Developing electrocatalysts (precious and non-precious) for PEM fuel cells applying metal organic frameworks
|Abstract:||Fuel cells have great potential for use as alternative energy conversion devices for a wide variety of applications. Proton exchange membrane fuel cells (PEMFCs) are considered to be potential replacements for internal combustion engines in automobiles, owing to their reduced emissions and better efficiency. A platinum (Pt)-based catalyst is required to facilitate both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) which occur at the anode and cathode of PEMFCs, respectively. The ORR kinetic is inherently very sluggish and is considered the limiting factor facing the performance of PEMFCs. In order to generate power at an acceptable rate for real world applications, a significant amount of Pt catalyst is required. This is traditionally in the form of Pt nanoparticles evenly distributed on a porous carbon support material (Pt/C). Pt is an extremely expensive noble metal with very limited natural abundance. Thus, large-scale commercialization of PEMFCs requires significant advances in catalyst development in order both to reduce the amount of Pt metal and to enhance catalyst durability. In this research work, we employed Metal-Organic Frameworks (MOFs) as a sole precursor for preparing PEMFC electrocatalysts. Owing to their crystalline, porous, hybrid structure, these materials have potential to be applied as PEMFCs electrocatalyst precursor. The clearly-defined three-dimensional structure of these materials can produce a high density of metal active sites evenly distributed through their regularly arranged structure. They can therefore enhance catalyst utilization. The organic linkers of the MOF-based precursor would be converted to carbon during thermal activation while maintaining the porous framework, leading to catalysts with high surface area and uniformly distributed active sites without the need for a carbon support. Pt and Fe containing MOF precursors were synthesized and used as the sole precursor to develop both Pt and non-precious (Fe)-based electrocatalysts for PEMFCs. A Pt-based electrocatalyst was the first reported on implementation of precious metal containing MOFs for developing PEMFC electrocatalyst. The Pt-based electrocatalyst derived from this Pt-containing MOF precursor demonstrated catalytic performance comparable to the commercially available Pt/C especially for HOR at the anode side. To prepare a non-precious electrocatalyst, Fe containing MOF belonging to a different class of MOF materials other than ZIFs was synthesized and used as the sole electrocatalyst precursor. This was the first report on using non-ZIF MOF precursor for ORR electrocatalyst development. This Fe-based electrocatalyst revealed promising ORR activity and PEM fuel cell performance when applied at the cathodic catalytic layer of the corresponding membrane electrode assembly (MEA). In addition, the effect of catalyst ink composition prepared from the MOF derived Pt-based electrocatalyst, in terms of Nafion ionomer content, on the overall performance of PEMFC was investigated via a macroscopic CFD model. The trend predicted from the model calculations was then surveyed experimentally in search for the optimum Nafion ionomer content. Furthermore, the products of thermal transformation of Pt-based MOF into carbon-black based electrocatalyst were studied using a.c. impedance spectroscopy. Along with the electrocatalyst precursor, thermolysis products of parent MOF-253 (Al-containing) were considered in these studies. The materials subjected to thermolysis at increasing temperatures were found to pass through different conduction states starting from insulator and ending up with a particular metal-like conductance with positive temperature dependence and high ambient conductivity.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||23 April 2018|
|Collection:||Thèses et mémoires|
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