The Grotthuss mechanism for bifunctional proton transfer in poly(benzimidazole).

Jittima Thisuwan,Kritsana Sagarik,Phorntep Promma

doi:10.1098/rsos.211168

Jittima Thisuwan, Kritsana Sagarik + Show 1 more

Open Access

https://doi.org/10.1098/rsos.211168

Copy DOI

Abstract

Poly(benzimidazole) (PBI) has received considerable attention as an effective high-temperature polymer electrolyte membrane for fuel cells. In this work, the Grotthuss mechanism for bifunctional proton transfer in PBI membranes was studied using density functional theory and transition state theory. This study focused on the reaction paths and kinetics for bifunctional proton transfer scenarios in neutral ([PBI]2), single (H+[PBI]2) and double-protonated (H2+[PBI]2) dimers. The theoretical results showed that the energy barriers and strength for H-bonds are sensitive to the local dielectric environment. For [PBI]2 with ε = 1, the uphill potential energy curve is attributed to extraordinarily strong ion-pair H-bonds in the transition structure, regarded as a ‘dipolar energy trap’. For ε = 23, the ion-pair charges are partially neutralized, leading to a reduction in the electrostatic attraction in the transition structure. The dipolar energy trap appears to prohibit interconversion between the precursor, transition and proton-transferred structures, which rules out the possibility for [PBI]2 to be involved in the Grotthuss mechanism. For H+[PBI]2 and H2+[PBI]2 with ε = 1, the interconversion involves a low energy barrier, and the increase in the energy barrier for ε = 23 can be attributed to an increase in the strength of the protonated H-bonds in the transition structure: the local dielectric environment enhances the donor–acceptor interaction of the protonated H-bonds. Analysis of the rate constants confirmed that the quantum effect is not negligible for the N–H+ … N H-bond especially at low temperatures. Agreement between the theoretical and experimental data leads to the conclusion that the concerted bifunctional proton transfer in H2+[PBI]2 in a high local dielectric environment is ‘the rate-determining scenario’. Therefore, a low local dielectric environment can be one of the required conditions for effective proton conduction in acid-doped PBI membranes. These theoretical results provide insights into the Grotthuss mechanism, which can be used as guidelines for understanding the fundamentals of proton transfers in other bifunctional H-bond systems.

Highlights

Fuel cells have been accepted as environmentally friendly electrochemical devices that can effectively transform chemical energy into applicable electrical energy
The B3LYP/DZP and B3LYP/TZP methods suggested that the equilibrium structures of the [PBI], H+[PBI] and H2+[PBI] monomers take on approximately the same planar structure (C-form in figure 1), and the structures of the dimers formed from these monomers are an approximately planar structure; the high proton conductivity of the PBI membrane was attributed to the linear PBI chains with a high tendency for coplanar aromatic rings [19]
The Grotthuss mechanism was hypothesized to occur through the bifunctional proton transfer process in the coplanar hydrogen bonds (H-bonds) networks, e.g. in H2+[PBI]2, as shown in figure 1e

Summary

Introduction

Fuel cells have been accepted as environmentally friendly electrochemical devices that can effectively transform chemical energy into applicable electrical energy. As an aromatic heterocyclic polymer with extraordinary thermal stability and high proton conductivity at relatively high temperatures (400–600 K), poly(benzimidazole) (PBI) has received considerable attention as an effective PEM in fuel cells [1]. Pristine PBI is not a proton-conducting polymer (σ = 10−12 S cm−1) and cannot be directly used as a PEM [2], acid-doped PBI membranes are excellent proton-conducting polymers [3]. Based on the Scatchard method [5], both imide groups in PBI (figure 1) were reported to be preferentially protonated with a considerably high protonation constant compared with sites with lower affinity. Experiments revealed that in anhydrous states in the temperature range between 298 and 433 K, proton conductivities for PBI membranes with acid doping levels of two range from 10−9 to 10−5 S cm−1 [7]

Methods

Results

Conclusion