NEW CLASS OF ELECTROCATALYST FOR OXYGEN REDUCTION ALBERT TSAI. A thesis submitted to the. Graduate School New Brunswick

Transcription

1 SYNTHESIS AND CHARACTERIZATION OF LiNiPO 4 NANOCRYSTALS VIA A MICROEMULSION METHOD AS A NEW CLASS OF ELECTROCATALYST FOR OXYGEN REDUCTION By ALBERT TSAI A thesis submitted to the Graduate School New Brunswick Rutgers, the State University of New Jersey in partial fulfillment of the requirements for the degree of Master of Science Graduate Program in Material Science and Engineering written under the direction of Professor Dunbar P. Birnie and approved by New Brunswick, New Jersey January, 2011

2 2011 Albert Tsai ALL RIGHTS RESERVED

3 Abstract of the Thesis SYNTHESIS AND CHARACTERIZATION OF LiNiPO 4 NANOCRYSTALS VIA A MICROEMULSION METHOD AS A NEW CLASS OF ELECTROCATALYST FOR OXYGEN REDUCTION by Albert Tsai Thesis Director: Dunbar P. Birnie This thesis examines the electrocatalytic properties shown by lithium nickel phosphate nanoparticles for the oxygen reduction reaction (ORR), an important cathode reaction in fuel cells. The main drawback to the continued development of fuel cells is the relatively slow rate of the ORR compared with a reaction at the anode of a fuel cell. Electrocatalysts are necessary to increase the rate of oxygen reduction and allow more power to be generated at higher efficiencies. Inspired by ideas from chemical catalysis, lithium nickel phosphate nanoparticles were synthesized to determine the possibility of its use as an electrocatalyst for oxygen reduction. Lithium nickel phosphate nanomaterials were synthesized using a microemulsion approach, with a novel washing and drying technique. The resulting powders were characterized using XRD and FESEM to determine composition and particle size, along with particle uniformity throughout the sample. It was determined that the microemulsion method could be used to predictably tailor the size, shape, and crystalline nature of the product by changing various variables within the synthesis method. The resulting products were studied electrochemically using the rotating ring disk electrode (RRDE) to observe the mechanism of oxygen reduction on ii

4 the LiNiPO 4 surface, along with the effect that the particle size had on the electrocatalysis of the oxygen reduction reaction. The mechanism of oxygen reduction on LiNiPO 4 is via the two electron peroxide pathway, followed by another two electron peroxide dissociation reaction to produce water. The electrocatalysis of oxygen using LiNiPO 4 catalyzes the two electron reaction of oxygen to form peroxide. RRDE results showed that larger particle sizes created larger amounts of electrocatalytic activity. At the largest particle sizes, it was found that peroxide would also be preferentially dissociated at the electrocatalyst surface, producing the full four-electron pathway and increasing the number of electrons transferred per oxygen molecule adsorbed. iii

5 ACKNOWLEDGMENTS I would like to express my thanks and appreciation to my advisor, Professor Dunbar P. Birnie, who took me on as a Masters student and helped me throughout my pursuit of this degree. He has been a great source of encouragement and insight, and even while in India has stayed very much in touch. I d also like to thank Dr. Yiyun Yang, who gave me the inspiration to pursue the research topic that I have worked on, and who has given me instruction and guidance throughout my time in the laboratory. I would never have been able to get this far without their combined efforts, and I m very grateful. I d like to thank all of my committee members for taking the time out of their busy schedules to participate in my defense, it is much appreciated. Professor Lisa Klein helped me a great deal throughout the pursuit of my degree, and I am very grateful for all of the advising that I ve received from her. Mrs. Claudia Kuchinow and Mr. John Yaniero have also helped me very much with my various questions during my time at Rutgers. I d like to also thank Dr. Jafar F. Alsharab for his assistance with the TEM. I couldn t have finished my degree without all of the support that I received from all of these people. Of course I d like to thank my family, who have been supportive and have provided everything that I ve needed throughout my life. I hope that my work has made you proud and that I can continue to make you proud throughout my professional career. iv

6 TABLE OF CONTENTS ABSTRACT OF THE DISSERTATION ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF ILLUSTRATIONS LIST OF TABLES ii iv v viii xi 1: INTRODUCTION 1 1.1: Fuel Cells : Fuel Cell Advantages: Environmental Benefits and Efficiency : Types of Fuel Cells : Low Temperature Fuel Cells (<120 C) : High Temperature Fuel Cells (>500C) : Fuel Cell Technology in the Future 8 1.2: The Oxygen Reduction Reaction : Reaction Pathways : Slow ORR Kinetics : The Need for Electrocatalysts : Environmental Impact and Safety 16 2: THEORY : Potential of Lithium Nickel Phosphate as an Electrocatalyst : Microemulsion Synthesis : Properties of Microemulsions : Microemulsion Synthesis Technique and Emulsion Drying Technique : Controlling Particle Size : Water to Surfactant Ratio : Annealing Temperature and Time 23 v

7 2.4: Physical Characterization Methods : X-Ray Diffraction (XRD) : Electron Microscopy: FESEM and TEM : Electrochemical Characterization Methods : Rotating Ring Disc Electrode (RRDE) : Experimental Determination of Collection Efficiency : Determination of ORR Pathway 33 3: PRODUCT SYNTHESIS: LiNiPO 4 VIA THE MICROEMULSION METHOD : Microemulsion reaction : Drying of Precursor : Heat Treatment of Precursor 38 4: PRODUCT CHARACTERIZATION : Product Phase Determination : Particle Size : Effect of Water to Surfactant Ratio : Effect of Annealing Temperature : Effect of Annealing Time : Electrochemical Properties : Cyclic Voltammetry : Efficiency : Rotating Ring Disk Electrode Collection Experiments : ORR Functionality as an Electrocatalyst : Water to Surfactant Ratio : Annealing Time : Annealing Temperature 59 5: ANALYSIS : Particle Size Effects on the Electrocatalyst : Catalytic Reaction Pathway : Future Research 66 vi

8 6: CONCLUSION 68 REFERENCES 69 vii

9 LIST OF ILLUSTRATIONS Figure 1: Figure 2: Figure 3: Figure 4: Figure 5: Carnot Efficiency Equation: E R thermal is Carnot Efficiency, W r is the reversible work performed, H is the enthalpy change of the reaction, and T 1 and T 2 are the operating temperatures of the heat engine [1]...2 Fuel Cell Efficiency Equation: E R cell is the thermodynamic efficiency, E V is the electrochemical efficiency, E F is the faradaic efficiency, U is the utilization of fuel, and E H is the heating value efficiency. All these factors produce the overall fuel cell efficiency, E fc. [1]...3 Efficiency of a typical fuel cell compared to the efficiencies of current electrical systems [1]...4 From left to right- the Griffiths model, the Pauling model, and the Bridge model[8]...12 Process showing the intermicellar exchange for the microemulsion synthesis of nanomaterials [6]...21 Figure 6: Particle size distributions of LiFePO 4 sintered at various temperatures [37]...23 Figure 7: XRD spectra of LiFePO 4 sintered at different temperatures for 24 hours: a 500 C, b 600 C, c 700 C [27]...24 Figure 8: Bragg Plane Diffraction [39]...26 Figure 9: LiNiPO 4 Secondary Electron Image...29 Figure 10: Velocity profile at a rotating ring disk electrode [47]...31 Figure 11: Figure 12: Figure 13: Diagram of full Microemulsion Method...35 XRD spectra of a sample of synthesized LiNiPO 4, matching the spectra found in the database...39 High Magnification Images (150,000x) of LiNiPO 4 with varying water-tosurfactant ratio (W 0 ) top left W 0 = 7.5, top right W 0 = 12.5, bottom W 0 = viii

10 Figure 14: Low magnification (25000x) images of LiNiPO 4 samples with W 0 values of 7.5 (top) and 17.5 (bottom)...42 Figure 15: XRD Data for products varying by water to surfactant ratios...43 Figure 16: Low magnification (25,000x) images of LiNiPO 4 sintered at 600 C (left) and 700 C (right)...44 Figure 17: Figure 18: High magnification (150,000x) images of LiNiPO 4 sintered at 600 C (left) and 700 C (right)...45 XRD Data for products varying by annealing temperature...46 Figure 19: High Magnification (150,000x) images of LiNiPO 4 sintered at 6 hours (top left), 9 hours (top right), and 12 hours (bottom)...47 Figure 20: Figure 21: Figure 22: Figure 23: Figure 24: Figure 25: Figure 26: Figure 27: Figure 28: Figure 29: XRD Data for samples varying by annealing time...48 LiNiPO 4 sintered at 600 C for six hours...49 TEM bright field image showing both crystalline and amorphous phases within an LiNiPO 4 sample sintered for only 6 hours, along with the corresponding diffraction image...50 Cyclic Voltammograms for coated and uncoated glassy carbon disks in electrolyte saturated by nitrogen and oxygen gas...52 RRDE Efficiency Data at Various RPM...53 RRDE disk current data comparing the electrocatalytic activity on a glassy carbon disk with a coating of LiNiPO 4 and an uncoated glassy carbon disk...55 RRDE data at the disk electrode for LiNiPO 4 with varying water-to-surfactant ratio...56 RRDE data at the ring electrode for LiNiPO 4 with varying water-to-surfactant ratio...57 RRDE data at the disk electrode for LiNiPO 4 with varying annealing time...58 RRDE data at the ring electrode for LiNiPO 4 with varying annealing time...58 ix

11 Figure 30: RRDE data for LiNiPO 4 sintered at different temperatures...59 Figure 31: Three-dimensional structure of LiNiPO 4 [48]...61 Figure 32: Mechanistic diagram of the Pauling model[49]...62 Figure 33: Number of Electrons transferred for each sample tested...63 Figure 34: RRDE disk currents at various water-to-surfactant ratios...65 x

12 1 1: Introduction The need for clean alternative energy is greater today than it has ever been. With the rising prices of oil and the increasing focus on preserving the environment that we live in, the sources in which we generate the energy necessary for everyday living must change. This need has led to the development and research of new, environmentally friendly alternative energy sources, including fuel cells.[1-2,14] The development of fuel cells is driven by this need for a new energy source. The invention of fuel cells as energy conversion systems dates back to the middle of the 19 th century, attributed to Sir William Grove and Christian Friedrich Schonbein [1-2]. However, making them cost effective and efficient enough for practical everyday use is the challenge facing many researchers today. One line of research has focused on the cathode reaction within fuel cells, which is the oxygen reduction reaction. This particular reaction has very slow kinetics, and electrocatalysts are needed to increase the rate of this reaction and avoid large overpotentials at the cathode.[3] Platinum is the best electrocatalyst for oxygen reduction, but it is extremely expensive and not practical for commercial usage.[2] New and less expensive materials are in development to catalyze the oxygen reduction reaction.[1-3] Olivine lithium nickel phosphates, along with other olivine metal phosphates, can have applications as a cost-effective electrocatalyst for the oxygen reduction reaction. Throughout the synthesis and characterization of the olivine lithium nickel phosphates, a great emphasis was placed on determining the effect of particle size on the performance of the product as an electrocatalyst. The microemulsion method specifically allows for control over

13 2 the particle size and the extent of crystal growth. [4-6] The effect of the particle size and shape on the electrocatalytic performance was examined. 1.1: Fuel Cells Fuel cells are galvanic cells, in which the free energy of a chemical reaction is converted into electrical energy.[1-2] With the rising need for alternative energy sources, the high efficiency and environmental benefits of fuel cell technology has made this a widely discussed topic among researchers. However, developing fuel cell technology into commercially practical products has been difficult. New materials for the components of fuel cells must be developed in order for this technology to advance.[2] Many different types of fuel cells are in development, and each has its own properties.[1-2] 1.1.1: Fuel Cell Advantages: Environmental Benefits and Efficiency One major advantage that fuel cells have over current electrical systems is their environmental benefits and high efficiency. In the case of a hydrogen/oxygen fuel cell, the only byproduct is water, and emissions are essentially eliminated. Fuel cells are an attractive energy option over fossil fuels as the focus on the environment is increased. Examination of the high efficiency of fuel cells over other electrical systems provides additional benefits. The efficiency of other electrical systems, with the internal combustion engine as an example, is related to the Carnot efficiency. The Carnot efficiency is the maximum efficiency of a heat engine system, and the equation is displayed in Figure 1.

14 3 Figure 1: Carnot Efficiency Equation: E R thermal is Carnot Efficiency, W r is the reversible work performed, H is the enthalpy change of the reaction, and T 1 and T 2 are the operating temperatures of the heat engine [1] In practice, this efficiency does not surpass 50% for even the most efficient heat engines. The efficiency of fuel cells can exceed these levels. The efficiency of a fuel cell is calculated from the Gibbs free energy and enthalpy of the electrochemical reaction. Theoretically, the entirety of the Gibbs free energy can be converted into electricity. However, realistically there are many natural factors that lower the efficiency of the fuel cell. Taking account the theoretical thermodynamic efficiency and correcting for other factors encountered due to nature, it is possible to determine the efficiency of a certain fuel cell. The overall efficiency for a fuel cell can be calculated by the equation below in Figure 2. Figure 2: Fuel Cell Efficiency Equation: E R cell is the thermodynamic efficiency, E V is the electrochemical efficiency, E F is the faradaic efficiency, U is the utilization of fuel, and E H is the heating value efficiency. All these factors produce the overall fuel cell efficiency, E fc. [1] The electrochemical efficiency corrects for electrode overpotentials and the electrolyte resistances found within each specific fuel cell. This allows for comparison of all fuel cells, regardless of their differing components. The faradaic efficiency corrects for the possibility of parallel reactions occuring within the fuel cell, resulting in a lower current yield than possible. The fuel utilization corrects for fuel cells that use only a portion of the inlet fuel, while the heating value efficiency accounts for inert components in the fuel that is utilized in the fuel cell. Even after all of these corrections, the efficiency of even the most inefficient fuel cell is

15 4 substantially greater than the most efficient current electrical system, demonstrated in Figure 3.[1] Figure 3: Efficiency of a typical fuel cell compared to the efficiencies of current electrical systems [1] The potential of fuel cell technology is enormous, with benefits both to the environment and future energy generation : Types of Fuel Cells There are many types of fuel cells that are currently in use and development. These fuel cells differ in operating temperature, electrolyte, needed fuel, among other properties.[1-2] A table of the properties of all of these fuel cells is shown in Table 1.

16 5 Table 1: Properties of currently commercialized and researched fuel cells[1] The electrolyte determines the name of the fuel cell, except for the case of the direct methanol fuel cell. The direct methanol fuel cell refers to the methanol, rather than hydrogen, used for fuel at the anode reaction. These different types of fuel cells have varying properties, from the anode reaction to the electrolyte to the operating temperature, but common denominator is that all of these fuel cells use oxygen to fuel the cathode reaction. Molten carbonate fuel cells also use carbon dioxide at the cathode, but oxygen reduction takes place at the cathode for each fuel cell. The oxygen reduction reaction is very important for the operation of every fuel cell.[1-2] 1.1.3: Low Temperature Fuel Cells (<120 C) Low temperature fuel cells include alkaline fuel cells (AFC, <100 C), polymer electrolyte membrane fuel cells (PEMFC, C), and direct methanol fuel cells (DMFC, C).[1] AFCs, PEMFCs, and DMFCs have been used in transportation, space, and energy storage. Due to

17 6 the low temperatures of these fuel cells, electrocatalysts are very important for increasing the rate of oxygen reduction. Alkaline fuel cells use an alkaline electrolyte, typically a weight % KOH solution. The advantage of using an alkaline electrolyte is that oxygen reduction kinetics increase in an alkaline environment. This provides the AFC with the highest electrical efficiencies of all fuel cells. However, it only works properly with very pure gases, which is an inconvenience for many applications. The fuel for AFCs is pure hydrogen, and platinum is used for the anode and cathode electrodes. Different electrode materials are in development to lower the cost of AFCs and replace platinum within the electrodes. Space applications for AFCs included the Apollo missions and the Space Shuttle program.[1] Polymer electrolyte membrane fuel cells use a proton exchange membrane as an electrolyte. This was the first type of fuel cell to be used in space systems, used by the Gemini program. The PEMFC was replaced by AFCs because the proton exchange membrane used at that time was found to be unstable after time. However, PEMFC technology advanced greatly with the development of Nafion membranes produced by DuPont. These membranes had a greater conductivity than previous membranes, and improved the operations of PEMFCs. The electrodes used in PEMFCs are porous gas diffusion electrodes to allow the gas to reach the catalyst at the anode and cathode. At the cathode, platinum is the best material to catalyze the oxygen reduction reaction. Other materials are in development to replace platinum as an electrocatalyst, again for cost reasons. At the anode, problems still arise with contaminants within the inlet fuel stream, but anode materials resistant to these contaminants are under development.[1-2]

18 7 Direct methanol fuel cells are special PEMFCs designed to use methanol as the inlet fuel stream instead of hydrogen gas. The fuel stream typically is a 1-2 molar methanol stream. Both catalysts and new proton exchange membranes must be developed to fit the specific needs of the DMFC. This is viewed as an attractive option for development because methanol is much easier to produce than hydrogen gas. However, specific methanol-tolerant catalysts must be developed before this concept can become a reality.[1] 1.1.4: High Temperature Fuel Cells (>500 C) High temperature fuel cells include phosphoric acid fuel cells (PAFCs, C), molten carbonate fuel cells (MCFCs, C), and solid oxide fuel cells (SOFC, C). These fuel cells produce heat as well as power, and have applications as heat and power generators for decentralized stationary energy stations. The high temperatures and heat generation prevent these fuel cells from having applications in mobile devices. The very high temperature fuel cells, MCFCs and SOFCs, also benefit from the increased oxygen reduction kinetics at these temperatures.[1] Phosphoric acid fuel cells are the most advanced system in regards to commercialization. The applications are mostly for stationary power plants and on-site generators. The advantages of the PAFC are its simple construction, stability, and low volatility of the electrolyte at its operating temperatures. The phosphoric acid electrolyte provides very high conductivity, and the concentration of the acid within the cell electrolyte is almost 100%. The electrodes for the PAFC are generally platinum dispersed onto a carbon support. To produce a high rate of oxygen reduction, the platinum electrocatalyst must be at a relatively high loading

19 8 level on the cathode. More cost-effective electrocatalysts are being developed to replace platinum or reduce the loading necessary for oxygen reduction at the cathode.[1] Molten carbonate fuel cells have applications for stationary power and the high temperatures at around C greatly improve the kinetics of oxygen reduction without the need for expensive catalytic solutions. The electrolyte for an MCFC is a molten carbonate stabilized by an alumina-based matrix. It is important to avoid degradation of the electrode materials and electrolyte at the high operating temperatures seen in MCFCs. Cathode materials for MCFCs are typically NiO cathodes. At high temperatures, the NiO cathodes exhibit a large enough rate for oxygen reduction without the use of an electrocatalyst. However, the challenge is the development of a stable anode material and the determination of high temperature cell materials for the actual construction of the fuel cell.[1-2] Solid oxide fuel cells have the same inherent problems due to high temperature that have slowed progress on MCFCs, but the solid oxide material used as the electrolyte is more stable than the molten carbonate electrolyte used in MCFCs. SOFCs operate at an even higher temperature, around C. Both SOFCs and MCFCs have very high efficiencies when paired with other power generators to take advantage of the heat produced by the fuel cells. The construction of the SOFC is very important because of the high temperatures, and much attention has been placed on new designs. The desire to decentralize electrical power sources has increased development on both MCFCs and SOFCs.[1-2] 1.1.5: Fuel Cell Technology in the Future Many questions must be answered for fuel cells to become a practical and cost-effective energy source. There are benefits to fuel cell technology, but the cost is much greater than

20 9 current options for electrical power. The development of new electrocatalysts for the oxygen reduction reaction in each fuel cell s specific environment would assist in making fuel cell technology a viable option. At low temperature operations, the oxygen reduction reaction needs to be catalyzed due to the low rate kinetics. This is true throughout all low temperature fuel cells. The improvement of oxygen reduction kinetics is one of the challenges currently facing fuel cell technology.[1-3] 1.2: The Oxygen Reduction Reaction The oxygen reduction reaction, or ORR, has been a widely studied reaction for fuel cells and other electrochemical applications. Oxygen, a chemically reactive element, is consumed during the oxygen reduction reaction via cathodic reduction. However, the electrochemical reactions involving oxygen are kinetically slow reactions. This has limited the utilization of oxygen in electrochemical reactions, and these processes are not widespread. The overall reaction for the oxygen reduction reaction is given below: This reaction can occur through two overall pathways, one being a direct 4-electron pathway as shown above, and the other being a 2-electron pathway producing hydrogen peroxide in solution before dissociating fully to complete the overall reaction.[3] 1.2.1: Reaction Pathways The oxygen reduction reaction can take place through two pathways, depending on the conditions in the electrochemical cell.[3,7-8] The first pathway is the direct 4-electron pathway,

21 10 and it takes place via the reaction pathway below. All potentials are given versus the normal hydrogen electrode (NHE) at 25 C. Alkaline Solution: O 2 + 2H 2 O + 4e - = 4OH - E = V Acidic Solution: O 2 + 4H + + 4e - = 2H 2 O E = V This pathway involves a number of intermediate steps resulting in the final overall reaction. As shown, the oxygen molecule is reduced to hydroxide ions or water, depending on the solution, and no peroxide is seen in solution.[3] The second pathway is the peroxide pathway, which is a two step process involving the 2-electron electrochemical reaction creating hydrogen peroxide and then the subsequent 2- electron decomposition or reduction of peroxide to water or hydroxide ions. These steps follow the reaction pathway below. Alkaline Solution: O 2 + H 2 O + 2e - = HO 2- + OH - E = V Reduction of peroxide: HO 2- + H 2 O + 2e - = 3OH - E = V Or decomposition of peroxide: 2HO 2- = O 2 + 2OH - Acid Solution: O 2 + 2H + + 2e - = H 2 O 2 E = 0.67 V Reduction of peroxide: H 2 O 2 + 2H + + 2e - = 2H 2 O E = 1.77 V

22 11 Or decomposition of peroxide: 2H 2 O 2 = 2H 2 O + O 2 This pathway results in the presence of hydrogen peroxide in solution, before it is either reduced or decomposed into hydroxide ions or water. Nonetheless, the overall reaction is still the 4-electron pathway.[3] The direct 4-electron path is predominant on noble metal electrocatalysts, metal oxides, and some transition-metal macrocyclics which are N4 chelates. The 2-electron peroxide pathway is predominant on graphite, most carbons, gold, mercury, most oxide covered metals, most transition metal oxides, and some transition metal macrocycles[3]. By controlling the electrocatalyst for the oxygen reduction reaction, it is possible to control the resulting pathway. The mechanism of the oxygen reduction reaction is very complicated, and it has been extensively studied in literature. Oxygen reduction is a multielectron reaction that may include a number of elementary steps involving various series-parallel pathways. Depending on the solution and the cathode material, the mechanism can have different rate determining steps. One important factor in the oxygen reduction reaction is the reaction site, or where the oxygen molecule adsorbs to the surface of the electrode. Three types of interactions were proposed for the adsorption of oxygen on the electrocatalyst surface.[3,7-8] These models are the Griffiths model, the Pauling model, and the Bridge model. The three models are shown below in Figure 4.

23 12 Figure 4: From left to right- the Griffiths model, the Pauling model, and the Bridge model [8] In the Griffiths model, the oxygen molecule interacts laterally to the surface transition metal ions, with its π orbitals interacting with empty 2d orbitals of the transition metal. The strength of the oxygen-oxygen bond is weakened by this strong metal-oxygen interaction, allowing for the breaking of the strong oxygen-oxygen bond. This model leads to the fourelectron direct oxygen reduction pathway. The transition metal is then reduced back to its initial state to regenerate the catalyst site. In the Pauling model, the oxygen molecule interacts endon to the surface transition metal ions, with the π orbital interacting with the 2d orbital of the transition metal. Partial charge transfer follows in this model, superoxide and peroxide intermediates are formed, and this model leads to the two-electron peroxide pathway for oxygen reduction. The last model, the bridge model, again exhibits lateral adsorption of O 2, and requires the oxygen molecule to adsorb to two neighboring adsorption sites. The π* orbitals of the oxygen molecule interact with partially filled d-orbitals at both adsorption sites. This interaction leads to oxidation and reduction of the electrocatalytic adsorption sites following the four-electron direct oxygen reduction pathway.[3,8] 1.2.2: Slow ORR Kinetics The kinetics of the oxygen reduction reaction at the surface of the cathode are very slow compared to the hydrogen oxidation reaction occurring at the anode of a typical fuel cell.

24 13 Depending on each reaction pathway, there is a different rate determining step. The kinetics of this reaction are complex, and remains unclear.[3,7-8] The determination of the reaction mechanisms for various surfaces is one of the major focal points of the research of this reaction. There still is no reliable information for the mechanism of adsorption of oxygen on an electrode. The oxygen-oxygen bond within an oxygen molecule is very strong, with a dissociation energy of 494 kj/mol. Because the bond is very difficult to break, the direct four electron pathway is seen only in certain metals.[8] It has been proposed that the mechanism for the breaking of this bond must involve dual adsorption sites for oxygen on the electrocatalyst, as shown in the Bridge model.[3,7-8] The dissociation energy of the H 2 O 2 molecule is only 146 kj/mol, much less than the dissociation energy of the oxygen molecule.[8] This explains the generation of H 2 O 2 on many surfaces, and mechanisms of this process have been proposed involving only one adsorption site on the electrode surface, such as the end-on adsorption in the Pauling Model.[3,7-8] The adsorption of molecular oxygen onto the surface of the electrocatalyst is necessary for electron transfer, and the mechanism of the adsorption determines which oxygen reduction pathway will follow.[3,8] One line of research has to do with increasing the adsorption sites on an electrocatalytic surface to increase the kinetics of oxygen reduction, and the focus is on determining the particle size effects.[3] As the particle size changes, the concentration of surface sites changes as well. Depending on the material and the particle size, the electrocatalytic activity may increase or decrease. Higher temperatures will also increase the kinetics of oxygen reduction, and applications such as SOFCs and MCFCs use high temperatures to bypass the need for an electrocatalyst for oxygen reduction.[1-2] However, for low temperature fuel cells, electrocatalysts must be available to increase the kinetics of the oxygen reduction reaction.

25 : The Need for Electrocatalysts The kinetics of the oxygen reduction reaction have prevented the development of high performance electrochemical systems using oxygen electrodes. The kinetic inhibition of the oxygen reduction reaction leads to high overpotentials within the cell, as the oxygen reduction reaction at the cathode cannot keep up with the pure hydrogen oxidation at the anode.[3,7] Because of the slow kinetics of the reaction, it is necessary to provide an electrocatalyst to increase the rate of the oxygen reduction reaction. An improved cathode electrocatalyst would have a major impact on the fuel cell efficiency, and would overcome the most limiting factor in the development of new fuel cells. [1-2,9] Many electrocatalysts have been tested, and they each have their own benefits and drawbacks. An electrocatalyst is a catalyst that participates in electrochemical reactions, modifying and increasing the rate of reaction while not being consumed in the reaction. These electrocatalysts function at the electrode surfaces, and assist in transferring electrons between the electrode and reactants. In the case of the oxygen reduction reaction, the electrocatalyst transfers electrons from the electrode to the adsorbed oxygen gas and increase the rate of the reaction. Because of the nature of the oxygen reduction reaction and the multi-electron transfers, the reaction has significant kinetic barriers. These barriers and the kinetics of the oxygen reduction reaction need to be understood in order for the development of a cheap and high performance electrochemical system to be practical. Depending on the type of electrocatalyst, electrolyte, and other reaction conditions, the kinetics of the oxygen reduction reaction can change. The best electrocatalyst for a pure hydrogen and air fuel cell is platinum metal.[9-11, 19-22] The major problem with platinum

26 15 metal is the cost and rarity of the material. Because of the high capital costs, the development new electrocatalysts to replace the platinum metal is important. Other electrocatalysts are made with more common elements, and can have comparable activity to platinum metal. Some of these developed electrocatalysts are discussed below. Experiments have been performed to attempt to supplement platinum with other materials in order to lower the cost and retain the electrocatalytic ability of platinum. This includes mixing platinum with high-surface area carbon and doping platinum with other elements. [9-11,22] Other methods have incorporated nanosized noble metal alloys, for example doping Palladium (Pd) with Iron (Fe). In searching for a replacement for platinum electrocatalysts, it was recently found that palladium, after morphology tailoring, had a comparable catalytic activity to platinum.[12-13] With the goal of reducing platinum usage in electrocatalysis, the usage of other nano-sized noble metal alloys is one leading area of research. All of these electrocatalysts catalyze the four-electron direct pathway in oxygen reduction. However, a different line of research has focused on the usage of non-noble metal materials for oxygen reduction. These materials include carbon, manganese oxides, silver, and some transition metal macrocycles. These non-noble metal materials mostly follow the 2- electron peroxide pathway for oxygen reduction.[3,18] These materials all have various benefits but also limitations. High surface area carbon has exhibited catalytic activity towards oxygen reduction, but does not have the capacity to support any high-energy applications.[3,18] Manganese oxides, although currently used in commercial Zn/air batteries for hearing aids, also cannot support high-energy applications.[3,18] Silver and some transition metal macrocycles, although exhibiting good catalytic activity towards the ORR, have also been found to be unstable

27 16 in their electrolytes and show signs of degradation over time.[3,18] These problems must be overcome when developing a new material. 1.3: Environmental Impact and Safety Many current energy options produce byproducts that are extremely harmful to the environment. The focus in alternative energy development has been to ease the world s dependence on oil and protect the environment from harmful byproducts. Industrial countries dependence on oil was made clear during the oil crises, and economies rise and fall based on oil prices. The growing global awareness of how greenhouse gases and emissions affect the environment has put the focus on developing more environmentally friendly and sustainable energy options.[1] The generation of greenhouse gases due to fossil fuels has been well documented. The primary greenhouse gas created due to the combustion of fossil fuels is carbon dioxide. Carbon dioxide has been a concern due to its possible impact on global warming. Being such a stable structure, carbon dioxide stays within the atmosphere for up to 100 years before it is reused in a natural biological process. It has been called the most important greenhouse gas contributing to climate change. Because of the combustion of fossil fuels for energy production, the concentration of carbon dioxide in the atmosphere is increasing by a shocking 0.4% per year. Projected studies say that if carbon dioxide continues to be released into the atmosphere at this rate, the global temperature will rise C in the next years. This is a drastic problem that must be solved by reducing the usage of fossil fuels and developing a new sustainable and environmentally friendly energy source.[14]

28 17 Hydrogen fuels have been praised for environmental friendliness. All fuels must produce water, carbon dioxide, or nitrogen as its byproduct. Hydrogen fuels only produce water, therefore eliminating emissions completely. Hydrogen gas is only a renewable source of energy if it is produced via renewable methods, i.e. solar, wind, or hydro power. Electricity from a renewable source is used for the electrolysis of water, producing hydrogen gas and oxygen. The hydrogen gas is then stored and combusted using oxygen, producing energy and a water byproduct. The water can then undergo electrolysis to produce hydrogen gas and oxygen, repeating the cycle. This is known as the hydrogen cycle, and it is analogous to the natural carbon cycle except that no carbon is involved. The main challenge facing researchers developing hydrogen fuels is the storage of hydrogen fuel, as hydrogen is a gas at ambient conditions. Research to advance the usage of hydrogen fuels is based on developing new methods of hydrolysis and transport. Fuel cells use hydrogen fuels at the anode, and they fill the combustion role in the hydrogen cycle.[15] Fuel cells are a sustainable and environmentally friendly option for future energy use. With the demand for energy growing, the push to develop functional and commercialized fuel cells has been greater than ever. If run on pure hydrogen, fuel cells will only produce water and zero emissions, eliminating all prospect of environmental harm. The combination of sustainable energy sources from the sun, wind, and water along with fuel cells may be the new method of power generation in the future.[1]

29 18 2: Theory 2.1: Potential of Lithium Nickel Phosphate as an Electrocatalyst The inspiration for using lithium nickel phosphate as an electrocatalyst comes from ideas in chemical catalysis. Chemical catalysis and electrocatalysis share similar catalytic mechanisms. Phosphates have applications in chemical catalysis, and have properties that might translate to electrocatalysis. Iron phosphates have surface effects during chemical catalysis that are similar to the desired surface effects in electrocatalysis. Their structures, incorporating a PO 4 framework, also are very favorable for the Fe 2+ to Fe 3+ redox reaction, which suggests that the chemical catalytic activity is related to its electrochemical activity. [16] Because lithium nickel phosphate has a very high electrochemical activity as the lithium metal phosphate with the highest energy density, [17] it is important to examine the electrocatalytic activity of this compound.[18] Many of the metals that allow for the direct four-electron pathway are very expensive. These metals include platinum, which is very cost prohibitive. The direct four-electron pathway is more efficient than the two-electron peroxide pathway, but much more difficult to produce. Metal oxides have been explored[23], but the electrocatalytic activity of these oxides does not reach the levels reached by platinum and some of the other noble metals.[18] One other way to approach this problem is by increasing the speed of the two-electron peroxide pathway. By increasing the rate of the two-electron pathway, the overall reaction can be as efficient as the four-electron pathway.[22,25] This approach allows for many different and less expensive electrocatalytic options.[25] LiNiPO 4 is one material that provides electrocatalysis of oxygen reduction through a 2-electron peroxide pathway.[18]

30 19 The cost of LiNiPO 4 and most other chemical compounds are very low compared to the cost of platinum and other noble metals. The motivation to develop these chemical compound electrocatalysts is driven by the desire to lower costs throughout the fuel cell. The electrocatalyst is the most important part of the fuel cell, and it will dictate a large portion of how much the fuel cell will cost as well as how it will operate.[26] 2.2: Microemulsion Synthesis Microemulsion synthesis is a chemical method where nanomaterials can be synthesized with a uniform size distribution without expensive instruments. Each microemulsion can be related to a micellar system, and each micelle in the microemulsion can act as a nano-reactor for the reactants added to the microemulsion.[6] 2.2.1: Properties of Microemulsions Microemulsions are a special type of emulsion, important because of their stability and small micelle size. A microemulsion is defined as a stable dispersion of one liquid in another in the form of spherical droplets, the diameters of which are less than one-quarter the wavelength of white light, or 1400 angstroms.[4-5] A mixed film adsorbs to the interface between the oil and water phases, allowing the adsorbed monolayer to spontaneously achieve zero interfacial tension. This mixed film, or interphase, allows the system to remain disperse, preventing the system from separating into its two oil and water phases.[4-6] Within the microemulsion, there exists a micellar system. In a water-in-oil microemulsion, droplets of water are present within an oil phase. At equilibrium in a microemulsion, there are three phases present. Two of these phases are the water and oil phases, and the remaining phase is called the interphase. The interphase is a film forming

31 20 surfactant, allowing the microemulsion to stabilize and reach the equilibrium state. To lower the interfacial tension between the oil and water, 1-alkanols are added to the solution and are distributed to the interphase. The combination of the surfactant and the alkanols lower the oil/water interfacial tension, forming the microemulsion and preventing the separate oil and water components from separating. The water phase forms the tiny micelles within the oil phase in the microemulsion.[4-6,33] 2.2.2: Microemulsion Synthesis Technique and Emulsion Drying Technique This experiment produced uniformly sized nanoparticles through the microemulsion synthesis method. The microemulsion synthesis method is used to achieve these goals because of the control of the micelle size within the microemulsion. By limiting the growth of the products using the size of the micelle, it is possible to create uniform sized nanoparticles. Micelle size remains uniform throughout the microemulsion, and the size is maintained by the surfactant and other interphase elements along with the water-to-surfactant ratio.[6,27-34] The initial reactants are contained within micelles in separate microemulsions. Upon combination of the microemulsions, the micelles containing the reactants collide and undergo intermicellar mixing. The mixing causes the reaction to begin within the micelles. The reaction happens very quickly due to the mixing mechanism. As the micelles collide within the aqueous solution, water channels form, resulting in the formation of a transient dimer. The reaction begins at the edges of the micelle, and this provides a nucleation point for growth as intermicellar mixing continues. Because ionic reactions happen very quickly, once the dimer forms, the ions quickly react to form the product. A diagram of the process is shown below in Figure 5. [6]

32 21 Figure 5: Process showing the intermicellar exchange for the microemulsion synthesis of nanomaterials [6] There are other methods to produce these nanomaterials, but there are advantages to the microemulsion method. The microemulsion method does not require any expensive equipment, as opposed to other physical methods. The product is uniform and microhomogenous, as the desired stoichiometry is retained within the water phase in the micelles. The largest advantage, however, is that it is possible to tune the particle size and morphology of the product by altering the properties of the microemulsion. I will discuss this more later in this section.[6,27-32] The microemulsion method creates an amorphous precursor which is sintered to form the crystalline final product. The precursor product precipitate must be removed from within the microemulsion solution. In the emulsion drying technique, the product precursors are removed from the emulsion in a series of steps.[28-30] The water phase is removed from the

33 22 system by adding the microemulsion solution dropwise into hot oil, heated above the boiling point of water. The water phase evaporates fully, and the leftover oil with the precursor powder precipitate is evaporated at a higher temperature. This leaves behind only the product precursor powders, and these are heat treated to form the final product.[6, 28-30] 2.3: Controlling Particle Size The major benefit of using the microemulsion system is the ability to control and tailor the size of resulting particles. By altering the size of the micelles, the resulting particle size will change. The micelle size is dependent on the water to surfactant ratio in the microemulsion system. After the precursor products are formed, the heat treatment controls the amount of crystallization within the final product, producing larger and smaller crystalline products based on the annealing conditions : Water to Surfactant Ratio By changing the water to surfactant ratio, the particle size of the resulting products can be controlled. Changing the water to surfactant ratio has been found to change the size of the micelles within the microemulsion, essentially changing the size of the water droplet where the reaction occurs. Because the size of the water micelle within the oil phase plays a major role in determining the size of the final product, the particle size can be controlled by altering these ratios.[6,32] Many groups have tried to relate the size of the water micelle to the resulting particle size. Some found linear correlations between the water to surfactant ratio and the resulting particle size.[32-33] Upon increasing the water to surfactant ratio, the particle size reached a maximum after a certain ratio was surpassed.[32] This would indicate that the water to

34 23 surfactant ratio controls the particle size of the resulting product. Monte Carlo simulations were performed on a well-modeled microemulsion system, and the results correlated well with the results from other experiments. The simulations found that the size of each micelle controls the resulting particle size, and the water to surfactant ratio changes the properties of the microemulsion.[35] 2.3.2: Annealing Temperature and Time The process of annealing the precursor particles has an effect on the resulting particle size of the crystalline product. Annealing is heating powder particles below their melting point to cause the powdered particles to adhere to each other.[36] Two factors within the annealing process that can be controlled are the annealing temperature and the annealing time. Both of these factors are shown to contribute to particle growth.[20,36-37] Figure 6: Particle size distributions of LiFePO 4 sintered at various temperatures [37]

35 24 The extent of reaction and crystallization, along with crystal growth, depend on the temperature at which the sample is sintered. As the temperature increases, the amount of crystal growth and particle size increases. This trend has been shown in literature, in similar synthesis experiments. During synthesis of another phospho-olivine, LiFePO 4, the same trend was seen and is shown in Figure 6.[37] In a similar microemulsion synthesis of LiFePO 4 /C[18], the effect of annealing temperature was observed as the sample precursors were sintered at 500, 600, and 700 degrees Celsius for a period of 24 hours. At a temperature of 500 degrees Celsius, it was found that the precursors had not fully reacted to form pure LiFePO 4, and traces of crystalline precursor were present. However, at temperatures of 600 and 700 degrees Celsius, the product was pure LiFePO 4, meaning that the reaction had been completed. However, upon comparing the size of the particles within each sample, it was found that the sample heated at 600 degrees Celsius had a significantly smaller particle size than the particles found in the sample heated at 700 degrees Celsius.[27] The extra heat leads to more particle growth and the extent Figure 7: XRD spectra of LiFePO 4 sintered at different temperatures for 24 hours: a 500 C, b 600 C, c 700 C [27] of crystallization is greater in samples exposed to higher temperatures. This was shown qualitatively within the XRD data provided of the samples, with the highest temperature providing the largest and sharpest peaks, indicating a much greater extent of crystallization. This

36 25 data is displayed in Figure 7. The annealing temperature is an important factor that must be accounted for when pursuing a product with a particular particle size. The annealing time is another important factor that must be taken into account when controlling the particle size of a product. A larger annealing time allows the precursor particles more time to nucleate and grow, producing larger particle sizes. The simple model of grain growth is given by the equation below. G is the final average particle size, G 0 is the initial average particle size, t is time, and K is a factor involving the molar activation energy, ideal gas constant, temperature, and a material constant.[36] If the annealing time increases, the Kt factor will increase, increasing the final average particle size and causing a larger extent of grain growth. 2.4: Physical Characterization Methods After the product is produced, it is necessary to determine the physical properties and chemical makeup of the powder. The methods used to characterize the product are X-ray diffraction (XRD), the Field Emission Scanning Electron Microscope (FESEM), and the Transmission Electon Microscope (TEM). The physical properties confirm that the product is uniform in size and pure olivine product : X-Ray Diffraction (XRD) Bragg diffraction applicable to x-ray diffraction occurs when x-rays incident with a crystal are scattered by the atoms in the system and in the structure.[38-39] X-rays undergo constructive interference in accordance to Bragg s law, which describes the condition for this

37 26 interference from successive crystallographic planes of the crystal structure. [38-39] These successive planes are identified by Miller indices, which describe the orientation of these planes, and are a distance d apart. When constructive interference occurs between two waves, they remain in plane because the path length of each wave is equal to an integer multiple of the wavelength. The difference between two paths that undergo constructive interference is described by Bragg s law, which is stated below.[39] A diagram of this phenomenon is also shown in Figure 8. where θ is the scattering angle, d is the distance between planes, n is an integer, and λ is the wavelength[39] Figure 8: Bragg Plane Diffraction [39] Where constructive interference occurs, very intense peaks are obtained in the diffraction pattern. These waves satisfy the Bragg condition, and from the Bragg peaks obtained, it is possible to determine the crystallographic structure of a sample. The model cannot solve for the arrangement of atoms within the unit-cell, because it does not interpret the relative intensities of the reflections. However, it allows for the identification of the structure s Miller indices. A

38 27 Fourier transform must be performed to get any more information about the arrangement of atoms within a unit-cell in the material. The goal of x-ray diffraction is to determine the density of electrons throughout the crystal structure. [38-40] The density of the electrons can be used to determine which atoms exist at various dense areas of electrons. X-ray scattering data collects data about the Fourier transform of the density of electrons. The Fourier transform of the electron density is the representation of the density of electrons in reciprocal space.[40] Therefore, the x-ray scattering data determines the reciprocal vectors in the lattice. To convert between reciprocal space to real space and to determine the density of the electrons within the sample requires backtracking from the Fourier transform. Because most crystals are idealized as perfect structures, with one unit cell repeated over and over again in a perfect lattice, the electron density should be periodic as well. This means that the Fourier transform would be zero in every case except that where Bragg s law applies.[39-40] This creates the Bragg peaks in the intensity vs. angle data plots for x-ray diffraction. Having found the electron density of the lattice along with its Miller indices, it is possible to put together a model for the position of specific atoms within the structure.[39-40] X-ray diffraction can also solve for the atomic model of a structure. The particular type of x-ray diffraction used in this application is powder diffraction.[41] X-ray powder diffraction is widely used for characterization and phase identification.[41] Every possible orientation of a crystal is present in its powder form, and as the X-rays are projected at the powder, the intensity of the diffraction scattering is measured as the diffraction angle changes. The resulting scattering is collected at a flat plate detector. The data is typically presented as a plot of

39 28 intensity versus scattering angle, producing Bragg peaks at their respective scattering angles.[39,41] By observing the resulting Bragg peaks, the phase and content of the examined powder can be determined : Electron Microscopy: FESEM and TEM It is impossible to observe the size or morphology of the resulting lithium nickel phosphate product via the naked eye, or even with a normal light microscope. The FESEM allows for clear pictures of the product lithium nickel phosphate at very high magnification. The Field Emission Scanning Electron Microscope uses a scanning electron beam to generate a topographical image.[42-43] As the electron beam strikes the surface of the sample, the interactions between the beam and the sample create signals that can be used to generate an image. The signals are picked up by various detectors within the microscope chamber. The signals produced include secondary electrons, backscattered electrons, and x-rays.[42-43] The secondary electron signals provide enough information for the creation of a clear topographical image of the lithium nickel phosphate product. A typical FESEM photo is shown below in Figure 9.

40 29 Figure 9: LiNiPO 4 Secondary Electron Image The FESEM can produce very clear, high-resolution topographic images of a sample. One benefit of the FESEM is the fact that it is possible to perceive depth in the image, also known as characteristic depth of field.[42-43] The image appears to be three dimensional because of the depth of field within the image, showing darkened areas within the particles. The electron beam produces interactions with the sample surface, and the secondary electrons produced from these interactions are detected by the secondary electron detector. Based on the number of collected electrons, the microscope adjusts the intensity of the signal seen on the viewing monitor. This produces the image that is seen by the operator. As the electron beam scans the sample, it rests briefly on each spot. If the spot on the sample is a crystal face oriented away from the detector, the spot will not have the same intensity as a crystal face oriented towards the detector. This creates the spotlight view of the sample and

41 30 allows for the operator to retain a sense of dimension while using the microscope, and helps to create the characteristic depth of field found in FESEM images.[42-43] The Transmission Electron Microscope (TEM) is another high resolution method of electron microscopy. The purpose of the TEM for characterization in this experiment was to determine whether or not there was amorphous product remaining within the sample. The TEM observes the effect on an electron beam transmitted through a thin specimen. The very high resolution of the TEM produces a magnified image of the sample, and diffraction pictures allow for the determination of the presence of amorphous product within the crystalline sample.[44] 2.5: Electrochemical Characterization Methods To determine the potential of the final product as an electrocatalyst for the oxygen reduction reaction, the product is characterized electrochemically. The rotating ring disc electrode (RRDE) is used to examine the effect that the electrocatalyst has on the oxygen reduction reaction. By testing the different particle size products produced by varying the synthesis conditions, we can determine how these synthesis conditions affect the electrocatalyst performance. These properties are important to determine whether lithium nickel phosphate is practical for use as an electrocatalyst for the oxygen reduction reaction : Rotating Ring Disc Electrode (RRDE) The rotating ring disc electrode (RRDE) is a useful tool for determining the oxygen reduction reaction pathway. The RRDE consists of a double working electrode that draws the solution towards itself using convection due to the electrode s rotating motion.[45] As the electrode rotates, it moves the solution towards the electrode and hydrodynamic transport equations at steady state apply.

42 31 Under steady state conditions, the spinning electrode flings the solution outwards from the center of the electrode in a radial direction.[45-47] The solution replenishes itself with a flow normal to the center of the electrode. The velocity profile of the fluid at the surface of a spinning disk was obtained theoretically by von Karman and Cochran by solving the hydrodynamic equations under steady state conditions.[45-47] The resulting velocity profile is depicted in Figure 10 below.[47] Figure 10: Velocity profile at a rotating ring disk electrode [47] One type of experiment that can be done with the rotating ring disk electrode is a collection experiment. The disk is held at a potential E D, and as the oxygen within the solution contacts the disk, it is reduced to its products, water and peroxide. The ring is held at a sufficiently positive potential E R, and as the products move outwards along the surface of the electrode, they contact the ring and are oxidized back to oxygen. When the reduction and oxidation occur at the disk and ring, respectively, current is produced. A cathodic current, i D, is produced at the disk, while an anodic current, i R, is produced at the ring. In the case of the oxygen reduction reaction the ring is set to oxidize any produced hydrogen peroxide.[45-47]

43 32 The two currents at the ring and disk are related by the collection efficiency, N.[45] The equation for the collection efficiency is below. The relationship between the current produced at the disk and at the ring is the collection efficiency. In an ideal situation, all of the reduced product would be oxidized at the ring, producing equal current at the disk and ring and producing 100% collection efficiency. The collection efficiency is a physical constant value that varies for every individual RRDE setup. It is dependent on the geometry and the properties of the specific setup at hand : Experimental Determination of Collection Efficiency To determine the collection efficiency of a certain RRDE system experimentally, a well understood electrochemical system will be used. In literature, the [Fe(CN) 6 ] 3- /[Fe(CN) 6 ] 4- electrochemical couple is widely used for this purpose.[9-10,19] This is a one step reduction process, and gives well defined current measurements at the disk and ring. To perform this experiment, the RRDE is set up in a three-electrode setup, and the electrolyte is 0.1M KOH with 10 mmol K 3 Fe(CN) 6. The potential at the disk was swept from 600mV to -200mV at a rate of 20 mv per second. The potential at the ring was kept at a constant 800mV to collect the product reduced at the disk. To ensure that the only signal picked up by the machine was from the K 3 Fe(CN) 6, the electrolyte was de-aerated by bubbling nitrogen gas through it for 30 minutes. The current produced on the ring and the disk allow for the calculation of the collection efficiency. This collection efficiency has been found to stay constant at various rotation rates as well.[9-10,19]

44 : Determination of ORR Pathway After the collection efficiency for the RRDE setup has been acquired, it is then possible to determine the amount of H 2 O 2 produced at the disk and acquired at the ring. This is done by first detecting the average number of electrons produced per oxygen molecule reduced.[9-10,19] If the pathway is fully the four-electron pathway, then the average number of electrons produced per molecule should be 4. Likewise, if the pathway is fully the peroxide pathway, then the average number of electrons produced per molecule should be 2. An equation for the average number of electrons produced per molecule can be derived using the definition of the collection efficiency.[9-10,19] The current produced at the disk (I D ) is the sum of the current created via the 4-electron H 2 O pathway (I H2O ) and the current created via the 2-electron H 2 O 2 pathway (I H2O2 ). Using the definition of the collection efficiency, the current created via the peroxide pathway can be related to the current produced at the ring (I R ). These two equations are shown below. (1) (2) Using a simple proportion, the average number of electrons passed at the disk (n e- ) can be related to the two different pathways where products are produced. (3) Substitution and rearrangement leaves the number of electrons passed at the disk (n e- ) in terms of the ring and disk current (I R and I D, respectively) and collection efficiency (N).

45 34 (4) Using the outputs of the RRDE system and equation (4), the number of electrons transferred per oxygen molecule reduced can be obtained. This allows for a simple determination of the pathway of the oxygen reduction reaction on a surface.[9-10,19,46]

46 35 3: Product Synthesis: LiNiPO 4 via the Microemulsion Method The preparation of lithium nickel phosphate occurs via a microemulsion method. There are three stages to the preparation of the lithium nickel phosphate: (1) the initial microemulsion reaction to produce the precursor, (2) the drying of the precursor powders, and (3) the heat treatment of the precursors to produce the final crystalline product. A flowchart of the entire process and details for each step are below in Figure 11. Figure 11: Diagram of full Microemulsion Synthesis Method

47 36 3.1: Microemulsion reaction The synthesis of lithium nickel phosphate was performed using a combination of principles found in both the microemulsion method and the emulsion drying method.[6,27-33] Three separate water-in-oil microemulsions were formed, each one containing one of the ions necessary to create lithium nickel phosphate. The first microemulsion contained Li + ions from LiOH, the second contained Ni 2+ ions from NiAc 2, and the third contained PO 3-4 ions from H 3 PO 4. The concentration of ions in each of these microemulsions was equal, and the resulting molar ratio of ions (Li +, Ni 2+, PO 3-4 ) was 1:1:1. Each of the three separate microemulsions contained 25 ml n-octane, 8g CTAB, 1ml of 5% by weight polyethylene glycol (PEG), and 10 ml 1-butanol. The reactants are 0.4M solutions of LiOH, NiAc 2, and H 3 PO 4. Equal amounts by volume were added to each separate microemulsion. These microemulsions were stirred rapidly until a clear solution was achieved. The clear solution demonstrates that the water is dispersed within the oil phase to the point where light is not diffracted as it goes through the solution. This indicates that the micelle size is extremely small, to the scale of about Angstroms.[4-6] Once these three microemulsions have been prepared, the two microemulsions containing the Ni 2+ and PO 3-4 ions are combined. Then the remaining microemulsion containing the Li + ions is added dropwise to the combination of microemulsions. As these microemulsions are added, the reaction begins and the previously clear solutions become cloudy. The cloudy nature of the resulting aqueous solution demonstrates that precipitate has been formed. Finally, 0.8M NH 4 OH is added to the combination to balance the extra protons from the H 3 PO 4, in an equal volume to that of the initially added 0.4M solutions to the separate microemulsions. The

48 37 resulting solution is left to stir for a few hours to ensure that the reaction has finished completely. 3.2: Drying of Precursor The second phase of the synthesis involves drying the precursor, and removing the powders from the aqueous solution. Because lithium ion is very soluble in water, the goal is to remove the water from the aqueous solution, and then afterwards remove the oil and surfactant, leaving only the powdered precursor for heat treatment. The reacted microemulsion was added dropwise to a dodecane oil bath that was heated to around C and stirred with a stirring rod. The added water evaporated from the oil bath, and the precursor powders fell to the bottom of the bath. Because the boiling point of the octane in the solution is around 125 C, the water is selectively evaporated from the solution. Once the boiling within the oil bath ends, the water has been fully evaporated and the only remaining components of the oil phase are dodecane, octane, and 1-butanol, along with the surfactant. The next step involves removing the oils and the surfactant from the resulting waterfree solution. Ethanol dissolves the surfactant, and does not affect the product powders. Ethanol is added to the resulting solution, and dissolves the surfactant into the aqueous phase. Upon centrifuging the product at 8000 rpms for a few minutes at a time, the surfactant can be washed out of the precipitated precursor powders along with the remaining oils (octane and dodecane). After washing the product thoroughly with ethanol, the precursor powders are allowed to dry and then are ready for the heat treatment process.

49 38 3.3: Heat Treatment of Precursor The heat treatment of the resulting washed precursors is where the precursors crystallize and the product is achieved. The resulting powders are placed in an oxygen free environment, and heated at high temperatures to sinter the powder. The high temperature is important to complete the synthesis reaction and produce the olivine LiNiPO 4. The precursor powders are placed into an oxygen free furnace, with flowing nitrogen gas. To reach the annealing temperature, the temperature is increased by a rate of about 100 C per hour. Once it reaches the annealing temperature, it holds at that temperature for the duration of the annealing time before cooling back down to room temperature, 100 C at a time. The heat treatment of the precursor involves two variables, the annealing time and temperature. The heat treatment is required to convert the precursor to the desired crystalline olivine LiNiPO 4 structure. If the annealing temperature is too low, then the precursor will not become one pure olivine LiNiPO 4 phase. However, if either of these variables is too high, then the crystal growth among the pure product will be more than desired, leaving a product with a larger particle size. This requires annealing at the correct temperature and time, ideally stopping the annealing process once the product has become a pure LiNiPO 4 product.

50 39 4: Product Characterization Once the synthesis is complete, the resulting product must be characterized to determine its physical and electrochemical properties. The physical properties of the product were characterized using the x-ray diffractometer, field emission scanning electron microscope, and transmission electron microscope; the electrochemical properties were characterized using the rotating ring-disk electrode. 4.1: Product Phase Determination After the product is produced, it is important to determine if the product created is the expected product. X-ray powder diffraction allows for the determination of the product phase and chemical makeup.[41] An x-ray slide is prepared by mounting the powder onto a glass plate and the diffraction was performed from 2θ values of 10 to 70. Based on the diffraction pattern that is produced from the x-ray diffraction machine, such as the one shown in Figure 12, it is possible to determine the composition and phase of a material. Figure 12: XRD spectra of a sample of synthesized LiNiPO4, matching the spectra found in the database

51 40 The diffraction pattern of olivine lithium nickel phosphate has been determined previously in literature, and comparing the peaks of the product diffraction pattern to the peaks of the diffraction pattern in literature allows for the determination of the phase and composition. 4.2: Particle Size The characterization of the particle size was done using the Field Emission Scanning Electron Microscope (FESEM). Visual comparison of images collected on the FESEM can provide information about the changes in the particle size when altering any one of three factors: the water to surfactant ratio, the annealing temperature, and the annealing time. All three of these factors can cause significant changes to the average particle size of the product. The images from the FESEM are shown throughout this next section : Effect of Water to Surfactant Ratio Theoretically, altering the water to surfactant ratio (W 0 ) during the synthesis process would cause a significant change in the final average particle size. This was examined in the laboratory, with three equivalent syntheses processes only differing in water-to-surfactant ratio. The three water-to-surfactant ratios were 7.5, 12.5, and As the water-to-surfactant ratio increased, an increase in particle size was seen in the resulting product, as expected from findings in literature. Figure 13 below shows FESEM images of the products at each different W 0 value.

52 41 Figure 13: High Magnification Images (150,000x) of LiNiPO4 with varying water-to-surfactant ratio (W0) top left W0 = 7.5, top right W0 = 12.5, bottom W0 = Based on high magnification images, it is clear that the particle sizes increase as the water-tosurfactant ratio increases. The size of the water droplet within the microemulsion determines the size that the particle can grow to. The particles seem to be around 100 nm in diameter with a W0 of 7.5, 200 nm in diameter with a W0 of 12.5, and greater than 300 nm in diameter with a W0 of

53 42 One interesting aspect of the changing water-to-surfactant ratio can be seen in the low magnification images of the samples with W0 values of 7.5 and These images are shown below in Figure 14. Figure 14: Low magnification (25000x) images of LiNiPO4 samples with W0 values of 7.5 (top) and 17.5 (bottom)

54 43 Not only are the particle sizes larger in the sample with W 0 of 17.54, but there are morphology differences as well. In the sample with W 0 of 7.5, the product is relatively uniform and produces spherical and oval shapes. However, in the sample with W 0 of 17.54, the sample seems to have many differently sized products, and seems to not exhibit the same uniformity as the sample with a lower W 0. This demonstrates the problem with having too large of a water droplet within the microemulsion. The larger water droplets will produce larger particles, but the larger water micelle will not constrain the shape of the product as much as a smaller water micelle. To achieve uniform nanoparticles, it seems better to use lower water-to-surfactant ratios. XRD data follows for the three samples of LiNiPO 4, and all the samples indicate olivine structure within their LiNiPO 4 products regardless of the water-to-surfactant ratio. Changing the W 0 should not have an effect on the product purity and the process of creating LiNiPO 4. This is shown by the very similar XRD spectra observed for each sample in Figure 15. Figure 15: XRD Data for products varying by water to surfactant ratios

55 44 Therefore, it seems that the water-to-surfactant ratio has a great impact on the particle size and morphology of the sample. All three XRD spectra for the different water-to-surfactant ratios showed an olivine product, meaning that the W0 has little to no impact on the crystal structure of the final product. The annealing will determine the crystallinity of the final LiNiPO4 product : Effect of Annealing Temperature During the synthesis, the effect of the annealing temperature on the particle size of the resulting product was observed. Two identical synthesis processes were performed, only changing the heating temperature of the product after washing. The initial product was heated at 600 C and the second product was heated at 700 C. Both of these products were heated at these temperatures for twelve hours. The images of the two resulting products are shown in Figure 16 and 17 in low and high magnification, respectively. Figure 16: Low magnification (25,000x) images of LiNiPO4 sintered at 600 C (left) and 700 C (right)

56 45 Figure 17: High magnification (150,000x) images of LiNiPO4 sintered at 600 C (left) and 700 C (right) Based on Figures 16 and 17, the product sintered at 700 C had more particle growth than the product sintered at 600 C. Figure 16 exhibits a very uniform product, both similarly shaped products. The average size of the product at 700 C appears to be greater than the average size of the product heated at 600 C. This is much more evident in Figure 17 at higher magnification, with much larger particles within the 700 C sample. The average particle size of the sample sintered at 600 C seems to be around nm, while the average size of the sample sintered at 700 C seems to be around 300 nm. This demonstrates that by changing the annealing temperature during the synthesis method, it is possible to manipulate the resulting particle size. It would also be interesting to observe the particle growth at even higher and lower temperatures, as 700 C produced particles only slightly larger than 600 C.

57 46 Figure 18: XRD Data for products varying by annealing temperature As shown in Figure 18, products sintered at both temperatures demonstrate very sharp peaks, indicating pure olivine products. Based on Figure 16 and 17, it is expected that these two samples would have similar XRD spectra. The shape of the particles was very similar in the low magnification images, and the only difference was an increase in particle size from 600 to 700 C : Effect of Annealing Time The effect of the annealing time during the synthesis was also examined. Identical samples were heated at 600 C for six, nine, and twelve hours. These samples were then viewed in the FESEM, and images of each sample are shown below in Figure 19.

58 47 Figure 19: High Magnification (150,000x) images of LiNiPO4 sintered at 6 hours (top left), 9 hours (top right), and 12 hours (bottom) Samples sintered for shorter periods of time showed significantly less particle growth. There is an increase in particle size from annealing times of 6 hours to 9 hours and finally to 12 hours. Particles sintered for 6 hours have an average particle size of about nm, particles sintered for 9 hours have an average particle size of nm, and particles sintered for 12 hours have an average particle size from nm. The increase in annealing time increases the time for the crystals to grow within the product, and it is demonstrated in these samples. The sharper peaks within the XRD spectra of the products sintered for longer periods of time also demonstrate a greater amount of crystallization within these products, shown in Figure 20.

59 48 Figure 20: XRD Data for samples varying by annealing time The annealing time also has one other important effect on the sample, it works to crystallize the amorphous precursor powders into the olivine LiNiPO 4. The sample sintered for only six hours exhibited different morphology than the other samples sintered for nine and twelve hours. A FESEM image of the sample sintered for six hours is shown below in Figure 21.

60 49 Figure 21: LiNiPO 4 sintered at 600 C for six hours Because the precursor is amorphous, the XRD spectra cannot detect any unreacted precursor within the final product, only the olivine LiNiPO 4. Looking at the XRD data presented earlier, this is clear as the only visible peaks are attributed to olivine LiNiPO 4. Previous research had shown that products not fully sintered had similar impurities due to unreacted precursor products. Any precursor with nickel ions should be amorphous nickel (II) phosphate. TEM studies revealed the presence of amorphous precursor within the crystalline LiNiPO 4 product, shown in Figure 22.

61 50 Figure 22: TEM bright field image showing both crystalline and amorphous phases within an LiNiPO 4 sample sintered for only 6 hours, along with the corresponding diffraction image The TEM diffraction pattern shows broad rings if the sample is amorphous, and these rings are observed in the sample. To synthesize pure olivine LiNiPO 4, a certain amount of annealing is necessary and six hours was not enough to crystallize the entire sample. 4.3: Electrochemical Properties LiNiPO 4 is an olivine transition metal phosphate with documented electrochemical properties as a cathode material for lithium-ion batteries. A rotating ring disk system was used to determine the electrocatalytic activity of synthesized LiNiPO 4 for the oxygen reduction reaction. The disk electrode is coated with a thin film of electrocatalyst, carbon black, and Nafion. The ratio of electrocatalyst to carbon black is 3:7 by weight. Because lithium nickel phosphate is an insulator by nature, its conductivity is very low. To increase the conductivity of

62 51 the electrocatalyst, carbon black is dispersed into the electrolyte. Nafion allows for the deposition of a thin film of the electrolyte/carbon black dispersion onto the surface of the disk electrode. The electrocatalyst, carbon black, and Nafion are added to a water/ethanol solution (9:1 ratio by volume), and then the electrocatalyst and carbon black are dispersed within the solution via ultrasonication. 7.5 mg of electrocatalyst/carbon black was added to 10 ml deionized water, along with 40 ul of Nafion. After ultrasonication for about 30 minutes, all of the powder was dispersed within the solution. 20 ul of the dispersion was drop coated onto the disk electrode, wetting the entire disk surface. The water and ethanol evaporates at room temperature, leaving a thin uniform layer of electrocatalyst and carbon black on the surface of the disk. All electrocatalyst samples were prepared by this method for electrochemical testing using the RRDE. RRDE experiments were run using a bipotentiostat and rotator (Pine Instruments). The reference electrode was a saturated calomel electrode, and all potentials are referred to the saturated calomel electrode : Cyclic Voltammetry Cyclic voltammetry for the oxygen reduction reaction was done for a prepared lithium nickel phosphate thin film deposited on a glassy carbon disk as well as the uncoated glassy carbon disk. The voltammograms were obtained in an alkaline environment, in a 0.1M KOH electrolyte. The resulting voltammograms are shown in Figure 23. In an oxygen saturated environment, a voltammogram taken from an uncoated glassy carbon disk produces much less current than a voltammogram taken from a glassy carbon disk coated with a thin layer of LiNiPO 4 and carbon black (curves 1A and 1B in Figure 23). In a nitrogen saturated environment,

63 52 voltammograms taken from both a coated and uncoated glassy carbon disk show no evidence of electrochemical activity (curves 2A and 2B in Figure 23). This attributes the current produced in an oxygen saturated environment to the activity due to the oxygen reduction reaction.[25] 2B 2A 1B 1A Figure 23: Cyclic Voltammograms for coated and uncoated glassy carbon disks in electrolyte saturated by nitrogen and oxygen gas These cyclic voltammograms are repeatable upon many scans, indicating the chemical and electrochemical stability of lithium nickel phosphate. This is expected considering the very stable olivine structure formed by the phosphate atoms. Comparing the electrocatalytic activity of an uncoated disk electrode in an oxygen saturated environment (1B) to that of an LiNiPO 4 coated disk electrode in the same environment (1A) demonstrates that the LiNiPO 4 does improve the rate of oxygen reduction and has significant electrocatalytic properties towards the reaction.[25]

64 : Efficiency The efficiency of the rotating ring disk electrode is dependent on the machine and the setup of the electrodes. Based on the geometry of the system, for instance the separation distance between the ring and disk electrode, the efficiency of the system can differ from machine to machine. Experiments designed to determine the efficiency of the system were performed following the procedure in Section 2.5.1, and were run at various rotation speeds using the uncoated glassy Figure 24: RRDE Efficiency Data at Various RPM carbon RRDE electrode. Varying the rotation speeds determined whether or not the efficiency of the system had any dependence on rotation speed. The rotation speeds varied from 500RPM to 2000RPM. The data resulting from these experiments is shown in Figure 24. From these experiments, the efficiency of the RRDE system has been determined to be ± The extremely small deviation due to electrode rotation rate again shows that in this setup the rotation rate has no impact on the RRDE system efficiency.

65 : Rotating Ring Disk Electrode Collection Experiments The setup of the rotating ring disk electrode includes the coating of the disk, along with the setup of the ring and disk potentials and the remainder of the electrode system (electrolyte, reference electrode, counter electrode). The reference electrode is a standard calomel electrode reference, the counter electrode is a platinum wire, and the electrolyte is 0.1M KOH. Having determined that the efficiency of the RRDE system does not depend on the rotation rate, each experiment was run at a constant rotation rate of 1000RPM. Before every collection experiment, the electrolyte was saturated with oxygen and oxygen gas was blown over the surface of the electrolyte throughout each experiment. 4.4: ORR Functionality of LiNiPO 4 as an Electrocatalyst Olivine LiNiPO 4 definitely can have applications as an electrocatalyst for the oxygen reduction reaction. Based on the cyclic voltammograms, the application of a thin film of LiNiPO 4 to an electrode increases the electrocatalytic activity by almost three times. The following RRDE disk current data displayed in Figure 25 demonstrates the effects of the LiNiPO 4 coating on the surface of the disk.

66 55 Figure 25: RRDE disk current data comparing the electrocatalytic activity on a glassy carbon disk with a coating of LiNiPO 4 and an uncoated glassy carbon disk An electrode with a thin layer of LiNiPO 4 coating produces almost three times the current than an uncoated electrode. LiNiPO 4 has significant electrocatalytic activity towards oxygen reduction. It is interesting to determine the effects of changing the size and morphology of the LiNiPO 4 electrocatalyst. By altering different properties of the microemulsion, it s possible to alter the electrocatalytic activity of the resulting product and determine the physical properties that produce the largest electrocatalytic activity possible : Water to Surfactant Ratio Changes in the water to surfactant ratio cause changes to the product particle size and shape. Smaller water to surfactant ratios lead to the formation of smaller particles and larger water to surfactant ratios lead to the formation of larger particles along with different

67 56 morphology. Investigation of the electrochemical effects of the changing water to surfactant ratio is desired. The RRDE disk current data for samples with varying water-to-surfactant ratios is shown in Figure 26 below. Figure 26: RRDE data at the disk electrode for LiNiPO 4 with varying water-to-surfactant ratio At -0.5V, the disk current has a maximum due to the formation of peroxide at the disk surface. As the water-to-surfactant ratio increases, the electrocatalytic activity increases. The increase in W 0 leads to a significant change in particle size, and the electrocatalytic activity increase corresponds to this particle size change.

68 57 Figure 27: RRDE data at the ring electrode for LiNiPO 4 with varying water-to-surfactant ratio The ring current, displayed in Figure 27, represents the amount of peroxide detected after formation at the disk. At higher water-to-surfactant ratios, the ring current decreases dramatically compared to lower ratios. This can be attributed to peroxide dissociating at the disk surface after formation. As the particle size increases, it becomes more favorable for peroxide to continue to dissociate at the disk. Therefore, a smaller amount of peroxide is detected at the ring electrode at larger water-to-surfactant ratios : Annealing Time A similar effect is seen when observing the effect of the annealing time. The annealing time affects the particle size and crystalline nature of the resulting product. Figure 28 is the RRDE disk current data from the samples sintered in 600 C temperatures for six, nine, and twelve hours.

69 58 Figure 28: RRDE data at the disk electrode for LiNiPO 4 with varying annealing time The effect of the annealing time seems to have a different effect on the LiNiPO 4 than the waterto-surfactant ratio. Instead of similarly shaped curves differing in magnitude, these three curves are all different shapes. Because the annealing time has a direct relation to the final crystalline nature of the product, the crystalline nature of each product must have a great deal to do with the resulting electrocatalytic performance.

70 59 Figure 29: RRDE data at the ring electrode for LiNiPO 4 with varying annealing time The performance at the ring mirrors the performance at the disk. This seemingly gives no definitive information, but with the idea in mind that the crystalline nature of the product changes with increasing annealing time, the shape of these curves in Figure 29 indicate that at larger potentials non-crystalline LiNiPO 4 precursor does not dissociate peroxide as well as crystalline LiNiPO : Annealing Temperature The annealing temperature also plays a role in the resulting particle size of the product LiNiPO 4 crystals. Based on the particle sizes seen through physical characterization, the sample sintered at 700 C had a larger particle size than the sample sintered at 600 C. The trend stays consistent as the particles sintered at 700 C produce a larger quantity of current than the particles sintered at 600 C. The full RRDE data is shown below in Figure 30. Figure 30: RRDE data for LiNiPO 4 sintered at different temperatures

71 60 At temperatures of 600 C and 700 C, the larger particles do produce a larger amount of current but only slightly. The FESEM images of the product indicate that there was only a slight increase in particle size from annealing temperatures of 600 and 700 C, so similar RRDE data is expected. It could also be that the annealing temperature does not have as large of an effect on the electrocatalytic ability of the LiNiPO 4 as was initially expected.

72 61 5: Analysis Based on the current vs. potential plots given by the RRDE, it is shown that decreasing the LiNiPO 4 particle size does not increase the electrocatalytic activity of LiNiPO 4. The crystalline nature of the LiNiPO 4 does play a large role in determining the electrocatalytic activity, a crystalline sample of LiNiPO 4 will produce a different characteristic RRDE disk current curve than a sample that has not fully crystallized or contains amorphous precursor products. 5.1: Electrocatalyst Particle Size Effects The RRDE data found demonstrates that smaller particles of LiNiPO 4 are less electrocatalytically active than larger particles of LiNiPO 4. The reason for this can be determined by observing the crystal structure of LiNiPO 4.[48,50] A diagram of the crystal structure is displayed in Figure 31. Figure 31: Three-dimensional structure of LiNiPO 4 [48] The structure of LiNiPO 4 consists of distorted NiO6 octahedral units that are corner shared and cross-linked with PO4 tetrahedron oxo-anions, forming a three-dimensional network with tunnels inhabited by Li ions along the (010) and (001) directions.[48,50] This structure causes