Publication Date

2020

Document Type

Dissertation/Thesis

First Advisor

Xu, Tao

Second Advisor

Liu, Di-Jia

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

Electrochemical carbon dioxide reduction reaction (CO2RR) is a promising technique for energy conversion and storage from intermittent electricity sources (ie. wind and solar). By coupling electrochemical CO2 reduction (CO2R) to a renewable energy source atmospheric CO2 could be converted back into a fuel such as ethanol, or a commodity chemical such as ethanol or acetic acid. These products could then be consumed for energy or used to generate more valuable chemical products effectively removing CO2 from the atmosphere. To commercialize this process, it is highly desired to prepare catalysts that selectively reduce CO2 at low overpotential with long durability. Research in this area is hindered by the experimental difficulty of comparing catalyst activity. This thesis begins with the development of a custom-made electrolysis cell that can be coupled to product detection with NMR and GC.

We report new fundamental chemistry involved in the synthesis of carbon supported metal single atoms and bimetallic nanoalloys via dissolving the pure bulk transition metals in molten lithium. It is revealed at the atomic level that when two pure bulk transition metals such as Cu, Sn, Rh, Ag, In, Au, Pd and Ce are placed in molten lithium (∼200 °C), they undergo a dissolution process in which the metal-metal bonds in pure bulk transition metals are completely ruptured, which results in the existence of individual metals atoms surrounded by lithium atoms, as is evident by synchrotron X-ray adsorption techniques. Then, upon the conversion of metal lithium to LiOH in humid air, the metal atoms undergo oxidization process. This method was further expanded to include CuxSny, which is showing obvious core-shell structure. Additionally, this lithium assisted “dissolution-alloying” method bypasses many complications intrinsic to conventional ion reduction-based nanoalloy synthesis including the necessity of ligated metal ions, the use of proper reducing agents and dispersing surfactants, and the presence of segregated phases due to different reduction potentials of the constituent metal ions.

Copper is the only known metal capable of reducing CO2 to hydrocarbons at appreciable rates and low overpotentials. This work aims to find new materials that produce similar hydrocarbons, but at lower overpotentials with higher rates and greater selectivity than current copper catalysts. Direct electrochemical conversion of CO2 to value added chemicals offers a promising strategy to mitigate carbon dioxide emission. Here we report a Cu single atom catalyst synthesized by a unique Cu-Li amalgm method over commercial carbon support that achieved Faradaic efficiency (FE) higher than 91 % at -0.7 V (RHE) and the active onset potential as low as -0.4 V (RHE) during the direct electrocatalytic CO2-to-ethanol conversion. The catalyst also demonstrated stabilities over an extended period of operation. A strong correlation between the catalytic selectivity and the initial Cu atoms dispersion was found and Operando X-ray absorption spectroscopy identified a dynamic and reversible transformation from the states of copper single atom to Cun (n = 3 and 4) under the electrochemical reaction. First-principles calculations further elucidate the catalytic mechanism of CO2 reduction over Cun producing overpotentials consistent with the experiments.

Electrochemical reduction of carbon dioxide (CO2RR) to liquid fuels provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Therefore, liquid products that form are in a mixture with the dissolved salts, requiring energy-intensive downstream separation. Here, we report a Sn single atom catalyst synthesized by a unique Sn-Li amalgam method over commercial carbon support that achieved Faradaic efficiency (FE) higher than 91 % at -0.6 V (RHE) for acetic acid, 93% at 0.4 V (RHE) for ethanol and 91% at -0.6 V (RHE) for formate with different Sn loading on carbon support. The active onset potential as low as -0.3 V (RHE) during the direct electrocatalytic CO2-to-ethanol conversion. In-situ Sn K3-edge X-ray absorption spectroscopy indicate electron transfer from Sn+4 to Sn2+ to Sn(0), which indicates positive oxidation states of Sn in the precatalyst of Sn dispersed on carbon under operating conditions. The catalyst also demonstrated stabilities over an extended period of operation for 28 hours. Finally, A theoretical thermodynamic analysis of the reaction energetics suggests that Tin shows different selectivity to various products with different kinds of dispersion formation.

The advantages of the electrochemical conversion of carbon dioxide to fuels using renewable energy sources are two-fold: (1) it has the potential to accomplish a carbon-neutral energy cycle and (2) it can provide an approach to tackle the environmental challenges caused by anthropogenic carbon dioxide emissions. Although thermodynamically possible, the kinetics of carbon dioxide reduction to fuels remains challenging and therefore, an efficient and robust electrocatalyst is needed to promote the reaction.

Improving the cost and durability of polymer electrolyte membrane (PEM) fuel cell materials is a hot topic of research today. The Nafion membrane and cathode catalysts are two areas where PEM fuel cells have issues of cost, durability, and efficiency. Achieving high catalytic performance with the lowest possible amount of platinum is critical for fuel cell cost reduction. Here we describe a method of preparing highly active yet stable electrocatalysts containing ultralow-loading platinum content by using cobalt or bimetallic cobalt and zinc zeolitic imidazolate frameworks as precursors. Another is related to Co-based MOF materials synthesis and design for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).

Extent

215 pages

Language

eng

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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