Vacancy Defects and Multilayer Shading in Graphene Monolayers: Enhancing Proton Transport in Centimeter-Sized Direct Methanol Fuel Cells

Proton selectivity of monolayer graphene offers promising opportunities in energy devices. The impermeability of graphene to methanol, on the other hand, offers potential as a membrane in direct methanol fuel cells (DMFCs). The integration of graphene in DMFCs requires samples larger than 1 cm² and therefore must be grown via chemical vapor deposition (CVD) instead of using exfoliation. CVD graphene─as opposed to exfoliated flakes─however, contains inherent defects and multilayer patches, both of which could be harnessed to tune the fuel cell performance controllably. Here, we investigated the impact of multilayer patches on the performance of centimeter-scale graphene films in DMFCs. While single-crystalline graphene (SCG) has no multilayer patches, polycrystalline graphene (PCG) can exhibit an areal ratio of multilayer patches up to ∼4%. While multilayer patches are less reactive to plasma etching, the monolayers within SCG and PCG exhibit similar reactivity, enabling precise control over the etching process. These shaded areas of the membrane contribute to higher proton selectivity, likely due to more constrained and controlled defect sites. Our findings indicate that plasma-induced defects yield a proton conductance 10% higher than pristine graphene, and we attribute the enhanced performance of defected PCG to the shading effects provided by the multilayer patches.