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  1. 1. Synthesis of Polymer DerivedBoron-Doped Rare Earth Stabilized Bismuth Oxide Nanocomposites for SOFC Applications Prof.Dr. İbrahim USLU Prof.Dr. İbrahim USLU
  2. 2. Outline• Fuel Cells and SOFCs• Solid oxide fuel cell history• Design and operation of SOFCs• R&D On Fuel Cell• RE Stabilized Bismuth Oxides• Techniques used in our studies – Electrospinning technique – Polymer precursor technique• Results of our studies Prof.Dr. İbrahim USLU
  3. 3. Fuel CELLs• Fuel cells convert chemical energy of a fuel gas directly into electrical work, and are efficient and environmentally clean, since no combustion is required.• Moreover, fuel cells have the potential for development to a sufficient size for applications for commercial electricity generation. Prof.Dr. İbrahim USLU
  4. 4. Solid oxide fuel cells• SOFC are based on the concept of oxygen ion conducting electrolyte through which the oxide ions (O2-) migrate from the air electrode (cathode) side to the fuel electrode (anode) side where they react with the fuel (H2, CH4, etc.) to generate an electrical voltage. Prof.Dr. İbrahim USLU
  5. 5. Christan Friedrich Schönbein• Schönbein discovered the principle of the fuel cell in 1838.• It was his Welsh friend Sir William Robert Grove who developed the first prototype using hydrogen and oxygen to create electricity in 1845. Prof.Dr. İbrahim USLU
  6. 6. Groves Fuel Cell• Grove discovered that by arranging two platinum electrodes with one end of each immersed in a container of sulfuric acid and the other ends separately sealed in containers of oxygen and hydrogen, a constant current would flow between the electrodes. Prof.Dr. İbrahim USLU
  7. 7. Ludwig Mond• Ludwig Mond (1839–1909) and assistant Carl Langer described their experiments with a hydrogen–oxygen fuel cell that attained 6 amps per square foot (measuring the surface area of the electrode) at 0.73 V. Prof.Dr. İbrahim USLU
  8. 8. Friedrich Wilhelm Ostwald• Ostwald (1853–1932), a founder of the field of physical chemistry, provided much of the theoretical understanding of how fuel cells operate. Prof.Dr. İbrahim USLU
  9. 9. NASA spent tens of millions of dollars• In connection with the space program Apollo in 1960, NASA spent tens of millions of dollars in a successful program that used hydrogen-based fuel cells to power the on-board electrical systems on the Apollo journey to the moon. Prof.Dr. İbrahim USLU
  10. 10. Solid oxide fuel cell history• The operation of the first ceramic fuel cell at 1000°C, by Baur &Preis, was achieved in 1937.• Researchers at Westinghouse, for example, experimented with a cell using zirconium oxide (zirconia) and calcium oxide in 1962. Prof.Dr. İbrahim USLU
  11. 11. Solid oxide fuel cell history• However, electrolytes based on zirconia have a relatively low oxide ion conductivity at temperatures below 800 K and require very high sintering temperatures (often higher than 2000 K).• For example δ-phase-Bi2O3 is two orders of magnitude higher conductivity than zirconia.• Replacement of zirconia with δ-phase-Bi2O3 based ion conductor, would give a signifcant reduction in the material and fabrication problems together with an improvement in the e•-ciency and longevity of the cell. Prof.Dr. İbrahim USLU
  12. 12. Fuel cell was built by Siemens Westinghouse• The fuel cell was built by Siemens Westinghouse and the microturbine by Northern Research and Engineering Corporation.• In a year of actual operating conditions, the 220 kW SOFC, running on natural gas is achieving an efficiency of 60%.• Also, a world record for SOFC operation, roughly eight years, still stands, and the prototype cells have demonstrated two critical successes: – the ability to withstand more than 100 thermal cycles, and – voltage degradation of less than 0.1% per thousand h. Prof.Dr. İbrahim USLU
  13. 13. Historical review of fuel cells Prof.Dr. İbrahim USLU
  14. 14. TODAY Fuel Cells• Today, fuel cells are common in spaceflight (Space Shuttle, Skylab and Gemini spacecrafts), transportation and make sense for use as portable power, home power generation and large power generation. Prof.Dr. İbrahim USLU
  15. 15. Design and operation of SOFCs• Cells are being constructed in two main configurations,• Tubular cells or rolled tubes, such as those being developed at Westinghouse Electric Corporation since the late 1950s,• Flat-plates configuration adopted more recently. Prof.Dr. İbrahim USLU
  16. 16. Tubular cells Prof.Dr. İbrahim USLU
  17. 17. Tubular Solid Oxide Fuel Cell Prof.Dr. İbrahim USLU
  18. 18. Flat-plates configuration Prof.Dr. İbrahim USLU
  19. 19. Flat-plates configuration Prof.Dr. İbrahim USLU
  20. 20. Components of the SOFCs• A SOFC is mainly composed of two electrodes (the anode and the cathode), and a solid electrolyte. Prof.Dr. İbrahim USLU
  21. 21. Anode and Cothode• The anode, conducts the electrons that are freed from the hydrogen molecules so that they can be used in an external circuit.• The cathode, distribute the oxygen to the surface of the catalyst. It also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. Prof.Dr. İbrahim USLU
  22. 22. Electrolyte and Catalyst• The electrolyte is the proton exchange membrane only conducts positively charged ions.• The membrane blocks electrons.• The catalyst facilitates the reaction of oxygen and hydrogen.• It is usually made of platinum porous nanoparticles very thinly coated onto carbon paper or cloth so that the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. Prof.Dr. İbrahim USLU
  23. 23. The key requirement for the solid electrolyte• it has good ionic conduction to minimize cell impedance, but also;• has little or no electronic conduction to minimize leakage currents, Prof.Dr. İbrahim USLU
  24. 24. How Fuel Cell Works• The pressurized H2 gas entering the fuel cell on the anode side. This gas is forced through the catalyst by the pressure. When an H2 molecule comes in contact with the platinum on the catalyst, it splits into two H+ ions and two electrons (e-). The electrons are conducted through the anode, where they make their way through the external circuit and return to the cathode side of the fuel cell.• On the cathode side, O2 gas is being forced through the cathode, where it forms two oxygen atoms. Each of these atoms has a strong negative charge and charge attracts the two H+ ions through the membrane, where they combine with an oxygen atom and two of the electrons from the external circuit to form H2O. Prof.Dr. İbrahim USLU
  25. 25. How Fueş Cell Works SOFC Technology & Advantages• SOFCs have a modular and do not present any moving parts, thereby are quiet enough to be installed indoors. Prof.Dr. İbrahim USLU
  26. 26. SOFC Technology and Advantages• SOFCs are the most efficient (fuel input to electricity output) fuel cell electricity generators currently being developed world- wide.• SOFCs are flexible in the choice of fuel such as carbon- based fuels, eg, natural gas.• SOFCs do not have problems with electrolyte management (liquid electrolytes, for example, which are corrosive and difficult to handle).• SOFCs have a potential long life expectancy of more than 40000–80000 h. Prof.Dr. İbrahim USLU
  27. 27. R&D On Fuel Cell• US, Canada and Japan significantly increased their funding for fuel cell R&D.• The superior fuel flexibility is due primarily to the higher operating temperature, which increases reaction rates in the fuel, but also increases the rates of undesired reactions and creates thermal stresses during thermal cycling.• Thus, the development and fabrication of materials to meet these requirements is a major challenge for the implementation of cost effective SOFCs Prof.Dr. İbrahim USLU
  28. 28. Development & fabrication Oxide ion conductivity• At present electrolyte materials used in SOFC Technology are based on doped zirconia systems.• Yttria stabilised zirconia, (YSZ) is a typical electrolyte material.• However, electrolytes based on zirconia have a low ionic conductivity compared to bismuth oxide (Bi2O3) based electrolytes. Prof.Dr. İbrahim USLU
  29. 29. Disadvantages of YSZ• Compared to Bi2O3 solid electrolytes based on ZrO2, – have a relatively low oxide ion conductivity and – require very high sintering temperatures (>2000 K). Prof.Dr. İbrahim USLU
  30. 30. Bi2O3 One of the best oxygen ion conductors• Cubic Bismuth oxide is one of the best oxygen ion conductors and is of considerable interest in SOFC and oxygen sensors. Prof.Dr. İbrahim USLU
  31. 31. δ- Bi2O3 high concentration of oxygen vacancies• The concentration of oxygen vacancies of Bi2O3 can be as high as 25% of the total amount of anions,• With such a high concentration of oxygen vacancies, the conductivity of δ-Bi2O3 can be two orders of magnitude higher than that of the common oxide conductor, YSZ. Prof.Dr. İbrahim USLU
  32. 32. Bi2O3 has high ionic conductivity• Bi2O3 has high ionic conductivity, but decomposes at low oxygen partial pressures, which prevents it from being used in SOFC.• The δ-phase is one of the four known polymorphs of Bi2O3 solid, and it is stable only in the temperature range from 730ºC- 825 ºC. Prof.Dr. İbrahim USLU
  33. 33. Bi2O3, cracking due to the volume changes• δ- Bi2O3, has exceptionally high ionic conductivity, but is subject to cracking due to the volume changes associated with phase transformation during cyclic heating and cooling.• The high-temperature form of Bi2O3 can be stabilized with REs to eliminate the fracture problem, but the ionic conductivity is reduced. Prof.Dr. İbrahim USLU
  34. 34. RE Stabilized Bi2O3• Stabilize Bi2O3 with REs the ion conductivities firstly increase and then decrease, therefore a maximum value is obtained• The conductivity increases along with oxygen vacancies.• However, if the number of oxygen vacancies reaches a certain value, the oxygen vacancies begin to become ordering.• This results in the weakening of oxygen ionic diffusion because of the decrease of effective vacancies. Prof.Dr. İbrahim USLU
  35. 35. The ionic conductivity of Dy2O3 Stabilized Bi2O3• The ionic conductivity of Dy2O3 stabilized Bi2O3 from x=0.25 to 0.60• It is apparent that the highest conductivity, for a stable fcc structure, was reported for the sample x=0.28, with a value of – 7.1 10-3 Scm-1 at 500 ºC, and – 0.14 Scm-1 at 700 ºC.• Doping does not result in increase in the oxygen ion vacancy concentration and increasing dopant amount just decreases the ionic conductivity Prof.Dr. İbrahim USLU
  36. 36. The oxygen ion conductivityvalues of Bi2O3• Bi2O3 stabilized with Neodymia, Lantania and Erbia and the samples are also compared to YSZ. Prof.Dr. İbrahim USLU
  37. 37. Conductivity values of RE oxide stabilized Bi2O3• Er2O3 stabilized Bi2O3 has been shown to have one of the highest oxygen ion conductivities systems in air; ~0.4 S cm-1. Prof.Dr. İbrahim USLU
  38. 38. RE stabilized Bi2O3 Oxide İon Conductivity• At high temperature, the material has a higher oxide ionic conductivity. Prof.Dr. İbrahim USLU
  39. 39. Boron oxide used as dopant• The addition of boron oxide to the composite materials as a dopant is very beneficial.• It is an effective sintering aid because of its low melting point (460 °C), which could help during the sintering process.• In this study, boric acid was chosen as the cheapest and nontoxic source of boric oxide Prof.Dr. İbrahim USLU
  40. 40. Boric Acid-PVA cross-linking reaction• White boric acid powder upon addition to water form tetrahedral [B(OH)4]-(aq) ions.•• B(OH)3(aq) + H2O(l) → [B(OH)4]-(aq) + H+(Aq) (1)• Boric acid is a monoprotic acid and hydroxyl groups (OH) of [B(OH)4]-(aq) tetrahedral ions.• Boric acid react with PVA and weak cross-linking within the polymer resulting in formation of the viscoelastic gel with tetrahedral [BO4]- ions . Prof.Dr. İbrahim USLU
  41. 41. In this study• Five kinds of rare earth stabilized bismuth oxide ceramics, (RE=Dy, Y, Ho, Er and La), were synthesized using – A) the polymeric precursor technique and – B) electrospinning technique.• Calcining and sintering of – A) the polymeric precursor technique or – B) the electrospun nano-fibers at 850 ºC,• Bi2O3 and RE oxide composite powder were obtained and their characterization and electrical properties were investigated. Prof.Dr. İbrahim USLU
  42. 42. A) Polymer precursor technique• In this study boron and RE-Bismuth acetate containing PVA polymer solution were used as a polymeric precursor.• Polymeric precursor was calcined to remove the organics and to obtain homogeneous crystalline ceramic powders.• The main advantages of such polymer-derived ceramics are the homogeneity of the precursors on a molecular level, the low processing temperatures compared to conventional powder mixing-milling and then sintering method. Prof.Dr. İbrahim USLU
  43. 43. B) Electrospinning technique• In this study boron containing Bi2O3-La2O3 composite PVA polymer solution was also electrospun to obtain ultrahomogenus and nanosized fiber structures. Prof.Dr. İbrahim USLU
  44. 44. Electrospun nanofibers• The polymer solution was poured in a syringe and connected to a high-voltage supply (applied voltage 18 kV).• The solution was delivered by a syringe pump with a flow rate 0.5 ml/h.• Then the fibers were dried in vacuum for 12 h at 80 C. Prof.Dr. İbrahim USLU
  45. 45. Electrospun fiber diameter distributions• The average fiber diameters for electropsun boron doped and undoped nanofibers were in the range of 200 nm and 500 nm. Prof.Dr. İbrahim USLU
  46. 46. nano-grain size composites were obtained from electrospinning technique Prof.Dr. İbrahim USLU
  47. 47. nano-grain size composites were obtained from electrospinning technique Prof.Dr. İbrahim USLU
  48. 48. Advantages of nano-grains• Nano-grains have greater ratios of surface area to volume, which means a greater ratio of grain boundary to dislocations.• The more grain boundaries that exist, the higher the strength becomes.• Thus, an easy way to improve the strength of a material is to make the grains as small as possible, increasing the amount of grain boundary Prof.Dr. İbrahim USLU
  49. 49. Sintering after the calcination• The grains of sintering samples increased with respect to calcined samples, which can be explained by the fact that the grains of Bi2O3 gradually grow larger after the second heat treatment of the samples.• that the grains of Bi2O3 gradually grow larger as the sintering temperature becomes higher. Prof.Dr. İbrahim USLU
  50. 50. Boron doped & undoped nano-composites using electrospinning technique• Boron undoped & the sample with very little boron addition consist of spheroidal shaped grains. Average grain diameters of boron doped and undoped nanocrystalline calcined powders were measured as 170 nm &120 nm respectively.• “Given that the radius of B3+ (0.023 nm) is much smaller than that of Bi3+ (0.117 nm), it is difficult for B3+ to replace the Bi3+ site. Boron ions may enter the interstitial site of Bi2O3 crystal structure and lead to the swell of the crystallite size. Prof.Dr. İbrahim USLU
  51. 51. Further increase in boron doping• Further increase of the boron may cause a decrease in crystallite size and transition to the amorphous glassy structure which is consisted with the literature. Prof.Dr. İbrahim USLU
  52. 52. FT-IR Spectrum• It is concluded from the FTIR spectrum that no water vapor can adsorb on the surface of the ceramic powder and all the carbons were removed after sintering process of the samples. Prof.Dr. İbrahim USLU
  53. 53. Boron Doped and Sintered Composites Prof.Dr. İbrahim USLU
  54. 54. Crystallite sizes• Crystallite sizes of the samples were evaluated using Scherrer’s equation.• This result shows that holmia stabilized bismuth oxide nanoceramic powders consisted of crystallites with a diameter of 37 nm.• In addition, the crystallite size the calculated microstrain (ε) and dislocation density (δ) values are also given in the Table. Prof.Dr. İbrahim USLU
  55. 55. BET Analysis• The BET results show that boron undoped and doped Bi2O3- La2O3 nanocrystalline powder ceramic structures sintered at 800 oC have surface area of 20.44 m2/g and 12.93 m2/g, respectively.• The surface area of undoped powder is thus about 1.6 times larger than that of the doped one.• The smaller grain size, as observed by SEM analysis given above, also supports high surface area of the undoped powders. Prof.Dr. İbrahim USLU
  56. 56. XRD Results of calcined &sintered Powders• The decrease in the width of the X-ray peaks of sintered samples with respect to calcined ones corresponds to a reduction in the crystallite size. Prof.Dr. İbrahim USLU
  57. 57. Electrical Conductivity values of Er stabilized Bi2O3• Measurements of temperature dependence electrical conductivity (sdc) for polymer-derived erbia stabilized bismuth oxide ceramic were carried out in the wide temperature range of 648-1175 K.• Electrical conductivity was measured by raising and decreasing temperature. Prof.Dr. İbrahim USLU
  58. 58. Electrical Conductivity values of Er stabilized Bi2O3• There are two distinct region of the conductivity curve which is indicate a transition occur at 650 C.• This transition can be attributed to the order-disorder transition which is a higher-order or critical point transition Prof.Dr. İbrahim USLU
  59. 59. Electrical Conductivity values of Er stabilized Bi2O3• At high temperature region (>650 C), the order-disorder transition may be attributed to a charge transfer between the oxide ions and vacant orbital of Er3+ cations in the distorted crystal structure.• The oxygen lattice points of the Er2O3 doped -phase Bi2O3 are not completely occupied with oxygen ions.• Thorough the solid state reactions of Bi2O3 doped with Er2O3, Er3+ cations preferentially substitute at the fcc sites in the crystal structure.• This was an indication that Er2O3 dissolves in -type Bi2O3 matrix.• Some of the oxygen lattice points located around fcc sites may be vacant forming an oxygen vacancy.• Prof.Dr. İbrahim USLU
  60. 60. Electrical Conductivity values of Er stabilized Bi2O3• In the references, the fcc crystal structure has an oxygen deficient fluorite structure with two units, and two oxygen ion vacant sites per unit cell and bismuth ions in the structure are located on fcc sites, and differ only in the location of the oxygen ions [K.R. Kendall, C. Navas, J.K. Thomas, H.C. zurLoye, Chem. Mater. 8 (1996) 642-649].• Therefore, oxygen vacancies are partly responsible for the order-disorder transition. Prof.Dr. İbrahim USLU
  61. 61. sol–gel prepared holmium doped Bi2O3 Prof.Dr. İbrahim USLU
  62. 62. Boron Doped holmium Stabilized Bi2O3 via Electrospun Prof.Dr. İbrahim USLU
  63. 63. Gadolinia Stabilized Bismuth Oxide with Boron via Electrospun Prof.Dr. İbrahim USLU
  64. 64. Sol-Gel Gadolinia doped Bismuth Oxide Prof.Dr. İbrahim USLU
  65. 65. Boron Doped Bismuth Oxide-Erbium Oxide Powder Prof.Dr. İbrahim USLU
  66. 66. Boron doped Bi2O3-La2O3 via electrospun Prof.Dr. İbrahim USLU
  67. 67. Prof.Dr. İbrahim USLU
  68. 68. Prof.Dr. İbrahim USLU
  69. 69. PVA (Bi2O3 doped) Prof.Dr. İbrahim USLU