Quantitative ¹H NMR Analysis of Chemical Stabilities in Anion-Exchange Membranes

Abstract

We compared the alkaline stability of three classes of anion exchange membranes that are leading candidates for applications in platinum-free fuel cells. A methodology is presented for the study of chemical stability of anion-exchange polymers in alkaline media that provides clear and quantitative 1H NMR spectroscopic data of dissolved polymers containing benzyltrimethylammonium functionalities. Recent studies have investigated the stabilities of benzimidazolium- and alkylimidazolium-bearing polymers using periodic 1H NMR sampling. These studies included varying alkaline concentrations, external heating sources, and excessive processing and contained no internal standard for absolute measurements. Key aspects of our time-resolved 1H NMR method include in situ heating and sampling within the spectrometer, fixed stoichiometric relationships between the benzyltrimethylammonium functionalities of each polymer and potassium deuteroxide (KOD), and the incorporation of an internal standard for the absolute measurement of the polymer degradation. In addition, our method permits the identification of the degradation products to find the underlying cause of chemical lability. Our results demonstrate that a styrene-based polymer containing benzyltrimethylammonium functional groups is remarkably stable when exposed to 20 equivalents per cation of KOD at 80 °C with a half-life (t 1/2) of 231 h. Under these standard conditions, functionalized poly(phenylene oxide) and poly(arylene ether sulfone) copolymers, both bearing benzyltrimethylammonium functionalities were found to degrade with a half-lives of 57.8 and 2.7 h, respectively.

Hydroxide based Benzyltrimethylammonium Degradation: Quantification of Rates and Degradation Technique Development

“…Hydroxide degradation rates increase with decreasing dielectric constant, 29 like those found in organic solvents. 11,12 The nucleophilicity of the hydroxide anion increases in the lower dielectric constant media and the observed increase in degradation rate may simply be due to partitioning of some fraction of BTMA and hydroxide into the organic phase accompanied by accelerated degradation rates. Additionally the possibility exists that the organic phase components also participate in degradation reactions thereby increasing degradation rates.…”

“…Degradation of BTMA has been the subject of several past studies. 5,[9][10][11][12][13] However, methodologies of degradation were not standardized or fully validated. Several reports that include BTMA degradation are presented in Table I to highlight the differences in reaction conditions and use of various experimental techniques.…”

Preparation and performance evaluation of novel alkaline stable anion exchange membranes

“…To overcome this challenge, various functional groups such as imidazole [6,7], guanidine [8], trimethylamine [9e11], benzimidazole [12], and phosphine [13] were usually used for synthesis of alkaline AEMs. Unfortunately, as revealed by the computational and experimental studies, the alkyl chain structure and structure around b hydrogen are more sensitive to degradation conditions and Hofmann elimination [14,15]. Cationic degradation causes the reduction in hydroxide conductivity and ion exchange capacity of the membrane.…”

“…Cationic degradation causes the reduction in hydroxide conductivity and ion exchange capacity of the membrane. This type of degradation was observed by 1 H NMR spectroscopy and Fourier transforms infrared spectroscopy [15]. Amduri's group found that synthesis of alkaline anion exchange membrane without hydrogen at b position to quaternary nitrogen could minimize membrane decomposition and Hofmann elimination [16,17].…”

Strategies for Developing New Anion Exchange Membranes and Electrode Ionomers

“…This polymer has been an excellent platform for AEM development due to its ease of chemical modification and large-scale synthesis. However, poly(phenylene oxide) is susceptible to degradation under the most stringent conditions of high temperature (above 80 °C) and high pH (1-4 M NaOH) 31 and under oxidative conditions. 32 Under milder conditions at 60 °C, many materials have demonstrated high pH stability, so it is difficult to find the most promising materials using low-temperature degradation measurements.…”