Improving the Alkaline Stability of Imidazolium Cations by Substitution

Abstract

Imidazolium cations are promising candidates for preparing anion-exchange membranes because of their good alkaline stability. Substitution of imidazolium cations is an efficient way to improve their alkaline stability. By combining density functional theory calculations with experimental results, it is found that the LUMO energy correlates with the alkaline stability of imidazolium cations. The results indicate that alkyl groups are the most suitable substituents for the N3 position of imidazolium cations, and the LUMO energies of alkyl-substituted imidazolium cations depend on the electron-donating effect and the hyperconjugation effect. Comparing 1,2-dimethylimidazolium cations (1,2-DMIm+) and 1,3-dimethylimidazolium cations (1,3-DMIm+) with the same substituents reveals that the hyperconjugation effect is more significant in influencing the LUMO energy of 1,3-DMIms. This investigation reveals that LUMO energy is a helpful aid in predicting the alkaline stability of imidazolium cations.

A review of the synthesis and characterization of anion exchange membranes

“…With respect to alkaline stability, in addition to S N 2 and deprotonation degradation mechanisms, imidazolium-based head groups can also be degraded via a ring opening mechanism ( Fig. 3e) [67,68]. Multiple literature sources have reported that the electron-deficient C2 position of imidazolium-based head groups is highly susceptible to nucleophile attack, which could be mitigated through the addition of large functional groups to sterically hinder OHattack [67,69,70].…”

Density functional theory study on the degradation of fuel cell anion exchange membranes via removal of vinylbenzyl quaternary ammonium head group

“…The spin‐unrestricted generalised gradient approximation proposed by Becke, Lee, Yang, and Parr (GGA‐BLYP) was used as the exchange‐correlation functional together with a double numerical basis set with polarisation (DNP) for all calculations . The convergence criteria for geometry optimisations were set to fine quality (ie, self‐consistent field (SCF) threshold value of 1×10 −6 , convergence tolerance energy of 1×10 −5 Ha, and maximum force and displacement of 2×10 −3 Ha/Å and 5×10 −3 Å, respectively) . The direct inversion in the iterative subspace value of 6 and a smearing value of 5×10 −3 Ha were also used to accelerate convergence of SCF iterations .…”

“…The use of water as an implicit solvent was implemented utilising a conductor‐like screening model (COSMO), wherein the solvent is represented as a homogeneous dielectric medium (ε water = 78.54). The transition state (TS) search was performed using the complete linear synchronous transit/quadratic synchronous transit method . Vibrational frequencies were calculated to ensure that the initial structures are in their lowest energy state and the TSs have only one imaginary frequency.…”

“…[48] The convergence criteria for geometry optimisations were set to fine quality (ie, self-consistent field (SCF) threshold value of 1×10 −6 , convergence tolerance energy of 1×10 −5 Ha, and maximum force and displacement of 2×10 −3 Ha/Å and 5×10 −3 Å, respectively). [29] The direct inversion in the iterative subspace value of 6 and a smearing value of 5×10 −3 Ha were also used to accelerate convergence of SCF iterations. [49,50] The use of water as an implicit solvent was implemented utilising a conductorlike screening model (COSMO), [51,52] wherein the solvent is represented as a homogeneous dielectric medium (ε water = 78.54).…”

“…The transition state (TS) search was performed using the complete linear synchronous transit/quadratic synchronous transit method. [29,53] Vibrational frequencies were calculated to ensure that the initial structures are in their lowest energy state and the TSs have only one imaginary frequency. After the TS search, a TS confirmation algorithm based on the nudged elastic band method [54,55] was performed to validate the obtained TSs by connecting them to their respective reactants and products.…”