The large-scale electrical energy storage (EES) technology is critical in increasing renewable energy utilization and achieving the peak carbon dioxide emissions before 2030. Among the EES technologies, vanadium redox flow battery (VRFB) is one of the most promising ones due to its excellent safety, good stability, long cycle life (20 years), and tremendous flexibility. However, the high capital cost of VRFBs significantly impedance its commercialization. Improving the power density and capacity retention rate of VRFBs is regarded as the most effective way to reduce capital costs.

Electrode working as the electrochemical reaction sites plays the dominant role in battery performance. Thus, firstly, we studied the essential effects of flow rates and electrode compression ratios on select...[ Read more ]

The large-scale electrical energy storage (EES) technology is critical in increasing renewable energy utilization and achieving the peak carbon dioxide emissions before 2030. Among the EES technologies, vanadium redox flow battery (VRFB) is one of the most promising ones due to its excellent safety, good stability, long cycle life (20 years), and tremendous flexibility. However, the high capital cost of VRFBs significantly impedance its commercialization. Improving the power density and capacity retention rate of VRFBs is regarded as the most effective way to reduce capital costs.

Electrode working as the electrochemical reaction sites plays the dominant role in battery performance. Thus, firstly, we studied the essential effects of flow rates and electrode compression ratios on selecting electrodes for VRFBs and proposed the working conditions-induced performance reversal phenomenon, which provides a significant reference for electrode selection for VRFBs. Then, we proposed a response time-based method to decouple the polarizations of RFBs. The results show that the ohmic polarization accounts for the most considerable voltage loss proportion ranging from 49.6%-78.8% with different Nafion series membranes and maintains almost the same under different current densities. Therefore, a highly efficient PBI membrane with high proton conductivity and ion selectivity is prepared through a secondary phosphoric acid-doping method, which exhibited a higher voltage and coulombic efficiency than that of Nafion 212 under 200 mA cm^-2.

Besides, the capacity decay rate also significantly influences the capital cost of the VRFBs in long-term cycling. Thus, we studied the capacity decay of the VRFBs with the PBI and Nafion 212 membranes and proposed an efficient valence regulation strategy, an electrolytes concentration strategy, and a membrane mixing strategy to suppress the capacity decay. The results show that the accumulated discharge capacities in 400 cycles were improved by 52.33%, 70.97%, and 141.54%, separately than that with conventional operations.

Keywords: Vanadium redox flow battery; polarizations; PBI membrane; suppress capacity decay; elevating average valance; electrolyte concentration