THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Wednesday, July 31, 2019

1:30 PM

in Marcus Nanotechnology Building (Room 1116)

will be held the

DISSERTATION DEFENSE

for

Ali Ahmed Abdelhafiz Mahmoud

“Enhancing Platinum-based Nanoarchitectures Electrocatalytic Activity and Durability for Oxygen Reduction Reaction at PEMFC: Investigating the Dual Role of Graphene as a Catalyst Support and a Protective Cap”

Committee Members:

Prof. Meilin Liu, Advisor, MSE

Prof. Faisal Alamgir, Co-Advisor, MSE

Prof. Preet Singh, MSE

Prof. Sundaresan Jayaraman, MSE

Prof. Mostafa El-Sayed, CHEM

Abstract:

Energy demand-supply relationship is a big concern with world’s consumption increased over 65% through the past two decades. Moreover, carbon dioxide emissions increased with an associated serious Global Warming effect. Therefore, a dire need for a renewable and green energy source captured a huge interest in the scientific community over the past three decades. Fuel cell technology arose as a prominent candidate where chemical energy is converted into electricity. Polymer Electrolyte Membrane Fuel Cell (PEMFC) is a class of fuel cell powered by hydrogen gas as a fuel, which generates zero carbonaceous emissions (i.e. produce H2O). PEMFC operates at lower temperature (typically 80-200 °C) compared to other fuel cell categories, which enables them to be used for mobile applications (e.g. vehicles or electronic devices). One of the major challenges in PEMFC is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode side. State of the art catalyst for ORR in PEMFC is based on a precious metal (i.e. Pt). PEMFC commercialization is suppressed due to high cost and short lifetime of Pt catalyst component (i.e. 40% of PEMFC cost is due to Pt catalyst with operation limits well below 100 hours). Thus, producing a cost-effective, ORR catalytically active and highly durable catalyst is crucial for PEMFC technology spread.

In the presented thesis herein, atomic scale hybrid catalyst architectures are synthesized. Catalyst architectures are composed of two distinctive components: catalyst material and catalyst support. Electrocatalysis exclusively occurs at the outmost atomic layer of the catalyst material. Therefore, synthesis of Pt-rich surface catalysts is believed to enhance Pt mass activity (i.e. activity per Pt loading). Herein, 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 dictates Pt growth which generates compressive strain induced 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 to Pt surface.

Graphene 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. graphene-Pt intimacy is thoughtful to generate an electronic coupling where a hybrid catalyst forms, on which ORR occurs without an activity loss. Moreover, graphene capping helps retaining full activity of Pt-catalyst beyond 5K testing cycles. Thickness of graphene capping layers shows influence on ORR intermediate accessibility to Pt underneath. Triple layer graphene thick enhances catalyst lift-time retaining 74% of ECSA after 20K testing cycles. Triple layer graphene thick marks the threshold to observe ORR activity when masking Pt adatoms. Thicker graphene (i.e. 5 layers thick), however enhances catalyst durability beyond 30K cycles, blocks reaction intermediates accessibility to Pt.