Preparation ò controlled one dimensional silver and silver palldium cỏe shell nanostructures and their electrocatalytic performance for ethanol oxidation in alkaline direct alcohol fuel cells

The major purpose of thesis is to synthesize the Ag@Pd nanomaterial, investigate the effects of synthesis conditions on the formation of Ag nanowire the core and Pd shell and preliminari

VIETNAM NATIONAL UNIVERSITY HCMC UNIVERSITY OF TECHNOLOGY

ALKALINE DIRECT ALCOHOL FUEL CELLS

Speciality: Chemical Engineering

Code: 60520301

MASTER’S THESIS

First, I would like to express my deep sense of gratitude to my supervisor, Dr Nguyen Truong Son, for his encouragement, great support and valuable advice for finalization of my thesis His knowledge about fuel cells help me grow my potential

I sincerely would like to thank M.Sc Nguyen Truong Xuan Minh for providing excellent material, experimental techniques and great mentor in the development and conclusion of this thesis

I am very grateful to M.Sc Luu Hoang Tam and Ms Nguyen Phuc Thanh Duy for supporting me of equipment as well as knowledge of operation

I would like to extend my sincere thanks to Assoc Prof Le Minh Vien and all group members at Inorganic Laboratory for their equipment support and warm hospitality

I would like to appreciate Mr Thomas Ng and Bronx Creative & Design Centre for their technical and equipment support

I would like to thank Vietnam National Foundation for Science and Technology Development (NAFOSTED) for the financial support under grant number 104.05-2017.34

Last, but not least, I would like to thank my parents, my sister, my friends, for their unconditional support, encouragement and love, and without which I would not have come this far

Fossil fuel reserves are likely depleting and their combustion produces a lot of emission gas Therefore, development of a new energy resource burning a low CO2 gas is an essential mission For decades, studies of alternative energy sources was extending, in which fuel cells have attracted more attention as effective power conversion In fuel cells, chemical reactions convert to electric energy Moreover, the gas emission of devices is low polluting This technology has great potential to become the major power in the future Platinum-based commonly catalyze in fuel cell reaction, but the platinum metal is high cost and low quantity Recently, there have been more and more studies on palladium, which is as active as platinum for electro-oxidation reactions [2] Especially, palladium displays as a good catalyst for ethanol electro-oxidation in alkaline media In this work, Pd modified with Ag to form core-

shell nanowire structures to improve the stability, the oxidation and the cost

The major purpose of thesis is to synthesize the Ag@Pd nanomaterial, investigate the effects of synthesis conditions on the formation of Ag nanowire (the core) and Pd shell and preliminarily examine their catalytic activity for the ethanol oxidation in alkaline media (crucial reaction in alkaline direct ethanol fuel cells) In this study, the Ag nanowire was synthesized by the polyol method Using sodium chloride and sodium bromide formed long and thin Ag nanowires The characterization of Ag nanowires carried out by the transmission electron microscopy(TEM) and X-ray Diffraction (XRD) The results showed that a AgNO3 – PVP molar ratio of 1:1.5, a temperature of 150oC and a reaction time of 2h were the appropriate condition for the formation of Ag nanowires (AgNWs) with the length

of 6 to 10 µm, and the diameter of 45 to 55nm

Continuously, the AgNW covered by a thin layer of Pd through sequential reductionaccompanied by the galvanic displacement reaction The products showed the Ag@Pd core-shell structures, as demonstrated by characterization High-resolution transmission electron microscopy (HRTEM) and energy dispersive spectrometry (EDS) demonstrated core-shell structure clearly Cyclic voltammetry (CV) test used to examine the catalytic activity of the samples and compare their activity to that of Pd nanocatalyst

ra một lượng lớn khí thải, vì vậy phát triển một nguồn năng lượng mới là một nhiệm

vụ quan trọng Trong những năm gần đây nghiên cứu các nguồn năng lượng mới đang được thúc đẩy trong đó pin nhiên liệu thu hút được nhiều sự quan tâm như một thiết

bị chuyển đổi năng lượng hiệu quả Năng lượng hóa học sẽ được chuyển đổi trực tiếp thành năng lượng điện thêm vào đó khí thải do thiết bị tạo ra là không đáng kể Kỹ thuật này có tiềm năng to lớn để trở thành nguồn cung cấp năng lượng trong tương lai Tuy nhiên thiết bị này hoạt động ở nhiệt độ cao làm hạn chế ứng dụng của chúng Khi sử dụng xúc tác thì nhiệt độ làm việc của pin nhiên liệu sẽ giảm Các xúc tác trên nền tẳng platin thường được sử dụng cho phản ứng trong thiết bị này nhưng platin có giá cao và trữ lượng thấp Có nhiều nghiên cứu chứng minh hoạt tính xúc tác của Paladi gần giống với Platin Nội dung chính của luận văn là tổng hợp vật liệu có cấu trúc dây nano dạng lõi vỏ, đây là một cấu trúc vật liệu tốt có khả năng ứng dụng để làm xúc tác trong pin nhiên liệu

Mục tiêu chính của luận văn tốt nghiệp là tổng hợp vật liệu có cấu trúc lõi vỏ và kiểm tra sơ bộ hoạt tính của vật liệu trong phản ứng oxi hóa ethanol trong môi trường kiềm.( phản ứng chính trong pin nhiên liệu kiềm) Dây nano bạc được tổng hợp bằng phương pháp polyol Sử dung natri clorua và natri bromua để tạo các dây nano dài hơn và mỏng hơn Qua quá trình tổng hợp tìm được điều kiện thích hợp để tạo ra các dây nano có đường kính từ 45-55nm, chiều dài từ 7-10µm ở nhiệt độ 150oC và thời gian phản ứng là 2h

Sau đó các dây nano sẽ được phủ một lớp paladi bằng phương pháp trao đổi ion tạo ra cấu trúc lỏi võ như mong muốn Các phương phân tích hiện đại được áp dụng

để chứng minh vật liệu đảm bảo các đặc tính Mẫu Ag-Pd lõi vỏ ở các tỷ lệ 6:100, 8:100, 10:100, 12:100 14:100 được tổng hợp Sử dụng kính hiển vi điện tử truyền qua

độ phân giải cao (HRTEM) và phổ tán sắc năng lượng tia X(EDS) để xác định cấu trúc vật liệu Hoạt tính xúc tác cho phản ứng oxy hóa ethanol của vật liệu được đo bằng điện thế quét(CV) Bên cạnh đó là so sánh hoạt tính của mẫu với paladi ở dạng nano

I declare that “Preparation of controlled one dimensional silver and

silver@palladium core-shell nanostructures and their electrocatalytic performance for ethanol oxidation in alkaline direct alcohol fuel cells ” thesis

was an original report of my research, which had write by me and submitted for any previous degree The experimental work is entirely my own work; the collaborative contributions indicated and acknowledged, clearly References provided on all supporting literatures and resources

TABLE OF CONTENTS

ACKNOWLEDGEMENT

SUMMARY

DECLARATION

TABLE OF CONTENTS 1

LIST OF FIGURES 3

LIST OF TABLES 5

ABBREVIATION 6

INTRODUCTION 7

CHAPTER 1 LITERATURE REVIEW 9

1.1 Overview 9

1.2 Overview of fuel cells and direct ethanol fuel cells 12

1.2.1 Fuel Cells 12

1.2.2 Direct ethanol fuel cells 13

1.2.3 DEFCs and challenges 15

1.3 Review of core-shell one-dimensional (1-D) nanostructure 17

1.3.1 One-dimensional (1-D) nanostructure 17

1.3.2 Core-shell one-dimensional (1-D) nanostructure 19

1.4 Review of Ag@Pd core-shell nanowires 24

1.4.1 Silver's Properties, Characteristics and Applications 24

1.4.2 Review of silver nanowires 24

1.4.3 Synthesis of silver nanowires 27

1.4.4 Palladium's Properties, Characteristics and Applications 28

1.4.5 Galvanic displacement method 30

1.4.6 Improving catalytic activity of Ag on ethanol oxidation 32

1.5 Applications and related studies 33

1.6 Electrochemical measurement 37

1.6.1 Cyclic voltammetry (CV) test 37

1.6.2 Mechanism of oxidation reaction on palladium in alkaline media 38

1.6.3 Mechanism of ethanol oxidation reaction on palladium in alkaline media 39

1.6.4 Electrochemical active surface area (ECSA) measurements 39

1.7 The scope of thesis 40

CHAPTER 2: EXPERIMENT 41

2.1 Chemicals 41

2.3 Experiment procedure 41

2.3.1 Synthesis of silver nanowires 41

Synthesis of silver nanowires 42

2.3.2 Synthesis of Ag@Pd core-shell nanowire 45

2.4 Characterization of Ag@Pd core-shell nanowires 47

CHAPTER 3: RESULTS AND DISCUSSION 48

3.1 The result of silver nanowires 48

3.1.1 Effect of PVP concentration on silver nanowire formation 48

3.1.2 Effect of AgNO 3 concentration on silver nanowire formation 49

3.1.3 Effects of reaction temperature on silver nanowire formation 50

3.1.4 Effects of reaction time on silver nanowire formation 51

3.1.5 XRD diagram of the silver nanowire 51

3.1.6 The optimal conditions synthesize the Ag nanowires 52

3.2 The result of Ag@Pd core - shell nanowire 53

3.2.1 Preparing Pd(NO 3 ) 2 and Ag nanowire in deionized water to synthesize the core-shell structure 53

3.2.2 Preparing Pd(NO3) 2 in ethylene glycol on the formation of Ag@Pd core shell nanomaterial 54

3.2.3 Preparing Pd(NO 3 ) 2 in deionized water to synthesize Ag@Pd core-shell nanowires 54

3.2.4 XRD pattern of Ag@Pd core- shell nanostructures 56

3.2.5 EDS analysis of Ag@Pd core-shell nanowire 57

3.2.6 HRTEM image of Ag@Pd core-shell nanowires 58

3.2.7 Distribution of material on Vulcan XC-72R 59

3.3 Performance of the synthesized Ag@Pd for ethanol electro-oxidation in alkaline media 59

3.3.1 Preparation for cyclic voltammetry(CV) test 59

3.3.2 Cyclic voltammetry(CV) result of silver nanowire 60

3.3.3 The catalytic activity of samples in 1M KOH + 1M C 2 H 5 OH 61

3.3.4 Comparison to CV plot of Ag@Pd nanowires to Pd/C nanoparticle in 1M KOH 63

3.3.5 Comparison to CV plot of Ag@Pd nanowires to Pd nanocatalyst in 1M KOH and 1M C 2 H 5 OH 63

CHAPTER 4: CONCLUSION AND RECOMMENDATION 65

4.1 Conclusion 65

4.2 Recommendation 66

LIST OF PUBLICATIONS 67

REFERENCES 68

LIST OF FIGURES

Fig 1.1 The image shows a reaction flowchart for direct ethanol fuel cells 16

Fig 1.2 The complete electro-oxidation of ethanol in ADEFC produce [3] 17

Fig 0.1 Schematic representation of 1D nanostructures From top to bottom: nanowire, nanorod, nanotube, and nanobelt nanoribbon……….…19

Fig 1.4 Concentric spherical core-shell nanoparticles [10] 20

Fig 1.5 Scanning electron microscopy (SEM) images of ZnO/CdSe nanowire arrays after different annealing treatments: 1 h at 250°C: (a) plan view and (b) cross section; 1 h at 400 °C : (c) plan view and (d) cross section; 1 h at 350°C plus 1 h at 400 °C: (e) plan view and (f) cross section In Figure 1b the circled area shows the local thickness of CdSe nanowire shell 21

Fig 1.6 Typical SEM images of Sn/Pt nanotube arrays 22

Fig 1.7 Schematic view of Ge-Si core-shell nanowires (a) Side view of Ge-Si core- shell nanowire; (b) top view of Ge-Si core-shell nanowire[14] 23

Fig 1.9 a) Schematic of the displacement process Transmission electron microscopy images of material, b) Ag nanowire, c) corresponding Pt nanotube following galvanic displacement[19] 32

Fig 1.10 Typical cyclic voltammogram where i pc and i pa show the peak cathodic and anodic current respectively for a reversible reaction 37

Fig 1.11 The cyclic voltammogram of the Pd/C in 1M KOH solution (scan rate: 20 mV s −1 ) [3] 38

Fig 1.12 The cyclic voltammogram of the Pd/C in 1M KOH + 1M EtOH solution 39

Fig 2.1 The synthsis of Ag nanowire 43

Fig 2.3 The synthesis of Ag@Pd core-shell nanowires 46

Fig 3.1 The TEM images of as-synthesized products with different PVP amounts of 0.3(a), 0.4(b), 0.5(c), 0.6(d), 0.65(e) and 0.7(f)M 48

Fig 3.2 The TEM images of the as-synthesized silver nanowires with AgNO 3 concentrations of 0.1(a), 0.2(b), 0.3(c), 0.4(d), 0.5(e), 0.6M(f), respectively 49

Fig 3.3 (a–e) depict the TEM images of the products obtained at 140(a), 150(b), 160(c), 170(d) and 180 o C(e), respectively 50

Fig 0.2 The TEM images of the silver nanowires synthesized with reaction times of 60(a), 90(b), 120(c), 150(d) and 180(e) minutes……….51

Fig 3.5 XRD diagram of the silver nanowires 52

Fig 3.6 The result of synthesis of optimal condition 53

Fig 3.7 Image TEM of the Pd weren't successfully used to coat the surface of the Ag nanowire 53

Fig 3.8 Presented representative TEM images of the as-synthesized Ag@Pd nanomaterial. 54

Fig 3.9 Ag nanowires and the TEM images of the Ag@Pd core-shell nanowires synthesized with the ratio of 6:100, 8:100, 10:100, 12:100, 14:100, respectively 55

Fig 3.10 Distribution of Ag@Pd core-shell nanowires in 5µm scale 56

Fig 3.11 XRD pattern of Ag@Pd core- shell nanostructures 57

Fig 3.13 The HRTEM image of Ag@Pd core – shell nanowire 58

Fig 3.14 Distribution of Ag@Pd material in Vulcan XC-72R 59

Fig 3.15 Detector, sample, working electrode of CV test 60

Fig 3.16 CV plots of Ag in 1M KOH and 1M KOH + 1M C 2 H 5 OH with a 61

Fig 3.17 Linear sweep voltammograms for ethanol oxidation on in 1M KOH + 1M C 2 H 5 OH with 50 mV/s scan rate with the ratio of 6:100(CS1), 8:100(CS2), 10:100(CS3), 12: 100(CS4), 14:100(CS5) and Pd nanocatalyst 62

Fig 3.18 CV plot of Ag@Pd nanowire(the ratio of 10:100) and Pd nanocatalyst 63

Fig 3.19 CV plot of Ag@Pd nanowires to Pd nanoparticle in 1M KOH and 1M C 2 H 5 OH 64

LIST OF TABLES

Table 1.1 Performance of various types of power generation[8] 12

Table 1.2 Fuel cells classification 13

Table 1.3 The nobility of metals follows their standard redox potential 31

Table 2.1 Detailed conditions of experiments in polyol method 44

Table 2.2 Detailed conditions of experiments Pd@Ag 45

ABBREVIATION

1D: One-Dimensional

ORR: Oxygen Reduction Reaction

HOR: Hydrogen Oxidation Reaction

DAFCs: Direct Alcohol Fuel Cells

DEFC: Direct Ethanol Fuel Cells

AEM: Alkaline Anion Exchange Membrane

GNs: Graphene Nanosheets

EG: Ethylene Glycol

PVP: Poly (Vinyl) Pyrrolidone

SEM: Scanning Electron Microscope

TEM: Transmission Electron Microscopy

HRTEM: High-resolution Transmission Electron Microscopy EDS: Energy dispersive spectrometry

XRD: X-ray Powder Diffraction

CV: Cyclic Voltammetry

INTRODUCTION

Development of heavy industries uses a lot of energy Power requirements are constantly increasing Fossil fuel is the main energy for engines but the product of combustion is carbon dioxide causing air pollution The problem go from bad to worse, emissions rise leads to environmental risks as the greenhouse effect, climate change… Moreover, fossil fuel is limited and not regenerated Nuclear power has enormous potential, in terms of total life-cycle greenhouse gas emissions per unit of energy generated, nuclear power has emission values comparable or lower than organic energy, but nuclear power poses many threats to people and the environment The demand for clean energy is an urgent mission

Fuel cells have attracted more attention with various applications, which can directly convert chemical reaction to electric energy The low-temperature fuel cells with a proton electrolyte membrane operating at below 100oC is widely investigated Methanol is widely proposed as possible fuels for mobile applications such as electric vehicles However, the question of the toxicity of methanol remains crucial Methanol considered since a long time as a toxic product, in addition to possible environmental problems in relation to its large miscibility to water Ethanol offers an attractive alternative as a fuel in low temperature fuel cells because it can be produced in large quantities from agricultural products and it is the major renewable biofuel from the fermentation of biomass[1] The reaction of direct ethanol fuel cell (DEFC) needs a catalyst to decay ethanol

The electro-catalyst materialsused for ethanol oxidation and oxygen reduction in direct alcohol fuel cells (DAFCs) are platinum-based It is well-known that platinum-based catalysts are the best ones for low-temperature fuel cells[2] However, using platinum catalysts is restricted due to the limited resource and high cost of platinum These reasons hinder commercialization of fuel cells Thus, new materials for oxidation of ethanol in fuel cells is a matter of concern Many research groups studied palladium-based materials as alternatives for platinum-based catalysts The abundance of palladium is higher than Pt about fifty times[3] Therefore, the cost of palladium is cheaper Palladium-based nano-materials can oxidize ethanol in fuel cell reaction effectively Moreover, their activity is high in alkaline media According to

Son et al., they showed the combination Pd with other metals enhanced its catalytic activity, stability and poison tolerance[3]

In this research, 1D-dimensional material examined as the catalyst for the oxidation reaction of ethanol in alkaline direct ethanol fuel cells (ADEFCs) Ag@Pd core-shell nanowires found as a good composite electro-catalyst for fuel cells The core-shell structures were special materials which not only improved activity performance but also decrease price of catalysts This new material can resolve the commercialization ability of fuel cells

1 CHAPTER 1 LITERATURE REVIEW

1.1 Overview

According to statistics [4], the demand energy of the world is increasing in this decade was approximately 12 billion tones by burning fossil fuels The CO2 gas of the atmosphere raising have being caused the global warming, destroying forests and killing fish, killing thousands of people every single year, making a tons of ill and degrading our quality of life in other way Moreover, The emission gas as nitrogen dioxide, sulfur dioxide, carbon monoxide that can have severe consequences on the habitats mentioned, the most in recent years have been the "greenhouse effect’, acid rain, air pollution etc… The fossil fuels considered non-renewable power resources, which need some million years to form on the environment In the next hurries, we will have no fuel to burn Many countries is having dependence a significant lower dependence of fossil fuel, like France with more than 80% nuclear or Brazil with 35% hydro [4] The raise of fossil fuel prices to record levels in real terms over the past decade inevitably lead to supply responses, by development and deployment of new technologies across a range of energy sources

The depletion of fossil fuels make a urgent requirement that new energy replace for petroleum-based energy We must decide the time we use the last drop oil, lump

of coal and cubic foot of natural gas collected from the Earth All of it how we manage

to use the energy By research is the important mission of scientific community, new technology must create the clean and renewable power sources, which is a low-cost, high efficiency and friendly environment There are many alternative energy sources as: solar, wind, hydroelectric and geothermal power They do not damage our environment They reduce emissions gas of atmosphere but it is poor flexibility to decrease our dependence on limited reserves The batteries have a limited self-time and contain more heavy metal and toxic chemical On the contrary, Fuel cell have a number of advantages to compare with the energy sources The fuel cells have long lifetime and high efficiency It can use of any conditions, included high or low temperature

In addition, the by-product of process is only water, which reduce the pollution of burning fossil fuel Hydrogen can produce easily, it need water and a sources of power to generate cell The fuel cells is a small device, it can be cleaned by moving part in the system, thus the process ensured smooth operation will maintain high efficiency

There are commonly fuel cell as polymer electrolyte membrane fuel cells ( PEMFCs), (alkaline fuel cells) AFCs, phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) They having different system modified to the flexible use Therein, alkaline fuel cells AFCs has more advantage as low temperature, raw material (alcohol) The reactant of AFCs is alcohol( methanol, ethanol…) which is product of agriculture, biomass process [5] Methanol has been investigated and many oxidation mechanism explanations of this alcohol Unfortunately, methanol is a toxic substances, it has highest eco-toxicity scores, only behind hydrazine Therefore, ethanol is the reasonable selection by rich

of hydrogen and similar structure Additionally, ethanol is more environmentally friendly and possesses a higher energy density compared to methanol (8.00 vs 6.09 kWh kg-1) Ethanol can be easily prepared in well quantity from renewable resources such as sugar cane, wheat, corn, and even straw[5] They increase the performance

of direct ethanol fuel cells for a cleaner energy industry in the future

According to Changwei Xu the Pt-based catalysts are recognized as the best catalysts for low temperature fuel cells It accepted that PtRu/C is an effective anode catalyst for DAFCs Here we report oxide-promoted (CeO2, NiO) Pt/C and Pd/C catalysts for direct electrochemical oxidation of ethanol in alkaline media The oxide has used to promote Pt activity for ethanol electrooxidation in alkaline media CeO2 and NiO promoted Pt for alcohol electro-oxidation reported However, the limitation

of the use of the Pt-based electro-catalysts also comes from the high cost and limited resources of Pt[2] The problem with platinum is high cost Example: the cost of the

Pt alone in a fuel cell for a small car 100 kW is higher than the cost of an entire 100

kW gasoline engine And another reason, During the course of the lifetime of a fuel cell, the Pt cathode suffers from oxidation, migration, loss of active surface area, and

corrosion of the carbon support[6] The challenges inherent in finding a catalyst as good as Pt, there is a great deal of interest in finding alternatives The Pt is to replace with an alternative precious metal such as palladium (Pd)

Although the activities of Pd is lower than Pt, The Pd is more abundance on the earth and price of Pd is the best selection Moreover, In alkaline, electrochemical catalysis of Pd is higher than Pt[7] For these reasons, substitute for Pt as anodic catalysts for alkaline DAFCs Among different Pd is a good composite, the combination of Pd with Ag has found to have great activity for ethanol electro-oxidation DAFCs in alkaline media

1.2 Overview of fuel cells and direct ethanol fuel cells

1.2.1 Fuel Cells

Fuel cells strongly linked to renewable energies, particularly to the so-called “Hydrogen Economy” For decades, the development of fuel cells able to convert hydrogen and oxygen in electrical energy with water as unique byproduct has motivated huge activity in fundamental and applied electrochemistry

Hydrogen is as the fuel of the future Although it is the most abundant element

in cosmos, it is not available in Earth in its stable elementary form, H2 That means one have to spend energy to obtain it from water or fossil fuels The way we produce H2 determines if it is “green” or “black” hydrogen Green hydrogen refers to that obtained from renewable energy sources, like wind or solar, by splitting water in its components Black hydrogen, on the other hand, is that obtained from hydrogen-containing hydrocarbons or biomass by chemical reactions (reforming) Nowadays reforming of natural gas is the less expensive and preferred method to produce molecular hydrogen Therefore, a really zero-emission hydrogen economy would be true once the cost of splitting water in electrolyser could compete with the actual reforming processes Obviously, the economical balance will depend not only on technical issues such as increasing the efficiency of photovoltaic conversor or wind-power generators, and developing new water electrolysis technologies, but on political and environmental factors[4]

Table 1.1 Performance of various types of power generation[8]

Technology Diesel Engine Micro

Turbine

Mini turbine Fuel cell

Size(MW) 0.03 – 10 0.03 – 0.2 0.5 – 10 0.1 – 3 OEM($/kWh) 0.005 – 0.015 0.004 – 0.010 0.003 – 0.008 0.002 – 0.015 Electric

ft/Kw)

1 Electric Efficiency LHV: Net electric output at lower heating value

2 Overall Efficiency: The sum of the electrical and thermal outputs assuming Combined Heat and Power (CHP)

Various technologies are available for DG, including turbine generators, internal combustion engine/generators, micro-turbines, photovoltaic/solar panels, wind turbines, and fuel cells The application of fuel cell technologies to advanced power generation systems portends the most significant advancement in energy efficiency, conservation, and environmental protection for the next decade[8]

The fuel cells classified according to the choice of electrolyte and fuel cells are available Presently six major different types of fuel:

Table 1.2 Fuel cells classification:

Characteristic

Polymer electrolyte membrane

Alkaline

Phosphoric acid

Molten carbonate Solid oxide

Mobilized or immobilized potassium

Immobilized liquid

Immobilized liquid molten

Perovskites (ceramics)

Electrodes

(common) Carbon Platinum Carbon Nickel and

nickel oxide

Pervoskite/ metal cermet

Catalyst Platinum

Palladium

Platinum Platinum

Electrode material

Electrode material

1.2.2 Direct ethanol fuel cells

Direct alcohol fuel cells (DAFCs) are a new source of energy that has recently attracted much attention DAFCs are a type of alkaline fuel cell (AFC) AFCs have shown that they can produce higher current densities proton exchange membrane fuel cells (PEMFCs) PEMFCs are an emerging fuel cell technology and much attention has given to them in recent years due to the flexibility of using solid electrolytes and avoidance of electrolyte leakage However, AFCs have other advantages, including their low cost and their low corrosiveness In addition, AFCs are able to use relatively

cheap and non-noble metal electro-catalysts, including nickel, silver and palladium, rather than platinum This is due to the faster reaction kinetics of oxygen reduction in comparison to PEMFCs Direct alcohol fuel cells (DAFCs) primarily use alcohol The alcohols that used as fuel in DAFCs are methanol, ethanol, ethylene glycol and 2-propanol Methanol, ethanol and 2-propanol have quite high energy densities of 6.09, 8.00 and 8.58 kWh.kg-1, respectively, and are as fuel The alcohols that used as fuel in DAFCs are methanol, ethanol, ethylene glycol and 2-propanol [1] Methanol, ethanol and 2-propanol have quite high energy densities of 6.09, 8.00 and 8.58 kW h

kg-1, respectively, and are comparable to hydrocarbons and gasoline, which have energy densities of 10 and 11kW.h.kg-1, respectively Matsuoka et al found that DAFCs show excellent performance when conducted in alkaline media Conventionally, DAFCs used acid proton-exchange membrane (PEM) or Nafion-type membrane such as Nafion 117 and platinum type catalysts This conventional DAFC let CO2 generated from anode reaction of DAFC easy to remove in acidic electrolyte membrane but kinetic constraints of alcohol electro-oxidation relatively lower the performance of the system Later on, some idea came out to shift the trend

of DAFC to try alkaline media electrolyte or anion-exchange membrane (AEM) In alkaline media, DAFCs show better polarization characteristics in the oxidation of e

on platinum than in acidic media Furthermore, using alkali electrolytes allows for a greater possibility for application of non-noble and less expensive metal catalysts By the way, alcohol permeation rate reduced by the reversing of direction of ionic current due to hydroxide ion conduction against conventional proton conducting system However, DAFCs face significant challenges due to the poor performance of electro-catalysts, in particular anode catalysts at lower temperatures, and anode surface poisoning by CO like intermediates Instead of acid and alkaline media DEFC, there are other type of DEFC which combine alkaline acid media in single cell where anode in alkaline media while cathode in acid media The problem arise with acid and alkaline media is low theoretical voltage (1.14 V) The membrane type is used for alkaline-acid DEFC is cation exchange membrane/PEM type Anion-exchange membrane (AEM) is neglected because of poor thermal stability and ionic conductivity lower than proton-exchange membrane (PEM)[9]

Ethanol is a good fuel choice for over coming the problems with methanol In fact, ethanol is less toxic and has a higher energy density It also can produce from agricultural bioprocesses and consider a renewable energy Additionally, ethanol prove by researchers having a lower crossover rate and affects cathode performance less severely than methanol The development of electro-catalysts has mainly focused

on Pt/Ru and Pt/Sn catalysts for DAFCs recently PtRu/C catalysts are suitable for methanol, and PtSn/C in acidic environments is particularly suitable for ethanol There are two types of direct ethanol fuel cells: proton exchange membrane DEFCs (PEM-DEFCs) and anion exchange membrane DEFCs (AEM-DEFCs) The primary challenge in PEM-DEFCs is the sluggish kinetics of the ethanol oxidation reaction (EOR) Using acidic media and Pt catalysts has not been able to overcome this problem Different observations have made in alkaline media The kinetics of both the EOR and oxygen reduction reaction (ORR) are faster than that in acidic media It has been shown that when the acid electrolyte is changed to alkaline media, the fuel cell efficiency increases[9]

1.2.3 DEFCs and challenges

The electro-oxidation of ethanol in DEFC produce the electrons hence conduct electricity in complete circuit All the reactions occurs in the (Membrane Electrode Assembly) MEA that connected to current collector to conduct the electricity through the system MEA consists of anode, cathode and membrane afterward sandwiched together The electrons move from anode to cathode that produce from the reaction Fig 1 shows a reaction flowchart for direct ethanol fuel cells The reactions occur in direct ethanol fuel cells:

Anode reaction: CH CH OH3 2 H O2 CO2 12H 12e,

Athode reaction: 3O2 12H 12e 6H O2 ,

Overall reaction: CH CH OH3 2  3O2  2CO2 3H O2 .[9]

Fig 1.1 The image shows a reaction flowchart for direct ethanol fuel cells

The primary objective of a DEFC is to convert ethanol into carbon dioxide via a complete oxidation reaction, thereby producing 12 electrons in a single reaction [4] This surpasses the pioneering DMFC; DMFCs produce six electrons for each complete oxidation of methanol to carbon dioxide The DEFC, however, displays a lower percentage of complete oxidation reaction compared to the DMFC This lower rate is because of setbacks such as the chain in ethanol structure, type of catalyst, fuel structure, membrane used and physical factors such as the concentration and temperature Thus, a huge amount of studies is still required to increase the efficiency per mole of the fuel before ethanol may be used primarily as a new fuel source [3]

Fig 1.2 The complete electro-oxidation of ethanol in ADEFC produce [3]

1.3 Review of core-shell one-dimensional (1-D) nanostructure

1.3.1 One-dimensional (1-D) nanostructure

Within the last decade, nano-science and nanotechnology have reached the status of leading sciences with fundamental and applied research in all basic physical, life, and earth sciences as well as engineering and materials science An important feature of nano-science is that it bridges the crucial dimensional gap between the atomic and molecular scale of fundamental sciences and the microstructural scale of engineering and manufacturing Accordingly, a vast amount of true multidisciplinary fundamental knowledge explored and linked It shall lead to a tremendous amount of in-depth understanding as well as to the fabrication of novel high technological devices in many fields of applications from electronics to medicine Therefore, it should improve tremendously the level of technological advance to a much greater rate than human history has ever experienced As a result, the technological, educational, and societal implications of nano-science and nanotechnology are of immense importance, which attested to by the tremendous interests, the major economic efforts, and the national initiatives of many countries around the world

One-dimensional (1D) nanostructures, a new class of low dimensional materials, have emerged recently These maintain one of their dimensions in the nanometer range, but with a much larger length scale, ranging from hundreds of nanometers to hundreds of microns and up to millimeters in certain cases Such dimensionality can give aspect ratios (length over diameter) of several thousand: for example, 10,000 for a 1D object of 10 nm in diameter and 0.1 mm in length With no specific nomenclature, the shortest 1D nano-materials called nano-rods and the longest called nanowires regardless of their diameter In addition, many straightforward names have used to describe their morphology and appearance based

nano-on their micro-scale analogs One may find them in the literature as quantum wires, nanofibers, nanopillars, nanocables, and nanolines They also referred to as nano-whiskers by analogy to their micrometer counterpart However, these names are essentially similar and all of them refer to 1D anisotropic nano-scale objects of circular cross-section For the sake of clarity, nano-rod will use in this chapter for 1D anisotropic objects with length below 1 mm and nanowire will be used for length exceeding such dimension Their diameter ranges from several nanometers to several hundreds of nanometers Such 1D objects could also produced with a hollow interior and referred to as nanotubes regardless of their length, inner, or outer diameters Such porous anisotropic nanostructures show very large specific surface areas Very recently, a new class of 1D building blocks reported Such novel objects called nanobelts and nanoribbons Compared with nanowires and nanotubes, they have extended length (up to millimeter scale) and exhibit a rectangular cross-section A schematic representation of 1D nanostructures displayed in Figure 1-3 The basic goals have been to develop synthetic techniques to produce a large quantity of 1D building blocks in a controllable and fashionable way The crucial challenge remaining for scientists and engineers is to develop varistors, resonators, dielectrics, piezoelectrics, pyroelectrics, ferroelectrics, magnets, transducers, thermistors, thermoelectrics, protective and anticorrosion coatings, fuel cells, alkaline and lithium batteries, and solar cells had developed The diversity of such applications originates from the more complex crystal and electronic structures of metal oxides compared with these of other classes of materials The main reasons related to their variety of

oxidation states, coordination number, symmetry, ligand–field stabilization, density, stoichiometry, and acid–base properties, which yield fascinating compounds exhibiting insulating, semiconducting, conducting, or magnetic behaviors with transitions among those states [3]

Fig 1.3 Schematic representation of 1D nanostructures From top to bottom:

nanowire, nanorod, nanotube, and nanobelt nanoribbon

1.3.2 Core-shell one-dimensional (1-D) nanostructure

1.3.2.1 The basic of core-shell nanostructures

More recently, during the early 1990s, researchers synthesized concentric multilayer semiconductor nanoparticles with the view to improving the property of such semiconductor materials Subsequently, the terminology “core-shell” adopted

by Wei Lu et al (2005) & Chaokang Gu et al (2009) Furthermore, there has been a gradual increase in research activity because of the tremendous demand for more and more advanced materials fueled by the demands of modern technology Simultaneously, the advancement of characterization techniques had greatly helped

to establish the structures of these different core-shell nanostructures[10]

The schematic representation of different classes of core-shell nanoparticles showed in Figure 1.4 Concentric spherical core-shell nanoparticles were the most

common (Figure 1.4a) where a simple spherical core particle completely coated by a shell of a different material Different shaped core-shell nanoparticles also given rise

to immense research interest because of their different novel properties Different shaped core-shell nanoparticles generally formed when a core is non-spherical as shown in Figure 1.4b Multiple core core-shell particles formed when a single shell material coated onto many small core particles together as shown in Figure 1.4c Concentric nano shells of alternative coating of dielectric core and metal shell material onto each other showed in Figure 1.4d Here nano-scale dielectric spacer layers separate the concentric metallic layers These types of particles also knowed

as multilayered metallo dielectric nanostructures are mainly important for their plasmonic properties (Yao et al 2012) It is also possible to synthesize a moveable core particle within a uniformed hollow shell particle A bilayer coating of the core material represented in Figure 1.4e Using the suitable method only the first layer removed[10]

Fig 1.4 Concentric spherical core-shell nanoparticles [10]

1.3.2.2 Core-shell one-dimensional

Core–shell nanostructure extensively studied for applications such as nanostructured solar cells based on ZnO/CdSe core–shell nanowires According to R Tena-Zaera, the role of CdSe primary particle size pointed out not only in the structural properties but also in the optical and electronic behavior of ZnO/CdSe nanowire arrays Annealing temperatures higher than 350°C result in a CdSe nanowire shell constituted of particles 9 nm, considerably enhancing the electronic performance of the ZnO/CdSe nanowires In this framework, external quantum

efficiencies higher than 70% in ferro-/ferricyanide solutions obtained for ZnO/CdSe nanowire arrays annealed at 400°C, demonstrating their potential in nanostructured solar cells In conclusion, this work not only demonstrates the enhanced photovoltaic performances of ZnO/CdSe nanowire arrays due to a controlled thermal treatment but also gives some insights about the fundamental mechanisms that promote the improvements[11]

Fig 1.5 Scanning electron microscopy (SEM) images of ZnO/CdSe nanowire arrays

after different annealing treatments: 1 h at 250°C: (a) plan view and (b) cross section; 1 h

at 400 °C : (c) plan view and (d) cross section; 1 h at 350°C plus 1 h at 400 °C: (e) plan view and (f) cross section In Figure 1b the circled area shows the local thickness of CdSe

nanowire shell

In addition, According to Yu-Guo Guo et al, They have demonstrated that the electrodeposition of Pt nanoparticle into Sn nanotube provides a simple and convenient method to design effective electrode materials for DMFC The so-prepared composite nanostructures contain a large of the Sn nanotubes The novel structure results in a better dispersion of Pt nanoparticles with smaller size and hence endows it with a higher electroactive surface area of Pt Electrocatalytic oxidation of methanol at the Sn/Pt-bimetalic-nanotube-array electrode shows remarkably enhanced activity compared with that at the Sn/Pt- film- electrode Furthermore, the

Sn/Pt composite nanotubes promote the oxidation of methanol by lowering its over potenital to that found at Pt-Sn alloy nanoparticle It will a feasible to design the structure, size, and composition of the composite nanotube Experiment are underway

to further improve the performance of the Pt-Sn nanotube array electrode by introducing cost effective Pt hollow nano spheres or by depositing Pt nanoparticle on the surfaces of the Sn nanotubes, and to determine the underlying mechanism for the improved performance The result described in the present report also demonstrate the ability of metallic nanotube arrays to serve as a new type of conductive support for fuel cell applications Owing to its rich surfaces and easy manipulation, the composite structure will find use in a number of application s such as fuel cells, sensors, and chemical analysis[12]

Fig 1.6 Typical SEM images of Sn/Pt nanotube arrays

The fabrication of core/shell nanowire arrays with well-defined morphologies and tunable function remains a challenge Progress achieved in developing cost-effective and simple methods for controlling nanowire growth as well as creating more complex heterostructures Core/shell nanowire arrays generally grew by the catalyst-assisted vapor-liquid-solid mechanism and template-based approaches However, there is still no simpleand high-efficiency method to synthesize transition metal oxide or hydroxide core/shell nanostructure arrays with precise structure control, even though a few successful strategies (electrodeposition, oxidation, wet-chemical methods based on sacrificial templates and physical technique such as sputtering and pulsed laser deposition) have been reported[13]

Core/shell nanostructure were pulling in more consideration, they applied in many fields, such as electronics, biomedical, pharmaceutical, optics, and catalysts Core/shell nanostructure was exceedingly practical materials with modified properties Sometimes properties emerging from either core or shell materials could

be quite dissimilar The properties could be adjust by altering the constituting materials or the core to shell proportion Because of the shell material covering, the core wire reactivity decrease or its thermal stability can be modified, so that the overall particle stability and disperse ability of the core particle increases Eventually, particles demonstrate particular properties of the distinctive materials utilized together This was particularly valid for the inborn capacity to control the surface capacities to meet the differing application requirements Coating the shell on the core wire have many purposes, such as surface modification, the ability to increase the functionality, stability, controlled release of the core, reduction in consumption

of precious materials, etc

In addition to the enhanced material properties, core/shell materials were likewise critical from a financial perspective A precious material could be coating over a cheaper material to lessen the consumption of the expensive material compared with making the same sized pure material Moreover, decorated nanostructures which had less precious metal coating on the core material than core/shell nanostructures was getting more and more attention since that route helps reducing the amount of costly metal

Fig 1.7 Schematic view of Ge-Si core-shell nanowires (a) Side view of Ge-Si core-

shell nanowire; (b) top view of Ge-Si core-shell nanowire[14]

1.4 Review of Ag@Pd core-shell nanowires

1.4.1 Silver's Properties, Characteristics and Applications

Silver (Ag), chemical element, a white lustrous metal valued for its decorative beauty and excellent conductor of heat and electricity Silver has the atomic number

47 which is located in Group 11 (Ib) and Period 5 of the periodic table, between copper (Period 4) and gold (Period 6), and its physical and chemical properties are intermediate between those two metals[23] It is not a chemically active metal, but it

is attacked by nitric acid (forming the nitrate) and by hot concentrated sulfuric acid

It has the highest electrical conductivity of all metals[15]

Silver is always monovalent in its compounds, but an oxide, a fluoride, and a sulfide of divalent silver are known It does not oxidize in air but reacts with the hydrogen sulfide present in the air, forming silver sulfide (tarnish) This is why silver objects need regular cleaning Silver is stable in water[15]

The principal use of silver is as a precious metal and its halide salts, especially silver nitrate, are widely used in photography The major outlets are photography, the electrical and electronic industries and for domestic uses as cutlery, jewelry and mirrors Silver nanomaterial applied in many fields such as diagnostic applications, antibacterial applications, conductive applications, optical applications, etc[15] Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions Choosing Ag as a catalyst for fuel cell is based on underneath reasons: Alkaline electrolytes was known to have faster oxygen reduction kinetics than acidic medium High densities and an electrical efficiency close to 60% could achieve with high loadings of platinum (Pt) catalysts at the electrodes Pt was referred to as a superior catalyst with high activity and steadiness The main troubles with Pt are the excessive price and restricted future supply, which led to the development of alternative catalyst materials for fuel cell applications[15]

1.4.2 Review of silver nanowires

Although a number of approaches (such as those based on vapor-solid and vapor-liquid-solid processes) have been successfully developed for generating nanowires from semiconductors to dielectrics, the most widely used method for generating metallic nanowires is still template-directed synthesis that involves either

chemical or electrochemical depositions A rich variety of templates have already been demonstrated for use with such synthesis, including those made of both “hard” and “soft” materials Typically, “hard” templates include channels contained in membrane of alumina or tracketched polycarbonate; pores within zeolite or mesoporous silca; carbon nanotubes; and steps or edges supported on solid substrates.Although these templates were effective in fabricating nanowires with uniform and controllable dimensions, many of them needed to be selectively dissolved under harsh conditions in order to harvest the nanowires On the other hand,

a range of “soft” templates had demonstrated for use in the synthesis of metallic nanowires Notable examples include polymer film of poly(vinyl alcohol) (PVA); DNA chains; mesostructures selfassembled from diblock copolymers;rod-shaped micelles of cetyltrimethylammonium bromide (CTAB); liquid crystalline phases of oleate or sodium bis(2- ethylhexyl) sulfosuccinate (AOT)/p-xylene/water; and arrays

of calix hydroquinone nanotubes The silver nanowires synthesized using these templates were often in the form of aggregated bundles, and the templates needed to remove in order to recover the individual nanowires To avoid the step of template removal, several direct chemical and electrochemical approaches explored recently For example, silver nanowires successfully synthesized by reducing AgNO3 with a developer in the presence of AgBr nanocrystallites, or by arc discharging between two silver electrodes immersed in an aqueous NaNO3 solution Silver nanorods produced by irradiating an aqueous AgNO3 solution with ultraviolet light in the presence of PVA, or by electroreduction of AgNO3 in aqueous solution with poly ethylene glycol (PEG) The products of these templateless methods were characterized by problems such as relatively low yields, irregular morphologies, polycrystallinity, and low aspect ratio[57]

Similar to other nanostructures made of noble metals (e.g Au and Ag), Ag nanowires exhibit strong surface plasmon resonances (SPRs) under photo-illumination due to strong coherent oscillation of free surface electrons in the nanowires, resulting in strong absorption and scattering of incident light As a result, dispersions of Ag nanowires always display a yellowish color The evanescent electrical fields near the surface of a Ag nanowire are usually very high, providing

capability to enhance Raman scattering and fluorescence of molecules or emitters (such as quantum dots and upconversion nanocrystals) adjacent to the nanowires For example, single Ag nanowire, randomly assembled networks, rafts of Ag nanowires assembled through Langmuir–Blodgett (LB) process and bundles of Ag nanowires evaluated to exhibit significant enhancement on Raman signals of molecules adsorbed on the surfaces of the Ag nanowires Due to the anisotropic geometry of the

Ag nanowires, the measured Raman scattering is strongly dependent on the polarization of excitation light[57]

In addition, coating surfaces of the Ag nanowires with metal nanoparticles can create ‘‘hot spots’’ (where the local electrical fields are drastically increased due to the strong coupling between the nanoparticles and nanowires) in the gaps formed between the nanowires and the nanoparticles for enhancing Raman scattering In addition to SPRs, surface plasmon polaritons (SPPs) can be excited with free-space photons by coupling the energy into Ag nanowires and can propagate in the nanowires along their longitudinal axes This property stimulates intensive interests

in using Ag nanowires as waveguides in photonic and optoelectronic circuits because the confinement of photons/SPPs in the Ag nanowires with diameters less than 100

nm restricted by the diffraction limit In contrast, it is always a problem in photonic waveguides when their dimensions minimized down to subwavelength Although Ag nanowires fabricated with varying methods including lithography could propagate SPPs, Ag nanowires synthesized through wet chemistry approaches exhibit much better performance with minimized losses due to their single crystallinity and atomic surface smoothness[57]

In combination of the guiding capability of Ag nanowires and ‘‘hot spots’’ created between the Ag nanowires and metal nanoparticles, surface-enhanced Raman scattering (SERS) of molecules on the Ag nanowires decorated with individual metal nanoparticles can be remotely excited by coupling external photons into the ends of the nanowires In a typical measurement, a laser beam focused to one end of Ag nanowire to excite SPPs, which propagate in the nanowire and re-emit as photons at the points contacting with metal nanoparticles These photons excite the SPRs in the

Ag nanowire and the metal nanoparticles and their strong coupling results in strong

local electrical field for SERS The remote excitation of SERS in Ag nanowire/metal nanoparticle hybrid structures might used as probes for potential high-resolution SERS imaging Properties and applications of Ag nanowires presented here (not limited to these examples) demonstrate the importance of Ag nanowires In addition,

Ag nanowires had demonstrated to serve as a unique class of templates for synthesizing 1D nanoparticles with novel structures and functionalities, which was difficult (or impossible) to synthesize via other methods[57]

1.4.3 Synthesis of silver nanowires

Researchers have proposed many synthetic methods to prepare nanowires during the past thirty years that were mainly derived from the strategy employed to prepare quantum wires AgNWs were mainly prepared via electrochemical methods during the early stages, but the AgNWs synthesized by this method were not uniform and they were obtained in low yield Based on this method, the hard-template and soft-template methods have been developed in the past twenty years The polyol method is a prominent example of the soft-template approach, and this strategy has been widely used by most researchers[57].

The template-based methods have first been widely used to generate AgNWs, these methods can be categorized as hard template and soft template approaches based on the nature of the template The advantage of the template method is that the AgNWs prepared through this approach can be synthesized in a well controlled manner[57]

Hard template method: Using a nanoporous membrane as a template, the

AgNWs can be grown within the pores of the membrane The diameter of the AgNWs can be varied by choosing membranes with pores of different sizes[57]

Soft template method: To overcome the shortcomings of hard template

methods, researchers have focused their efforts on strategies that rely on soft templates, such as various kinds of surfactants, micelles, and many other polymers

A key feature of many of these soft templates is that they can dissolve in solution Because most soft template methods are conducted in the solution phase, these strategies may have excellent potential for applications in industry In addition,

surfactants were first chosen as soft templates for the synthesis of silver nanowires[57]

Polyol method: This method has been the most successful route to produce

AgNWs both at large scales and of high quality This reaction proceeds with heating via the reduction of a metal salt by a polyol During these reactions poly(vinyl pyrrolidone) (PVP) is used both as a capping agent and to prevent the aggregation of silver nanoparticles As an extension of this work, Xia noted that , the chloride anions facilitated AgNW growth in two significant ways Firstly, the chloride anions stabilized the silver seed particles via electrostatic interactions Secondly, the chloride anions also prevented the accumulation of undesirably high concentrations of free Ag(I) ions and seeds by forming poorly soluble AgCl salts, thus regulating the reagent concentrations without the need for a syringe pump [57]

Fig 1.8 Characterization of Ag nanowires synthesized through a polyol process

1.4.4 Palladium's Properties, Characteristics and Applications

Palladium is a soft, rare, silvery-white metal that is valued for its catalytic properties and shares a number of the characteristics common to the platinum group metal (PGM), which include a relatively high melting point and high density Even though high for a metal, palladium's melting point and density are the lowest of the PGMs

The atomic number of palladium is 46, the density at room temperature is 120.23 kg/m3 Whereas, its density at its melting point is 10.38 g/cm3 Its boiling point is

2963°C Palladium is over 70% reflective The melting point of this element is 1554.9°C The value of thermal conductivity is 72 W/m/K

At room temperatures, the metal has an unusual property of absorbing nearly 900 times its own volume of hydrogen It dissolves slowly in concentrated nitric acid, sulfuric acid and hydrochloric acid when finely divided

Palladium, similar to platinum, is very resistant to oxidation and corrosion and has great catalytic properties That is mainly because of the fact that palladium has

an abnormal - and remarkable - ability to absorb hydrogen gas at a rate of 900 times its own volume Soft and ductile when annealed, palladium increases with electricity and hardness when cold annealed Palladium is also chemically stable and conductive, making it useful for applications in the electronics industry

Global sales of palladium were estimated to be around 300,000kgs (660,000lbs)

in 2010 Auto-catalysts were the largest application for the metal, accounting for an estimated 57% of palladium used in 2010 In fact, palladium-based alloys used in fuel cell technology applications as follows[18]

The catalyst desires to have the highest possible surface region Thus, the active phase is dispersed on a conductive support including excessive surface area carbon powders However, due to the expanded rate and restricted resources, Pt can not used for large-scale applications and alternative substances are needed In addition, platinum itself is known to be unexpectedly poisoned on its surface by strongly adsorbed species coming from the dissociative adsorption of ethanol For a majority

of these reasons Non platinum-based catalysts had been examined as electrode materials for low temperature fuel cells[16]

Pd is of outstanding interest as a substitute material for Pt as electrode catalyst in fuel cells, not only because of its chemical similarity to Pt (same group of the periodic table, same the FCC crystal structure, similar atomic length), however also due to its drastically reduced cost (about 1/5 that of Pt) Furthermore, Pd is exciting as it is at least fifty times greater abundance on the earth than Pt Pd, as other platinum-group metals, provides electro-catalytic activity for both the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), but the HOR and ORR activities

of Pd are significantly lower than those of Pt

Even though Pd promise to be a substitute material for Pt, it does not appear to

be active enough for utilization as a catalyst in most oxidation or reduction reactions

It had reported that via addition of a suitable metal, such as Co and Fe, the ORR activity of Pd would become comparable to that of Pt Certainly, in various papers a considerably lower ORR activity of Pd-based catalysts than Pt reported Wang et al [17], He found the ORR over-potential on all of Pd– Co/C electro-catalysts with different Pd/Co atomic ratios prepared by using a modified polyol reduction is higher than that on the Pt/C catalyst, indicating that the ORR activity on Pd-based catalysts

is lower than that on the Pt-based catalysts in pure acidic solution

Pd-Pt bimetallic catalysts have been determined to outperform Pt in lots of reactions, and Pd prove to be far advance catalyst than Pt in formic acid oxidation For the oxygen reduction reaction, Pd-alloys have also confirmed improved performance while in comparison to Pt, especially in the presence of methanol The transition from Pt-based to Pd-based catalysts has commenced and could probable result in the development of greater cost effective materials with enhanced activity toward the commercialization of fuel cells[18]

1.4.5 Galvanic displacement method

Catalysts formed by (spontaneous) galvanic displacement are poised (and in a few cases have shown the ability) to produce high mass activities The material displaced similar characteristics to other demonstrated extended surface catalysts It has shown similar promise in the area of long-term durability to potential cycling Extended surfaces, unlike nanoparticles, have the potential for high surface areas with less lower coordinated (more dissolution prone) catalyst sites Extended surface catalysts allow for the potential of proton conduction across their surfaces without the addition of an ionomer or electrolyte and potential losses associated with its presence Much of the focus to date for galvanic displacement as an electro-catalyst synthesis route has focused on acidic, hydrogen fuel cells (Pt based catalysts)[18] Galvanic displacement also holds great promise in the development of catalysts for a variety of other electrochemical processes In hydroxide exchange membrane (HEM) fuel cells, these materials studied for activity in ORR and HOR In direct alcohol fuel cells, these materials studied for methanol oxidation activity in acidic electrolytes and

for methanol, ethanol, and ethylene glycol oxidation activity in basic electrolytes The research to date clearly shows these materials have great promise and in some cases have already demonstrated state of the art electro-catalytic performance The work discussed here focused on the development of catalysts for fuel cells The use

of galvanic displacement and extended surfaces is also potentially beneficial for a variety of electrochemical devices, including electrolysis, photo electrochemical, and chemical synthesis systems[18]

Galvanic displacement is a highly promising route for novel catalyst synthesis Galvanic displacement occurs spontaneously when a metal “template” comes into contact with a more noble metal cation In this case, it is thermodynamically favorable for the more noble metal cation to “steal” electrons from the less noble metal (the nobility of metals follows their standard redox potential, Table 2) The galvanic displacement process combines aspects of corrosion of the template metal and electrodeposition of the more noble metal cation An advantage of galvanic displacement as a synthesis route is that the less noble metal provides a template for the resulting structure This enables the creation of extended surface catalysts through the implementation of extended structure templates (tubes, wires, plates) The galvanic displacement process also tends to form a continuous layer, beneficial for durability[19]

Table 1.3 The nobility of metals follows their standard redox potential

V and RHE 3

Galvanic displacement is shown schematically in Figure 1.9 as a general case

for the displacement of a nanowire by a more noble metal cation The schematic shows increasing time into displacement, moving from left to right As an example,

a less noble nanowire (for example, Ag depicted in blue) is displaced by higher

nobility metal ions (Pt, depicted in red) Depending on reaction variables, different final structures can be obtained; However for full Pt displacement of Ag nanowires,

Pt nanotubes are typically obtained Galvanic displacement allows for the synthesis

of catalysts (structures and compositions) which cannot be synthesized directly: that are shape-controlled, including the morphologies of wires, tubes, and plates; and with tuneable compositions[19]

Fig 1.9 a) Schematic of the displacement process Transmission electron microscopy

images of material, b) Ag nanowire, c) corresponding Pt nanotube following galvanic displacement[19]

1.4.6 Improving catalytic activity of Ag on ethanol oxidation

According to ST Nguyen et al, There have been some achievement to enhance performance of Pd by using Pd and Ag in a structure Improvement based on d-band theory of Nørskov and co-workers When metals with small lattice constants are overlayed or alloyed on metals with larger lattice constants, the d-band center shifts

up and vice versa, which subsequently affects the reaction rate If the d-band center

is shifted up, the adsorption ability of the adsorbate onto the metals will be stronger and this may help to improve the electrooxidation of ethanol on the surface of the metals[3]

Ag is a metal which is much cheaper and more abundant than Pt According

to Hammer and Nørskov’s calculation [29], d-band center of Pd with a lattice value

of 3.89 A˚ will be shifted up when combining with Ag (a = 4.09 A˚) Therefore, Ag was chosen to be modified with Pd in this research[3]

1.5 Applications and related studies

N.A Khan et al, they have synthesized Pd-Ag particles, supported on thin alumina films, These results indicate that Pd-Ag system is more selective towards ethylene then pure Pd, although the overall activity decreases significantly In general, this behavior is similar to what has being observed for real Pd-Ag catalysts, thus suggesting that the model Pd-Ag catalysts studied here mimic the properties of the real catalysts[20]

Zhongwei Chen et al, they have synthesized PtNTs and PtPdNTs (50 nm diameter, 5–20 µm long and 4–7 nm wall thickness) and tested their suitability as catalysts for ORR in PEMFCs Pt/NTs were synthesized by a galvanic replacement reaction of silver nanowires (AgNWs) developed by Xia and co-workers The AgNWs were synthesized using a polyol method and subsequently heated at reflux with Pt(CH3COO)2 in an aqueous solution The mass activity of the PtPdNTs is 1.4 and 2.1 times higher than that of Pt/C and platinum black, respectively and the specific activity of the PtPdNTs iseven 5.8 and 2.7 times higher than that of the Pt/C and platinum-black electro-catalysts[21]

Son Truong Nguyen et al, they have synthesized alloy structure for the Pd–Ag/C Electrochemical measurements indicated that it possessed a better intrinsic activity towards ethanol oxidation than Pt/C and Pd/C and the best performance was observed for 10%Pd–10%Ag/C This makes Pd–Ag/C a promising anodic catalyst for alkaline DEFC[3]

According to Hong Ji et al, The nanoporous bimetallic Ag80Pd20 alloy can be fabricated by dealloying a ternary M60Ag32Pd8 alloy in the HCl solution The addition

of the third element Pd into Mg–Ag has a significant influence on the dealloying process of the ternary Mg– Ag–Pd alloy and on the nanoporosity evolution during dealloying through control over surface diffusion of released Ag and Pd atoms along the alloy/solution interface The nanoporous Ag80Pd20 alloy exhibits superior catalytic activity towards ethanol electro-oxidation in alkaline media Our present findings, including the design of a solid solution and/or intermetallic compound and control over surface diffusion of the more noble atom through elemental additions,

pave the way for low-cost fabrication and functionalization of ultrafine hierarchical nanoporous metals or alloys[22]

Jianfeng Huang et al, they have synthesized Pd-Ag bimetallic dendrites have been synthesized via a galvanic replacement reaction of Ag dendrites in a Na2PdCl4 solution Many small crystalline Pd nanoparticles are protruding on the partially depleted Ag dendrites underneath Because of the large surface area characteristic for the dendrites, the bimetallic interfaces formed between Pd and Ag, and the excellent hydrogen storage capacity of Pd, these Pd-Ag dendrites are highly active in hydrogen reduction reactions as has been benchmarked via the model 4-NP/NaBH4 system Regarding the capability to tune composition and morphology, this type of compound

is promising to provide a reaction selective catalyst[23]

Yizhong Lu and Wei Chen have synthesized nanoneedle-covered silver nanotubes were synthesized through a galvanic displacement reaction with Ag nanorods at 100°C (Pd/Ag-100) and room temperature (Pd/Ag-25) Such nanostructures with a large surface area makes them the promising electrocatalysts applied in fuel cells The PdAg nanotubes show high electrocatalytic activity toward the formic acid oxidation in the electrochemical cyclic voltammetric studies[24] Karaked Tedsree et al, they coated with a thin layer of Pd atoms can significantly enhance the production of H2 from formic acid at ambient temperature The catalysts dramatically promote hydrogen production from formic acid decomposition, allowing processing and separation at room temperature The results offer a number of exciting possibilities for the development and exploitation of small portable PEM fuel cell devices[25]

palladium-According to M.C Oliveira et al, Pd–Ag alloys containing different amounts

of Ag (8, 21 and 34 at.%) were prepared in order to evaluate their catalytic activity towards the ethanol oxidation (EOR) and oxygen reduction (ORR) reactions The Pd–

Ag alloys under study, also revealed a higher ORR activity at room temperature, compared to Pd The maximum activity for the ORR was observed at the alloy composition of 8 at.% Ag, which can be in part ascribed to the reduced OH coverage

on this electrode material In the presence of ethanol all Pd–Ag alloys showed a good ORR selectivity and activity, much superior to pure Pd[26]