At present, the traditional CF and GF product need to go through processes such as spinning, oxidation, carbonization, graphitization, surface treatment, and sizing. [7a,55] The process flow is various and complicated, and further performance Review

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The iron–chromium flow battery (ICFB), the earliest flow battery, shows promise for large-scale energy storage due to its low cost and inherent safety. However, there is no specific membrane designed that meets the special requirements of ICFBs. To match the harsh operation parameters of ICFBs, we designed and fabricated a composite membrane with high mechanical, chemical, and thermal stability. In the design, a commercial porous polyethylene membrane is selected as the framework material, offering high mechanical stability and reducing the cost. Meanwhile, the Nafion resin is filled in the pores of a porous membrane, which inhibits the transfer of redox-active ions and creates the proton channels via hydrophobic/hydrophilic phase separation. As a result, the composite membrane exhibits high conductivity, selectivity, and stability, especially with almost no swelling at high operating temperatures. Thus, an ICFB with the prepared membrane exhibits a coulombic efficiency of 93.29% at the current density of 80 mA cm−2 and runs stably for over 300 cycles. This work provides an easy method to fabricate high-performance and low-cost membranes specifically for ICFBs and has the potential to promote the development of ICFBs.

Vanadium flow batteries (VFBs) are well suited for energy storage due to the attractive features of high safety and long cycle life. Electrodes are a key component of a VFB, directly affecting the energy efficiency and power density of the battery. However, the carbon felt electrode commonly used suffers from poor hydrophilicity and electrochemical activity. Herein, a simple ammonium sulfate hydrothermal method was presented to improve the electrochemical activity and hydrophilicity of carbon felt. In the design, hydrophilic nitrogen‐containing and oxygen‐containing functional groups were introduced in the hydrothermal process, in favor of optimizing the hydrophilicity of carbon felt. The doping of nitrogen with negative charge density and the increasing defects promoted the electrochemical reactions of vanadium ions. Furthermore, the effects of the experimental parameters on the performance of the carbon felt and the battery were carefully studied. As a result, the VFB with 90‐1 nitrogen‐doped carbon felt exhibited the best performance; the energy efficiency achieved 87.34 % at the current density of 80 mA/cm2, which was 3.91 % higher than that with the original one.

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