Deconstructing proton transport through atomically thin monolayer CVD graphene membranes

Selective proton (H+) permeation through the atomically thin lattice of graphene and other 2D materials offers new opportunities for energy conversion/storage and novel separations. Practical applications necessitate scalable synthesis via approaches such as chemical vapor deposition (CVD) that inevitably introduce sub-nanometer defects, grain boundaries and wrinkles, and understanding their influence on H+ transport and selectivity for large-area membranes is imperative but remains elusive. Using electrically driven transport of H+ and potassium ions (K+) we probe the influence of intrinsic sub-nanometer defects in monolayer CVD graphene across length-scales for the first time. At the micron scale, the areal H+ conductance of CVD graphene (similar to 4.5-6 mS cm(-2)) is comparable to that of mechanically exfoliated graphene indicating similarly high crystalline quality within a domain, albeit with K+ transport (similar to 1.7 mS cm(-2)). However, centimeter-scale Nafion|graphene|Nafion devices with several graphene domains show areal H+ conductance of similar to 339 mS cm(-2) and K+ conductance of similar to 23.8 mS cm(-2) (graphene conductance for H+ is similar to 1735 mS cm(-2) and for K+ it is similar to 47.6 mS cm(-2)). Using a mathematical-transport-model and Nafion filled polycarbonate track etched supports, we systematically deconstruct the observed orders of magnitude increase in H+ conductance for centimeter-scale CVD graphene. The mitigation of defects (>1.6 nm), wrinkles and tears via interfacial polymerization results in a conductance of similar to 1848 mS cm(-2) for H+ and similar to 75.3 mS cm(-2) for K+ (H+/K+ selectivity of similar to 24.5) via intrinsic sub-nanometer proton selective defects in CVD graphene. We demonstrate atomically thin membranes with significantly higher ionic selectivity than state-of-the-art proton exchange membranes while maintaining comparable H+ conductance. Our work provides a new framework to assess H+ conductance and selectivity of large-area 2D membranes and highlights the role of intrinsic sub-nanometer proton selective defects for practical applications.

Automated summary

Selective proton permeation through atomically thin materials like graphene is important for energy conversion/storage and separations. This study investigates the influence of sub-nanometer defects in chemically vapor deposited (CVD) graphene on proton transport and selectivity. The results show that mitigating defects and using interfacial polymerization significantly increase proton conductance and selectivity, making the atomically thin membranes suitable for practical applications.