Effect of the orientation of sulfonated graphene oxide (SG) on the gas-barrier properties and proton conductivity of a SG/Nafion composite membrane

In a proton-exchange membrane fuel cell, the proton-exchange membrane is located between the oxidant and the fuel, which facilitates proton conduction and prevents the penetration of gas molecules. In this study, sulfonated graphene oxides (SG) were synthesized, and a series of SG/Nafion composite multilayer membranes were prepared using layer-by-layer assembling. This multilayer membrane-structure has an efficient orientation effect on the SG sheets. The thinner the layer is, the more significant is the orientation effect. A statistical angle between the SG sheets and the membrane surface (Theta) is used to represent the orientation of SG sheets, and a correlation between the thickness of the layers and Theta is found. Under various conditions, these multilayer composite membranes improve the gas-barrier properties (H-2, O-2, and water vapor) and proton conductivity. The effect of Theta on the gas-barrier properties and proton conductivity of the membranes is discussed. Our results reveal that the SG sheets have excellent gas-barrier properties in the longitudinal direction and good proton conductivity in the lateral direction. Furthermore, based on positron-annihilation lifetime spectroscopy, it is found that the diffusion coefficients of gas molecules in all the SG/Nafion composite membranes are almost identical. In other words, the orientation of the SG sheets changes the gas-barrier properties of the membranes by altering the morphology of the diffusion paths for the gas molecules during the permeation.

Automated summary

In this study, sulfonated graphene oxides (SG) were synthesized and used to prepare SG/Nafion composite multilayer membranes through layer-by-layer assembling. These membranes demonstrate improved gas-barrier properties and proton conductivity, with the orientation of the SG sheets playing a significant role in determining these properties. The diffusion coefficients of gas molecules in the membranes remained consistent despite changes in SG sheet orientation, indicating that morphology of diffusion paths is altered by the orientation of SG sheets.