Doctoral Dissertations

Date of Award

12-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Kenneth D. Kihm

Committee Members

Doug S. Aaron, Seungha Shin, Georgios Polyzos, Kashif Nawaz

Abstract

Redox flow batteries (RFBs) are a promising technology for large-scale energy storage. In contrast to conventional vanadium-based systems, organic redox flow batteries (ORFBs), offer advantages such as lower material costs, wider electrochemical windows, and higher energy densities, making them strong candidates for commercialization. However, their performance is limited by sluggish ion and mass transport within the cell, particularly with organic reactants.

This study introduces directionally porous graphene aerogels (DGAs) as electrode materials to address the mass transport challenges in organic electrochemical systems. DGAs are hypothesized to enhance mass and charge transport through highly aligned pore structures while maintaining graphene’s high surface area and electrical conductivity. The DGAs were synthesized via a two-step process involving the chemothermal reduction of graphene oxide (GO) using ascorbic acid (AA) and directional freeze-casting of graphene hydrogels (GH).

Microstructural analysis revealed that the synthesis parameters such as freezing temperature, GO concentration, and container aspect ratio strongly influence the pore architecture. Optimal conditions for DGA fabrication were found to be a freezing temperature of -70°C, a GO concentration of 4 mg/ml or below, and an aspect ratio under unity. Electrical conductivity of the DGA was maximized (~5 S/m) with a GO ratio of 1:4 and a 24hrs. second reduction process.

Additionally, copper-functionalized DGAs (Cu-DGA) were synthesized without significantly altering the base method. Factors such as pH, copper precursor concentration, and reduction duration were shown to affect the particle size and distribution. Neutral pH promoted smaller, uniformly distributed copper particles, while higher precursor concentrations led to agglomeration. Despite a 30% decrease in electrical conductivity due to charge scattering and partial recovery of sp² domains, Cu-DGA exhibited enhanced electrochemical performance in Na2Sx-based flow cells with organic electrolyte.

This research advances the design of graphene aerogels and their nanocomposites as electrode materials for electrochemical systems and offers potential improvements in areas such as catalysis, energy storage, and separation technologies.

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