University of Illinois Urbana-Champaign

Rheology, conductivity, and structure of complex fluids in flow batteries

Wang, Yilin

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https://hdl.handle.net/2142/120354

Description

Title
Rheology, conductivity, and structure of complex fluids in flow batteries
Author(s)
Wang, Yilin
Issue Date
2023-04-12
Director of Research (if dissertation) or Advisor (if thesis)
Ewoldt, Randy H
Doctoral Committee Chair(s)
Ewoldt, Randy H
Committee Member(s)
  • Rogers, Simon
  • Schroeder, Charles M
  • Feng, Jie
Department of Study
Mechanical Sci & Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
  • complex fluids
  • rheology
  • flow battery
  • non-Newtonian fluid mechanics
  • viscosity
  • conductivity
  • carbon black
  • colloidal suspension
  • thixotropy
  • antithixotropy
  • viscoelasticity
Abstract
The development of intermittent renewable energy requires reliable energy storage methods, among which the use of flow batteries is a competitive one due to their scalable energy capacity, large electrochemical stability windows, and potentially long operating lifetimes. However, the largest drawback of flow batteries is low energy density, due to the low concentration of redox-active materials in their working fluid. Here, two approaches to increase the concentration of the working fluid are studied: one is to use redox-active organic molecules (ROMs) with higher solubility, and another is to use redox-active colloids that can be suspended in the working fluid and therefore break the solubility limit. For both fluids, the rheological and conductive properties are complex, and the underlying microstructures are poorly understood, while the performance of flow batteries heavily depends on their rheology and conductivity. Therefore, the main objective of this dissertation is to study the rheological and conductive properties of complex fluids that are relevant to flow batteries and to infer molecular or structural information from the bulk properties measurements. There are two systems of interest in this dissertation. In the first part of this dissertation, MEEPT in different supporting salt electrolytes is studied. MEEPT, short for N-(2-(2-methoxyethoxy)ethyl)phenothiazine, is a ROM (∼ 1 nm) that shows promise for meeting grid-scale energy storage requirements, as evidenced by its high solubility. However, its dramatically increased viscosity and reduced ionic conductivity at high concentrations can be detrimental to flow batteries. Here the concentration-dependent viscosity of the solution is measured using a microchannel viscometer, from which the microstructure information, such as hydrodynamic diameter and Huggins coefficient, can be inferred, and the conductivity is predicted using mathematical models. In this study, a standard fitting and inference protocol, evaluating the fit using Bayesian information criterion, is established. The part two of this dissertation focuses on a conductive colloid, carbon black, which is commonly used in flow batteries as conductive additive. The transient rheology of carbon black suspension is complicated and controversial, and whether the stress decay observed during step-down in shear rate tests on carbon black suspensions is anti-thixotropy or viscoelasticity remains unknown. In this dissertation, to identify the dominating dynamics of carbon black suspensions and differentiate between thixotropy, anti-thixotropy, and viscoelasticity, three test protocols are developed using a combination of mathematical models and experiments: step-shear rate, hysteresis, and orthogonal superposition. Subsequently, using a bespoke attachment to the stress-controlled (combined-motor-transducer) rotational rheometer, the rheology and electrical impedance of carbon black suspensions under shear are measured simultaneously. As a result, a micro-structure model of carbon black is proposed, considering its short-time thixotropy, long-time anti-thixotropy, and anisotropy. With a combination of rigorous experiments and mathematical models, this dissertation offers novel ways to measure and quantify rheology and impedance under shear simultaneously, to infer microstructure from the bulk measurements with the aim of designing complex fluids in flow batteries, and to differentiate time-dependent rheological dynamics in complex fluids.
Graduation Semester
2023-05
Type of Resource
Thesis
Copyright and License Information
Copyright 2023 Yilin Wang

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