Monitoring chemical processes in redox flow batteries employing in situ and in operando analyses†

Redox flow batteries (RFBs) are promising solutions for large-scale stationary energy storage due to their scalability and long cycle life. The efficient operation of RFBs requires a thorough understanding of the complex electrochemical processes occurring during charging and discharging. This review provides an overview and perspective of in situ and in operando analytical techniques to monitor RFBs. In more detail, these advanced techniques allow for real-time observation of redox reactions, ion transport, and electrode–electrolyte interactions under working conditions, offering insights into formation of intermediate species and mechanisms of electrolyte degradation, State-of-Charge (SoC), and ion crossover. By discussing the principles, capabilities, and limitations of techniques such as nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), ultraviolet-visible (UV-vis) spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), electrochemical impedance spectroscopy (EIS), tomography and radiography, mass spectrometry (MS), and fluorescence microscopy this review highlights the essential role of in situ and in operando approaches in advancing RFB technology.