ChE PhD Dissertation Defense: Kevin Yang

Name:
Kevin Yang

Title:
Structural Investigation of Single-Atom Catalysts in HCl Electrolysis, CO₂ Reduction, and Li-S Batteries

Date:
05/08/2026

Time:
01:00:00 PM

Committee Members:
Prof. Sanjeev Mukerjee (Advisor)
Prof. Joshua Gallaway
Prof. Hannah Sayre
Prof. Magda Barecka

Location:
EXP 202

Abstract:
Transition metal single-atom catalysts have emerged as a promising class of materials for electrochemical energy conversion and storage due to their high atomic utilization, tunable electronic structure, well-defined active sites, and use to high earth abundant metals. Metal nitrogen carbon (M-N-C) catalysts are practical in a wide range of electrochemical systems. However, the development of M-N-C catalysts into industrial systems still requires much effort, in part, due to the lack of durability and stability studies. M-N-C catalysts can be used in oxygen depolarized cathode (ODC) HCl electrolysis, CO₂ reduction, and lithium sulfur (Li-S) batteries. Understanding the mechanisms of M-N-C degradation, durability, and effects of modifications to such catalysts would ultimately benefit their implementation in each electrochemical system.   Chapter 1 introduces M-N-C catalysts and the various electrochemical systems they will be used in (ODC HCl electrolysis, CO₂ reduction, and Li-S batteries).

In chapter two, we investigate the durability of the Fe-N-C catalyst in ODC HCl electrolysis. Fe-N-C exhibits high oxygen reduction activity and strong resistance to chloride poisoning relative to most noble metal catalysts. However, its durability and degradation mechanisms in HCl electrolysis are not well studied. Through a combination of durability studies, accelerated stress tests, and multimodal spectroscopic techniques, we identified two main degradation pathways: an operational demetallation of Fe-N₄ active sites under sustained polarization and a carbon-corrosion-induced demetallation that occurs during the transient conditions of uncontrolled shutdown. Spectroscopic analysis reveals one unstable FeN₄ moiety and two stable Fe-N-C moieties that can withstand the HCl electrolysis operating conditions. These findings establish a mechanism for Fe-N-C degradation to drive future catalyst design.

In chapter three, we modify Fe-N-C catalyst and Ni-N-C catalyst with heteroatom dopants to observe their effects on CO₂ reduction activity, product selectivity, and the correlation with the changes in electronic and coordination structure. Using a post-pyrolysis treatment process, we dope the environment around the metal active center either by introducing an axial ligand or binding to the carbon structure. Utilizing in situ/operando XAS, we found that the dopants are generally not stable and can introduce a site-blocking effect at low overpotentials. Through these findings, we find that the axial ligand dopants are not durable during CO₂ reduction and do not make large contributions to the activity or product distribution of Ni-N-C and Fe-N-C in CO₂ reduction.

In chapter four, we investigate the effects of metal centers for M-N-C in polysulfide conversion and the changes in the active site structure after operation. The catalytic activity of M-N-C catalysts varies largely with different metal centers and coordinating environments.  We find that Co-N-C and Fe-N-C favor the oxidation of short-chain polysulfides to elemental sulfur, while Sn-N-C and Ni-N-C make a larger contribution to the reduction of elemental sulfur to short-chain polysulfides. Mo-N-C, which had the presence of Mo nanoparticles, exhibited the lowest increase in performance compared to the others. This finding emphasizes the catalytic capability and importance of synthesizing purer M-N-C catalysts. All M-N-C catalysts were able to impact the conversion of lithium polysulfides to gain performance greater than baseline carbon. X-ray spectroscopic methods were used to analyze the structure of the M-N-C catalyst at various cycles to find that the active site structure of Fe-N-C undergoes a partial change to form Fe₂O₃, while Co-N-C and Ni-N-C remain relatively stable. This change in the active site could be a cause of capacity decay in Li-S batteries.

Chapter 5 summarizes the findings and offers suggestions for future work.

Kevin Yang is a Ph.D. candidate in Chemical Engineering at Northeastern University, where his research focuses on understanding structure–property relationships in single-atom catalysts for electrochemical energy conversion and storage. His work spans multiple electrochemical systems, including oxygen depolarized cathodes for hydrochloric acid electrolysis, CO₂ electroreduction, and lithium–sulfur batteries, with an overarching emphasis on how catalyst active sites evolve under operating conditions and how those structural changes govern activity, selectivity, and durability. Kevin’s research combines electrochemical engineering with advanced multimodal characterization, including in situ and ex situ X-ray absorption spectroscopy (XANES/EXAFS), X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy, and electrochemical diagnostics. Through this work, he has developed mechanistic insights into active site degradation pathways in Fe–N–C and other transition metal–nitrogen–carbon single-atom catalysts, helping bridge fundamental catalyst chemistry with practical reactor operation.