Design and optimization of nanoporous carbon for electrochemical applications

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The goal of this research was to design porous carbon materials for application as Pt supports in proton exchange membrane fuel cells (PEMFCs) and to understand how the porosity of these Pt/C catalysts influences their activity towards the oxygen reduction reaction (ORR). Through the use of these novel carbon supports, another goal of this work was to improve the Pt nanoparticle utilization vs. commercial carbon supports. Additionally, the supercapacitive properties of some of these porous carbons were also studied. Initial investigations ( experimental work and modelling) of a catalyst layer prepared from commercial Pt/C catalysts confirmed that changes in the catalyst layer thickness (average pore length), conductivity, exchange current density, and average interparticle pore radius can all influence the observed ORR Tafel slope due to ohmic losses through the layer. To gain more control over these four parameters, two types of porous carbons were synthesized and evaluated: colloid imprinted carbons (CICs) and ordered mesoporous carbons (OM Cs). After loading the CICs (having pore diameters of 15, 26, 50, and 80 nm) with Pt, it was determined that the carbon wall thickness has a larger influence on ORR activity than does the pore diameter. This was supported by modelling, which demonstrated that the resistance of the CIC walls must be included in the model for the simulated and experimental ORR Tafel slopes to overlap. Pt-loaded OMCs, all having the same bimodal pore size distribution of ea. 1.8 and 3.5 nm, but different carbon wall (i.e., 'nano-string') diameters were also investigated, showing a precipitous drop in ORR activity for nanostring diameters < 3 nm, due to their high ohmic resistance. A direct comparison between the ORR activity of the most promising Pt/CIC and Pt/OMC catalysts, as well as a conventional Pt/Vulcan carbon catalyst, proved that Pt/CICs are the most promising materials for PEMFC cathodes. This is because CICs have thicker and more crystalline walls, as well as larger pore diameters, vs. OMCs. The CICs were also found to have larger surface areas and pore diameters vs. conventional Vulcan carbon (VC), allowing for more uniform Pt deposition and enhanced Pt utilization. As a further advantage of CICs vs. both OMC and VC, it was shown that the pore depth of CICs can be easily tuned, allowing for control over the depth to which Pt is deposited. This is anticipated to greatly reduce any mass transport limitations that may be encountered in a membrane electrode assembly (MEA). While the OMC supports were not found to be suitable as PEMFC catalyst supports, their extremely high surface areas (> 1000 m2/g) make them ideal as supercapacitor materials. It was shown that OMCs prepared using sucrose as a precursor have a high density of electroactive surface functional groups, resulting in a large ( ~ 0.15 F/m2) specific capacitance. The combination of a high surface area and high specific capacitance resulted in OMC-16 having the largest as yet reported gravimetric capacitance (260 F/g) for a templated carbon material.
Bibliography: p. 196-212
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Banham, D. W. (2012). Design and optimization of nanoporous carbon for electrochemical applications (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/4714