The size of the pore size on the diaphragm affects the Coulomb efficiency (CE) on one hand. The larger the pore size, the faster the flow rate of vanadium ions, causing direct contamination of the electrolyte through the porous membrane, thereby reducing CE. On the other hand, aperture size also affects voltage efficiency and antagonizes its impact on Coulomb efficiency. Compared with the PSF membrane with a content of 16 wt.% (inner pore size: 30 nm), protons find it difficult to pass through the ultra small inner pore size (10 nm) of the PSF membrane with a content of 20 wt.%, resulting in a decrease in voltage efficiency (VE). Therefore, by considering the optimal pore size function to obtain the optimal energy efficiency (EE=CE x VE), a 16 wt.% PSF membrane was used as a model membrane for further addition of GO nanosheets. From the subsequent results, it can be seen that the addition of graphene oxide nanosheets increases the thickness from 140 μ m to 300 μ m, thereby improving the Coulombic efficiency of the membrane. In addition, due to the carboxyl group and high conductivity of graphene oxide, adding graphene oxide nanosheets can also increase the voltage efficiency of the membrane. The Raman spectroscopy results observed characteristic peaks of graphene oxide, indicating that GO was successfully mixed into the 16 wt.% PSF membrane.
From the mechanical performance testing of polysulfone membranes with different contents of GO added, it can be seen that the mechanical properties are significantly improved after adding GO, and at 0.4wt.% When the optimal strain occurs, the subsequent increase in GO content will lead to a more brittle proton exchange membrane and a decrease in mechanical properties.

The author subsequently conducted thermogravimetric analysis on the prepared samples. The results show that the PSF porous membrane undergoes the first visible weight loss due to the decomposition of sulfonyl groups at 100-230 ℃, while the PSF/GO porous membrane can still maintain thermal stability, mainly because GO nanosheets are well dispersed in the PSF matrix, which can significantly improve thermal stability by controlling the fluidity of polymer chains. Moreover, the excellent thermal stability of the PSF/GO porous membrane results in incomplete degradation of the PSF porous membrane when the temperature increases, especially for the 0.6 wt.% membrane, which remains about 38% at 700 ° C, significantly higher than the original PSF membrane (30%).
Then, the author tested the vanadium ion permeability of the membrane, and in order to achieve high Coulombic efficiency and low self discharge rate, the membrane used in the VRFB system should have low vanadium ion permeability. The results showed that due to transmembrane transfer, the VO2+concentration in the right reservoir increased over time, and under the same conditions, the permeation rate of vanadium ions through the PSF membrane was much higher than that through the PSF/GO membrane (about 56%). The low permeability of vanadium ions in the modified membrane can be attributed to the blockage of surface pores by GO on the PSF membrane. In addition, the PSF/GO membrane has more - COOH groups, leading to channel filling blockage and slower migration rate of vanadium ions in the PSF/GO membrane.

Subsequently, the author used electrochemical impedance spectroscopy (EIS) to analyze the internal resistance of VRFB single cell batteries. The results showed that the introduction of GO into the PSF membrane significantly reduced the internal resistance, and the GO membrane with a content of 0.6 wt.% had the lowest internal resistance. This indicates that the hydrophilic properties of GO lead to more absorption of vanadium electrolyte into the battery, thereby reducing the internal resistance of the battery. However, when the amount of GO increases to 1 wt.%, due to the low conductivity of GO, the resistance increases. In order to improve the electrochemical reaction of graphene oxide (GO), Cui et al. reported the preparation of graphene oxide (GO) nanosheets (GONPs) using an improved Hummers process. Compared with original graphene, GONPs exhibit good catalytic ability for V2+/V3+redox reactions due to the introduction of oxygen-containing groups. Especially for samples treated at 50 ℃ and vacuum environment, due to the highest content of hydroxyl and carboxyl functional groups, the catalytic performance for VO2+/VO2+and V2+/V3+redox reactions is the best.

Finally, the author tested the electrochemical performance of PSF/GO composite films. EE is a very important indicator for the electrochemical performance of all vanadium flow batteries, as it reflects the energy conversion efficiency of large energy storage devices, and the variation of EE depends on CE and VE. From the results, it can be seen that the Coulombic efficiency and overall energy efficiency of the membrane doped with GO are lower than those of Nafion membranes on the market, but the voltage efficiency is higher. In long-term testing, it shows the same performance as the exchange membrane on the market, which can maintain about 200 cycles. The CE, VE, and EE of 16 wt.% PSF porous membranes containing 0.4 wt.% or 0.6 wt.% GO are higher than those without GO, and can maintain CE at 96% during long-term testing. This is because the composition of GO nanosheets not only improves mechanical properties, but also stabilizes the pores inside the porous membrane, allowing the vanadium electrolyte to remain in the structure for a long time. The higher the CE, the less crossing of vanadium ions. In the PSF+0.6 wt.% GO membrane, the pores in the PSF membrane are blocked by GO nanosheets, thereby reducing the permeability of VO2+.

In addition to ion exchange membranes (IEMs) such as cation exchange membranes (CEMs), anion exchange membranes (AEMs), and zwitterionic ion exchange membranes (AIEMs), non ionic porous (microporous or nanoporous) membranes have been developed for VRFB. Porous membranes hinder the transport of vanadium ions, but allow protons to be transported through the pore size exclusion effect. Compared to commercial IEMs such as Nafion membranes, porous membranes have much lower costs, making them an excellent alternative to expensive IEMs. In this study, GO nanosheets were uniformly filled in the PSF matrix to form a composite film of PSF/GO. The unique two-dimensional structure and - COOH functional group of GO nanosheets make them well compatible with PSF matrix, while also improving the performance of VRFB system.

In summary, this work successfully prepared PSF/GO porous nanohybrid membranes using different proportions of PSF and GO nanosheets. As the concentration of PSF increases, the pore size decreases, and smaller pore sizes lead to higher CE but lower VE. Therefore, the optimal aperture is 16 wt.% PSF, which exhibits excellent overall EE. After adding GO nanosheets, the mechanical properties were improved, the internal structure of the porous membrane was strengthened, the conductivity increased, and a larger EE was obtained. After doping 0.6 wt.% GO nanosheets, the CE could be maintained at 96%, indicating high stability.
At present, the commonly used membrane in VFRB is perfluorosulfonic acid proton exchange membrane, such as Nafion membrane, because perfluorosulfonic acid membrane has high proton conductivity and excellent chemical and electrochemical stability. However, the cost of Nafion series membranes used for large-scale VRFB systems is still very high, and modified Nafion membranes are not suitable for the commercialization of VRFB systems. Therefore, the wider application of VRFB urgently requires high-performance low-cost alternative membranes. Non ionic porous membranes have been studied for VRFB applications due to their low-cost characteristics. PSF/GO porous membranes have low costs and can separate vanadium ions from protons through pore size exclusion effects, resulting in low vanadium ion permeability and relatively high VRFB performance. Continuing to research and develop low-cost porous membranes is an important step in reducing the cost of all vanadium flow batteries and accelerating the commercialization of all vanadium flow batteries in the energy storage field. How to further improve the performance of porous membranes to reach or even exceed commonly used perfluorosulfonic acid exchange membranes is an important direction for future research on porous membranes.