(Invited) The Use of Broadband Electric Spectroscopy to Probe the Relaxations and Polarizations in Phase Seperated Polymer Electrolytes

Abstract

There has been a recent surge in interest in the use of anion exchange membranes (AEMs) as the separator in many electrochemical conversion devices such as fuel cells, electrolyzers, redox flow cells, and for uses in electrochemical water purification processes. This is because electrocatalysis is generally more facile in alkaline conditions, and so these devices could potentially operate with non-precious metal catalysts and with catalysts capable of oxidizing or generating fuels beyond hydrogen. An AEM with high anionic conductivity would overcome many of the disadvantages of proton exchange membrane devices that need precious metal catalyst and work best with hydrogen. Improved AEMs are also required for water purification applications. Realization of AEM devices, therefore, would revolutionize electrochemical energy conversion, as inexpensive versatile devices could be made available. While much attention has been focused on generating new AEMs with stable cations and backbones, very little attention has been paid to the design of AEMs to maximize ionic conductivity and water transport. The work presented here represents the first detailed study of the thermomechanical, polarization, and dielectric phenomena related to morphology in a series of AEMs. In detail in this presentation, the micro-structure is studied by Small Angle X-ray Scattering (SAXS), the thermal stability and transitions are detected by High-Resolution Thermogravimetry (HR-TG) and by Modulated Differential Scanning Calorimetry (MDSC), respectively. The mechanical properties and transitions are investigated by Dynamic Mechanical Analysis (DMA). Broadband Electrical Spectroscopy (BES) is used to study the electric response and the conductivity mechanisms of the membranes in the frequency and temperature range of 0. 01—107 Hz and -105 — 100 °C, respectively. BES is a powerful technique, which allows us to fully understand the overall electric response and dielectric relaxations of the membranes, both in a completely dry state and after full hydration, thus allowing clarification of the role played by both ion solvation phenomena and water cluster structure on performance of these materials. The integration of the information acquired by the above techniques allows us to propose a reasonable mechanism for the long-range charge migration processes in polyvinylbenzyltrimethylamne bromide-block-polymethylbutylene [PVBTMA][Br]-b-PMB, it hydoxide analogue and the random polymers of the same chemistry crosslinked by UV and thermal methods. We have also extended this study to polyphenylene-b-[PVBTMA][OH] and the pentablock quaternary ammonium functioalized analogue of the sulfonated polymer, Nexar. In amny of these polymers we observe an order-disorder transition, Tδ, where dispersed cations aglomorate due to the interactions of their dipoles. As Tδ often occurs above 0°C this phenomenom is of nterset to the development of practical devices. This study has important consequences for the design of next generation revolutionary AEMs.