Sulfonated Lignin-Based Ionomer Biocomposite Membranes for Redox Flow Batteries

Author

Aiswarya Mechoor, Clemson UniversityFollow

Date of Award

5-2024

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical and Biomolecular Engineering

Committee Chair/Advisor

Eric M. Davis

Committee Member

Mark C. Thies

Committee Member

Mark E. Roberts

Abstract

The current benchmark membrane for vanadium redox flow batteries (VRFBs), Nafion™, accounts for up to 40% of the cost of the battery and suffers from poor ion selectivity, leading to diminished battery performance. Consequently, alternative ionomer membranes, such as sulfonated poly(ether ether ketone) (SPEEK), have recently garnered attention due to their lower cost and similar performance properties to Nafion. However, like Nafion, SPEEK also suffers from poor ion selectivity. In this study, we attempt to address the issue of poor ion selectivity by developing SPEEK–lignin composite membranes containing fractionated and functionalized lignin. Specifically, two lignin fractions – a low molecular weight (LMW) lignin and high molecular weight (HMW) lignin, which were obtained from fractionation of bulk softwood Kraft lignins, were functionalized with sulfonic acid groups. This was achieved by reacting the fractionated and purified lignin with sulfuric acid, thereby adding sulfonic acid groups onto both the α position of phenylpropanoid units and to the aromatic ortho position (relative to the hydroxide) in lignin.

To limit the number of factors that could impact membrane performance, one degree of sulfonation (DS 82%) of the SPEEK ionomer and two lignin concentrations (5 and 15 mass %) were selected. The DS of the SPEEK was measured by both titration and ¹H nuclear magnetic resonance spectroscopy, while the DS of the functionalized lignin was measured by elemental analysis. After fabrication, the permeability of vanadium ions – specifically the vanadyl ion (VO²⁺) – through the membranes was measured using ultraviolet-visible spectroscopy, where both the DS and lignin concentrations were seen to impact VO²⁺ permeability. Most notably, the VO²⁺ permeability was reduced by approximately three-fold with the addition of 15 wt % sulfonated LMW and HMW lignins. Accompanying this reduction in VO²⁺ permeability was an improvement in the ion exchange capacity of the ionomer membranes containing sulfonated lignin independent of the lignin MW. In addition, an increase in the equilibrium water uptake was also observed for SPEEK composites containing sulfonated lignin when compared to neat (or lignin-free) SPEEK. Finally, the dispersion of sulfonated lignin in the SPEEK matrix was analyzed using transmission electron microscopy (TEM). The TEM images showed low aggregation and good dispersion of sulfonated lignin with the SPEEK ionomer, which can be attributed to the similarity in sulfonic acid groups on the ionomer backbone and on the functionalized lignin. These results help elucidate the relationship between ionomer processing and the final salient performance properties of these green ionomer biocomposites, providing a pathway for developing the next generation of ionomer membranes for redox flow batteries.

Recommended Citation

Mechoor, Aiswarya, "Sulfonated Lignin-Based Ionomer Biocomposite Membranes for Redox Flow Batteries" (2024). All Theses. 4279.
https://tigerprints.clemson.edu/all_theses/4279