The utilization of intermittent renewable energy sources needs low-cost, reliable energy storage systems in the future. Among various electrochemical energy storage systems, redox flow batteries (RFBs) are promising with merits of independent energy storage and power generation capability, localization flexibility, high efficiency, low scaling-up cost, and excellent long charge/discharge cycle life. RFBs typically use metal ions as reacting species. The most exploited types are all-vanadium RFBs (VRFBs). Here, we discuss the core components for the VRFBs, including the development and application of different types of membranes, electrode materials, and stack system. In addition, we introduce the recent progress in the discovery of novel electrolytes, such as redox-active organic compounds, polymers, and organic/inorganic suspensions. Versatile structures, tunable properties, and abundant resources of organic-based electrolytes make them suitable for cost-effective stationary applications. With the active species in solid form, suspension electrolytes are expected to provide enhanced volumetric energy densities.

Sol–gel Ru_0.3Sn_0.7O₂ electrode coatings with crack-free and mud-crack surface morphology deposited onto a Ti-substrate are prepared for a comparative investigation of the microstructural effect on the electrochemical activity for Cl₂ production and the Cl₂ bubble evolution behaviour. For comparison, a state-of-the-art mud-crack commercial Ru_0.3Ti_0.7O₂ coating is used. The compact coating is potentially durable over a long term compared to the mud-crack coating due to the reduced penetration of the electrolyte. Ti L-edge X-ray absorption spectroscopy confirms that a TiO_x interlayer is formed between the mud-crack Ru_0.3Sn_0.7O₂ coating and the underlying Ti-substrate due to the attack of the electrolyte. Meanwhile, the compact coating shows enhanced activity in comparison to the commercial coating, benefiting from the nanoparticle-nanoporosity architecture. The dependence of the overall electrode polarization behaviour on the local activity and the bubble evolution behaviour for the Ru_0.3Sn_0.7O₂ coatings with different surface microstructure are evaluated by means of scanning electrochemical microscopy and microscopic bubble imaging.

Electrolyte chemistry is critical for any energy‐storage device. Low‐cost and sustainable rechargeable batteries based on organic redox‐active materials are of great interest to tackle resource and performance limitations of current batteries with metal‐based active materials. Organic active materials can be used not only as solid electrodes in the classic lithium‐ion battery (LIB) setup, but also as redox fluids in redox‐flow batteries (RFBs). Accordingly, they have suitability for mobile and stationary applications, respectively. Herein, different types of electrolytes, recent advances for designing better performing electrolytes, and remaining scientific challenges are discussed and summarized. Due to different configurations and requirements between LIBs and RFBs, the similarities and differences for choosing suitable electrolytes are discussed. Both general and specific strategies for promoting the utilization of organic active materials are covered.

A comparative investigation (oxyfluoride vs. oxide) demonstrates the positive impact of F⁻ incorporation on the cathode performance including capacity, voltage, hysteresis and Li⁺ diffusion.