THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Monday, November 8, 2021

12:00 PM

in MoSE G021

and via

BlueJeans Video Conferencing

https://bluejeans.com/8695020379/

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Christopher Sewell

"Investigation into Strategies for Enhanced Electrocatalytic Activities and Stabilities of Spinel-Based Transition Metal Oxide Nanoparticles"

Committee Members:

Prof. Zhiqun Lin, Advisor, MSE

Prof. Seung Soon Jang, MSE

Prof. Meilin Liu, MSE

Prof. Vladimir Tsukruk, MSE

Prof. Angus Wilkinson, CHEM/MSE

Abstract:

As the severity of global climate issues continues to build, the need for clean energy storage and conversion devices has become increasingly pressing. The production of green hydrogen through water electrolysis is a promising route to alleviating these challenges. However, the high cost and scarcity of the state-of-the-art noble metal-based electrocatalysts utilized in such processes represents one of the critical hurdles to be overcome prior to their practical implementation. A promising direction is to utilize transition metal-based nanoparticles (NPs), which offer superior electrocatalytic performance over their bulk counterparts. The goal of my research is to systematically investigate strategies to improve the electrocatalytic performance and stability of transition metal-oxide (TMO) NPs.

Capping the surface of NPs with polymers is widely recognized as an effective means towards their dispersion and stabilization. However, it is often circumvented due to its tendency to lower the electrocatalytic activity of the ligated NPs. In this context, I will present the first systematic investigation into the impact of the chain density and hydrophilicity of the surface-capping polymers, which can be judiciously regulated, on the oxygen evolution reaction (OER) activity. By capitalizing on star-like diblock copolymers as nanoreactors, spinel CoFe2O4 (CFO) NPs permanently ligated with polymers of interest (i.e., varied chain density and characteristic) are crafted. The correlation between the chain density and hydrophilicity of surface-capping polymers and the OER activity of CFO NPs are scrutinized. Intriguingly, decreasing the number of surface-capping chains and increasing the chain hydrophilicity result in significantly decreased overpotential, caused by an increased exposure of the active material (CFO) to the electrolyte and reduced diffusion resistance. This study provides insight into the strategies for mitigating the activity-limiting properties of surface polymers and tailoring the electrocatalytic properties of polymer-ligated NPs.

Recently, the use of externally applied magnetic fields has garnered significant attention as a promising strategy to enhance OER electrocatalytic performance. OER exhibits spin-dependent kinetics, producing triplet O2 from singlet reactants (OH-, H2O). Notably, magnetization can reduce this kinetic barrier by aligning the spin ordering of ferromagnetic (FM) electrocatalysts. Unfortunately, some of the most active OER catalysts, namely transition metal oxyhydroxides, are paramagnetic (PM). This can be circumvented by utilizing a spin pinning effect in FM/PM core/shell materials, which has already been successfully demonstrated in a bulk CFO/CoFeOxHy system. In this work, I aim to build upon previous research by examining a similar system at the nanoscale. By utilizing the star-like diblock copolymer nanoreactors, several sets of CFO NPs of varying sizes and tailorable degrees of surface reconstruction will be crafted, thus enabling a systematic study on the effects of NP size and core-to-shell ratio on the magnetic field-rendered OER enhancement.

In addition to externally applied magnetic fields, other external forces can be introduced during electrocatalysis to improve performance. Previous research has found that some spinel NPs, NiFe2O4 (NFO) for example, experience a photothermal effect upon near-infrared light irradiation which promotes the dynamic generation of active OER sites. Thus, in this phase of my research, both the magnetic field-based enhancement and photothermal effect will be collectively exploited to further improve the OER electrocatalytic ability of NFO NPs.