Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Chemistry

Major Professor

John N. Kuhn, Ph.D.

Co-Major Professor

Randy Larsen, Ph.D.

Committee Member

Venkat R. Bhethanabotla, Ph.D.

Committee Member

Brian Space, Ph.D.

Committee Member

Humberto R. Gutierrez, Ph.D.

Keywords

Surface Functionalization, Vacancy Defects

Abstract

Current energy crisis has dramatically shifted the focus of technological advancements towards clean and renewable forms of energy. Continued dependence and utilization of fossil fuels has created global awareness on harmful greenhouse gas emissions and climate change. A need for sustainable technology has gained a lot of significance in the recent years. This has led to the development of devices and technologies that rely on environmentally friendly electrochemical conversion and storage of energy. One such advancement that generates electrical energy from chemical reactions is known as fuel cell technology. While fuel cells have demonstrated potential in replacing the conventional technologies involving combustion of natural gas, concerns arise due to its cost of commercialization and durability. High cost of this technology is primarily due to the use of expensive platinum group metals as electrode materials. Hence, substantial progress in this field relies on developing alternate cheaper and abundantelectrode materials, without compromising the performance. Lately, non-noble metal based catalysts have gained increasing attention as electrode materials in fuel cell reactions. However, sluggish kinetics of electrocatalytic oxygen evolution and reduction reactions (OER/ORR) pose a great challenge towards achieving excellent activity and stability. This dissertation aims towards synthesis of low cost electrode materials to improve performance of oxygen electrocatalysis.

Transition metal-based catalysts have demonstrated widespread potential as alternate electrode materials for oxygen electrocatalysis. A crucial aspect that is identified to affect overall performance is the binding strength of reaction intermediates at the active metal site. Keeping this in mind, the focus of this dissertation is directed towards tuning the surface electronic properties of the active metal site towards enhanced improved kinetics and performance. This is achieved using various strategies like doping, generation of defects and surface functionalization. Finally, insights on forming highly active metal sites during the electrochemical process are studied. Three different catalyst systems used in this work are tungsten oxide, nickel phosphide and cobalt chalcogenides.

Using crystal engineering technique, the intrinsic properties of WO3 are altered via rare-earth doping of La into the oxide matrix. The significance of La addition is shown to influence the morphology of WO3 nanoparticles and the amount of lattice oxygen vacancies. Further, selective exposure of catalytically active crystallographic facet and its growth mechanism elucidated. Introduction of basic sites via La doping drives the ORR performance by favoring the adsorption of surface hydroxyl ions and improves overall catalytic activity. In addition to this, La doped WO3 with lattice oxygen vacancies provide additional binding sites for ORR.

Surface electronic properties of the active metal site also depend on the electronegativity of interacting ligands. Towards this effect, Ni and Co based catalysts have been explored with ligands like nitrogen and phosphorous. The hybrid superstructure, consisting of a conducting polymer, introduces a new synthesis route for PPy analogs of metal phosphides, for applications in OER. In combination with PPy, the electron affinity of metal site is modified to generate synergic M-N sites that are illustrated to be more active towards catalysis. Highly electropositive Ni sites facilitate adsorption of the hydroxyl and thereby, decrease the Tafel slopes. Introduction of conducting polymer also affects the overall charge transfer properties of the catalyst. Presynthesized polymers intercalate amongst the transition metal catalysts, providing a constant conducting matrix. Instead of employing multi-step synthesis for two metal sites, simultaneous synthesis of CoSe2-MoSe2 along with PEDOT is demonstrated to generate anion vacancy defects. These defect sites are demonstrated to significantly alter the hydroxyl binding energy properties.

Metal chalcogenides are a class of non-noble metal catalysts that provide a wide range of stable stoichiometries in distinct crystal structure and tunable properties. This work focusses on elucidating the structural relation of cobalt chalcogenides towards OER, in hexagonal and cubic crystal systems. It is demonstrated that differences in coordination number and bond lengths allow varying degree of stable surface oxidation during OER. Cubic chalcogenides favor the formation of Co3+ by adsorption of hydroxyl ions. In particular, selenides exhibit excellent OER performance as higher metallic character aids electron conductivity in the core of the catalyst. Additionally, effect of varying anions and crystal structure on the catalytic performance is provided in detail. This dissertation demonstrates the ability of these catalysts to lower the overpotential required for oxygen electrocatalysis, by selectively altering material properties. Fundamental insights on tuning the material properties help design better catalysts for OER.

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