(Georgia Institute of Technology, 2020-01-03)
Mahmoud, Ali Ahmed Abdelhafiz
Energy demand-supply relationship is a big concern with world’s consumption increased over 65% through the past two decades. Polymer Electrolyte Membrane Fuel Cell (PEMFC) possess high energy density with zero carbonaceous emissions. One of the major challenges in PEMFC is concerned with the oxygen reduction reaction (ORR) at the cathode side, where low mass activity and poor stability are key challenges to overcome. Catalyst architectures are composed of two distinctive components: catalyst material and catalyst support. Herein, our objectives are enhancing catalyst stability, catalyst-support interaction, and maximizing catalyst active surface area. In the presented thesis, strain-tuned atomically precise hybrid catalyst architectures are synthesized. Mixed single atomically dispersed up-to a few thick atomic layers Pt catalyst architecture are synthesized using electrochemical layer by layer synthesis or magnetron DC-sputtering techniques. Graphene, due to its mechanical and chemical stability, is used to demonstrating its role as a platform for Pt growth (i.e. catalyst support) and as a protective cap.
Graphene/Pt vicinity dictates Pt adatoms structure. Templated Pt adatoms growth is dictated by graphene epitaxy. TEM analysis shows ripening-free of low-dimensional Pt adatom deposited on graphene, where Pt is fully wetting graphene at ~1 nm thick layer over 1 cm2 area. Pt/Graphene epitaxy induces compressive strain on Pt-Pt bond distance up to 4%. Compressive strain enhances ORR activity due to down-shifting the d-band center of Pt adatoms, weakens reactions intermediates adsorption at Pt surface. Templated Pt adatoms grown on graphene shows higher stability due to covalent chemical bond formation between Pt adatoms and underlying graphene substrate. Single layer graphene utilized as a protective cap shows chemical transparency to ORR electroactivity and suppresses catalytic deactivation, wherein graphene does not restrict the access of the reactants but does block Pt from dissolution or agglomeration. Single layer graphene cap enables survival of Pt active surface area by almost zero-loss after 1K testing cycles with on observable mass activity loss within marginal error bars. Sandwiching Pt adatoms between two sets of multi-layered graphene sheets boosts ORR activity and catalyst stability. Thickness of graphene cap influences ORR activity, 3-layers graphene thick marks the threshold to observe ORR activity, where 5-layers graphene shows blockage or ORR activity. Three-layers graphene/Pt Sandwich retains 460% of ECSA after 15K testing cycles and 975% higher mass activity, compared to state-of-the-art commercial Pt/carbon catalyst. Ex-situ Raman analysis suggests catalyst degradation to occur when sufficient point defects are generated within graphene structure, through which Pt single atoms hop through towards the surface. DFT based calculations suggests that dissolution of trapped Pt adatoms at defect-center is activated by Pt clustering right underneath the defect.
(Georgia Institute of Technology, 2015-11-16)
Lai, Samson Yuxiu
Solid oxide fuel cells (SOFCs) represent a major piece of a next-generation, renewable, clean energy economy and contribute to combating anthropogenic climate change by efficiently converting chemical energy into electrical energy through electrochemical reactions. However, despite adding significant chemical, mechanical, and microstructural complexity to push SOFC performance ever higher, cost and durability remain significant barriers to SOFC commercialization. Two of these issues are cathode stability in atmospheres containing carbon dioxide and water vapor and anode stability in fuel containing hydrogen sulfide. With regards to those aspects, state-of-the-art SOFC cathodes (La1-xSrxMnO3-δ and La1-xSrxCo1-yFeyO3-δ) and anodes (NiO and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ) are studied to understand the interactions between contaminant and electrode.
In this work, powerful in situ and operando x-ray spectroscopy and scattering experiments provide deep insight into the physiochemical phenomena that define the behavior of SOFC electrode materials. These studies demonstrate that proper combination of in situ and operando experiments, due partially to the powerful intensity and capabilities of synchrotron x-rays, can provide unique information that has never before been possible and is critical to gaining new perspectives and to better understand data where a single perspective may only lead to ambiguous conclusions. Such a multi-pronged characterization approach is vital to gaining a better understanding of complex SOFC materials and providing critical insights for rational design of next-generation SOFC electrode materials.