As a potential carrier for large-scale energy storage technology, redox flow batteries (RFBs) have garnered significant attention in the fields of renewable energy grid integration and grid peak shaving, thanks to their unique characteristics of power-capacity decoupling and long cycle life. The selection and performance optimization of the core component, the membrane material, directly determine the efficiency, lifespan, and cost of the battery. Currently, the mainstream technological routes include all-vanadium flow batteries, zinc-based flow batteries, and organic flow batteries, each with distinct requirements for membrane materials. The perfection of technological innovation and validation systems has become the key to breaking through industry bottlenecks.
I. All-Vanadium Flow Batteries: Balancing Anti-Vanadium Permeation and Durability
Material Selection: Nafion series perfluorosulfonic acid membranes have traditionally been the first choice, with an ionic conductivity as high as 85 mS/cm. However, they suffer from high vanadium ion permeability (>10?7cm2/s) and high cost (nearly ￥20,000 per square meter). Improvement directions focus on:
Filling Modification: Utilizing SiO? nanoparticles (as studied by Xi et al.) or organic-inorganic hybrid layers (e.g., ORMOSIL) to fill the hydrophilic clusters in Nafion, reducing vanadium permeability by a factor of 7.
Surface Engineering: Constructing a positively charged barrier layer on the surface of Nafion through polyethyleneimine (PEI) interfacial polymerization (as studied by Luo et al.), using electrostatic repulsion to inhibit the migration of high-valence vanadium ions.
Non-Fluorinated Alternatives: For example, the quaternized polysulfone/PVDF composite membrane developed by Harbin Institute of Technology (as reported in Ren Jing's thesis); and the non-fluorinated ion exchange membrane series developed jointly by ZH Energy and Central South University, including PBI and SPEEK flow battery membranes.
Validation System:
Anti-Vanadium Oxidation Test: Immersing membrane samples in a 3 mol/L H?SO? solution containing V(V) for 7 days, and detecting the concentration changes of V(IV) using a UV spectrophotometer. If the absorbance is higher than the detection limit (usually <0.01 A), the membrane is deemed to have degraded.
Chemical Stability Evaluation: Using Fenton reagent (H?O?/Fe2?) for accelerated oxidation testing, with a membrane mass loss rate of <5% and an ionic conductivity decay of <10% as the pass criteria.
Electrochemical Validation: Testing according to NBT 42081 single-cell standards, requiring an energy efficiency of >75% at 100 mA/cm2, and a capacity decay of <15% after 2000 cycles.
II. Zinc-Based Flow Batteries: Dual Challenges of Zinc Dendrite Suppression and Ion Sieving
Material Selection: Alkaline zinc-iron flow batteries must withstand strong alkaline corrosion (pH >14) and the risk of zinc dendrite penetration. Porous membranes, with a cost advantage of <￥500 per square meter, have become mainstream:
Polyether Sulfone/Polyacrylonitrile-Based Asymmetric Membranes: With a skin layer pore size of 50-150 nm (as reported in Liu Zaichun's thesis), they utilize size sieving to block Fe(CN)?3?/?? (diameter ～1.2 nm).
Chitosan-Modified Layer: In a patent from the Dalian Institute of Chemical Physics, a 20-μm chitosan skin layer guides the uniform deposition of Zn(OH)?2? through hydroxyl complexation, increasing the battery cycle life from 20 to over 150 cycles.
Three-Dimensional Cross-Linked Structure: For example, the PBI/polyether sulfone composite membrane in CN111261913A, with a bending modulus >2 GPa, has an anti-penetration strength three times higher than that of Nafion.
Evaluation Methods:
Dendrite Suppression Capability: Observing the morphology of zinc deposition using SEM, with surface roughness controlled at <50 nm (data from CN111261913A).
Ion Selectivity: Using a Zn2?/OH? migration number ratio >500 as the benchmark (compared to ～10 for Nafion membranes), with a coulombic efficiency >98%.
Mechanical Testing: Applying a cyclic pressure of 2 MPa, with a membrane thickness change rate <10% (according to GB/T32509-2016).
III. Organic Flow Batteries: Innovation in Molecular Sieving and Confined Mass Transfer
Material Evolution: Targeting active molecules such as quinoline derivatives (e.g., BQDS/TEMPO) (as studied by Yang Dawei), traditional ion exchange membranes face the bottleneck of high permeability (>10?8cm2/s). New paradigms include:
Intrinsic Microporous Membranes (PIMs): The trisubstituted PIM membrane developed by Tsinghua University (as reported by Yang et al.), with a pore size of 0.8-1.2 nm, has a quinoline permeability coefficient as low as 3×10?11cm2/s, while maintaining a proton conductivity of 103 mS/cm.
Microporous Framework Membranes: For example, the covalent organic framework (COF-DQTB) with rigid channels (0.7 nm × 0.9 nm) achieves sub-angstrom-level ion sieving, with an energy efficiency breakthrough of 87% (as reported by Zuo et al.).
Zwitterionic Membranes: The SPEEK/PAES composite membrane from Toray Industries (as reviewed by Peng Kang), with a Zeta potential of +15 mV, can simultaneously repel quinone/phenothiazine molecules from both the positive and negative electrodes.
Key Indicators:
Electroactive Substance Permeability Coefficient: Must be <1×10?12cm2/s (verified by Xe adsorption and BET specific surface area testing).
Surface Resistance Control: Less than 1 Ω·cm2 in a 1 mol/L supporting electrolyte to avoid voltage efficiency loss due to ionic conduction losses.
Long-Term Stability: Continuous operation for 500 hours at an oxidation potential of 1.5 V, with a sulfonic acid group retention rate >95% analyzed by FTIR.
The core demands for membranes in the three systems show a gradient difference (see the figure below). The all-vanadium system focuses on chemical stability, the zinc-based system emphasizes mechanical strength, and the organic system pursues extreme sieving capability. It is worth noting that composite modification has become a common direction: for example, the Nafion/GO membrane (as studied by Lee et al.) reduces the surface resistance in all-vanadium batteries to 0.53 Ω·cm2, while the chitosan/ceramic composite membrane (CN201611088069.X) increases the ion selectivity ratio by 20 times in the zinc-based system. In the future, AI-assisted molecular simulations (such as DFT calculations in Chen Yuning's thesis) to optimize pore structures will accelerate the iterative process of high-performance membranes.
In response to the differentiated validation needs of multiple technological routes and core material systems in flow batteries, the ZH Energy 's self-innovated flow battery experimental testing platform (bench-top type) is built based on mature flow battery energy storage system integration technology. It features modular design, easy disassembly and assembly, and contains various monitoring sensors, meeting the testing requirements for materials or processes. It is suitable for research institutions and manufacturers to test flow battery materials (membranes, bipolar plates, electrodes, electrolytes, etc.), as well as cell stacks or systems.
Currently, the ZH Energy flow battery experimental testing platform has been successfully delivered to manufacturers and research institutions in the fields of all-vanadium, iron-chromium, all-iron, organic, and other flow batteries. It has helped customers shorten their R&D cycles and pushed multiple technologies towards the commercialization stage, earning widespread recognition from clients.
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