Matching Items (6)

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Electrocatalytic and photoelectrosynthetic hydrogen production using metalloporphyrins and molecular-modified gallium phosphide photocathodes

Description

Metalloporphyrins represent a class of molecular electrocatalysts for driving energy relevant half-reactions, including hydrogen evolution and carbon dioxide reduction. As electrocatalysts, they provide a strategy, and potential structural component, for

Metalloporphyrins represent a class of molecular electrocatalysts for driving energy relevant half-reactions, including hydrogen evolution and carbon dioxide reduction. As electrocatalysts, they provide a strategy, and potential structural component, for linking renewable energy sources with the production of fuels and other value-added chemicals. In this work, porphyrins are used as structural motifs for exploring structure-function relationships in electrocatalysis and as molecular building blocks for assembling photoelectrochemical assemblies leveraging the light capture and conversion properties of a gallium phosphide (GaP) semiconductor. These concepts are further covered in Chapter 1. A direct one-step method to chemically graft metalloporphyrins to GaP surfaces is described in Chapter 2. Structural characterization of the hybrid assemblies is achieved using surface-sensitive spectroscopic methods, and functional performance for photoinduced hydrogen production is demonstrated via three-electrode electrochemical measurement combined with product analysis using gas chromatography. In Chapter 3, preparation of a novel cobalt porphyrin modified with 3-fluorophenyl groups at all four meso-positions of the porphyrin ring and a single 4-vinylphenyl surface attachment group at one of the β-positions is described. Electrochemical measurements show the 3-fluorophenyl groups perturb the reduction potentials of the complex to more positive values as compared to non-fluorinated analogs, illustrating synthetic control over the redox properties of the catalysts. The use of grazing angle attenuated total reflectance Fourier transform infrared spectroscopy to characterize chemically modified GaP surfaces containing grafted cobalt fluoro-porphyrins is presented in Chapter 4. In these hybrid constructs, porphyrin surface attachment is achieved using either a two-step method involving coordination of cobalt fluoro-porphyrin metal centers to nitrogen sites on an initially applied thin-film polypyridyl surface coating, or via a direct modification strategy using a cobalt fluoro-porphyrin precursor bearing a covalently bonded 4- vinylphenyl surface attachment group. Finally, Chapter 5 describes binuclear copper porphyrins in which two copper porphyrin macrocycles are doubly fused at the meso-β positions are shown to be active electrocatalysts for the hydrogen evolution reaction. The enhancement in catalytic performance over analogous non-fused copper porphyrins indicates extended macrocycles provide an advantageous structural motif and design element for preparing electrocatalysts that activate small molecules of consequence to renewable energy.

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Date Created
  • 2019

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Hybrid Materials and Interfaces for Artificial Photosynthetic Assemblies

Description

Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of

Chemical modification of (semi)conducting surfaces with soft-material coatings containing electrocatalysts provides a strategy for developing integrated constructs that capture, convert, and store solar energy as fuels. However, a lack of effective strategies for interfacing electrocatalysts with solid-state materials, and an incomplete understanding of performance limiting factors, inhibit further development. In this work, chemical modification of a nanostructured transparent conductive oxide, and the III-V semiconductor, gallium phosphide, is achieved by applying a thin-film polymer coating containing appropriate functional groups to direct, template, and assemble molecular cobalt catalysts for activating fuel-forming reactions. The heterogeneous-homogeneous conducting assemblies enable comparisons of the structural and electrochemical properties of these materials with their homogeneous electrocatalytic counterparts. For these hybrid constructs, rational design of the local soft-material environment yields a nearly one-volt span in the redox chemistry of the cobalt metal centers. Further, assessment of the interplay between light absorption, charge transfer, and catalytic activity in studies involving molecular-catalyst-modified semiconductors affords models to describe the rates of photoelectrosynthetic fuel production as a function of the steady-state concentration of catalysts present in their activated form. These models provide a conceptual framework for extracting kinetic and thermodynamic benchmarking parameters. Finally, investigation of molecular ‘proton wires’ inspired by the Tyrosine Z-Histidine 190 redox pair in Photosystem II, provides insight into fundamental principles governing proton-coupled electron transfer, a process essential to all fuel-forming reactions relevant to solar fuel generation.

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Date Created
  • 2020

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Pt/Pt alloy and manganese dioxides based oxygen reduction reaction catalysts for low-temperature fuel cells

Description

The fuel cell is a promising device that converts the chemical energy directly into the electrical energy without combustion process. However, the slow reaction rate of the oxygen reduction reaction

The fuel cell is a promising device that converts the chemical energy directly into the electrical energy without combustion process. However, the slow reaction rate of the oxygen reduction reaction (ORR) necessitates the development of cathode catalysts for low-temperature fuel cells. After a thorough literature review in Chapter 1, the thesis is divided into three parts as given below in Chapters 2-4.

Chapter 2 describes the study on the Pt and Pt-Me (Me: Co, Ni) alloy nanoparticles supported on the pyrolyzed zeolitic imidazolate framework (ZIF) towards ORR. The Co-ZIF and NiCo-ZIF were synthesized by the solvothermal method and then mixed with Pt precursor. After pyrolysis and acid leaching, the PtCo/NC and PtNiCo/NC were evaluated in proton exchange membrane fuel cells (PEMFC). The peak power density exhibited > 10% and 15% for PtCo/NC and PtNiCo/NC, respectively, compared to that with commercial Pt/C catalyst under identical test conditions.

Chapter 3 is the investigation of the oxygen vacancy (OV) effect in a-MnO2 as a cathode catalyst for alkaline membrane fuel cells (AMFC). The a-MnO2 nanorods were synthesized by hydrothermal method and heated at 300, 400 and 500 ℃ in the air to introduce the OV. The 400 ℃ treated material showed the best ORR performance among all other samples due to more OV in pure a-MnO2 phase. The optimized AMFC electrode showed ~ 45 mW.cm-2, which was slightly lower than that with commercial Pt/C (~60 mW.cm-2).

Chapter 4 is the density functional theory (DFT) study of the protonation effect and active sites towards ORR on a-MnO2 (211) plane. The theoretically optimized oxygen adsorption and hydroxyl ion desorption energies were ~ 1.55-1.95 eV and ~ 0.98-1.45 eV, respectively, by Nørskov et al.’s calculations. All the configurations showed oxygen adsorption and hydroxyl ion desorption energies were ranging from 0.27 to 1.76 eV and 1.59 to 15.0 eV, respectively. The site which was close to two Mn ions showed the best oxygen adsorption and hydroxyl ion desorption energies improvement with the surface protonation.

Based on the results given in Chapters 1-4, the major findings are summarized in Chapter 5.

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Date Created
  • 2019

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The investigation and characterization of redox enzymes using protein film electrochemistry

Description

Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface

Redox reactions are crucial to energy transduction in biology. Protein film electrochemistry (PFE) is a technique for studying redox proteins in which the protein is immobilized at an electrode surface so as to allow direct exchange of electrons. Establishing a direct electronic connection eliminates the need for redox­active mediators, thus allowing for interrogation of the redox protein of interest. PFE has proven a versatile tool that has been used to elucidate the properties of many technologically relevant redox proteins including hydrogenases, laccases, and glucose oxidase.

This dissertation is comprised of two parts: extension of PFE to a novel electrode material and application of PFE to the investigation of a new type of hydrogenase. In the first part, mesoporous antimony-doped tin oxide (ATO) is employed for the first time as an electrode material for protein film electrochemistry. Taking advantage of the excellent optical transparency of ATO, spectroelectrochemistry of cytochrome c is demonstrated. The electrochemical and spectroscopic properties of the protein are analogous to those measured for the native protein in solution, and the immobilized protein is stable for weeks at high loadings. In the second part, PFE is used to characterize the catalytic properties of the soluble hydrogenase I from Pyrococcus furiosus (PfSHI). Since this protein is highly thermostable, the temperature dependence of catalytic properties was investigated. I show that the preference of the enzyme for reduction of protons (as opposed to oxidation of hydrogen) and the reactions with oxygen are highly dependent on temperature, and the enzyme is tolerant to oxygen during both oxidative and reductive catalysis.

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Date Created
  • 2014

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The investigation and characterization of the group 3 [NiFe]-Hydrogenases using protein film electrochemistry

Description

Hydrogenases, the enzymes that reversibly convert protons and electrons to hydrogen, are used in all three domains of life. [NiFe]-hydrogenases are considered best suited for biotechnological applications because of their

Hydrogenases, the enzymes that reversibly convert protons and electrons to hydrogen, are used in all three domains of life. [NiFe]-hydrogenases are considered best suited for biotechnological applications because of their reversible inactivation with oxygen. Phylogenetically, there are four groups of [NiFe]-hydrogenases. The best characterized group, "uptake" hydrogenases, are membrane-bound and catalyze hydrogen oxidation in vivo. In contrast, the group 3 [NiFe]-hydrogenases are heteromultimeric, bifunctional enzymes that fulfill various cellular roles. In this dissertation, protein film electrochemistry (PFE) is used to characterize the catalytic properties of two group 3 [NiFe]-hydrogenases: HoxEFUYH from Synechocystsis sp. PCC 6803 and SHI from Pyrococcus furiosus. First, HoxEFUYH is shown to be biased towards hydrogen production. Upon exposure to oxygen, HoxEFUYH inactivates to two states, both of which can be reactivated on the timescale of seconds. Second, we show that PfSHI is the first example of an oxygen tolerant [NiFe]-hydrogenase that produces two inactive states upon exposure to oxygen. Both inactive states are analogous to those characterized for HoxEFUYH, but oxygen exposed PfSHI produces a greater fraction that reactivates at high potentials, enabling hydrogen oxidation in the presence of oxygen. Third, it is shown that removing the NAD(P)-reducing subunits from PfSHI leads to a decrease in bias towards hydrogen oxidation and renders the enzyme oxygen sensitive. Both traits are likely due to impaired intramolecular electron transfer. Mechanistic hypotheseses for these functional differences are considered.

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Date Created
  • 2012

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Structural and photoelectrochemical characterization of gallium phosphide semiconductors modified with molecular cobalt catalysts

Description

The molecular modification of semiconductors has applications in energy

conversion and storage, including artificial photosynthesis. In nature, the active sites of

enzymes are typically earth-abundant metal centers and the protein provides a

The molecular modification of semiconductors has applications in energy

conversion and storage, including artificial photosynthesis. In nature, the active sites of

enzymes are typically earth-abundant metal centers and the protein provides a unique

three-dimensional environment for effecting catalytic transformations. Inspired by this

biological architecture, a synthetic methodology using surface-grafted polymers with

discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional

environments is presented. The use of polymeric coatings to interface cobalt-containing

catalysts with semiconductors for solar fuel production is introduced in

Chapter 1. The following three chapters demonstrate the versatility of this modular

approach to interface cobalt-containing catalysts with semiconductors for solar fuel

production. The catalyst-containing coatings are characterized through a suite of

spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection

Fourier transform infrared spectroscopy (GATR-FTIR) and x-ray photoelectron (XP)

spectroscopy. It is demonstrated that the polymeric interface can be varied to control the

surface chemistry and photoelectrochemical response of gallium phosphide (GaP) (100)

electrodes by using thin-film coatings comprising surface-immobilized pyridyl or

imidazole ligands to coordinate cobaloximes, known catalysts for hydrogen evolution.

The polymer grafting chemistry and subsequent cobaloxime attachment is applicable to

both the (111)A and (111)B crystal face of the gallium phosphide (GaP) semiconductor,

providing insights into the surface connectivity of the hard/soft matter interface and

demonstrating the applicability of the UV-induced immobilization of vinyl monomers to

a range of GaP crystal indices. Finally, thin-film polypyridine surface coatings provide a

molecular interface to assemble cobalt porphyrin catalysts for hydrogen evolution onto

GaP. In all constructs, photoelectrochemical measurements confirm the hybrid

photocathode uses solar energy to power reductive fuel-forming transformations in

aqueous solutions without the use of organic acids, sacrificial chemical reductants, or

electrochemical forward biasing.

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Created

Date Created
  • 2018