Energy storage is crucial for utilizing renewable energies like wind and solar. Aqueous zinc-iodine flow batteries (Zn-I FBs) are promising due to their safety, high theoretical specific capacity (268 Ah L⁻¹), and high energy density. However, the high cost of widely used fluorinated Nafion membranes limits their overall cost-effectiveness. While Nafion membranes offer high selectivity, improving Coulombic efficiency (CE), they also increase membrane resistance, reducing voltage efficiency (VE), energy efficiency (EE), and power density, especially at high currents. To address this, low-cost polyolefin-based porous membranes (LPPMs) have emerged as a potential alternative, offering high ionic permeability and low resistance. However, LPPMs lack selectivity, leading to active material crossover, capacity loss, and irreversibility, particularly when integrated with renewable energy sources and operating at high temperatures. Functional coatings on LPPMs can improve selectivity but increase ionic resistance and add to the cost. This research explores a novel approach: regulating the size of active materials in the electrolyte to enhance membrane selectivity. By leveraging the strong interaction between starch and iodine, the study hypothesizes that tunable, large-sized colloidal iodine-starch (IS) active species can prevent active redox loss through LPPMs, addressing the crossover issue.

Existing literature highlights the challenges and opportunities in aqueous Zn-I FBs. High energy density and safety are advantageous features, but the cost of Nafion membranes remains a significant barrier to large-scale adoption (Darling et al., 2014; Yuan et al., 2022). Research efforts have focused on membrane engineering and electrolyte modifications to enhance performance. Membrane modifications, such as designing new polymeric membranes, face challenges in precise pore-size modulation and cost (Machado et al., 2020; Xiong et al., 2021). LPPMs offer a cost-effective alternative, but their low selectivity necessitates strategies to mitigate polyiodide crossover (Park et al., 2016; Yuan et al., 2018). Previous work has investigated methods to improve Zn-I FB performance by addressing the polyiodide crossover issue through various strategies like improving membrane selectivity, modifying the electrolyte, or introducing protective layers on the membrane (Li et al., 2015; Zhao et al., 2013; Ma et al., 2021; Yang et al., 2022). However, these approaches often involve complex synthesis or increased costs. This study departs from these approaches by focusing on creating a colloidal system that addresses the crossover issue while maintaining cost-effectiveness and high performance.

The research developed colloidal chemistry for the iodine catholyte in Zn-I FBs using renewable and cost-effective starch. The iodine-starch (IS) active materials were regulated to form aggregated colloidal nanoparticles, balancing high ionic selectivity and conductivity. The study characterized the colloidal electrolytes via methods such as observing the Tyndall effect, measuring viscosity and ionic conductivity, and evaluating permeability across polypropylene (PP) membranes. Atomic force microscopy (AFM) determined nanoparticle sizes, while Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) examined the interaction between starch and iodine. Density functional theory (DFT) calculations were employed to investigate bonding energies and ionic transference numbers. Electrochemical performance was assessed using 2 × 2 cm² cells, measuring Coulombic efficiency (CE), voltage efficiency (VE), energy efficiency (EE), and power density at various current densities. Cycling performance was evaluated at high current density and volumetric capacity. Asymmetric flow batteries were studied to assess Zn anode reversibility and stability. The electrochemical performance was also tested at elevated temperatures (50 °C) to evaluate its high-temperature adaptability. Finally, a scaled-up flow battery module with photovoltaic panels demonstrated practical application and cost analysis compared the installed cost of a 1-MW flow stack using Nafion and PP membranes. Specific characterization techniques included UV-visible spectroscopy for permeability measurements, electrochemical impedance spectroscopy (EIS) and DC resistance analysis for impedance characterization, and SEM and XRD for surface morphology and composition analysis. DFT calculations used Gaussian 09 W and Multiwfn 3.3.8 software packages.

The study successfully demonstrated the effectiveness of starch-mediated colloidal chemistry in improving the performance of Zn-I FBs. Key findings include: 1. The formation of colloidal iodine-starch complexes was confirmed, exhibiting larger sizes (approximately 138.49 nm at 50% SOC) compared to bare starch (78.56 nm), facilitating the size-sieving effect. 2. The optimized 1 M starch and 2 M ZnI₂ electrolyte showed significantly reduced polyiodide permeability, as evidenced by reduced iodine crossover. 3. The Zn-I FBs with the starch-based colloidal electrolyte showed superior electrochemical performance, achieving a high power density of 41.58 mW cm⁻² (compared to 28.41 mW cm⁻² for the Nafion-based battery) and a high Coulombic efficiency of over 98%. 4. Long-term cycling stability was achieved, with over 350 cycles at 30 mA cm⁻² and a volumetric capacity of 6 Ah L⁻¹, and over 250 cycles at 22.5 mA cm⁻² and a high volumetric capacity (33.5 Ah L⁻¹, 50% SOC). 5. High performance was maintained even at 50 °C, with stable cycling over 200 cycles at 30 mA cm⁻² (CE of 98.9%) and a high volumetric capacity of 32.4 Ah L⁻¹ (50% SOC). 6. The starch-based electrolyte also improved Zn anode reversibility, mitigating side reactions and enhancing cycle life. 7. The cost analysis showed a substantial reduction (14.3 times) in the installed cost of a 1-MW flow stack by using the low-cost PP membranes instead of Nafion membranes. 8. The successful integration of a scaled-up flow battery module with photovoltaic panels validated the system's potential for practical renewable energy storage applications.

The results directly address the research question of improving the performance and cost-effectiveness of Zn-I FBs. The use of starch-mediated colloidal chemistry offers a novel and effective approach to mitigate polyiodide crossover, a major limiting factor in the performance of LPPM-based Zn-I FBs. The size-sieving effect achieved by the enlarged colloidal particles effectively prevents polyiodide crossover while maintaining high ionic conductivity, enabling the use of low-cost porous membranes. This approach not only enhances the electrochemical performance, including power density, Coulombic efficiency, and cycle life, but also significantly reduces the overall system cost. The findings are significant because they demonstrate a promising path towards commercially viable, large-scale energy storage solutions that are both efficient and cost-competitive. The superior performance at elevated temperatures further strengthens the practical applicability of this technology for outdoor and high-temperature environments.

This research successfully demonstrated a starch-based colloidal electrolyte approach to enhance the performance and reduce the cost of Zn-I FBs. The strategy effectively suppresses polyiodide crossover while maintaining high ionic conductivity, achieving high power density, Coulombic efficiency, and long-term cycling stability. Cost analysis highlights the significant economic advantages of using low-cost membranes. Future research could focus on further optimizing the starch-iodine colloid, exploring alternative low-cost membranes, and developing strategies for even larger-scale integration with renewable energy sources.

While this study demonstrates significant improvements in Zn-I FB performance, some limitations exist. The study mainly focuses on laboratory-scale testing, and further large-scale testing is necessary to validate the scalability and long-term reliability of the system. The impact of long-term operation on the stability of the starch-iodine colloid under varying environmental conditions also warrants further investigation. Furthermore, although the cost analysis highlights significant cost reduction, factors like manufacturing processes and material availability might affect the actual cost in large-scale production. Finally, a more detailed investigation of the interaction mechanisms between starch, iodine, and the membrane at a molecular level might further inform the optimization process.