Joel Rosenthal

Department of Chemistry and Biochemistry,

University of Delaware, Newark, DE, 19716

Abstract: Development of new electrosynthetic tools and methods has attracted much interest
in recent years as a means to prepare chemicals and materials that are either
inaccessible or whose preparation is inefficient using traditional thermal chemistries. In
addition to opening up routes to new compounds and materials, implementation of
electrosynthetic strategies can enable reduced waste streams and more streamlined
synthetic routes, while circumventing the use of expensive, acutely toxic, and highly
reactive reagents. Driving synthetic chemistry with electric current as opposed to heat
also represents a direct way to power chemical processes using renewable energy (such
as electricity from wind or sunlight), and therefore provides an opportunity for more
sustainable chemical syntheses and renewable energy storage.
Our lab has developed efficient electrosynthetic routes to prepare commodity
chemicals and fuels, fine chemicals, and new inorganic materials. In this presentation, we
will provide an overview of our efforts in each of these areas, which include 1) controlling
the electrochemical reduction of carbon dioxide to switch between the formation of either
formic acid or carbon monoxide depending on the electrolysis conditions; 2) the
electrosynthesis of α,β-ynones en route to polyphenols that show anti-cancer and anti-
HIV activity; and 3) the electrochemical synthesis of new classes of metal-organic
frameworks and other porous materials that are based upon non-traditional metal ions
and organic linkers. Throughout the presentation, we will show how the ability to drive
challenging transformations that require the activation of strong bonds or access to highly
reactive chemical intermediates is greatly facilitated through an electrochemical
approach. We will also demonstrate how controlling the chemical dynamics and
environments at working electrode interfaces can be leveraged to promote interesting
energy conversion processes, solar fuel generation, and porous material construction.
Implications for the future development of efficient electrosynthetic strategies and
platforms will also be discussed.

Abstract: Non-emissive, sustainable energy sources such as solar, wind, and geothermal power have continued to provide an increasing amount of electricity to support electrical grids. Due to the intermittent nature of renewable energy sources like wind and solar, grid energy storage systems must adjust for variations in and mismatches between electricity production and consumption. Among the available energy storage technologies, redox flow batteries (RFBs) are expected to play a critical role in the grid energy storage due to their decoupled energy and power, long service life, and simple manufacturing. However, the worldwide market penetration of RFB systems is still limited due to technical and economic challenges. The commercially available aqueous vanadium redox flow batteries offer durable performance but suffer from low energy density and high chemical costs. A key advantage of transitioning from aqueous to nonaqueous systems is the possibility of achieving higher energy density through the wider windows of electrochemical stability associated with organic solvents. Further, nonaqueous systems would provide a greater selection of redox materials which do not fit into the aqueous systems due to lower solubility, instability or redox potentials outside the stability window of water. Despite these promises, nonaqueous flow batteries are still an immature concept and, to date, no redox chemistry has proven competitive due to a combination of low solubility and stability of redox couples and a lack of selective membranes/separators. This thesis focuses on designing and testing robust, redox active organic molecules intended for use as either positive or negative active materials in nonaqueous RFBs. The two main redox active cores evaluated in this study are phenothiazine (as a positive active material) and viologen (as a negative active material) where both served as learning platforms. The molecules were functionalized through simple and scalable molecular synthetic approaches with particular emphasis on increasing solubility, ionic conductivity, redox potential, and chemical stability. Further, change in chemical stability of variably functionalized electron donating redox active organic cores (phenothiazine, triphenylamine, carbazole, dialkoxybenzene, and cyclopropenium) with different oxidation potentials was explored to identify the correlation between chemical stability of charged forms (radical cation) and coulombic efficiency in galvanostatic cycling. The analysis of chemical and electrochemical stabilities of developed redox active materials were conducted through a variety of spectro-electro analytical technique including cyclic voltammetry, UV-vis spectroscopy, bulk electrolysis, and flow cell cycling.

This project was motivated by an in situ heating experiment in the transmission electron
microscope (TEM) in which gold (Au) nanoparticles were observed to dissolve tin dioxide (SnO2)
nanowires (NWs) under vacuum. The explanation for this observation was that the hightemperature
and low-pressure environment of the TEM caused the reverse reaction of the wellknown
vapor-liquid-solid (VLS) method commonly used to grow NWs. In the VLS process, a
metal catalyst absorbs reactant vapor until it becomes supersaturated. The precipitation of the NW
occurs at the liquid-solid interface, which ceases when there is no longer reactant vapor, and the
diameter of the NW is determined by the diameter of the original catalyst. The reverse process, the
solid-liquid-vapor (SLV) method occurs when atoms in a solid NW diffuse into the metal catalyst.
Eventually, the metal catalyst becomes supersaturated and the vapor escapes at the liquid-vapor
interface. In this dissertation we demonstrate the combination of the SLV and the VLS mechanisms
to create embedded heterogeneous interfaces in a variety of metal oxides. Metal catalysts are first
used to etch metal oxide surfaces producing hollow channels that we term “negative nanowires”,
and after etching the metal catalyst is reused to grow a NW of a different material from within the
channel to form a crystalline interface. Understanding the chemical structure at these interfaces is
both crucial and fascinating because diverse materials may interact in a variety of ways, including
atomic mixing of the two structures and/or the formation of an abrupt crystalline interface or gap.
We present our approach, therefore, towards gaining a comprehensive understanding of the
structure-function relationship of these materials, focusing on particular on the interfacial region,
to allow the design of new nanomaterials with tailored functionality.