Water self-diffusion anisotropy and electrical conductivity of perfluorosulfonic acid/SiO 2 composite proton exchange membranes Carmen Filipoi a , Dan E. Demco a,c,⇑ , Xiaomin Zhu a,⇑ , Rostislav Vinokur a , Oliver Conradi b , Radu Fechete c , Martin Möller a a Functional and Interactive Polymers, DWI RWTH Aachen University, Forckenbeckstraße 50, D-52074 Aachen, Germany b 3M Deutschland GmbH, Carl-Schurz-Straße 1, 41453 Neuss, Germany c Technical University of Cluj-Napoca, Memorandumului 28, R-400114 Cluj-Napoca, Romania article info Article history: Received 25 July 2012 In final form 8 October 2012 Available online 17 October 2012 abstract Water self-diffusion anisotropy in perfluorosulfonic acid (PFSA)/SiO 2 composite membranes was investi- gated by pulsed gradient spin-echo NMR as a function of diffusion time, temperature and relative humid- ity. The anisotropy of water diffusion that is related to the ion channel orientation decreases with the increase of silica content, diffusion time and relative humidity. Activation energies of water diffusion and proton conductivity were evaluated for the temperature range of 313–363 K. Empirical dependences for proton conductivity and proton diffusion coefficient as a function of water uptake were established and correlated with the silica content of membranes. The effect of in situ grown sulfonated silica nano- particles on the orientation of water channels is also discussed. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Proton exchange membranes (PEM) play an essential role in polymer electrolyte fuel cells preventing mixing of feed gases and providing transport of protons from the anode to the cathode [1–3]. The benchmark PEM material is Nafion, which has good pro- ton conductivity only in the wet state, where the absorbed water molecules serves as transport carriers for protons [4–7]. A current goal in fuel cell research is the development of materials that perform effectively within PEM fuel cells at high temperature and low relative humidity. The approach we are using to prevent the loss of water from the ionic regions of perfluorosulfonic acid ionomer (PFSA) [8], is the incorporation of silica nanoparticles into PFSA membranes via an in situ sol–gel technology using hyper- branched polyethoxysiloxane (PEOS) as silica precursor [9]. Small silica nanoparticles with silanol groups on the surface are expected to improve the connectivity of ionic channels forming a pathway for proton transport. Nafion/SiO 2 composite membranes showed an improved proton conductivity and better fuel cell performance than the pure membrane when operated at reduced humidity condi- tions [10–16]. One important key in designing PEM is related to the under- standing of the transport mechanism and its correlation with water content and the PEM morphology. Zawodzinski et al. [17] re- ported water diffusion coefficients in Nafion membranes of con- trolled states of hydration, in comparison with proton mobility estimated from the conductivity data and concluded that proton transport by Grotthuss hopping mechanism is negligible at low water contents, but becomes significant at high water contents. Kreuer et al. [18,19] studied more the proton and water diffusion in sulfonated polymers over a range of water contents and from the nonequilibrium statistical mechanics-based calculations showed that the major proton conduction mechanism at high de- gree of hydration is Grotthuss mechanism and at intermediate and low degrees of hydration is vehicle mechanism. Recently, ‘channel orientation’ has been introduced as an impor- tant feature of the fuel cell performance of the PEMs [20–23]. The study of translational motion of water molecules in PEMs by pulsed gradient spin-echo (PGSE) NMR diffusometry revealed the anisot- ropy of water mass transport related to the channel orientation [22–25]. Previously, we investigated water diffusion in fully hy- drated 3M PFSA/silica composite membranes by means of non-inva- sive PGSE NMR and determined the anisotropy of water channel alignment in correlation with the silica content of the membranes [24]. Furthermore, the effect of water exchange between free and bound water on the diffusion measurements was analyzed. The aim of this study is to investigate the micrometer-scale structure of PFSA/silica composite membranes, reflected in anisot- ropy of water channel orientation as a function of diffusion time, temperature, relative humidity, and to correlate the water self-dif- fusion with the proton conductivity of the membranes. Further- more, we present our preliminary results on the effect of in situ 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.031 ⇑ Corresponding authors. Address: Functional and Interactive Polymers, DWI RWTH Aachen University, Forckenbeckstraße 50, D-52074 Aachen, Germany (D.E. Demco). Fax: +49 241 233 01. E-mail addresses: demco@mc.rwth-aachen.de (D.E. Demco), zhu@dwi.rwth- aachen.de (X. Zhu). Chemical Physics Letters 554 (2012) 143–149 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett