Backbone effects on the charge transport in poly-imidazole membranes: a theoretical study

Abstract

In this paper we present a theoretical investigation of proton conduction in a 2-tethered poly-vinylimidazole system, an N-heterocyclic-based membrane used as electrolyte in proton exchange membrane fuel cells (PEMFCs). In particular a detailed analysis, combining Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations, shows the underpinning role of the backbone in determining the proton transport mechanisms. DFT calculations, carried out on a neutral trimer, suggest that proton conduction is based on a Grotthuss-like mechanism, which requires a series of proton transfers along the chain followed by a rotation of all the imidazoles. This latter represents the limiting step and it is clearly observed during the MD simulations on a larger model (oligomer of 15 imidazoles) at a typical operative temperature. From the computational point of view, this is the first time that such a mechanism is clearly evidenced in N-heterocycle-based membranes and the results give a clear explanation for the difference in proton mobility conductivities observed between P2VI and the parent poly(4-vinyl-imidazole) (P4VI). These results underline the relevant role of imidazole connectivity to the polymer backbone in determining proton conduction and could give valuable insights for the design of new and better performing protogenic groups with minimum reorientation barriers.

Study of Chemical Modified Amphoteric Ion Exchange Membrane from Taurine

“…The Grotthuss mechanism was only slightly affected by the existing cations in AIEMs. [14][15][16] Therefore, the AIEMs can show good ability in preventing the permeation of vanadium ions while maintaining acceptable proton conductivity.…”

Proposing Efficient New Pendant Group Polymer Electrolyte Membranes for Fuel Cells: A Computational Study

“…This low value compares very favorably with the proton transfer barriers that have been calculated in the past for other PEM systems. [16][17][18]20,46 Following this first proton transfer step, an intermediate complex is formed, referred to as "int.1" in Figure 6, which is a structure stabilized by hydrogen bonding between the two nitrogen atoms, with the difference that it is now the middle nitrogen that has the proton. The hydrogen-bond stabilized intermediate int.1 is more stable than the reactant structure by 2.6 kcal/mol (see Figure 6).…”

“…However, despite their attractiveness, such PEMs, having pendant groups on a polymer backbone, have a disadvantage in that the reorientation step (ii) is the slow step, with a significant barrier, − ranging from 8.0 to 39.2 kcal/mol . The reason a rotation or reorientation barrier is required in such membranes can be understood when one considers an example, such as the case of the proton transfer reaction between two nitrogen containing moieties, which has been studied and reported − for model systems such as imidazole ,, and triazole .…”

A new sulfonated poly(ether sulfone) hybrid with low humidity dependence for high‐temperature proton exchange membrane fuel cell applications

“…Hybrids often prepared by combination of a polymer with high proton conductivity and another with high water uptake or moisture trapping in the membrane . Investigation of the mechanism of proton transfer and performance of different protogenic groups helps to improve membranes efficiency . In our previous works, we reported two theoretical studies on the role of different protogenic groups and functional groups on proton conductivity that confirms the presence of amphoteric groups in polymer backbone improve hydrogen bonding and proton transfer.…”