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Abstract:
The decarbonisation of the energy sector requires the widespread introduction of zero-emission power sources based on renewable energies. Especially in the heavy-duty sector, proton exchange membrane fuel cells (PEMFCs) powered by green hydrogen could become a core part of a successful transformation to a sustainable infrastructure. However, long lifetimes and a strong performance at high power densities are required under the demanding operating conditions, placing increased importance on the stability and transport behavior of the electrocatalysts.
In this work, the properties of the established hollow graphitic spheres (HGS) are modified individually, ultimately enabling the determination of structure-performance relationships through collaborative efforts. After an initial scale-up of the synthesis, the graphitization of HGS was tailored by varying the annealing temperature. Post-functionalization and the chemical vapor deposition of heteroatom-containing precursors allowed the preparation of O-, N- and S-doped hollow carbon spheres, which are being investigated as support materials beyond the oxygen reduction reaction (ORR). While the preparation of HGS with different shell thickness and particle size was achieved by synthesizing new siliceous hard templates, the modification of the pore size likely requires a change in the preparation strategy.
The evaluation of the prepared support materials and catalysts is ongoing, but Pt/HGS and a similar catalyst based on a fully mesoporous primary particle (Pt/MGS) were used to study transport phenomena occurring under ORR catalysis. The stability evaluation using different half-cell techniques and varying pH and gas-saturation of the electrolyte revealed information about the local reaction environment. The results suggest an increased pH and concentration of dissolved Pt ions in the proximity of pore-confined active sites, questioning the strongly acidic conditions commonly assumed for the catalyst layer of PEMFCs. Further work focused on the visualization of the ionomer distribution by electron microscopy. The obtained information was essential during the investigation of the catalysts in a PEMFC. While Pt/HGS performed poorly in the realistic reaction environment of a single-cell, Pt/MGS surpassed commercial catalysts in activity and performance at high current densities and low humidity. Ultimately, the comparison of Pt/HGS, Pt/MGS, and commercial catalysts resulted in unexpected insights on the optimal catalyst morphology for PEMFC electrocatalysts. A new perspective for the facile preparation of such catalysts in industrially-relevant quantity was provided using resorcinol-formaldehyde gels as precursor material.
Overall, the uniform, tailored carbon supports lead to an improved electrocatalyst, but more importantly enabled further understanding of the influence of the catalyst and catalyst layer structure on catalytic performance in different reaction environments.
