Insights into Proton Permeation Mechanism in Graphene and Graphene Oxide for PEM Fuel Cells

Graphene and graphene oxide (GO) are promising materials in fuel cell applications due to their selective proton transport and exceptional mechanical and thermal stability properties. In this study, we explored the different mechanisms of electrochemically driven proton permeation through graphene and graphene oxide by using ReaxFF molecular dynamics simulations. Our findings reveal that the primary mechanism for proton transport in graphene and graphene oxide is the flipping mechanism, in which the proton first adsorbs onto the surface and flips through the basal graphene ring. However, direct permeation of protons through the center of the graphene ring is also observed. Additionally, our results show that the proton conductivity increases with temperature due to the high thermal energy and a less negative surface charge density. In the case of graphene, as the temperature increases from 300 to 350 K, the proton conductivity rises from 0.014 to 0.020 mS/cm, showing a 43% improvement. In the case of graphene oxide, the proton conductivity at 300 K is two folds that of graphene. Furthermore, the proton conductivity of graphene oxide improves from 0.031 to a maximum of 0.038 mS/cm, showing a 23% improvement as the hydroxyl group concentration increases (0.75OH:0.25OE). Our results highlight the crucial role of functional groups; a higher concentration of the hydroxyl groups enhances the proton conductivity, while epoxy groups influence the surface geometry and charge density distribution of graphene oxide. Our study offers valuable insights into tunable proton conductivity in graphene-based materials.