In this work the degradation behavior of sulfonated polyimide-hydrogen type (SPI-H) membrane for vanadium redox flow battery (VRFB) usage was investigated through in-situ and ex-situ measurements. Morphological, mechanical and physico-chemical properties, together with structure information of the SPI-H membrane or its oligomer before and after degradation were characterized. Both optical photographs and SEM images show more serious destruction of surface morphology of SPI-H membrane during in-situ degradation than ex-situ one. Decrease in mechanical property and viscosity average molecular weight of SPI-H membrane after degradation in different immersion solutions verifies that both H+ and V(V) play important roles in accelerating the decomposition. FTIR results demonstrate that –SO3H groups of SPI-H membrane in VRFB aren’t destructed. 1H-NMR spectra verify the scission of polymer chain and the hydrolysis of imide ring of SPI-H membrane after degradation. XPS spectra evidence the oxidation of –NH2 groups of one hydrolysis product of SPI-H. According to all the experimental results, the 2-step degradation mechanism including hydrolysis and oxidation of SPI-H membrane in VRFB is proposed.

•Effect of non-sulfonated diamine monomer on bSPI membrane in VRFB is disclosed.•bSPI membranes own better VO2+ blocking performance than Nafion 117.•bSPI (BAPP) membrane has the highest proton selectivity among all membranes.•bSPI (BAPP) membrane has the highest chemical stability among all bSPI membranes.•bSPI (BAPP) membrane shows excellent performance in VRFB.Branched sulfonated polyimide (bSPI) membranes based on three different non-sulfonated diamine monomers were fabricated for vanadium redox flow battery (VRFB) usage. Both FT-IR and 1H NMR spectra confirm that bSPIs were successfully synthesized. TGA results show that bSPI membranes have good thermal stability. FESEM pictures indicate that the surface of membrane in contact with catholyte keeps better integrity than that in contact with anolyte after 500-time cycling charge-discharge test. The water uptake, ion exchange capacity, mechanical strength of bSPI membranes are larger than or close to those of Nafion 117 membrane, and bSPI membranes exhibit remarkably lower vanadium ion permeability compared with Nafion 117 membrane. In addition, bSPI membranes have outstanding chemical stability in contrast with linear SPI membrane in ex-situ degradation test. Further, the bSPI (BAPP) membrane displays the best chemical stability and highest proton selectivity (1.23 × 105 S min cm−3) among all bSPI membranes, thus it is seen as an optimum one for further single VRFB test. During 500-time cyclic charge-discharge test, the coulomb efficiency (CE: 97.1 ∼ 99.7%) of VRFB containing bSPI (BAPP) membrane is higher than that containing Nafion 117 (CE: 95.2 ∼ 98.6%) at 30 ∼ 120 mA cm−2. The VRFB using bSPI (BAPP) membrane presents higher or comparable energy efficiency (EE) and capacity retention compared with that using Nafion 117 membrane at low or high current density. Based on the excellent physico-chemical properties and single cell performance together with much lower cost compared with widely used Nafion 117 membrane, the as-prepared bSPI (BAPP) membrane has great potential to be applied in VRFB.Download high-res image (133KB)Download full-size image

We studied the relationship between the sulfonation degree (SD) of the sulfonated polyimide and the properties of sulfonated polyimide/chitosan (SPI/CS) composite membranes for application to a vanadium redox flow battery (VRFB). The SD of SPI was altered from 30 to 70%. The structure and morphology were characterized using attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIR) and atomic force microscope, respectively. The physicochemical properties of membranes were also measured. The as-prepared SPI30/CS membrane had the lowest proton conductivity and negligible vanadium ion permeability. The VRFB performance of the SPI30/CS membrane was acceptable and stable at 10 mA cm−2, but the charge–discharge process could not be completed at a higher current density, such as 20 and 30 mA cm−2, because of its proton conductivity being too low. Although the SPI70/CS membrane had the highest proton conductivity (4.88 × 10−2 S cm−1), the coulombic efficiency (CE) of the VRFB containing SPI70/CS was the lowest because of its highest vanadium ion permeability (10.47 × 10−7 cm2 min−1). In particular, the VRFB assembled with the SPI40/CS membrane underwent 100 charge–discharge cycles at 50 mA cm−2 and exhibited good stability in CE (99.3%) and energy efficiency (EE) (70.5%). Both SPI40/CS and SPI50/CS membranes are potential candidates for VRFB applications because of their high proton selectivity, good chemical stability and excellent VRFB performance.

A sulfonated poly(imide-siloxane) (SPI-PDMS) membrane was prepared and evaluated for its performance in a vanadium redox flow battery (VRFB). Fourier transform infrared spectroscopy analysis verified the successful synthesis of SPI-PDMS, and scanning electron microscopy images and energy-dispersive spectroscopy results illustrated the homogeneity of the membrane. The chemical stability of the SPI-PDMS membrane was superior to that of the pristine sulfonated polyimide membrane. The as-prepared SPI-PDMS membrane showed an order-of-magnitude lower permeability of VO2+ ion (1.92 × 10−7 cm2 min−1) in contrast to Nafion 117 (17.1 × 10−7 cm2 min−1) and it possessed a much longer self-discharge time (550 h above 1.3 V) than Nafion 117 (65 h above 1.3 V). The SPI-PDMS-containing VRFB exhibited higher coulombic efficiency (98.50%–99.10%) at current densities ranging from 20 to 80 mA cm−2 than that of Nafion 117 (80.6%–94.8%). At current densities lower than 70 mA cm−2, the energy efficiency of the SPI-PDMS-containing VRFB was higher than that of Nafion 117. Cyclic charge–discharge testing demonstrated that the VRFB containing the SPI-PDMS membrane had good operational stability. Overall, this low-cost SPI-PDMS membrane exhibits excellent battery performance and has considerable potential for applications in VRFBs.

A sulfonated polyimide (SPI)/TiO2 composite membrane was fabricated by a blend way to improve its performance in vanadium redox flow battery (VRB). Both EDS and XRD results verify the successful preparation of the SPI/TiO2 composite membrane. The surface SEM image shows its homogeneous structure. TG analysis identifies its thermal stability. The SPI/TiO2 composite membrane possesses much lower permeability of VO2+ ions (2.02 × 10−7 cm2 min−1) and favorable proton conductivity (3.12 × 10−2 S cm−1). The VRB single cell with SPI/TiO2 composite membrane shows higher coulombic efficiency (93.80–98.00 %) and energy efficiency (83.20–67.61 %) at the current density ranged from 20 to 80 mA cm−2 compared with that with Nafion 117 membrane. And the operational stability of the as-prepared composite membrane is good after 50 times of cycling tests. Therefore, the low-cost SPI/TiO2 composite membrane with excellent battery performance exhibits a great potential for application in VRB.

In order to probe the effect of the infiltration time with chitosan solution, a series of sulfonated polyimide/chitosan (SPI/CS) composite membranes with different infiltration time including 6 h, 12 h, 24 h and 36 h were prepared and applied in vanadium redox flow battery (VRB) system. SEM images demonstrate the presence of thin CS layer on the surface of SPI membrane, and the thickness of the CS layer increases with the infiltration time. The proton conductivity and the vanadium ion barrier property of SPI/CS composite membranes increase with the increment of the infiltration time. The SPI/CS-24 and SPI/CS-36 membranes show similar proton selectivity which is over eight times of that of Nafion® 117 membrane. VRB single cell using SPI/CS membrane reveals higher coulombic efficiency (CE) and energy efficiency (EE) than that using Nafion® 117 membrane. In particular, the CE and EE for SPI/CS-24 membrane achieve 97.8% and 88.6% respectively. All experimental results indicate that the SPI/CS composite membranes are promising proton conducting membranes for VRB, among which SPI/CS-24 membrane exhibits the best combination property.Highlights► Effect of infiltration time of CS solution on SPI/CS membrane was investigated. ► The SPI/CS-24 membrane has the best combination property for VRB usage. ► The VRB single cell using SPI/CS-24 membrane shows energy efficiency of 88.6%.

•Cs+ ions were removed by a four-chamber electrodeionization at constant current.•The optimum operation parameters were experimentally obtained.•The effluent Cs+ concentration in the dilute chamber is achieved below 0.05 mg L− 1.•The highest removal percentage of Cs+ is about 99.9%.The synthetic low radioactive wastewater containing Cs+ was treated by a four-chamber electrodeionization process at constant current. Effects of various operation parameters including electrical current, feed flow rate, the initial concentration and pH value, and the volume reduction factor are investigated respectively. The optimum variables are as follows: the electrical current is 0.14 A, the feed flow rate is 6.0 L h− 1, the initial Cs+ concentration is less than 50 mg L− 1, the initial feed pH is 7.0, and the volume reduction factor is lower than 40. Under these conditions, the effluent Cs+ concentration in the dilute chamber is below 0.05 mg L− 1, and the removal percentage of Cs+ can be achieved to be 99.9%.