(2jk) Sulfonated Ionomer Composite Membranes for Use in Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) have garnered considerable attention as scalable energy storage technology for their flexible design, fast response, and relatively low maintenance. A critical component in VRFBs is proton exchange membrane (PEM), which significantly affects battery efficiencies as well as its cost. Among all types of membrane materials, a perfluorosulfonic acid polymer, Nafion, is the current benchmark sulfonated ionomer that is used most widely as PEM for VRFBs. With a hydrophobic, poly(tetrafluoroethylene)-based backbone and side chains terminated with hydrophilic sulfonic acid groups, Nafion forms a nanophase segregated morphology when hydrated, where the hydrophilic domains provide a continuous pathway for water and ion transport. However, with high proton conductivity as well good mechanical stability, Nafion has suffered from high vanadium ion crossover rate, leading to reduced battery coulombic efficiency and lifetime. To address this issue, in our work, silica nanoparticles (SiNPs) have been introduced into Nafion as nanofiller, where SiNPs functionalized with different surface chemistry were incorporated into polymer matrix at different loading, where significant reduction in vanadium ion permeability was observed with membranes containing neat SiNP (hydroxyl groups) and amine functionalized SiNP. To better understand how vanadium ion permeability was tuned by SiNPs, vanadium ion diffusivity in both pristine and SiNP incorporated Nafion membranes were studied with quasi-elastic neutron scattering (QENS), where jump diffusion of vanadium ion was observed while the diffusion coefficient decreased with increasing nanoparticle loading, which is consistent with the reduced permeability. While SiNP containing Nafion membranes demonstrated decreased vanadium ion crossover rate, the high cost of Nafion itself still prevents the further application. Therefore, sulfonated poly(ether ether ketone) (SPEEK)-based composites have emerged as a promising class of PEM for VRFBs due to their low cost and comparable proton conductivity to that of Nafion. In our work, SPEEK with different degree of sulfonation (DS) was prepared via sulfonation reaction of PEEK as well as SiNPs modified with amine, acid, hydroxyl surface were introduced into SPEEK polymer matrix to tune transport properties of the composite membrane. In addition to regular inorganic nanoparticles, lignin, a by-product of the paper industry was studied as a filler in SPEEK based membranes, as lignin has an abundance of functional groups (i.e., hydroxyl, carboxylic acid) that interact with the SPEEK polymer matrix and therefore affect the nanostructure of the composite membranes. Notably, SPEEK membranes containing 5 mass % lignin exhibited proton conductivities that were approximately 90% and 110% greater than neat SPEEK and Nafion, respectively, while significant decrease was observed for vanadium ion permeability. Further, the property-structure interplay of SPEEK-lignin membranes was systematically investigated, where lignin molecular weight and loading have been varied and structural study of these membranes were conducted with transmission electron microscopy (TEM) and small-angle neutron scattering (SANS), helping establish a better understanding of the fabrication parameter – structure – property relationship. Along with the structural study, water transport kinetics in these membranes have been studied using in situ time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR) spectroscopy. A three-element viscoelastic relaxation model was applied to capture the water-induced swelling dynamics of the ionomer nanocomposites, while a combined diffusion-relaxation model was used to determine water diffusion coefficient in these membranes. The relaxation time constant was seen to decrease markedly after the introduction of lignin, where this decrease was seen to be proportional to the lignin concentration, indicating a stiffening of the ionomer network while water diffusion coefficient was also observed to decrease with the introduction of lignin. With property characterizations, structural analysis as well as kinetic studies, this systematic investigation of sulfonated ionomer composite membranes will contribute to development of novel membranes with improved performances.

Through my research in phD, I have gained expertise in membrane fabrication, membrane property characterizations (i.e., permeability conductivity.), and transport property analysis. Along with this, I have gained expertise in nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), small-angle neutron scattering analysis, and quasi-elastic scattering analysis of soft materials.

Research Interests

My research interests involve polymer membrane for fuel cells and redox flow batteries. I strive to expand my membrane background into electrochemistry studies, such as designing membranes for batteries and conducting battery performance test, where the effect of membrane on battery properties (i.e., coulombic efficiency and energy efficiency) can be discussed from membrane and transport perspectives, which can eventually contribute to development of energy storage technology with better properties.