Abstract
The hydration of proton-conducting ionomers is described in terms of a simplified model, where only osmotic and elastic contributions to the Gibbs free energy of hydration are considered. Although only two physically meaningful parameters are used - the deformation parameter, inversely proportional to the elastic modulus of the ionomer, and the free volume parameter – simulated hydration isotherms are in good agreement with the experiment. The proton mobility u inside the electrolyte solution of the ionomer is calculated from the proton conductivity determined at various hydration numbers. Its variation with the proton concentration c reveals the percolation threshold of hydrated nanometric channels and the tortuosity of the membrane. Above the percolation threshold, a power law u ~ c-3 is observed, in agreement with the “universal” law for 3-dimensional percolation. The proton conductivity shows at 100°C a maximum of 0.2 S/cm at a hydration number ~90. The = f(c) plot allows to predict which hydration conditions are necessary for a desired area specific resistance.
Highlights
ENERGY RESEARCHHydration and proton conductivity of ionomers: the model case of sulfonated aromatic polymers
Polymer electrolytes are a fascinating class of materials with many promising applications, especially in the field of energy and environment (Armand, 1994; Meyer, 1998; Fergus, 2010, 2012; Couture et al, 2011; Hou et al, 2011; Li et al, 2011; Merle et al, 2011; Wang et al, 2011; Di Vona and Knauth, 2013; Hickner, 2013)
Whereas polymer electrolytes for lithium batteries must work in absence of water to avoid corrosion and decomposition reactions, polymer electrolytes used in fuel cells and redox flow batteries contain significant amounts of water in hydrated nanometric domains that assure the proton or anion conduction inside a matrix made by the polymer backbones (Springer et al, 1991; Zawodzinski et al, 1993; Kreuer, 2001; Kreuer et al, 2004; Smitha et al, 2005; Diat and Gebel, 2008; Peckham et al, 2008; Hickner, 2012; Wu et al, 2013)
Summary
Hydration and proton conductivity of ionomers: the model case of sulfonated aromatic polymers. The hydration of proton-conducting ionomers is described in terms of a simplified model, where only osmotic and elastic contributions to the Gibbs free energy of hydration are considered. Only two physically meaningful parameters are used – the deformation parameter, inversely proportional to the elastic modulus of the ionomer, and the free volume parameter – simulated hydration isotherms are in good agreement with the experiment. The proton mobility u inside the electrolyte solution of the ionomer is calculated from the proton conductivity determined at various hydration numbers. Its variation with the proton concentration c reveals the percolation threshold of hydrated nanometric channels and the tortuosity of the membrane. The proton conductivity σ shows at 100°C a maximum of 0.2 S/cm at a hydration number ~90.
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