Mechanisms of proton transfer in Nafion®: elementary reactions at the sulfonic acid groups

Abstract

Proton transfer reactions at the sulfonic acid groups in Nafion were theoretically studied, using complexes formed from triflic acid (CF3SO3H), H3O+ and H2O, as model systems. The investigations began with searching for potential precursors and transition states at low hydration levels, using the test-particle model (T-model), density functional theory (DFT) and ab initio calculations. They were employed as starting configurations in Born-Oppenheimer molecular dynamics (BOMD) simulations at 298 K, from which elementary reactions were analyzed and categorized. For the H3O+-H2O complexes, BOMD simulations suggested that a quasi-dynamic equilibrium could be established between the Eigen and Zundel complexes, and that was considered to be one of the most important elementary reactions in the proton transfer process. The average lifetime of H3O+ obtained from BOMD simulations is close to the lowest limit, estimated from low-frequency vibrational spectroscopy. It was demonstrated that proton transfer reactions at -SO3H are not concerted, due to the thermal energy fluctuation and the existence of various quasi-dynamic equilibria, and -SO3H could directly and indirectly mediate proton transfer reactions through the formation of proton defects, as well as the -SO3- and -SO3H2+ transition states.

Dynamics and mechanism of structural diffusion in linear hydrogen bond

“…Since the vibrational energies for the interconversion between the oscillatory shuttling and structural diffusion motions (Dm OH;MD BA ) in the intermediate states (n ¼ 1 and 3) are ranging from 4.8 to 10.5 kJ/mol, it is reasonable to anticipate that the thermal energy fluctuation and dynamics in the short H-bond chains are sufficient to activate the structural diffusion process (at 298 K the experimental activation enthalpy is 8.4 kJ/mol), without the requirement of H-bond weakening or cleavage anywhere. As our previous and present results concluded that proton transfer reactions in H-bonds are not concerted, [7][8][9]19] covalent bond breaking and forming need not occur in a single step, and the ratedetermining steps are associated with the lifetimes of the quasi-dynamic equilibriums between the shared-proton and close-contact structures (not the proton displacement within a single H-bond), elementary steps which involve fluctuation of the number of the CH 3 OH molecules in the H-bond chains (n ¼ 3 and n ! 4) could be formulated, using the H-bond chains with n ¼ 3-5 as examples.…”

“…It was iterated that static proton transfer potentials cannot provide complete description of proton transfer reactions and the thermal energy fluctuation and dynamics must be incorporated in the model calculations. [7][8][9] To continue the series of theoretical studies on small H-bond systems, [7][8][9]19] as well as to obtain additional explanations for the proposed mechanisms, [13,15] structures, energetic and dynamics of proton transfer processes in protonated H-bond chains were investigated in the present work, using the CH 3 OH þ 2 (CH 3 OH) n complexes, n ¼ 1-4, as model systems, and B3LYP/TZVP calculations and BOMD simulations as model calculations. Since the effects of electron correlations could be important in the present model systems and it has been our intention to apply the DFT method in larger H-bond complexes, some static and dynamic results, obtained based on B3LYP/TZVP calculations, were examined using ab initio calculations at the RIMP2/TZVP level of accuracy.…”

“…[12][13][14] It was demonstrated that the structural diffusion in aqueous solution is not concerted due to the thermal energy fluctuation and dynamics in the H-bond network. [9] Being the smallest alcohol with hydrophobic (CH 3 ) and hydrophilic (OH) groups in the same molecule makes the H-bond structure in liquid methanol (CH 3 OH) different from liquid water; one-dimensional H-bond chains with occasional branches are the predominant structures from 153 K to room temperature. [11] CH 3 OH has therefore been frequently selected as model molecule for the investigation of proton transfer in H-bond chain.…”

“…Two mechanisms have been proposed to explain the high mobility of proton in the three-dimensional H-bond network of water. [1,[8][9][10][11] The vehicle mechanism is represented by hydrated proton traveling through the solvent via mutual diffusion, whereas the structural diffusion mechanism involves the diffusion of the H-bond structure, in which an excess proton is shuttling back and forth in the H-bond. [4] These two mechanisms have been widely accepted as the most important processes in the proton transfer reaction in aqueous solution.…”

Modeling the ion transfer and polarization of ion exchange membranes in bioelectrochemical systems

“…Additionally to the limited number of published experimental data [12,22] the diffusion coefficient of any ion appears to be strongly dependent on its concentration and the presence of other ions. This especially applies to protons, whose exact transport mechanism in Nafion under different conditions may change and is still in debate [25].…”