Electrochemical Insights into Platinum Catalysts for Fuel Cells

Research output: Book/ReportPh.D. thesis – Annual report year: 2017Research

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Development of sustainable energy production, conversion and storage technologies must be considered one of the major challenges of the 21st century. Insight and understanding of the oxygen reduction reaction is imperative in these pursuits. In this work electrochemical investigations and physical characterization of various model systems ranging from extended surfaces, to thin films and nanoparticle electrocatalysts have been presented and discussed. This have been done with a special focus on governing factors controlling the electroreduction of oxygen.
Preparation of Cu/Pt(111) near-surface alloys was conducted and compared to earlier results from our group. In lieu of the Cu amount in the 2nd atom layer the OH adsorption energy could be tuned. This was done for a range of Cu/Pt(111) samples which were investigated in alkaline media, revealing a Sabatier volcano relationship to the relative shift in OH adsorption potential. This work  demonstrates, for the first time, that the OH binding energy indeed is a descriptor in alkaline as well as in acidic media. The apparent synergistic effects between the alkali cations and the electrodesurface of the Cu/Pt(111) combined with the optimization of the OH binding energy, resulted in extremely high specific oxygen reduction activities. The maximum ORR activity was recorded to be 100.7 ±7.5mA/cm2 at 0.9V vs. the reversible hydrogen electrode.
The Cu/Pt(111) system was also used to investigate the oxygen reduction reaction in the presence of poisoning anions from the electrolyte. These experiments revealed that catalyst with optimum activity in non-adsorbate-adsorbate interacting electrolytes, such as HClO4, also resulted in catalytic surfaces with superior tolerances for phosphate. Suggesting scaling between OH and phosphate adsorption energies.
Results on Gd/Pt(111) samples revealed that compressive strain from Pt overlayer formation is of major importance for the observed oxygen reduction activity enhancement. In-situ GI-XRD studies revealed that the overlayer forms almost instantaneous once the electrode is immersed into the acidic electrolyte. Furthermore, the overlayer appeared to be very stable after accelerated stability test. 
Pt and Pt-Gd thin film investigation was also conducted. Here X-ray characterization played a central role in ascertaining that oxygen incorporation into the alloy, due to the oxyphilic nature of Gd, was kept at a minimum. Stable Pt and Pt5Gd thin films were produced and electrochemical characterization experiments revealed specific activities of 0.9V vs. RHE of 9.0 ± 0.6mA/cm2 and stability retention of 83 %. Both of these metrics were comparable to those reported for bulk polycrystalline Pt5Gd samples in HClO4 electrolyte.
A preliminary electrochemical study of in-house synthesized Pt-Y nanoparticles have also been presented revealing specific mass actives of 0.3 ± 0.1A/mgPt in HClO4. The study revealed that extensive optimizations of the Pt-Y nanoparticles are required and their performance is severely impeded by poor electrochemically active areas and maybe also non-conformity of their crystal phase structure.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages254
Publication statusPublished - 2017

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