Molecular Engineering of Organic Species for Aqueous Redox Flow Batteries

The design engineering of nanocatalysts for high power redox flow batteries

Redox flow batteries (RFBs) are one of the most promising long-term energy storage technologies which utilize the redox reaction of active species to realize charge and discharge. With the decoupled power and energy components, RFBs exhibit high battery pile construction flexibility and long lifespan. However, the inherent slow electrochemical kinetics of the current widely applied redox active species severely impedes the power output of RFBs. Developing high performance electrocatalysts for these redox active species would boost the power output and energy efficiency of RFBs. Here, we present a critical review of nanoelectrocatalysts to improve the sluggish kinetics of different redox active species, mainly including the chemical components, structure and integration methods. The relationship between the physicochemical properties of nanoelectrocatalysts and the power output of RFBs is highlighted. Finally, the future design of nanoelectrocatalysts for commercial RFBs is proposed.

Exploring the Landscape of Heterocyclic Quinones for Redox Flow Batteries

Redox flow batteries (RFBs) rely on the development of cheap, highly soluble, and high-energy-density electrolytes. Several candidate quinones have already been investigated in the literature as two-electron anolytes or catholytes, benefiting from fast kinetics, high tunability, and low cost. Here, an investigation of nitrogen-rich fused heteroaromatic quinones was carried out to explore avenues for electrolyte development. These quinones were synthesized and screened by using electrochemical techniques. The most promising candidate, 4,8-dioxo-4,8-dihydrobenzo[1,2-d:4,5-d']bis([1,2,3]triazole)-1,5-diide (-0.68 V(SHE)), was tested in both an asymmetric and symmetric full-cell setup resulting in capacity fade rates of 0.35% per cycle and 0.0124% per cycle, respectively. In situ ultraviolet-visible spectroscopy (UV-Vis), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopies were used to investigate the electrochemical stability of the charged species during operation. UV-Vis spectroscopy, supported by density functional theory (DFT) modeling, reaffirmed that the two-step charging mechanism observed during battery operation consisted of two, single-electron transfers. The radical concentration during battery operation and the degree of delocalization of the unpaired electron were quantified with NMR and EPR spectroscopy.

A benzoquinone-imidazole hybrid organic anolyte for aqueous redox flow batteries

A benzoquinone derivative annelated by two imidazole rings was investigated as an organic anolyte of aqueous redox flow batteries. The anolyte showed a high solubility of 0.18 M in 1 M KOH aqueous solution and exhibited a one-step two-electron reversible redox wave with a half-wave potential of -0.59 V VS. SHE. An aqueous redox flow cell comprising the benzoquinone-imidazole hybrid as the anolyte and potassium ferrocyanide as the catholyte showed an operating voltage of ∼1.1 V and minimum capacity fading of over 220 cycles.

Recent Development of Electrolytes for Aqueous Organic Redox Flow Batteries (Aorfbs): Current Status, Challenges, and Prospects

In recent years, aqueous organic redox flow batteries (AORFBs) have attracted considerable attention due to advancements in grid-level energy storage capacity research. These batteries offer remarkable benefits, including outstanding capacity retention, excellent cell performance, high energy density, and cost-effectiveness. The organic electrolytes in AORFBs exhibit adjustable redox potentials and tunable solubilities in water. Previously, various types of organic electrolytes, such as quinones, organometallic complexes, viologens, redox-active polymers, and organic salts, were extensively investigated for their electrochemical performance and stability. This study presents an overview of recently published novel organic electrolytes for AORFBs in acidic, alkaline, and neutral environments. Furthermore, it delves into the current status, challenges, and prospects of AORFBs, highlighting different strategies to overcome these challenges, with special emphasis placed on their design, composition, functionalities, and cost. A brief techno-economic analysis of various aqueous RFBs is also outlined, considering their potential scalability and integration with renewable energy systems.

Functional materials for aqueous redox flow batteries: merits and applications

Redox flow batteries (RFBs) are promising electrochemical energy storage systems, offering vast potential for large-scale applications. Their unique configuration allows energy and power to be decoupled, making them highly scalable and flexible in design. Aqueous RFBs stand out as the most promising technologies, primarily due to their inexpensive supporting electrolytes and high safety. For aqueous RFBs, there has been a skyrocketing increase in studies focusing on the development of advanced functional materials that offer exceptional merits. They include redox-active materials with high solubility and stability, electrodes with excellent mechanical and chemical stability, and membranes with high ion selectivity and conductivity. This review summarizes the types of aqueous RFBs currently studied, providing an outline of the merits needed for functional materials from a practical perspective. We discuss design principles for redox-active candidates that can exhibit excellent performance, ranging from inorganic to organic active materials, and summarize the development of and need for electrode and membrane materials. Additionally, we analyze the mechanisms that cause battery performance decay from intrinsic features to external influences. We also describe current research priorities and development trends, concluding with a summary of future development directions for functional materials with valuable insights for practical applications.

Identifying structure-function relationships to modulate crossover in nonaqueous redox flow batteries

Nonaqueous redox flow batteries (NARFBs) offer a promising solution for large-scale storage of renewable energy. However, crossover of redox active molecules between the two sides of the cell is a major factor limiting their development, as most selective separators are designed for deployment in water, rather than organic solvents. This report describes a systematic investigation of the crossover rates of redox active organic molecules through an anion exchange separator under RFB-relevant non-aqueous conditions (in acetonitrile/KPF6) using a combination of experimental and computational methods. A structurally diverse set of neutral and cationic molecules was selected, and their rates of crossover were determined experimentally with the organic solvent-compatible anion exchange separator Fumasep FAP-375-PP. The resulting data were then fit to various descriptors of molecular size, charge, and hydrophobicity (overall charge, solution diffusion coefficient, globularity, dynamic volume, dynamic surface area, clogP). This analysis resulted in multiple statistical models of crossover rates for this separator. These models were then used to predict tether groups that dramatically slow the crossover of small organic molecules in this system.

A Physical Organic Chemistry Approach to Developing Cyclopropenium-Based Energy Storage Materials for Redox Flow Batteries

ConspectusRedox flow batteries (RFBs) represent a promising modality for electrical energy storage. In these systems, energy is stored via paired redox reactions of molecules on opposite sides of an electrochemical cell. Thus, a central objective for the field is to design molecules with the optimal combination of properties to serve as energy storage materials in RFBs. The ideal molecules should undergo reversible redox reactions at relatively high potentials (for the molecule that is oxidized during battery charging, called the catholyte) or low potentials (for the species that is reduced during battery charging, called the anolyte). Furthermore, anolytes and catholytes must be highly soluble in the electrolyte solution and stable to extended electrochemical cycling in all battery-relevant redox states. The ideal candidates would undergo more than one reversible electron transfer event. Finally, the optimal structures should be resistant to crossover through a selective separator in order to maintain isolation of the two sides of the cell. This Account describes our design and optimization of organic molecules for this application. We first provide background for the metrics and experiments used to characterize anolytes/catholytes and to progress them toward deployment in flow batteries. We then use our studies of aminocyclopropenium-based catholytes to illustrate this workflow and approach.We identified tris(dimethylamino) cyclopropenium hexafluorophosphate as a first-generation catholyte for nonaqueous RFBs based on literature reports from the 1970s describing its reversible chemical and electrochemical oxidation. Cyclic voltammetry and electrochemical cycling experiments in acetonitrile/LiPF₆ confirmed that this molecule undergoes oxidation at relatively high potential (0.86 V versus ferrocene/ferrocenium) and exhibits moderate stability toward charge-discharge cycling. Replacing the methyl groups with isopropyl substituents led to enhanced cycling stability but poor solubility of the radical dication (<0.1 M in acetonitrile). Solubility was optimized using quantitative structure-property relationship modeling, which predicted derivatives with ≥10-fold enhanced solubility. Cyclopropeniums with 300-500 mV higher redox potentials were identified by replacing one of the dialkylamino substituents with a less electron-donating thioalkyl or aryl group. Multielectron catholytes were developed by creating hybrid structures that contain a di(amino) cyclopropenium conjugated with a phenothiazine moeity. Finally, oligomeric tris(amino) cyclopropeniums were designed as crossover resistant catholytes. Optimization of their solubility enabled the deployment of these oligomers in high concentration asymmetric redox flow batteries with energy densities that are comparable to the state-of-the-art commercial aqueous inorganic systems.