In Situ and Operando Studies as Mechanistic Toolbox for Redox Flow Batteries, Redox Targeting, and CO ₂ Capture Systems: Progress and Perspective

As renewable energy deployment accelerates, safe and scalable long-duration energy storage (LDES) becomes increasingly critical. Redox flow batteries (RFBs) offer intrinsic nonflammability and decoupled energy–power architectures, yet multihour operation and multidecade lifetimes demand precise and state-resolved diagnostics. These challenges are intensified for organic redox-active species and emerging architectures such as redox-targeting flow batteries and electrochemical CO 2 capture systems. This perspective highlights recent advances in in situ and operando spectroscopic and electrochemical tools, including UV–vis–NIR, NMR/EPR, vibrational spectroscopy, X-ray technique, fluorescence microscopy, neutron imaging, ultramicroelectrodes, and SECM, that resolve electrolyte speciation, transient intermediates, aggregation, and interfacial kinetics. These methodologies establish a mechanistic framework for rational electrolyte design and next-generation RFB development.