Publication Date

2024

Document Type

Dissertation/Thesis

First Advisor

Li, Tao

Second Advisor

Victor Ryzhov

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

The microstructure of energy conversion and storage materials is a key determinant of physical properties and electrochemical performance. Synchrotron X-ray techniques, with their strengths of high flux and exceptional brightness, play an irreplaceable role in understanding nano and sub-nano structures. This helps the study of reaction mechanisms and benefits material design in energy conversion and storage area. Electrifying CO2 transformation using renewable energy offers a carbon-neutral solution to CO2 recycling and fuel generation. Among various feedstocks from the CO2 reduction reaction, CO, HCOOH, and CH4 have approached the performance thresholds for industry implementation. The application of single atom catalysts (SACs) and nano catalysts in CO2 reduction reaction (CO2RR) has attracted significant attention because of their promising properties. Here we applied in-situ X-ray absorption spectroscopy (XAS) to study the evolutions of active sites under different working conditions. We have developed different transition metal single-atom catalysts on N doped carbon (M-N-C) by pyrolysis method. Among M-N-C catalysts, Ni-N-C was found to deliver the highest FECO (97.9%) at −0.8 V vs. RHE along with CO formation current density as high as 48.4 mA cm-2 at −1.2 V vs. RHE. The in-situ X-ray absorption near edge structure (XANES) spectra reveals inconspicuous oxidation state changes of Ni-N-C under open circuit voltage (OCV) accompanied by subsequent reduction. Further in-situ Fourier transform (FT) of extended X-ray absorption fine structure (FT-EXAFS) spectra analysis in coordination number discloses the undercoordinated Ni SACs at −0.8 V vs. RHE. Density-functional theory (DFT) calculations suggest that the reconstructed Ni-N3 coordination structure with the amine group could enhance CO2 activation. In the following in-situ XAS measurement work, we demonstrated a unique dynamic reconstruction of atomically dispersed Cu sites to Cu3 clusters immobilized by nitrogen of the carbon support and hydroxyl species from the electrolyte under the operating CO2 reduction conditions. The in-situ constructed Cu3 species is the actual catalytic site for efficient CO2-to-CH4 reduction. This work holds great promise for practical CO2-to-fuel electrolysis implementation. The rational catalyst design principles could create an effective platform for building advanced catalysts with optimal structures at a multiscale capable of regulating reaction pathways and selectivity for complex catalytic reactions associated with multiple elemental steps and intermediates. In addition to the transition metal single atom catalysts, we applied in-situ XAS to study the nano sized SnO2 under working conditions. Different from the gas reduction products (CO or CH4) when employing SACs, the CO2 reduction product in this system is liquid formate. Moreover, we have demonstrated the size effect on SnO2 toward CO2RR by preparing SnO2 NPs on a carbon sphere with different loadings. The in-situ XAS measurement reveals the reduction of SnO2 NPs under the applied voltage range with the presence of SnOx and metallic Sn. This study disclosed SnOx as an active site for CO2-to-formate conversion and provided helpful guidance for designing highly efficient and stable CO2RR electrocatalysts. Different from atom information revealed by XAS, SAXS uncovers the information at nanometer scale. We also applied SAXS to study the solvation structure of organic electrolyte for Li-S batteries and aqueous electrolyte for lithium ion batteries. By using Raman spectroscopy, we found that more contact ion pairs (CIPs) are formed, and fewer free solvent molecules exist in high-concentration electrolytes, thus suppressing the dissolution and shuttle of lithium polysulfides (LiPSs). For the study of the physicochemical properties of imide-based asymmetric lithium salts with different concentrations in water, the SAXS spectra show the blue shifts of the lower q peak with decreased intensity as the increasing of concentration, indicating a decrease in the average distance between solvated anions. The increase in the population of anions and reduction of water molecules in highly concentrated solution induce the formation of FSI networks, which is indicated by the formation of additional peak at higher q value. Significantly, an exponential decrease in the d-spacing as a function of concentration was observed. The results obtained in this work benefit the understanding of the solvation structure of water in salts (WISs) electrolytes and promote the development of high-energy density batteries.

Extent

145 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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