Trust is good, control is better: a review on monitoring and characterization techniques for flow battery electrolytes

Realizing one-step two-electron transfer of naphthalene diimides via a regional charge buffering strategy for aqueous organic redox flow batteries

Naphthalene diimide derivatives show great potential for application in neutral aqueous organic redox flow batteries (AORFBs) due to their highly conjugated molecular structure and stable two-electron storage capacity. However, the two-electron redox process of naphthalene diimides typically occurs via two separate steps with the transfer of one electron per step ("two-step two-electron" transfer process), which leads to an inevitable loss of voltage and energy. Herein, we report a novel regional charge buffering strategy that utilizes the core-substituted electron-donating group to adjust the redox properties of naphthalene diimides, realizing two electron transfer via a single-step redox process ("one-step two-electron" transfer process). The symmetrical battery testing of NDI-DEtOH revealed exceptional intrinsic stability lasting for 11 days with a daily decay rate of only 0.11%. Meanwhile, AORFBs with NDI-DMe/FcNCl and NDI-DEtOH/FcNCl exhibited a remarkable 40% improvement in peak power density at 50% state of charge (SOC) in comparison to NDI/FcNCl-based AORFBs. In addition, the battery's energy efficiency has increased by 24%, resulting in much more stable output power and significantly improved energy efficiency. These results are of great significance to practical applications of AORFBs.

Zinc-Bromine Rechargeable Batteries: From Device Configuration, Electrochemistry, Material to Performance Evaluation

Zinc-bromine rechargeable batteries (ZBRBs) are one of the most powerful candidates for next-generation energy storage due to their potentially lower material cost, deep discharge capability, non-flammable electrolytes, relatively long lifetime and good reversibility. However, many opportunities remain to improve the efficiency and stability of these batteries for long-life operation. Here, we discuss the device configurations, working mechanisms and performance evaluation of ZBRBs. Both non-flow (static) and flow-type cells are highlighted in detail in this review. The fundamental electrochemical aspects, including the key challenges and promising solutions, are discussed, with particular attention paid to zinc and bromine half-cells, as their performance plays a critical role in determining the electrochemical performance of the battery system. The following sections examine the key performance metrics of ZBRBs and assessment methods using various ex situ and in situ/operando techniques. The review concludes with insights into future developments and prospects for high-performance ZBRBs.

Organic Redox Targeting Flow Battery Utilizing a Hydrophilic Polymer and Its In-Operando Characterization via State-of-Charge Monitoring of The Redox Mediator

The hydrophilic poly(2,2,6,6-tetramethylpiperdinyloxy-4-yl-methacrylamide) (PTMAm) was utilized as redox target material in an aqueous organic redox targeting flow battery (RTFB). This polymer is processed into granules, which contain a conductive agent and an alginate binder. By this, a hydrophilic, yet water-insoluble redox target can be obtained. The target was combined with the redox mediator molecule N,N,N-trimethyl-2-oxo-2-[(2,2,6,6-tetramethylpiperidin-4-yloxyl)amino]ethan-1-ammonium chloride (TEMPOAmide), that has been reported earlier as flow battery active material. This target/mediator combination has been characterized electrochemically and flow battery testing has been done. Furthermore, in-operando characterization of the redox target via electrolyte state-of-charge (SOC) monitoring has been performed for the first time. The approach provides estimates for the redox target's SOC changes during cycling. In addition, a figure of merit - the "redox targetivity" - is proposed, which provides insights into the efficiency of the targeting reaction and supports the future optimization of materials, cell designs, and operational parameters for RTFBs.

Addressing Practical Use of Viologen-Derivatives in Redox Flow Batteries through Molecular Engineering

In practical scenarios, viologen-derivatives face an accelerated degradation in the unavoidable presence of traces of oxygen in large-scale redox flow batteries. Herein, we confirm the primary degradation mechanism and propose a straightforward, cheap, and fast method to evaluate the stability of viologen-derivatives toward this degradation. Considering that the cleavage of the N-substituent is the main proposed pathway for viologen degradation, a new viologen-derivative, bearing an alkylsulfonate chain with a secondary carbon center joined to the N atom, is synthesized to illustrate how molecular engineering can be used to improve stability.

Scientific Challenges and Improvement Strategies of Zn-Based Anodes for Aqueous Zn-Ion Batteries

Aqueous zinc-ion batteries (ZIBs) have attracted widespread attention due to the intrinsic features of Zn-based anodes, mainly including high capacity, low cost, and low working potential together with high over-potential for hydrogen evolution reaction. Aqueous ZIBs are considered to be strong competitors and substitutes for lead-acid, nickel-metal hydrogen, nickel-cadmium, and even lithium-ion batteries. Great efforts have been made in the past few years towards the issues existed in aqueous ZIBs, mainly including alkaline and mild acidic systems. In this perspective, we illustrate the advantages, the main challenges, and the corresponding solution strategies of Zn-based anodes in various aqueous rechargeable ZIBs with alkaline and mild acidic electrolytes. Furthermore, feasible aqueous ZIBs for practical use are prospected.

State of Charge and State of Health Assessment of Viologens in Aqueous-Organic Redox-Flow Electrolytes Using In Situ IR Spectroscopy and Multivariate Curve Resolution

Aqueous-organic redox flow batteries (RFBs) have gained considerable interest in recent years, given their potential for an economically viable energy storage at large scale. This, however, strongly depends on both the robustness of the underlying electrolyte chemistry against molecular decomposition reactions as well as the device's operation. With regard to this, the presented study focuses on the use of in situ IR spectroscopy in combination with a multivariate curve resolution approach to gain insight into both the molecular structures of the active materials present within the electrolyte as well as crucial electrolyte state parameters, represented by the electrolyte's state of charge (SOC) and state of health (SOH). To demonstrate the general applicability of the approach, methyl viologen (MV) and bis(3-trimethylammonium)propyl viologen (BTMAPV) are chosen, as viologens are frequently used as negolytes in aqueous-organic RFBs. The study's findings highlight the impact of in situ spectroscopy and spectral deconvolution tools on the precision of the obtainable SOC and SOH values. Furthermore, the study indicates the occurrence of multiple viologen dimers, which possibly influence the electrolyte lifetime and charging characteristics.

Half-Cell State of Charge Monitoring for Determination of Crossover in VRFB-Considerations and Results Concerning Crossover Direction and Amount

Membranes play a crucial role in efficiency and longevity of flow batteries. Vanadium flow batteries suffer self-discharge and capacity fading due to crossover of electrolyte components through the membrane from one battery half-cell to the other. We consider the impact of vanadium species crossing ion exchange membranes on state of charge of the battery and we present a simple method to determine crossoverll open circuit potential measurements. State of s. State of charge for the negative and positive half-cell is simulated based on assumptions and simplifications for cation and anion exchange membranes and different crossover parameters. We introduce a crossover index "IndXovr" which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs. Furthermore IndXovr allows statements on crossover amount in dependence on state of operation. Simulated case studies are compared to experimental state of charge values estimated from half-cell potential measurements. Our results reveal that half-cell potential monitoring respectively half-cell SOC estimation, is a simple and suitable tool for the identification of crossover direction and relative amount of crossover in VFB.