Alkaline Degradation Studies of Anion Exchange Polymers to Enable New Membrane Designs

Open Access
Author:
Nunez, Sean Andrew
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 21, 2015
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • James Patrick Runt, Committee Member
  • T C Mike Chung, Committee Member
  • Alexander Thomas Radosevich, Special Member
Keywords:
  • fuel cell
  • polymer chemistry
  • exchange membrane
  • FTIR
  • NMR
Abstract:
Current performance targets for anion-exchange membrane (AEM) fuel cells call for greater than 95% alkaline stability for 5000 hours at temperatures up to 120 °C. Using this target temperature of 120 °C, an incisive 1H NMR-based alkaline degradation method to identify the degradation products of n-alkyl spacer tetraalkylammonium cations in various AEM polymers and small molecule analogs. Herein, the degradation mechanisms and rates of benzyltrimethylammonium-, n-alkyl interstitial spacer- and n-alkyl terminal pendant-cations are studied on several architectures. These findings demonstrate that benzyltrimethylammonium- and n-alkyl terminal pendant cations are more labile than an n-alkyl interstitial spacer cation and conclude that Hofmann elimination is not the predominant mechanism of alkaline degradation. Additionally, the alkaline stability of an n-alkyl interstitial spacer cation is enhanced when combined with an n-alkyl terminal pendant. Interestingly, at 120 °C, an inverse trend was found in the overall alkaline stability of AEM poly(styrene) and AEM poly(phenylene oxide) samples than was previously shown at 80 °C. Successive small molecule studies suggest that at 120 °C, an anion-induced 1,4-elimination degradation mechanism may be activated on styrenic AEM polymers bearing an acidic α-hydrogen. In addition, an ATR-FTIR based method was developed to assess the alkaline stability of solid membranes and any added resistance to degradation that may be due to differential solubilities and phase separation. To increase the stability of anion exchange membranes, Oshima magnesate–halogen exchange was demonstrated as a method for the synthesis of new anion exchange membranes that typically fail in the presence of organolithium or Grignard reagents alone. This new chemistry, applied to non-resinous polymers for the first time, proved effective for the n-akyl interstitial spacer functionalization of poly(phenylene oxide) and poly(styrene-co-ethylene-co-butylene-co- styrene) polymer backbones. The comprehensive methodologies for the assessment of alkaline stability in AEMs as well as the new synthetic methodologies are intended as a guide toward robust AEM synthetic designs that approach current performance standards.