(103g) A Novel Sulfonated Aromatic Polymer Membrane with Different Pendant Groups for Vanadium Redox Flow Batteries (VRFBs)

A novel sulfonated aromatic polymer membrane with different pendant
groups for vanadium redox flow batteries (VRFBs)

Tongshuai Wang¹, Junyoung Han²,
Kihyun Kim², Janice Lee¹, Chulsung Bae²*, and
Sangil Kim¹*

¹Department of
Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA

²Department of Chemistry & Chemical Biology, Rensselaer Polytechnic
Institute, Troy, NY 12180, USA

Vanadium
redox flow batteries (VRFBs) have received extensive attention due to its
attractive features for the large-scale energy storage because of their design
flexibility, high energy efficiency, absence of intercalation/deintercalation
and stress build-up in electrodes, and safe operation. One of the most critical components in VRFB is an ion exchange
membrane (IEM) which can prevent cross mixing of the multivalence vanadium ions, while it still
allows the transport of protons (or anions) to complete the circuit. However,
the trade-off relationship of proton exchange membrane (PEM) limits the energy efficiency
(EE) of the VRFB due to low coulombic efficiency (CE) caused by vanadium ion
cross over through the membrane or high membrane resistances. Anion exchange
membrane (AEM) suffers from poor chemical stability and low voltage efficiency
due to low ion conductivity.

In this
study, we synthesized a series of novel sulfonated aromatic polymer membranes
having high proton conductivity as well as proton selectivity.
Vanadium/hydronium ion permeability, proton conductivity, area resistance, and
ion exchange capacities of the membranes were measured. The single cell VRFB
performance with the prepared membranes was evaluated and compared with the one
using a commercial Nafion 212 membrane. The effect of
the pendent group structure on the membrane ion selectivity and battery
performance was also discussed.

It was found that the incorporation of aromatic structures on
the pendent groups effectively improved the ion selectivity of membranes which
can overcome the limitations of the trade-off relationship between the proton
conductivity and selectivity. The membrane with aromatic pendent groups (BP-Ar-SH) shows 7 times higher vanadium/proton ion selectivity
than that without aromatic groups (BP-SH) (5.18 vs. 0.75) and 3 times higher than Nafion
212 (5.18 vs. 1.78). As a result, the
VRB single cell equipped with the BP-AR-SH membrane shows significantly better
performance in comparison to the cell with Nafion
membranes, as high as 90.79% EE (vs. 79.96% EE of Nafion
212) at 40 mA/cm². The cycle stability test shows that efficiencies
of the membrane remain almost unchanged after running more than 50 cycles. Our
results strongly suggest that the BP-Ar-SH membrane
may provide a breakthrough toward the development of high-efficiency VRFB
systems.