Ab initiostudies on the proton dissociation and infrared spectra of sulfonated poly(ether ether ketone) (SPEEK) membranes

• Published in: Physical chemistry chemical physics : PCCP 2013-12, Vol.16 (3), p.1041-1049
• Main Authors: Zhao, Yuan-yuan, Tsuchida, Eiji, Choe, Yoong-Kee, Ikeshoji, Tamio, Barique, Mohammad Abdul, Ohira, Akihiro
• Format: Article
• Language: English
• Subjects: Atomic structure; Dynamical systems; Dynamics; Electronic structure; Ethers; Infrared spectra; Mathematical models; Membranes
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c3cp53146e

SPEEK is known to possess high proton conductivity at high water content, being comparable with other popular membranes used in fuel cells, such as Nafion and sulfonated polyethersulfone (SPES). However, much less is known about its fundamental properties, including the status of proton dissociation and spectral features. In this work, the properties of two model molecules of SPEEK, M1 (20 atoms), M2 (50 atoms) and their hydrated systems, M1 + nH sub(2)O and M2 + nH sub(2)O (n= 1-9), have been investigated using static electronic structure calculations and the ab initiomolecular dynamics (MD) method. Optimized structures for all of the systems and the trajectories of M1 + nH sub(2)O (n= 3-6) at finite temperatures have been computed using density functional theory at the B3LYP level of theory. Proton dissociation has been discussed in detail, especially for n= 3 and n= 4. In addition, the infrared spectra of SPEEK and its hydrated systems have been studied using a combination of theory and experiment. The characteristic bands of SPEEK and the surrounding water clusters have been assigned with emphasis on their relationship with the degree of proton dissociation. We have found that the hydronium ion stretching modes, which appear in the 2000-3000 cm super(-1) region in static electronic structure calculations, are not observed experimentally. This discrepancy is explained by the stationary structure and the temperature effect.