PhD, University of Cincinnati, 2022, Arts and Sciences: Chemistry

The contents of this dissertation are primarily focused on the evaluation of molecular electrocatalysts and their intrinsic properties during electroreduction reactions such as carbon dioxide (CO2) reduction, hydrodehalogenation, and hydrogen evolution. The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO). This electrocatalyst converted CO2 to CO with a Faradaic efficiency of 99% and a current density of –6.2 mA/cm2 at –2.4 V vs Fc/Fc+ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO2-catalyst adduct in order to facilitate intramolecular proton transfer. This demonstrates that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO2 conversion. The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here we report an electrocatalyst (ZnPEG8T) with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO2 reduction. Using foot-of-the-wave analysis, the ZnPEG8T catalyst displayed CO2-to-CO activity (TOFmax) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with volta (open full item for complete abstract)

Committee: Jianbing Jiang Ph.D. (Committee Member); Ryan White Ph.D. (Committee Member); Hairong Guan Ph.D. (Committee Member) Subjects: Chemistry

Doctor of Philosophy, Case Western Reserve University, 2022, Macromolecular Science and Engineering

The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are critical electrocatalytic reactions for clean and renewable energy technologies, such as fuel cells, metal-air batteries, and water-splitting. Current commercial applications of these reactions utilize noble-metal-based catalysts (e.g., Pt, Pd, RuO2, IrO2). The high cost of these precious metal-based catalysts and their limited reserve have precluded these renewable energy technologies from large-scale applications. Therefore, research efforts have focused on the development of alternative catalysts that are readily available and cost-effective, with superior electrocatalytic performance compared to noble-metal-based catalysts. In 2009, nitrogen-doped carbon nanotubes (N-CNTs) were discovered to demonstrate electrocatalytic ORR activity attributed to the doping-induced charge transfer from carbon atoms adjacent to the nitrogen atoms to change the chemisorption mode of O2. More recent studies have further demonstrated that certain heteroatom-doped carbon nanomaterials can even act as multi-functional metal-free electrocatalysts for ORR/OER/HER, leading to the potential development of low-cost, highly efficient, and multi-functional electrocatalysts for advanced clean and renewable energy technologies. The work presented herein develops new carbon-based metal-free electrocatalysts (C-MFECs) by utilizing different design strategies. Chapter two demonstrates carbonization of a newly-synthesized pair of enantiotopic chiral metal-organic frameworks (MOFs) to produce Co-coordinated N-doped carbon materials with a hierarchical rod-like morphology and remarkable bi-functional electrocatalytic activity and stability for both OER and ORR – comparable to both commercial RuO2 for OER and Pt/C electrocatalysts for ORR. The observed excellent electrocatalytic activities were attributed to their unique hierarchical rod-like structure with homogeneously distributed cob (open full item for complete abstract)

Committee: Liming Dai (Advisor); Gary Wnek (Committee Chair); Lei Zhu (Committee Member); Hatsuo Ishida (Committee Member); Chung-Chiun Liu (Committee Member) Subjects: Chemistry; Materials Science

Doctor of Philosophy (PhD), Ohio University, 2020, Chemical Engineering (Engineering and Technology)

Lignin is one of the main byproducts of pulp and paper industry and biorefineries. Depolymerization of lignin can lead to producing valuable low molecular weight compounds with different functional groups, which are mainly achieved from crude oil sources. Lignin electrolysis could address issues of other lignin depolymerization methods such as complexity, lignin combustion, and low selectivity. On the other hand, lignin electrolysis can occur at significantly lower overpotentials than those required for water electrolysis, which leads to lower-voltage electrolyzer operation and as a result lower energy consumption for hydrogen production. This study includes research and experimental works on developing a continuous electrochemical process for both lignin electrolysis and hydrogen production in an electrolyzer. At the first step of this project, high surface area TiO2 or carbon-supported NiCo electrocatalysts were synthesized and applied for lignin depolymerization at room temperature and pressure. The electrocatalysts were characterized by Brunauer-Emmett-Teller (BET), X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDS) techniques. In addition, a three-electrode rotating disc electrode (RDE) system was used to test the performance and durability of 6 electrocatalysts individually and among them 1:3NiCo/TiO2 was selected as the most effective catalyst for lignin depolymerization. In the second step, a continuous electrochemical cell with 10 cm2 electrodes, separated by an anion exchange membrane (AEM), was applied for lignin electrolysis in the anode and hydrogen generation in the cathode. The effects of temperature, lignin concentration, cell voltage, and electrolysis time on hydrogen production, oxygen evolution, lignin conversion, products with different functional groups, and energy efficiency of the electrochemical reactor were investigated. Although applying high cell voltages increases the rate of electr (open full item for complete abstract)

Committee: John Staser (Advisor) Subjects: Chemical Engineering; Chemistry; Engineering; Environmental Engineering; Wood Sciences

Master of Science, University of Akron, 2018, Polymer Science

Electrochemical water splitting has drawn a lot of attention for renewable energy generation.1 Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are two half-cell reactions for water splitting. In order to drive these two reactions under reasonable overpotential, noble metals electrocatalysts such as platinum (for HER) and ruthenium oxide/iridium oxide (for OER) were used. However, the high price of the noble metals hampered the used electrochemical water splitting in large scale.2 It is therefore urgent to develop efficient electrocatalysts with low cost and earth-abundant materials. Recently, transition metal phosphides (TMPs) have been reported as a promising family of highly active and low-cost electrocatalysts for water splitting.3 In this research, a facile fabrication method was developed to obtain TMPs loaded OER or HER electrode. The electrode scaffold is commercial carbon cloth, on which nickel phosphide (Ni-P) and nickel-iron phosphide (Ni-Fe-P) were coated by electroless deposition. The Ni-Fe-P catalysts exhibited better OER performance than that of Ni-P catalysts. The highly efficient catalyst reached a low OER overpotential of 229 mV under an anodic current density of 10 mA cm-2 in alkaline solutions. The HER performances obtained from Ni-P and Ni-Fe-P based catalysts exhibited similar performance, with the best HER catalysts having a overpotential of 248 mV under a cathodic current density of 100 mA cm-2 in 1 M KOH solutions.

Committee: Yu Zhu (Advisor); Steven Chuang (Committee Member) Subjects: Energy; Polymer Chemistry

Doctor of Philosophy, The Ohio State University, 2016, Chemistry

Layered transition metal dichalcogenides (TMDs), especially molybdenum disulfide (MoS2), have been of great interest for a long time. MoS2 naturally occurs as the mineral molybdenite, which has been involved in diverse research fields, such as electronics, optoelectronics, spintronics, energy storage, lubrication, and catalysis. In MoS2 crystals, a sheet of molybdenum atoms is sandwiched between sheets of sulfur atoms. The covalent Mo-S bonding is strong, but the interaction between the sandwich-like tri-layers is weak van der Waals force, resulting in easy exfoliation of a single layer or a few layers. These MoS2 ultrathin layers (less than 10 atoms thick) belong to the family of two-dimensional (2D) materials. As dimensionality reduced, these MoS2 ultrathin layers have unique charge transport properties, which solve the limitation of conventional Si and other bulk materials as scaling down to microelectronic and nanoelectronic devices. However, the synthesis of orientated single- or few- layer TMDs with large area remains challenging. The first part of this dissertation focuses on the design and controlled synthesis of 2D TMDs and study of their electronic device applications. A facile vapor-solid method was employed to get single crystalline few-layer MoS2 films on (0001)-oriented sapphires with excellent structural and electrical properties over centimeter length scale. A carrier density of ~2.0E11 cm -2 and a room temperature mobility as high as 192 cm2/Vs were extracted from space-charge limited transport regime in the films. In addition, transition metal doped 2D MoS2 films were successfully synthesized by one-step process. These doped films enable the tuning of the properties 2D MoS2 films. By substituting to other substrates or film transferring, 2D/3D heterojunction diodes have also been made with excellent rectification. MoS2 has also been explored as catalysts such as for hydrogen evolution reaction (HER). The edges of MoS2 has been identified a (open full item for complete abstract)

Committee: Yiying Wu Dr (Advisor); Patrick Woodward Dr (Committee Member); Joshua Goldberger Dr (Committee Member) Subjects: Chemistry

Doctor of Philosophy, Case Western Reserve University, 2008, Chemistry

A quantum-chemistry based constrained variation theory and a local reaction center model were applied to analyze the mechanisms for electron transfer reactions on platinum electrode surfaces in fuel cells. Electrode potential-dependent electron transfer activation energies and transition state structures were calculated, as well reversible potentials for reactions of reaction intermediates. In the mechanistic study of hydrogen oxidation and evolution on platinum electrodes in basic electrolyte, the Tafel step, Volmer step and Heyrovsky step were studied. Tafel step: 2H(ads) ⇌ H2(g) Volmer step: H+(aq) + e- ⇌ H(ads), Heyrovsky step: H(ads) + H+(aq) + e- ⇌ H2(ads) The results are consistent with the Tafel-Volmer mechanism for H2 oxidation and a mixture of Tafel-Volmer and Heyrovsky-Volmer mechanism for H2 evolution on platinum surfaces in basic electrolyte. The four one-electron transfer steps were studied for oxygen reduction on platinum in basic electrolyte. The interesting result is that superoxide, O2-(ads), forms as the first reduction intermediate in base rather than peroxyl, OOH(ads), which forms in acid electrolyte. Two- and four-electron O2 reduction mechanisms in acid were explored using 1-fold and 2-fold Pt adsorption site models. Potentials, for O(ads) and OH(ads) reduction steps were related for acid and base by the Nernst equation plus a constant. A systematic study of OH(ads) reduction to H2O in acid electrolyte on platinum and the reverse reaction was carried out in the local reaction center model approach using B3LYP and MP2 calculations employing basis sets with and without diffuse functions. It was found that electrode potential-dependent electron transfer activation energy curves predicted by this method are robust toward shifts caused by changing the external potential. The potential-dependent conversion of OH(ads)⋯H+(aq) + e- ⇌ OH2(ads) on a Pt surface was calculated using a new method combining density functional theory and modified Poisson (open full item for complete abstract)

Committee: Alfred B. Anderson PhD (Advisor); Clemens Burda PhD (Committee Chair); John E. Stuehr PhD (Committee Member); Anthony J. Pearson PhD (Committee Member); Heidi B. Martin PhD (Committee Member) Subjects: Chemistry