Doctoral Dissertations

Date of Award

12-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Seungha Shin

Committee Members

Douglas S. Aaron, David J. Keffer, Kenneth D. Kihm

Abstract

Effective use of renewable energy resources has been regarded as the most promising solution to climate emergency and energy crisis. However, the fluctuating and intermittent nature of renewable resources causes stability issues in the electric grid. High-capacity electrical energy storage is essential to stabilize the electric power supply using renewable resources. Among various types of energy storage systems, organic redox flow battery (ORFB) has attracted attentions due to their high stability, flexibility, low cost, and environmental compatibility, but the performance of the ORFB still needs a significant improvement due to their low energy or current density. Specifically, even though the efficiency of the electrode, where the redox reactions occur, is critical to the ORFB performance, the disordered structures of widely used carbon-based electrodes hinder the convective flux of electrolytes. This makes the ion transport through the electrodes very restricted, which is a major challenge in ORFB.

To overcome the major challenge while enhancing or maintaining other electrode properties, directional graphene aerogel (DGA) was proposed as an electrode material, and the DGA synthesis and ion transport for its electrode application are discussed here. As DGAs are synthesized by freeze casting, the control of water-ice phase change and the effect of graphene-water interaction on freezing and structuring are important to develop the DGA synthesis. Also, understanding of ion transport through graphene channels with convective flux should be enhanced for the design of effective DGA electrodes.

Multiscale simulations employing molecular dynamics and finite volume methods were conducted to investigate the propagation and shape of the water-ice interface under various freezing conditions (e.g., temperature, crystal orientation, container geometry, etc.) and the ion electric mobility with convective flux under different graphene-surface interactions and channel conditions. Through this research, I identified 1) various ice propagation control methods and structuring mechanisms during the freezing, 2) the roles of the Rayleigh-Bénard convection on the propagation and shape of the water-ice interface, and 3) the effects of surface interaction on the ion mobility in graphene nanochannels under the coupling of field- and pressure-driven transport. This research explores new possibilities to enhance the ion transport of DGA electrodes for ORFB.

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