Proton transport in perfluorosulphonic membranes

The transport properties of perfluorosulphonic membranes are largely influenced by the water content of the membrane, particularly when the membrane is in the acid form. In the dry state, the Nafion membrane behaves like an insulator but, when hydrated, the membrane becomes conductive as a function of the water content. Yeo established that the minimum threshold corresponds to about six molecules of water per sulphonic site, whereas Pourcelly et al. estimated about seven molecules. Randin has shown that membranes with six molecules of water per sulphonic site have suflicient conductivity for use as a semi-solid proton 

The mobility of protons in Nafion perfluorosulphonic membranes is strongly assoeiated with the water content of the membrane phase. This mobility is directly related to the molality of fixed charged sulphonic sites. By IR measurements, Jenard has observed the protonated form of sulphonic sites for low water contents in these membranes. Excepting these extreme conditions, the proton transfer process in perfluorosulphonic membranes must take account of the ionic composition of the membrane phase. Two cases must be examined on the one hand, the membrane eontains counter-ions, lone protons or protons with other eations, and on the other hand, the membrane contains sorbed acids. In the first case, the conduction is due only to the counter-ions and proton motion is considered to involve the jump from one site located in the 

3 Proton transport when only counter-ions are present in the membrane 

For a given water content and by applying the Stokes-Einstein equation and the rate process to ionic migration s , it is possible to calculate, for two dilferent ions, the difference between their activation free energies for the elementary ion transfer reactions, the proton being chosen as the reference ion 

An attempt has been made to analyse the results for membrane conductivity obtained in the presence of two counter-ions, the proton and an alkali ion. From the results of self-diffusion flux measurements by radiotracer techniques, Gavach et al. obtained a value for the individual mobility of Na ions in the membrane containing both Na and counter-ions. By measuring the conductivity of the membrane containing only as the mobile ion and applying the Nemst-Einstein relationship, they determined the relative mobilities of these two counter-ions and concluded that the individual mobility of the proton increases by up to 150% as the number of Na ions increases in the membrane. This increase with mole fraction of Na ions has been attributed to an increase of the proton hydration, Na being less hydrated than H.  

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