Ammonium Polyphosphate Based Protonic Electrolytes

The widespread interest in fuel cells stems from their high efficient in energy conversion. Although various types of high temperature (above 400℃) and low temperature (below 150℃) fuel cells have been developed, there still exist lots of material issues to be solved for commercialization. Intermediate temperature solid state fuel cells, which are operated in the range of 150-400℃, are promising to solve these problems and offer many advantages such as suppression of CO poisoning of Pt catalyst, higher efficiency of energy conversion than that of polymer electrolyte membrane fuel cells, utilization of metals and plastics, and lower materials degradation than that of solid oxide fuel cells. Therefore, it can not overstate the importance on developing intermediate temperature fuel cells. One of the key points to develop intermediate temperature fuel cells is to develop electrolytes which can be used in the temperature range of 150-400℃.This thesis aims to study electrochemical characterizations and proton conduction mechanism of ammonium polyphosphate composite, which is one of the proton conductors that exhibit high conductivity and can be used as potential electrolytes in intermediate temperature fuel cells.In chapter 1, the theory of fuel cell and the importance of developing intermediate temperature fuel cells were introduced at first. The latest progress in the study of intermediate temperature electrolyte materials and electrode materials has been reviewed. The development in electrolyte materials for intermediate temperature fuel cells was especially highlighted.In chapter 2, 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ was synthesized and its conductivity behavior was characterized in dry and wet atmospheres. It was the first time that the proton transference numbers of NH₄PO₃-based composite were determined. And we also investigated the possibility of using 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ in fuel cells as electrolyte. It was found that the conductivity of 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ was always higher in wet atmosphere than that in dry atmosphere. For example, its conductivity in wet hydrogen is about 0.08S/cm and only 0.018S/cm in dry hydrogen. A hydrogen concentration cell was fabricated using 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ as electrolyte and Pt/C as electrode catalyst material. The emfs of the concentration cell almost equal to theoretical values calculated from Nernst equation. Therefore 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ was confirmed to be a pure proton conductor. The proton transference numbers were determined to be 1.0 at 150℃, 0.99 at 200℃, and 0.99 at 250℃. A fuel cell with 6NH₄PO₃-(NH₄)₂SiP₄O₁₃ as an electrolyte showed power density of 6.6mW/cm² at 250℃and open circuit voltage of 0.82V at 150℃. Increases in electrolyte density and cathode activity were supposed to enhance the cell performance drastically.In chapter 3, a novel NH₄PO₃-based composite, xNH₄PO₃-(NH₄)₂MnP₄O₁₃(x=2, 4, 6), was synthesized using solid-state reaction. Its electrochemical behavior and thermal stability were studied. Thermal gravimetric analysis and X-ray diffraction showed that (NH₄)₂MnP₄O₁₃ was very stable in dry nitrogen under 300℃. After xNH₄PO₃-(NH₄)₂MnP₄O₁₃ were treated at 250℃for 24 hours, X-ray diffraction shows that (NH₄)₂MnP₄O₁₃ was chemically compatible with NH₄PO₃ or HPO₃, which was thermal decomposition residue of NH₄PO₃ and responsible for high conductivity of NH₄PO₃-based composites. These results showed that (NH₄)₂MnP₄O₁₃ was stable enough to serve as a supporting matrix for NH₄PO₃. The proton conductivity of the composite electrolytes was improved by increasing the molar ratio of NH₄PO₃ under both dry and wet atmospheres. Provided at 250℃, the conductivity of xNH₄PO₃-(NH₄)₂MnP₄O₁₃ was increased from 0.27mS/cm to 1mS/cm in dry nitrogen and from 4mS/cm to 7mS/cm in wet nitrogen (3ï¼…H₂O) when x was increased from 2 to 6. And, the conductivity of (NH₄)₂MnP₄O₁₃ is one or two orders of magnitude lower than that of xNH_aPO₃-(NH₄)₂MnP₄O₁₃ in both dry and wet nitroten. So, It was believed that NH₄PO₃ or HPO₃ was responsible for high conductivity of xNH₄PO₃-(NH₄)₂MnP₄O₁₃ and (NH₄)₂MnP₄O₁₃ only served as support material. Comparing to nitrogen and oxygen, 2NH₄PO₃-(NH₄)₂MnP₄O₁₃ showed a higher conductivity in hydrogen. Hence, it was supposed to be a proton conductor. The proton transference number of 2NH₄PO₃-(NH₄)₂MnP₄O₁₃ was determined to be 0.95 at 250℃using a hydrogen concentration cell. Fuel cells utilizing 2APP-(NH₄)₂MnP₄O₁₃ as electrolyte were constructed and tested. Maximum power density of 16.8mW/cm² was observed at 250℃when dry hydrogen and dry oxygen were used as fuel and oxidant, respectively. And no decrease was observed when the output current density was fixed at 20mA/cm² for 10 hours. These results suggest that (NH₄)₂MnP₄O₁₃ is an excellent supporting matrix and NH₄PO₃-(NH₄)₂MnP₄O₁₃ composites are potential candidate electrolytes for intermediate temperature fuel cells. The chemical compatibility between NH₄PO₃-(NH₄)₂MnP₄O₁₃ and Pt/C catalyst was tested using thermal gravimetric analysis. Though, catalytic degradation of (NH₄)₂MnP₄O₁₃ and NH₄PO₃ happened with Pt/C catalyst presence, HPO₃, which is responsible for high conductivity of the composite, was still chemically compatible with Pt/C catalyst. It indicates that NH₄PO₃-based composites are still potential candidate electrolytes for intermediate temperature fuel cells if non-Pt catalysts for NH₄PO₃-based electrolyte were developed.In chapter 4, NH₄PO₃-based proton conductors, 2NH₄PO₃-(NH₄)₂MnP₄O₁₃ and 2NH₄PO₃-(NH₄)₂SiP₄O₁₃, were synthesized and their proton conduction mechanism was investigated. Their conductivities not only increase with temperature, but also increase with water vapor pressure. In any atmosphere, the relation of conductivity of NH₄PO₃-based composites and temperature obeys Arrhenius equation. And at a given temperature, conductivity of NH₄PO₃-based composites is proportional to natual logarithm of partial water pressure. Water molecules were believed to be adsorbed into NH₄PO₃-based composite through chemisorptions and participate in proton conduction by "external H-bond". Then, the conductivity activation energy, Ea, was decreased. Because amount of adsorbed water molecules depended on water partial pressure and amount of "external H-bond" depended on amount of adsorbed water molecules, the amount of"external H-bond" was a function of partial water pressure. As a consequence, the conductivity activation energy was a function of partial water pressure. Both experimental results and theoretical analysis showed that Ea decreased linearly with natural logarithm of partial water pressure. The adsorbed water also contributed to the conductivity by providing extra protons through ionization. Because in proton conductors, the pre-exponential factor of Arrhenius equation was proportional to proton concentration, the pre-exponential factor for NH₄PO₃-based composites was higher in wet atmosphere than that in dry atmosphere and was also a function of partial water pressure. Hence, the conductivity of NH₄PO₃-based cgmposites was increased in wet atmospheres. Our theoretical analysis agreed well with experimental results and gave reasonable explanations for higher conductivity of NH₄PO₃-based composites in wet atmosphere. Finally, a modified Arrhenius equation, which contains water pressure, was obtained (Please refer chapter 4 for details):ln(σT)=ln(k₄[H⁺]₀)+ k₅k₂/[H^]₀²-Ea₀/RT+(k₅k₃/[H⁺]₀²+m/RT)ln(P_H2O/P^φ)where,σis conductivitu, T is temperature with unit K, k₂, k₃, k₄, k₅ and m are constants, [H⁺]₀ is the proton concentration coming from HPO₃ in NH₄PO₃-based composites, Ea₀ is the activation energy when P_H2O equals to p^φ, the atmospheric pressure (101 kPa), R is gas constant, 8.314 J/mol-K.