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Development of new proton conducting materials for intermediate temperature fuel cellsaoxiang, Xiaoxiang January 2010 (has links)
The work in this thesis mainly focuses on the preparation and characterization of several phosphates and solid oxide systems with the aim of developing new proton conducting materials for intermediate temperature fuel cells (ITFCs). Soft chemical methods such as sol-gel methods and conventional solid state methods were applied for the synthesis of these materials. Aluminum phosphate obtained by a solution method is single phase and belongs to one of the Al(H₂PO₄)₃ allotropies with hexagonal symmetry. The material is stable up to 200°C and decomposes into Al(PO₃)₃ at a higher temperature. The electrical conductivity of pure Al(H₂PO₄)₃ is on the order of 10⁻⁶-10⁻⁷ S/cm, very close to the value for the known proton conductors AlH₃(PO₄)₂•3H₂O and AlH₂P₃O₁₀•2H₂O. Much higher conductivity is observed for samples containing even a trace amount of excess H₃PO₄. It is likely that the conduction path gradually changes from grain interior to the surface as the acid content increases. The conductivity of Al(H₂PO₄)₃-0.5H₃PO₄ exhibited a good stability over the measured 110 hours. Although tin pyrophosphate (SnP₂O₇) has been reported to show a significantly high conductivity (~10⁻² S/cm) at 250°C in various atmospheres, we observed large discrepancies in the electrical properties of SnP₂O₇ prepared by different methods. Using an excess amount of phosphorous in the synthetic procedure generally produces SnP₂O₇ with much higher conductivity (several orders of magnitude higher) than samples with stoichiometric Sn:P ratios in their synthetic procedure. Solid state ³¹P NMR confirmed the presence of residual phosphoric acid for samples with excess starting phosphorous. Transmission Electron Microscope (TEM) confirmed an amorphous layer covered the SnP₂O₇ granules which was probably phosphoric acid or condensed phases. Thereby, it is quite likely that the high conductivity of SnP₂O₇ results mainly from the contribution of the residual acid. The conductivity of these samples exhibited a good stability over the measured 80 hours. Based on the observations for SnP₂O₇, we developed a nano core-shell structure based on BPO₄ and P₂O₅ synthesised by solid state methods. The particle size of BPO₄ using this method varied between 10-20 nm depending on the content of P₂O₅. TEM confirmed the existence of an amorphous layer that is homogeneously distributed. The composite exhibits the highest conductivity of 8.8×10⁻² S/cm at 300°C in air for 20% extra P₂O₅ and demonstrates a good stability during the whole measured 110 hours. Polytetrafluoroethylene (PTFE) was introduced into the composites in order to increase malleability for fabrication. The conductivity and mechanical strength were optimized by adjusting the PTFE and P₂O₅ content. These organic-inorganic composites demonstrate much better stability at elevated temperature (250°C) over conventional SiC-H₃PO₄-PTFE composites which are common electrolytes for phosphoric acid fuel cells (PAFCs). Fuel cells based on BPO₄-H₃PO₄-PTFE composite as the electrolyte were investigated using pure H₂ and methanol as fuels. A maximum power density of 320 mW/cm² at a voltage of 0.31 V and a maximum current density of 1.9 A/cm² at 200°C were observed for H₂/O₂ fuel cells. A maximum power density of 40 mW/cm² and maximum current of 300 mA/cm² 275°C were observed when 3M methanol was used in the cell. Phosphoric acid was also introduced into materials with internal open structures such as phosphotungstic acid (H₃PW₁₂O₄₀) and heteropolyacid salt ((NH₄)₃PW₁₂O₄₀), for the purpose of acquiring additional connections. The hybrids obtained have a cubic symmetry with enlarged unit cell volume, probably due to the incorporation of phosphoric acid into the internal structures. Solid state ³¹P NMR performed on H₃PW₁₂O₄₀-xH₃PO₄ (x = 0-3) showed additional peaks at high acid content which could not assigned to phosphorus from the starting materials, suggesting a strong interaction between H₃PW₁₂O₄₀ and H₃PO₄. The conductivity of hybrids was improved significantly compared with samples without phosphoric acid. Fourier transform infrared spectra (FT-IR) suggest the existence of large amount of hydrogen bonds (OH••••O) that may responsible for the high conductivity. A H₂/O₂ fuel cell based on H₃PW₁₂O₄₀-H₃PO₄-PTFE exhibited a peak power density of 2.7 mW/cm² at 0.3 V in ambient temperature. Solid oxide proton conductors based on yttrium doped BaZrO₃ were investigated by introducing potassium or lanthanum at the A-sites. The materials were prepared by different methods and were obtained as a single phase with space group Pm-3m (221). The unit cell of these samples is slightly smaller than the undoped one. The upper limit of solid solution formation on the A-sites for potassium is between 5 ~ 10% as introducing more K results in the occurrence of a second phase or impurities such as YSZ (yttrium stabilized zirconium). K doped Barium zirconates showed an improved water uptake capability even with 5% K doping, whereas for La doped ones, water uptake is strongly dependent on particle size and synthetic history. The conductivity of K doped BaZrO₃ was improved by a factor of two (2×10⁻³ S/cm) at 600°C compared with undoped material. Fuel cells based on Pt/Ba₀₋₉₅K₀₋₀₅Zr₀₋₈₅Y₀₋₁₁Zn₀₋₀₄O[subscript(3-δ)]/Pt under humidified 5% H₂/air conditions gave a maximum power density 7.7 mWcm⁻² at 718°C and an interfacial resistance 4 Ωcm⁻². While for La doped samples, the conductivity was comparable with undoped ones; the benefits of introducing lanthanum at A-sites may not be so obvious as deficiency of barium is one factor that leads to the diminishing conductivity.
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Synthesis, adsorption and catalysis of large pore metal phosphonatesPearce, Gordon M. January 2010 (has links)
The synthesis and properties of metal phosphonates prepared using piperazine-based bisphosphonic acids have been investigated. The ligands N,N’-piperazinebis(methylenephosphonic acid) (H₄L), and the 2-methyl (H₄L-Me) and 2,5-dimethyl (H₄L 2,5-diMe) derivatives have been prepared using a modified Mannich reaction. Hydrothermal reaction of gels prepared from metal (II) acetates and the bisphosphonic acids results in the synthesis of four structures: STA-12, Ni VSB-5, Co H₂L.H₂O and Mg H₂L. STA-12, synthesised by reaction of Mn, Fe, Co or Ni acetate with H₄L or H₄L-Me, has been investigated further. STA-12 crystallises in the space group R⁻₃, and Ni STA-12 is the most crystalline version. Its structure was solved from synchrotron data (a = b = 27.8342(1) Å, c = 6.2421(3) Å, α = β = 90°, γ = 120°), and it has large 10 Å hexagonal shaped pores. Helical chains of Ni octahedra are coordinated by the ligands, resulting in phosphonate tetrahedra pointing towards the pore space. Water is present, both coordinated to the Ni²⁺ cations and physically adsorbed in the pores. Mixed metal structures based on Ni STA-12, where some Ni is replaced in the gel by another divalent metal (Mg, Mn, Fe or Co) can also be synthesised. Dehydration of STA-12 results in two types of behaviour, depending on the metal present. Rhombohedral symmetry is retained on dehydration of Mn and Fe STA-12, the a cell parameter decreasing compared to the as-prepared structures by 2.42 Å and 1.64 Å respectively. Structure solution of dehydrated Mn STA-12 indicates changes in the torsion angles of the piperazine ring bring the inorganic chains closer together. Fe and Mn STA-12 do not adsorb N₂, which is thought to be due to the formation of an amorphous surface layer. Dehydration of Ni and Co STA-12 causes crystallographic distortion. Three phases were isolated for Ni STA-12: removal of physically adsorbed water results in retention of rhombohedral symmetry, while dehydration at 323 K removes some coordinated water forming a triclinic structure. A fully dehydrated structure (dehydrated at 423 K) was solved from synchrotron data (a = 6.03475(5) Å, b = 14.9156(2) Å, c = 16.1572(7) Å, α = 112.5721(7)°, β = 95.7025(11)°, γ = 96.4950(11)°). The dehydration mechanism, followed by UV-vis and Infra-red spectroscopy, involves removal of water from the Ni²⁺ cations and full coordination of two out of three of the phosphonate tetrahedra forming three crystallographically distinct Ni and P atoms. No structural distortion takes place on dehydration of Ni and Co STA-12 prepared using the methylated bisphosphonate, and the solids give a higher N₂ uptake as a result. Dehydrated Ni and Co STA-12 were tested for adsorption performance for fuel related gases and probe molecules. Investigations were undertaken at low temperature with H₂, CO and CO₂, and ambient temperature with CO₂, CH₄, CH₃CN, CH₃OH and large hydrocarbons. Due to the presence of lower crystallinity, Co STA-12 has an inferior adsorption performance to Ni STA-12, although it has similar adsorption enthalpies for CO₂ at ambient temperature (-30 to -35 kJ mol⁻¹). Ni STA-12 adsorbs similar amounts of CO₂ and N₂ at low temperature, indicating the adsorption mechanisms are similar. Also, it adsorbs 10 × more CO₂ than CH₄ at low pressure, meaning it could be used for separation applications. Ni STA-12 adsorbs 2 mmol g⁻¹ H₂ with an enthalpy of -7.5 kJ mol⁻¹, the uptake being due to adsorption on only one-third of the Ni²⁺ cations. The uptake for CO is 6 mmol g⁻¹, with adsorption enthalpies ranging from -24 to -14 kJ mol⁻¹. This uptake is due to adsorption on all the Ni²⁺, meaning the adsorption enthalpies are high enough to allow the structure to relax. This is also observed for adsorption of CH₃CN and CH₃OH, where there is a return to rhombohedral symmetry after uptake. The adsorption sites in dehydrated Ni and Co STA-12 were investigated via Infra-red spectroscopic analysis of adsorbed probe molecules (H₂, CO, CO₂, CH₃CN and CH₃OH). The results indicate the adsorption sites at both low and ambient temperature are the metal cations and the P=O groups. The metal cation sites are also characterised as Lewis acids with reasonable strength. STA-12 was shown to have acidic activity for the liquid phase selective oxidations of 1-hexene and cyclohexene, although there is evidence active sites are coordinated by products and/or solvents during the reaction. STA-12 also demonstrates basic activity for the Knoevenagel condensation of ethyl cyanoacetate and benzaldehyde. Modification of STA-12 by adsorption of diamine molecules causes a slight increase in the basicity, and the highest conversions are where water and diamine molecules are both present.
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