Ion transfer asymmetry in Nafion membranes with gradient distribution of acid salts of heteropoly acids
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We describe the effect caused by the modification of Nafion membranes with nanoparticles of rubidium and cesium acid salts of phosphotungstic and silicotungstic heteropoly acids (HPAs) using layer-by-layer casting on the properties of the membranes depending on the dopant type and the percent modified layer thickness. The ionic conductivity and diffusion permeability of hybrid materials have been studied. The modification is shown to increase the ionic conductivity at low relative humidity. The highest conductivity is observed for the membranes where the modified layer amounts to 35–50% of the total membrane thickness. On the contrary, diffusion permeability is reduced upon modification. Asymmetric ion transfer is observed: the ion transfer rate from the modified layer is lower than from the unmodified layer. This asymmetry arises from the anion concentration gradient in the membrane, which is directed from the modified to unmodified side of the membrane. An asymmetric membrane oriented so that its modified side faces the anode can potentially increase the power of a fuel cell.
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