Proton transfer reactions and dynamics of sulfonic acid group in Nafion®.
Proton transfer reactions and dynamics of the hydrophilic group (-SO(3)H) in Nafion® were studied at low hydration levels using the complexes formed from CF(3)SO(3)H, H(3)O(+) and nH(2)O, 1 ≤n≤ 3, as model systems. The equilibrium structures obtained from DFT calculations suggested at least two structural diffusion pathways at the -SO(3)H group namely, the "pass-through" and "pass-by" mechanisms. The former involves the protonation and deprotonation at the -SO(3)H group, whereas the latter the proton transfer in the adjacent Zundel complex. Analyses of the asymmetric O-H stretching frequencies (ν(OH)) of the hydrogen bond (H-bond) protons showed the threshold frequencies (ν(OH*)) of proton transfer in the range of 1700 to 2200 cm(-1). Born-Oppenheimer Molecular Dynamics (BOMD) simulations at 350 K anticipated slightly lower threshold frequencies (ν(A)(OH*,MD)), with two characteristic asymmetric O-H stretching frequencies being the spectral signatures of proton transfer in the H-bond complexes. The lower frequency (ν(A)(OH,MD))) is associated with the oscillatory shuttling motion and the higher frequency (ν(B)(OH,MD))) the structural diffusion motion. Comparison of the present results with BOMD simulations on protonated water clusters indicated that the -SO(3)H group facilitates proton transfer by reducing the vibrational energy for the interconversion between the two dynamic states (Δν), resulting in a higher population of the H-bonds with the structural diffusion motion. One could therefore conclude that the -SO(3)H groups in Nafion® act as active binding sites which provide appropriate structural, energetic and dynamic conditions for effective structural diffusion processes in a proton exchange membrane fuel cell (PEMFC). The present results suggested for the first time a possibility to discuss the tendency of proton transfer in H-bond using Δν(BA)(OH,MD)) and provided theoretical bases and guidelines for the investigations of proton transfer reactions in theory and experiment.