Molecular Engineering and Electrochemical Characterization of Redox-active Electrolytes for Aqueous Redox Flow Batteries

• Author / Creator

  Wang, Hao

• Energy is one of the most indispensable ingredients in modern economies and is a critical cornerstone for the development of human society. In contrast to carbon-intensive legacy fossil fuels, low-carbon renewable energy sources (such as solar and wind) provide a significant opportunity to alleviate carbon emissions and global anthropogenic climate change. Due to the intermittent nature of solar and wind energy, the need for energy storage to complement these vast but fluctional sources of wind and solar energy is pressing. Redox flow batteries (RFBs) are promising thanks to their inherent decoupling of stored chemical energy and tunable power density, as well as their low cost, safe operation, and long lifetimes. Traditional inorganic RFBs, such as all-vanadium RFBs, have been studied for over three decades and have undergone successful commercialization. These batteries are, however, subject to problematic issues related to their chemistry, including corrosivity (e.g., sulfuric acid is the solvent), cost (ca. $27/kg for vanadium (V) oxide raw material and $500/m2 for Nafion membranes), parasitic reactions, and crossover of electrolytes that reduces efficiency. RFBs based on organic electrolytes may be promising alternatives for the next-generation of aqueous RFBs due to the tunability of their chemical and physical properties. To achieve high performance aqueous flow batteries, much development is needed as well as detailed fundamental studies of the electrochemical properties of these redox electrolytes. This thesis focuses on developing new organic materials and improved electrochemical methods for the next generations of aqueous RFBs for storage of low-carbon energy.
  The thesis can be divided into two parts. In the first part of the thesis, mathematical modeling was applied to simulate the electrochemical behavior of electrochemically reversible, quasi-reversible, and irreversible systems by using different electrochemical techniques. The advantages and limitations of these electrochemical methods to acquire kinetics parameters were analyzed and discussed, and their limitations addressed and reconsidered. The result of this work was a new method and protocols to obtain electrochemical rate constants and diffusion coefficients for both reduced and oxidized species. The effects of heterogeneous rate constants of redox compounds on the performance of the flow batteries also were simulated and evaluated.
  In the second part, a water-soluble octahedral bis(imino)pyridine cobalt complex was synthesized. Due to the introduction of carboxylic groups to the bis(imino)pyridine ligand periphery, the complex is water soluble, with two redox couples within the water splitting window. Its electrochemical kinetics, pH-dependent cyclic voltammetric behavior, and solubility in the water solution were investigated thoroughly. Symmetric aqueous RFBs with the complex acting as both catholyte and anolyte were fabricated, and they were found to have high capacity retention and good Coulombic efficiencies over 100 cycles. Upon completion of this work on the cobalt complex, the project was extended to a series of phenazine derivatives with different numbers of sulfonated terminated short carbon chains that were designed and prepared. The redox potentials, solubility, and stability of the phenazine moieties are modulated through the tailoring of the number and position of the functional groups. The electrochemical properties and the performance of full cell aqueous RFBs also were investigated.

• Subjects / Keywords

  • Redox flow battery
  • Electrochemical characterization
  • Molecular engineering
  • Electrochemical energy storage

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-rdgr-jj52

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.