Abstract
The effect of relative humidity (RH) and degree of sulfonation () on the ionic conductivity and water uptake of proton-exchange membranes based on sulfonated multiblock copolymers composed of polysulfone (PSU) and polyphenylsulfone (PPSU) is examined experimentally and numerically. Three membranes with a different and ion-exchange capacity are analyzed. The heterogeneous structure of the membranes shows a random distribution of sulfonated (hydrophilic) and non-sulfonated (hydrophobic) domains, whose proton conductivity is modeled based on percolation theory. The mesoscopic model solves simplified Nernst–Planck and charge conservation equations on a random cubic network. Good agreement is found between the measured ionic conductivity and water uptake and the model predictions. The ionic conductivity increases with RH due to both the growth of the hydrated volume available for conduction and the decrease of the tortuosity of ionic transport pathways. Moreover, the results show that the ionic conductivity increases nonlinearly with , experiencing a strong rise when the is varied from 0.45 to 0.70, even though the water uptake of the membranes remains nearly the same. In contrast, the increase of the ionic conductivity between and is significantly lower, but the water uptake increases sharply. This is explained by the lack of microphase separation of both copolymer blocks when the is exceedingly high. Encouragingly, the copolymer membranes demonstrate a similar performance to Nafion under well hydrated conditions, which can be further optimized by a combination of numerical modeling and experimental characterization to develop new-generation membranes with better properties.
Highlights
Fuel cells (FCs) are electrochemical devices that convert fuels into electric power in an efficient and environmentally friendly way
The effect of relative humidity (RH) and degree of sulfonation (DS) on the ionic conductivity and water uptake of proton-exchange membranes based on sulfonated multiblock copolymers composed of polysulfone (PSU) and polyphenylsulfone (PPSU) is examined experimentally and numerically
Among the various types of FCs, proton-exchange membrane fuel cells (PEMFCs) have drawn significant attention as alternative clean energy conversion devices because of their high efficiency, low operating temperature, fast start-up, and their potential to operate with fuels from renewable sources [4,5]
Summary
Fuel cells (FCs) are electrochemical devices that convert fuels into electric power in an efficient and environmentally friendly way. Eikerling et al [37] presented a random network model of charge transport in PEMs based on effective medium theory to examine the membrane complex impedance as a function of water content. Tongwen et al [41] presented a three-phase model based on percolation theory, i.e., accounting for the pure-gel phase (active region), inert-gel phase (inactive region), and the inter-gel phase (the interstitial region between the other two regions) Their results highlighted the percolative nature and the importance of cluster connectivity on ionic conduction in sulfonated poly(phenylene oxide) (SPPO) membranes. Mobile protons attached to sulfonic groups were replaced by Pb2+ ions for a better visualization of the hydrophilic domains To this end, dried membranes in Na+ form were immersed in a 1 M HCl solution several times for 48 h to replace Na+ with H+ and washed with deionized water. The experimental data obtained from the EIS measurements were analyzed using the Z-View analysis impedance software (Scribner Associates, Inc., Southern Pines, NC, USA)
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