Development of Iron Complex-based Aqueous Redox Flow Batteries for Large-scale Energy Storage

Abstract

The world's reliance on fossil fuels has led to significant environmental problems and economic costs. Consequently, the global economy has made it a top priority to transition to clean energy. Although renewable energy sources such as solar and wind power are growing rapidly, their intermittent nature presents a challenge to their widespread use. To fully capitalize on the advantages of renewable energy, energy storage technologies must be safe, efficient, and scalable. Redox flow batteries (RFBs), including aqueous redox flow batteries (ARFBs), offer a sustainable solution for large-scale energy storage. ARFBs are particularly appealing due to their extended cycling lifetime, discharge durations, and inherent fire safety. Iron complex-based flow batteries are a promising option for large-scale energy storage, as they utilize an electrolyte solution containing iron-based complexes to store energy. This technology offers potential cost and environmental advantages when used in ARFBs, making it an attractive solution for grid-scale energy storage. However, further research and development are necessary to improve the performance, efficiency, and cost-effectiveness of iron complex-based flow batteries. Through structural modifications, the electrochemical properties of iron-based complexes can be optimized to achieve stable and high-potential redox-active species for sustainable energy storage applications. Chapter 1 provides an overview of the motivation for utilizing aqueous redox flow batteries for energy storage, as well as an introduction to redox flow batteries. This chapter also reviews previous research on the use of iron complex-based electrolytes for redox flow batteries. Chapter 2 describes the synthesis and characterization of a novel iron complex, tris(4,4'-bis(hydroxymethyl)-2,2'-bipyridine) iron dichloride (Fe(Bhmbpy)3). The redox potential of Fe(Bhmbpy)3 is determined to be 0.985 V vs. SHE. When paired with bis(3-trimethylammonio)propyl viologen tetrachloride (BTMAP-Vi), the BTMAP-Vi | Fe(Bhmbpy)3 ARFB system displays impressive electrochemical properties, including a high open-circuit voltage of 1.3 V, excellent cycling performance with a capacity fade rate of 0.07% per day, and a peak galvanic power density exceeding 120 mW/cm2. Notably, the study reveals that the iron complex's crossover rate is suppressed, which is essential for achieving a long lifetime of ARFB. Finally, the study proposes molecular decomposition mechanisms, based on thorough post-mortem analyses of the BTMAP-Vi | Fe(Bhmbpy)3 cell. Chapter 3 describes a new posolyte redox species for aqueous redox flow batteries (ARFBs) that is both highly soluble and stable: tetrakis(2-pyridylmethyl)ethylenediamine iron(II) dichloride, with a redox potential of 0.788 V vs. SHE. When paired with BTMAP-Vi at neutral pH, this posolyte demonstrates good performance, with an open-circuit voltage of 1.19 V and a capacity fade rate of just 0.28% per day, while maintaining a coulombic efficiency of 99.3% at a 0.6 M reactant concentration. This iron complex holds significant promise for large-scale energy storage due to its low cost, high solubility, and high redox potential compared to other posolyte species. Chapter 4 describes the synthesis and development of Fe(TPTA), an iron complex, using a simple and effective synthetic method. The complex's structure was confirmed through single-crystal XRD analysis, and its stability is attributed to the chelation effect of TPTA, which forms a six-membered ring structure around the iron ion. Fe(TPTA) displayed reversible redox behavior, with a redox potential of 0.96 V vs SHE, and attained a maximum OCV of 1.3 V when paired with BTMAP-Vi. Chapter 5 outlines potential iron complexes that could serve as cost-effective electrolyte candidates. These complexes were developed using commercially available ligands, including Tris(2-pyridylmethyl)amine (TPA) and quinolinic acid (QA). The section also highlights the challenges that need to be addressed to consider these two complexes as viable candidates for further investigation.