Zirconium phosphate has been extensively studied as a proton conductor for proton exchange membrane (PEM) fuel cell applications. Here we report the synthesis of mesoporous, templated sol–gel zirconium phosphate for use in PEM applications in an effort to determine its suitability for use as a surface functionalised, solid acid proton conductor in the future. Mesoporous zirconium phosphates were synthesised using an acid–base pair mechanism with surface areas between 78 and 177 m2 g−1 and controlled pore sizes in the range of 2–4 nm. TEM characterisation confirmed the presence of a wormhole like pore structure. The conductivity of such materials was up to 4.1 × 10−6 S cm−1 at 22 °C and 84% relative humidity (RH), while humidity reduction resulted in a conductivity decrease by more than an order of magnitude. High temperature testing on the samples confirmed their dependence on hydration for proton conduction and low hydroscopic nature. It was concluded that while the conductivity of these materials is low compared to Nafion, they may be a good candidate as a surface functionalised solid acid proton conductor due to their high surface area, porous structure and inherent ability to conduct protons.
Operation of polymer electrolyte membrane fuel cells (PEMFCs) with dry feeds is demonstrated to depend on the gas flow field design, gas pressure, temperature and flow rate. We demonstrate auto-humidified operation of the channel-less, self-draining fuel cell, from 25 to 115ºC. Traditional serpentine flow channel designs cannot sustain autohumidified operation above 60ºC. Auto-humidified (or self humidifying) PEMFC operation is improved in the channel-less fuel cell where axial dispersion enhances "back-mixing" of the product water with the dry feed. The auto-humidified self-draining fuel cell design offers substantial benefits for simplicity of operation and control including: the ability to self drain reducing flooding, the ability to uniformly disperse water removing current gradients and the ability to operate on dry feeds eliminating the need for humidifiers.
Hydrogen fuelled Proton Exchange Membrane Fuel Cells (PEMFCs) are being developed as next generation energy delivery devices to replace conventional combustion technology and enable the development of a hydrogen economy. The attraction of the technology is its ability to operate at higher efficiencies than traditional combustion technology, reducing overall harmful gas and particulate emissions. When hydrogen derived from renewable resources is used as the energy vector, an environmentally benign system is created. Despite the promise held by PEMFC technology, important materials design and system engineering challenges still exist, preventing commercialisation. From a materials perspective, proton exchange membranes capable of operating at or above 140oC need to be developed. This minimum operating temperature is necessary for specific applications in transportation, to reduce catalyst poisoning by carbon monoxide and to increase the system’s ability to dissipate heat. System engineering challenges are due to poor understanding of the water balance in the fuel cell, the lack of design metrics for fuel cell systems and a poor understanding of dynamic operation. This thesis explores each of the aforementioned challenges as follows: • A review of the latest developments in solid acid membranes for PEMFCs operating at 140oC is presented. The most promising areas for future advancements are identified, analysed and discussed and the concept of a thin film, inorganic membrane fuel cell is introduced. • Sol-gel synthesised zirconium phosphate membranes are investigated as an alternative to layered zirconium phosphate proton conductors. The materials show conductivity values similar to α-zirconium phosphate. The advantage of the solgel synthesis route is that the materials can be manufactured as thin film proton conducting membranes. • Sol-gel titanium phosphate membranes are synthesised and their proton conduction mechanism and chemical stability are investigated. The materials show stable conductivity values up to 0.0044 S.cm−1 at 100oC and 100% relative humidity. 31P MAS NMR was used to elucidate the functional groups and develop a model for the Grotthuss proton conduction mechanism. • The STR fuel cell, which can fully self humidify when operating with dry feeds, is characterised and demonstrated and design equations are developed. The STR design cedes only a small performance penalty to fully humidified designs and eases the requirement to develop PEMs capable of operating independent of water in conditions below 130oC. • Dynamic operation of the STR autohumidified fuel cell is investigated. Current ignition and extinction phenomena and multiple steady states are characterised. It is shown that through careful choice of the operating parameters the dynamic response of the autohumidified fuel cell can replicate that of hydrated feed systems. • A spatially resolved current density distribution and impedance analysis of a 20 cm2 autohumidified STR fuel cell system is undertaken. The analysis demonstrates that the STR fuel cell design reduces current density gradients in the fuel cell under reduced relative humidity operation and that the performance of the system is superior to a single channel serpentine system. Through its multi-scale approach, this thesis demonstrates the importance of considering the fuel cell as a system. New thermally and chemically stable fuel cell membranes which are capable of operating at temperatures above the glass transition temperature of current state of the art polymers are needed. Additionally, there is a requirement to operate fuel cells with atmospheric condition feed streams while maintaining the hydration of the membranes. Together, the sol-gel titanium phosphate membranes and autohumidified STR fuel cell developed fill this knowledge gap, leading to a closer realisation of a commercial fuel cell system.
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