Influence Of Carbon Spacers And Alkyl Pendant Chains On The Stability Of Quaternary Ammonium Cations For Anion Exchange Membranes

Open Access
Author:
Capparelli, Clara
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 10, 2015
Committee Members:
  • Michael Anthony Hickner, Thesis Advisor
Keywords:
  • anion exchange membranes
  • degradation
  • small molecule cations
  • NMR
  • LCMS
  • membrane degradation
Abstract:
Proton and anion exchange membranes are of great importance in the function of fuel cells, one of the most promising technologies for renewable energy conversion. Proton exchange membrane fuel cells (PEMFC) have been studied extensively in the past couple of decades, and there have been tremendous advances in the development of these systems, especially in industries such as automotive and portable power. Anion exchange membranes (AEM) have caught the attention of scientists because they would allow for the development of fuel cells without costly precious metal catalysts, among other advantages. Efforts are being made in developing long-lived and high performance AEMs for fuel cell applications. Primarily, the focus in AEM research has been membrane stability. It has been observed that AEMs are not as stable as the state-of-the-art NAFION® PEM and demonstrations of cell performance beyond 1000 hours is rare. For this reason, scientists are in the search for more stable AEMs. The first step to developing more stable membranes is to understand the mechanisms by which these membranes degrade – both in ex-situ and in-situ stability assessments. It is the focus of this thesis to provide insight in the degradation mechanisms of AEMs under highly basic conditions. The topic of this thesis is the use of alkyl spacers between the polymeric backbone and cationic group, and alkyl pendant chains replacing one of the methyl groups in the quaternary ammonium moiety, to provide steric hindrance around the cation and lower the degradation rate of the nitrogen-centered cation. Samples were developed with systematically differing architectures, for example, different lengths of alkyl spacers. The samples were then degraded under highly basic conditions at high temperatures. The strategy chosen to detect degradation was the analysis of the degradation by-products of the degraded small molecules, by two different characterization techniques: 1H NMR and LC-MS. 1H NMR was chosen to provide quantitative information of the degradation rates of various analogue small molecules with distinct chemistries; LC-MS was chosen to identify the degradation by-products and thus, determine the degradation mechanisms. These results on small molecules can be extended to membranes as described in some preliminary membrane experiments and in future work. It is the aim of this work to deeply probe the steric hindrance strategy to stable ammonium cations as well as provide a clear methodology for determining the degradation rates and mechanisms of analogue small molecules for AEMs.