Investigation of local environments in Nafion-SiO2 composite membranes used in vanadium redox flow batteries

Proton conducting polymer composite membranes are of technological interest in many energy devices such as fuel cells and redox flow batteries. In particular, polymer composite membranes, such as SiO2 incorporated Nation membranes, are recently reported as highly promising for the use in redox flow batteries. However, there is conflicting reports regarding the performance of this type of Nation-SiO2 composite membrane in the redox flow cell. This paper presents results of the analysis of the Nafion-SiO2 composite membrane used in a vanadium redox flow battery by nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier Transform Infra Red (FUR) spectroscopy, and ultraviolet-visible spectroscopy. The XPS study reveals the chemical identity and environment of vanadium cations accumulated at the surface. On the other hand, the F-19 and Si-29 NMR measurement explores the nature of the interaction between the silica particles, Nation side chains and diffused vanadium cations. The Si-29 NMR shows that the silica particles interact via hydrogen bonds with the sulfonic groups of Nation and the diffused vanadium cations. Based on these spectroscopic studies, the chemical environment of the silica particles inside the Nation membrane and their interaction with diffusing vanadium cations during flow cell operations are discussed. This study discusses the origin of performance degradation of the Nafion-SiO2 composite membrane materials in vanadium redox flow batteries. (C) 2011 Elsevier Inc. All rights reserved.