential of compound. As the applied potential reaches the
reduction potential the graph starts falling down due to increase in current. This pattern is
observed until the potential reaches the reduction peak. When the potential reaches the
reduction peak the entire compound around the electrode is reduced and thereafter the
compound reduced has to diffuse towards the working electrode. So, the current drops
due to diffusion. As soon as reverse scan is started there will be a small increase in
current due to the fact that the potential is far away from reduction potential and at the
same time reduction goes on. Complete oxidation will be started when the potential
21
crosses reduction peak. Current peaks as the entire compound around electrode starts reoxidation after reduction peak. After potential passes through the oxidation peak current
falls back to zero as there is no reduction and oxidation occurs.
22
Chapter 3
Experimental Procedure
The experiment set-up described in this section served as the basis for meeting the
objective of this thesis. The experimental setup was designed to prepare nanoporous
nickel on zirconium metal. Results of this experiment would test the feasibility of
proposed process and help in identifying important process parameters that govern the
process. This in turn would help to meet objective of this thesis.
In the first part of this experiment electrochemically copper is removed from Ni-Cu alloy
wire to prepare a nanoporous nickel wire. Then both nickel and copper are electroplated
on zirconium substrate with various compositions of electrolyte and then copper is
successfully dealloyed from the surface by leaving nanoporous nickel on it. These sample
films are studied for surface morphology and chemical compositions by SEM techniques.
3.1 Materials
Various materials used in this process are described in this part.
1) Nickel sulfate hexahydrate (98%).
2) Copper sulfate pentahydrate (99%).
3) Boric acid (98%).
4) Nickel chloride hexahydrate (99.3%).
23
5) Nickel-copper alloy wire (0.5mm & 55:45 wt percent).
6) Platinum wire (0.5mm & 99.999% purity).
7) Ag/AgCl reference electrode.
Copper-nickel wire was purchased from Alfa Aesar, Ward Hill, Maryland. Platinum
wire was purchased from Chem Instrument, Austin, Texas. The Ag/AgCl electrode is
nothing but silver wire coated with AgCl. This wire is in turn sealed in glass tubing with
Teflon heat shrinkable tubing. The reference electrode compartment is filled with 1.0 M
KCl. The solution in the tubing is refillable even if the KCl leaks from the reference
electrode.
3.2 Electrochemical Dealloying of Copper
POWER
SOURCE
AMMETER
A
VOLTMETER
V
ANODE
(Ni-Cu WIRE)
CATHODE
(Pt)
ELECTROLYTE
(25%HNO3)
REFERENCE
ELECTRODE
(Ag/AgCl)
Figure 3.1.1: Setup for Dealloying of Copper
24
In this experiment copper is dealloyed to study the process and parameters affecting the
process. A nickel-copper alloy wire of 0.5 mm was purchased from Alfa Aesar is used for
this purpose. Electrochemical etching of copper is carried out in a three electrode cell
with Ag/AgCl reference electrode, Ni-Cu alloy wire as working electrode and platinum
electrode as counter electrode as shown in Figure 4.1.1. Nickel is more passive towards
the nitric acid and the oxidation of nickel avoids it from etching, so the dealloying of
nickel can be avoided by using nitric acid.
To study the rate of dealloying and formation of nanoporous nickel various experiments
with different parameters are conducted. An electrolyte of HNO3 with 25% concentration
is prepared. Electrolyte with concentration can make etching independent of power
source. Ni-Cu wire is connected to positive terminal and platinum electrode is connected
to negative terminal of the power source. Under controlled potentials generated by
positive cyclic voltammetry (CV) selective etching of copper is achieved successfully.
During the process the changes in anodic current is recorded by Xplore GLX Data
acquisition system. The experiment was carried out in two stages; in first stage time is
varied by keeping voltage constant and in later voltage is kept constant by varying time.
Voltage is varied from 0.2 V to 0.5V and time is varied from 200 seconds to 500 seconds.
The total surface area of specimen in each case is measured by Venire calipers after
running the experiment.
25
Table 3.1: Various Cases Studied for Dealloying of Copper
Case 1
Case 2
Case 3
Voltage:0.2V
Voltage:0.2V
Voltage:0.2V
Time: 200 sec
Time: 300 sec
Time: 500 sec
Length: 22.45mm Length: 21.25mm Length: 21.25mm
Case 4
Case 5
Case 6
Voltage:0.25V
Voltage:0.25V
Voltage:0.25V
Time: 200 sec
Time: 300 sec
Time: 500 sec
Length: 30.10mm Length: 30.10mm Length: 31.10mm
Case 7
Case 8
Case 9
Voltage:0.275V
Voltage:0.275V
Voltage:0.275V
Time: 200 sec
Time: 300 sec
Time: 500 sec
Length: 30.25mm
Length: 29.5mm
Length: 25.40mm
Case 10
Case 11
Case 12
Voltage:0.3V
Voltage:0.3V
Voltage:0.3V
Time: 200 sec
Time: 300 sec
Time: 500 sec
Length: 28.40mm Length: 33.50mm Length: 32.12mm
Various cases are conducted to study the effects of current and voltage on etching of
copper. These cases are shown in the above table. From the table we can see that time is
changing as we move from left to right and voltage varies as we move from top to
bottom. The optical micrograph was recorded using a ProScope HRTM purchase from
PASCO Scientific Inc to know more about the working area on the alloy wire.
26
Figure 3.1.2: Dealloying of Copper at 0.2 Volts for 200 Seconds
Figure 3.1.3: Dealloying of Copper at 0.5 Volts for 500 Seconds
27
3.3 Preparation of Nanoporous Nickel on Zirconium Substrate
From the literature review it is clear that nanoporous nickel film can be obtained by
selective etching of more active element from a homogeneous alloy. In order to obtain
this both nickel and copper alloy is selected and copper is dealloyed from the
composition. Zirconium plate of 4mm width and 0.25 mm thick is selected as a substrate.
Study of fabricating nanoporous Ni/Zr alloy by electrodeposition technique is carried in 3
cases in this experiment work. One case will study the effects of changing electrolyte and
the second one is used to study the sample film by changing the counter electrode and the
final case is to study the annealing effects on the film. A three electrode system was used
for this process as shown in Fig 3.2.1
Figure 3.2.1: Setup for Electrodeposition of Ni and Cu and Dealloying of Cu
28
Zirconium metal strip is used as working electrode, Nickel wire and platinum
electrodes are used as counter electrode and Ag/Agcl is used as the reference electrode.
Voltage is supplied by an external source called electrochemical analyzer. Electrolyte is
prepared from nickel sulfate hexahydrate or nickel chloride hexahydrate, copper sulfate
pentahydrate and boric acid. Boric acid is used to maintain the acidity level in the
electrolyte. Reactions involved in this process are
At Anode:
Ni → Ni2+ + 2eCu → Cu2+ + 2eAt Cathode
Ni2+ + 2e- → Ni
Cu2+ + 2e- → Cu
Case 1:
In this part of experiment let’s discuss various parameters and materials used.
Zirconium metal strip of 4mm width and 0.25 mm thick is used. The main difference of
this experiment with others is the counter electrode, which is platinum. Electrolyte is
prepared with 1.5 M NiSO4.6H2O, 0.1 M CuSO4.5H2O and 0.6 M BO3. Experiment is
carried at room temperature and the working area is calculated to be 60.2 mm2. Voltage is
applied to this setup with a range of -1.5 to 0.5 and a total of 60 segments to ensure a
required level of thickness is obtained. Sample graph for this experiment is shown below
29
Figure 3.2.2: Cyclic Voltammogram of Sample 1 for 1 Segment
Figure 3.2.3: Cyclic Voltammogram of Sample 1 for 60 Segments
This graph is drawn with the help of electrochemical analyzer and is between
voltage and current. From this graph we can clearly see that the current gradually
30
decreases when voltage goes from negative to positive and increases when it comes back
to negative. It remains same for all the experiments and it is shown above. The gradual
changes in the values of current can tell us about the amount of material transferred in the
solution
Case 2:
Lets discuss various parameters and materials used. Zirconium metal strip of 4mm
width and 0.25 mm thick is used. The main difference of this experiment with others is
the counter electrode, which is nickel. Nickel is used as counter electrode to maintain the
mass flow rate constant throughout the experiment. This can help us to make the film
homogeneous and rich in nickel. Electrolyte is prepared with 1.5 M NiSO 4.6H2O, 0.1 M
CuSO4.5H2O and 0.6 M BO3. Experiment is carried at room temperature and the working
area is calculated to be 60.2 mm2. Voltage is applied to this setup with a range of -1.5 to
0.5 starting at 0.5 to make sure that all the impurities from the surface are removed.
Sample graph for this experiment is shown below
Figure 3.2.3: Cyclic Voltammogram of Sample 1 for 60 Segments
31
This experiment is carried out to a total of 60 segments to ensure a required level
of thickness is obtained and The associated graph shown below is drawn with the help of
electrochemical analyzer and is between voltage and current. From this graph we can
clearly see that the current gradually decreases when voltage goes from negative to
positive and decreases when it comes back. We can see that the amount of metal
deposited and etched from the surface are more when compared with the other cases. The
graph remains same for all the experiments and it is shown below. The gradual changes
in the values of current can tell us about the amount of material electroplated and
dealloyed from the solution.
Figure 3.2.4: Cyclic Voltammogram of Sample 2 for 60 Segments
32
Case 3:
Again in this case zirconium metal strip of 4mm width and 0.25 mm thick is used. The
difference is in the electrolyte preparation. Nickel is used as counter electrode to maintain
the mass flow rate constant throughout the experiment. This can help us to make the
sample film homogeneous and rich in nickel. Electrolyte is prepared with 1.5 M
NiCl2.6H2O, 0.1 M CuSO4.5H2O and 0.6 M H3BO3. Experiment is carried at room
temperature and the working area is calculated to be 57 mm2. Voltage is applied to this
setup with a range of -1.5 to 0.5 starting at 0.5 to make sure that all the impurities from
the surface are removed. This experiment is carried out to a total of 60 segments to
ensure a required level of thickness is obtained. Sample graph for this experiment is
shown below
Figure 3.2.5: Cyclic Voltammogram of Sample 2 for 1 Segment
33
This graph is drawn with the help of electrochemical analyzer and is between
voltage and current. From this graph we can clearly see that the current gradually
decreases when voltage goes from negative to positive and decreases when it comes back.
We can see that the amount of metal deposited and etched from the surface are more
when compared with the other cases. The graph remains same for all the experiments and
it is shown below.
Figure 3.2.6: Cyclic Voltammogram of Sample 3 for 60 Segments
Samples prepared from all these cases are then prepared for the study of
morphology and composition using SEM (Scanning electron microscope) images.
3.4 Morphology and Composition Analysis
In this step of experimental procedure SEM image analysis is carried out to
extract more accurate information on characteristics and parameters of porous nickel
structure. These can be done by studying the morphology and composition analysis with
34
the help of images taken by Scanning electron microscope. The Hitachi S-4800 High
Resolution Scanning Electron Microscope is used to take images of films at various
magnitudes. The Hitachi HD-2300 is a high throughput dedicated STEM with an
accelerating voltage of 200 kV is shown in figure 4.3.1. Images at selected portion on the
film were taken at different resolutions to get the best output.
Figure 3.3.7 Hitachi S-4800 High Resolution Scanning Electron Microscope
35
Samples of these images are shown below:
(a)
(b)
Figure 3.3.8 (a, b): Shows the SEM Images Taken on Samples Prepared by Using Nickel
as Cathode and Zirconium as Anode
36
Chapter 4
Results and Discussion
Experiments were performed with the aim of depositing nanoporous nickel on zirconium
substrate of 4mm width and 0.25mm thick. In the first part lets discuss the
electrochemical dealloying of copper (Cu) from nickel-copper alloy wire. Cyclic
voltammograms were prepared in each case to identify the electrochemical reactions
related to selective dissolving process. Later in the section electrochemical deposition of
nickel and copper on zirconium substrate and selective dealloying of copper from it were
discussed. Finally characterization of these films was studied from the Scanning electron
microscope images and X-ray diffraction analysis.
4.1 Cyclic Voltammograms of Electrochemical Dealloying of Copper
Cyclic Voltammetry is the best way to know number of concepts in electrochemistry.
These graphs can be obtained from series of experiments containing measurements of
potentials with the help of a pair of chemical electrodes. In the first part of experiment,
cyclic voltammograms were prepared by dealloying copper from a nickel-copper wire.
The counter electrode is platinum and electrolyte is 25% concentrate nitric acid (HNO3).
The I~V response in electrochemical dealloying, time-dependent linear potential
waveform, anodic current density vs. time graphs were drawn in each case to study the
37
effects and changes taking place in the process. During initial stage of dealloying both
nickel and copper were dissolved into the solution, by increasing potential only copper is
etched due to the formation of nickel-oxide (NiO). So with increase in potential nickel
will become passive and etching of nobler element (copper in this case) is carried out.
38
Figure 4.1.1: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand for 200 Seconds
39
40
Figure 4.1.2: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand For 300 Seconds
41
Figure 4.1.3: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.20V of Voltageand for 500 Seconds
42
43
Figure 4.1.4: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.25V of Voltageand for 200 Seconds
44
Figure 4.1.5: Dealloying of Copper From Copper-Nickel Wire With an Electrolyte of
25%HNO3, 0.25V of Voltageand for 300 Seconds
45
46
Figure 4.1.6: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.25V of Voltageand for 500 Seconds
47
Figure 4.1.7: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.275V of Voltageand for 200 Seconds
48
49
Figure 4.1.8: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.275V of Voltageand for 300 Seconds
50
Figure 4.1.9: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.275V of Voltageand for 500 Seconds
51
52
Figure 4.1.10: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 200 Seconds
53
Figure 4.1.11: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 300 Seconds
54
55
Figure 4.1.12: Graphs Showing Dealloying of Copper From Copper-Nickel Wire With an
Electrolyte of 25%HNO3, 0.30V of Voltageand for 500 Seconds
4.2 Morphology of Nanoporous Nickel
The optical micrograph as shown in Fig.5.2 (a) was recorded using a ProScope HRTM
purchase from PASCO Scientific Inc. In the picture, the left-hand side is the Ni-Cu alloy
wire before dealloying. The right-hand side part of the picture is the material after
dealloying. From the picture shown, it can be seen that the dealloying caused significant
removal of materials from the surface layer of the Ni-Cu alloy wire. The increase in
brightness of the right hand part of the wire indicates the removal of copper because
copper is in red-yellow color, while nickel is bright.
56
In order to confirm that nanoporous nickel is formed, SEM images are shown in Fig.5.2
(b) and Fig.5.2(c). From Fig.5.2 (b), a low magnification SEM micrograph, it can be seen
that the surface of Ni-Cu alloy are porous. Such a global porous feature is due to the
passivity of the Ni-based alloy under anodic polarization. With the increase in the level of
polarization, protective oxide film (called passive film) forms at the Ni-Cu anode. Further
increase in the potential results in the anodic dissolution of the passive film. In the
transpassive region, there exists competition between dissolution and passive film
protection of the material. Thus some areas are well protected by the passive film, while
other areas have an increased materials dissolution rate. Therefore, electrochemical
dealloying is not uniform on the surface of the anode. Eventually, the passivation film is
completely dissolved after the potential increased to a sufficiently high value. At higher
magnification, the nano-structures can be seen, as shown by the electron microscopic
image of Fig.5.2(c). From this SEM image, the micro-scale pores (dark regions of celllike features) are found around 5 microns in diameters. The walls of the cell-like features
contain numerous small ligaments and voids. These small ligaments and voids are
nanostructure. The dimensions of these structures are in the range of several hundred
nanometers.
57
Figure 4.2.1: Microscopic Analysis of the Dealloyed Ni-Cu Alloy; (a) Optical
Micrograph at 50X (The left part is before dealloying; the right part is
after dealloying.), (b) Low Magnification SEM Image Showing the
Global Porous Feature, (c) High Magnification SEM Image Showing the
Nanostructures of the Dealloyed Ni-Cu, (d) EDX Spectrum
Elemental analysis was conducted using energy-dispersive X-ray spectroscopy. As shown
in Fig.4.2 (d), the major composition of dealloyed materials is nickel (two high peaks
located at 0.8 keV and 7.8 keV). This confirms the formation of porous nickel. However,
very weak signals from copper residue (small peaks at 1.0 keV, 8.1 keV and 9.0 keV) and
58
very small amount of oxygen (a very small peak at 0.2 keV) were found in the EDX
spectrum. From the quantitative analysis (converting the atomic ratio to the weight
percentage), it is found that the relative composition in weight percent is
Ni:Cu:O=92.4%:7.2%:0.4%. This indicates that some copper still left after dealloying.
Fully dealloying to remove copper completely is unlikely to be done using this method.
Multicyclic electrochemical alloying/dealloying processes may be used to fully dealloy
an element. It should be pointed out that even if the surface dealloying was completed,
there should still be some signal for copper in the EDX spectrum due to the possible
response of copper from the Ni-Cu alloy underneath the surface layer, especially when
the acceleration voltage of the SEM used for EDX analysis is high enough to activate a
relatively large volume of the material. The existence of trace of oxygen is reasonable
because it comes from the anodic reaction of oxide film formation on the surface of the
electrode.
4.3 Electrodeposition of Ni and Selective Etching of Copper on Zirconium Substrate
Electrochemical behaviours related to the deposition of both nickel and copper and then
dealloying of copper were monitered and recorded by electrochemical analyser purchased
from Ch Instruments.As discussed earlier the experimental procedure is divided into three
cases with changes in counter electrode and electrolyte.
Case 1:
First case results are obtained by using 1.5 M NiSO4, 0.1 M CuSO4, 0.6 M H3BO3,
Zirconium strip as working electrode and Platinum wire as counter electrode. The
electrochemical behaviour of electroplating of Nickel and Copper on zirconium substrate
and selective etching of Copper is monitered with the help of electrochemical analyzer.
59
Figures 4.3.1, 4.3.2, 4.3.3a, 4.3.3b shown below are various plots showing time
dependent current density, time dependent voltage and current density vs voltage. In this
case triangular wave varrying between -1.5 V to 0.5 V. Cyclic voltammetry was
performed to understand the electrochemical behaviour of Nickel and Copper on
Zirconium substrate. From the literature review it is clear that the deposition and etching
of material on a working electrode is directly proportional to the flow of current between
the two electrodes, distance between the working and counter electrode and the
concentration of electrolyte. In the first case the counter electrode is platinum. From the
figure 4.3.1 we can clearly see that the current varies proportional to the voltage and it
changes from -0.04A to 0.09A in which the positive current belongs to the etching of
copper and negative part of current belongs to the deposition of both copper and nickel.
From the figure 4.3.2 we can see that applied potential between electrodes varies
accordingly.
Figure 4.3.1: Graph Showing the Change of Current Density With Respect to Time Using
Platinum Electrode
60
Figure 4.3.2: Graph Showing the Change of Voltage With Respect to Time Using
Platinum Electrode
Current Density (A/m2)
1500
1000
500
0
-500
-1000
-1.5
-1
-0.5
Voltage (V)
0
0.5
Figure 4.3.3 a): Cyclic Voltammogram Showing the Changes of Current Density With
Respect to Voltage Using Platinum Electrode for 1 Segment
61
Current Density (A/m2)
2000
1500
1000
500
0
-500
-1000
-1.5
-1
-0.5
Voltage (V)
0
0.5
Figure 4.3.3 b): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Voltage Using Platinum Electrode for 60 Segments
Cyclic voltammetry was perfomed to understand the electrochemical behaviour of Ni and
Cu species on zirconium electrode. The cyclic voltomogram recorded at an zirconium
electrode for a solution containing nickel and copper is shown in figure 4.3.3. On the
cathode potential of -1.5V, the reduction of nickel occurs and it will be gradually
decreases as the voltage becomes anodic. Thus there will be deposition of nickel is more
when the voltage is more cathodic. As the voltage approaches -1 there will be low
reduction of nickel and the reduction of copper starts at this point and it decreases as the
voltage reaches 0V.In the anoidic potential there will be a little oxidation of nickel
because of passivation of nickel. As the voltage becomes more anodic the oxidation of
copper starts at 0.1 V and it will reaches to a maximun at 0.5V. In the reverse scan due to
less avaialbility of deposited copper and due to moving away from oxidation peak the
etching of copper is decreased. When the voltage approaches reduction peak of copper i.e
62
-0.45 there will be deposition of copper and as the voltage moves away from it and
towards the reduction peak of nickel the deposition of nickel has become prominent.
From the graph since energy associated with the oxidation of copper is more when
compared to reduction of copper, it is suggested that the selective anodic stripping of
copper is done.Due to passivation of nickel and etching of more nobler copper,
nanoporous nickel on zorconium is successfully achieved.
Case 2:
As we discussed earlier this set of experiment is carried out by changing the counter
electrode from platinum to nickel. This change is made to study the electrodeposition of
nanoporous nickel on zirconium substarte with nickel counter electrode and to maintain
the equilibrium of mass tranfer in the system. With this change we can clearly see that
there is a constant flow of current in the system, where as there is slightly increase in
current in previous case. From the Figures 4.3.6 a, 4.3.6 b we clearly see that the
procedure is unifrom and steady through out the process with less variation. With this
change we obtained a current density is constant between 1000 A/m2 and -500 A/m2,
where as in the previous case it varyied between 1200 A/m2 and -600 A/m2. This
studyness in the results are obtained from the equilibrium of mass transfer by keeping the
amount of nickel in the electrolyte constant at all the time. From the cyclic
voltammogram we can also see that the reduction peak associated with the copper is
slightly lower when compared with the previous case, thus it is clear that there is an
increase in the percentage of nickel present in the sample film.
63
Figure 4.3.4: Graph Showing the Change of Current Density With Respect to Time Using
Nickel Electrode
Figure 4.3.4: Graph Showing the Change of Current Density With Respect to Time Using
Nickel Electrode
64
1200
Current Density (A/m2)
1000
800
600
400
200
0
-200
-400
-600
-1.5
-1
-0.5
Voltage (V)
0
0.5
Figure 4.3.6 a): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Elctrode for One Segment
Figure 4.3.6 b): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode for 60 Segments
65
Case 3:
In this case the electrolyte is made with 1.5 M NiCl2. 6H2O, 0.1 M CuSO4.5H2O and 0.6
H3BO3.Due to the presence of nickel chloride in the electrolyte the deposition process is
rapid and material deposition is random leaving a bulk material at few places. From the
time dependent current density graph (Fig 4.3.7), we can clearly see that there is an
increase in current density from 1000 A/m2 to approximatley 1800 A/m2. We can also see
that the current is not linear with the potential applied between electrodes from figure
4.3.7.
Figure 4.3.7: Graph Showing the Change of Current Density With Respect to Time Using
Uickel Electrode and Nickel Chloride Instead of Nickel Sulphate in
Electrolyte
66
Figure 4.3.8: Graph Showing the Change of Voltage Density With Respect to Time
Using Nickel Electrode and Nickel Chloride Instead of Nickel Sulphate in
Electrolyte
2000
Current Density (A/m2)
1500
1000
500
0
-500
-1000
-1500
-2000
-1.5
-1
-0.5
Voltage (V)
0
0.5
Figure 4.3.9 a): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode and Nickel Chloride Instead
of Nickel Sulphate for one Segment
67
Figure 4.3.9 b): Cyclic Voltammogram Showing the Change of Current Density With
Respect to Volatge Using Nickel Electrode and Nickel Chloride Instead
of Nickel Sulphate for 60 Segments
From the cyclic voltammogram, when the cathodic potential is -.15 V the current is very
high until potential reaches to -1.2 V. When the potential reaches to 0V from -1.2 V the
current rapidly decreases to 0A. In the anodic poteantial range ( from 0V to 0.5V) the
change in current is again rapid. On the reversal scan, change in current is very low when
compared to the forward scan. When the anodic potential reaches to -0.2V etching of
nickel occurs and there is an increase in current upto a potenatial of -0.5v. After -0.5V the
current decreases as we move away from oxidation poteantial of nickel.When the anodic
potential is -0.8V etching of copper starts and increases with voltage. Sample film
prepared from this process had rapid deposition and dealloying of materials leaving bulk
materials at several places.
68
4.4 Morphology of Ni-Zr Film
Case 1:
In order to study the morphology of these sample films, SEM images were
captured by using an Hitachi HD-2300 Scanning electron microscope at various
magnitudes. These images were taken at different positions on the film to study the
porosity, film and substrate interface etc. Due to the presence of conducting materials in
the film a bright light is observed in the images. Images are also taken from a range of
2.00µm to 1 mm to obtain vital information. films in this case are prepared from a
electrolyte solution containing 1.5 M NiSO4 , 0.1 M CuSO4 and 0.6 M H3BO3.
Nanoporous nickel formed on zirconium substrate can be observed by SEM
images as shown in Fig 4.4.1(a,b), Fig 4.4.2(a,b), Fig 4.4.3(a,b). It is noted that the
surface of film is both porous as well as cracked in certain areas. During the initial stages,
the deposition is uniform all over the surface of the substrate but as the number of
segments increase the deposition is increased in certain areas leading to a crystal
structure. Large number of ligaments and voids were found on the film as shown in Fig
4.4.1. Figures 4.4.2, 4.4.3 shows the images of low magnification when compared with
previous ones. In these pictures we can see a dense crystal structure surrounded by a
cracked surface. Working area and substrate interface is clearly identified in figure 4.4.4.
From all these SEM images it observed that the electrodeposited material is crystalline
structure surrounded by a solid cracked surface. This feature will helps us to increase the
surface area by more than 50%, which in turn increases the current density when used as
anode material in solid oxide fuel cell.
69
(a)
(b)
Figure 4.4.1(a,b): SEM Images at 25.0KX and 11.0KX Magnitude on a Film Prepared
From the Electrolyte Solution Containing Nickel Sulphate and
Copper Sulphate Using Platinum Electrode
(a)
(b)
Figure 4.4.2(a,b): SEM Images at 3.50KX and 1.20KX Magnitude on a Film Prepared
From the Electrolyte Solution Containing Nickel Sulphate and
Copper Sulphate Using Platinum Electrode
70
(a)
(b)
Figure 4.4.3(a,b): SEM Images at 3.5KX and 2.2KX Magnitude on a Film Prepared From
the Electrolyte Solution Containing Nickel Sulphate and Copper
Sulphate Using Platinum Electrode
(a)
(b)
Figure 4.4.4(a,b): SEM Images at 45X Magnification of a Sample Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate
Using Platinum Electrode
71
(a)
(b)
Figure 4.4.5(a,b): SEM Images at 450X and 600X Magnitude on a Film Prepared From
the Electrolyte Solution Containing Nickel Sulphate and Copper
Sulphate Using Platinum Electrode
(a)
(b)
Figure 4.4.6(a,b): SEM Images at 1.80KX Magnification of a Sample Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate
Using Platinum Electrode
72
Case 2:
As discussed in previous chapters this case of experiment is carried out with electrolyte
solution containintg 1.5 M NiSO4, 0.1 M CuSO4, and 0.6 M H3BO3 with nickel as
countered electrode.With the change in counter electrode from platinum to nickel, the
mass transfer in maintained at equilibrium resulting in a smooth surface without crakes.
The only change observed in this case from the previous one is obsence of crakes and
increase in desnity of crystal structure. Figure 4.4.7 shows us a high magnified image
where as figure 4.4.8(a) shows a low maginified image. This type of experimental setup
is recommended for the preparation of anode material of solid oxide fuel cell due to the
formation of smooth and densed surface.
(a)
(b)
Figure 4.4.7(a,b): SEM Images at 4.0K X and 2.0K X on a Film Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper Sulphate
Using Nickel Electrode
73
(a)
(b)
Figure 4.4.8(a,b): SEM Images at 60X and 2.20KX on a Film Prepared From the
Electrolyte Solution Containing Nickel Sulphate and Copper
Sulphate Using Nickel Electrode
Case 3:
These SEM images are captured from a sample film which is prepared by using an
electrolyte containing 1.5 M NiCl2.6H2O, 0.1 M CuSO4 and H3BO3 with nickel as
counter electrode. Heat treatment was carried out on this film to make the material strong
and to make small grains combine and became to large grains. This method is also used
to remove or lessen the internal stresses formed due to the electrodeposition process.
Figure 4.4.9(a) shows us the interface of film and substarte and it is seen that the material
is gouped together at different places due to the effect of heat traetment. All the images
shown in figures 4.4.9 and 4.4.10 has crakes due to the effect of heat treatment which is
carried out by heating film to 3000C for 30 min and leaving it to be cooled on natural air.
This process makes the material to become less porous and more hardened.
74
(a)
(b)
Figure 4.4.9(a,b): SEM Images at 30X and 250X on a Film Prepared From the Electrolyte
Solution Containing Nickel Chloride and Copper Sulphate Using
Nickel Electrode
(a)
(b)
75
(c)
Figure 4.4.10(a,b,c): SEM Images 1.10KX, 1.20KX and 300X Magnifications of a Film
Prepared From the Electrolyte Solution Containing Nickel
Chloride and Copper Sulphate Using Nickel Electrode
4.5 Composition Analysis of Ni-Zr Film
Case 1:
Elemental analysis was conducted using X-ray diffraction technique. As shown in
Fig 4.5.1,the major composition of film materials is nickel( as shown in bar graph). This
confirms the formation of nanoporous nickel. However, very small amount of copper and
oxygen were found in the spectrum area. The existence of trace of oxygen is reasonable
because it comes from the anodic reaction of oxide film formation on the surface of the
electrode.A small area conatianing the grain or crystal structure is selected for
composition analysis in second case as shown in Fig 4.5.2. In this case a large amount of
copper and a small amount of nickel, oxygen and carbon is observed. This change in
compisition is occurred due to the selection of copper rich area.From the quantitative
analysis shown in Fig 4.5.3, it is found that the relative composition in weight percent of
76
nickel is more than copper and oxygen. Fully dealloying to remove copper completely is
unlikely to be done using this method.
Figure 4.5.1: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 with
Platinum Electrode
Figure 4.5.2: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With
Platinum Electrode
77
Table 4.5.1: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 with
pltainum electrode
Processing option: All elements analyzed
All results in weight %
Spectrum
Spectrum
1
Spectrum
2
In Stats
C
O
Ni
Cu
Total
Yes
20.93
52.16
84.12
439.72
596.94
144.21
136.27
9.71
290.19
Yes
Max
20.93
144.21
136.27
439.72
Min
20.93
52.16
84.12
9.71
Case 2:
This case of composition analysis was carried out on a film made from an
electrolyte solution containing 1.5 M NiSO4.6H2O, 0.1 M CuSO4.5H2O and 0.6 H3BO3
and an counter electrode of Nickel.In this case composition analysis was carried out on
both grain structure and solid base. From the figures 4.5.4 and 4.5.5, the composition of
copper is more when compared to nickel due to the passivation of nickel in last segments.
Very few quantity of oxygen is present in the film because of oxidation of nickel as
discussed in previous cases. The over all composition when considering the two small
selected area, the composition of copper is more when compared to nickel in this case
78
along with small amounts of oxygen,carbon and aluminium.The presence of carbon and
alluminium was mainly due to the contamination of electrolyte or film after the process.
Figure 4.5.3: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 with
Nickel Electrode
Figure 4.5.4: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With
Nickel Electrode
79
Table 4.5.2: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiSO4, 0.1 M CuSO4 With
Nickel Electrode
Processing option: All elements analyzed
All results in weight %
Spectrum
Spectrum
1
Spectrum
In
Stats
C
Yes
Al
26.49
Ni
Cu
Zr
Total
65.66
283.76
4.93
380.84
88.81
24.01
9.33
0.89
5.75
16.65
32.18
Max
24.01
26.49
0.89
65.66
283.76
32.18
Min
24.01
9.33
0.89
5.75
16.65
4.93
2
Yes
O
Case 3:
The sample film in this case are prepared from an electrolyte containing 1.5 M
NiCl2.6H2O, 0.1 M CuSO4 and H3BO3 with nickel as counter electrode. This film are
then heat treated to reduce the residual stresses and to harden the material. As shown in
figure 4.5.7 and 4.5.8, the major composition of film is nickel with minute quantiities of
oxygen, copper, aluminium, nitrogen, chloride. Presence of chlorine is observed in the
sample due to the contact of film and electrolyte through out the process. Special
80
cleaning procedure can be adopted to remove the chlorine from film.From the
morphology and composition analysis results we can clearly see that nanoporous nickel is
prepared by this technique. Multicyclic electrochemical alloying/dealloying process may
be used to fully dealloy copper.
Figure 4.5.5: Composition Analysis Carried Out at Spectrum 1 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO4 With Nickel
Electrode
Figure 4.5.6: Composition Analysis Carried Out at Spectrum 2 on a Film Prepared With
an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO2 With Nickel
Electrode
81
Table 4.5.3: Chart Showing Weight Percentage of Different Elements on Film Prepared
With an Electrolyte Solution Containing 1.5 M NiCl2, 0.1 M CuSO4 With
Nickel Electrode
Processing option: All elements analyzed (Normalized)
(All results in weight %)
Spectrum
In Stats
Spectrum 1
Yes
Spectrum 2
Yes
N
O
Al
Cl
29.36
Ni
Cu
60.24
10.40
Zr
100.00
4.89
14.60
0.48
4.32
5.34
1.89
68.47
Max
4.89
29.36
0.48
4.32
60.24
10.40
68.47
Min
4.89
14.60
0.48
4.32
5.34
1.89
68.47
82
Total
100.00
Chapter 5
Conclusions
Nanoporous nickel has been successfully prepared using electrochemical dealloying
method. The concentration of nitric acid is kept constant to better understand the effect of
voltage and time on the dealloying process. The applied potential is varied from 0.2 V to
0.5V while changing time period from 200 sec to 500 sec. In all the cases the change in
current or material dealloyed is always same. The effect of time change from 200 sec to
500 sec and voltage change from 0.2V to 0.5V is almost negligible.
An experimental study was conducted to explore the electrodepositing of the nanoporousNi on zirconium substrate. Compositionally modulated crystalline or grain like structures
were obtained using cyclic voltammetry. Other variations in the deposition procedure
included change in the counter electrode between platinum and nickel, change in the
electrolyte from NiSO4.6H2O to NiCl2.6H2O. The Ni deposition potential was between 1.5V and -0.9V and Cu deposition potential was between -0.8V and -0.2V. The
dealloying potential of Cu was between 0V and 0.5 V. The nanoporous nickel film
prepared from a electrolyte solution of 1.5 M NiSO 4.6H2O, CuSO4.6H2O and with
platinum electrode were found to have better uniform crystal structure than other
methods. Ni- rich nanoporous alloy films were fabricated under cyclic voltammetry.
83
The morphology of the sample film was studied using SEM images, which gave us better
understanding about the structure of deposited material. These SEM images confirm the
formation of rich Ni nanoporous nickel on zirconium substrate. Crystalline or grain like
structural growth is observed in all the cases. Few cracks were also found in the film
deposited by using platinum counter electrode. These cracks are eliminated by using
nickel as counter electrode which maintains mass equilibrium in the system. Film
deposited by using nickel chloride hexa-hydrate in electrolyte has irregular deposited
sites with non-homogenous structure. The dimensions of these pores are in the range of
several hundred nanometers. This method of experiment may be used to electroplate
nanoporous nickel on the outer surface of anode of solid oxide fuel cell to obtain surface
area increase.
Elemental analysis was conducted using X-ray diffraction technique to better understand
the composition of each element. These results from all the cases clearly show us that the
amount of nickel in the film was more than 50 % by weight when compared with all
other elements. Minute quantities of elements like oxygen, carbon, aluminum etc were
present in the film due to the contamination of solution.
84
Chapter 6
Future Work
Even though it has been shown that it is possible to produce an active and selective Ni-Zr
anode for solid oxide fuel cell using electrodeposition there is further work to be done.
1. The thickness of the deposited layer and the durability of the deposited surface for
the required application should be investigated.
2. Investigation need to be done on these films to completely dealloy copper from it.
3. Thermal analysis should be carried out to better understand the stresses and
strains induced in it at high temperatures
4. The effects of H2O and CO2 should be investigated independently to further
clarify their effects on catalyst performance.
85
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