Doctoral Dissertations

Date of Award

12-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Mechanical Engineering

Major Professor

Matthew M. Mench

Committee Members

Matthew Mench, Kenneth Kihm, Feng-Yuan Zhang, Vasilios Alexiades

Abstract

All-vanadium redox flow batteries (VRFBs) are a promising technology for grid-level energy storage, however, there are still several limitations in the forms of durability, efficiency, and overall costs, which are barriers to its commercial viability. With both bulk electrolyte flowing through its porous matrix and species flux at the solid-electrolyte interface, electrodes are the component of VRFB systems which host electrochemical reactions and facilitate contact between the liquid phase electrolyte and the electronically conductive solid phase. While the more limiting electrode in VRFB systems is dependent on the material, for polyacrylonitrile (PAN)-based carbon felts, the anode constitutes a larger portion of the total overpotential than the cathode. In-situ characterization of modified felts can pro-vide both a path towards understanding the source improvements to the anode but also their transient behavior, thereby creating a path to higher voltage and energy efficiencies and higher commercial viability.The primary experimental components of this work are the symmetric cell con-figuration and electrochemical impedance spectroscopy (EIS). The symmetric cell is an in-situ experimental configuration, which utilizes a single electrolyte at both half-cells of a reactor. The symmetric cell allows an experiment to maintain a constant state of charge, incur no net-crossover and isolate a single redox couple.Electrochemical impedance spectroscopy is the focal experimental diagnostic technique in this work, owing to both its minimal perturbation of the state of a cell and to the insights into electrochemical interfaces it provides. The combination of these two techniques is utilized in this work to both characterize several material modifications and to enhance the understanding of the sensitivity of this diagnostic for the purposes of model selection. A novel experimental configuration is also developed, which combines a cell-in-series approach with the symmetric cell in order to continuously characterize VRFB electrodes as a function of state of charge.The outcomes of this work elucidate methodologies for in-situ characterization of anode modifications from an experimental perspective and also frameworks for characterizing porous electrochemical interfaces, investigating sensitivity, and elucidating systematic approaches to separate electrochemical phenomena occurring at a VRFB anode interface.

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