Anion conducting methylated aliphatic PBI and its calculated properties

Abstract

A methylated polybenzimidazole with an aliphatic chain in the backbone (Me‐PBI‐C10) was synthesized and formed into membranes. Literature suggests that alkyl chains on C2 of imidazolium ions increase their alkaline stability. While this may be true for model compounds or ions attached as a side chain, both our DFT calculations and experimental results show that Me‐PBI‐C10 does not withstand alkaline conditions. To increase the alkaline stability, blend membranes with PBI‐OO were fabricated. A blend membrane with 50% PBI‐OO showed a chloride conductivity of up to 6 mS/cm, indicating that these membranes could find use in non‐alkaline applications like vanadium redox flow batteries (VRFB). The high mechanical stability (tensile strength: 70.25 ± 14.85 MPa, Young modulus: 1.65 ± 0.16 GPa) would be an advantage over currently used Nafion membranes. Finally, three different models were successfully applied to qualitatively predict the water uptake of Me‐PBI‐C10 exchanged with different anions. The results match with experimental data. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 256–265

Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity

“…Because the nickel complex had a smaller Δ H tot , but likely a similar T Δ S term, the Δ G of counterion exchange for the nickel complex was more negative than for the other metals, suggesting that there was more of a thermodynamic driving force for chloride ion release, a parameter that has previously been difficult to measure for AEMs and PEMs. Additionally, the lower Δ H tot indicated that less energy needed to be absorbed by the nickel system to facilitate counterion exchange, strongly suggesting that nickel had weaker initial binding to its chloride counterion . Weaker initial counterion binding and a stronger thermodynamic driving force for counterion release should increase the number of free ions and increase the AEM’s conductivity. , Given that ion release is critical to any conductivity mechanism, and that the percentage of free ions is directly correlated to conductivity, this thermodynamic driving force for ion release appears to be the prevailing factor for the nickel-based AEM’s superior ion conductivity. , …”

“…Additionally, the lower ΔH tot indicated that less energy needed to be absorbed by the nickel system to facilitate counterion exchange, strongly suggesting that nickel had weaker initial binding to its chloride counterion. 61 Weaker initial counterion binding and a stronger thermodynamic driving force for counterion release should increase the number of free ions and increase the AEM's conductivity. 1,49 Given that ion release is critical to any conductivity mechanism, and that the percentage of free ions is directly correlated to conductivity, this thermodynamic driving force for ion release appears to be the prevailing factor for the nickel-based AEM's superior ion conductivity.…”

From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides

“…313 Another attempt to improve the stability was to incorporate longer aliphatic segments in the structure (Table 1; Entry 9), but the concept did not circumvent the major degradation mode. 98 A more effective stabilization approach was reported by Thomas et al, 90 who introduced methyl substituents on the m-phenylene group adjacent to the polybenzimidazolium C2 position (see Table 1; Entry 6). After N-methylation, as shown in Scheme 9, the dihedral angle between the benzimidazoliums and the phenylenes was changed so that the methyl groups on the phenylene moieties were positioned above and under the benzimidazolium plane and sterically protected the benzimidazolium C2 position from nucleophilic attack.…”

Photodegradation of polybenzimidazole/polyvinyl chloride composites and polybenzimidazole: Density functional theory and experimental study

“…A hybrid B3LYP functional method along with 6–311 G** basis set was used for geometry optimization and frequency calculations of all the species, 16 and the natural charge was analyzed by natural bond orbital. Another major goal of this investigation was to calculate enthalpy (∆ H ) and Gibb's free energy (∆ G ) of photooxidation of PVC under 1 atm and 298.15 K. An oligomer containing four repeated units for PVC 12 and one structure unit for PBI were selected as the model compound 17 …”