Hydrocarbon membranes with high selectivity and enhanced stability for vanadium redox flow battery applications: Comparative study with sulfonated poly(ether sulfone)s and sulfonated poly(thioether ether sulfone)s

Robust Adamantane-Based Membranes with Enhanced Conductivity for Vanadium Flow Battery Application

Membranes with high conductivity, high selectivity, and high stability are urgently needed for high-power-density vanadium flow batteries (VFBs). Enhancing membrane conductivity presents many challenges, often resulting in sacrificing membrane selectivity and mechanical strength. To overcome this, new robust adamantane-based membranes with enhanced conductivity are constructed for VFB. Low-content basic piperazine (IEC = 0.78 mmol g⁻¹) and hydrophilic hydroxyl groups are introduced into highly rigid, hydrophobic adamantane containing poly(aryl ether ketone) backbone (PAPEK) and then selectively swelled to induce microphase separation and form ion transport pathways. The highly rigid and hydrophobic PAPEK exhibits high swelling resistance and provides the membranes with slight swelling, high selectivity, and high mechanical strength. The selective swelling temperature has a significant influence on the areal resistance of the resulting membrane, e.g., the PAPEK-130 membrane, when selectively swelled at 130 °C, has low areal resistance (0.22 Ω∙cm²), which is approximately two-fifths that of the PAEKK-60 membrane (treated at 60 °C, 0.57 Ω∙cm²). Consequently, the resulting PAPEK membranes exhibit low swelling, high selectivity, and low areal resistance, with the VFB constructed with a PAPEK-90 membrane exhibiting excellent energy efficiency (91.7%, at 80 mA∙cm⁻², and 80.0% at 240 mA∙cm⁻²) and stable cycling performance for 2000 cycles.

Amphoteric-Side-Chain-Functionalized "Ether-Free" Poly(arylene piperidinium) Membrane for Advanced Redox Flow Battery

To solve the stability issue of cost-effective nonfluorinated membranes, an ether-free poly(arylene piperidinium) (PBPip)-based membrane is first applied in redox flow batteries (RFBs). For improved efficiencies of RFB, amphoteric side chains are introduced onto the PBPip. Without an ether bond in the polymer backbone, the membrane shows a good stability in a strong oxidation environment. The Fourier transform infrared (FTIR) spectra exhibit no obvious changes over 30 days of oxidation test. Different from traditional blended amphoteric membranes, the amphoteric side chain allows both cation- and anion-exchange capacities to increase with grafting degree, which leads to a very high total ion-exchange capacity (IEC) (4.19 mmol g^-1). Outstanding ion-conduction ability (area resistance: 0.22 Ω cm²) comparable to Nafion 212 (0.24 Ω cm²) is consequently achieved. Ionic cross-linking structure between cationic and anionic groups results in a low swelling rate (13.9%). Combined with the repelling effect of positively charged piperidinium, a low VO²⁺ permeability (1.31 × 10^-8 cm² s^-1) is accomplished. On the basis of these good properties, the membrane exhibits excellent vanadium battery performances, especially at high current densities. The VE and EE both exceed 80% even at 200 mA cm^-2. The battery performances have no obvious reductions after 500 cycles. These results indicate that this work provides a new orientation to design the membrane for RFB.

Novel Triple Tertiary Amine Polymer-Based Hydrogen Bond Network Inducing Highly Efficient Proton-Conducting Channels of Amphoteric Membranes for High-Performance Vanadium Redox Flow Battery

A novel amphoteric membrane was designed by blending triple tertiary amine-grafted poly(2,6-dimethyl-1,4-phenylene oxide) (PPO-TTA) with sulfonated poly(ether ether ketone) (SPEEK) for vanadium redox flow batteries. An "acid-base pair" effect is formed by the combination of the tertiary amine group and sulfonic group, and extra nonbonding amine groups could be protonated. Both of them constitute a hydrogen bond network, which facilitates proton conduction and also hinders vanadium permeability because of the lowered swelling ratio and Donnan effect. All these contribute to improve the ion selectivity of the membrane while maintaining ionic conductivity. Compared with other amphoteric and SPEEK-based membranes, the membrane exhibits an excellent performance. The amphoteric membrane containing 15% PPO-TTA exhibits an ultralow vanadium permeability of 3.4 × 10^-9 cm² s^-1 and a low area resistance of 0.39 Ω cm^-2. Consequently, the cell assembled with this membrane shows excellent performances far superior to SPEEK and Nafion 212. The Coulombic efficiency and energy efficiency of the cell are 94.3-98.3 and 90.3-77.1% at 40-200 mA cm^-2, respectively, and have no significant reductions after 200 cycles. This performance is at a high level among the amphoteric and SPEEK-based membranes reported in recent years. The cell's open circuit voltage is maintained for up to 165 h. In addition, the membrane's chemical stability is improved by the effective barrier to the vanadium ion.

Sulfonated component-incorporated quaternized poly(phthalazinone ether ketone) membranes with improved ion selectivity, stability and water transport resistance in a vanadium redox flow battery

Novel poly(phthalazinone ether ketone)-based amphoteric ion exchange membranes with improved ion selectivity, stability and water transport resistance were prepared for vanadium redox flow battery (VRB) applications. The preparation method ensured the absence of electrostatic interaction. A small amount of sulfonated poly(phthalazinone ether ketone) (SPPEK) with different ion exchange capacity (IEC) values was mixed with brominated poly(phthalazinone ether ketone) (BPPEK) to prepare base membranes with the solution casting method, and they were aminated in trimethylamine to obtain the resulting membranes (Q/S-x, x represents the IEC value of SPPEK). Compared with the AEM counterpart (QBPPEK) prepared from the amination of the BPPEK membrane, Q/S-1.37 showed lower swelling ratio and area resistance (R). The R value of Q/S-1.37 (0.58 Ω cm²) was close to that of Nafion115. The VO²⁺ and V³⁺ permeability values of Q/S-x were 96.7–97.6% and 98.5–99.2% less than those of Nafion115, respectively, demonstrating the excellent ion selectivity of Q/S-x. Compared with Nafion115 and QBPPEK, Q/S-1.37 displayed 90.0% and 92.1% decrease in the static water transport volume and 93.2% and 66.7% decrease in the cycling transport rate, respectively, revealing good water transport resistance. Compared with Nafion115, Q/S-1.37 exhibited an increase of 1.0–5.7% in the coulombic efficiency (CE) and an increase of 2.5–8.7% in the energy efficiency (EE) at 20–200 mA cm⁻². Q/S-x showed better chemical stability in VO₂⁺ solutions than QBPPEK. VRB with Q/S-1.37 could be steadily operated for 400 h without sudden capacity and efficiency drop, while VRB with QBPPEK could hold for only around 250 h. Q/S-1.37 retained higher CE, EE and capacity retention than Nafion115, displaying good long-term stability. Thus, the Q/S-x are promising for use in commercial VRBs.

Novel AIEMs were prepared through successive blending and amination processes, and they exhibited good ion selectivity, stability and water transport resistance.

Amphiprotic Side-Chain Functionalization Constructing Highly Proton/Vanadium-Selective Transport Channels for High-Performance Membranes in Vanadium Redox Flow Batteries

A novel amphiprotic side-chain-functionalized membrane was for the first time designed for vanadium redox flow battery (VFB). Different from frequently used blending amphiprotic membranes, the one proposed here is allowed to possess high anion-exchange capacity (IEC_a) without sacrificing the cation-exchange capacity (IEC_c) because both IEC_a and IEC_c increased with the grafting degree of side chains. Having a high IEC_a, the membrane prepared here exhibits an ultralow vanadium permeability (<10^-8 cm² s^-1), which leads to very high Coulombic efficiencies (97-98% at 40-200 mA cm^-2) of VFB and good cell self-discharge durability. Moreover, the high IEC_c contributes to a decent ionic conductivity (area resistance: 0.5 Ω cm^-2), which ensures a high-voltage efficiency of the cell. On the basis of these good properties, the VFB single cell with this membrane achieves a high energy efficiency (e.g., 77.4% at 200 mA cm^-2) that is higher than those of Nafion 212 and other reported amphiprotic membranes. These results indicate that the approach proposed here is an ideal option to prepare amphiprotic membranes for VFBs with high efficiency and good durability.

Hydrophilic Channel Alignment of Perfluoronated Sulfonic-Acid Ionomers for Vanadium Redox Flow Batteries

It is known that uniaxially drawn perfluoronated sulfonic-acid ionomers (PFSAs) show diffusion anisotropy because of the aligned water channels along the deformation direction. We apply the uniaxially stretched membranes to vanadium redox flow batteries (VRFBs) to suppress the permeation of active species, vanadium ions through the transverse directions. The aligned water channels render much lower vanadium permeability, resulting in higher Coulombic efficiency (>98%) and longer self-discharge time (>250 h). Similar to vanadium ions, proton conduction through the membranes also decreases as the stretching ratio increases, but the thinned membranes show the enhanced voltage and energy efficiencies over the range of current density, 50-100 mA/cm². Hydrophilic channel alignment of PFSAs is also beneficial for long-term cycling of VRFBs in terms of capacity retention and cell performances. This simple pretreatment of membranes offers an effective and facile way to overcome high vanadium permeability of PFSAs for VRFBs.

Polybenzimidazole/Nafion hybrid membrane with improved chemical stability for vanadium redox flow battery application

In order to increase the chemical stability of polybenzimidazole (PBI) membrane against the highly oxidizing environment of a vanadium redox flow battery (VRFB), PBI/Nafion hybrid membrane was developed by spray coating a Nafion ionomer onto one surface of the PBI membrane. The acid–base interaction between the sulfonic acid of the Nafion and the benzimidazole of the PBI created a stable interfacial adhesion between the Nafion layer and the PBI layer. The hybrid membrane showed an area resistance of 0.269 Ω cm² and a very low vanadium permeability of 1.95 × 10⁻⁹ cm² min⁻¹. The Nafion layer protected the PBI from chemical degradation under accelerated oxidizing conditions of 1 M VO₂⁺/5 M H₂SO₄, and this was subsequently examined in spectroscopic analysis. In the VRFB single cell performance test, the cell with the hybrid membrane showed better energy efficiency than the Nafion cell with 92.66% at 40 mA cm⁻² and 78.1% at 100 mA cm⁻² with no delamination observed between the Nafion layer and the PBI layer after the test was completed.

Novel polybenzimidazole (PBI)/Nafion hybrid membranes for the VRFB are made by spray coating a Nafion layer to protect PBI from chemical degradation.