Development of Polybenzimidazole/Sulfonated Nanolignin Composite Membrane for PEM Redox Flow Battery
Authors: Brito dos Santos Fernanda , McMichael Philip Spencer , Whitbeck Alex , Jalaee Adel , Gyenge Elod L. , Foster Johan 
Published: December 2024

Renewable energy is a key component to the global decarbonization strategy, which requires green alternatives to conventional energy sources along with progress in governmental regulations and technological advances. In this context, electrochemical cells are critical enabling technologies for energy management, conservation, and storage, as well as pollution control/monitoring and greenhouse gas reduction. Redox flow batteries (RFBs) are gaining popularity in various energy storage applications, and such devices rely on proton exchange membranes (PEMs). Commercial PEMs are often perfluorinated polymers like Nafion®, which has a hydrophobic tetrafluoroethylene backbone interlaced with hydrophilic sulfonic acid groups to imbue the polymer with mechanical and thermal stability while still providing the necessary proton conductivity. However, all PEMs suffer from certain limitations, often having limited chemical and thermal durability, dimensional instability, and negative ion and molecular (e.g., methanol, ethanol, etc.) crossover. Nafion has limited long-term stability, while costing over $500 USD per square meter, and most commercial PEMs for RFB applications are complex synthetic polymers derived from petroleum oils, which are extracted, isolated, and processed at a high environmental cost. Acid-doped polybenzimidazoles (PBIs) are potential alternatives to Nafion membranes and are considered as one of the most promising polymeric materials for use as PEMs. These acid- or alkali-doped PBI membranes have outstanding properties that allow them to be used as high-temperature PEMs up to 200 ◦C. Moreover, PBI is a relatively low cost non-perfluorinated polymer and exhibits excellent oxidative and thermal stability. However, the performance of high-temperature PEMs using pure PBI is still below the commercial requirements. Therefore, PBI composite membranes containing various multifunctional inorganic, organic, and hybrid fillers are being actively explored to produce high-efficiency PEMs. Alternative PEMs that utilize biobased materials, including lignin and sulfonated lignin (SL), low-cost byproducts of the wood pulping process, have struggled to balance electrochemical performance with dimensional stability. Herein, SL nanoparticles are demonstrated for use as a nature-derived, ion-conducting PEM material. SL nanoparticles (NanoSLs) can be synthesized for increased surface area, uniformity, and miscibility compared with macrosized lignin, improving proton conductivity. Our previous work showed that a NanoSL membrane can have an ion exchange capacity of 1.26 meq/g, above that of the commercial PEM Nafion 112 (0.98 meq/g), along with a conductivity of 80.4 mS/cm in-situ, above that of many biocomposite PEMs. NanoSL membranes also demonstrated a coulombic efficiency (CE), energy efficiency (EE) and voltage efficiency (VE) of 91%, 68% and 78% respectively at 20mA.cm-2. This present work seeks to synthesize a PEM using NanoSL and PBI to apply in a zinc iodine redox flow battery (ZIRFB). Here, our target is to understand how we can improve the ionic conductivity and hydrophilicity of PBI membranes by adding NanoSL into its molecular structure while retaining its thermal and mechanical properties. We produced negatively charged lignin nanoparticles and mixed it with PBI in different percentages (5, 7.5, 15, 20, and 25%). We also characterized these membranes in terms of their mechanical and thermal stability as well as their chemical, physical, and electrochemical properties. In general, the proton transportation in the membranes occurs by the interaction of the functional groups where the water molecules act as a bridge. Considering this, the mechanism and behavior of the ionic conductivity in different humidities and water uptake contents were also evaluated. Our study showed a promising perspective and understanding of the application of lignin in a PEM composition considering their environmental compatibility, renewable origin, and low gas crossover. These novel, biobased PEMs demonstrate the potential for valorization of forest biomass waste streams for high value clean energy applications and have potential for further tuning through optimization of support structure material, blend compositions, and synthesis methods.

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