(541d) Durability and Performance Of An Anion Exchange Membrane Separator For Electrochemical Energy Storage and Conversion Applications

All-vanadium RFBs (VRFB) are very interesting from an operational point of view because the intermixing of positive and negative solutions does not cause irreversible damage. In this work, we synthesized a novel AEM based on cardo-polyetherketone (PEK-C), and evaluated its stability under RFB working conditions. The membrane stability was evaluated by measuring the changes in the chemical structure and mechanical properties after immersion in vanadium (V) solutions (strongly oxidizing) for 1 month. Post-mortem analysis of the membrane using 2D NMR techniques confirmed that there was no degradation in chemical structure after such immersion. The vanadium (IV) membrane permeability of quaternized PEK-C/TMA⁺ membrane (8.2±0.2 ×10^-9 cm² s^-1) was 35 times lower than for Nafion^® membranes (2.9±0.2 ×10^-7 cm² s^-1) confirming the suitability of anion exchange membranes for RFB applications.  Water absorption isotherms (VTI-SA, TA instruments) showed moderate water uptakes (30wt%)  - this is a necessary requisite for adequate mechanical integrity during long term RFB operation. No changes were observed in the ultimate tensile strength of the membranes after 1 month immersion in the vanadium (V) solution (19 ± 2 and 21 ± 1 MPa respectively). Redox flow battery (5 cm² single cell) testing was carried out and the charge/discharge of the battery was performed at a current density of 100mA cm^-2 over 20 cycles initially. The coulombic efficiency was close to 100%, and the voltage and energy efficiencies were around 80% for every cycle. The stability of the anion exchange membrane under RFB operating conditions is a promising result that will facilitate widespread use of inexpensive AEMs as separators in RFBs.

Likewise, the same membrane chemistries can also serve as separators in anion exchange membrane fuel cells. Studies on this front are currently ongoing; the presentation will also discuss the efficacy and stability of the above membranes in alkaline fuel cell applications.