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.

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

Flow batteries (FBs) currently are one of the most promising large-scale energy storage technologies for energy grids with a large share of renewable electricity generation. Among the main technological challenges for the economic operation of a large-scale battery technology is its calendar lifetime, which ideally has to cover a few decades without significant loss of performance. This requirement can only be met if the key parameters representing the performance losses of the system are continuously monitored and optimized during the operation. Nearly all performance parameters of a FB are related to the two electrolytes as the electrochemical storage media and we therefore focus on them in this review. We first survey the literature on the available characterization methods for the key FB electrolyte parameters. Based on these, we comprehensively review the currently available approaches for assessing the most important electrolyte state variables: the state-of-charge (SOC) and the state-of-health (SOH). We furthermore discuss how monitoring and operation strategies are commonly implemented as online tools to optimize the electrolyte performance and recover lost battery capacity as well as how their automation is realized via battery management systems (BMSs). Our key findings on the current state of this research field are finally highlighted and the potential for further progress is identified.