Construction of high-performance membranes for vanadium redox flow batteries: Challenges, development, and perspectives

A groundbreaking review published in Nano-Micro Letters provides a comprehensive overview of the role of advanced membranes in shaping the future of vanadium redox flow batteries (VRFBs). Authored by Tan Trung Kien Huynh, Jiaye Ye and Hongxia Wang from Queensland University of Technology, the review highlights the potential of next-generation membranes to overcome the limitations of commercial Nafion and unlock large-scale, long-duration energy storage.

Why This Research Matters
• Overcoming Nafion Limitations: As the VRFB industry targets >20-year lifespan and <$150 kWh^-1 system cost, commercial perfluorinated membranes suffer from high vanadium-ion crossover (10^-6–10^-7 cm² min^-1) and prices of US $500–1000 m^-2. Advanced membranes—via precise ion-selective nanochannels—emerge as the pivotal component to slash crossover, cut cost, and sustain cycle life beyond 10,000 cycles.
• Enabling Renewable Grid Integration: Beyond stationary storage, global utilities demand flexible, MW-to-GW scale batteries to firm solar and wind output. High-selectivity, low-cost membranes enable modular VRFB stacks that can be deployed in 4-hour to 12-hour duty cycles, meeting modern grid codes and frequency-response markets.

Innovative Design and Mechanisms
• Membrane Taxonomy for VRFBs: The review systematically analyzes ion-exchange membranes (CEM, AEM, AIEM), porous size-exclusion membranes, and ion-solvating membranes (ISM). Each class exploits distinct transport mechanisms—vehicle, Grotthuss, ion-sieving, and solvation coordination—to balance proton conductivity (>50 mS cm^-1) with vanadium rejection (>10⁵ S min cm^-3).
• Advanced Material Strategies: The study explores hierarchical structures such as sulfonated poly(ether ether ketone) (SPEEK) grafted with lignin, 2D MoS₂ nanosheets, UiO-66-NH₂@PWA metal–organic frameworks, and electrospun CNT nanofiber networks. These architectures create tortuous pathways that elongate vanadium-ion diffusion length while preserving continuous proton highways.
• 3D Integration and Green Chemistry: The review highlights solvent-free cross-linking, hot-pressing bilayers (PBI/Nafion), and biomass-derived additives (cellulose nanocrystals, lignin) to deliver mechanically robust (<15 % swelling) and chemically stable membranes (negligible degradation in 2 M VO₂⁺ + 3 M H₂SO₄ at 60 °C for 1000 h).

Applications and Future Outlook
• In-Battery Performance Validation: Lab-scale VRFB cells employing SPEEK/lignin composites achieve 99.5 % coulombic efficiency (CE) and 83.5 % energy efficiency (EE) at 120 mA cm^-2 over 500 cycles, outperforming pristine Nafion 117 (CE 92 %, EE 78 %). MIL-101-NH₂@PWA hybrids deliver an ion selectivity of 2.5 × 10⁵ S min cm^-3—three orders of magnitude above commercial benchmarks.
• Techno-Economic Pathways: Cost modeling shows that replacing Nafion with hydrocarbon-based membranes reduces stack CAPEX by 30–40 % while maintaining >80 % EE at 80 mA cm^-2. Scalable roll-to-roll slot-die coating and phase-inversion fabrication are identified as key enablers for GW-level manufacturing.
• Future Research Directions: Priorities include (i) standardized ASTM/ISO test protocols for vanadium permeability and oxidative stability, (ii) AI-guided polymer design to decouple conductivity–selectivity trade-offs, and (iii) cradle-to-gate LCA to quantify environmental benefits of bio-based membranes.

Conclusions
This review by Huynh, Ye and Wang provides a comprehensive roadmap for next-generation VRFB membranes. By elucidating ion-transport mechanisms, advanced material chemistries, and scalable fabrication routes, the work underscores the pivotal role of membrane innovation in accelerating the global transition to renewable, resilient, and cost-effective energy storage. Stay tuned as researchers translate these lab triumphs into multi-megawatt deployments across continents.