Redox flow batteries are a critical technology for large-scale energy storage, offering the promising characteristics of high scalability, design flexibility and decoupled energy and power. In recent years, they have attracted extensive research interest, with significant advances in relevant materials chemistry, performance metrics and characterization. The emerging concepts of hybrid battery design, redox-targeting strategy, photoelectrode integration and organic redox-active materials present new chemistries for cost-effective and sustainable energy storage systems. This Review summarizes the recent development of next-generation redox flow batteries, providing a critical overview of the emerging redox chemistries of active materials from inorganics to organics. We discuss electrochemical characterizations and critical performance assessment considering the intrinsic properties of the active materials and the mechanisms that lead to degradation of energy storage capacity. In particular, we highlight the importance of advanced spectroscopic analysis and computational studies in enabling understanding of relevant mechanisms. We also outline the technical requirements for rational design of innovative materials and electrolytes to stimulate more exciting research and present the prospect of this field from aspects of both fundamental science and practical applications.
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G.Y. acknowledges the financial support from the Welch Foundation award F-1861, Camille Dreyfus Teacher-Scholar Awards and Sloan Research Fellowship. R.F. and W.W. acknowledge the financial support from the US Department of Energy, Office of Electricity (under contract no. 70247) and Energy Storage Materials Initiative, which is a Laboratory-Directed Research and Development project at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogramme national laboratory operated by Battelle for the US Department of Energy under contract DE-AC05-76RL01830.
The authors declare no competing interests.
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Similar to regeneration, the recovery of active species following membrane crossover.
- Electrode passivation
A decrease in active electrode surface area and an increase in contact and charge transfer resistance. Passivation is generally caused by the deposition and accumulation of side products on the surface of the electrode.
- Hydrotropic effect
The use of suitable organic additives (for example, urea) to help increase the solubility of redox-active solute in aqueous solution through intermolecular interactions (for example, hydrogen bonding).
- Capacity utilization
The percentage of delivered capacity over theoretical capacity.
- Crossover effect
An effect, including, for example, capacity decay and side reactions, that occurs as a result of penetration of the redox species through the membrane.
- Capacity fade rate
The loss in capacity, usually expressed as a percentage, over the total cycle number (% per cycle) or test time (% per day).
The recovery of active species after crossover without sacrificing capacity, which is similar to rebalancing.
- Symmetric cell
A symmetric cell uses the same redox species (but exploits different oxidation states) on both sides of the cell. In the unbalanced configuration, using excess charge for the counter electrolyte minimizes its negative impact on the cell capacity decay.
- Asymmetric cell
The asymmetric full-cell configuration uses different active materials with suitable potential difference in the catholyte and the anolyte, respectively. In a charge-unbalanced cell, one is the working electrolyte (limited charge) and the other serves as the counter electrolyte (excess charge).
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Zhang, L., Feng, R., Wang, W. et al. Emerging chemistries and molecular designs for flow batteries. Nat Rev Chem 6, 524–543 (2022). https://doi.org/10.1038/s41570-022-00394-6
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