Recent Advances for Electrode Modifications in Flow Batteries: Properties, Mechanisms, and Outlooks.

Flow batteries (FBs) have been demonstrated in several large-scale energy storage projects, and are considered to be the preferred technique for large-scale long-term energy storage in terms of their high safety, environmental friendliness, and long life, including all-vanadium flow batteries (VFBs) and Fe-Cr flow batteries (ICFBs). As the electrochemical reaction site, the electrode parameters, such as the specific surface area, active site, and so on, have a significant impact on the flow battery performance and reliability. Extensive research has been carried out on electrode modification to improve the current density and energy efficiency of the FBs. In this review, the reaction mechanisms of VFBs and ICFBs are discussed in detail firstly, and then the electrodes modification methods are overviewed and summarized from four aspects: self-modification, carbon-based electrocatalysts, metal-based electrocatalysts and composite electrocatalysts. Finally, the recent catalytic mechanism, in situ characterization technology, and future research directions are presented.

Boosting the Areal Capacity of Titanium-Manganese Single Flow Battery by Fe2+ /Fe3+ Redox Mediator.

Aqueous manganese-based flow batteries (AMFBs) have attracted great attention due to the advantages of low cost and environmental friendliness. Extending the cycle life of AMFBs has long been a challenging theme. The titanium-manganese single-flow batteries (TMSFB) are promising due to their special structure and electrolyte composition. However, TMSFB with high areal capacity faces capacity decay for unknown reasons. In this work, the capacity decay mechanism (accumulation and growth of MnO2 ) is clarified by a homemade in situ microscope system. Given that, a redox mediator of Fe2+ /Fe3+ is specially designed to boost the areal capacity of TMSFB without side reaction. The directional promoting principle of the Fe2+ /Fe3+ is elaborated in detail. Fe2+ chemically reacts with the residual MnO2 to form Fe3+ , which is reduced to Fe2+ by the electrochemical reaction. And then Fe2+ continues reacting with MnO2 until MnO2 is consumed completely. As a result, the TMSFB with the areal capacity of ≈55 mA h cm-2 can stably operate at a current density of 40 mA cm-2 , which is the highest areal capacity reported in aqueous manganese-based batteries. This work provides a new strategy for boosting the capacity of manganese-based batteries, shedding light on the improvement of other deposition-type batteries.