Role of carbon nanotubes (CNTs) in electrochemistry

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BACKGROUND

Fuel Cell (FC) research and investigation of the electrochemical kinetics of FC molecules such as methanol (MeOH), ethylene glycol (EG) and formic acid (FA) have attracted great interest worldwide. This is due to their promising potential replacement for batteries in low power electronic applications such as laptops, computers and cellphones. However, during the electrooxidation of these FC molecules on platinum (Pt) electrocatalyst, the catalytic surface is poisoned by strongly adsorbed reaction intermediates, mainly carbon monoxide (CO) that eventually blocks the active sites [1]. Nanoparticulate alloys of Pt and other metals notably ruthenium (Ru) have shown to improve the electrocatatalytic activity and tolerance to CO poisoning compared to pure Pt [2]. Other existing problems in fuel cells include the high cost and the lack of the catalyst support and electrode materials. In an attempt to solve these problems, efforts are being intensified on materials structured on the nanoscale since they are becoming a critical tool in the development of cleaner and higher performance energy devices.
These nanostructured materials display a range of remarkable and unique properties, namely high surface area, mechanical strength, electronic conductivity and thermal stability which enhance the chemical reactivity of materials such as electrocatalyst [3, 4]. Among the energy devices that could benefit from the unique properties of nanomaterials are fuel cells, photovoltaics, hydrogen storage systems, supercapacitors and rechargeable lithium ion batteries. Nanomaterials promise to improve the performance and reduce the cost of the electrocatalyst and electrode used in FCs [5-9]. Therefore in this study platinum-ruthenium carbon-nanotubes (PtRu- CNT) or ruthenium tetrakis (diaquaplatinum) octacarboxyphthalocyanine carbon nanotube (RuOcPcPt-CNTs) nanocomposite materials have been developed to improve the efficiency of the fuel cell electrocatalytic oxidation. The drawbacks associated with carbon black based electrodes are overcomed by choosing a CNT based graphite electrode. Carbon nanotubes are regarded as high quality pure supports and are essential for high catalytic activity due to their high surface area. Graphite electrode has the ability to interact with carbon nanotubes via π interactions. CNTs possess admirable conductive properties and also exhibit faster electrode kinetics compared to other electrodes such as the glassy carbon, boron-doped or carbon black electrode. Finally, this work systematically explores the electron transport and electrocatalytic properties of fMWCNTs integrated with PtRu or RuOcPcPt nanoparticles supported on basal plane pyrolytic graphite electrode (BPPGE) platform towards electrooxidation of MeOH, EG, FA and in oxygen-reduction reaction (ORR). This introductory chapter provides information on the general overview of the thesis, overview of electrochemistry, voltammetric techniques, electrochemical impedance spectroscopy, chemically modified electrodes, electrode modification techniques, metallophthalocyanine, fuel cells, microscopy and spectroscopy techniques and the analytes (MeOH, EG, FA and oxygen) used in this work as analytical probes. Chapter two describes the experimental procedure adopted while; chapter three to six discuss the results obtained.

Basic concepts

Electrochemistry is a broad field of physical science, encompassing those processes that involve the passage of charge across the interface between two phases. These involve electron transfer from a metal electrode to a redox active species in an electrolyte solution, and the transfer of an ion from an aqueous phase to an immiscible organic phase. It also deals with the study of chemical changes caused by the passage of an electric current and the production of electrical energy by chemical reactions and vice versa. The fundamental process in electrochemical reactions is the transfer of electrons between the electrode surface and molecules in the interfacial region, either in solution or immobilized at the electrode surface. Therefore, electrochemistry is one effective technique to study electron transfer properties [10-19]. When electron transfer is between a solid substrate and a solution species, it is termed heterogeneous process. Inversely, if electron transfer reaction occurs between two species, both of which are in solution, the reaction is homogeneous. Electrochemistry involves the measurement of potential (potentiometry) or current response (voltammetry) [12, 14-16]. Therefore, the work described in this thesis involves a number of voltammetric techniques, such as cyclic voltammetry (CV), chronoamperometry (CA), and Rotating Disk Electrode (RDE) for studying the mechanism of electron transport and the electrocatalytic behaviour of the studied fuel cell (FC) molecules and oxygen at the electrode.

The electrode-solution interface

Charged particles exist at every material interface called the electrical double layer. In electrochemistry, this layer reflects the ionic zones formed in solution to compensate for the excess of charge on the electrode. The electrical double layer is made up of several layers when the electrode is immersed in solution as illustrated in Figure 1.1. Whether the charge on the metal is negative or positive with respect to the solution depends on the potential across the interface and the composition of the solution. A positively charged electrode thus attracts a layer of negative ions and vice versa. The inner layer closest to the electrode contains solvent molecules that are specifically adsorbed on the electrode. This layer is called the Inner Helmholtz Plane (IHP) or compact layer [17]. The outer layer called the Outer Helmholtz Plane (OHP) is the imaginary plane passing through the solvated cations. These planes cannot be measured nor do they exist so it can be assumed that the distance from the electrode to the IHP will be the radius of the ion. The IHP and OHP represent the layer of charges which is

Mass transport processes

Reaction at electrode surface can be complicated, and takes place in a sequence that involves several steps. In simple reactions, only mass transport of the electroactive species to the electrode surface, the electron transfer across the interface, and transport of the product back to the bulk solution occurs. However in complex reactions, additional chemical and surface processes that follow the actual electron transfer take place. Therefore, the overall measured current, may be limited by either mass transport of the reactant or by the rate of electron transfer. Hence, when the overall reaction is controlled solely by the rate at which the electroactive species get to the electrode, the measured current is said to be limited by mass transport [10, 17, 21]. Mass transport is a process which governs the movement of charged or neutral species and contributes to the flow of electricity through an electrolyte solution in an electrochemical cell. Mass transfer in solution occurs by diffusion, migration, and convection. Diffusion and migration result from a gradient in electrochemical potential. Convection results from an imbalance of forces on the solution. Each of these processes is futher discussed below.
(a) Diffusion This is a mass transport process which is aimed at minimising concentration differences at the electrode electrolyte interface. Diffusion is simply the movement of material from a high concentration region of the solution to a low concentration region, or the transport of particles as a result of difference in their chemical potential [10, 17, 21]. If the potential at an electrode oxidizes or reduces the analyte, its concentration at the electrode surface will be lowered, and therefore, mor analyte moves to the electrode from the bulk of the solution, which makes it the main current-limiting factor in voltammetric process. This process is represented in Figure 1.2a.
(b) Migration Migration is a mass transport process involving movement of charge particles along an electrical field. The charge particles are carried through the solutions by ions according to their transference number. Migration of electroactive species can either enhance or diminish the current flowing at the electrode during reduction or oxidation of cations. It helps reduce the electrical field by increasing the solution conductivity, and serves to decrease or eliminate sample matrix effects. Addition of high concentration of inert salt (i.e. hundred times higher than the electroactive ions concentration) as supporting electrolyte helps in eliminating current due to electromigration such that the current measured at the electrode is purely due to the diffusion of the electroactive species to the electrode surface. The supporting electrolyte also ensures that the double layer remains thin with respect to the diffusion layer, and it establishes a uniform ionic strength throughout the solution [14, 21]. Migration process is represented in Figure 1.2b.
(c) Convection Convection is mass transport movement to the electrode by either physical movement such as fluid flow by stirring, or flow of solution and simultaneous vibration or rotation of the electrode (i.e forced convection) as found during the rotating disk electrode experiment [14, 22]. Convection can also occur naturally due to density gradient. In voltammetry, convection is eliminated by maintaining the cell under quiet and stable condition. Mass transport due to convection is represented in Figure 1.2c below.

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Cyclic voltammetry

Cyclic voltammetry (CV) has become a very popular technique for initial electrochemical studies of new systems and has proven very useful in obtaining information about fairly complicated electrode reactions. It is often the first experiment performed in an electroanalytical study, particularly due to its ability to rapidly provide considerable information on the thermodynamics of redox processes and the kinetics of heterogeneous electron transfer reactions. It offers a rapid location of redox potentials of the electroactive species and convenient evaluation of the effect of electrolyte on the redox process. In voltammetry the potential of the working electrode is ramped at a selected scan rate, . The resultant trace of current against potential is termed a voltammogram. During cyclic voltammetry, the potential is similarly ramped from an initial potential Ei but, at the end of its linear sweep, the direction of the potential scan is reversed, usually stopping at the initial potential Ei (or it may commence an additional cycle). The potential at which the reverse occurs is known as the switch potential (E). Almost universally, the scan rate between Ei and E is the same as that between E and Ei. Values of the scan rates forward and backward are always written as positive numbers. There are three cyclic voltammetric processes that could take place, namely, reversible, irreversible and quasi-reversible [20].

TABLE OF CONTENTS :

  • Title Page
  • Declaration
  • Dedication
  • Acknowledgements
  • Abstract
  • Table of Contents
  • List of Abbreviations
  • List of Symbols
  • List of Figures
  • List of Schemes
  • List of Tables
  • INTRODUCTION
    • 1.1 Background
    • 1.2 Overview of Electrochemistry
    • 1.2.1 Basic concepts of Electrochemistry
    • 1.2.2 The electrode-solution interface
    • 1.2.3 Faradaic and Non-Faradaic process
    • 1.2.4 Mass transport processes
    • 1.3 Voltammetric techniques
    • 1.3.1 Cyclic Voltammetry
      • 1.3.1.1 Reversible process
      • 1.3.1.2 Irreversible process
      • 1.3.1.3 Quasi-reversible process
    • 1.3.2 Chronoamperometry
    • 1.3.3 Linear Sweep Voltammetry
    • 1.3.4 Rotating disk electrode
    • 1.4 Electrochemical impedance spectroscopy
    • 1.4.1 Principle of impedance spectroscopy
    • 1.4.2 Application and data presentation
    • 1.5 Introduction to Phthalocyanines (Pcs)
    • 1.6 Introduction to MetallophthalocyanineS (MPcs)
    • 1.7 Novelty of Ruthenium (II) tetrakis (diaquaplatinum) octacarboxyphthalocyanine (RuOcPcPt)
    • 1.8 Modification of electrodes
    • 1.8.1 Carbon electrodes
    • 1.8.2 Carbon nanotubes as electrode modifiers
    • 1.8.3 Metal-catalyst modified electrodes for fuel cell application
    • 1.9 Role of carbon nanotubes (CNTs) in electrochemistry
    • 1.9.1 The importance of Platinum-Carbon Nanotube over Platinum Carbon Black
    • 1.10 Electrode modification techniques
      • 1.10.1 Electrodeposition
      • 1.10.2 Drop-dry
      • 1.10.3 Dip-dry
      • 1.10.4 Spin coating
    • 1.10.5 Vapour deposition
    • 1.10.6 Langmuir-Blodgett
    • 1.10.7 Electropolymerisation
    • 1.11 Overview of Fuel Cell (FC) technology and its shortcomings
    • 1.12 Justification of this work
    • 1.13 Fuel Cell Types
    • 1.13.1 Polymer Electrolyte Membrane Fuel Cell (PEMFC)
      • 1.13.1.1 Principle of PEMFC
      • 1.13.2 Direct Methanol Fuel Cell (DMFC)
      • 1.13.2.1 Principle of DMFC
      • 1.13.3 Alkaline Fuel Cell (AFC)
    • 1.13.4 Phosphoric Acid Fuel Cell (PAFC)
    • 1.13.5 Molten Carbonate Fuel Cell (MCFC)
    • 1.13.6 Solid Oxide Fuel Cell (SOFC)
    • 1.14 Application of fuel cells
    • 1.15 Advantages and disadvantages of fuel cell systems
    • 1.16 Mechanism of adsorption of methanol oxidation intermediates at Pt-modified electrodes
    • 1.17 Microscopy and spectroscopic techniques
    • 1.17.1 Raman Spectroscopy
    • 1.17.2 Scanning Electron Microscopy (SEM)
    • 1.17.3 Fourier Transform Infra-Red Spectroscopy (FTIR)
    • 1.17.4 Transmission Electron Microscopy (TEM)
    • 1.17.5 Atomic Force Microscopy (AFM)
    • 1.17.6 Energy Dispersive X-RAY Spectroscopy (EDX)
    • 1.17.7 X-RAY Diffraction Spectroscopy (XRD)
    • 1.18 Background of the studied Analytes
    • 1.18.1 Methanol electrooxidation
    • 1.18.2 Formic acid electrooxidation
    • 1.18.3 Ethylene glycol electrooxidation
    • 1.18.4 Oxygen reduction reaction (ORR)
    • 1.19 Problem statement
    • 1.20 Hypothesis
    • 1.21 Aims and Objectives
    • 1.22 Structure of the thesis
    • REFERENCES
  • CHAPTER TWO EXPERIMENTAL
    • 2.1 Materials and Reagents
    • 2.1.1 Synthesis of functionalised CNTs
    • 2.1.2 Synthesis of Potassium tetra-chloroplatinate (II), K2PtCl
    • 2.1.3 Synthesis of ruthenium (II) tetrakis (diaquaplatinum)octacarboxyphthalocyanine
    • 2.1.4 Synthesis of fMWCNT Ruthenium (II) tetrakis (diaquaplatinum)tetracarboxyphthalocyanine (RuTetPcPt)
    • 2.1.5 Synthesis of Pt/fMWCNT catalyst
    • 2.2 Equipments and methods
    • 2.3 Electrode Modification and Pretreatments
    • 2.3.1 Electrode cleaning
    • 2.3.2 Electrode modification
      • 2.3.2.1 Drop-dry / electrodeposition techniques
      • 2.3.2.2 Electrode modification with synthesisedRuOcPcPt nanoparticles
      • 2.3.2.3 Electrode modification with synthesisedRuTetPcPt nanoparticles
    • 2.4 Electrocatalytic and Electroanalysis
    • REFERENCES
  • CHAPTER FOUR Dynamics of electrocatalytic oxidation of ethylene glycol, methanol and formic acid at MWCNT platform electrochemically modified with Pt/Ru nanoparticles
    • 4.1 SEM characterization
    • 4.2 Comparative cyclic voltammetric evolutions
    • 4.3. Studies on the Catalysts Tolerance to CO Poisoning
    • 4.4 Impedance spectroscopy measurements
    • REFERENCES
  • CHAPTER FIVE Efficient oxygen reduction reaction using ruthenium tetrakis (diaquaplatinum) octacarboxyphthalocyanine catalyst supported on MWCNT platform
    • 5.1. Spectroscopic and microscopic characterisation
    • 5.2 Oxygen reduction reaction (ORR)
    • REFERENCES
  • CHAPTER SIX Electrocatalytic Oxidation of Methanol and Formic Acid at MWCNT Platform Modified with synthesized Pt/Ru and Pt/RuOcPc Nanoparticles
    • 6.1 Spectroscopic and microscopic characterisation
    • 6.2 Methanol and Formic acid oxidation
    • 6.3 Impedimetric Experiment
    • 6.4 Effects of scan rate
    • 6.5 Effects of scan numbers
    • REFERENCES
    • CONCLUSIONS AND RECOMMENDATIONS
    • CONCLUSIONS
    • APPENDIX A: List of publications in peer-reviewed journals from this thesis
    • APPENDIX B: List of conference presentation from this thesis

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