Controlling the Conductivity and Stability of Azoles: Proton and Hydroxide Exchange Functionalities

Open Access
Author:
Chaloux, Brian Leonard
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 04, 2014
Committee Members:
  • Michael Anthony Hickner, Dissertation Advisor
  • Ralph H Colby, Committee Member
  • James Patrick Runt, Committee Member
  • Harry R Allcock, Committee Member
Keywords:
  • azoles
  • imidazolium
  • anhydrous proton conduction
  • impedance spectroscopy
  • polymers
  • polymer chemistry
  • chemical stability
Abstract:
For low temperature hydrogen fuel cells to achieve widespread adoption in transport applications, it is necessary to both decrease their cost and improve the range of environmental conditions under which they effectively operate. These problems can be addressed, respectively, by either switching the catalyst from platinum to a less expensive metal, or by reducing the polymer exchange membrane’s reliance upon water for proton conduction. This work focuses on understanding the chemistry and physics that limit cation stability in alkaline environments and that enable high proton conductivity in anhydrous polymer exchange membranes. Polystyrenic 1H-azoles (including 1H-tetrazole, 1H-1,2,3-triazole, and 1H-imidazoline) were synthesized to investigate whether pKa and pKb of an amophoteric, proton-conductive group have a systematic effect on anhydrous proton conductivity. It was discovered that the 1H-tetrazole (PS-Tet) exhibited distinct phase separation not seen in its carboxylic acid analog (PS-HA) or reported for other 1H-azole–containing homopolymers in literature. The resulting microstructured polymer, hypothesized to be the result of regions of high and low clustering of azoles, analogous to the multiplet-cluster model of ionomer microstructure, resulted in proton conductivity coupled with simultaneous rubbery behavior of the polymer well above its glass transition (Tg). Phase separation was similarly observed in PS-Tri and PS-ImH2 (the triazole- and imidazoline-containing polymers); soft phases with similar Tgs and hard phases with varying Tgs lend support to this hypothesis of aggregation-driven phase separation. Electrode polarization exhibited in the impedance spectra of PS-Tet and PS-HA was modeled to determine the extent of proton dissociation in undoped 1H-tetrazoles and carboxylic acids. Dry polymers (0% relative humidity) retained ~1% by weight residual water, which was observed to act as the proton acceptor in both cases. Despite doping by residual water molecules, the extent of 1H-tetrazole dissociation in PS-Tet (~0.02–0.08%) was found to be the primary factor limiting proton conductivity, suggesting that further doping should substantially increase proton conductivity. Although small molecules can provide this doping, polymer–polymer doped blends were investigated to prevent evaporation and leeching of dopant over time. It was found that the pKa and pKb of the 1H-azoles do not directly have an effect on proton conductivity, but instead play a large role with respect to dopants. Polymers containing low pKa or pKb azoles (e.g. 1H-tetrazole, 1H-imidazoline) were more easily doped by residual acceptors / donors (e.g. water, acetic acid), leading to high conductivity compared to counterparts with high pKa and pKb (e.g. triazole). Doping PS-Tet with poly(styrenesulfonic acid) lead to conductivities higher by an order of magnitude than those in undoped PS-Tet, but which originated from water of hydration associated with the strong acid. Although acidic PS-Tet and alkaline PS-ImH2 might represent an ideal combination of 1H-azoles to improve mobile proton density, the presence of strongly interacting, residual small molecules (e.g. water, acetic acid) prevented full association of tetrazole and imidazoline moieties in their blends. Finally, N,N’-symmetrically disubstituted imidazoliums with bulky alkyl substitutents were synthesized to determine how substituents and substitution patterns improve alkaline stability of these species. Although literature often cite rates of reaction (and thus stability) as being trivially linked to orbital energies, it was found that stability in practice was more strongly correlated with steric bulk of the imidazolium substituents, consistent with traditional chemical intuition. Both isopropyl and neopentyl groups significantly slowed degradation compared to benzyltrimethylammonium (a control compound), with substitution at the N and N’ positions improving stability more significantly than substitution at the 2-position in their absence. 1,2,3-Triisopropylimidazolium and N,N’-bis(neopentyl)-2-isopropylimidazolium were found exclusively to be more stable than benzyltrimethylammonium in ex situ, high temperature alkaline degradation experiments.