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MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY ------------------------------ Pham Hong Hanh STUDY AND FABRICATED ELECTROCATALYSTS ON THE IrO2 BASIS FOR OXYGEN EVOLUTION REACTION IN PROTON EXCHANGE MEMBRANE WATER ELECTROLYZER Major: Theoretical and Physical Chemistry Code: 9.44.01.19 SUMMARY OF CHEMICAL DOCTORAL THESIS Ha Noi – 2019 The work was completed at : Graduate University Science and Technology – Vietnam Academy of Sicence and Technology Science instructor: Doc. Nguyen Ngoc Phong Doc. Le Ba Thang Reviewer 1: Reviewer 2: Reviewer 3: The thesis will be protected before the doctoral dissertation thesis, meeting at the Academy of Science and Technology - Vietnam Academy of Science and Technology at ... hour ... ', date ... month ... 2019 The thesis can be found at: - Library of the Academy of Science and Technology - National Library of Vietnam 1 PREFACE 1. Urgency of the thesis Population growth and rapid industrialization of countries, especially emerging countries such as China and India, lead to rapid demand for energy worldwide. Currently, more than 80% of energy needs are met from fossil fuel sources such as oil, coal and natural gas because they are available in nature, easily and conveniently in transport and storage. However, fossil fuel sources are gradually exhausted while increasing demand for energy use has been and will threaten the energy security of many countries as well as bring to the seed of one. On the other hand, the use of fossil fuels also creates gas products that pollute the environment. Therefore, the need to develop new and alternative energy sources, capable of regenerating and not polluting the environment is becoming urgent for humankind. Hydrogen is one of the potential new energy sources in the future, hydrogen is the most abundant element, its combustion efficiency is higher than petroleum (60% versus 25%). When burning hydrogen, there is only one byproduct, water, without any waste that is harmful to the environment. In short, hydrogen is the cleanest, most efficient and endless source of renewable energy. So hydrogen-based economics will gradually replace the oil economy and will be the most ideal sustainable economy of mankind. There are many ways to produce hydrogen, the proton exchange membrane water electrolysis method (PEMWE) using uses electricity to split the pure water into hydrogen at cathode and oxygen at anode. It is a method with many outstanding advantages: high efficiency (possibly more than 90%), high purity (about 99%), safe, low energy consumption, can operate with high current density (up to 2 A.cm-2) and 1 ability to combine with renewable energy sources such as wind power, solar energy…There are a lot of intensive researches and developments which have been done on PEMWE and commercialized product s(with hydrogen production capacity of 0,01‒50000 Nm3.h-1) were provided by globe companies. However, high cost of investment to use precious and expensive catalysts material has limited their mass commercialization. In addiction, the overpotential loss on anode of PEMWE for oxygen evolution reaction (OER) is still relatively high and this reduce the efficiency of water electrolysis processes. Therefore, in recent PEMWEs research have been focused on new catalysts material in order to improve the anode active surface area, catalyst utilization, and stability by use nanosize powder materials, thereby improving the performance and capacity of PEMWE. In Vietnam, studies of hydrogen production electrolysis using proton exchange membranes have not been given much attention. In order to continue gradually with the hydrogen economy and keep up with the research trend of catalytic materials for PEMWE. Stemming from that reason, the thesis is aimed at: “Study and fabbricated electrocatalysts on the IrO2 basis for oxygen evolution reaction in proton exchange membrane water electrolyser”. 2. Scope of thesis Fabbrication of electrocatalyst materials on the IrO2 basis for oxygen evolution reaction in proton exchange membrane water electrolyser PEMWE. Applying to fabbricated proton exchange membrane water electrolyser PEMWE to produce hydrogen. 2 3. Main contents of thesis - Introduction of proton exchange membrane water electrolyser (PEMWE), research and development of catalytic materials in PEMWE based on IrO2: unary, binary and ternary system. - Introduction of preparation IrO2 powder catalyst by hydrolysis anh Adams method, the effect of temperature on the characteristics of synthetic catalyst materials, from which to choose the method and making the suitable process to sythesis catalyst IrO2. - Stydy on sythesis IrxRu(1-x)O2 (x =0; 0,5; 0,6; 0,7; 0,8; 1) binary mixture powder. Evaluation and comparison of the effect of component ratio RuO2: IrO2 to the activity and durability of catalytic materials. Select the optimal component with high activity and durability. - Research and development of IrRuMO2 ternary catalyst (with M = Ti, Sn and Co). - Applying to fabbricated proton exchange membrane water electrolyser PEMWE with 5 cm2 working area including parts: current collector, separator plate, membrane electrode assembly, shell plate...to produce high-capacity hydrogen. Test and evaluate the performance of single PEMWE when operating. CHAPTER 1. INTRODUCTION - Introduction of development history, structure, operational principles and application of PEMWE. - Present the mechanisms and kinetics of oxygen evolution reaction and hydrogen evolution reaction on the catalytic materials based on IrO2 in PEMWE. - Introduction of development history of catalytic materials used in PEMWE and research & development of catalytic materials at the anode and cathode electrodes of PEMWE. 3 CHAPTER 2. EXPERIMENTAL AND RESEARCH METHOD 2.3. Fabrication of electrocatalyst materials on the IrO2 basis Three methods applied to catalyst synthesis are hydrolysis, Adams and Adams modified method. - Hydrolysis method: At first, metal precursors were dissolved in deionization water with the exactly calculated metal precursors. The aqueous solution was then heated (100°C) under air atmosphere and magnetically stirred for 1 hour. Afterward, sodium hydroxide (1 M) was added to the solution in order to obtain the precursor- hydroxide. This mixture was maintained under stirring and heat (100°C) for 45 minutes. The given precipitate was then filtered and washed with deionization water. After washing the precursor-hydroxide was dried for 5 hours at 80°C. Finally, the dryed paste was calcined in air at 450°C for 1 hour with a heating ramp of 5°C.min-1. - Adams method: metal precursors were added to isopropanol to obtain a total metal concentration of 0.01 M, this solution was ultrasonic for 30 minutes and magnetically stirred for 1-2 hours to ensure complete dissolution of the precursors, followed by the addition of 10 gram of finely ground NaNO3. The mixture was heated at 70oC in air until completely dry. Using isopropanol as the precursor solvent appeared to give a more homogenous mixture than if water was used. The dry salt mixture was then placed into a preheated furnace. The fused salt oxide mixture was cooled slowly to room temperature then washed in deionised water to remove the remaining salts, filtered and dried in an oven at 80oC. - Modified Adams method: the steps are the same as the Adams method differs only at the furnace stage: before heating at 500°C for 1 hour the 4 salt mixture need pre-heated at 325°C for 30 minutes at a rate of 5°C.min-1. 2.4. Physical and electrochemical cheracterization of electrocatalyst 2.4.1. Physical cheracterization The mechanisms of the thermal decomposition process of metal precursors to form oxide powders were studied by means of thermal gravity analysis (TGA). The physical phase and structure of the oxide powder catalysts were determined by x-ray diffraction (XRD). The surface morphology and particle size of the catalysts materials was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy-dispersive X-ray spectroscopy (EDX) is used to determine the composition of the catalyst mixture. 2.4.2. Electrochemical cheracterization Catalytic layers preparations: The catalyst layer deposited on the carbon paper (AvCarrb 1071-USA) and the anode electrocatalyst loading was 4 mg.cm-2; the cathode electrocatalyst loading was 1 mg.cm-2. To prepare the thin-film electrocatalyst layers, a homogeneous ink composed of powder catalyst, Nafion solution, isopropanol and deionised water were homogenized by stirring and ultrasonicing. The mixture was cast on a cacbon paper surface by sweep method and dried in air, this process was repeated until enough the loading. CV measurements are measured with three purposes: determination of catalytic activation and electrochemical processes on catalytic surfaces: examine the reversibility of the redox process; determination of catalystic degradation. The anodic polarization curve is carried out by scanning the linear potential over time with a constant speed of 0.5 mV.s-1 from - 250 VAg/AgCl to 1.3 VAg/AgCl (compared to the corrosion potential) in 0.5 5 M H2SO4. This method allows the determination of the steady-state current density, the starting potential of OER. Galvanostatic: the catalytic samples were polarized at high current density (200 mA.cm-2) in 0.5 M H2SO4 medium to accelerate the dissolution or inactivation process of anode, thereby quickly assessing elctrode’s lifetime. The voltage is recorded over time, the lifetime of the electrode is the measured time until the electrode is destroyed. 2.5. Fabrication and cheracterization of MEA Membrane electrode assembly is fabricated by heat-press method, pressing Nafion film between two diffusion gas layers covered by catalytic ink. The catalytic layer is fabricated by brushing on the cacbon paper surface and dried in air. This process was repeated until reaching enough the electrocatalyst loading of 4mg.cm-2. In this thesis, a simple PEMWE electrolyte is designed, manufactured and installed. Materials and specifications of PEMWE are given in Table 2.5. Table 2.5. Materials and specifications of PEMWE Component Material Size (mm) MEA Gasket Separator plate Shell Current collector Bolt Made in section 2.5.1 Silicon 23 × 23 50 × 50 × 1 Graphite AXF- 5Q (Poco) 50 × 50 × 3,2 Acrylic 50 × 50 × 8 Gold plated copper 50 × 50 × 1 Stainless steel plastic wrap ⏀5 6 CHAPTER 3. RESULTS AND DISCUSSUIONS 3.1. Fabrication of IrO2 electrocatalyst Fig. 3.1. TGA and DTA diagram Fig. 3.2. TGA and DTA of (H2IrCl6.xH2O + NaOH) diagrams of (H2IrCl6.nH2O + precursor mixture follows NaNO3) precursor mixture hydrolysis method follows Adams method Observed on both TGA graphs of the two synthetic methods are divided into two stages, the first stage occurs at low temperature, salt mixtures of both methods have a rapid weight reduction. Currently, it is mainly due to the evaporation of water molecules which adsorb physics and in the form of hydrate in Ir's salt. The second stage on the DTA and TGA is the complete thermal decomposition of the salts forming IrO2 powder. For hydrolysis method, this process occurs in the temperature range of 300-394oC. For Adams method, the decomposition temperature to create IrO2 powder in the range of 350-605.6oC. The result obtained by spectra of X-ray diffraction spectra have proved that the final product is IrO2 (Figure 3.3 and 3.4). From here, IrO2 catalysts will be furnaced at 300oC, 400oC, 500oC and 600oC follow hydrolysis method and 400oC, 500oC and 600oC follow Adams method. 7 Fig. 3.3. XRD patterns of IrO2 fabricated hydrolysis method Fig. 3.4. XRD patterns of IrO2 fabricated Adams method Figure 3.3 and 3.4 are X-ray diffraction pattern of IrO2 powder samples synthesized by hydrolysis and Adams methods at different furnacing temperatures. At the furnace temperature values lower than 500oC, the peaks on the X-ray diffraction pattern of both synthesized methods have an unclear peak signal and narrower. This is because the IrO2 formed at these furnacing temperatures has a very fine structure or anatas structure. When the furnacing temperature is increased to 500°C and 600°C, the size of the peaks is smaller and the more peak signals that represent the crystal structure. The peaks on the X-ray diffraction pattern have signal peaks at 2θ angle values: 28(110); 35.1(101); 54.3(211) are similar and all peaks match a rutile-structure as indexed. Thus, at the furnacing temperature of 500oC or more, the IrO2 catalytic material changes from amorphous structure to rutil crystal structure. Fig. 3.5. SEM pictures of IrO2 electrocatalytic powers by hydrolysis method, degree of magnification 80.000 times 8