Research background

The large-scale energy storage technology represented by vanadium flow batteries (VFB) has the advantages of high safety, long cycle life, minimal self discharge, and flexibility. It provides a practical and feasible method to utilize renewable energy such as wind energy, tidal energy, and solar energy to meet the requirements of sustainable development and environmental protection. Ionic conductive membrane (ICM) is one of the most critical components in liquid flow batteries, which plays a role in separating the active substances in the anode and cathode while conducting protons to complete the current circuit.

The ideal ICM of VFB needs to meet the requirements of low vanadium ion permeability, high proton conductivity, and excellent chemical durability to achieve efficient energy conversion. At present, perfluorosulfonic acid membranes (such as Nafion) are the most commonly used ICM for VFB due to their excellent conductivity and physicochemical properties, but there are still drawbacks such as high cost and severe vanadium crossover. Sulfonated polyether ether ketone (SPEEK), as a low-cost and highly ion selective non perfluorinated material, has broad application prospects in the next generation of ICMs. However, SPEEKs with high degree of sulfonation (DS) typically exhibit good proton conductivity, with reduced ion selectivity and mechanical properties. Another approach is to regulate the physicochemical properties of the membrane by hybridizing nanofillers. Traditional inorganic fillers, such as TiO2 and SiO2, can greatly improve the penetration resistance of vanadium ions and enhance ion selectivity. However, due to the prolongation of ion transport pathways, the conductivity of protons is inevitably limited. Porous materials with sub nanoscale channels, such as metal organic frameworks (MOFs), covalent organic frameworks (COFs), and zeolites, can provide internal intrinsic channels for selective ion transport and additional proton transfer pathways, which is beneficial for breaking the trade-off between ion selectivity and proton conductivity in ICMs.
Research Highlights

The author Xu et al. synthesized channel stable and chemically stable s-pCTF (sulfonated piperazine covalent triazine framework) through a one-step ultrasound method and modified it. Polar interconnected channels below 2-nm were formed to promote proton transport. The results showed that s-pCTF significantly improved the physicochemical properties of the membrane and battery performance. The battery with S/s-pCTF-3 (SPEEK contains 3% s-pCTF) membrane had the longest self discharge voltage retention time of 134.2 hours, outstanding single cell performance (EE was 92.41% to 78.53% at 40-200 mA cm-2), excellent long-term stability (EE was 88.2-85% at 120 mA cm-2900 cycles), and the best capacity retention, superior to Nafion212 and the original SPEEK membrane.

Figure 2

Under high current density, low vanadium crossover and short charge discharge time can lead to reduced capacity loss, and Coulombic efficiency (CE) increases with increasing current density. Meanwhile, overpotential and Ohmic polarization also increase with increasing current density, leading to a decrease in voltage efficiency and energy efficiency. S/s-pCTF-3 exhibits excellent CE at different current densities (97.4% at 40 mA cm-2 and 98.98% at 200 mA cm-2), which is superior to the original SPEEK film (92.25% at 40 mA cm-2 and 98.85% at 200 mA cm-2) and Nafion212 (96.66% at 40 mA cm-2 and 98.53% at 200 mA cm-2), indicating that the hybrid film has good inhibitory ability against vanadium ions (Figure 3a). Meanwhile, the VE values of S/s-pCTF-3 at 40 mA cm-2 and 200 mA cm-2 were 94.87% and 79.34%, respectively; The VE of SPEEK membrane at 40 mA cm-2 and 200 mA cm-2 were 93.58% and 63.8%, respectively; The VE values of Nafion212 at 40 mA cm-2 and 200 mA cm-2 were 93.3% and 73.72%, respectively. Ultimately, EE exhibited a similar trend to VE, with the S/s-pCTF-3 membrane exhibiting the best performance, with EE values of 92.41% and 78.53% at 40 mA cm-2 and 200 mA cm-2, respectively. A long-term cycling experiment was conducted at 120mA cm-2, and the battery with S/s-pCTF-3 membrane showed the best cycling stability in 900 cycles, with excellent EE (EE: 88.2-85.0%) and capacity retention ability, superior to the original SPEEK membrane (740 cycles, EE ≈ 79%), S/pCTF-3 membrane (820 cycles, EE ≈ 81%), and Nafion212 (530 cycles, EE ≈ 81%). After cycling, the morphology of the S/s-pCTF-3 membrane was intact, and no visible interface gaps were found, reflecting superior chemical stability and good filler matrix compatibility.

Figure 3

In addition, the vanadium ion permeability test results show that the vanadium ion permeability of the hybrid membrane is lower than that of the original SPEEK membrane. The self discharge voltage retention time of S/s-pCTF-3 membrane is the longest, reaching 134.2 h, which exceeds that of commercial Nafion212 membrane (26.5 h) and original SPEEK membrane (39.3 h), as shown in Figure 4, confirming that the filler has a good blocking effect on the crossing of vanadium ions, which is consistent with the Coulomb efficiency results. Moreover, in addition to having a higher vanadium ion resistance, the hybrid membrane also exhibits higher proton conductivity than the original membrane. Overall, the abundant external attraction of active sites and internal proton channels in s-pCTF are conducive to rapid proton selective transport. The "external internal" synergistic effect makes s-pCTF an ideal candidate for optimizing the ion selectivity of hybrid membranes and provides a reasonable explanation for superior single cell performance.

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