With the increasing need to seamlessly integrate renewable energy with the current electricity grid, which itself is evolving into a more intelligent, efficient, and capable electrical power system, it is envisioned that energy‐storage systems will play a more prominent role in bridging the gap between current technology and a clean sustainable future in grid reliability and utilization. Redox flow battery technology is a leading approach in providing a well‐balanced solution for current challenges. Here, recent progress in the research and development of redox flow battery technology, including cell‐level components of electrolytes, electrodes, and membranes, is reviewed. The focus is on new redox chemistries for both aqueous and non‐aqueous systems.

exceptional design fl exibility, excellent scalability and modularity, and high energy effi ciency. These technical merits underline RFBs a well-suitable choice to stabilize the power grid and overcome the intermittency of renewable energy sources (e.g., solar, and wind).Aqueous redox fl ow batteries (ARFBs), because of safety operation and high power density, have attracted both governmental and industrial investments for technical development and applications. Currently, vanadium (V) based ARFBs are the most populated systems and have been commercialized by numerous companies. [ 1a,b ] However, high cost and volatile supply of V 2 O 5 raw material lead to high system capital cost and thus limit wide implementation of the vanadium ARFBs. [ 2 ] Current capital cost for V-ARFB is ≈$450/kWh according to a cost analysis [ 3 ] while DOE target cost is below $150/kWh. Therefore, there are increasing efforts to identify new fl ow battery systems with affordable material cost and high electrochemical performance to replace vanadium ARFBs. Hybrid nonaqueous redox fl ow batteries (NRFBS) using Li metal as solid-state anode [ 4 ] and various solution catholytes (e.g., anthraquinone, [ 5 ] ferrocene, [ 6 ] TEMPO, [ 7 ] ployhalides, [ 8 ] and polysulfi des [ 9 ] have emerged. Fullly organic NRFBs were also reported. [ 10 ] These NRFBs, albeit potential high energy densities, encounter a number of challenges for practical applications: safety issues associated the use of highly reactive Li metal and fl ammable organic solvents, low current density due to Li dendrite formation, and limited cycling life.To overcome the cost and sustainable issues and retain safety features and high power density of vanadium ARFBs, redox active quinonoid molecules have been employed in several acidic ARFBs in the last few years. In 2009, Xu et al. fi rst reported the concept of the organic ARFB by adopting 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) or 1,4-dihydrobenzoquinone-2-sulfonic acid (BQS) as cathode and conventional PbSO 4 as anolyte in an acid ARFB. In 2014, Aziz and co-workers reported an ARFB study using anthraquinon-2,7-disulfonic acid (AQDS) as anolyte and bromine as catholyte. [ 12 ] The AQDS/Br 2 ARFB can be operated at impressively high current densities (>0.5 A cm −2 ), highlighting the possibility of using organic redox active materials to generate high power output. However, the use of Br 2 is concerned with Increasing worldwide energy demands and rising CO 2 emissions have motivated a search for new technologies to take advantage of renewables such as solar and wind energies. Redox fl ow batteries (RFBs) with their high power density, high energy effi ciency, scalability (up to MW and MWh), and safety features are one suitable option for integrating such energy sources and overcoming their intermittency. However, resource limitation and high system costs of current RFB technologies impede wide implementation. Here, a total organic aqueous redox fl ow battery (OARFB) is reported, using low-cost and sustainable met...

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A TEMPO-based non-aqueous electrolyte with the TEMPO concentration as high as 2.0 m is demonstrated as a high-energy-density catholyte for redox flow battery applications. With a hybrid anode, Li|TEMPO flow cells using this electrolyte deliver an energy efficiency of ca. 70% and an impressively high energy density of 126 W h L(-1) .