Experimental study of non equilibrium electrodeposition of nanostructures on copper and nickel used for fuel cell application

Transcription

1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2011 Experimental study of non equilibrium electrodeposition of nanostructures on copper and nickel used for fuel cell application Rajesh Kumar Shanmugam The University of Toledo Follow this and additional works at: Recommended Citation Shanmugam, Rajesh Kumar, "Experimental study of non equilibrium electrodeposition of nanostructures on copper and nickel used for fuel cell application" (2011). Theses and Dissertations This Thesis is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Thesis entitled Experimental Study of Non Equilibrium Electrodeposition of Nanostructures on Copper and Nickel Used for Fuel Cell Application by Rajesh Kumar Shanmugam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science in Mechanical Engineering Dr. Yong Gan, Committee Chair Dr. Mehdi Pourazady, Committee Member Dr. Matthew Franchetti, Committee Member Dr. Patricia R. Komuniecki, Dean College of Graduate Studies The University of Toledo May 2011

3 Copyright 2011, Rajesh Kumar Shanmugam This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 Abstract An Abstract of Experimental Study of Non Equilibrium Electrodeposition of Nanostructures on Copper and Nickel Used for Fuel Cell Application by Rajesh Kumar Shanmugam Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science in Mechanical Engineering The University of Toledo May 2011 Fuel cells are one of the most promising technologies of the future which can be used to generate electricity using natural sources. With intent to increase the performance of a fuel cell, non-equilibrium electrodeposition is performed on the Cu and Ni electrode materials. The process parameters viz. electrolyte concentration, electrode potential and electrode material; governing the electrodeposition were studied using cyclic voltammetry, SEM image analysis and X-ray Spectroscopy (EDS) to understand their effect on the nanostructures deposited on these metals. An increase in the conductivity of the electrode is observed for an increase in electrolyte concentration (from 0.05 M to 1.0 M) and electrode potential (from -0.5 V to -4.5 V). The magnitude of reduction potential (E r ) of the electrode was observed to increase in the range of 0.5 V to 1.3 V for the change in operating conditions. The effect of electrode material showed growth of porous fractal structures on copper and surface cracks on nickel. The fractal structures deposited on copper electrode improved the surface area of an electrode. The electrodeposited copper samples tested as cathode for fuel cell application showed an increase in iii

5 magnitude of DC output voltage. However, electrodeposited nickel showed a decrease in the magnitude of DC output voltage. iv

6 To my Family.for their love, support and encouragement to let me achieve what I desire.

7 Acknowledgments I would like to thank my Adviser Dr. Yong Gan for an opportunity to work under him. His perseverance, advice and support through my research helped me achieve my goal. A special vote of thanks to Dr. Mehdi Pourazady and Dr. Matthew Franchetti for being a part of my thesis defense committee. Last, by no means least, would like to thank all my friends Boya Zhang, Hemant, Purnima and Srinivasa Rao for their support and guidance through the course of my degree. vi

8 Table of Contents Abstract...iii Acknowledgments... vi Table of Contents... vii List of Tables... ix List of Figures... x Introduction Fuel Cell Architecture Classification of Fuel Cell... 3 Literature Survey Electrodeposition Electrodeposition Mechanism Diffusion Limited Aggregation Cyclic Voltammetry Scope and Objective Experimental Materials and Methods vii

9 4.1 Experimental Setup Electrodes Electrolytes Instrumentation Experimental Method Electrodeposition Morphology and Composition Analysis Fuel cell application test Results and Discussion Cyclic Voltammetry of Electrodeposition of Copper Effect of Electrolyte Concentration Effect of Electrode Potential Effect of Electrode Material Characterization of Electrodeposits Characterization of electrodeposits on copper electrode Characterization of electrodeposits on Nickel electrode Characterization of Ni electrodeposits on Nickel and Copper electrode Fuel Cell Applications Conclusion Future Recommendations References viii

10 List of Tables Table 4-1. Electrolyte Properties Table 5-1. Comparison of Reduction Potential (E r ) and Cathode Current (I c ) for Various Molar Concentrations of CuSO 4 5H 2 O Table 5-2. EDS spectrum denotations ix

11 List of Figures 1-1. Basic Fuel Cell Architecure PEM Fuel Cell Alkaline Fuel Cell Molten Carbonate Fuel Cell Solid Oxide Fuel Cell Electrodeposition Cell Experimental Setup Hitachi S-4800 High Resolution Scanning Electron Microscope Fuel Cell Application Setup Cyclic Voltammogram of 0.05 M CuSO4 5H2O solution using Cu electrode at -2 V electrode potential Cyclic Voltammogram of 0.10 M CuSO4 5H 2 O solution using Cu electrode at -2 V electrode potential Cyclic Voltammogram of 0.25 M CuSO 4 5H 2 O solution using Cu electrode at -2 V electrode potential Cyclic Voltammogram of 0.50 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential x

12 5-6. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -0.5 V electrode potential Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -1.0 V electrode potential Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -3.0 V electrode potential Cyclic Voltammogram of 1 M CuSO 4 5H 2 O solution using Cu electrode at -4.5 V electrode potential Comparison of Cyclic Voltammogram of Cu and Ni in (a) 0.05 M (b) 0.1 M (c) 0.25 M and (d) 0.5 M CuSO 4 5H 2 O solution at -2.0 V electrode potential Electrodeposited Cu electrode using 0.25M CuSO 4 5H 2 O at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum Electrodeposited Cu electrode using 1M CuSO 4 5H 2 O solution at -3.0 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Cu electrode using 1 M CuSO 4 5H 2 O solution at -3.0V (a) X 3µ m (b) ) X 10µ m (c) ) X 50µ m Electrodeposited Cu electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Cu electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) X 3µ m (b) X 20µ m (b) X 100µ m xi

13 5-17. Electrodeposited Ni electrode using 0.25 M CuSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Ni electrode using 0.25 M CuSO4 5H2O solution at -2.0 V (a) X 500 nm (b) X 30µ m (c) X 50µ m (d) X 100µ m Electrodeposited Ni electrode using 0.50 M CuSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Ni electrode using 0.5 M CuSO 4 5H 2 O solution at -2.0 V (a) X 30µ m (b) X 50µ m (c) X 100µ m (d) X 400µ m Electrodeposited Ni electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Ni electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) X 5µ m (b) X 50µ m (c) X 300µ m Electrodeposited Cu electrode using 0.50 M NiSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Cu electrode using 1.0 M NiSO 4 5H 2 O solution at -2.0 V (a) X 3µ m (b) X 20µ m (c) X 30µ m (d) X 50µ m Electrodeposited Ni electrode using 0.50 M NiSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum High Magnification SEM images of electrodeposited Cu electrode using 1.0 M NiSO 4 5H 2 O solution at -2.0 V (a) X 10µ m (b) X 20µ m (c) X 40µ m (d) X 200µ m Fuel Cell Output Voltage using Copper electrodes in 1.0 M CuSO 4 5H 2 O ( E P = -4.5 V) (a) without UV (b) with UV xii

14 5-28. Fuel Cell Output Voltage using Copper electrodes in 1.0 M CuSO 4 5H 2 O ( E P = -3.0 V) (a) without UV (b) with UV Fuel Cell Output Voltage using Nickel electrodes in 1.0 M CuSO 4 5H 2 O ( E P = -4.5 V) (a) without UV (b) with UV xiii

15 Chapter 1 Introduction With the growing uncertainty surrounding the availability of fossil fuels, their increasing prices and in an effort to reduce carbon monoxide emissions and greenhouse effect, a need to research on alternative energy sources has risen. Alternative energy resources are renewable and use the natural sources of energy such as sun, air and water. Investing in technologies, that use alternative energy resources for high energy generation efficiency, has increased over the last two decades. Fuel cell is one of the most promising technologies in the future to generate electricity using the natural sources. 1.1 Fuel Cell A fuel cell is a device that converts chemical energy into electricity through an electrochemical process that combines hydrogen and oxygen to produce electricity. Since no combustion of fuel is involved in fuel cells, the process is clean, quiet and highly efficient with the generation of water and heat as its by-product. A fuel cell provides a DC (direct current) voltage that can be used to power electrical appliances Architecture Fuel cell architecture consists of two electrodes cathode and anode sandwiched around an electrolyte. Fuel is injected through the anode section and oxygen (from air) through the 1

16 cathode section. Hydrogen for the fuel cell can be used from a number of hydrocarbon fuels including natural gas, methanol, propane, biomass and gasoline. The anode is oxidizing that should hold hydrogen and the cathode is reducing. Using a catalyst, the anode oxidizes the fuel splitting the Hydrogen atom into a proton (hydrogen ion) and an electron. Figure 1-1. Basic Fuel Cell Architecure The reactions occurring at the two electrodes are shown below At anode H 2 2H + +2e - At cathode 2H + + ½ O 2 + 2e - H 2 O 2

17 The proton passes through the electrolyte and the electrons pass through an external wire creating a separate current that can be utilized before transferring them to the cathode. The hydrogen ion upon reaching the cathode reunites with the electron and oxygen to form water molecule that is released as a by-product Classification of Fuel Cell Fuel cells are classified based on the type of electrolytes used and the temperature of operation. They are as follows a. Polymer Electrolyte Membrane (PEM) Fuel Cell: Figure 1-2. PEM Fuel Cell 3

18 PEM fuel cells (also called as proton exchange membrane fuel cell) use a solid polymer as an electrolyte and porous carbon electrodes along with platinum catalyst (Figure 1.2). Platinum is used as a catalyst as it is the most chemically active substance for low temperature hydrogen separation. The cell uses hydrogen, oxygen and water to operate. PEM fuel cells operate at low temperatures (around 60 o C to 100 o C) and usually require less warm up time. Hence they are primarily used for transportation application. b. Direct methanol Fuel Cell Direct methanol fuel cells (DMFCs) use liquid methanol fuel instead of hydrogen. Pure methanol is mixed with steam and fed directly to the fuel cell anode. On reacting with the steam, methanol breaks down to hydrogen ions and electrons with the liberation of carbon dioxide gas. Protons transported through the proton exchange membrane react with the oxygen to produce water. The half-reactions are At anode At cathode CH 3 OH + H 2 O 6 H + + 6e - + CO O H + + 6e - 3H 2 O DMFCs operate at a slightly higher temperature of 50 o C to 120 o C. DMFCs mainly focus on small mobile power applications such as laptops and cell phones. 4

19 c. Alkaline Fuel Cell Alkaline Fuel Cells (AFCs) are designed to operate in an electrolyte solution of potassium hydroxide (KOH) using hydrogen gas as fuel. AFCs use a variety of nonprecious metal as catalyst thus reducing the need for expensive platinum catalysts (Figure 1.3). The operating temperatures of these fuel cells are usually high (23 o C to 250 o C). AFCs are used in controlled aerospace and underwater applications. Figure 1-3. Alkaline Fuel Cell d. Phosphoric Acid Fuel Cell Phosphoric acid fuel cell (PAFC) are the most widely used fuel cell technology that s generates power and useful steam for heating purposes. PAFC use liquid phosphoric acid 5

20 as an electrolyte and platinum as the catalyst. The operating temperature is in the range of 150 o C to 200 o C. e. Molten Carbonate Fuel Cell Figure 1-4. Molten Carbonate Fuel Cell Molten Carbonate Fuel Cell (MCFCs) are high-temperature fuel cells that use an electrolyte composed of molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide (LiAlO 2 ) (Figure 1.4). Due to high operating temperatures (600 o C to 750 o C), MCFCs are being developed for coal based and natural gas power plants for electrical utility and military applications [3]. 6

21 f. Solid Oxide Fuel Cell Figure 1-5. Solid Oxide Fuel Cell Solid Oxide fuel cells (SOFCs) use a non-porous ceramic compound as the electrolyte. SOFCs operate in the temperature range of 800 o C to 1000 o C. Metal catalysts are not needed for these fuel cell operations mainly due to the high operation temperature. Heat is recycled from the steam produced during the operation of SOFCs thus increasing its operating efficiency. SOFCs are used for medium to large scale applications like on-site power generation and auxiliary power units for military and telecommunications. 7

22 Chapter 2 Literature Survey This chapter contains a literature pertinent to the electrodeposition process, materials and methods, techniques used to study and explain the mechanisms of electrodeposition. Material deposition can be achieved using different techniques and methodology. They are chemical vapor deposition technique (CVD) [1], molecular beam epitaxy (MBE) [2], closed space vapor transport technique [3], sonoelectrochemical method [4-6], chemical bath deposition, and electrodeposition. Electrodeposition offers advantages over competing technologies such as physical and chemical vapor deposition, in that it requires simpler instrumentation and operating conditions [7]. With the trend toward miniaturization, electrodeposition has established itself as the manufacturing technology of choice [8,9]. It is important to study the structure, composition, properties, performance and applications of different materials using electrodeposition technique. 2.1 Electrodeposition Electrodeposition is the process of depositing a material having a desirable form on a substrate by means of electrolysis. This process is used to alter the characteristics of a surface so as to provide improved appearance, resistance to corrosion and abrasion and to improve the thermo electric characteristics of the material [10]. Another application of electrodeposition is to develop the thickness of materials so as to provide increased 8

23 surface area for better conductivity and reduction properties in the application of fuel cells. Electrodeposition Process is interdisciplinary in nature and is used for study in the fields of electrochemistry, electrochemical engineering, solid state physics, metallurgy and materials and electronics [11]. Electrochemistry applies to the study of the electrode processes while Electrochemical engineering studies the transport phenomenon involved during the electrodeposition process. Solid State physics uses quantum mechanical solid state concepts to study the electrode process. Metallurgy and material science study the properties of deposits and electronics use this fabrication process for modern instrumentation. Figure 2-1. Electrodeposition Cell A typical electrodeposition cell (Figure 2.1) consists of two electrodes dipped in an electrolyte containing the metal to be deposited. Generally the cathode is the substrate on 9

24 which material has to be deposited. A voltage is applied across the two electrodes in an electrolysis cell which results in a potential difference. This potential gradient result in the oxidation of anode to release positively charged cations and negatively charged electrons. The negatively charged electrons flow through an external circuit and are available at the cathode. The cations transported to the cathode through the electrolyte medium combine with the electrons and form a deposit layer on the cathode. 2.2 Electrodeposition Mechanism The rate of electrodeposition reaction, is the rate at which cations are delivered to the cathode, depends in the prevailing hydrodynamic conditions near and at the cathode surface. The three main mechanisms involved in the delivery of ions are migration, convection and diffusion [12]. Migration refers to the movement of charge through a solution due to a potential gradient. The magnitude of the potential gradient defines the rate of reaction and usually increases with the increase in potential gradient. The velocities of ion transfer in this mechanism are very low and hence its contribution is usually neglected for mathematical simulations [13]. Convective mass transport is the mechanism which involves the movement of the bulk solution (electrolyte) which can be forced (by stirring) or by the natural circulation of liquid due to difference in solution density caused by thermal effects. The movement of ions is limited only to the bulk state and it usually ceases to be significant in the region close to the electrode surface. Convection is an important mechanism as it not only 10

25 involved with the movement of ions through the movement of the electrolyte, but it also determines the thickness of diffusive layer. Diffusion mechanism refers to the movement of ions close to the electrode surface as a result of concentration gradient existing. In an open circuit condition, the concentration of ions at the electrode surface is the same as that of the bulk solution. Once current flows through the circuit, the oxidation of the anode results in a high concentration region while the reduction of cathode results in the formation of low concentration region. This difference in the concentration gradient acts as the driving force of the diffusion process and is described by Fick s law, which states the flux of matter (Φ) is directly proportional to concentration gradient: Where A is the Area of the electrode D 0 is the diffusion coefficient C 0 is the initial concentration Φ= AD ( C x ) For a reversible electrochemical reaction, the concentration of redox species at the electrode surface (C 0 and C r ) and rate of reaction (K 0 ) are controlled by the electrode potential (E) applied. This is explained using Nernst equation: = ln( / ) According to this equation, as the applied potential (E) is varied the fraction C r /C 0 changes as E is directly proportional to ln(c r /C 0 ). This ratio increases as the applied potential becomes negative and decreases when the potential becomes positive. 11

26 The thickness of the diffusive layer [14] for current densities is given by the equation Where Γ is the gamma function = 4 3 (3 ύ ) / l is the intermembrane distance y is the linear velocity of solution (For low current densities) In a state of static state the value of δ would be approximately 0.2mm but under the conditions of forced convection (stirring), δ can reach values as low as mm [12]. 2.3 Diffusion Limited Aggregation In materials such as copper, diffusion limited aggregation is the driving force of the electrodeposition process [15]. Diffusion limited aggregation is a process in which particles, undergoing a random walk due to Brownian motion, cluster together to form aggregates of the same. These processes produce a relatively compact cluster (known as Fractals) whose density correlations are independent of distance in the limit of large size [16]. Fractals structures are observed in wide range of disciplines like Biology (cells, lungs, and nerves), nature (mountain ranges, snowflakes, tree roots) and chemistry (electrodeposition of Cu). Investigations of diffusion limited aggregation on the growth of fractal structures through electrodeposition of Cu are available in a number of texts [17-22]. 12

27 The properties of these structures depend on a number of factors such as composition grain size and growth orientations of the fractals, which are in turn are affected by the deposition conditions. These parameters include current density, electrolyte composition, and temperature. A detailed study of these factors is essential for the successful electrodeposition of high aspect ratio geometries with well-defined properties. An understanding of metal electrode properties will help us to identify the right process conditions to produce high quality nanostructures. 2.4 Cyclic Voltammetry Cyclic Voltammetry (CV) is an electrochemical method used to understand the basic electrochemistry of a reaction. CV incorporates the chemistry of a reaction into a circuit and controls the reaction using the voltage as the circuit parameter. It was first practiced using a hanging mercury drop electrode [23]. This technique provides information on the thermodynamics of redox processes, the kinetics of heterogeneous electron transfer reactions and coupled reactions. A typical CV setup would consist of a Reference electrode, Auxiliary electrode, Working electrode and Potentiostat. The Potentiostat is an instrument that controls the potential of the working electrode with respect to the reference electrode while measuring the current flow between the working electrode and counter electrode. In CV experiment, the potential of the working electrode is swept using a triangular wave form as shown in the figure. The corresponding current flowing through the working electrode is recorded as a function of potential and presented as plot of current versus potential known as Voltammograms. Several monographs [24-27] offer excellent information on fundamentals of Cyclic Voltammetry. 13

28 Chapter 3 Scope and Objective The surface area of an electrode plays an important role in the performance of a fuel cell. The amount of current generated by a fuel cell depends on the effective surface area of the fuel cell electrode exposed to the fuel. This aspect can be increased by growing fractal structures on the surface of the material so as to increase the porosity. The characterization of fractal structures have been studied in the context of dimensions, surface roughness and nucleation mechanisms [28]. We have studied the growth of fractal structures using electrodeposition technique with an intention to increase the surface area of the electrode samples. The deposition process uses simple instrumentation and operating conditions. The operating conditions: electrolyte concentration, electrode potential and electrode material, that effect the growth of these structures were studied using cyclic voltammetry and SEM image analysis. The deposited samples were tested as the cathode electrode in a fuel cell setup and their results are discussed in the following chapters. 14

29 Chapter 4 Experimental Materials and Methods In this chapter, we discuss the materials, electrolytes and apparatus equipments used for the experimental setup. A typical three electrode system consisting of a working electrode, counter electrode and reference electrode is used for the electrodeposition process. The experiments conducted used two types of metals as working electrodes: Copper and Nickel under varying operating conditions of electrolyte composition and electrode potential. The results of the experiments are used as a basis to identify and compare the process parameters that govern the non-equilibrium electrodeposition of microstructures on the two metals. The process parameters under verification are the electrolyte composition and varying electrode potential. Copper and Nickel are electroplated on the working electrode with varying compositions of electrolyte and electrode potential. The samples are then studied for surface morphology and chemical compositions by using SEM techniques. 4.1 Experimental Setup The experimental setup uses a standard three electrode cell consisting of a working electrode (WE), auxiliary electrode (AE) and a reference electrode (RE) dipped in an electrolyte solution (Figure 4.1). The WE is connected to the negative terminal (cathode) 15

30 and AE is connected to the positive terminal of an external source called electrochemical analyzer (Potentiostat). The RE is connected in line with the AE( anode) to control the voltage input to the system. A Computer is connected to the electrochemical analyzer for Data acquisition. The materials and roles of each part is explained in the following sections. Figure 4-1. Experimental Setup 16

31 4.1.1 Electrodes The cathode and anode electrodes used for electrodeposition were Copper and Nickel and were purchased from Alfa Aesar, Ward Hill, Maryland. Both the wire s are 0.25mm in diameter and equal lengths of 25mm were cut to use for all the experiments. Ag/AgCl which was used as the reference electrode was purchased from Chem Instrument, Austin, Texas. The reference electrode is Silver wire coated with Silver Chloride (AgCl). This electrode is in turn sealed in glass tubing with Teflon heat shrinkable tubing and the electrode compartment is filled with 1.0 M Potassium Chloride (KCl) solution Electrolytes Two electrolytes were used for the electrodeposition of Cu and Ni. They are Copper (II) sulphate pentahydrate (99%) and Nickel (II) sulphate hexahydrate (98%).They were purchase from Alfa Aesar, Ward hill, Maryland and their properties are listed in Table 4.1. Table 4-1. Electrolyte Properties Electrolyte Formulae Appearance Molar Mass Density Copper(II)sulphate pentahydrate (99%) CuSO 4.5H 2 O Crystalline Nickel(II)sulphate hexahydrate (98%) NiSO 4.6H 2 O Crystalline g/mol (anhydrous) g/mol (pentahydrate) g/mol (anhydrous) g/mol (hexahydrate) g/cm 3 (anhydrous) 2.28 g/cm 3 (pentahydrate) 3.68 g/cm 3 (anhydrous) 2.07 g/cm 3 (hexahydrate) 17

32 Molar concentrations of the electrolytes measuring to 0.05 M, 0.1 M, 0.25 M, 0.50 M and 1.0 M were prepared to study the effect of molar concentration on the electrodeposition process. Molar concentration is defined as a measure of the concentration of a solute in a solution. Mathematically it is defined as the amount of solute per unit volume of solution. = = / Where c is molar concentration n is the amount of solute in moles N A is Avogadro constant C is the number concentration In our case the electrolyte crystals are the solute and distilled water is the solution Instrumentation The Potentiostat used for all our experiments is a Electrochemical analyzer manufactured by CH instruments, Inc. The Electrochemical analyzer uses a computer for data recording and presents the results in the form of both graphs and data files. A wide range of voltammetric techniques can be analyzed using this instrument. It basically applies a potential difference between the working electrode and reference electrode while measuring the current flow between the working electrode and auxiliary electrode. 18

33 4.2 Experimental Method Our experimental methodology is divided into three main sections as discussed below Electrodeposition The first part of our experimental procedure involved the electrodeposition of Cu and Ni electrodes with either Cu or Ni. Similar metal to metal electrodeposition is done with the aim to increase the efficiency of the electrode by increasing their surface area. The experimental setup is shown if Figure 4.1. At the start of the experiment, the electrochemical analyzer is calibrated to the required scan rate. A low and high electrode potential is specified as per the need of study. We used a range of low potential from V to -4.5 V to study the effect of voltage on the electrodeposition process. During the experiment, the current data is collected by the computer and plotted against the voltage input to present a cyclic voltagramms. These cyclic voltagramms are used to analyze the current-voltage trend and estimate reduction potentials and currents of the electrodes. The experiments were repeated for different concentrations of the electrolyte as well to study their effect on the electrodeposition process Morphology and Composition Analysis The second step of our experimental procedure included the morphological and compositional study of the structure deposited on the electrodes. Optical micrograph was recorded using a ProScope HR to obtain low magnification pictures of electrodeposits. Selected test samples were collected for SEM image analysis to extract more accurate information on characteristics and parameters of Cu and Ni electrodes. Hitachi S

34 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 Figure 4-2. Hitachi S-4800 High Resolution Scanning Electron Microscope accelerating voltage of 200 kv is shown in Figure 4.2. Finally X-ray spectroscopy (EDS) was used to obtain detailed information of the composition of the deposits. A high acceleration voltage of SEM is used for EDS analysis to activate a large volume of material and map the material composition on a EDS spectrum Fuel cell application test The last step of our experimental method was to test the electrode for a fuel cell application. A fuel cell setup as shown in the Figure 4.3 is setup and the electrodeposited material is connected to the cathode section as shown. Platinum is used as the anode for its wide known knowledge of hydrogen generation and chemical inertness. A reference 20

35 electrode Ag/AgCl is used to record the voltage and time data. The electrolyte used is a mixture of 20% ethanol, 20% phosphoric acid and 60% distilled water. An external Figure 4-3. Fuel Cell Application Setup energy in the form of ultraviolet rays is supplied through an ultraviolet energy source. In the first setting, the potential versus time graph is recorded for a plain metal electrode. Then the electrodeposited sample was tested for the same operating conditions to verify the effectiveness of the fractal structures deposited on the metal. 21

36 Chapter 5 Results and Discussion The Experiments were performed with the aim of studying the microstructures deposited on nickel and copper metals. The first section of this chapter includes the study of Cyclic Voltammograms to understand the electrochemical properties of the electrode under different experimental conditions of electrolyte concentration and electrode potential (V). The Second half discusses the characterization of electrodeposits using images from Scanning electron microscope (SEM) and X-ray diffraction analysis. The final section discusses the validation of the samples as the cathode electrode in a fuel cell setup. 5.1 Cyclic Voltammetry of Electrodeposition of Copper Cyclic Voltammetry (CV) is an effective electroanalytical technique used to study the basic electrochemistry of a reaction. It is used to incorporate the chemistry of a reaction into a circuit and control the reaction using the voltage as the circuit parameter Effect of Electrolyte Concentration The effect of electrolyte concentration on electrodeposition is explained in the following sections using Cyclic Voltammograms. The Cyclic Voltammograms discussed correspond to 0.05 M, 0.10 M, 0.25 M, 0.50 M and 1.0 M electrolyte concentrations of 22

37 CuSO 4 5H 2 O.The experiments are conducted by cycling the potential of the working electrode linearly between a start potential of 0 V and Low Potential of -2.0 V. Negative potential is applied to favor for electrodeposition. The scan rate used for all the experiments in this section is 0.1 V/s. Figure 5-1. Cyclic Voltammogram of 0.05 M CuSO4 5H2O solution using Cu electrode at -2 V electrode potential. Figure 5.1 shows a Cyclic Voltammogram of 0.05 M CuSO 4 5H 2 O solution using Cu as the working and auxiliary electrode. The start and end electrode potentials for the experiment are 0 V and -2.0 V respectively. The graph signifies two scan curves namely forward scan curve and reverse scan curve. In the forward scan, an initial increase in cathode current (I c ) is observed due to the fact that working electrode potential is greater than the reduction potential (E r ). A peak signifying the reduction potential (E r ) is observed at V with a Cathode current (I c ) measurement of A followed by a gradual decrease in the current. The decrease in current can be explained due to the 23

38 decrease in concentration of the electrolyte compound close to the electrode surface. The reduced compound close to the electrode surface diffuses towards the working electrode and accounts for the drop in current. At anode, the Cu metal is oxidized introducing more Cu +2 anions and negatively charged electrons into the electrolyte, the current once again increases due to the availability of electrons. During the reverse scan, the cathode current (I c ) decreases gradually and then changes to anodic current (I a ). The shape of the reverse scan curve is very similar to the forward scan curve with a difference in magnitude of the current. No distinctive peaks were observed during the reverse scan. The current decreases to the original start value signifying the complete oxidative dissolution of Cu at the electrode surface. Figure 5-2. Cyclic Voltammogram of 0.10 M CuSO4 5H 2 O solution using Cu electrode at -2 V electrode potential. In Figure 5.2, the electrolyte used is 0.10 M CuSO 4 5H 2 O solution with the same electrode and experimental setup. During the forward scan, the reduction potential (E r ) is 24

39 observed at the same value of V with an increase in the Cathode current (I c ) to A. The increase of current at this peak is observed for the increased molar concentration of the electrolyte due to the fact that more electron charge carriers are available in this system. However, in the reverse scan, a crossover occurs between the cathodic and anodic current traces at a value of V. The oxidation of Cu in the reverse scan starts from the cathode where copper has been deposited, resulting in a potential close to its equilibrium potential. This difference in deposition and dissolution potential accounts for the crossover and the potential at this point is called crossover potential (E x ). Figure 5-3. Cyclic Voltammogram of 0.25 M CuSO 4 5H 2 O solution using Cu electrode at -2 V electrode potential. For an electrolyte concentration of 0.25 M and 0.50 M CuSO 4 5H 2 O, there is a slight decrease in the reduction potential to V (Figure 5.3) and V (Figure 5.4) respectively. An increase in the cathode current (I c ) at these potentials is observed with 25

40 their values being A and A respectively. Crossover between the two scans occurs at a potential of V for only 0.25 M CuSO 4 5H 2 O electrolyte solution, the reasons being the same as explained above. No Crossover is observed in the cyclic Voltammogram of 0.50 M CuSO 4 5H 2 O electrolyte solution. Figure 5-4. Cyclic Voltammogram of 0.50 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution is shown in Figure 5.5. The graph denotes a reduction potential (E r ) of V and cathode current (I c ) of 0.13 A.There is a gradual increase in the cathode current after the reduction peak which is due to the increase in electrolyte concentration. The electrolyte in this region dissolute and release excess electron which favors for the increase in current. Crossover is observed at a lower potential value of V. This is due to the difference in dissolution and deposition potential. 26

41 Figure 5-5. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential. Table 5-1. Comparison of Reduction Potential (E r ) and Cathode Current (I c ) for Various Molar Concentrations of CuSO 4 5H 2 O Electrolyte Concentration Reduction Potential (E r ) Cathode Current (I c ) 0.05 M V A 0.10 M V A 0.25 M V A 0.50 M V A 1.0 M V A 27

42 Table 5.1 lists the reduction potential (E r ) and Cathode current (I c ) obtained from the Cyclic Voltammograms of different molar concentrations of the electrolyte. It is observed that as the concentration of the electrolyte increases, the magnitude of reduction potential (E r ) increases from 0.73 V to 1.25 V. This increase in magnitude is due to the availability of more number of moles of solute per liter of solution. The cathode current (I c ) increases as the concentration of the electrolyte increases due to the increase in the magnitude of reduction potential (E r ) Effect of Electrode Potential The effect of electrode potential on the non-equilibrium electrodeposition process is studied by keeping the concentration of the electrolyte constant. The electrolyte used for this study is Copper Sulphate (CuSO 4 5H 2 O) and Cu is used as the working and auxiliary electrode. Different electrode potentials ( -0.5 V, -1.0 V, -2.0 V, -3.0 V, -4.5 V ) are applied to the system using the electrochemical analyser. The response of the system to various electrode potentials is interpretted using the cyclic voltammograms. At an electrode potential of -0.5 V applied between the working and reference electrode, a voltage-current response shown in Figure 5.6 is observed. At the start of forward scan, there is no change in current till -0.2 V because the applied potential is far from the reduction potential of the compound. Then the current gradually increases and reaches a maximum cathode current value of 0.05 A at a reduction potential of -0.5 V. A crossover occurs early in the reverse scan stage at a value V. 28

43 Figure 5-6. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -0.5 V electrode potential. Figure 5-7. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -1.0 V electrode potential. 29

44 When Copper is used as the cathode in a 1.0 M CuSO 4 5H 2 O solution at a higher electrode potential of -1.0 V (Figure 5.7), forward and reverse scan are smooth with cross over occurring at a value which is a bit less than half value of the electrode potential applied. During the forward scan, there is no steep increase in current until -0.2 V and there on it increases gradually to form a reduction peak at -0.9 V with a cathode current value of 0.14 A. Figure 5-8. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -2.0 V electrode potential. At an electrode potential of -2.0 V applied to Cu, the reduction potential value shifts to the right in the Cyclic Voltammogram as shown in Figure 5.8 with its value being -1.2 V. Compared to the previous case it is observed that reduction potential shifts to a lower value as the magnitude of electrode potential applied increases. This shift in reduction potential value indicates that reduction of cathode occurs at a later stage of forward scan. 30

45 The magnitude of crossover potential also increases implying dissolution to occur early in the reverse scan. Figure 5-9. Cyclic Voltammogram of 1.0 M CuSO 4 5H 2 O solution using Cu electrode at -3.0 V electrode potential. For an increase in the electrode potential value to -3.0 V and -4.0 V, no crossover is observed between the forward scan and reverse scan (Figure 5.9 and Figure 5.10). At an electrode potential of -3.0 V applied to the system, the reduction potential is observed to be -1.3 V with a cathode current value of 0.1 A and for an electrode potential of -4.0 V, the reduction potential occurs at -1.2 V with a cathode current value 0.12 A. For both the cases, no major difference in the reduction potential value is observed. When -4.0 V is applied during the electrodeposition, the current becomes constant when a potential of V is reached. This is observed in the case of reverse scan as well which is due to saturation state reached by the system where no more electrons are available to conduct 31

46 Figure Cyclic Voltammogram of 1 M CuSO 4 5H 2 O solution using Cu electrode at -4.5 V electrode potential. current. The reverse scan curve is a straight line implying that the electrode behaves as a conductor material obeying Ohm s law. Hence as the magnitude of electrode potential is increased, it is clearly observed that its corresponding reduction potential magnitude increases with an increase in current value. The reverse scans for the voltammogram in this section were smooth straight line with crossover occurring for electrode potential cases of -0.5 V, -1.0 V and -2.0 V Effect of Electrode Material The effect of electrode material on Voltammograms is presented in Figure The voltammograms to the left are of Copper material and the ones to the right are of Nickel. Each row indicates the electrodeposition using different electrolyte concentration of Copper (II) Sulphate pentahydrate at a electrode potential value of -2.0 V. 32

47 For 0.05M electrolyte concentration, a slight difference in the magnitude of reduction potential and cathode current is observed. Nickel has a E r = -0.8V and I c = A when compared to Copper with E r = -0.7V and I c = A. The crossover potential in Nickel occurs earlier in the reverse scan at Ex=-1.15V indicating that oxidation of material starts earlier when compared to Copper which has a E x = -0.91V. Figure Comparison of Cyclic Voltammogram of Cu and Ni in (a) 0.05 M (b) 0.1 M (c) 0.25 M and (d) 0.5 M CuSO 4 5H 2 O solution at -2.0 V electrode potential. 33

48 No major difference is observed in the Voltammograms of Copper and Nickel at 0.1M and 0.25 M concentration. However a difference in the cross over potential is observed for 0.5 M concentration in Nickel and Copper. Crossover occurs early in the reverse scan of Nickel metal when compared to Copper indicating the start of dissolution in Ni at an early stage. The value of crossover potential for copper and nickel are V and -1.1 V. Hence it is observed that difference in cross over potential exists for the two metals under similar deposition conditions indicating the dissolution of Nickel occurs faster than Copper during the reverse scan. This implies the deposition on Copper material is more when compared to Nickel. 5.2 Characterization of Electrodeposits Optical and Compositional analysis of the electrodeposits were carried out. An optical micrograph was recorded using a ProScope HR TM which shows the image of the electrodeposits under low magnification. The SEM provides the image of the deposit under high magnification and the X-ray element mapping furnishes information about the element distribution. Hitachi S-4800 High Resolution Scanning Electron Microscope and an electron microprobe (EMP) with energy dispersive X-ray spectroscopy (EDS) were used to obtain detailed information of the composition of the deposits. The denotations used in the EDS spectrum are tabulated in Table

49 Table 5-2. EDS spectrum denotations Metal Carbon Aluminium Oxygen Sulphur Nickel Copper Tantalum Denotation C Al O S Ni Cu Ta Characterization of electrodeposits on copper electrode Three samples of electrodeposited Cu electrodes under different operating conditions (concentration and electrode potential) were analyzed. The three cases used CuSO 4 5H 2 O solution at 0.25 M, 1.0 M and 1.0 M concentration with an applied electrode potential of V, -3.0 V and -4.0 V respectively. The figures presented in the following sections show (a) Low magnification scans using optical ProScope HR TM (b) High magnification SEM images using a Scanning electron Microscope and (c) EDS spectrum using a Scanning Electron Microscope and an electron microprobe (EMP) with energy dispersive X-ray spectroscopy. Figure 5.12 presents the results of characterization of Cu sample electrodeposited in 0.25 M concentration of CuSO 4 5H 2 O at an electrode potential of -2.0 V. The electrodeposition of Cu in this conditions show uneven deposition of material on the electrode surface. SEM image show growth of fractal structures which are not distinctive and triangular in shape. Some void areas where no electrodeposition occurred are observed in these images. The complete surface area of the electrodes was not effectively increased due to the low concentration of electrolyte used. The EDS spectrum confirms 35

50 the deposit of Cu at three peak values of 1keV, 8 kev and 9 kev. High peaks were observed at 1 kev and 8 kev and a weak signal was observed at 9 kev. Figure Electrodeposited Cu electrode using 0.25M CuSO 4 5H 2 O at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum Figure 5.13 presents the results of characterization of Cu sample electrodeposited in 1M concentration of CuSO 4 5H 2 O at an electrode potential of -3.0 V. The optical micrograph shows even deposition of material on the electrode surface with a uniform thickness. High magnification SEM images (Figure 4.14) show growth of dendrite like fractal structures with certain low area fractal deposits. The dendrites are observed to be spherical root like structures. The surface area of the electrode in this operating condition 36

51 is improved compared to the previous case and can be explained due to the increased concentration and increased electrode potential. The EDS spectrum confirms the deposit of Cu at three peak values with high peaks occurring at 1 kev, 8 kev and low peak at 9 kev. Figure Electrodeposited Cu electrode using 1M CuSO 4 5H 2 O solution at -3.0 V (a) Optical image (b) SEM image and (c) EDS spectrum 37

52 Figure High Magnification SEM images of electrodeposited Cu electrode using 1 M CuSO4 5H2O solution at -3.0V (a) X 3µ m (b) ) X 10µ m (c) ) X 50µ m 38

53 Figure Electrodeposited Cu electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) Optical image (b) SEM image and (c) EDS spectrum In the case of electrodeposition of Cu in 1.0 M CuSO 4 5H 2 O at -4.5 V, the thickness of electrodeposition increases as shown in Figure The Fractal growth in increased in this condition with high density dendrite structure observed in the SEM images. The high magnification SEM images (Figure 5.16) show fern tree like copper deposit structures. These deposited fractal structure increase the porosity of the material which is necessary for the use as an electrode in fuel cell applications. The EDS spectrum confirms the electrodeposition of Cu at the same peak values when compared to the other two cases. 39

54 Figure High Magnification SEM images of electrodeposited Cu electrode using 1.0 M CuSO4 5H2O solution at -4.5 V (a) X 3µ m (b) X 20µ m (b) X 100µ m Characterization of electrodeposits on Nickel electrode Three samples of Cu electrodeposited on Ni electrodes under different operating conditions (concentration and electrode potential) were analyzed. The three cases used CuSO4 5H2O solution at 0.25 M, 0.50 M and 1.0 M concentration with an applied electrode potential of -2.0 V, -2.0 V and -4.0 V respectively. The figures presented in the 40

55 following sections show (a) Low magnification scans using optical ProScope HR TM (b) High magnification SEM images using a Scanning electron Microscope and (c) EDS spectrum using a Scanning Electron Microscope and an electron microprobe (EMP) with energy dispersive X-ray spectroscopy. Figure Electrodeposited Ni electrode using 0.25 M CuSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum In the electrodeposition of Cu on nickel wire at an electrode potential value of -2.0 V and 0.25 M electrolyte concentration, a coat of Cu over the nickel base metal is observed (Figure 5.17). No distinctive fractal structures were observed during the SEM analysis. 41

56 Figure High Magnification SEM images of electrodeposited Ni electrode using 0.25 M CuSO4 5H2O solution at -2.0 V (a) X 500 nm (b) X 30µ m (c) X 50µ m (d) X 100µ m The high magnification SEM images (Figure 5.18) show clusters of Cu deposits in certain sections of the electrode sample. The EDS spectrum shows high peak deposition of copper at 1 kev and 8 kev and a low peak at 9 kev. Weak signals of Carbon (C), oxygen 42

57 (O), aluminum (Al) and sulphur (S) deposits were observed. Carbon is mapped from the tape used to fix the sample to the holder during EDS, Oxygen from the Copper oxide layer forming over the electrodeposition, Aluminium from the sample holder and Sulphur is from the electrolyte used during deposition. Figure Electrodeposited Ni electrode using 0.50 M CuSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum In the case of 0.50 M CuSO 4 5H 2 O and electrode potential of -2.0 V (Figure 5.19), the optical micrograph shows non uniform electrodeposition of Cu on Nickel. During the initial stages of the cycle, a uniform deposition of metal is observed but as the voltage increases, the deposition increases only in certain areas resulting in the void formation. 43

58 Figure High Magnification SEM images of electrodeposited Ni electrode using 0.5 M CuSO4 5H2O solution at -2.0 V (a) X 30µ m (b) X 50µ m (c) X 100µ m (d) X 400µ m High magnification SEM images (Figure 5.20) show uneven surface structure with fractal nature deposits. The deposits form branched clusters which are ovular in shape. The EDS 44

59 spectrum shows high peak deposition of Cu and Ni occurring at values 1 kev, 8 kev and 9 kev. Other deposits observed are Carbon, Sulphur, Aluminium, Tantalum and Oxygen which are impurities occurring due to the SEM sample preparation and electrolyte contamination. Figure Electrodeposited Ni electrode using 1.0 M CuSO 4 5H 2 O solution at -4.5 V (a) Optical image (b) SEM image and (c) EDS spectrum In the case of electrodeposition of Cu on Ni in 1.0 M CuSO 4 5H 2 O at -4.5 V, the thickness of electrodeposition increases as shown in Figure High magnification SEM images (Figure 5.22) show honeycomb like structures which are circular in shape. The EDS spectrum confirms the electrodeposition of Cu at the same peak values when 45

60 compared to the other two cases with a small peak deposition of Carbon, oxygen and sulphur. Figure High Magnification SEM images of electrodeposited Ni electrode using 1.0 M CuSO4 5H2O solution at -4.5 V (a) X 5µ m (b) X 50µ m (c) X 300µ m 46

61 5.2.3 Characterization of Ni electrodeposits on Nickel and Copper electrode Nickel is electrodeposited on Copper and Nickel electrode metals using 0.50 M NiSO 4 5H 2 O concentration with an applied electrode potential of -2.0 V. The figures presented in the following sections show (a) Low magnification scans using optical ProScope HR TM (b) High magnification SEM images using a Scanning electron Microscope and (c) EDS spectrum using a Scanning Electron Microscope and an electron microprobe (EMP) with energy dispersive X-ray spectroscopy. Figure Electrodeposited Cu electrode using 0.50 M NiSO 4 5H 2 O solution at -2.0 V (a) Optical image (b) SEM image and (c) EDS spectrum 47