Metal-Organic Frameworks (MOFs) are porous materials consisting of organic ligands connected by inorganic nodes. Their structural uniformity, high surface area, and synthetic tunability, position these frameworks as suitable active materials to achieve efficient and clean electrochemical energy storage. In spite of recent demonstrations of MOFs undergoing diverse electrochemical processes, a fundamental understanding of the mechanism of electron, proton, and ion transport in these porous structures is needed for their application in electronic devices. The current work focuses on contributing to such understanding by investigating proton-coupled electron transfer, capacitance performance, and the relative contribution of electron and ionic transport in the voltammetry of zirconium-based MOFs.
First, we investigated the effects that the quinone ligand orientation inside two new UiO-type metal-organic frameworks (2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF) have on the ability of the MOFs to achieve proton and electron conduction. The number of electrons and protons transferred by the frameworks was tailored in a Nernstian manner by the pH of the media, revealing different electrochemical processes separated by distinct pKa values. In particular, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in the MOFs, lead to different PCET processes. The ability of the MOFs to transport discrete numbers of protons and electrons, suggested their application as charge carriers in electronic devices.
With that purpose in mind, we assembled 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF into two different types of working electrodes: a slurry-modified glassy carbon electrode, and as solvothermally-grown MOF thin films. The specific capacitance and the percentage of quinone accessed in the two frameworks were calculated for the two types of electrodes using cyclic voltammetry in aqueous buffered media as a function of pH. Both frameworks showed an enhanced capacitance and quinone accessibility in the thin films as compared to the powder-based electrodes, while revealing that the structural differences between 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF in terms of defectivity and the number of electrons and protons transferred were directly influencing the percentage of active quinones and the ability of the materials to store charge.
Additionally, we investigated in detail the redox-hopping electron transport mechanism previously proposed for MOFs, by utilizing the chronoamperometric response (I vs. t) of three metallocene-doped metal-organic frameworks (MOFs) thin films (M-NU-1000, M= Fe, Ru, Os) in two different electrolytes (TBAPF6 and TBATFAB). We were able to elucidate, for the first time, the diffusion coefficients of electrons and ions (De and Di, respectively) through the structure in response to an oxidizing applied bias. The application of a theoretical model for solid state-voltammetry to the experimental data revealed that the diffusion of ions is the rate-determining step at the three different time stages of the electrochemical transformation. Remarkably, the trends observed in the diffusion coefficients (De and Di) of these systems obtained in PF61- and TFAB1- based electrolytes at the different stages of the electrochemical reaction, demonstrated that the redox hopping rates inside frameworks can be controlled through the modifications of the self-exchange rates of redox centers, the use of large MOF channels, and the utilization of smaller counter anions. These structure-function relationships provide a foundation for the future design, control, and optimization of electronic and ionic transport properties in MOF thin films. / PHD / The necessity of implementing new energy storage systems that enable the utilization of clean energy in diverse technologies such as electric vehicles and smart power grids, has generated great research efforts in the field of materials science. In particular, the development of nanoscale-based materials that can be utilized in batteries and supercapacitors is essential for achieving effective and clean electrochemical energy storage. Two of the main desired properties for such materials to be employed as electrodes in energy storage devices are high surface area and the possibility of incorporating redox-active moieties that are able to store electricity.
Metal-Organic Frameworks (MOFs) are a relatively new kind of porous materials with high surface area and structural uniformity, consisting of organic ligands connected by inorganic nodes. The application of these materials in charge transport and storage is still in its early stages. Therefore, fundamental understanding of the mechanism of electron, proton, and ion transport in MOFs is necessary for a rational design of these porous structures. In order to contribute to such understanding, the present work is focus on two main concepts: (1) elucidating the effect that the tridimensional orientation of redox moieties inside the MOF could have on the charge storage performance and the ability of the material to achieve proton and electron conduction; and (2) quantifying for the first time the individual relative contribution of electron and ionic transport in MOF materials.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/96312 |
| Date | 16 July 2018 |
| Creators | Celis Salazar, Paula Juliana |
| Contributors | Chemistry, Morris, Amanda J., Tanko, James M., Tissue, Brian M., Morris, John R. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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