(50a) Crosslinked Anion Exchange Membranes for Redox Flow Batteries

All-vanadium redox flow batteries (VRFB) have been very popular as they use the same electrochemical species in both electrode chambers and therefore they do not suffer from electrolyte cross contamination.  A core component of any RFB is the electrolyte membrane (PEM or AEM) used to separate the positive and negative electrolytes.  The key requirements for the separator are good chemical and electrochemical stability, good mechanical stability, high ionic conductivity, and low permeability for the vanadium ions.  Nafion^® membranes have been the most popular separators in RFBs because of their excellent oxidative stability and high proton conductivity, however, they exhibit high vanadium crossover.  Anion exchange membranes (AEMs) are ideal candidates as RFB separators as they have much lower vanadium ion crossover than proton exchange membranes (10-100 times less) due to Donnan exclusion.  Moreover, the reduced penetration of VO₂⁺ into the AEM minimizes chemical degradation.

We selected poly (2,6-dimethyl 1,4-phenylene) oxide (PPO) based AEMs as battery separators as they possess good conductivity, low water uptake and good mechanical stability.  PPO is commercially available, relatively inexpensive, and can be readily modified by attaching bromine to the backbone to induce a phase-separated morphology. PPO can be easily functionalized into an AEM by bromination followed by reaction with a tertiary amine. In this talk, we will discuss the effect of polymer crosslinking on the conductivity, water uptake, vanadium (IV) permeability, ultimate tensile stress, glass transition temperature and storage modulus of PPO- based AEMs.  Polymer crosslinking has been performed by the reaction of brominated PPO (Br-PPO) with tetramethylhexanediamine (TMHDA) in liquid phase.  Non crosslinked AEMs have been prepared by reaction of the Br-PPO with quinuclidine (PPO-ABCO⁺) and trimethylamine (PPO-TMA⁺).  Nuclear Magnetic Resonance techniques [¹H-NMR, ¹³C-NMR, correlation spectroscopy (COSY), and ¹H-¹³C heteronuclear multiple quantum correlation (HMQC) spectroscopy] have been used to investigate the polymer structure before and after exposure to VO₂⁺ solution (over time) and to identify any possible degradation products and mechanisms.  Crosslinked membranes will be tested (and compared against Nafion^® and non crosslinked AEMs) in a vanadium redox flow battery to evaluate its performance and durability.  Postmortem analysis of the membranes via NMR, conductivity/IEC measurements, and mechanical testing will be performed to unearth key degradation modes.