Characterization and Modeling of Free Volume and Ionic Conduction in Multiblock Copolymer Proton Exchange Membranes

Abstract

Free volume plays a key role on transport in proton exchange membranes (PEMs), including ionic conduction, species permeation, and diffusion. Positron annihilation lifetime spectroscopy and electrochemical impedance spectroscopy are used to characterize the pore size distribution and ionic conductivity of synthesized PEMs from polysulfone/polyphenylsulfone multiblock copolymers with different degrees of sulfonation (SPES). The experimental data are combined with a bundle-of-tubes model at the cluster-network scale to examine water uptake and proton conduction. The results show that the free pore size changes little with temperature in agreement with the good thermo-mechanical properties of SPES. However, the free volume is significantly lower than that of Nafion®, leading to lower ionic conductivity. This is explained by the reduction of the bulk space available for proton transfer where the activation free energy is lower, as well as an increase in the tortuosity of the ionic network.

On the optimal cathode catalyst layer for polymer electrolyte fuel cells: Bimodal pore size distributions with functionalized microstructures

“…The value of σ p in finite-sized ionomer domains depends on the size of the conductive medium and the confined morphology of protogenic groups (thin film vs. nanofiber). 2D thin ionomer films in conventional CLs show a lower conductivity than bulk PEMs (2-10 times lower) due to the more difficult percolation of protons through finite thickness domains (Siroma et al, 2009;Paul et al, 2014;Gostick and Weber, 2015;Chen et al, 2019), increasing with the film thickness-the bulk value of PEMs is reached when the film thickness is significantly larger than the size of protogenic nanodomains (around 10 nm) (Gomaa et al, 2022). However, ionomer nanofibers show an opposite behavior, reaching a higher ionic conductivity for small fiber diameters compared with bulk PEMs (up to 10 times higher) and a similar ionic conductivity to that of PEMs for significantly high fiber diameters (Pan et al, 2008;Sun et al, 2019).…”

“…= 1 s cm −1 and ASR pem = 0.02 cm 2 S −1 , respectively (see, e.g. (Owejan et al, 2013(Owejan et al, , 2014Ureña et al, 2021;Gomaa et al, 2022)); 2) an analysis of the exterior mass and proton transport resistances, R chcl O 2 and ASR pem , where the baseline values are reduced to R chcl O 2 = 0.5 − 0.1 s cm −1 and ASR pem = 0.01-0.002 cm 2 S −1 (x0.5 and x0.1); 3) an analysis of the catalyst activity, i.e., the exchange current density, i o,c , where the baseline value and x10).…”

Positron annihilation lifetime spectroscopic analysis of Nafion and graft‐type polymer electrolyte membranes for fuel cell application

“…[25,28] Some works have proposed only one o-Ps component existing in the Nafion membranes [17,18,29,30] and other PEMs. [15,[31][32][33] In this case, only an average value of o-Ps lifetime for different amorphous arrangements is assumed. In contrast, other works have argued that there are two o-Ps components, which are present in at least two different amorphous arrangements in PEMs.…”