Properties of Water and Protons in PEMs

Studies of the dynamical behavior of water molecules and protons in PEMs rationalize the influence of random morphology and water uptake on effective physicochemical properties, that is, proton conductivity, water flux, and electro-osmotic drag. 

Phase separation and spatial organization of membrane domains determine the state of water, fundamental interactions in the polymer/water/ion system, vibration modes of fixed sulfonate groups, and mobilities of water molecules and protons. Dynamic properties of the membrane can be probed at the microscopic scale with spectroscopic techniques, including Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) (Mauritz and Moore, 2004). FTIR 

FIGURE 2.6 Activation energies of proton transport Nafion 117, extracted from the Arrhenius representation of condnctivity data at varying water contents. (With kind permission from Springer Science+Business Media Fuel Cells I, Fhoton-conducting polymer electrolyte membranes Water and structnre in charge, 2(X)8, pp. 15-54, Eikerling, M., Kornyshev, A. A., and 

QENS studies on the dynamics in PEMs suggest that water and protons attain microscopic mobilities that are similar to those in bulk water, with mean jump times of protons in the order of 1 ps. The experimental scattering data include two components, which correspond to fasf (At O (1 ps)) and slow (At 0 (100 ps)) motions. Observation of the latter type of motion suggests that hydronium ions exist as long-lived entities in Nation (Perrin et al., 2006). 

The comparison of local and long-range diffusion demonstrates that in well-hydrated PEMs the major geometric constraints for water mobility unfold at the scales probed by QENS, which is up to several tens of nanometers. This implies that there are no significant restrictions of mobility arising between the nanometric and the micrometric scales, when the membrane is sufficiently hydrated. Similar considerations could be made for proton diffusivities. 

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